The demographics of a wild animal or plant population

Population ecology and population size

When looking at the demographics of a population, there are many parameters you can look at. Today we are taking a look at the population size itself. How is this influenced and what strategies there are between different species. We start with some definitions. A population is the number of organisms of the same species that live in a particular geographic area at the same time and can breed and produce fertile offspring.

The population size changes are defined by birth, death, immigration and emigration. Where (birth-dead) + (immigration-emigration) = the change in population size. Either a negative, neutral or positive increase. In population ecology, the term immigration and emigration are a bit different than we use when talking about human actions. In ecology, emigration is the leaving of a place of residence/habitat with the intention of living in another place. The act of leaving their habitat is called emigration, when arriving in their new habitat it is called immigration.

Birth and death

But we can spice this up a bit, even though it is a simple topic the reality is slightly less straight forward. Birth and death can be influenced by a realized and potential result. Looking at the birth of new individuals. Each species has potential fertility, the maximum number of offspring this animal can have, but this number is rarely met. The animal might die younger, there might not be enough resources to sustain the pregnancy or she was unable to find a mate. This makes a realized fertility more realistic, but this means that each individual processes less offspring than they could. A decrease of new individuals born is a potential decrease in population size

The same is for death, there is potential longevity. We, humans, have the potential to be 100+ before we pass away, but how many people make this? The realized longevity is a lot lower, we consider 80/85 as an ‘old age for humans. This realized longevity results in a great number of deaths within a population than you would expect with potential longevity. Another potential decrease in population size.

Immigration and emigration

Immigration and emigration are influenced by a few more different things, both natural and due to human activity. The movement of animals from one place to another is called dispersal. In many animal species, it is the male individuals that move to a new territory, but there are also species where females move. The movement is also influenced by the range of the species and its core distribution range. The larger its own range already is, the less reason there is to move to other areas. As well as the population size itself plays a role, although this will have different effects for different species.

For some species, large population size will lead to more dispersal, while for another species a large population size will lead to less dispersal. As each species have their own specific dispersal traits. The last one that influences the immigration and emigration of species is due to human activity. We have destroyed great areas of forest and other habitats. Creating a barrier for animals, who are no longer able to emigrate to other areas.

Demographic strategies

Different species have different techniques to ensure their survival, we can measure this using ‘life tables’. In these tables we look at the observed number of species alive, the proportion surviving, the number of individuals that died and the rate of mortality. Let’s take a look at the three different types of species: birds, humans and trees, as these three have very different strategies.

Birds typically have a short life span but also a high mortality rate. Very few animals will reach their potential longevity and most birds will die in the first year of their life. Within each year, roughly the same proportion of animals that were alive will die. The opposite is for humans. There is relatively low mortality under humans until they reach about 80 years, after which the mortality increases sharply. Trees show an opposite curve. The mortality is very high in the first period, but when reaching a certain age the mortality rate decreases.

Population growth

When the birth and immigration of new individuals outweigh the deaths and emigrations, there will be population growth. Under normal conditions, mortality will wipe out these extra individuals, but when the conditions are favourable these extra individuals can survive, the population growth is evident.

But not all population growth looks the same or is stable. There are 4 different types of population growth, (i) stable, (ii) irruptive, (iii) irregular and (iv) cyclic. The stable population will have very little difference over time. Where irruptive shows a very sudden and short peak in population size, which will go back to normal within a short time. An irregular population growth shows many fluctuations, both an increase as a decrease in size. The last one is cyclic population growth, where there is a repetitive pattern of increase and decrease of size. This one can be linked to, for example, the lemming. They have a very high reproduction in the summer, but few individuals survive the winter.

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