Ecosystems

?
  • Created by: Phoebeacb
  • Created on: 10-05-18 16:02

What is an ecosystem?

Any group of living, and non-living things, and the interrelationships between them, can be thought of as an ecosystem. Ecosystems can be on a large scale (like an African grassland), a medium scale (like a playing field) or on a smaller scale (like a rock pool or a large tree). The components of an ecosystems include:

  • Habitat - the place where an organism lives. 
  • Population - all of the organisms of one species, so live in the same place at the same time, and who can breed together. 
  • Community - all the populations of different species, who live together in the same place at the same time, and who can interact with each other. 

The role of each species in an ecosystem is its niche. Because each organism interacts with both living and non-living things, it is almost impossible to define its niche entirely. A description of its niche could include things like how and what it feeds on, what it excretes, and how it reproduces. It is impossible for two species to occupy exactly the same niche in the same ecosystem. Ecosystems do not have clear edges.

1 of 22

Biotic factors

Depending on their niche, the living organisms in an ecosystem can affect each other: 

  • Producers - Plants (and some photosynthetic bacteria), which supply chemical energy to all other organisms. 
  • Consumers - Primary consumers are herbivores, which feed on plants, and which are eaten by carnivorous secondary consumers. These in turn are eaten by carnivorous tertiary consumers. 
  • Decomposers - Decomposers (bacteria, fungi and some animals) feed on waste material or dead organisms. 

Because these components of the ecosystem require their own source of materials and energy they can affect other organisms food supply. They can also be responsible for predation and disease.

2 of 22

Abiotic factors

Abiotic factors describe the effects of the non-living components of an ecosystem: pH, relative humidity, temperature and the concentration of pollutants are all examples. These can vary in space and time. Such factors could also include disturbance to the ecosystem by other factors such as turbulence and storms. Abiotic factors may also be influenced by the biotic components of the ecosystem. For example, in a rainforest, the forest canopy influences the temperature and humidity of the ecosystem. 

At extreme values of an abiotic factor, a species may perform better or worse, or even die. The abiotic factor shown in the graph could be pH, temperature or any variable that has some sort of optimum level, and where there are lethal levels at both extremes.

When there not a lethal level at both extremes, an organism's response can be plotted differently. For example, at low levels of pollutants, an organism may survive without any detrimental effect. But at high levels, the pollutants may be lethal.

3 of 22

Ecosystems are dynamic

Because ecosystems change, we refer to them as dynamic. The non-living elements change, and the living elements grow and die, with populations of particular species rising and falling. In most ecosystems, population sizes rise and fall very slightly or very noticeably. Living things in an ecosystem interact with each other and with their physical environment. Any small changes in one can affect the other. 

  • Cyclic changes - These changes repeat themselves in a rhythm. For example, movement of tides and changes in day length are cyclic. The way in which predator and prey species fluctuate is cyclic. 
  • Directional changes - They go of in one direction, and tend to last longer than the lifetime organisms within the ecosystem. Within such change, particular variables continue to increase or decrease. Examples include the deposition of silt in an estuary, or the erosion of coastline. 
  • Unpredictable/erratic changes - These have no rhythm and no constant direction. For example, such changes may include the effects of lightning or hurricanes. 

Living things respond to changes in ecosystems. 

4 of 22

Energy and materials in an ecosystem

Materials are constantly recycled within an ecosystem - nutrient cycles, such as the nitrogen cycle and carbon cycle, are good examples. Energy is not recycled - it flows through the ecosystem. 

All living things need energy and materials. Energy is captured by plants in photosynthesis to produce organic molecules like glucose from water and carbon dioxide; such energy is released from glucose during respiration. The products of photosynthesis are not only used immediately for respiration, but incorporated into tissues and organs (e.g. cellulose - the building block of plant cell walls - is made up of large numbers of glucose molecules). Mineral ions are also absorbed through plant roots. 

Together, the organic components (such as glucose molecules) and inorganic components (such as mineral ions, but excluding water) of the plant make up its biomass. So when a plant is eaten, its biomass is consumed by a primary consumer. 

Each level of the food chain is a trophic level. Tracking how biomass changes in a food chain helps us to track the movement of materials and energy through the food chain. We can do that for one food chain or for a whole food web in an ecosystem.

