Ecosystem - Any group of living organisms and non-living occuring together, and the interrelationships between them.
The components of an ecosystem:
- Habitat - The place where an organism lives.
- Population - all of the organisms of one species that live in the same place at teh same time, and can breed together.
- Community - all the populations of different species that live in the same place at the same time, and can interact with each other.
- Niche - The role that each species plays in an ecosystem
Depending on their niche, the living organisms in an ecosystem can effect each other. Such biotic factors inclucde food supply, predation & disease. Abiotic factors (non-living components of ecosystem) such pH, temperature and soil type.
The community of living things in an ecosystem interact with each other and with their physical environment. Any small chaneges in one can affect the other:T
- If a predator's population size goes up, the population size of the prey will go down (because more are being eaten more quickly).
- The nitrogen levels in soil can affect the population sizes of plants growing there. Nitrogen-fixing plants would grow successfully in nitrogen-deficient soil, but they would affect their environment by increasing the soil nitrogen levels. This change would then help other plants to grow there as well.
Energy and Ecosystems
Matter is recycled with an ecosystem, enegy is not recycled- it flows through the ecosystem.
Light energy -------------------> Biotic component --------------------> Heat energy
At the start of nearly all food cghains is a plant, which captures light energy through photosynthesis, and converts it to chemical energy stores in molecules like glucose.
- Because plants & other photosynthetic organisms (algae & other bacteria), supply chemical energy to all other organisms - Producers
- Other organisms (animals & fungi) are Consumers. Primary consumers are herbivores (diet of plants), & who are eaten by carnivorous Secondary consumers. These in turn are eaten by carnivorous Tertiary consumers.
- Decomposers (bacteria, fungi & some animals) feed on waste material or dead organisms.
Transfer of energy in an ecosystem
A food chain shows how energy is transferred from on living organisms to another. The level at which an organism feeds is called its trophic level.
The arrows in a food chain show the direction of energy transfer.
* The level at which an organism feeds in a food chain is called a trophic level.
Oak leaves ----> Aphid ----> Ladybird ----> Spider
Producer Primary Secondary Tertiary
consumer consumer consumer
Living organisms are usually members of more than one food chain, and often feed at different trophic levels in different chains. Therefore, food webs are used to help understand how enegy flows through the whole ecosystem.
Efficiency of energy transfer
At each trophic level, some energy is lost from a foods chain, & is therefore unavailable to the organism at next trophic level.
- Respirtation releases energy from organic molecules like glucose. Some of this energy is eventually converted to heat.
- Energy remains stored in dead organisms and waste material, which is then only available to decomposers. This waste material includes parts of animals and plants that cannot be digested by consumers.
Because of this, there is less energy avaiable to sustain living tissue at higher levels of the food chain, and so less living tissue can be kept alive. When organisms in a food chain are the same size, the will be fewer consumers at the higher levels.
Ecologists draw a pyramid of numbers to represent this idea. The area of each bar in the pyramid is proportional to the number of individuals.
Measuring efficiency of energy transfer
- Pyramids of biomass - The area of the bards is proportional to the dry mass of all the organisms at that trophic level.
An ecologist would collect all the organisms and put them in an oven at 80°C until all water in them has been evaportated. Unfortuantely, this is destructive to the ecosystem being studied, so they often just measure the wet mass of organisms and calculate the dry mass on the basis of previously published data.
- Pyramids of energy - However, even puramids of biomass present problem, as different species may release different amounts of energy per unit mass. This involves burning the organisms in a Calorimeter and working out how much heat energy is released per gram - this is calculated from the temp. rise of a known mass of water. However, this is destructive and time-consuming, ecologists often revert to using pyramids of biomass instead.
Measuring efficiency of energy transfer (2)
- Productivity - Even pyramids of energy have limitations
- They only take a snapshot of an ecosystem at one moment in time
- Population sizes can fluctuate over time, this may provide a distorted idea of the efficiency of energy transfer.
