3.7.4 Populations in ecosystems

Ecology is the study of the inter-relationships between organisms and their environment. The environment includes the abotic/non-living factors (e.g rainfall and temperature, light, pH, humidity) and the biotic/living factors, (e.g. competition and predation).

Ecosystems are dynamics made up of a community and all the non-living factors of its environment. A good example is a pond as it has its own community of plants to harbor sunlight energy to supply organisms in it and nutrients (e.g nitrate/phosphate ions) are recycled within the pond. There are two main processes to consider within ecosystems:

• the flow of energy through the system
• the cycling of elements within the system

Each species within an ecosystem is made up of a group of individuals that make up a population. A population is a group of individuals of one species that  occupy the same habitat at the same time and are potentially able to interbreed. An ecosystem can only support a certain size of population - this is known as the carrying capacity and it can vary as a result of:

• the effect of abiotic factors
• interactions between organisms (e.g intraspecific and interspecific competition/predation)
• Intraspecific competition - occurs when individuals of the same species compete with one another for resources such as food/water/breeding ground. the greater the availability the larger the population can grow.
• Interspecific competition - occurs when individuals of different species compete for resources. One species will usually have a competitive advantage over the other and the population size of this species will gradually increase as the other diminishes, leading to the complete removal of the weaker species (competitive exclusion principle). The principle states that when two species are competing for limited resources the one that uses these resources most effectively will eliminate the other. 

A community is all the populations of different species living and interacting in a particular place at a particular time.

A habitat is the place where an organism normally lives and is characterized by physical conditions and other types of organism present. There can be many habitats in a single ecosystem. Within each habitat there are smaller units with their own microclimate known as microhabitats. 

An ecological niche describes how an organism fits into the environment. it refers to where an organism lives and what it does there and includes all the biotic and abiotic conditions to which an organism is adapted to survive/reproduce/maintain a viable population. No two species occupy the exact same niche. This is known as the competitive exclusion principle.

Population size

The population size is the number of individuals in a population.

It is tricky to plot the growth of microorganisms on a graph of population against time as the populations may grow rapidly. In cases such as these we can use a logarithmic scale to represent the number of bacteria.

However, population growth rate can change if limiting factors are present. For example (I am using bacterium as an example):

• As the population grows mineral ions are consumed
• The population becomes so large that bacteria at the surface prevent light reaching those at lower levels
• Other species in the pond may use bacteria as food/compete for light/minerals
• Environmental factors ,at bring limiting factors such as: Winter might bring cooler temperatures and lower light intensity/less light availability.

Over winter the population size is often fairly constant as conditions do not really change. The carrying capacity of a population that can be sustained depends on the limiting factors. Each population has optimum abiotic factors which influence it's carrying capacity for that particular ecosystem.

Predation

This occurs when an organism is consumed by another. The effect of the predator-prey relationship on population size:

• predators eat prey reducing population size of prey
• fewer prey available so predators are in greater competition for leftover prey
• predator population reduces
• with fewer predators, prey population increases
• with more prey, the predator population increases.

Investigation population

The abundance of a population is the number if individuals of a species in a given space. To measure population size we take a representative sample and use the following sampling techniques:

Non-motile organisms

• random sampling using frame quadrats or point quadrats
• A frame quadrat is a square divided by wire/string into equally sized subdividions
• A point quadrat is a horizontal bar supported by one, or two, legs. there are 10 holes at set intervals along the bar through which a long pole can be dropped and the number of species that touch each pin is recorded
• How we do this randomly is lay out two tape measures at right angles along the study area/obtain a series of coordinates from a random number generator/place a quadrat at the intersection of each pair of coordinates and record the species within it
• systematic sampling along a belt transect
• It may be more useful sometimes to measure species abundance and distribution in a systematic rather than random way e.g when there is gradual change).
• A belt transect can be made by laying tape across the ground in a straight line. A frame quadrat is laid down alongside the line and the species within it are recorded giving a record of species in a continuous belt.

Random sampling with quadrats and transects measures abundance. It can be measured in a number of ways including:

• frequency (the likelihood of the species appearing in the quadrat). It is useful for abundant species such as grass but des not give information about the quantity or detailed distribution of the species
• percentage cover (an estimate of the area within a quadrat that a species covers). It is useful when a species is hard to count. This means that data can be collected rapidly and individual plants need not be counted but is a pain when organisms overlap.

Motile organisms

• Mark-release-recapture
• A known number of animals are caught, marked, and released back into the community and some time later a number of individuals are collected again and the number of marked individuals is recorded.
• The population size can be calculated as it = ((total no. of individuals in first sample + total no. of individuals in second sample)/number of marked individuals recaptured).
• There are various assumptions that this technique relies on that we must learn:
• The proportion of marked to unmarked individuals in the second sample is the same as the proportion of marked to unmarked individuals in the population as a whole
• The marked individuals distribute themselves evenly when reintroduced
• The population has a definite boundary so there is no immigration/emigration
• There are few (if any) births/deaths in the population
• The method of marking is not toxic/will not rub off/does not make the individual more liable to predation etc

Succession

ecosystems are dynamic - they change very often (day to day) as population sizes fluctuate. The term we use to describe these changes is succession, it takes place in a series of stages. At each stage a new species colonises the area which changes the environment making it less suitable for the existing species (so the last one is outcompeted) and more suitable for more adapted species. Eventually these changes result in a less hostile environment which is easier for species to survive in.

1. The first stage is colonisation by pioneer species. These have special adaptations to live in the harsh conditions:
• asexual reproduction to allow rapid multiplication from a single organism
• vast production of wind dispersed seeds/spores to reach isolated lands
• rapid germination (no period of dormancy)
• ability to photosynthesise as other food is usually not available.
• ability to fix nitrogen from the atmosphere (as, if there is soil, it will not have much nutrients)
• tolerance to extreme conditions
2. As the pioneer species die and decompose they release sufficient nutrients to support a community of small plants, for example. As these die and new species come the environment keeps changing until the climax community contains species such as deciduous oak woodlands. This state is stable and usually lasts a long period of time.

The standard pattern for succession is as follows:

1. non-living (abiotic) environment becomes less hostile
2. a greater number and variety of habitats and niches
3. increased biodiversity
4. more complex foo webs
5. increased biomass

Secondary succession

This occurs when land that has been cleared returns to it's climax community - it usually occurs more rapidly than the above as soil already exists etc. Because the land has been altered, e.g b a forest fire, the climax community will be different.

Conservation

This is the management of the Earth's natural resources by humans. It involves managing succession in a way that prevents a change to the next stage, conserving the current stage of succession. E.g in moorland at higher land in the UK the grazing by sheep has prevented the land from reaching it's climax community. If the factor that is preventing further succession is removed then the ecosystem develops naturally into its climax community (secondary succession).