Populations and Sustainability

Population sizes of many species are kept in check through factors such as water availability or the abundance of predators. Competition between species (interspecific) and between members of the same species (intraspecific) also regulates population size. Ecologists can estimate population size using methods like quadrats and transects.

Factors which affect population size

The abundance (or population size) is defined as the number of individuals of one species in a particular area. It depends on both abiotic (non-living) and biotic (living) factors.

Abiotic factors, such as light intensity and water availability, affect population size because when these conditions are ideal then organisms can grow and reproduce successfully. When these conditions fluctuate, for example if the temperature of a habitat is too cold, then the organisms in that habitat cannot grow as efficiently. More energy will be spent maintaining body temperature which means less energy will be available for growth and reproduction.

Biotic factors also affect species abundance. These include interspecific competition, intraspecific competition and predation.

Interspecific competition describes competition for the same resources between different species. Interspecific competition means that resources have to be shared between the different species so there will be less available to both. This means both species will have less energy for growth and reproduction, so population numbers of both species will decrease. For example, leopards and lions show interspecific competition since they both feed on the same prey.

Intraspecific competition describes competition for the same resources between the same species. When resources are abundant, the population size increases. This increases intraspecific competition since resources will need to be shared between more organisms, which causes population size to decrease. This pattern causes population numbers to fluctuate around the carrying capacity (the maximum number of organisms that an ecosystem can support).

Predation affects the abundance of the prey species and vice versa. Whenever the population size of prey increases, this causes the population size of the predator species to increase because more food is available. The increase in the predator population causes a decrease in the prey population because there are more predators to feed on the prey. This causes a drop in the prey population size, which reduces the food availability for the predators, which in turn leads to a reduction in the predator population.

Factors which affect species distribution

Abiotic and biotic factors also influence the distribution of different species within a habitat. Species will only be able to live in parts of the habitat where abiotic conditions, such as food availability or soil pH, are ideal. Biotic factors, such as interspecific competition, means that only the better-suited species will live in an area because they will out-compete other, less-adapted species.

Investigating population size

Random and systematic sampling

Measuring the population size of a species by trying to count all of the individuals in the habitat would not only be pretty tricky but also considerably time-consuming. To save time, ecologists take a sample of the habitat and use that to estimate the size of the population in the whole habitat.

For species which don’t move, such as plants, quadrats are typically used. It is important that quadrats are placed randomly within the habitat to avoid bias. This can be done by using a tape measure to plot out the habitat as a grid and using a random number generator to plot coordinates. A large number of quadrats will be randomly placed throughout the habitat to ensure the data collected is reliable.

Sometimes it may be better to place the quadrats non-randomly - this is called systematic sampling. For example quadrats can be placed one next to each other along a transect (see below). Systematic sampling is needed wherever there is an environmental gradient i.e. the abiotic factors change gradually from one end of the sample to the other.

Using transects

To see how the distribution of different species changes along a habitat, we use something called a transect, which is essentially a line placed from one part of the habitat to another. There are three different ways of using transects:

1. Line transect - a tape measure is placed along the line and any species which touch the tape measure will be recorded.

2. Belt transect - quadrats are placed along the transect one after another, so that each quadrat is touching

3. Interrupted transect - quadrats are placed at regular intervals along the transect

Once data has been gathered about the abundance and distribution of particular organisms, these can be displayed on a kite diagram. On kite diagrams, the abundance of each organism is shown by the thickness of the kite shape. The x-axis shows the distance along the transect, so indicates how that species is distributed through the habitat.

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