POPULATION IN ECOSYSTEMS

Ecology – study of the inter-relationships between organisms and their environment. It includes abiotic and biotic factors. Abiotic factors – non-living factors in the environment, e.g. temperature and rainfall. Biotic factors – living factors in the environment, e.g. competition and predation.

Ecosystems – are dynamic systems made up of a community and all the non-living factors of its environment. They can be very small to very large. Within an ecosystem there is a flow of energy and cycling of elements throughout the system.

Habitat – place where an organism normally lives and is characterised by physical conditions and by types of organisms present. Within an ecosystem there are many habitats. Within each habitat there are smaller units who have their own microclimate – microhabitats.

Niche – role of species within its habitat. This includes biotic interactions (e.g. where it is in the food chain/interaction with other organisms) and abiotic factors (e.g. temperature range an organism can live or the time of day it is active) which it is adapted to in order to survive, reproduce and maintain a viable population. No two species occupy exactly the same niche; this is known as the competitive exclusion principle. Even if two species appear to do one thing similar, something else would be different. If two species would occupy the same niche at some point, they will be compete against each other. One of them being more successful that another due to it being better adapted to the environment, and one species would be left.

Population – group of organisms of one species that occupy the same habitat at the same time and can interbreed with each other.

Population size – number of individuals in a population; total number of organisms of one species in a habitat.

Population Growth curves

Carrying capacity, k – a certain size of a population that an ecosystem is able to support; the maximum stable population size of a species that an ecosystem can support. The size of population can vary as a result of:

effect of abiotic factors Interactions between organisms, e.g. intraspecific and interspecific competition and predation.

Community – all the populations of different species living and interacting in a particular place at the same time.

When a population grows in size slowly over a period of time, it is possible to plot a graph of number in a population against time. When a population grows in size quickly over a short period of time, for example growth of bacteria, this may not be possible and so logarithmic scale is used.

No population continues to grow indefinitely, even if at the start there is not limiting factors, with increase in size the limiting factors may appear. The limiting factors include availability of food, light, water, oxygen, shelter, accumulation of toxic waste, disease and predators. The population size that can be sustained for a long time is determined by the living factors of that habitat.

Abiotic factors that influence population size include: temperature – each species has their own optimum temperature at which it is best able to

survive, as it effects action of enzymes. If the temperature is further from optimum, the population size decreases. For ectotherms and plants, if the temperature falls lower or is higher than needed, their metabolic rate would be reduced due to enzymes working slower or denaturing. In endotherms, though they maintain their own body temperature, the further the external temperature deviates from the optimum, the more energy needed to maintain constant temperature and so less energy is spent on growth and reproduction. Therefore, population size is reduced.

Light – this is ultimate source of energy, therefore growth of organisms that photosynthesize increases and those who consume there organisms as well. Therefore population size increases.

pH – effects action of enzymes, therefore metabolic rate, so effects population size. Water and humidity – in dry, low humid conditions, the population size is small and

consists only of species that are adapted well in living in low-water and low-humidity environment. Humidity affects transpiration rates in plants and evaporation of water from animals.

The basic factors that affect the growth and size of human population are the birth rate and the death rate. The balance of these two factors decide if a human population increases, decreases or remains the same.

Individual populations are further affected by migration – when individuals move from one population to another. There are two types:

- Immigration – individuals join a population from outside- Emigration – individuals leave a population

Population growth = (births + immigration) – (deaths + emigration)

= x 100

Population pyramids – they show percentage of males and females in each age group in a population; they are useful to predict future trends of different populations. Three typical types of population are represented in the age population pyramids:

- Stable population – birth rates and death rate are in balance, no increase or decrease in population size

- Increasing population – birth rates are higher than death rates, which is show by a wide base for young people and narrow apex for older people.

- Decreasing population – when birth rates (narrow base of population) and a lower mortality rate (wider apex to pyramid).

Competition between two or more individuals results when the resource they share (e.g. light, food, oxygen, space) is in insufficient amount to satisfy all their requirements fully.