Ecologists often look at the rate at which energy passed through each trophic level, drawing a pyramid of energy flow - This rate of energy flow is called productivity.
- Productivity gives an idea of how much energy is available to the organisms at a particular trophic level, per unit area (one square metre), in a given amount of time (one year) - it is measured in kilojoules or megajoules or energy per square metre per year.
- At the base of the food chain, the productivity of plants is called the primary productivity.
- The gross primary productivity is the rate at which plants convert light energy into chemical energy. However, because energy is lost when the plant respires, less energy is available to the primary consumer. This remaining energy is called the net primary productivity (NPP)
Manipulating energy transfer
The energy captured by leaves for photosynthesis is called the primary productivity. Some of this will be used by the plant and loss as respiratory heat (R). The differences between primary productivity and R is the net primary productivity (NPP). NPP is the reate of production of new biomass available for consumption by heterotrophs.
* Primary productivity - is the total amount of energy fixed by photosynthesis. It is the net flux of carbon from the atmosphere to plants, per unit time. It is a rate and may be measured in terms of energy per unit time, such as MJ m-2 yr-1.
* Net primary productivity - is rate at which carbohydrate accumulates in the tissue of plants of an ecosystem and is measured in dry organic mass, such as kg ha-1 yr-1.
Net primary productivity = primary productivity - respiratory heat loss.
Net primary productivity is the amount of energy available to heterotrophs in the ecosystem.
Improving primary productivity
By manipulating environmental factors, humans can increase NPP - making energy conversion more efficient, reducing energy loss and increasing crop yields.
- Light intensity can limit the rate of P-S, and hence NPP. Some crops are planted early to provide a longer growing season to harvest more light. Others are grown under light banks.
- Lack of water is important in many countries, therefore drought-resistant strains have been bred.
- Temp. can limit the speed of chemical reactions in a plant. Greenhouses can provide a warmer temp. for growing plants & therefore increase NPP. Planting field crops early to provide growing season also helps to avoid the impact of temp. on final yield.
- Pests (caterpillars, insects) eat crop plants. They remove biomass & store energy from the food chain, and lower the yield. Spraying with pesticides can help to reduce this loss. Some plants have also been bred to be pest-resistent, or been genetically modified with a bacterial gene (Bt gene).
- Fungal diseases of crop plants can reduce NPP. Fungi cause root rot (reducing water absorption), damage to xylem vessels (water transport), damage to foliage tubes (translocation of sugars), or damage to flowers and fruit (reproduction). Farmers spray crops with fungicides. Many crops have been bred to be resistant to fungal infections.
- Competition from weeds for light, water & nutrients can reduce a crop's NPP. Farmers use herbicides to kill weeds.
Improving secondary productivity
Transfer of energy from producers to consumers is inefficient, as is transfer of energy from primary consumers to secondary consumers and beyond. Primary consumers don't make full use of plants' biomass, they don't digest everything & egest most of it in their faeces.
Much of the stored energy is used to keep the animal alive, with only a small amount being stored when it grown. It is this small amount that is available to the next consumers. However, it is possible for humans to mainipulate energy transder from producer to consumer:
- Young animal invests a larger proportion of its energy into growth than an adult does. Harvesting animals just before adulthood minimises loss energy from the food chain.
- Farm animals used to be treated steroids to make them grow more quicker, increasing the proportion for energy allocated to growt. However, this practive has been outlawed in EU.
- Selective breeding has been used to produce breeds with faster growth rates, increased egg production and increased milk production.
- Animals may be treated with antibiotics to avoid unnecessary loss of energy to pathogens etc.
- Mammals & birds waste a lot og energy walking around to find food, & keeping body temp. stable. Zero grazing for pig cattle farming maximise energy allocated to muscle production by stopping animals from moving about, by supplying food to them, & keeping environment temp. constant.
* Succession - is a directional changed in a community of organisms over time.
Development of a community from bare ground as primary succession, & comes about by:
- Algae & lichens begin to live on the bare rock - Pioneer community.
