Darwin focused some of this work in regards to the population size of a species, and what factors may affect them. He definedthese factors as environmental resistance, which can be split into two areas;
Density Dependant Factors
These types of factors are a direct result of a species' population size. Usually these factors arise because the population of the species have to compete for the same resources in a limited geographical niche. They can be as follows
Shortage of Food
Members of a species will require a minimum amount of food in order to survive. These members may have to compete with fellow members of the same species (intraspecific competition), or may have to compete with other species (interspecific competition). As food will become limited when the population reaches a size, the death rate will increase until the food supply is sustainable for the population sizes continued survival.
An increase in population size will result in more prey for the species' predators, therefore more chance of that species' members of being killed. This is also the case if the predators' species population increases, as their will be a higher occurrence of both species coming across one another
Disease - A disease can spread rapidly across the densely populated area, especially across the same members of a species (Foot and Mouth for example). Disease it at its' most deadly when a species population is densely dispersed, and the distance between each of the diseases host means that it is easier for the disease to spread
Particularly important for organisms living in a small-enclosed ecosystem, a large population in a species means that the waste produced by them begins to come into play, as the waste increases the risk of disease, and reduces hygiene. This is still the case in some human societies, such as villages in Africa, which are over-populated, as sewage systems flow right through the living area, and is one of the main causes of death in these areas.
Density Independent Factors
These factors occur regardless of a population size in a species. Usually these are environmental disasters, such as a forest fire, tidal wave etc that destroy the natural ecosystem that a species survives in. The result is that ecosystem can no longer sustain the species' population size and they begin to die out until the population size is at a more sustainable level. Human intervention also has increasingly played a part in determining the population of species, either via domestication or consequence from our actions. Pollution is such a factor that is not determined by a species' population, but by its external environment.
However, in Darwin's work, he identified that species tend to produce more offspring than the external environment can support, so effectively, homeostatic control comes into place as the maximum population of a species is the 'norm', taking into account that one of a species primary reasons for being alive is to reproduce.
However, with this in hand, some species are unable to increase their population levels through breeding. This is most usually because of climatic change, or environmental damage brought about by the Industrial Age and human intervention since then. For example, some rare birds do produce offspring, but since they are rare, the material value of their eggs is high, so opportunistic humans steal them in the attempt to profit. Species living in the tropical rain forests may also face danger soon, because of the alarming rate that their ecosystem is being chopped down for the material value that wood has to humans.
Such actions endanger species towards extinction, where the population size of a species becomes critically low. It is essential to keep the balance of an ecosystem, and the extinction of a species can unbalance the food chain in an area, and directly put other species in the ecosystem under threat, and possibly become the beginning of a chain reaction. As we begin the 21st century, humans have realised the importance of maintaining and preserving the world we live in, and to ensure that we are not unintentionally destroying natures' species, scientists use indicator species which indicate the state of an ecosystem.
Humans rely on many species for our own survival, most notably as a food source. Although we wish to yield an economy of scale when we are selling these foodstuffs', part of the population must be left to reproduce in order to preserve the long term survival of the species, and stay as a in-exhaustible source of food. Species can not be exploited too much, as excessively hunting them can damage the genetic diversity of them, which poses a greater risk to their long term survival.