Environmental Systems and Society Guide Part 1 by Nigel Gardner

Environmental Systems and Society

Topic 1: Systems and models 1.1. 1: Systems Why Environmental Systems? SYSTEM: an assemblage of parts and their relationship forming a functioning entirety or whole. During the 1970’s, British chemist James Lovelock and American biologist Lynn Margulis came up with the GAIA HYPOTHESIS: That the world acts like a single biological being made up of many individual and interconnected units ( A SYSTEM ). Gaia was the Greek Earth goddess

figure 1. A systematic view of the Earth’s biological and chemical components

The Components The Earth’s systems comprise interactions between the living ( Biotic ) and nonliving ( Abiotic ) constituent parts. As in any system these interactions involve

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INPUTS, PROCESSING of the inputs to create OUPUTS. Even if we look at the starting point of all food chains on Earth, photosynthesis and conversion of light energy to stored chemical energy in the leaf, this to can be viewed as a system component within a bigger system.

So Photosynthesis comprises inputs, a process and outputs

But photosynthesis is also a component in a larger system. A food chain the initial light energy gets processed and converted into chemical energy (food) that is passed along the system.

Yet if you take each of the organisms in the diagram above and place them in individual plant pots or cages at a zoo and the system breaks down: the interactions between the components are what make the system not the components themselves

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1.1. 2: Types of System Systems can be thought of as fitting into one of three types: Open (exchange matter and energy with its surroundings), Closed and Isolated Open Systems: exchange matter and energy with its surroundings. Most systems are open, including ecosystems. In forest ecosystems plants fix energy from light entering the system during photosynthesis. Nitrogen is fixed by soil bacteria. Herbivores that live within the forest canopy may graze in adjacent ecosystems such as a grassland, but when they return they enrich the soil with feces. After a forest fire top soil may be removed by wind and rain. Mineral nutrients are dissolved out of the soil and transported in ground water to streams and rivers. Open system models can even be applied to the remotest oceanic island - energy and mater is exchanged with the atmosphere, surrounding oceans and even migratory birds. It is important to remember that if we are thinking in the terms of systems, then each component of a system is surrounded by a larger environment. A single tree ( a system in its own right ) within a forest system exchanges energy and material with the surrounding forest. Closed Systems: exchange energy but not matter. Closed systems are extremely rare in nature.

No natural closed systems exist on Earth but the planet itself can be thought of as an “almost” closed system. Light energy in large amounts enters the Earth’s system and some is eventually returned to space a long wave radiation (heat). Biosphere 2 was a human attempt to create a habitable Closed system on Earth. build in Arizona at the end of the 1980’s Biosphere 2 was intended to explore the use of closed biospheres in space colonization. Two major “missions” were conducted but both run into problems. The Biosphere never managed to produce enough food to adequately sustain the the participants and at times oxygen levels became dangerously low and needed augmenting. Isolated Systems: An isolated system exchanges neither matter nor energy. These do not exist naturally. Though it is possible to think of the entire Universe as an isolated system.

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1.1. 3: Energy in Systems Energy in all systems is subject to the Laws of Thermodynamics. According to the First Law of Thermodynamics: Energy is neither created or destroyed. What this really means is that the total energy in any system including the entire universe is constant all that can happen is that the form the energy takes changes. This first law is often called the law of conservation of energy.

In the food chain above the energy enters the system as light energy, during photosynthesis it gets converted to stored chemical energy (glucose). It is the stored chemical energy that is passed along as food. No new energy is created it is just passed along. Even if we look at the sunlight falling on Earth not all of it is used for photosynthesis. 30% is reflected, around 50% is converted to heat, and most of the rest powers the hydrological cycle rain, evaporation, wind, etc. Less than 1% of incoming light is use for photosynthesis. The Second Law of Thermodynamics states that the entropy of an isolated system not in equilibrium will tend to increase over time. what this really means is that the energy conversions are never 100% efficient: When energy is transformed into work, some energy is always dissipated as waste heat.

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If you examine the food chain again in terms of the second law then: when the lion chases the zebra, the zebra attempts to escape changing the stored chemical energy in its cells into useful work. But during its attempted escape some of the stored energy is converted to heat and lost from the food chain.

This process can be summarized by a simple diagram showing the energy input and outputs. The Second Law can also be thought off as a simple word equation: ENERGY = WORK + HEAT (and other wasted energy) So what does the term ENTROPY mean? Entropy refers to the spreading out or dispersal of energy. Using the above example the energy spreads out - the useful energy consumed by one level is less than the total energy at the level below - energy transfer is never 100% efficient. Depending on the plant their efficiency at converting solar energy to stored sugars is around 2%. Herbivores on average only use around 10% of the total plant energy they consume the rest is lost in metabolic processes and a carnivores efficiency is also only around 10%. So the carnivores total efficiency in the chain is 0.02 x 0.1 x 0.1 = 0.0002 This means the carnivore only uses 0.02% of incoming solar energy that went into the grass. The rest of the energy is dispersed into the surrounding environment.