- Erosion of rock, and a build-up of dead & rotting organisms, produces enough soil for larger plants like mosess & ferns to grow. These succeed, the algae and lichens.
- Larger plants succeed these small plants, until a final, stable community is reached - climax community.
Succession doesn't always start from bare ground. Secondary succession takes place on a previously colonised, but distrubed or damaged, habitat.
Succession on sand dunes
Sand dunes display all the stages of succession in the same place & at the same time.
Sand just above the high water mark is at the start of the succession, whereas the sand further away already hosts its climax community.
1) Pioneer plants like sea rocket and prickly sandwort colonise the sand just above the high water mark. These can tolerate salt water spray, lack of fresh water and unstable sand.
2) Wind-blown sand builds up around the base of these plants, forming a mini sand dune. As plants die and decay, nutrients accumlate in this mini dune. As the dune gets bigger, plants like sea sandwort and sea couch grass colonise it. Because sea couch grass has underground steams, it helps to stablise the sand.
3) With more stability and accumulation of more nutrients, plants like sea spurge and marram grass start to grow. Marram grass is special: its shoots trap wind-blown sand and, as the sand accumulates, the shoots grow taller to stay above the growing dune. This traps more sand.
4) As the sand dune and nutrients build up, other plants colonise the sand. Many, such as hare's-foot clover & bird's-foot trefoil, are members of the bean family. Bacteria in their root nodules convert nitrogen into nitrates. With nitrates available, more species, like sand fescue, and viper's bugloss, colonise the dunes. This stablises them further.
- Sampling - Ecologists study ecosystems to find out whether the abundance and distribution of a species is related to that of other species, or environmental factors. They take samples from habitat - selecting small portions of habitat & studying them carefully.
Random sampling - laying out 2 tape measures on 2 edges of the study site, use a calculator, or dice to generate pairs of numbers. Use these pairs as coordinates to place quadrats.
- Quadrats - To compare the abundance and distribution of plant species in two different field. A quadrat is a square frame that defines the sample area. It can record presence of absence of each species (distribution), or estimate or count the number individuals (abundance) of species. For plants like grass or moss, it's difficult to count individuals, so ecologists estimate percentage cover.
- Transects - To look at more systematically changes in vegetation across a habitat. A transect is a line take across a habitat. Easiest to stretch out a tape measure, and then take samples at regular intervals along the tape. The distance between samples will depend on the length of the line you want to look at, & density of plants in habitat.
2 approaches to using a transect:
- Line transect - at regular intervals, make a note of which species is touching the tape.
- Belt transect - at regular interval, place a quadrat next to the line (interruped belt transect), studying each, as described above. Alternatively, place a quadrat next to the line, moving it along the line after looking at each quadrat (continuous belt transect).
How to use Quadrats
Before starting to sample, it is important to decide where to place the quadrats, how many samples to take & how big the quadrat should be
- If you take samples only from one corner of the field, it may be the soil in this corner is rich in nitrates & the species growing there are different from those in the rest of the field. To avoid biasing the sample, & provide a sample representative of the whole habitat, either: (1) randomly position the quadrats across the habitat, using random numbers to plot coordinates for each one, or (2) take samples at regular distances across the habitat, so you cample every part of the habitat to the same extent.
- Looking at just one quadrat won't give an accurate representation of the whole habitat. To work out how many are needed, ecologists carry out a pilot study. They take random samples from across the habitat and make a cumlative frequency table. They then pilot cumulative frequency against quadrat number. The point where the curve levels off tells them the minimum number of quadrats to use. Ecologists double this number.
- To do this similarly, count the number of species you find in larger and larger quadrats. Plot quadrat area on the x-axis, against the number of species you find in each one on the y-axis. Read the optimal quadrat size at the same point where the curve starts to level off.