Intraspecific competition – competition arises between members of the same species over resources. The availability of these resources is what determines the size of a population, as it would affect the degree of competition between individuals. The greater the availability, the larger is the population. Examples:

- Limpets competing for algae, which is their main food. - Oak trees – in a large population of oak trees some will grow larger and restrict the

availability of light, water and minerals to the rest, which then die. In time the population will be reduced to relatively few large dominant oaks.

- Robins competing for breeding grounds – female birds are normally only attracted to males who have established territories. Each territory provides adequate food for one family of bids. When food is scarce, territories become larger to provide enough food. If there are fewer territories in a given area and fewer breeding pairs, it creates a small-sized population.

Interspecific competition – competition arises between members of different species. One of these species will have a competitive advantage over the other. Its population size will gradually increase while the population of other diminish. If the conditions remain the same than the diminishing species will be completely removed. This is called competitive exclusion principle.

Percentage population growth rate (in a given period)

population change during the period/population at the start of the period

Competitive exclusion principle – states that where two species are competing for same limited resources, the one that uses there resources most effectively will ultimately eliminate the other i.e. they occupy the same niche.

To show how a factor influences the population size it is necessary to link it to the birth and death rate of individuals in a population, as an increase or decrease of that factor may not necessarily changed the population size.

A type of interspecific relationship is predator-prey relationship/predation. Predation – occurs when one organisms is consumed by another. Predator – an organism that feeds on another organism, prey. Predators are adapted for capturing their prey – faster movement, more effective camouflage, better means of detecting prey; and prey is adapted at avoiding predators – better camouflage, more protective features, concealment behaviour. Their relationship is relatively balanced and the prey rarely becomes extinct.

- Predators each their prey, reducing the prey’s population- With fewer prey available the predators are in greater competition with each other for the prey

that is left. - The predators population is reduced as some individuals are unable to get enough prey to

survive- With fewer predators left, fewer prey are eaten and so more survive and are able to reproduce. - The prey population therefore increases. - With more prey available as food, the predator population also increases.

It is difficult to replicate the predator-prey relationship in a laboratory as in natural conditions there greater variety of habitat, the area is greater over which the population can travel, so some of the prey can escape predation. In a laboratory all would be killed. In natural environment it is difficult to obtain reliable data as it is not possible to count all individuals in a population, so the size can be only estimated from sampling and surveys.

Although predator-prey relationships are significant reasons for cyclic fluctuations in populations, they are not the only reasons. Disease and climatic factors also play a part. The periodic population crashes are important in evolution to make the population between adapted to the prevailing conditions. Selection pressure – the environmental force altering the frequency of the alleles in a population

Investigating populations

Abundance – the number of individuals of a species in a given space/area/habitat. It is almost impossible to identify and count every organism, as it would be time consuming and cause damage to the habitat. Therefore only small samples of the habitat are studied in detail.

Quadrats are used for sampling. The quadrats are placed in different locations within the area being studied; the abundance of each species is recorded in these locations. There are two types:

- Point quadrat – consists of a horizontal bar supported by two legs. Along the bar at set intervals there are two holes; through each a long pin may be dropped. Each species that the pin touches is then recorded.

- Frame quadrat – square frame divided by string or wire into equally sized subdivisions. It is often designed so in can be folded, compact for storage and transport.

Factors considered when using a quadrat:

- Size – larger species require larger quadrats. Where a population is not evenly distributed throughout the area, a large number of small quadrats give a more representable result than a smaller number of large quadrats.

- Number of quadrats – the larger the number the more reliable the results will be. Sampling is time consuming, so a balance between accuracy and time available need to be gained. The greater the number of different species in the area studied, the greater the number of quadrats required to produce reliable results for a valid conclusion.

- Position of the quadrats within the area – to produce statistically significant results a technique known as random sampling must be used.