5 of 22

Biomass transfers through ecosystems

At each trophic level, some biomass is lost from a food chain and is therefore unavailable to the organism at the next trophic level:

  • At each trophic level, living organisms need energy to carry out life processes. Respiration releases energy from organic molecules like glucose. Some of this energy is eventually converted to heat, and materials are lost in carbon dioxide and water. 
  • Biomass is also lost from a food chain in dead organisms and waste material, which is then only available to decomposers such as fungi and bacteria. This waste material also includes parts of animals and plants that cannot be digested by consumers, such as bones and hair.

Therefore, biomass is less at higher levels of the food chain. When the organisms in a food chain are about the same size, this means there 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, as an approximation for the total biomass at that level. Pyramids can be drawn for individual food chains or for an ecosystem as a whole.

6 of 22

Calculating the efficiency of biomass transfer

Counting the number of organisms does not always provide an accurate picture of how much biomass exists at each level. A better approach is to draw a pyramid of biomass, where the area of each bar is proportional to the dry mass of all the organisms at that trophic level. To do this properly, an ecologist collects all the organisms and puts them into an oven at 80C until all the water has been evaporated. Unfortunately, doing this is rather destructive to the ecosystem being studied, so ecologists often just measure the wet mass of the organisms and calculate the dry mass on the basis of previously published data. 

                                       Biomass at the higher trophic level

Ecological efficiency =                                                             X 100

                                       Biomass at the lower trophic level

7 of 22

Increasing primary productivity - the entry of bio

The rate at which energy passes through each trophic level in a food chain is a measure of its productivity. Gross primary productivity is the rate at which plants convert light energy into chemical energy through photosynthesis. Even at the start of a food chain, this is inefficient. Because photosynthesis produces glucose, entry of biomass into the food chain is also inefficient. In optimal conditions, only 40% of light energy from the Sun enters the light reaction of photosynthesis, and only half of this is involved in glucose production. Only two-thirds of the glucose is then used for production of starch, cellulose, lipids and proteins, contributing to growth. The rest is respired. Hence, only a small proportion (between 1 and 8%) of the energy from the Sun remains to enter the food chain: the net primary productivity (NPP) is 8%. By manipulating environmental factors, humans make energy conversion more efficient, reduce energy loss and increase the amount of biomass which is incorporated into plants. 

Light levels limit the rate of photosynthesis and hence production of biomass. Some crops are planted early to provide a longer growing season to harvest more light. Others are grown under light banks.

8 of 22

Increasing primary productivity - the entry of bio

As well as irrigating crops, drought-resistant strains have been bred, for example drought-resistant barely in North Africa, wheat in Australia and sugar beet in the U.K. Water is a reactant in photosynthesis when glucose is produced. 

Growing plants in greenhouses provides a warmer temperature increases the rate of photosynthesis, and increases the rate production of biomass. Planting field crops early to provide a longer growing season also helps to avoid the impact of temperature on final yield. For example, winter wheat has a longer growing season than spring wheat.

Lack of available nutrients slows the rate of production of biomass through photosynthesis. Crop rotation can help - growing a different crop in each field on a rotational cycle. This stops the reduction in soil levels of inorganic materials such as nitrate or potassium. Including a nitrogen-fixing crop like peas or beans in that cycle replenishes nitrogen levels. Many crops have been bred to respond to high levels of fertiliser which provides ammonium, nitrate, potassium and phosphorus.

9 of 22

Increasing primary productivity - the entry of bio

Pests like insects or nematodes eat crop plants, removing biomass from the food chain and lowering yield. Spraying with pesticides may help. Some plants have also been bred to be pest-resistant or have been genetically modified with a bacterial gene (Bt gene) from Bacillus thuringiensis. In Bt cotton in the USA, this confers resistance against bollworm, and in maize against corn-borers. 

Fungal disease reduces biomass. Fungi cause root rot (reducing water absorption), damage xylem vessels (interfering with water transport), damage foliage through wilt, blight or spotting (interfering with photosynthesis directly), damage phloem tubes (interfering with translocation of sugars), or damage flowers and fruit (interfering with reproduction). Farmers spray crops with fungicides. Many crops have been bred to resist fungal infections (e.g. Rhizomania resistance in sugar beet). Potatoes have been genetically modified to resist potato blight. 

