1.4+Energy+Flow+in+Ecosystems

= Ecosystems =

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Ecology is the study of living organisms and their environment. Its aim it to explain why organisms live where they do. To do this ecologists study __ecosystems__, areas that can vary in size from a pond to the whole planet. **Energy and Matter ** Before studying ecosystems, it is important to appreciate the difference between energy and matter. Energy and matter are quite different things and cannot be inter-converted.
 * **Ecosystem ** || **A reasonably self-contained area together with all its living organisms. ** ||
 * **Habitat ** || The physical or __abiotic__ part of an ecosystem, i.e. a defined area with specific characteristics where the organisms live, e.g. oak forest, deep sea, sand dune, rocky shore, moorland, hedgerow, garden pond, etc. ||
 * **Community ** || The living or __biotic__ part of an ecosystem, i.e. all the populations of all the different species living in one habitat. ||
 * **Biotic ** || Any living or biological factor. ||
 * **Abiotic ** || Any non-living or physical factor. ||
 * **Population ** || The members of the __same species__ living in one habitat. ||
 * **Species ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A group of organisms that can interbreed and produce fertile offspring. ||
 * __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Energy __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> comes in many different forms (such as heat, light, chemical, potential, kinetic, etc.) which can be inter-converted, but energy can never be created, destroyed or used up. If we talk about energy being "lost", we usually mean as heat, which is radiated out into space. Energy is constantly arriving on earth from the sun, and is constantly leaving the earth as heat, but the total amount of energy on the earth is constant.
 * __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Matter __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> comes in three states (solid, liquid and gas) and again, cannot be created or destroyed. The total amount of matter on the Earth is constant. Matter (and especially the biochemicals found in living organisms) can contain stored chemical energy, so a cow contains biomass (matter) as well as chemical energy stored in its biomass.

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">All living organisms need energy and matter from their environment. Matter is needed to make new cells (growth) and to create now organisms (reproduction), while energy is needed to drive all the chemical and physical processes of life, such as biosynthesis, active transport and movement. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Food Chains and Webs ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The many relationships between the members of a community in an ecosystem can be described by __food chains__ and __webs__. Each stage in a food chain is called a__trophic level__, and the arrows represent the flow of energy and matter through the food chain. Food chains always start with photosynthetic __producers__ (plants, algae, plankton and photosynthetic bacteria) because, uniquely, producers are able to extract both energy and matter from the abiotic environment (energy from the sun, and 98% of their matter from carbon dioxide in the air, with the remaining 2% from water and minerals in soil). All other living organisms get both their energy and matter by eating other organisms.

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Although this represents a "typical" food chain, with producers being eaten by animal consumers, different organisms use a large range of feeding strategies (other than consuming), leading to a range of different types of food chain. Some of these strategies are defined below, together with other terms associated with food chains. <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">(=consumers + decomposers) || <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">So food chains need not end with a consumer, and need not even start with a producer, e.g.:
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Producer ** || **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An organism that produces food from carbon dioxide and water using photosynthesis. Can be plant, algae, plankton or bacteria. ** ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Consumer ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An animal that eats other organisms ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Herbivore ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A consumer that eats plants (= primary consumer). ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Carnivore ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A consumer that eats other animals (= secondary consumer). ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Top carnivore ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A consumer at the top of a food chain with no predators. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Omnivore ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A consumer that eats plants or animals. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Vegetarian ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A human that chooses not to eat animals (humans are omnivores) ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Autotroph ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An organism that manufactures its own food (= producer) ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Photoautotroph ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An organism that manufactures its own food using light energy ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Chemoautotroph ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An organism that manufactures its own food using energy derived from chemical reactions (e.g. sulphur reducing bacteria) ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Heterotroph ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An organism that obtains its energy and mass from other organisms
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Plankton ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Microscopic aquatic organisms. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Phytoplankton ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">"Plant plankton" i.e. microscopic aquatic producers. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Zooplankton ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">"Animal plankton" i.e. microscopic aqautic consumers. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Predator ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An animal that hunts and kills animals for food. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Prey ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An animal that is hunted and killed for food. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Scavenger ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An animal that eats dead animals, but doesn't kill them ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Detritus ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Dead and waste matter that is not eaten by consumers ||
 * **<span style="color: red; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Decomposer ** || <span style="color: red; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An organism that consumes detritus (= detrivores + saprophytes) ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Detritivore ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An animal that eats detritus. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Saprophyte ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A microbe (bacterium or fungus) that lives on detritus. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Symbiosis ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Organisms living together in a close relationship (= parasitism, mutualism, commensalisim). ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Mutualism ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Two organisms living together for mutual benefit. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Commensalism ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Relationship in which only one organism benefits ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Parasite ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An organism that feeds on a larger living host organism, harming it ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Pathogen ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A microbe that causes a disease. ||