Having collected the data, this quation shows how to esimate the size of each species' population in the whole habitat
Population size of a species =
Decomposers and recycling
Bacteria & fungi involved in decomposition feed in a different way from animals. They feed saprotrophically so they are described as saprotrophs.
- Saprotrophs secrete enzymes onto dead and waste material.
- These enzymes digest the material into small molecules, which are then absorbed into the organism's body.
- Having been absorbed, the molecules are stored or respired to release energy.
If bacteria & funfi didn't break down dead organisms, energy & valuable nutrients would remain trapped within the dead organisms. By digesting dead & waste material, microbes get a supply of energy to stay alive, & the trapped nutrients are recycled.
Recycling nitrogen within an ecosystem
Living things need nitrogen to make proteins & nucleic acids. Nitrogen is very unreactive - this means it's impossible for plants to use it directly. Instead they need a supply of fixed nitrogen such as ammonium ions (NH4+) or nitrate ions (NO3-).
Nitrogen-fixing bacteria account for most of fixed nitrogen. Many of these live in the soil & fix nitrogen gas in the soil, using it to make amino acids. This bacteria also live inside the root nodules of plants such of plants that are members of the bean family.
- They have a mutualistic relationship with the plant: the bacteria provide the plant with fixed nitrogen & receive carbon compounds, such as glucose, in return.
- Proteins in the nodules absorb oxygen & keep the conditions anaerobic. Under these conditions the bacteria use an enzyme, nitrogen reductase, to reduce nitrogen gas to ammonium ions that can be used by the host plants.
Recycling nitrogen within an ecosystem (2)
This happens when chemoautotrophic bacteria in the soil absorb ammonium ions.
- Ammonium ions are released by bacteria involved in putrefaction of proteins found in dead or waste organic matter.
- Instead of getting energy from sunlight, chemoautotropgic bacteria obtain it by ammonium ions to nitrates, or by oxidising nitrites to nitrates.
- Because this oxidation requires oxygen, these reactions only happen in well-aerated soils.
- Nitrates can be absorbed from the soil by plants & used to make nucleotide bases (for nucleic acids) & amino acids (for proteins).
Other bacteria convert nitrates back to nitrogen gas. When the bacteria involved are growing under anaerobic conditions, such as in waterlogged soils, they use nitrates as a source of oxygen for their respiration & produce nitrogen gas (N2) and nitrous oxide (N2O)
What affects population size
The size of a population depends upon the balance between the death rate (mortality) & the rate of reproduction.
What affects population size
- At the lag phase, there may only be a few individuals, still acclimatising to theri habitat. At this point, the rate of reproduction is low, and the growth in population size is slow.
- At the log phase, resources are plentiful and conditions are good. The rate of reproduction is fast and exceeds mortality. The population size increases rapidly.
- At the stationary phase, the population size has levelled out at that carrying capacityof the habitat - the habitat itself cannot support a larger population. In this phase, the rates of reproduction and mortality are equal. The population size therefore stays stable, or fluctuates very slightly up and down in response to small variations in environmental conditions each year.
The habitat cannot support a larger population because of factors that limit the growth in population size - limiting factors, and include the availability of resources, water, light, oxygen, nesting sites or shelter. They may also include the effect of other species (parasites & pathogens), or intensity of competition for resources. The carrying capacity is the upper limit that these factors place on the population size.
* Where the rate of a natural process is affected by a number of factors, the limiting factor is the one whose magnitude limits the rate of the process. It is often the factor in shortest supply.
Predators and prey
Predation can act as a limiting factor on a prey's population size, which in its turn can affect the predator's population size:
1) When the predator population gets bigger, more prey are eaten
2) The prey population then gets smaller, leabing less food for the predators
3) With less food, fewer predators can survive and their population size reduces
4) With fewer predators, fewer prey are eaten, and their population size increases
5) With more prey, the predator population gets bigger, and the cycle starts again
* Competition - happens when resources (like food & water) are not present in adequate amounts to satisfy the needs of all the individuals who depends on those resources.