1.1. 4: Equilibria Open systems tend to exist in a state of balance: Equilibrium. Equilibrium avoids sudden changes in a system, though this does not mean that all systems are none changing. If change exists it tends to exist between limits. We can therefore think of equilibrium states in two ways STATIC and “STEADY STATE”. Static Equilibrium is where the components of a system remain constant over a long period of time. Possibly the best example of static equilibrium in the environmental system in which we ourselves have to survive is the oxygen content of the atmosphere. Around 4 billion years ago there was very little oxygen in the atmosphere. Why? Our planet was void of life. Then life appeared and importantly photosynthesizing life, first cyanobacteria (bacteria with chlorophyll) and later plants. Both of which produce molecular oxygen a a waste product. As the oxygen levels rose so a new type of organism appeared that could use the external oxygen in respiration - animals - and so the Oxygen cycle was born. Eventually over time a balance was achieved in the level of atmospheric oxygen and for the last 2 billion years, plants and animals have held the oxygen level stable at 21% of the atmosphere. Steady State equilibria: this is a much harder concept to define and there are still arguments for what a dynamic equilibrium really is. The best way to think about it is that a system is in a steady state because the inputs and outputs that affect it approximately balance over a long period of time. An example of this can be seen in a classic study of the populations of Snowshoe Hares and Lynx in Canada. As the population of the Lynx rises the Hare population falls this is then followed by a fall in the Lynx population which in itself is followed by a rise in the Hare population etc. etc.

1.1. 5: Feedback systems Systems are continually affected by information they have to react to from both within and outside. Two simplistic examples, if you start to feel cold you can either put on more clothes or turn the heating up. The sense of cold is information putting on clothes is the reaction. Secondly if you feel hungry, you have a choice of reactions that you can take to this “information” Natural systems act in exactly the same way. The information starts a reaction which in turn may input more information which may start another reaction. This is called a Feedback Loop. Negative Feedback:this tends to damp down, neutralize or counteract any deviation from an equilibrium, and promotes stability. Using the example of the Snowshoe Hare / Lynx population cycle presented in the last section When Hare the population is high, there is surplus food for the Lynx so their numbers go up. This puts a pressure on the Hare population as more are eaten and their numbers fall. Less food for the Lynx so they start to starve and their numbers fall. Fewer Lynx means fewer hares are eaten and their numbers start to go back up. And so it continues as a loop.

Positive Feedback amplifies or increases change; it leads to exponential deviation away from an equilibrium. An example of this is the possible effect that rising global temperature could have by adding more water vapor to the atmosphere. Water is a powerful greenhouse molecule trapping heat in the atmosphere. If there is a global temperature rise more water will evaporate trapping more heat making more water evaporate trapping more heat and on and on. Again a diagram helps explain the idea.

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1.1. 6: Transfers and Transformations Both Material and Energy move or flow through ecosystems. A transfer is when the flow does not involve a change of form and a transformation is a flow involving a change of form. Both types of flow use energy, transfers being simpler use less energy and are therefore more efficient than transformations. Transfers can involve: The movement of material through living organisms (carnivores eating other animals) The movement of material in a non-living process (water being carried by a stream) The movement of energy (ocean currents transferring heat) Transformations can involve: Matter (glucose converted to starch in plants) Energy (Light converted to heat by radiating surfaces) Matter to energy (burning fossil fuels) Energy to matter (photosynthesis)

1.1. 7: Flows and Storages Both energy and matter flows (inputs and outputs) through ecosystems but at times is also stored (stock) within the ecosystem: The Biogeochemical Cycle illustrates the general flows in an ecosystem. Energy flows from one compartment to another. E.g. a food chain. But when one organism eats another organism the energy that moves between them is in the form of stored chemical energy: Flesh Energy Flows through an ecosystems in the form of carbon窶田arbon bonds within organic compounds. These bonds ae broken during respiration when carbon joins with oxygen to produce carbon dioxide. Respiration releases enrgy that is either used by organisms (life processes) or is lost as heat.

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The origin of all the energy in an ecosystem is the sun and the fate of the energy is eventually to be released as heat In the diagram the flow of energy is shown by the red arrows. Unlike energy MATTER cycles through the system as minerals (blue arrows). Plants absorb mineral nutrients from the soil. These nutrients are combined in to cells. Consumers eat plants and other consumers egest the minerals they contain and recombining them in cells. Eventually decomposers break down dead organic matter (DOM) and then return the minerals to the soil. These minerals may betaken out of the soil quickly by plants or can eventually through geological processes become locked within rocks until erosion eventually returns them to new soil. The geochemical cycles illustrate the flows and storage of energy and matter: The carbon cycle shows the flow of both where as the other geochemical cycles e.g. nitrogen only show the flow and storage of matter. In both cases though the direction of the flow - producer to consumer, and the magnitude loss of material up a food chain, amount of carbon dioxide moving from respiration and combustion to the atmosphere, can be described.

Topic 2: The ecosystem 2.1. 1: Biotic and Abiotic. The components of an ecosystem Ecosystems are made up of the interactions between the living and non-living components within them. It is impossible to think of an ecosystem without including these interactions The living components of an ecosystem are known as the “biotic factors” - living biological factors that influence the other organsims or environment of an ecosystem. This is a lot more than just listing the plants, animals or micro-organisms found in an ecosystem. It includes the roles played by the organisms. Biotic factors interact as : Producers, consumers, detrivores, decomposers, parasite, host, predator, competitor, herbivore, symbiant and pathogen.