**<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Ecological Pyramids ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">In general as you go up a food chain the size of the individuals increases and the number of individuals decreases. These sorts of observations can be displayed in ecological pyramids, which are used to __quantify__ food chains. There are three kinds: **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">1. Pyramids of Numbers. ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%; margin-left: 0.5in;">These show the numbers of organisms at each trophic level in a food chain. The width of the bars represent the numbers, or the bars may be purely qualitative. The numbers should be normalised for a given area for a terrestrial habitat (usually m²), or volume for an aquatic habitat (m³). Pyramids of numbers are most often triangular shaped, but can be almost any shape. In the pyramids below, A shows a typical pyramid of numbers for carnivores; B shows the effect of a single large producer such as a tree; and C shows a typical parasite food chain. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%; margin-left: 0.5in;"> **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">2. Pyramids of Biomass ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%; margin-left: 0.5in;">These convey more information, since they consider the total mass of living organisms (i.e. the biomass) at each trophic level. The biomass should be __dry mass__(since water stores no energy) and is measured in kg m-2. The biomass may be found by drying and weighing the organisms at each trophic level, or by counting them and multiplying by an average individual mass. Pyramids of biomass are __always__ pyramid shaped, since if a trophic level gains all its mass from the level below, then it cannot have more mass than that level (you cannot weigh more than you eat). The "missing" mass, which is not eaten by consumers, becomes detritus and is decomposed.

**<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">3. Pyramids of Energy ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%; margin-left: 0.5in;">Food chains represent flows of matter __and__ energy, so two different pyramids are needed to quantify each flow. Pyramids of energy show how much energy flows into each trophic level in a given time, so the units are usually something like kJ m-2 y-1. Pyramids of energy are always pyramidal (energy cannot be created), and always very shallow, since the transfer of energy from one trophic level to the next is very inefficient The "missing" energy, which is not passed on to the next level, is lost eventually as heat.

**<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Energy Flow in Ecosystems ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Three things can happen to the energy taken in by the organisms in a trophic level: <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">These three fates are shown in this energy flow diagram:
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">It can be passed on to the next trophic level in the food chain when the organism is eaten.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">It can become stored in detritus. This energy is passed on to decomposers when the detritus decays.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">It can be converted to heat energy by inefficient chemical reactions, radiated by warm bodies, or in friction due to movement. The heat energy is lost to the surroundings, and cannot be regained by living organisms.

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Eventually __all__ the energy that enters the ecosystem will be converted to heat, which is lost to space. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Material Cycles in Ecosystems ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Matter cycles between the biotic environment and in the abiotic environment. Simple inorganic molecules (such as CO2, N2 and H2O) are __assimilated__ (or __fixed__) from the abiotic environment by producers and microbes, and built into complex organic molecules (such as carbohydrates, proteins and lipids). These organic molecules are passed through food chains and eventually returned to the abiotic environment again as simple inorganic molecules by decomposers. Without either producers or decomposers there would be no nutrient cycling and no life.

