What determines the abundance of organisms?

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.

I would now like to shift attention to literature regarding the determinants of abundance. There have been two prevalent schools of thought concerning theories of abundance which have dominated ecological discussion for a large part of the twentieth century; some argue, such as A.J Nicholson, that populations have remained relatively constant in terms of size over time, and others argue that populations have experienced significant fluctuations which can be explained by external factors such as weather. The regulation of population size at a constant level will only occur when one or more density dependant processes impact on birth, death or movement rates, and thus the exact abundance of a population is determined by the compounded effects of all the processes (both density dependant and independent) affecting the population. The two theories of abundance, previously conflicting, are now seen as complementary to one another. The argument of stability suggests that while there are examples of density independent factors influencing population abundance, the most important factors in determining and regulating population size and abundance are the interactions that exist between biotic factors and those that are density dependent. Nicholson summarised suggesting that density dependence was the “influence which adjusts population densities in relation to environmental favourability”1. In contrast the other theory suggested that populations experienced continuous and repetitive series of ups and downs and that the main factor in population regulation was the shortage of time which occurred when population levels were increasing. This view was based on research into insect pests, and thus this theory is particularly relevant in explaining abundance patterns in pests. However, these theories are no longer thought of as contradictory as the theory of stability is thought to examine what regulated population abundance while the theory of fluctuation examines more closely what determines population size.  This brings me to an important point; regulation and determination are often confused, with one being viewed as more important than the other. However, we must understand that populations are subject to many factors, both density dependent and independent, which determine and regulate the population.

Ultimately abundance is determined by a multitude of factors, which have differential effect depending upon the age, sex, size and dominance of the population. These factors are both biotic and abiotic and indeed density dependent and density independent. The combination and relative impact of each factor results in a variety of abundance across species, across time and across space. Finally, there is a wider importance in determining the factors behind abundance; this knowledge ensures a responsible and appropriate policy of conservation which is essential if we want to maintain biodiversity and limit extinction.