Ecosystems

Definitions

• Ecosystem-an environment including all the living organisms interacting within it, the cycling of nutrients and the physical and chemical environment in which the organisms are living.
  
• Habitat-a place where an organism lives
• Community-all the populations of all the different species of organisms living in a habitat at any one time
• Abiotic factors-are the non living elements of the habitat of an organism.
• Biotic factors-are the living elements of a habitat that affect the ability of a group of organisms to survive there.
• Biosphere-all the areas of the surface of the Earth where living organisms survive.
• Succession-the process by which the communites of organisms colonising an area change over time.
• Climax community-a self sustaining community with relatively constant biodiversity and species range-most productive group of organisms.
• Plagioclimax-a climax community that is at least in part the result of human intervention.

Primary succession

• Primary succession starts with an empty inorganic surface, such as bare rock of sand dunes. This type of succession is seen after a volcanic eruption or landslide.
• The first organisms are pioneer species such as algae, mosses and fungi. These organisms can penetrate the rock surface, helping to break it into small grains, and trap organic material that will break down to form humus. The inorganic rock grains and the organic humus are the start of the formation of soil.
• Once there is soil, other species such as grasses and ferns can establish root systems. The action of their roots, and the humus they form when they die and decay, add to the soil.
• As the soil layers develop, more water and nutrients are retained and gradually larger plants can be supported and the diversity if species increases.
• Eventually a climax community is reached, where the biodiversity and range of species are generally constant. 
• Biomass increases as more plants colonise the land, so more nutrients become available and can support larger plants.

Secondary succession

• Secondary succession is the development of an ecosystem from existing soil that is clear of vegetation. It occurs as rivers shift their courses, after fires and floods.
  
• The soil is already formed and contains seeds, roots and soil organisms, so the number of plants and animals present right from the beginning of the succession is much higher than in primary succession.
• The time it takes to go from an area of bare earth to a climax communitiy varies enormously. It depends on many different factors, including temperature, rainfall and underlying soil fertility.
• At each stage, different plants and animals that are better adapted for the improved conditions move in, out-compete the plants and animals that are already there, and become the dominant species in the ecosystem.
• As succesion goes on, the ecosystem becomes more complex as new species move in alongside existing species, meaning biodiversity increases.
• A secondary climax community differs from the original climax community as the climax commmunity formed will depend on the climatic factors and plants and microorganisms that are within the area.

Abiotic factors

• Light intensity (for photosythesis)
• Temperature (rate of enzyme-controlled reactions)                                                                      
• Wind and water currents (wind increases water loss from body)                                                
• Water availability                                                                                                                                    
• Oxygen availability                                                                                                                                
• Edaphic factors: soil structure and mineral content (soil that contains a large proportion of sand are very easily drained, so water passes thorugh it carrying minerals that may be needed by plants-leaching of minerals)

Biotic factors

• Predation                                                                                                                                      
• FInding a mate                                                                                                                                      
• Territory                                                                                                                                        
• Parasitism and disease

Random sampling-to avoid bias

• Frame quadrat
• Point quadrat
• Use a random number generator
• Abundance-the relative representation of a species in a particular ecosystem. Use the ACFOR scale: A=abundant, C=common, F=frequent, O=occasional, R=rare
• Limitations of this scale: it is subjective, there are no set definitions of the terms, species can easily be rated by how obvious they are rather than how abundant they are.
• Percentage cover-the area covered by the above-ground parts of a particular species

For animals: mark-release-capture to work out estimated population size= 

number in first sample x number in second sample/number of marked individuals recaptured

• Assumptions: spread evenly once released after marking, no births, deaths, immigration or emmigration and marks not rubbed off or toxic.

Spearman's rank correlation coefficient

• Used to measure whether there is significant correlation between two variable. 
• Gives a correlation coefficient (rs) between -1 and +1-a value of -1 indicates a perfect negative correlation and a value of +1 indicates a perfect positive correlation.

Step 1: State the null hypothesis

There is no correlation between the two variables, so rs is equal to 0

Step 2: Calculate the correlation coefficient

Step 3: Decide whether to accept the null hypothesis

Find the p value that relates to your observed value and if p<0.05, the correlation is considered to be statistically significant, so you can reject the null hypothesis.

Spearman's rank example:

The Student's t-test

• Used to determine whether the mean of a variable in one group differs significantly from the mean of the same variable in a different group. 

Step 1: State the null hypothesis

There is no significant difference between the mean of the interval variable for the two categories.

Step 2: Calculate your observed value, t

Step 3: Decided whether to accept the null hypothesis

Degrees of freedom=total number of data values-2. Find the probability and if p<0.05 the difference is considered statistically significant and the null hypothesis is rejected.

T-test example:

Food chain

• Producers-make food
• Primary consumers-the organisms that eat producers, they are herbivores  
• Secondary consumers-the animals that feed on herbivores, they are carnivores
• Tertiary consumers-animals that feed on other carnivores
• Decomposers/saprobiants-they microorganisms, such as bacteria and fungi, that break down the remains of animals and plants and return the mineral nutrients to the soil.

Pyramid of numbers

• Area of each bar is proportional to the number of individuals at each trophic level.
• Energy is lost between each trophic level and so there is little energy available at the top of the food chain (tertiary consumers) meaning it is always the smallest trophic level.
• Limitations: may not show an accurate picture of what is happening in the food chain, for example one tree may support a very large population but will only have a small trophic level.