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The simple inorganic molecules are often referred to as __nutrients__. Nutrients can be grouped as: __major nutrients__ (molecules containing the elements C, H and O, comprising >99% of biomass); __macronutrients__ (molecules containing elements such as N, S, P, K, Ca and Mg, comprising 0.5% of biomass); and __micronutrients__ or__trace elements__ (0.1% of biomass). Macronutrients and micronutrients are collectively called __minerals__. While the major nutrients are obviously needed in the largest amounts, the growth of producers is usually limited by the availability of minerals such as nitrate and phosphate. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">There are two groups of decomposers: <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Detailed material cycles can be constructed for elements such as carbon, nitrogen, oxygen or sulphur, or for compounds such as water, but they all have the same basic pattern as the diagram above. We shall only study the carbon and nitrogen cycles in detail.
 * __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Detrivores __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> are animals that eat detritus (such as earthworms and woodlice). They digest much of the material, but like all animals are unable to digest the cellulose and lignin in plant cell walls. They break such plant tissue into much smaller pieces with a larger surface area making it more accessible to the saprophytes. They also assist saprophytes by excreting useful minerals such as urea, and by aerating the soil.
 * __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Saprophytes __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> (or decomposers) are microbes (fungi and bacteria) that live on detritus. They digest it by extracellular digestion, and then absorb the soluble nutrients. Given time, they can completely break down any organic matter (including cellulose and lignin) to inorganic matter such as carbon dioxide, water and mineral ions.

**<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The Carbon Cycle **

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">As this diagram shows, there are really many carbon cycles here with time scales ranging from minutes to millions of years. Microbes play the major role at all stages. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The Nitrogen Cycle **
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Far more carbon is fixed by microscopic marine producers (algae and phytoplankton) from CO2 dissolved in the oceans than by terrestrial plants from CO2 in the air.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A large amount of the fixed carbon is used by marine zooplankton to make calcium carbonate shells. These are not eaten by consumers and cannot easily be decomposed, so turn into carboniferous rocks (chalk, limestone, coral, etc). 99% of the Earth's carbon is in this form.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The decomposers are almost all microbes such as fungi and bacteria. Most of the detritus is in the form of cellulose and other plant fibres, which higher organisms cannot digest. Only a few bacteria posses the __cellulase__ enzymes required to break down plant fibres. Herbivorous animals such as cows and termites depend on these bacteria in their guts.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Much of the CO2 that was fixed during the carboniferous era (300 MY ago) was sedimented and turned into fossil fuels. The recent mining and burning of fossil fuels has significantly altered the carbon cycle by releasing the carbon again, causing a 15% increase in CO2 in just 200 years. Many people believe this is largely responsible for global warming.

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Microbes are involved at most stages of the nitrogen cycle: **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Nitrogen Fixation. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">78% of the atmosphere is nitrogen gas (N2), but this is inert and can’t be used by plants or animals. __Nitrogen fixing bacteria__ reduce nitrogen gas to ammonia (N2 + 6H Õ 2NH3), which dissolves to form ammonium ions. This process uses the enzyme __nitrogenase__ and ATP as a source of energy. The nitrogen-fixing bacteria may be free-living in soil or water, or they may live in colonies inside the cells of root nodules of leguminous plants such as clover or peas. This is an example of __mutualism__ as the plants gain a source of useful nitrogen from the bacteria, while the bacteria gain carbohydrates and protection from the plants. Nitrogen gas can also be fixed to ammonia by humans using the Haber process, and a small amount of nitrogen is fixed to nitrate by lightning. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Nitrification. **__<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Nitrifying bacteria __<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> can oxidise ammonia to nitrate in two stages: first forming nitrite ions then forming nitrate ions. These are chemosynthetic bacteria, which means they use the energy released by nitrification to live, instead of using respiration. Plants can only take up nitrogen in the form of nitrate. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Denitrification. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The anaerobic __denitrifying bacteria__ convert nitrate to N2 and NOx, which is then lost to the air. This represents a constant loss of "useful" nitrogen from soil, and explains why nitrogen fixation by the nitrifying bacteria and fertilisers are so important. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Ammonification. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Microbial saprophytes break down proteins in detritus to form ammonia in two stages: first they digest proteins to amino acids using extracellular__protease__ enzymes, then they remove the amino groups from amino acids using __deaminase__ enzymes. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Population Ecology ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Population Ecology in concerned with the question: why is a population the size it is? This means understanding the various factors that affect the population. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Population Growth ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">When a species is introduced into a new environment its population grows in a characteristic way. This __growth curve__ is often seen experimentally, for example bees in a hive, sheep in Tasmania, bacteria in culture. The curve is called a __logistic__ or __sigmoid growth curve__.