A tree in a woodland is a producer providing the basic unit of energy for the rest of the ecosystem. But at the same time it competes for light with other trees and may be the host to parasitic plants such as mistletoe or decomposing fungi. During the annual cycle in the wood, the tree will at times take water and mineral nutrients from the soil and at others return nutrients from fallen wood and leaves.

The Physical and Chemical components of an ecosystem are called the “abiotic factors” and include:

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The atmosphere

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Climate and water

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Soil structure and chemistry

•

Water chemistry

•

Seasonality

These factors operate at a broad scale but within ecosystems smaller component abiotic factors also work. The relative humidity within the bowl of an oak tree is higher than that of a woodland as a whole. This provides the physical and chemical conditions needed for a community of mosses, lichens and ferns to develop. In a very simplistic form it is the availability of suitable abiotic environment that provides the conditions for a distinct biotic community to exist. Importantly thought, the biotic community can greatly influence and even change the abiotic one. Commercial Forestry in parts of Scotland illustrates this well. Until the 1970’s large areas of Scottish Blanket Bog was viewed beyond the reach of commercial forest operations. It was to wet for Sitka spruce the predominant cash wood crop to grow and too expensive to drain. Then it was discovered that if a “nurse” crop of Lodgepole pine was planted ahead of the Sitka, even though the pines would eventually die in the very wet conditions, they would dry the soil enough to allow Sitka to take hold. Along with this the drying of the area and closing in of the canopy with trees planted tightly in rows would prevent continued growth and accumulation of sphagnum moss. This in turn aided the drying process.

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Many different abiotic factors an animal or plant species and also interact and change with time themselves. E.g. temperature is dependent upon: solar radiation, wind speed, time of year, time of day, altitude and aspect. Temperature affects water loss from organisms and respiration, and for plants the rate of photosynthesis. Changes in temperature affect relative humidity and evaporation from water bodies and soils.

It is the abiotic conditions in an environment which ultimately give rise to the biotic community present. This is illustrated below with examples of six different ecosystems, including an ecosystem found on the surface of some rocks, each of which is the result of the initial controlling abiotic factors which operate.

Alpine Grassland

Acidic Heath

Mediterranean Maquis or Chaparral

Temperate Deciduous Forest

Sand dune system

Lichen rock face ecosystem

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2.1. 2: Trophic Levels Trophic level:The position that an organism occupies in a food chain, or a group of organisms in a community that occupy the same position in food chains. It is possible to classify the way organisms obtain energy into two categories.

Producers or Autotrophs: These manufacture their own food from simple inorganic substances (plants) Consumers or Heterotrophs: Feed on autotrophs or other heterotrophs to obtain energy (herbivores, carnivores, omnivores, detrivores and decomposers But within the consumers their is a feeding hierarchy of feeding Plants capture the suns energy and convert it to glucose, herbivores eat plants and carnivores eat herbivores - different feeding levels (Greek for food is trophe) Trophic level 1 - producer Trophic level 2 - herbivore (primary consumers) Trophic level 3 - carnivore (secondary consumers) Trophic level 4 - carnivore (tertiary consumer)

The first trophic level, the autotrophs supports the energy requirements of all the other trophic levels above.

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2.1. 3: Food chains and Food webs Ecosystems have an hierarchy of feeding relationships (trophic levels) that determine the pathway of energy flow in the ecosystem. The energy flow in the ecosystem can be illustrated as a Food chain.

It is possible to construct food chains for an entire ecosystem, but this starts to create a problem. The food chains below are form a European Oak Woodland. In fact they are based on real food

chains at Wytham Wood in Oxford . In the four different food chains only ten species are listed and some of them are in more than one food chain. If we continued to list all the species in the wood and their interactions in every food chain the list would run for many pages. Food chains only illustrate a direct feeding relationship between one organism and another in a single hierarchy. The reality though is very different. The diet of almost all consumers is not limited to a single food species. So a single species can appear in more than one food chain. A further limitation of representing feeding relationships by food chains is when a species feeds at more than one trophic level. Voles are omnivores and as well as eating insects they also eat plants. We would then have to list all the food chains

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again that contained voles but moving them to the second trophic level rather than the third in a shorter food chain. The reality is that there is a complex network of interrelated food chains which create a food web.

2.1. 4: Ecological pyramids Pyramids of number A bar diagram that indicates the relative numbers of organisms at each trophic level in a food chain. The length of each bar gives a measure of the relative numbers. Pyramids begin with producers, usually the greatest number at the bottom decreasing upwards.

Advantages This is a simple easy method of giving an overview and is good at comparing changes in population numbers with time or season.

Disadvantages All organisms are included regardless of their size, therefore a system say based on an oak tree would be inverted (have a small bottom and bet larger as it goes up trophic levels). Also they do not allow for juvenilles or immature forms. Numbers can be to great to represent accurately.

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Pyramids of biomass As pyramids of number but uses dry mass of all organisms at each trophic level. Advantages Overcomes the problems of pyramids of number. Disadvantages Only uses samples from populations, so it is impossible to measure biomass exactly.also the time of the year that biomass is measured affects the result. Pyramids of energy The bars are drawn in proportion to the total energy utilized at each trophic level. Also the productivity of producers in a given area measured for a standard time, and the proportion utilized by consumers can be calculated. Advantages Most accurate system shows the actual energy transferred and allows for rate of production. Disadvantages It is very difficult and complex to to collect energy data. Why use ecological pyramids. Ecological pyramids allow you to examine easily energy transfers and losses. They give an idea of what feed s on what and what organisms exist at the different trophic levels. They also help to demonstrate that ecosystems are unified systems,that they are in balance.