Sampling at random is important to avoid any bias in collecting data, therefore making sure the data collected is reliable. When taking samples to investigate the effect of grazing animals on species of plants on fields, for example, you would choose fields are close together as possible to minimise soil, climatic and other abiotic differences. To make sure the samples taken on that area is random, can do the following:

- Throw a quadrat over your shoulder- Divide the area into coordinates by two long tape measurements, generate the coordinates by

a computer, use the intersection of coordinates to get a sample. Sometimes it is more informative to ensure the abundance and distribution of a species in a systematic rather than random manner. This is especially important when gradual change/transition in the communities takes place. A belt transect can be used long a line of succession, e.g. sand dunes by the edge of the sea and inland up into woodland; a quadrat is laid against and the number of species is counted. This gives a record of species in a continuous belt.

Both of these are useful way of measuring abundance.

Ways of measuring abundance when species don’t move around, non-motile (sessile) or very slow moving animals that remain in one place: Method Definition Advantages DisadvantagesFrequency Likelihood of a

particular species occurring in a quadrat.

Useful when a species is difficult to count.Gives a quick idea of the species present and their general distribution.

Does not provide density and detailed distribution

Percentage cover

Estimates the area within a quadrat that a particular species covers.

Useful when the species are particularly abundant or difficult to count.The data is collected rapidly as individual plants do not need to be counted

Less useful where organisms occur in several overlapping layers

To obtain reliable results it is necessary to ensure that the sample size is large (large number of samples obtained).

Other methods are used for motile organism, as they are difficult to identify. Motile organisms – move away when approached; often hidden.

Mark-release-recapture technique can be used when a known number of animals are caught, marked in some way, and then released back into the community. After some time, a given number of individuals randomly collected and number of marked individuals is recorded. From this population size can be calculated.

Estimated population size =Total number of individuals in the first sample x total number of individuals he second sample

÷

This relies on several assumptions:- The proportion of marked to unmarked in the second sample is the same as the proportion of

marked to unmarked individuals in the whole population. - Marked individuals released from the first sample distribute themselves evenly and have

sufficient time to do so. - The population has a definite boundary so that there is no immigration into or emigration out

of the population. - The population has a definite boundary, so there is no immigration or emigration - There are few, if any, birth and death within the population- The method of marking is no toxic to the individual nor does it make the individual more

conspicuous and so more liable to predation. - The mark or label is not lost or rubbed off

Ecosystems are dynamic – they change from day to day as populates fluctuate, sometimes slowly and sometimes rapidly. Succession – a term used to describe to describe changes over time in the species that occupy a particular area.

Succession takes place in a series of stages. At each stage new species colonize the area and may alter the abiotic environment, either making is less suitable for the existing species or make it more suitable for other species with different adaptations. In both cases the new species would out-compete the existing one and take over a given area. These alternations can result in a less hostile environment that makes it easier for other species to survive. So ne communities are formed and biodiversity by changed or increased.

The first stage of this type of succession is the colonisation of an inhospitable environment, barren land, by pioneer species. The barren land can arise from glacier retreating and depositing rock, sand being piled into dunes by wind or sea, volcanoes erupting and deposing lava, lakes or ponds being created by land subsiding, and silt and mud being deposited at river estuaries. Pioneer species make up a pioneer community and often have features that suit colonisation, including:

- Asexual reproduction so a single organism can rapidly multiply to build up a population. - Production of vast quantities of wind-dispersed seeds or spores, so they can easily reach

isolated situation e.g. volcanic islands- Rapid germination of seeds on arrival as they do not require a period of dormancy. - Ability to photosynthesise, as light is normally available but other food is not. They are

therefore not dependent on animal species- The ability to fix nitrogen from the atmosphere because, even if there is soil, it has few or no

nutrients- Tolerance to extreme conditions.

Lichens are an example of pioneer species. They weather rocks producing sand or soil, this is not enough to support other plants. But, as lichens die and decompose they release sufficient nutrients to support a community of small plants. When they die and decompose they release sufficient nutrients to support a community of small plants. Lichens therefore change the abiotic environment by creating soil and nutrients. Mosses are usually the next stage in succession, followed by ferns. With continuous erosion of rocks and increase in organic matter available from death of these plants, the layer of soil is build up.