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The growth curve has three phases, with different factors being responsible for the shape of each phase. The actual factors depend on the ecosystem, and this can be illustrated by considering two contrasting examples: yeast in a flask (reproducing asexually), and rabbits in a field (reproducing sexually). <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">At the end of phase 3 the population is stable. This population is called the __carrying capacity__ of the environment (K), and is the maximum population supported by a particular ecosystem. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Factors Affecting Population Size ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Many different factors interact to determine population size, and it can be very difficult to determine which factors are the most important. Factors can be split into two broad group: abiotic factors and biotic factors. We’ll look at 7 different factors. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">1. Abiotic Factors ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The population is obviously affected by the abiotic environment such as: temperature; water/humidity; pH; light/shade; soil (edaphic factors); mineral supply; current (wind/water); topography (altitude, slope, aspect); catastrophes (floods/fire/frost); pollution. Successful species are generally well adapted to their abiotic environment. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">In harsh environments (very cold, very hot, very dry, very acid, etc.) only a few species will have successfully adapted to the conditions so they will not have much competition from other species, but in mild environments lots of different species could live there, so there will be competition. In other words in harsh environments abiotic factors govern who survives, while in mild environments biotic factors (such as competition) govern who survives. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">2. Seasons ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Many abiotic factors vary with the seasons, and this can cause a periodic oscillation in the population size. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">This is only seen in species with a short life cycle compared to the seasons, such as insects. Species with long life cycles (longer than a year) do not change with the seasons like this.
 * || **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Yeast in a flask **  ||  **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Rabbits in a field **  ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">1. Lag phase ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Little growth while yeast starts transcribing genes and synthesising appropriate enzymes for new conditions. || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Little growth due to small population. Individuals may rarely meet, so few matings. Long gestation so few births. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">2. Rapid Growth Phase ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Rapid exponential growth. No limiting factors since relatively low density. || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Rapid growth, though not exponential. Few limiting factors since relatively low density. ||
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">3. Stable Phase ** || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Slow growth due to accumulation of toxic waste products (e.g. ethanol) or lack of sugar. || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Slow growth due to intraspecific competition for food/territory, predation, etc. ||

**<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">3. Food Supply ** **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">4. Interspecific Competition ** __<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Interspecific competition __<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> is competition for resources (such as food, space, water, light, etc.) between members of __different species__, and in general one species will out-compete another one. This can be demonstrated by growing two different species of the protozoan //Paramecium// in flasks in a lab. They both grow well in lab flasks when grown separately, but when grown together //P.aurelia// out-competes //P.caudatum// for food, so the population of //P.caudatum// falls due to interspecific competition:
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A population obviously depends on the population of its food supply: if there is plenty of food the population increases and vice versa. For example red deer introduced to an Alaskan island at first showed a population increase, but this large population grazed the vegetation too quickly for the slow growth to recover, so the food supply dwindled and the deer population crashed. ||
 * [[image:file:///C:/DOCUME~1/JONBUL~1/LOCALS~1/Temp/msohtmlclip1/01/clip_image017.gif width="263" height="158" align="center" caption="http://www.mrothery.co.uk/ecology/Image32.gif"]] ||

__<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Intraspecific competition __<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> is competition for resources between members of the __same species__. This is more significant than interspecific competition, since member of the same species have the same niche and so compete for exactly the same resources. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Intraspecific competition tends to have a stabilising influence on population size. If the population gets too big, intraspecific population increases, so the population falls again. If the population gets too small, intraspecific population decreases, so the population increases again: <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Intraspecific competition is also the driving force behind natural selection, since the individuals with the "best" genes are more likely to win the competition and pass on their genes. Some species use aggressive behaviour to minimise real competition. Ritual fights, displays, threat postures are used to allow some individuals (the "best") to reproduce and exclude others (the "weakest"). This avoids real fights or shortages, and results in an optimum size for a population. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">6. Predation ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The populations of predators and their prey depend on each other, so they tend to show cyclical changes. This has been famously measured for populations of lynx (predator) and hare (prey) in Canada, and can also be demonstrated in a lab experiment using two species of mite: //Eotetranchus// (a herbivore) and //Typhlodromus//(a predator). If the population of the prey increases, the predator will have more food, so its population will start to increase. This means that more prey will be eaten, so its population will decrease, so causing a cycle in both populations:
 * 1) **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Intraspecific Competition **