2.1. 5: Pyramids and Ecosystem Function Bioaccumulation Story of Minamata Bay. Minamata is a small factory town in Japan, dominated by one factory, The Chisso Factory. Chisso make petrochemical based substances from fertilizer to plastics. Between 1932 and 1968 Chisso dumped an estimated 27 tons of mercury into Minamata Bay. Beginning in the 1950’s, thousands of people started to suffer from mercury poisoning. What had happened? Some bacteria can change mercury to a modified form called methylmercury. Methylmercury is easily absorbed into the bodies of small organisms such as shrimp. When the shrimp are eaten by fish, the methylmercury enter the fish. The methylmercury does not break down easily and can stay in the fish bodies for a long time. As the fish eat more and more shrimp, the amount of methylmercury increases. The same increase in concentration happens when people then eat the fish. fish are a major part of the diet of people around Minamata bay. This process is known as bioaccumulation. There is a slow magnitude build up along the food chain: Very many bacteria absorb very small amounts of mercury - many shrimp eat a lot of bacteria building up the mercury concentration - lots of fish eat lots of shrimp again building up the concentration and finally a small number of humans at the top of the food chain eventually eat a lot of fish and absorb high levels of methylmercury. The end of the food chain It is the often the highest trophic level in a food chain that is the most susceptible alterations in the environment. Another example of the effects of toxins on a food chain was DDT (a pesticide) and Peregrine folcans in Britain in the 1950’s and 60’s. Follow this link to find out more PEREGRINES IN YORKSHIRE The top of the food chain is always vulnerable to the effects of changes further down the chain. Top carnivores often have a limited diet so a change in their food prey has a knock on effect. Their population numbers are low because of the fall in

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efficiency alone a food chain, therefore their ability to withstand negative influences is more limited than species lower in the food chain with larger populations.

2.1. 7: Population interactions COMPETITION 1. All the organisms in any ecosystem have some effect on every other organism in that ecosystem. 2. Also any resource in any ecosystem exists only in a limited supply. When these two conditions apply jointly, competition takes place. In a seagull colony on an oceanic outcrop, as the population grows, so the pressure for good nesting sites increases. This can affect the number of eggs that each female can successfully hatch, and so affects the birth rate of the population as a whole. This sort of interaction is called a Density Dependent factor - the effect is depends on the population density ( low density small effect, high density large effect). This mainly associated with pressure for food, nutrients or space. Competition between members of the same species is INTRASPECIFIC COMPETITION. When the numbers of a population are small, there is little real competition between individuals for resources. Provided the Small population

numbers are not too small for individuals to find mates, population growth will be high. As the population grows, so does the competition between individuals for the same resources until eventually the carry capacity of the ecosystem is reached. In this situation, often the stronger individuals claim the larger share of the resources.

Large population

Some species deal with intraspecific competition by being territorial. An individual or pair hold an area and fend off rivals. Individuals that are the most successful reproductively will hold the biggest territory and hence have access to more resources.

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Intraspecific competition tends stabilize populations dependent upon the controlling resources. It produces something called logistic growth. The graph illustrates this for a colony of yeast grown in a constant but limited supply of nutrient. During the first few days the colony grows slowly as it starts to multiply (lag phase) then it starts to grow very rapidly as the multiplying colony has a plentiful nutrient supply (exponential phase). Eventually the population size stabilizes as only a set number of yeast cells can exploit the limited resources (stationary phase). Anymore yeast cells and there is not enough food to go around. Competition does not only occur between individuals of the same species. Individuals of different species could be competing for the same resource. This is INTERSPECIFIC COMPETITION. Interspecific competition may result in a balance, in which both species share the resource. The other outcome is that one species may totally out compete the other, this is the principal of competitive exclusion. An example of both of these outcomes can be seen in a garden that has become overrun by weeds. A number of weed species coexist together, but often the original domestic plants have been totally excluded.

In a woodland light is a limiting resource. Plant species that can not get enough light will die out in a woodland. This is especially true of small flowering plants on the woodland floor that are not only shaded out by trees but by shrubs and bushes as well. Beech trees have very closely overlapping leaves, resulting in an almost bare woodland floor.

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But even in beech woods flowers manage to grow in the spring. Carpets of Snowdrops, Primroses and Bluebells an integral part of all Northern European deciduous woodlands in the spring. The key to these species success is that the grow, flower and reproduce before the shrub and tree species burst into leaf. They avoid competing directly with species that would out compete them for light by completing the stages of their yearly cycle that require the most energy and therefore the greatest photosynthesis when competition is less. The amount of competition depends on how much each species need for the resource overlaps: Interspecific competition may result in a balance, in which both species share the resource. But with the population size of each species reduced compared to without competition The other outcome is that one species may totally out compete the other. This is the principal of competitive exclusion

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2.4. 1: Biomes What is a biome? A collection of ecosystems sharing similar climatic conditions, eg tundra, tropical rainforest, desert How many biomes are there? Opinion differs slightly on the number of biomes, but it is possible to group biomes into six major types with sub divisions in each type. Freshwater Marine Desert Forest Grassland Tundra