Number of marked individuals recaptured

Organic material holds water making it easier for other plants to grow. Small flowering plants like grasses and in turn shrubs and trees can grow. These species provide more sources of food, leading to move food chains that develop into complex food webs and lead to more stable communities. This state has a balanced equilibrium of species with few, if any, new specie replacing the established ones. It has high biodiversity and contains animals and plants. This is called a climax community which remains more or less stable over a long period of time. In UK, this is most likely to be deciduous oak woodland. Climax communities have prevailing climate. Climate and other abiotic factors determine the dominant species of the community.

The plants available determine the species of animals that inhabit there: - Dead lichens provide food for animals like detritus-feeding mites - Growth of mosses and grasses provides food and habitats for insects, millipedes and worms,

who would be eaten by secondary consumers like centipedes. - Development of flowering plants, including trees, help to support butterflies, moths, reptiles,

mammals and birds.

During any succession there are a number of common features that emerge:- The non-living (abiotic) environment becomes less hostile, e.g. soil forms (which helps

retain water) nutrients are more plentiful, and plants provide shelter from the wind. - Greater number and variety of habitats and niches- Increased biodiversity – this is especially evident in early stages, reaching a peak in mid-

succession, but decreasing as climax communities is reached. This is because the dominant species out-competing pioneer and other species.

- More complex food webs- Increased biomass, especially during mid-succession

Another type of succession occurs when land that has already sustained life is suddenly altered e.g. land clearance for agriculture or a forest fire. The return to the climax community is same but more rapid. This is because the soil already exists with spores and seeds that often remain alive; and there is influx of animals and plants through dispersal and migration. This type of succession is classed secondary succession. Some of the species in climax community would be different from the original.

Succession of an Artic glacierThis occur as glacier in northern hemisphere have been melting over 200 years, increasingly due to additional global warming since industrial revolution and fossil burning. Stages:

- Pioneer stage – after the ice has retreated, photosynthetic bacteria and lichens colonise patches of land. Both fix nitrogen, which is essential as nitrogen is virtually absent from glacial moraines. They also form tough mats that help to stabilise the loose surface of the moraines. These pioneer species decompose to form humus, which provides nutrients that enable mosses to colonise. The pioneer stage occurs when the land has been ice free for 10-20 years.

- Dryas stage – after around 30 years after the ice had retreated, the ground is an almost continuous mat of herbaceous plant Dryas . Its roots stabilise the thin and fragile soil layer formed from the erosion of the rocks that make the moraine. Bacteria in the root nodules of Dryas also fix nitrogen, further adding nitrogenous nutrients to this poor-quality soil. Other plants found at this stage are the Artic poppy and moss companion.

- Alder stage – after about 50-70 years after ice has retreated, Alder is able to grow. Alder is a tree that has nitrogen-fixing nodules on its roots and that allows it to grown in nitrogen-poor soil. Alder sheds its leaves, which decompose into nitrogen-rich humus that fuerth enriches the soil.

- Spruce stage – about 100 years after the ice has retreated, spruce trees develop amongst the alder. A period of transition takes place and during the next 50 years or so the taller spruce out-competes the alder and ultimately displaces it altogether.

Conservation – management of the Earth’s natural resources by humans in such a way to maximize its usage in the future. This involves active intervention by humans to maintain ecosystems and biodiversity. It is management of existing resources and reclamation of those already damaged by human activities. Main reasons for conservation:

- Personal – maintain our planet, and so our life support system - Ethical – other species have occupied far longer than we have and should be allowed to

coexist with us. Respect for living things. - Economical – living organisms contain a gigantic pool of genes with the capacity to make

millions of substances, many of which may prove valuable in the future. Long-term productivity is greater if ecosystems are maintained in their natural balanced state.

- Cultural and aesthetic – habitats and organisms enrich our lives, adding interest and inspiring creativity.

Many of species that existed in earlier stages are no longer present as they are no longer part of climax community. This because their habitats have disappear as a result of succession or the species was out-competed by other species or due to human activities. One way of conserving these habitats and so species they containing is preventing the next stage. This is for example preventing moorland reaching its climax community of deciduous woodland by burning heather and grazing of sheep. If the factors preventing further succession are removed than the ecosystem would develop into its climax community.

Sand dunes can be managed to prevent woodland leaving wet areas where species like natterjack toads can thrive.