**<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">7. Parasitism and Disease ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Parasites and their hosts have a close symbiotic relationship, so their populations also oscillate. This is demonstrated by winter moth caterpillars (the host species) and wasp larvae (parasites on the caterpillars). If the population of parasite increases, they kill their hosts, so their population decreases. This means there are fewer hosts for the parasite, so their population decreases. This allows the host population to recover, so the parasite population also recovers:

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A similar pattern is seen for pathogens and their hosts. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The Ecological Niche ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">A population’s __niche__ refers to its role in its ecosystem. This usually means its feeding role in the food chain, so a particular population’s niche could be a producer, a predator, a parasite, a leaf-eater, etc. A more detailed description of a niche should really include many different aspects such as its food, its habitat, its reproduction method etc, so gerbils are desert seed-eating mammals; seaweed is an inter-tidal autotroph; fungi are asexual soil-living saprophytes. Identifying the different niches in an ecosystem helps us to understand the interactions between populations. Members of the same population always have the same niche, and will be well-adapted to that niche, e.g. nectar feeding birds have long thin beaks. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Species with narrow niches are called __specialists__ (e.g. anteater). Many different specialists can coexist in the same habitat because they are not competing, so this can lead to high diversity, for example warblers in a coniferous forest feed on insects found at different heights. Specialists rely on a constant supply of their food, so are generally found in abundant, stable habitats such as the tropics. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Species with broad niches are called __generalists__ (e.g. common crow). Generalists in the same habitat will compete, so there can only be a few, so this can lead to low diversity. Generalists can cope with a changing food supply (such as seasonal changes) since they can switch from one food to another or even one habitat to another (for example by migrating).

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The niche concept was investigated in some classic experiments in the 1930s by __Gause__. He used flasks of different species of the protozoan //Paramecium//, which eats bacteria. <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Conclusion: These two species of//Paramecium// share the same niche, so they compete. //P. aurelia// is faster-growing, so it out-competes //P. caudatum//. || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Conclusion: These two species of//Paramecium// have slightly different niches, so they don't compete and can coexist. || <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">It is important to understand the distribution in experiment 2. //P. caudatum// lives in the upper part of the flask because only it is adapted to that niche and it has no competition. In the lower part of the flask both species could survive, but only //P. bursaria// is found because it out-competes //P. caudatum//. If //P. caudatum// was faster-growing it would be found throughout the flask. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The niche concept is summarised in the __competitive exclusion principle__: Two species cannot coexist in the same habitat if they have the same niche. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Succession ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Ecosystems are not fixed, but constantly change with time. This change is called __succession__. Imagine a lifeless area of bare rock. What will happen to it as time passes?
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Experiment. 1:
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Experiment. 2:

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">These stages are called __seral stages__, or __seral communities__, and the whole succession is called a __sere__. Each organism modifies the environment, so creating opportunities for other species. As the succession proceeds the community becomes more diverse, with more complex food webs being supported. The final seral stage is stable (assuming the environment doesn’t change), so succession stops at the climax stage. In England the natural climax community is oak or beech woodland (depending on the underlying rock), and in the highlands of Scotland it is pine forests. In Roman times the country was covered in oak and beech woodlands with herbivores such as deer, omnivores such as bear and carnivores such as wolves and lynxes. It was said that a squirrel could travel from coast to coast without touching ground. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Humans interfere with succession, and have done so since Neolithic times, so in the UK there are few examples of a natural climax left (except perhaps small areas of the Caledonian pine forest in the Scottish Highlands). Common landscapes today like farmland, grassland, moorland and gardens are all maintained at pre-climax stages by constant human interventions, including ploughing, weeding, herbicides, burning, crop planting and grazing animals. These are examples of an artificial climax, or __plagioclimax__. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Ecological Impact of Farming ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">One of the main reasons for studying ecology is to understand the impact humans are having on the planet. The huge increases in human population over the last few hundred years has been possible due to the development of __intensive farming__, including monoculture, selective breeding, huge farms, mechanisation and the use of chemical fertilisers and pesticides. However, it is apparent that this intensive farming is damaging the environment and is becoming increasingly difficult to sustain. Some farmers are now turning to environmentally-friendly __organic farming__. We’ll examine 5 of the main issues and their possible solutions. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">1. Monoculture ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Until the middle of the 20th century, farms were usually small and __mixed__ (i.e. they grew a variety of crops and kept animals). About a third of the population worked on farms. The British countryside was described by one observer in 1943 as "//an attractive patchwork with an infinite variety of small odd-shaped fields bounded by twisting hedges, narrow winding lanes and small woodlands//". Today the picture is quite different, with large uninterrupted areas of one colour due to specialisation in one crop - __monoculture__. Monoculture increases the productivity of farmland by growing only the best variety of crop; allowing more than one crop per year; simplifying sowing and harvesting of the crop; and reducing labour costs. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">However, monoculture has a major impact on the environment: <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Some farmers are now returning to traditional crop rotations, where different crops are grown in a field each year. This breaks the life cycles of pests (since their host is changing); improves soil texture (since different crops have different root structures and methods of cultivation); and can increase soil nitrogen (by planting nitrogen-fixing legumes).
 * 1) <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Very few species can live on bare rock since it stores little water and has few available nutrients. The first __colonisers__ are usually __lichens__, which are a mutualistic relationship between an alga and a fungus. The alga photosynthesises and makes organic compounds, while the fungus absorbs water and minerals and clings to the rock. Lichens are such good colonisers that almost all "bare rock" is actually covered in a thin layer of lichen. __Mosses__ can grow on top of the lichens. Between then these colonisers start to erode the rock and so form a thin soil. Colonisers are slow growing and tolerant of extreme conditions.
 * 2) __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Pioneer species __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> such as __grasses__ and __ferns__ grow in the thin soil and their roots accelerate soil formation. They have a larger photosynthetic area, so they grow faster, so they make more detritus, so they form better soil, which holds more water.
 * 3) __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Herbaceous Plants __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> such as dandelion, goosegrass ("weeds") have small wind-dispersed seeds and rapid growth, so they become established before larger plants.
 * 4) <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Larger plants (__shrubs__) such as bramble, gorse, hawthorn, broom and rhododendron can now grow in the good soil. These grow faster and so out-compete the slower-growing pioneers.
 * 5) __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Trees __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> grow slowly, but eventually shade and out-compete the shrubs, which are replaced by shade-tolerant forest-floor species. A complex food web is now established with many trophic levels and interactions. This is called the __climax community__.
 * __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Primary succession __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> starts with bare rock or sand, such as behind a retreating glacier, after a volcanic eruption, following the silting of a shallow lake or seashore, on a new sand dune, or on rock scree from erosion and weathering of a mountain.
 * __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Secondary succession __<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;"> starts with soil, but no (or only a few) species, such as in a forest clearing, following a forest fire, or when soil is deposited by a meandering river.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Using a single variety of crop reduces genetic diversity and renders all crops in a region susceptible to disease.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Fertilisers are required to maintain soil fertility. This is expensive and can pollute surrounding groundwater due to leaching.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Pesticides are required to keep crops healthy. Again this is expensive and potentially polluting.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Monoculture reduces species diversity. This has many knock-on effects such as allowing a pest species to get out of control, fewer plants due to lack of pollinating insects and loss of species that may be useful to humans.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Less attractive countryside.