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If we only consider the terrestrial biomes we can split the major groups up again: Deserts - Hot and Cold Forests - Tropical, Temperate and Boreal (Taiga) Grasslands - Tropical or Savannah and Temperate Tundra - Arctic and Alpine

2.4. 2: Why are Biomes where they are? Ecologist Robert Whittaker plotted records of annual precipitation against annual temperature for locations around the planet and then grouped them within the biomes generally found in those places. The result was the graph on the left. This helps to illustrate that biomes that form anywhere in the world are mainly the result of the combination of rainfall and temperature found in those areas. The graph also illustrates that the concept of biome may not be as precise and clear cut as it at first appears. Marginal areas exist where a continuum of climatic conditions can give rise to a gradient of ecosystems as one biome is slowly replaced by another. Large areas of Boreal forest in Northern Europe slowly change

At the Poles the suns energy is spread over a large area

as you move South into areas that support predominantly Deciduous forest.

At the Equator the suns energy is spread over a small area

This is because the climatic conditions across the planet are not distinct but themselves show a gradients. From the equators out both North and South temperature drops until the permanently frozen regions

Northern Hemisphere Winter

at the poles is encountered. The result of incoming solar radiation being spread over greater and greater area with the curvature of the Earth. The Earth also tilts at an angle of 23.5˚, creating summer and winter in each hemisphere. During the Northern Winter almost no solar energy reaches

Southern Hemisphere Summer

the high arctic. This again reduces productivity at the poles

23.5˚ Tilt

The maximum incoming solar radiation at the equator gives rise to high temperatures which in turn lead to maximum evaporation of water from the large expanses of ocean found here. As the moisture laden warm air at the equator rises in the atmosphere it the water condenses out as clouds and falls back to Earth as exceptionally high rainfall. the rainfall which when combined with high temperatures and maximum sunlight creates the perfect conditions for maximum plant growth. The result equatorial or tropical rainforest. This rapidly rising warm air sucks in air from both Southern and Northern latitudes along the planets surface. In the atmosphere the still warm but now dry air moves away from the equator. These two air currents set up an atmospheric cell with descending warm dry air at around 30° north and south. This leads to the establishment of desert biomes at these latitudes.

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2.5. 3: Energy flow through the ecosystems The sun produces immense amounts of energy in the form of electromagnetic radiation. This is a broad spectrum from X rays to radio waves, though most exists in the ultraviolet, visible light and infrared radiation bands. Almost half of the sunâ&#x20AC;&#x2122;s total radiation is visible light. The distance of the earths orbit around the sun is fairly constant, around 1.5Ă&#x2014;10km, and the amount of energy reaching the outer atmosphere is within 5% of a constant energy quantity of 1400 J/m2/s, this is the Earths solar constant. The second law states that the efficiency of energy conversion to useful work is never perfect: when energy changes from one form, some of the energy is not available to do useful work in the system. It leaves the system mainly as useless heat. This is called entropy. This is true for all energy changes even those involving the living organisms. Only a very small part of the total sunlight reaching the Earths surface is ever transformed into energy used by living organisms. A study of an Illinois cornfield Boxes and Circles represent Storage and Arrow thickness represents magnitude of flow

suggested only 1.6% was actually used by the corn. Some of that energy becomes stored chemical energy (sugars, fats and proteins)

within the plants, some is used to maintain life processes and ultimately is dissipated as heat during respiration. The stored energy can then either be passed on to consumers as food or die and enter the detritus food chain. Consumers again are inefficient processors of the energy consumed with respiration taking place and some losses immediately as faeces and so the process of flow, storages and losses continues up the food chain.

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Organisms that gain energy from inorganic sources and fix it as energy rich organic molecules are called Autotrophs. Most are plants obtaining their energy directly from light, carbon dioxide and water: Photosynthetic autotrophs. Some bacteria obtain energy directly from Cyclamen in an Alpine forest

inorganic chemicals: chemosynthetic autotrophs.

Organisms that utilize energy rich organic molecules, edible food, for their energy supply are termed Heterotrophs. The hetrotrophs can be split into two kinds, consumers that obtain their from living organisms and decomposers that obtain theirs from dead organisms or from organic material dispersed in the environment Autotrophs and particularly plants are the base unit of all stored energy in any ecosystem. Light energy is converted into chemical energy by photosynthesis within the cells of plants. Pigments in plant cells of which chlorophyll is the most important catalyse this reaction

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2.5. 4: Transfers and Transformations - Global Cycles The biogeochemical cycles. Movement of nutrients and energy through the ecosystem is quite different. Energy travels from the sun, through food webs and is eventual lost to space as heat. Nutrients are recycled and reused. (Via the decomposer food chain) Organisms die and are decomposed Nutrients are released Eventually become parts of living things again, when they are taken up by plants These are the BIOGEOCHEMICAL CYCLES

The Carbon cycle. The balance between Photosynthesis, Respiration and incorporation into the lithosphere Carbon is an essential element in living systems, providing the chemical framework to form molecules that make up living organisms. Carbon makes up around 0.03% of the atmosphere as carbon dioxide, and is present in the Oceans as carbonate and bicarbonates and in rocks such as limestone and coal. Carbon cycles between living (biotic) and non-living (abiotic) chemical cycles: carbon is fixed by photosynthesis and released back to the atmosphere through respiration. Carbon is also released back to the atmosphere through combustion, including fossil fuels and biomass.

