### Ecology

Ecosystem: a community of living organisms and the non-living (abiotic) factors that affect them.

Population: a group of organisms belonging to one species found in a particular place at a particular time.

Community: all the populations of organisms belonging to one species found in a particular place at a particular time.

Environment: the conditions that surround an organism, consisting of both living and non-living components.

Habitat: where a particular population or community lives.

Niche: where an organism is found and what it does there.

Abundance: how many organisms are found in an area.

Distribution: whereabouts organisms may be found in a particular area.

Sampling techniques for plants

1. Quadrats: a sample area is marked out so that the organisms it contains can be studied. Quadrats are placed at random in the sample area.
1. Open quadrats are simple square frames, typically 50 cm x 50 cm. Gridded quadrats are divided into 100 equal squares so that percentage cover can be determined.
2. Point quadrats consist of a bar or frame attached to 10 long pins about 5 cm apart. Used to study plant distribution- the quadrat is positioned so that the 10 pins just touch the ground. By recording the number of times a pin hits a plant, percentage cover can be calculated. Often used with transects.
2. Transect: a line along which samples are taken. Quadrat samples will be taken at regular intervals along the transect line. Used to study variation of species over environmental gradients (e.g. down a beach).

Sampling techniques for animals: the mark-release-recapture technique is commonly used. An initial sample of animals is captured in a particular area. All the captured animals are then marked and released. A second sample of animals is then captured. The population size (N) can then be estimated using the Lincoln Index:

N = no. individuals in first capture x no. individuals in second capture / no. individuals recaptured

There are several limitations to this method, however:

1. Only works for non-territorial animals
2. Obviously, you need to leave some time between release and recapture so it is implicitly assumed that the population size remains constant during this time, which most likely would not be the case
3. Estimates are often too high, especially if the number of recaptures is low (if there are no recaptures N will equal infinity!)
4. Marked animals may undergo increased predation (if marked with a bright paint, for example, they will be more visible to predators).

Methods of capturing animals:

1. Sweep netting. Capture airborne insects or underwater organisms by sweeping a net across the sample zone.
2. Kick sampling. Disturb the sample area (e.g. a riverbed) and capture the disturbed organisms with a net.
3. Trapping. Use sticky traps for insects, light traps for nocturnal insects, pitfalls for ground creatures or Longworth traps for mammals.

Biodiversity is a measure of the number of different species in a community and the number of individuals of each of these species. The measurement of biodiversity allows assessment of a habitat and its colonisation over time, as well as the comparison of different habitats.

The Simpson Diversity Index is a quantitative measure of diversity.

D= diversity index, N= total number of individuals, n = number of individuals of a particular species (sum over all species present).

Abiotic factors relate to the non-living part of the environment. Some important abiotic factors are listed below:

1. Temperature: average ambient living temperature is 0-40ÂºC. Temperature is important because many reactions are enzyme-controlled and enzymes will not function if the temperature is too extreme (see Enzymes for further details).
2. Light is needed for photosynthesis- therefore it affects the distribution of plants and thus also animals. Light can also control animal reproductive patterns. Plants living in areas of low light intensity have lower compensation points and so are more efficient at photosynthesis in these low intensities.
3. Water is necessary for all life- annual rainfall and pH of rainwater are important factors in determining where land-dwelling organisms can live.

In dry areas, organisms have specialised mechanisms to conserve what little water is available.

For aquatic organisms, temperature, salinity, oxygen and nutrient content and current flow must also be taken into consideration.

1. Wind: strong winds can prevent the establishment of small trees and flatten larger ones. Winds also serve to pollinate and disperse seeds.
2. Humidity: the relative humidity of the atmosphere can vary greatly. Low humidity increases evaporation rate and thus the risk of dehydration. High humidity and temperature can be harmful as evaporation/transpiration (and therefore cooling) are reduced.
3. Soil: there are several abiotic factors relating to the soil- these are known as edaphic factors.
1. Inorganic particles include fine clay, silt, fine sand, coarse sand and gravel. Clay has poor aeration and drainage, but good nutrient content and water retention. Sand has fast drainage, good aeration, but poor nutrient content and water retention. Loamy soils contain a mixture of sand, silt and clay, and are the best for plant growth. Soil is bound together into crumbs by fungal mycelia and bacterial polysaccharides.
2. Water: soils with good water retention can be up to 15% water. Some is tightly bound to soil particles by surface tension, whilst the rest is free and accessible to plants.
3. Air is contained in pores between soil particles. It is usually saturated with water vapour, and contains less O2 and more CO2 than atmospheric air. NH3 and CH4 may also be present.
4. Organic material includes living organisms such as bacteria, protoctists, fungi and earthworms. These organisms help to aerate the soil and decompose the dead organic material that is also present.
5. Dissolved materials from parent rock and organisms in the soil means that soil pH can vary from 3-8.

