What determines the abundance of organisms

Abundance can simply be defined as the relative representation of a species in a particular ecosystem; or rather the average density of a particular species across all occupied areas, and thus average abundance excludes unoccupied areas.

There are fundamental questions one must ask when studying abundance of organisms; firstly, why are some species common and others are rare, why are species at low population densities in some places and higher in others, and finally what factors cause these fluctuations on abundance of species?These questions face us with a problems as we require huge amounts of data to answer these for one species in one location alone; we would need to know levels of all available resources across the range, physiochemical conditions of the environment, details of the organisms life cycle, impact of competition, parasites and predation, and a general understanding of how all these factors impact upon birth and death rates and rates of movement, and thus abundance. This highlights the difficulty of approximating the determinacy of abundance, as ultimately it is an incredibly vast interaction of factors which determines the abundance of organisms. In this essay I shall examine the differing theories of abundance and closely focus on density-dependent effects on abundance of organisms.In order to examine what determines the abundance of organisms it is necessary to measure abundance, which is in many respects more difficult than one may assume. It would be a vast simplification to merely count the size of the population in a particular area as this does not take into account the age, sex, size and dominance of the population. Furthermore, sampling is often inadequate and feasibly difficult in that it is often hard to track individuals of a species for an entire lifecycle, such as young rabbits in warrens, or indeed seeds in soil.

These shortfalls result in limited and often misleading data.Instead, many studies attempt to shed light onto the determinants of abundance by examining and detecting density-dependant processes of which there are many kinds. These processes can be differentiated according to their cause, effect, severity, response and mechanism. Negative density-dependant effects occur if a vital rate decreases while density increases, whereas positive density-dependant effects occur if both vital and density increase. Examples of mechanisms of density dependence are competition and predation. However, it is important to remember that organisms do not directly detect and respond to the density of their population, but instead they respond to the shortage of resources or aggression; in this sense density is rarely the direct factor changing birth or death rates.

In identifying the determinants of abundance it is important to directly observe individuals of a species and then incorporate this data into mathematical population models; if the model behaves similarly to the real population in question then there is strong evidence for the hypothesis. An example of one of the very few long term studies into abundance is the observation of the abundance of a species of swift, Micropus apus in Selborne in Southern England; in 1778 Gilbert wrote about a swift population in his village in which there was a constant of 8 swift pairs every year. 200 years later Lawton and May returned to observe the swift population in the same village, only to find that there had been dramatic changes to the village, but there was still a relatively constant number of 12 pairs of swift in the village. Similarly studies have shown that herons in the British Isles have a remarkably stable population over long periods of time. In contrast, mice have unstable population levels with long periods of low abundance with intermittent periods of sporadic and dramatic irruptions.Competition, both inter-specific and intra-specific, primarily arises from limited availability of resources or space.

Thus when density is high, the population must respond with a decrease in one or more vital rate; predation usually affects survivorship, while competition for resources may decrease growth rates, survivorships and birth rates (fertility or fecundity). In this sense abundance is density-dependant. However, when population levels are low, individuals are not limited by resources or space; in this sense population growth is density-independent.When population size increases, and thus resources become limited, and mechanisms such as competition and predation increase, negative density dependence is revealed. Similarly predation can have a negative density-dependent effect; when predators are at low density, then prey populations remain relatively stable, however when predators are at high density, then the prey can reach a minimum level, known as giving-up density, or indeed become extinct.

Essentially, both competition and predation do not affect population growth at low population density, however at high density, growth is reduced. This results in the regulation of population about a maximum density, which is often known as the carrying capacity (K) of the environment.However, positive density-dependent effects do not regulate population density or abundance. An example of this is the Allee effect (Allee, 1931) which states that a low density population has an extremely low growth rate due to the reduced possibility of finding mates, or in the case of plants, a low possibility of being pollinated. This is an unstable state of affairs as when the populations grow (admittedly very slowly) in density, density dependence will undoubtedly begin once a resource becomes limited. Similar examples of positive density dependence can be seen when animals congregate together as a form of 'safety in numbers' as seen in bird colonies and schools of fish.

An important aspect of abundance is that individuals of a species in different stages of their life-cycle within a population usually differ in size and in some cases even in shape, thus their ability to gather resources and in response to varying environmental conditions. This results in stage-specific differences in vital rates such as survivorship, growth and fertility, and thus density-dependant processes will have differential impacts. For example, Arcese and Smith found that at the beginning of a Canadian song sparrows life cycle the chicks huddle in the nest to keep warm, however, later in their life cycle chicks suppress weaker siblings and sometimes exile them from the nest. They suggest that it was fecundity, rather than survivorship, of the females within the population that are affected by nest density due to decreased food supply among breeding birds. This varying effect of density dependant processed on different individuals depending upon their stage in the life cycle highlights that the phenomena of density dependence not only changes rates of population growth, but it also affects the stage distributions and fluctuations within the population.