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Carbon can remain locked in either cycle for long periods of time. ie in the wood of trees or as coal and oil. Human activity has disrupted the balance of the global carbon cycle (carbon budget) through increased combustion, land use changes and deforestation. Nitrogen cycle Nitrogen gas in the atmosphere N2

$WPRVSKHULF Ă&#x20AC;xation during lightning storms

Animals consume plants

Plant and Animal Biomass containing nitrogen

Excretion

Nitrates taken up by plants

Dead Organic Matter DOM

Nitrogen Fixing bacteria Denitrifying bacteria

Decomposition

Ammonia NH3 Nitrite NO2 Nitrifying bacteria

Nitrate NO3 Nitrifying bacteria

Nitrogen Cycle

All living organisms use nitrogen to make molecules such as protein and DNA. Nitrogen is the most abundant gas in the atmosphere but atmospheric nitrogen is unavailable to plants and animals, though some specialized micro-organisms can fix atmospheric nitrogen. The nitrogen cycle can be thought of in three basic stages. Nitrogen fixation: atmospheric nitrogen is made available to plants through the fixation of atmospheric nitrogen as ammonia by nitrogen fixing bacteria either free living in the soil (Azotobacter) or symbiotically in root nodules (Rhizobium).

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Nitrogen gas in the atmosphere N2

$WPRVSKHULF Ă&#x20AC;xation during lightning storms

lightening also causes the oxidation of nitrogen gas to nitrate which is washed into the soil. Denitrification: denitrifying bacteria (Pseudomonas

Nitrogen Fixing bacteria Denitrifying bacteria

denitrificans)in waterlogged and anearobic conditions reverse this process converting ammonia, nitrate and nitrite to nitrogen gas which escapes to

Ammonia NH3 Nitrite NO2 Nitrifying bacteria

Nitrate NO3 Nitrifying bacteria

Nitrogen Cycle

the atmosphere.

Nitrification: some nitrifying bacteria are able to Animals consume plants

convert ammonium to nitrites (Nitrosomonas) while other convert the nitrites to nitrates

Plant and Animal Biomass containing nitrogen

(Nitrobacter) which is then available to be

Excretion

Nitrates taken up by plants

Dead Organic Matter DOM

absorbed by roots. Decomposition

Humans have intervened in the nitrogen cycle by applying large amounts of nitrogen fertilisers to

Ammonia NH3 Nitrite NO2 Nitrifying bacteria

Nitrate NO3 Nitrifying bacteria

Nitrogen Cycle

the land, either as organic sources (manure and green crops) or as inorganic chemical fertilisers. overuse of fertilisers can lead to serious pollution problems particularly to water supplies. (Eutrophication) The Hydrological cycle The hydrological or water cycle involves the transfer of water between atmosphere rivers lakes and oceans and the lithosphere. To move between these different sections water molecules often need to be transformed from one phase to another. E.g. Liquid water evaporates from surfaces allowing transfer to the atmosphere as gas where it condenses out in clouds to fall as liquid precipitation again. It is easy to think about Lakes, Oceans and Ground water as stores (sinks), but the largest store of fresh water is held within snow and ice. The polar ice caps and mountain glaciers in particular are an enormous sinks of water temporarily removed

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from the cycle ( though this could be for thousands of years) and unavailable for use by organisms. As well as being essential for life it is the water cycle that drives the worlds weather systems.

2.5. 6: Primary and Secondary Productivity Organisms that use inorganic sources of energy, and particularly plants are the base unit of stored energy in any ecosystem. Light energy is converted to chemical energy by photosynthesis within the cells of plants Because all the energy fixed by plants is converted to sugars it is in theory possible to calculate a plantâ&#x20AC;&#x2122;s energy uptake by measuring the amount of sugar produced. This is Gross Primary Production (GPP), because it occurs in the primary producers, an abstract that is difficult to measure. More useful is the measure of Net Primary Production (NPP). An ecosystems NPP is the rate at which plants accumulate dry mass, usually measured in kg,m-2,yr-1, or as the energy value gained per unit time kJ,m-2,yr-1. This store of energy is potential food for consumers within the ecosystem.

NPP represents the difference between the rate at which plants photosynthesize (GPP) and the rate, which they respire (R). This is because the glucose produced in photosynthesis has two main fates. Some provides for growth, maintenance and reproduction with energy being lost as heat during processes such as respiration. The remainder is deposited in and around cells represents the stored dry mass (NPP=GPP-R).

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The accumulation of dry mass is more usually termed biomass, and provides a useful measure of the production and use of resources. Primary production is the foundation of all metabolic processes in an ecosystem, and the distribution of production has a key part in determining the structure of an ecosystem. Biological communities include more than just plants, they also include herbivores, carnivores and detritivores.

