Population growth

A population size can be thought of as a system. It has inputs which increase the size of the population and outputs which decrease the size of the populations:

Populations increase in size when the rate of Births and Immigration are greater than the rates of Death and Emigration.

They decrease when Death and Emigration are greater than Birth and Immigration.

This means that over time the numbers within a population change. Returning to the example of bacterial cells from the previous page bacterial cells, place them in a suitable supply of nutrients and then under a microscope cont the number of cells every hour we would find that there would be many more bacteria at the end of a 24 hour period than at the start.

It is possible to model this growth as a mathematical equation:

Population growth = change in population number / change in time

or dN/dt

Where dN = change in numbers and dt = change in time

This equation can also be written using the symbol delta to represent time:

Thinking about the bacteria above, if I started out with one bacteria (bacteria reproduce asexually so a population can start with one)and if the bacteria reproduced one after 5 minutes and then died every, after 30 minutes I would have 64 bacteria – the population size would double every 5 minutes. This means that the each time the population changes it increases the amount of population change next time. More simply the rate of population growth increases as the population grows or exponential growth. This can be expressed as a rate equation – another model.

“There is no exception to the rule that every organic being increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally not be standing room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds – and there is no plant so unproductive as this – and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned to be the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase: it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pairs of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair.”

All resources in an ecosystem are limited. there is only so much food, only so much space, only so many mates even. The results of these ecological limits or ECOLOGICAL RESISTANCE is that no population can keep growing forever. There is a ceiling limit that each ecosystem sets. This limit set by the resources of the ecosystem (the available niche resource) is the CARRYING CAPACITY, confusingly given the symbol K in ecology.

In natural habitats the resources available are not infinite, so as the population grows it starts to experience environmental resistance. Lack of resources become limiting factors that have an effect on continued population growth. The result is that population growth slows down. 

Eventually the population reaches a maximum number that the limited resources in the habitat can support (carrying capacity). When carrying capacity is reached the number of gains in population size (birth and immigration) equals the number of losses (death and emigration) and the population size stabilises. 

These limits are set by the environment and include both abiotic and biotic factors. Competition for limiting recourse include:

Until the 1940’s the British population of Peregrine falcons had remained fairly constant – about 820 breeding pairs 

This was limited by available nest sites (same places each year). The average brood size was about 2.5 fledged young each summer 

Come the next breeding season the population was back to about 820 pairs. 

Competition for suitable nesting sites regulated the population of Peregrine falcons 

Limestone ecosystems make interesting population study sites. Where they are exposed to rain a broken habitat with gaps between limestone rocks known as limestone pavement can develop. It is a bare dry hostile environment except in the gaps between rocks where a small amount of soil develops and a small populations of alkaline tolerant species develop. Each population is limited by both available space and the ability to grow in an alkaline environment that experiences rapid drying out in warm seasons.

Removal of Rainforest in Malaysia for both Urban growth and Palm Oil plantation has pushed many Monkey species into direct contact with humans.

Whilst some adapt and feed in parks and from road side rubbish, many more die as individual troops are pushed into smaller areas where they directly compete with each other for food.