6.5- ecosystems

What is an ecosystem?

A community of animals, plants and bacteria interrelated with the physical and chemical environment

What are the components of an ecosystem?

Habitat- the place where an organism lives

Population- all of the organisms of one species, who live in the same place at the same time, and who can breed together

Community- all the populations of different species, who live in the same place at the same time, and who can interact with each other

What is a niche?

The role of each species in an ecosystem. A description of a niche could include how and what an organism feeds on, what it excretes and how it reproduces. Two species cannot occupy the same niche in the same ecosystems.

What are the biotic factors in an ecosystem?

Producers- plants and photosynthetic bacteria which supply chemical energy to other organisms

Primary consumers- herbivores which feed on plants and are eaten by carnivorous secondary consumers, which are eaten by carnivorous tertiary consumers

Decomposers- bacteria, fungi and some animals which feed on waste material or dead organisms

What are the abiotic factors in an ecosystem?

Humidity, pH, temperature, concentration of pollutants

Abiotic factors can be influenced by biotic factors

At extreme values of an abiotic factor, a species may perform better, worse, or die

Why are ecosystems dynamic?

They are constantly changing- the non living elements change, and the living elements grow and die, with populations rising and falling. Small changes in one thing can affect another- nitrogen levels in soil affect the population sizes of plants growing there- nitrogen fixing plants grow successfully in nitrogen deficient soil, but affect their environment by increasing the soil nitrogen levels

What are the three types of changes in an ecosystem which affect population size?

Cyclic changes- these repeat in a rhythm , for example movement of tides and changes in a day length

Directional changes- these changes go in one direction and tend to last longer than the lifetime of organisms in an ecosystem, for example coastal erosion or silt deposition in an estuary

Erratic changes- these have no rhythm or direction, for example lightning or hurricanes

What is biomass?

Organic and inorganic components making up a plant- e.g, glucose levels and mineral ions

What is biomass transfer?

Transfer of biomass from one tropic level to another

What is a trophic level?

The level at which an organism feeds in a food chain

What idea does a pyramid of numbers represent?

• At each trophic level, living organisms have to carry out life processes- respiration releases energy from organic molecules, which is eventually converted to heat, and materials are lost in CO2 and H20

• Biomass is lost from a food chain in dead organisms and waste material, which is then only available to decomposers

• 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

What does a pyramid of numbers contain?

* 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

What is a pyramid of biomass?

The area of each bar is proportional to the dry mass of all the organisms at that trophic levels

How do you calculate biomass transfer?

Ecological efficiency = (Biomass at the higher trophic level / Biomass at the lower trophic level) x 100

Ecological efficiency = (Biomass of primary consumer / Biomass of producer) x 100

What is productivity?

The rate of production of new biomass by producers

The rate at which energy passes through each trophic level in a food chain

What is gross primary productivity?

The rate at which plants convert light energy into chemical energy through photosynthesis

Why is gross primary productivity inefficient?

• Because photosynthesis produces glucose, entry of biomass into the food chain is 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 this glucose is then used for production of starch, lipids, cellulose and proteins, contributing to growth, and the rest is respired

• Only a small proportion of the energy from the sun remains to enter the food chain- the net primary productivity is 8%

Why do humans manipulate environmental factors?

To make energy conversion more efficient, reduce energy loss and increase the amount of biomass which is incorporated into plants

How do humans get over light levels as a limiting factor?

Planting crops early to provide a longer growing season to harvest more light, using light banks

How can you avoid the impact of temperature on the final yield?

Growing plants in greenhouses, planting field crops early

How can nutrients be maximised?

Crop rotation, which stops the reduction of inorganic materials such as nitrate and potassium in soils

Nitrogen fixing crops that replenish nitrogen levels

Breeding crops to respond to high levels of fertiliser which provides ammonium, nitrate, potassium and phosphorous

How can pests be mitigated against?