Pyramid of biomass

• Area of each bar is proportional to the dry mass of all the organisms at that trophic level (gm^-2)
• Wet mass is very inaccurate as it is affected by water uptake in the soil, transpiration, drinking, urinating and defaecating.
• Using dry mass eliminated these inaccuracies, but involves destroying the material.

Pyramid of energy

• Even pyramids of biomass have their limitations as they dont take into account the reproduction rate of an organism.
• For example, in a sample of water there appears to be a greater biomass of zooplankton than the phytoplankton on which it feeds. The sample fails to show that the phytoplankton reproduces much more rapidly than the zooplankton and so its biomass over a period of time is much greater.
• A pyramid made up of observations over time is called a pyramid of energy.

Losses along a food chain

1. Some is lost to the animal as undigested and therefore ununsed material in their faeces.             

2. Much of the material that is digested is used to drive respiration.                                              

3. Some of the plant material is lost in metabloic waste products such as urea

Gross primary productivity

• GPP is the rate at which plants convert light energy into chemical energy
• The total amount of energy made by producers 
• Plants use at least 25% of the material they produce for their own metabolic needs-they respire, breaking down glucose to ATP. The rest of the material is stored as new plant body tissues. This energy store is known as the net primary productivity.

Net primary productivity

• NPP is the amount of chemical energy a producer stores as biomass
• This is the total amount of energy available to the next trophic level
• NPP=GPP-R (respiratory loss)
• Respiratory loss is the energy used by organisms for respiration: active transport, movement and muscle contraction, heat

The nitrogen cycle

• Nitrogen is a vital part of the structure of many molecules, including amino acids and proteins and DNA and RNA.
• Nitrogen gas is very unreactive and so cant be used by plants in this state. Therefore it is converted into nitrates or ammonium compounds which are useful to plants.
• Nitrogen is fixed in 3 main ways:
• 1. In the Haber process where it is converted to nitrates or ammonium ions                                                                                                                
• 2. By lightning where it is converted to NOx and then nitrates                                                  
• 3. By bacteria (free-living or nitrogen-fixing bacteria) in the soil which can convert nitrogen from the soil air into ammonia, and this is then converted into nitrates by the nitrifying bacteria. 
• The legumes, plants such as peas, beans and clover, have nodules on their roots that are full of nitrogen-fixing bacteria. 
• Some decomposers act specifically on the proteins, breaking them down to form ammonium compounds. These ammonium compounds are then oxidised by nitrifying bacteria that convert them to nitrates.
• Denitrifying bacteria use nitrates as an energy source and break them down again into nitrogen gas, so reduce the amount of nitrates in the soil. (Anaerobic conditions)

The carbon cycle

• Carbon dioxide is removed from the air by green plants in the process of photosynthesis. It is used to make proteins, carbohydrates and fats.
• When the plants are eaten by animals, this carbon is passed on and become part of the animals' bodies.
• When green plants respire, any carbon dioxide not used in photosynthesis is released into the atmosphere. 
• Similarly when animals respire, they release carbon dioxide as a waste product into the air.
• When these plants and animals die, saprophtic bacteria decompose the dead plants and animals. When these microbes respire they release carbon into the atmosphere as carbon dioxide, ready to be taken up again by plants for photosynthesis.
• Carbon dioxide is also released into the atmosphere bu combustion when anything that has been living is burnt. 

Carbon sinks

• These are massive reservoirs where carbon is removed from the atmosphere and locked up in organic or inorganic compounds. 
• In the biotic part of the system, carbon is removed from the atmosphere by photosynthesis and stored in the bodies of living organisms. Soil also contains great amounts of organic, carbon-rich material in the form of humus.
• In the abiotic part of the system, rocks such as limestone and chalk, and fossil fuels, such as coal, oil and natural gas, hold vast stores of carbon.
• The oceans also act as massive reservoirs of carbon dioxide. The carbon dioxide is in continual exchange at the air-water interface.
• Carbon dioxide dissolved in the water is taken up by photosynthesis by the phytoplankton that live in the surface waters of the oceans. 
• Large amounts of carbon are also stored in the calcium carbonate shells produced by many different marine organisms and coral reefs.

Depletion of resources

Farming:

• When we farm we remove the crop before the plants die and decompose and therefore break the natural cycles that return the minerals to the soil.
• As a result, soil mineral concentrations can decrease rapidly, especially when a monoculture (where one crop is grown over a large area) absorbs large quantities of particular minerals.
• Artifical fertilisers can replace the minerals used by plants, but they do not support the structure of the soil. Once soil biodiversity is lost, the soil structure breaks down and it become infertile even when fertilisers are used.

Depletion of resources 

Fishing:

• If we take too many fish, or fish at the wrong times of year, the fish cannot breed and replenish the populations, and fishing becomes unsustainable.
• A number of factors are causing the large-scale depletion of fish stocks:

1. The size of the global fishing fleet

2. Open-ocean fishing ships that take huge catches of fish

3. Techniques such as bottom trawling, where nets are dragged along the seabed, damaging the habitat and catching a wide variety of species, many of which are not wanted.

4. Enormous drift nets that are almost invisible and catch and kill many species by accident

5. Nets with small mesh sizes that catch immature fish as well as adult fish

Depletion of resources

Conserving fish stocks:

1. Controlling the size of the mesh in fishing nets, so only the largest fish are caught

2. Banning fishing during the breesing seasons of different fish

3. Imposing very strict quotas on fishing fleets and individual fishing vessels

4. Encouraging the use of fishing methods that are less damaging to the ecosystems

5. Banning the catching of severely endangered species of fish altogether 

6. Fish farming or aquaculture- sustainable fish stocks and preserving the biological resources in our coastal waters and oceans