Production also occurs in animals as

Gross Secondary Production = Energy assimilated Food eaten - faeces

Secondary Production. Importantly though animals do not use all the biomass they

Total energy taken in (food eaten)

consume. Some passes

Usable energy (Assimilation)

through to become feces. Gross production in animals equals

Waste (faeces)

the amount of biomass or energy assimilated or biomass eaten less feces. As with plants some of the energy assimilated by animals is used to drive cellular processes via respiration the remainder is available to be laid down as new biomass. This is Net Secondary Production. Net secondary productivity (NSP ) = food eaten - faeces - respiration energy so NSP = GSP- R (just like plants)

NSP = GSP - R (Food eaten - Energy in faeces) - Respiration Energy to drive cellular processes (Respiration)

Total energy taken in (food eaten)

New Biomass

Waste (faeces)

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2.6. 2: Population Growth December 15th, 2009

Over time the numbers within a population change. If we were to collect a few bacterial cells, place them in a suitable supply of nutrients and then under a microscope cont the number of cells every hour we would find that there would be many more bacteria at the end of a 24 hour period than at the start. It is possible to model this growth as a mathematical equation: Population growth = change in population number / change in time or dN/dt Where dN = change in numbers and dt = change in time This equation can also be written using the symbol delta to represent time:

Exponential Growth Thinking about the bacteria above, if I started out with one bacteria (bacteria reproduce asexually so a population can start with one)and if the bacteria reproduced one after 5 minutes and then died every, after 30 minutes I would have 64 bacteria - the population size would double every 5 minutes. This means that the each time the population changes it increases the amount of population change next time. More simply the rate of population growth increases as the population grows or exponential growth. This can be expressed as a rate equation.

This produces a J shaped curve. As long as their is a plentiful supply of the resources that the organism needs. in the case of bacteria: sugars, moisture and warm the population would keep growing indefinitely. Obviously in nature this does not happen. Darwin in Origin of Species (p117 -119) recognizes that this would be an absurd proposition “There is no exception to the rule that every organic being increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally not be standing room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds - and there is no plant so unproductive as this - and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned to be the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase: it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pairs of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair.” So what stops the planet being knee deep in elephant dung? Limited resources Al resources in an ecosystem are limited. there is only so much food, only so much space, only so many mates even. The results of these ecological limits or ECOLOGICAL RESISTANCE is that no population can keep growing forever. There is a ceiling limit that each ecosystem sets. This limit set by the resources of the ecosystem is the CARRYING CAPACITY, confusingly given the symbol K in ecology.

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2.6. 3: Population Regulation The number of organisms in any population continually change. They never remain constant for ever. Many factors can affect population size, but their combined effect is going to be seen in an alteration in one of four conditions: birth rate, death rate, gains from immigration and losses from emigration. Changes in these four conditions determine later population numbers. These conditions in-turn are affected by the available resources in any ecosystems. As populations grow then competition tends to come into play and resources start to become limited, this can continue until a theoretical point is reached where the amount resources available can not support the current population. The resource available define the maximum number of species (or individuals within a species) that habitat can support throughout their complete life cycle. This is the Carrying Capacity of that ecosystem. It is the environmental carrying capacity that limits how large a population can grow to. I reality, often the number of individuals in a population is greater than the carrying capacity. When this occurs competition between individuals for the limiting resources takes place. These resources could be; food, mates, breeding sites, water, soil nutrients or anything else. This is an example of Density dependent competition (see 2.1.7: Population Interactions) With Density dependent competition the larger the population the greater the degree of competition for the limited resources. As the population grows so fewer individuals will get the resources they require to survive.

This need not in itself result increased mortality but may also cause emigration of individuals to areas where the resource is in greater supply or a lowering of reproductive success. This can be illustrated by examining the declining Swallow population in Europe. Since the 1970â&#x20AC;&#x2122;s concern has been raised about an apparent decline in the breeding populations of these birds across Europe. Two possible explanations are linked to changes in farming practices. Extensive use of pesticides on crop plants has reduced the number of insects that swallows can feed on.

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However a another factor is the loss of old brick and stone farm buildings that Swallows require for nest sites: this is an effect on their reproductive success, fewer breeding sites means fewer young and so eventually fewer adults.

2.6. 5: Succession ECOLOGICAL SUCCESSION After the retreat of glaciers following the last ice age, new virgin land was exposed with nothing living on it. It didnâ&#x20AC;&#x2122;t remain that way for long. Soon the land was covered with mosses and lichen. Gradually organic material was added to the simple mineral soils left behind by the glaciers and from the erosion of bare rock. This created conditions that allowed, first grasses and small herbs to establish, and eventually over time for northern Europe to be covered by woodland. This directional change in community is termed succession. PRIMARY SUCCESSION Involves the colonization of newly created land by organisms. o River deltas o Volcanic larva fields o Sand dunes o Glacial deposits Simple mineral soils evolved from erosion are, slowly invaded by mosses and lichen. These and other early plants are adapted to survive periods of drought as water drains quickly away from the mineral soils. These contribute organic matter to the soil as they die and spread, this creates conditions that allow larger

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mosses to invade. These help add more organic matter to the soil, which improves its water holding capacity, and provide a habitat for soil organisms that help speed up the breakdown of organic matter and release of nutrients. Conditions then become favorable for ferns and higher plants to establish on the primitive soils and humus forms as more organic matter is added. Eventually shrubs and trees invade, first from winddispersed seeds and eventfully by animal dispersal. Eventually over time a stable woodland community develops. Succession progress in stages from; Pioneer species that are adapted to develop in limiting environments to a stable developed community. This final community is termed a CLIMAX COMMUNITY . As the community develops, so biodiversity also increases. The entire process from bare rock to climax is called a SERE and that progress directionally through SERAL STAGES.