Pesticides, breeding pest resistant plants, genetically modifying plants

How to mitigate against fungal disease

Genetically modifying, breeding crops to resist infection

How to reduce competition from weeds

Herbicides- herbicides bind to an enzyme, stopping it from working and leading to a toxic build up of the enzyme’s substrate

Why is transfer of biomass between trophic levels inefficient?

• Primary consumers do not make full use of plants’ biomass- some plants die, consumers don’t make use of every part of the plant, and they don’t digest everything they eat

• 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

How can humans manipulate energy transfer?

• Harvesting animals just before adulthood minimises loss of energy from the food chain

• Selective breeding improves animal breeds with faster growth rates, increased egg production and increased milk production

• Animals can be treated with antibiotics to avoid unnecessary loss of energy to pathogens and parasites

• Mammals waste energy by finding food and keeping a stable body temperature- zero grazing and stopping animals from moving allows maximum energy for meat

Steps in saprotrophic decomposition

1. Saprotrophs secrete enzymes onto dead and waste material

2. Enzymes digest the material into small molecules, which are then absorbed into the saprotroph’s body

3. Having been absorbed, the molecules are stored or respired to release energy

Nitrogen fixation

• 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 as Rhizobium also live inside root nodules of bean plants

• 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 in return

Ammonification and nitrification

• ammonium ions are released through ammonification of bacteria involved in putrefaction of proteins found in organic matter

• Nitrosomonas obtain energy by oxidising ammonium ions to nitrites

• Nitrobacter bacteria obtain energy by oxidising nitrites to nitrates

Denitrification

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 and produce nitrogen gas and nitrous oxide

Recycling carbon

• Carbon is cycled between the abiotic and biotic components of an ecosystem

• Carbon cycle driven by photosynthesis and respiration

• Animals, plants and microorganisms respire to release carbon dioxide.

• Terrestrial plants use gaseous CO2 in photosynthesis, whereas aquatic plants use dissolved carbonates

• Carbon is exchanged between the air and water when CO2 dissolves in water and reacts to form carbonic acid

• Carbon enters rivers and lakes from limestone weathering in the form of hydrogen carbonate

What is succession?

Progressive change in a community of organisms over time

What is primary succession?

Development of community of plants from bare ground

Process of succession

1. Algae and lichens live on bare rock- pioneer community

2. Erosion of rock and build up of dead organic material produce soil for bigger plants, e.g. mosses and lichens to grow. These replace or succeed the algae / lichens

3. Larger plants succeed small plants until a final, stable community is reached- climax community

What is secondary succession?

Takes place on a previously colonised but disturbed or damaged habitat

What is a climax community?

The final stable community that exists after the process of succession has occurred

What is deflected succession?

Happens when succession is stopped or interfered with, such as by grazing or when a lawn is mowed

What is a pioneer species?

The species that begins the process of succession, often colonising an area as the first living things there

Stages of succession on a sand dune

1. Pioneer species like sea rocket and prickly sandwort colonise the sand just above the high water mark. These can tolerate salty water and unstable sand

2. Wind-blown sands build up around the base of these plants, forming a mini sand dune. When plants die, nutrients accumulate here. As the dune gets bigger, sea sandwort and sea couch grass coloniser it. Sea crouch grass has underground stems which stabilise the sand

3. Sea spurge / marram grass grow. Marram grass has shoots that trap sand, and as sand accumulates the shoots grow taller to stay above the dune so they can trap more sand

4. Other plants colonise the sand as the dune builds up. Many are leguminous such as hare’s foot clover and bird’s foot trefoil, which convert nitrogen into nitrate. With nitrate available, more species colonise the dunes, like sand fescue and viper’s bugloss which stabilise them further

Deflected succession

• Landscape dominated by agriculture- hard to work out whether a particular location has reached its climax community

• E.g. when grass is cut the area is being kept at one stage in succession- if the grass were left unmown succession would continue and the area would reach a climax community of woodland

• The sub-climax community that results is a plagioclimax

• Grazing, burning, application of fertiliser, application of herbicide, exposure to lots of wind

• Succession is often deflected by human activity which makes it hard for conservationists to decide which habitats warrant conservation