**<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">2. Hedgerows ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Hedges have been planted since Anglo-Saxon times to mark field boundaries and to contain livestock. As they have matured they have diversified to contain a large number of different plant and animal species, some found nowhere else in the UK. Since the second world war much of the hedgerow has been removed because: <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">However it has now become clear that hedgerows served an important place in the ecology of Britain. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The importance of hedgerows is now being recognised, and farmers can now receive grants to plant hedgerows. However it takes hundreds of years for new hedgerows to mature and develop the same diversity as the old ones. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">3. Fertilisers ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Since the rate of plant growth in usually limited by the availability of mineral ions in the soil, then adding more of these ions as fertiliser is a simple way to improve yields, and this is a keystone of intensive farming. The most commonly used fertilisers are the soluble __inorganic fertilisers__ containing nitrate, phosphate and potassium ions (NPK). Inorganic fertilisers are very effective but also have undesirable effects on the environment. Since nitrate and ammonium ions are very soluble, they do not remain in the soil for long and are quickly leached out, ending up in local rivers and lakes and causing eutrophication. They are also expensive. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">An alternative solution, which does less harm to the environment, is the use of __organic fertilisers__, such as animal manure (farmyard manure or FYM), composted vegetable matter, crop residues, and sewage sludge. These contain the main elements found in inorganic fertilisers (NPK), but in organic compounds such as urea, cellulose, lipids and organic acids. Of course plants cannot make use of these organic materials in the soil: their roots can only take up inorganic mineral ions such as nitrate, phosphate and potassium. But the organic compounds can be digested by soil organisms such as animals, fungi and bacteria, who then release inorganic ions that the plants can use (refer to the nitrogen cycle). Some advantages of organic fertilisers are: <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Some disadvantages are that they are bulky and less concentrated in minerals than inorganic fertilisers, so more needs to be spread on a filed to have a similar effect. They may contain unwanted substances such as weed seeds, fungal spores, heavy metals. They are also very smelly! **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">4. Pesticides ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">To farmers, a __pest__ is any organism (animal, plant or microbe) that damages their crops. Some form of pest control has always been needed, whether it is __chemical__(e.g. pesticides), __biological__ (e.g. predators) or __cultural__ (e.g. weeding or a scarecrow). Chemicals pesticide include:
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">As mixed farms converted to arable farms, hedgerows are no longer needed to contain livestock.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Many small farms have been amalgamated into large farms, allowing larger fields, which in turn allows greater mechanisation and lower labour costs. One farmer found that by removing 1.5 miles of hedges, he increased his arable land by 3 acres and reduced harvesting time by one third.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Hedgerows reduce the space available for planting crops, and their roots compete with those of crops for water and minerals in the soil.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Hedgerows provide shelter for pests such as rabbits and insects, and they are a reservoir of weeds and disease.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Hedgerows need to be maintained, which is a skilled job, costing time and money.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">They provide habitats for at least 30 species of trees and shrubs, 65 species of nesting birds, 1500 species of insects and 600 species of wildflowers. These in turn provide food for small mammals.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">They act as corridors, allowing animals to move safely between woodlands.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Some of the animals they shelter are predators of plant pests, so they may reduce pests, not increase them.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">They are efficient windbreaks, providing shelter for animals and plants, and reducing soil erosion. During storms in recent years large amounts of topsoil was blown away from large unsheltered fields.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">They provide habitats for pollinating insects, so removing hedgerows can indirectly reduce the populations of other local plant species.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">In the UK we have surpluses of many crops, and farmers can receive grants to reduce their food production.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Since the compounds in organic fertilisers are less soluble than those in inorganic fertilisers, the inorganic minerals are released more slowly as they are decomposed. This prevents leaching and means they last longer.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The organic wastes need to be disposed of anyway, so they are cheap. Furthermore, spreading on to fields means they will not be dumped in landfill sites, where they may have caused uncontrolled leaching.
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The organic material improves soil structure by binding soil particles together and provides food for soil organisms such as earthworms. This improves drainage and aeration.

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Pesticides have to be effective against the pest, but have no effect on the crop. They may kill the pests, or just reduce their population by slowing growth or preventing reproduction. Intensive farming depends completely on the use of pesticides, and some wheat crops are treated with 18 different chemicals to combat a variety of weeds, fungi and insects. In addition, by controlling pests that carry human disease, they have saved millions of human lives. However, with their widespread use and success there are problems, the mains ones being __persistence__ and __bioaccumulation__. <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Both of these are illustrated by DDT (DichloroDiphenylTrichloroethane), an insecticide used against the malaria mosquito in the 1950s and 60s very successfully, eradicating malaria from southern Europe. However the population of certain birds fell dramatically while it was being used, and high concentrations of DDT were found in their bodies, affecting calcium metabolism and causing their egg shells to be too thin and fragile. DDT was banned in developed countries in 1970, and the bird populations have fully recovered. Alternative pesticides are now used instead, but they are not as effective, and continued use of DDT may have eradicated malaria in many more places. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Persistence **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">. This refers to how long a pesticide remains active in the environment. Some chemicals are broken down by decomposers in the soil (they’re__biodegradable__) and so are __not persistent__, while others cannot be broken down by microbes (they’re __non biodegradable__) and so continue to act for many years, and are classed as __persistent__ pesticides. The early pesticides (such DTT) were persistent and did a great deal of damage to the environment, and these have now largely been replaced with biodegradable insecticides such as carbamates and pyrethroids. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Bioaccumulation (or Biomagnification) **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">. This refers to the built-up of a chemical through a food chain. DDT is not soluble in water and is not excreted easily, so it remains in the fat tissue of animals. As each consumer eats a large mass of the trophic level below it, DTT accumulates in the fat tissue of animals at the top of the food chain. This food chain shows typical concentrations of DDT found in a food chain (in parts per million, ppm):
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">herbicides anti-plant chemicals
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">insecticides anti-insect chemicals
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">fungicides anti-fungal chemicals
 * <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">bactericides anti-bacterial chemicals