As the intensity of competition increases, the rate of reproduction decreases, whilst the death rate increases. There are 2 types of competition: Intraspecific & Interspecific.
Intraspecific competition - happens between individuals of the same species. As factors, such as food supplies, become limiting, individuals have to compete for them. Those best adapted to obtaining food will survive & reproducem while those not well adapted will die or fail to reproduce. This slows down population grwoth & the population enters the stationary phase.
- If the population size drops, competition increases, and the population size then increases
- If the population size increases, competition increases, and the population size then drops.
Interspecific competition - happens between individuals of different species, and can affect both the population size of a species & the distribution of species in an ecosystem.
Competition - Russian scientist Gause
He grew 2 species of Paramecium, both seperately and together. When together, there was competition for food, with Paramecium aurelia obtaining food more effectively than Paramecium caudatum. Over a 20 days period, the population of Paramecium caudatum reduced and died out, where as the population of Paramecium aurelia increased, eventually being the only species remaining.
Gause concluded that more overlap between 2 species' niches would result in more intense competition. If 2 species have exactly the same niche, one would be out-competed by the other & would become extinct in that habitat. This idea is known as the competitive exclusion principle, and can be used to explain why particular species only grow in particular places.
However, other observations & experimets suggest that extinction is not necessarily inevitable:
- Sometimes, interspecific competition could result in one population being much smaller than the other, with population sizes remaining constant.
- It is also important to realise that in the lab it is easy to exclude the effects of other variables, so the habitat of the two species remains very stable. In the wild, however, a wide range of variables may act as limiting factors for the growth of different populations. These variables may change on a daily basis, or over the course of a year.
Due to human population size getting larger increasingly quicker - more intensive methods had to be used to exploit our environment for resources. These approaches can disrupt/destroy ecosystems and reduce biodiversity.
Sustainable management make exploitation of forests and woodlands possible, and this means that biodiversity is maintained, and that wood and timber companies can have a financially secure and sustainable supply of wood.
Managing small-scale timber production
* Coppicing involves cutting a tree trunk close to the ground to encourage new growth. Once cut, serval new shoots grow from the cut surface, and eventually mature into stems of narrow diameter. These can be cut and used fo fencing or firewood. After cutting, new shoots start to grow again, and the coppice cycle continues.
* Pollarding involves cutting the trunk higher up. It is useful wehn the population size of deer is high, as they like to eat the emerging shoots from a coppiced stem. If cut higher up, deer cannot reach the shoots.
Managing small-scale timber production
To procide a continous supply of wood, woodland managers divide a wood into sections adn cut one section each year until they've all been cut. This is called rotational coppicing. By the time they want to coppice the first section again, the new stems have matured and are ready to be cut. In each section, some trees are left to grow larger without being coppiced. These trees are called standards, and they are eventually harvested to supply pieces of timber.
Rotational coppicing is very good for biodiversity. Left unmanaged, woodland goes through a process of succession, blocking out light to the floor of woodland, and reducing the number of species that can grow there.
By using rotational coppicing, different areas of woodland provide different types of habitat, letting more light in and increasing the number and diversity of species.
Managing large-scale timber production
Large-scale production of wood for timber often involves clear-felling all the trees in one area. This can destroy habitats on a large scale, This is because trees usually:
- Remove water from soil and stop soil being washed away be rain
- Maintain soil nutrient levels through the trees' role in the carbon and nitrogen cycles.
Modern sustainable forestry works on the followin principles:
- Any tree which is harvested is replaced by another tree, either grown naturally or planted.
- Even with extraction of timber, the forest as a whole must maintain its ecological function regarding biodiversity, climate and mineral and water cycles.
- Local people should derive benefit from the forest
Selective cutting involves removing only the largest, most valuable trees. This means that the habitat is broadly unaffected
To enable continued supply of wood and to maintain biodiversity foresters:
- Control pests and pathogens
- Only plant particular tree species where they know they will grow well
- Position trees and optimal distance apeart. If trees are too close, this will cause too much competition for light, and will grow tall & thin, producing poor quality timber.