An example of primary succession can be seen in the development of the natural broad-leafed forest that covered much of Northern Europe following the end of the last Ice Age. We know that following the retreat of ice around 10,000 years until around 7,500 years ago, a Boreal community formed. First of Juniper then birch and later pine. As

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the climate warmed so the community changed from a dominance of birch to Oak with abundant wych elm, alder and lime, marking a change to warm, moist Atlantic period until about 5,000 years ago. Much of Northern Europe would still be covered in this mixed broad leaf forest if Neolithic man had not started changes the plant community around him as agriculture developed. Ecological period

Years ago

Community

Pre-Boreal

10,000 - 9,500

Tundra with patches of willow, birch and pine

Boreal

9,500 - 7,500

Hazel, Pine

Atlantic

7,500 - 5,000

Hazel, Oak, Elm Lime , Ash, Alder

Sub-Boreal

5,000 - 2,500

Mixed Oak with many cleared areas either being farmed or abandoned and returned to woodland

If primary succession starts on dry land it is a XEROSERE If it starts in water (a pond) it is a HYDROSERE.

Pond and lakes get continuous inputs of sediment from streams and rivers that open into them. Some of this sediment passes through but a lot sinks to the pond bottom. As plant communities develop they add dead organic material to these sediments. Over time these sediments build up allowing rooted plants to invade the pond margins as the pond slowly fills in. This eventually leads to the establishment of climax communities around the pond margins and in smaller ponds the eventual disappearance of the pond. In regions where rainfall is high, the xerosere climax community mat not establish after a hydrosere. The wet conditions creat the development of raised bogs as the climax following hydrosere succession.

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SECONDARY SUCCESSION Where an already established community is suddenly destroyed, such as following fire or flood or even human activity (ploughing) an abridged version of succession occurs. This secondary succession occurs on soils that are already developed and ready to accept seeds carried in by the wind. Also there are often dormant seeds left in the soil from the previous community. This shortens the number of seral stages the community goes through .

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Good examples of secondary succession have been studied in abandoned form land in North Carolina in the United States. The farmland had become infertile through not enough nutrients being returned after crops had been taken and through wind erosion. As the land became unproductive and uneconomical to farm, farmers simply abandoned the land. This left patches of former farmland of various ages. Years after

Dominant species

Predominant Forest Vegetation

Crabgrass

Developing grassland community

Crabgrass,

Community starts to be dominated by

Horseweeds, Pigweed

Horseweed

Aster, Crabgrass,

Grass scrub is developing

abandonment

Ragweed 3-18

Broomsedge, Pine

Developed grass scrub community with

later

later invading pine

18-30

Pine

Immature pine forest

30-70

Pine, Young Hickories

Mature pine wood with understorey of

and Oaks later

young hardwoods later

Pine, Hickories, Oaks

Transitonal stage between Pine and

70-100

Hardwood forest 100+

Oak, Hickory

Mature mixed hardwood forest

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2.6. 6: Succession, Productivity and Diversity. Productivity

During succession Gross Primary Productivity tends to increase through the pioneer and early wooded stages and then decreases as climax community reaches maturity. This increase in productivity is linked to growth and biomass. Early seral stages are usually marked by rapid growth and biomass biomass accumulation grasses, herbs and small shrubs. Gross Primary Productivity is low but Net Primary Productivity tends to be be a large proportion of GPP as with little biomass in the early seral stages respiration is low. As the community develops towards woodland and biomass increases so does productivity. But NPP as a percentage of GPP can fall as respiration rates increase with more biomass. Studies have shown that standing crop (biomass) in succession to deciduous woodland reaches a peak within the first few centuries. Following the establishment of mature climax forest biomass tends to fall as trees age growths slows and an extended canopy crowds out ground cover. Also Older trees become less photosynthetically efficient and more NPP is allocated to none photosynthetic structural biomass such as root systems. Biomass Accumulation and Successional Stage: Early Stage

Middle Stage

Late Stage

Low GPP but High

Gross Productivity high

Tree reach their maximum size

percetage NPP

increased

Little increase in biomass

Ratio of NPP to R is roughly equal

photosynthesis Increases in biomass as plant forms become bigger

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Biodiversity Early stages of succession tend to be marked by few species within the community. As the community passes through subsequent seral stages so the number of species found increases. Very few pioneer species are ever totally replaced as succession continues. The result is increasing diversity - more species. This increase tends to continue until a balance is reached between possibilities for new species to establish, existing species to expand their range and local extinction. Evidence following the eruption of the Mount St Helens volcano in 1980 has provided ecologists with a natural laboratory to study succession. In the first 10 years after the eruption species diversity increased dramatically but after 20 years very little additional increase in the diversity occurred1 Disturbance Early ideas about succession suggested that the Climax community of any area was almost self perpetuating. This is unrealistic as communities are affected by periods of disturbance to greater or lesser extent. Even in large forests trees eventually age, die and fall over leaving a gap. Other communities are affected by flood, fire, land slides earthquakes, hurricanes etc. All of these have an effect of making gaps available that can be colonised by pioneer species within the surrounding community. This adds to both the productivity and diversity of the community.

Carey, Susan, John Harte & R. del Moral. 2006. Effect of community assembly and primary succession on the species-area relationship in disturbed systems. Ecography 29:866-872. [â&#x2020;Š]