<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">The high concentration of DDT in birds explains why the toxic effects of DDT were first noticed in birds. **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">5. Eutrophication ** <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Eutrophication refers to the effects of nutrients on aquatic ecosystems. These naturally progress from being __oligotrophic__ (clean water with few nutrients and algae) to__eutrophic__ (murky water with many nutrients and plants) and sometimes to __hypertrophic__ (a swamp with a mass of plants and detritus). This is in fact a common example of succession. In the context of pollution "eutrophication" has come to mean a sudden and dramatic increase in nutrients due to human activity, which disturbs and eventually destroys the food chain. The main causes are fertilisers leaching off farm fields into the surrounding water course, and sewage (liquid waste from houses and factories). These both contain dissolved minerals, such as nitrates and phosphates, which enrich the water. <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">However, the fast-growing algae will out-compete larger plants for light, causing the plants to die. The algae also grow faster than their consumers, so many will die without being consumed, which is not normal. These both lead to a sudden increase in detritus. Sewage may also contain organic matter, which adds to the detritus. <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Decomposing microbes can multiply quickly in response to this, and being aerobic they use up oxygen faster than it can be replaced by photosynthesis or diffusion from the air. The decreased oxygen concentration kills larger aerobic animals and encourages the growth of anaerobic bacteria, who release toxic waste products. || **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Biochemical Oxygen Demand (BOD). **<span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">This measures the rate of oxygen consumption by a sample of water, and therefore gives a good indication of eutrophication. A high BOD means lots of organic material and aerobic microbes, i.e. eutrophication. The method is simple: a sample of water is taken and its O2concentration is measured using an oxygen meter. The sample is then left in the dark for 5 days at 20°C, and the O2 is measured again. The BOD is then calculated from: original O2 concentration – final O2 concentration. The more oxygen used up over the 5 days (in mg.dm-3) the higher the BOD, and the higher the BOD the more polluted the water is. This table shows some typical BOD values. <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">polluted water <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">cleaned sewage <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">raw sewage || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">3 <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">10 <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">20 (legal max) <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">300 || <span style="color: black; font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Aquatic ecosystems can slowly recover from a high BOD as oxygen dissolves from the air, but long-term solutions depend on reducing the amount of minerals leaching into the water. This can be achieved by applying inorganic fertilisers more carefully, by using organic fertilisers, by using low-phosphate detergents, and by removing soluble minerals by precipitation in modern sewage plants. As a last resort eutrophic lakes can be dredged to remove mineral-rich sediment, but this is expensive and it takes a long time for the ecosystem to recover. This has been done in the Norfolk Broads
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">[[image:file:///C:/DOCUME~1/JONBUL~1/LOCALS~1/Temp/msohtmlclip1/01/clip_image027.gif width="275" height="540" caption="http://www.mrothery.co.uk/ecology/Image42.gif"]] || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">Since producer growth is generally limited by availability of minerals, a sudden increase in these causes a sudden increase in producer growth. Algae grow faster than larger plants, so they show a more obvious "bloom", giving rise to spectacular phenomena such as red tides. Algae produce oxygen, so at this point the ecosystem is well oxygenated and fish will thrive.
 * || <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">BOD (mg.dm-3) ||
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 150%;">clean water