Competition from weeds for light, water and nutrients reduces a crop's NPP. Farmers use herbicides to kill weeds. The herbicide usually binds to an enzyme, stopping it from working, and frequently leading to a toxic build-up of the enzyme's substrate. 

10 of 22

Improving secondary productivity 1

Primary consumers do not make full use of plants' biomass - some plants die, consumers do not eat every part of the plant, and they do not digest everything they eat (such as cellulose), egesting lot of it in their faeces. Even when food is digested and absorbed, much of it is respired, with only a small amount contributing to an increase in biomass and being available to the next consumer in the food chain.  

Humans can manipulate energy transfer: A young animal invests a larger proportion of its energy into growth than an adult. Harvesting animals just before adulthood minimises loss of energy from the food chain. Selective breeding has been used to produce improved animal 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 and parasites. Mammals and birds waste a lot of energy finding food and keeping their body temperature stable. Zero grazing for pig and cattle farming maximises energy allocated to muscle (meat) by stopping the animals from moving about, by supplying food to them, and by keeping the environmental temperature constant. 

11 of 22

Improving secondary productivity 2

Grain could be used to feed humans directly as opposed to feeding cattle or pigs first, in some infertile areas grain cannot be grown but animals can survive; for example, sheep often live on mountainsides, producing food for humans.

Many people have serious concerns about modern farming practices and animal welfare. Deciding where the balance lies between welfare and efficient food production is a contentious topic that is constantly kept under review and should include topic that is informed public debate.

12 of 22

Recycling within ecosystems

Dead and waste organic material can be broken down by decomposers - microorganisms such as bacteria and fungi. Bacteria and fungi involved in decomposition feed in a different way animals. They feed saprotrophically, so from they are described as saprotrophs. The steps in saprotrophic decomposition are: 

  • Saprotrophs secrete enzymes onto dead and waste material. 
  • Enzymes break down material into small molecules, which are digested then absorbed into the saprotroph's body. 
  • Having been absorbed the molecules are stored or respired to release energy. 

If bacteria and fungi did not break down dead organisms, energy and valuable nutrients would remain trapped within the dead organisms. By digesting dead and waste material, microorganisms obtain a supply of energy to stay alive, and the trapped nutrients are recycled. Microorganisms have a particularly important role to play in cycling carbon and nitrogen within ecosystems. 

13 of 22

Recycling nitrogen

Living things need nitrogen to make proteins and nucleic acid. Bacteria are involved in ammonification, nitrogen fixation, nitrification and denitrification.

14 of 22

Nitrogen Fixation

Although nitrogen gas makes up 79% of the Earth's atmosphere, it is very unreactive. This means it is impossible for plants to use it directly (even though it is so abundant). Instead, plants need a supply of 'fixed' nitrogen such as ammonium ions (NH4+) or nitrate ions (NO3-). Nitrogen fixation can occur when lightning strikes or through the Haber process in making fertiliser. However, these processes only account for about 10% of nitrogen fixation around the world. 

Nitrogen-fixing bacteria supply the rest of the fixed nitrogen. Azotobacter are bacteria that live freely in the soil and fix nitrogen gas, which is in the air within soil, using it to manufacture amino acids. Nitrogen-fixing bacteria such Rhizobium also live inside the root nodules of plants such as peas, beans and clover, which are all members of the bean family. These nitrogen-fixing bacteria have a mutualistic relationship with the plant: the bacteria provide the plant with fixed nitrogen and receive carbon compounds such as glucose return. 

Proteins such as leghaemoglobin in the nodules absorb oxygen and keep the conditions anaerobic. Under these conditions, the bacteria use an enzyme, nitrogen reductase, to reduce nitrogen gas to ammonium ions can be used by the host plants.

15 of 22

Ammonification and nitrification

Ammonium ions are released through ammonification by bacteria involved in putrefaction of proteins found in dead or waste organic matter. Rather than getting their energy from sunlight (like photoautotrophic bacteria, algae and plants), some chemoautrophic bacteria in the soil (Nitrosomonas bacteria) obtain it by oxidising ammonium ions to nitrites, while others (Nitrobacter bacteria) obtain it by oxidising nitrites to nitrates. These processes are called nitrification. 