Biotic factors relate to the living part of the environment, e.g. competition, predation, parasitism (will be covered in another section) and disease.

1. Competition is the interaction of different organisms for the same resources. There are two types of competition:
1. Intraspecific competition occurs between members of the same species.
2. Interspecific competition occurs between members of different species. One consequence of this type of competition is summarised by Gause’s Competitive Exclusion Principle, which states that if two species occupy the same ecological niche, the interspecific competition leads to the extinction of one or the other of the species.
2. Predation: as a prey population increases, the number of individuals preyed upon naturally increases, but the percentage of individuals preyed upon decreases. This is because prey must be caught, eaten and digested- a process which takes a set amount of time. Therefore, even though more prey is available, time constraints and the fact that predators will only hunt what they need to survive, means that the percentage of prey caught is lower.

Indicator species: if a species is thriving and/or out-competing other species in a particular habitat, it indicates that conditions in that habitat are the optimum for that species, i.e. a presence of this species gives an indication as to conditions. The species in question is known as an indicator species.

Population growth: populations are dynamic, and are subject to change over a period of time, as described by a sigmoid growth curve.

1. Lag phase: Birth rate exceeds death rate but population increase is slow because there are few organisms available to reproduce.
2. Exponential phase: Rapid population increase. Short doubling time means population doubles at regular intervals, barring any limiting factors.
3. Stationary phase: birth rate equals death rate, so no further population growth possible. Population has reached carrying capacity- one or more factors have become limiting.
4. Death phase: death rate exceeds birth rate. A vital factor has become severely limited, leading to death from lack of vital nutrients or an excess of toxic substances.

Doubling time: the time taken for a population to double in size.

Environmental resistance factors limit the growth of a population. There are two types:

1. Density dependent: the effect of these factors depends on population size (generally greater in larger populations) e.g. availability of water, food light, oxygen an shelter; disease; waste accumulation and predation.

2. Density independent factors have the same effect on any population, no matter what the size, e.g. natural disasters such as fires, floods and frosts.

Absolute/actual growth of an organism or population can be represented by the graph below:

Growth rate of a population or organism can be represented by a graph of increase in size against time (effectively plotting the gradient of the above graph against time).

Succession occurs when communities of plants and animals colonise an area, and over time are replaced by other species.

1. Pioneer species/colonisers interact with an abiotic environment, changing the habitat so that it is able to support other organisms. This begins a new ecosystem.
2. Seres are stages of ecosystem development. As more organisms come to occupy a habitat, diversity and the number of niches in that habitat increase.
3. Climax community: this is the stable ecosystem formed at the completion of succession.

Primary succession occurs when organisms colonise a habitat that has not previously supported life, e.g. land laid down in a volcanic eruption or exposed after a landslide.

Note: as plant diversity increases, there is an increase in habitat and food available for animals, so animal diversity will also increase.

Secondary succession occurs when organisms recolonise an area where a community has been wiped out due to a natural disaster or removed due the area undergoing a change in function, e.g. an industrial area now fallen into disuse. A greater range of species is initially present because soil is already there.

Zonation: the occurrence of organisms in bands or zones. The most obvious example is a rocky shore, where different areas spend different lengths of time underwater. This creates a range of habitats which different organisms can inhabit.

Food Chains are arranged into trophic, or feeding levels; energy is lost at each level. There are rarely more than four trophic levels in a food chain, as there simply isn’t enough energy available to support another level of organisms.

Trophic Level 1: Producers (usually green plants): these are organisms which can produce organic molecules from simple inorganic ones using energy from sunlight and chemical reactions.
Trophic Level 2: Primary consumer/herbivore: feeds directly on producers.
Trophic Level 3: Secondary consumer/carnivore feeds on primary consumer.
Trophic Level 4: Tertiary consumer/top carnivore feeds on secondary consumer.

Energy flow through an ecosystem

1. 42% of light energy from the Sun reaches the Earth’s surface. The rest is absorbed or reflected by the atmosphere.
2. Only 2% of the light energy reaching the Earth’s surface is absorbed by plants for use in photosynthesis. The rest either does not reach a chloroplast, or is of an unsuitable wavelength.
3. Only around 2% of the energy used in photosynthesis is used in growth and thus incorporated into plant tissue. The rest is lost as heat in respiration, or used in active transport.
4. Only around 20% of the energy obtained when herbivores eat plant tissue is incorporated into herbivore tissue. The rest is either indigestible, lost as heat in respiration, or used in movement and metabolic reactions.
5. Only around 30% of the energy obtained when carnivores eat herbivore tissue is incorporated into carnivore tissue. The rest is either indigestible, lost as heat in respiration, or used in movement and metabolic reactions.