* Conservation involves the maintenance of biodiversity, including diversity between species, genetic diversity with species, and maintenance of a variety of habitats and ecosystems.
Unfortunately, a steadily increasing human population can threaten biodiversity through:
- Over-exploitation of wild populations for food, for sport and for commerce: species are harvested faster than they can replenish themselves.
- Habitat disruption and fragmentation as a result of more intensive argicultural practices, increased pollution, or widespread building.
- Species introduced to an ecosystem by humans, deliberately or accidentally. These may out-compete native species, which may become extinct.
Many conservations would argue that these kinds of problems make conservation essential. They believe that every spcies has value in its own right - right to survive, and humans have an ethical responsibility to look after them.
These arguments are subjective as the arugments in favour of human activities that work against conservation are driven by ecomonics. Expressing the value of conservation in economic terms is likely to be more effective in making governments give priority to conservation.
Economic and social reasons
Many species have direct economic value when harvested. Others may also have direct value that is yet unrecognised, and may provide benefit in the future:
- Many species provide a valuable food source, and were originally domesticated from wild species. Genetic diversity in wild strains may be needed in future to breed for dsease resistance, drought tolerance or improved yield.
- Natural environments are a valuale source of potentially beneficial resources. Many of the drugs we use today were discovered in wild plant species.
- Natural predators of pests can act as biological control agents. This can have advantages over the use of synthetic chemicals, although each situation is different.
Many species also have indirect economic value. E.g. Wild insect species responsible for pollinating crop plants. Other communties maintain water quality, protect soil and break down waste products. There is evidence that a reduction in biodiversity ay reduce climatic stability, resulting in drought or flooding and associated economic costs.
Ecotourism & recreation in the countryside also have significant social and financial value, which derives from aesthetic value. Ecotourism dpeends on maintenance of biodiversity.
Perservation is important to maintaining biodiversity - involves protecting areas of land yet used by humans.
What does conservation involve
Successful conservation requires consideration of the social and economic costs to the local community, as well as effective education and liaison with the community.
It can also involve giving legal protection to endangered species, or conserving them ex-situ in zooes or botanic gardens. However, maintaining biodiversity in dynamic ecosystems requires careful management to maintain a stable community, or even to reclaim an ecosystem by reversing the effects of human activity.
Some management strategies:
- Raise carrying capacity by providing extra food
- Move individuals to enlarge populations, or encourage natural dispersion of individuals between fragmented habitats by developin dispersal corridors of appropriate habitat
- Restrict dispersal of individuals by fencing
- Control perdators and poachers
- Vaccinate individuals against disease
- Preserve habitats by prenting pollution or disruption, or intervene to restrict the progress of succession. E.g. by coppicing, mowing or grazing.
Sometimes, simple management is inappropriate, as disruption of a community may have gone too far. It is often easier & more successful to replace a disrupted community with a slightly different community
Humans and the Galapagos
The islands' isolation and small population size provide optimal conditions for rapid evolutionary change for Darwin to study.
The dramatic increase in populations size has place huge demands on water, energy and sanitation services, which the authorites are struggling to meet. More waste & pollution have been produced, and the demand for oil has increased.
In the 19th Century species were harvested faster than they could replenish themselves. Giant tortoises were taken because they could survive on little good in the hold of a ship for a long time, before being killed and eaten. This has a catastrophic effect on tortoise populatins.
Rapid increase in fishing for exotic species in 1990s has left populations depeleted. Depletion of sea cumcumber populations has a drastic effect on under-water ecology, and the international markers for shark fin has lead to a large number of deaths of sharks each year.
With humans come non-native species that may have an impact on existing communities. Species such as goats, cats, fruit & vegetables were brought to the island deliberately, other insects have been carried to the islands accidentally.
As well as out-competing local species, alien species can eat native species, destrou native species' habitats, and bring diseases.