Oxidation requires oxygen therefore these reactions only happen in well-aerated soils. Nitrates can be absorbed from the soil by plants and used to make nucleotide bases (for nucleic acids) and amino acids (for proteins).

16 of 22

Denitrification

Other bacteria convert nitrates back to nitrogen gas. When the bacteria involved are growing their anaerobic conditions, such as in waterlogged soils, they use nitrates as a source of oxygen for respiration and produce nitrogen gas (N2) and nitrous oxide (N2O).

17 of 22

Recycling carbon

The carbon cycle is driven by the processes of respiration and photosynthesis, with carbon dioxide being the main vehicle for the cycling of carbon between biotic and abiotic components of the cycle. Animals, plants and microorganisms respire to release carbon dioxide. Microorganisms are particularly important in decomposition of dead organisms and waste.

Terrestrial plants use gaseous carbon dioxide in photosynthesis, whereas aquatic plants use dissolved carbohydrates. Carbon is exchanged between the air and water when carbon dissolves in water and then reacts to form carbonic acid. Carbon also enters rivers and lakes from weathering of limestone and chalk in the form of hydrogen carbonate. 

Combustion of fossil fuels has increased across the last century, so that the balance of the carbon cycle has changed and atmospheric carbon dioxide levels are higher. This change is responsible for global warming. 

18 of 22

Succession

Any change in a community of organisms can cause a change in their habitat. Any change in a habitat can also cause a change in the make-up of the community. These ideas can help explain why gradual directional changes happen in a community over time. Such a process of directional change is called succession. 

19 of 22

How does succession happen?

1. Algae and lichens begin to live on the bare rock. This is called a pioneer community. 

2. Erosion of the rock and a build-up of dead and rotting organic material produce enough soil for larger plants like mosses and ferns to grow. These replace, or succeed, the algae and lichens.

3. In a similar way larger plants succeed these small plants, until a final, stable community is reached. This is called a climax community. In the UK, climax communities are often woodland communities. 

Succession does not always start from bare ground. Secondary succession takes place on a previously colonised but disturbed or damaged habitat.

20 of 22

Succession on sand dunes

Pioneer species like sea rocket (Cakile maritima) and prickly sandwort (Salsola kali) colonise the sand just above the high water mark. These can tolerate being sprayed with salty water, lack of fresh water and unstable sand.

Wind-blown sand builds around the base of these plants, forming a mini sand dune. As plants die and decay, nutrients accumulate in this mini dune. As the dune gets bigger, plants like sea sandwort (Honkenya pepeloides) and sea couch grass (Agropyron junceiforme) colonise it. Because sea couch grass has underground stems, it helps to stabilise the sand. 

With more stability, and accumulation of more nutrients, plants like sea spurge (Euphorbia paralias) and marram grass (Ammophila arenaria) 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, trapping more sand in the process. 

As the sand dune and nutrients build up, other plants colonise the sand. Many are leguminous such as hare's foot clover (Trifolium arvense) and bird's-foot trefoil (Lotus corniculatus), which convert nitrogen into nitrate. With nitrate available, more species colonise the dunes, like sand fescue (Festuca rubra) and viper's bugloss (Echium vulgare), which stabilise them further.

21 of 22

Deflected succession

The landscape in the UK is heavily influenced by agriculture; therefore it can be difficult to work out whether a particular location has reached its climax community. For example, when a groundsman cuts the grass on a golf course, he is keeping that particular area at one stage in a succession. If the grass were left unmown over a number of years, succession would continue without interference and the course would most likely reach a climax community of woodland. When succession is stopped or interfered with in this way, we refer to it as deflected succession. The sub-climax community that results is called a plagioclimax. 

Other ways in which succession can be deflected include grazing, burning, application of fertiliser, application of herbicide, and exposure to excessive amounts of wind. Succession in many locations is deflected by human activity and has been for centuries, which can make it hard for preservationists and conservationists to decide which habitats warrant preservation hard or conservation. 

22 of 22

Comments

No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Ecosystems resources »