Gross primary production (GPP) is the rate at which producers capture light energy to build biomass, and is the theoretical maximum of energy available.

Net primary productivity (NPP) equals the GPP minus the energy used by producers to live. This is the actual energy available to the next trophic level.

Reasons for energy losses between trophic levels:

1. Many organisms in one trophic level will not be eaten by organisms in the next level but will instead die (from something other than predation, obviously) and provide food for decomposers instead.
2. Some of the food eaten is not digested or absorbed but is lost as faeces.
3. Energy-rich compounds obtained from feeding are lost through excretion or used in respiration.

Pyramid of numbers: this shows the number of organisms at each trophic level, arranging successive trophic levels on top of each other. The advantage of this method is that it is very easy to collect data (simply count organisms). However, not only is a pyramid of numbers is very difficult to draw to scale, but inverted pyramids may also arise where many small organisms (e.g. aphids) live off one large organism (e.g. oak tree) because in this system every organism has the same status regardless of size.

Pyramid of biomass: this shows the biomass of organisms at each trophic level, arranging successive trophic levels on top of each other. The advantage here is that size is taken into account, allowing for a more accurate representation of an ecosystem. However, it is more difficult to gather data for a pyramid of biomass as organisms must be dried out to determine their dry mass.

Pyramid of energy: this shows the energy present at each trophic level in kJ m-2 yr-1. This is the most accurate representation of an ecosystem as organism’s status only depends on its energy (not mass) and productivity over time can be calculated. However, it is very difficult to gather data as organisms must be burnt to determine their energy content.

The carbon cycle:

The nitrogen cycle:

Eutrophication is a process in which excess nitrates cause the death of water-dwelling organisms. Eutrophication occurs when highly soluble nitrates (e.g. from fertilisers) in the soil are leached into waterways. These excess nitrates enrich the nutrient content of rivers and lakes, causing increased algal growth. When these algae die, the bacteria that decompose dead matter flourish, and use up all the oxygen in the water, meaning that animal species in the river now die from lack of O2.

Fluctuations in O2 and CO2 levels in the atmosphere occur over the short-term due to respiration and photosynthesis. Over the long-term, these effects cancel out, however, more recently, human activities (e.g. deforestation, burning of fuels) may be permanently affecting the balance. Currently, oxygen makes up 21% of the atmosphere, while CO2 makes up 0.03-0.04%).

Deforestation: humans cut down areas such as rainforests to provide wood for furniture and paper, and to create farmland. This leads to habitat destruction and eventual soil erosion in the deforested area, resulting in decreased biodiversity. Burning wood and removing plants also increases atmospheric CO2 levels. Conservation methods are in place to prevent this to some extent but deforestation is still a problem.

Effects of increased CO2 concentration: CO2 is a greenhouse gas- it traps heat radiated by the sun in the Earth’s atmosphere. Therefore, too much CO2 leads to an increase in atmosphere temperature and may be responsible for a rise in sea level due to the melting of the polar ice caps.

CFC’s form free radicals which deplete the protective ozone layer in the upper atmosphere. This means more UV light from the sun penetrates through to the lower atmosphere, increasing the incidence of skin cancer. CFC’s are now prohibited but previous use still poses a problem today as they can remain in the atmosphere for up to 40 years.

Pesticides: if animals from one trophic level are poisoned by pesticides, the creatures that feed on them will also be damaged, but because the biomass of each successive level decreases, the concentration of pesticide increases, so the damage inflicted to higher trophic levels is greater. This is known as the concentration effect.

Nuclear fuels: radioactive waste products with long half-lives take years to decay, and pose a problem as to where they can be stored. Accidents can also result in radiation poisoning.

Combustion of forest fuels produces CO2. Impurities in these fuels also lead to the production of sulphur oxides, whilst the temperature of many combustion chambers means that nitrogen and oxygen can also react to form nitrogen oxides. Sulphur and nitrogen oxides will dissolve in water to form acid rain. Fossil fuel use also results in smog formation and oil spills, which damage living organisms.

Effects of acid rain: erosion of buildings, lowering of soil and waterway pH which can damage plants and animals.

Biological control: if a species introduced to a new environment becomes a pest (number increase rapidly due to lack of predation/competition), a parasite or predator species may be introduced to reduce the size of the pest species. This procedure is known as biological control. Biological control organisms must be able to survive and establish themselves in the new conditions without eating or affecting the existing species other than the target pest species.

The advantages of biological control over chemical pesticides are as follows:

1. Biological control species are specific and do not harm benign organisms
2. Biological control species only need to be introduced once (as opposed to needing to respray an area with pesticides)

Pests do not usually evolve resistance to parasites and predators the way they would to chemicals.