Population Ecology - Introduction

The central question in population ecology is: what determines structure and functioning of populations and their variation in time and space? The aim of this course is to acquire the appropriate tools to deal with this question in any real-life situation through scientific research.

The term population, like all ecological entities, requires definition. This has two purposes: a) to differentiate populations from other entities that are not populations, and b) to differentiate between different populations. The most widely accepted definition is that a population is a collection of individuals of the same species occurring together in a particular space and time. Since populations consist of individuals, which occur in communities together with other organisms, populations should be examined at a different organizational level than both individual organisms and communities. This is a matter of "focus" of observation: only overall quantities of the collection of individuals and the interactions among the individuals are considered. From the point of view of the population, whatever occurs within individuals is "detail" and interactions with other organisms are part of the "background". Both are essential for understanding populations, but describe phenomena that can be examined by using a different focus.

The second purpose of the definition is more problematical. This is due to a high degree of fuzziness of all components of the definition: individual, species, space and time. This is a problem inherent in biological systems and reflects their diverse and dynamic character. The solution is to know the target organisms well and to use ad hoc definitions that fit the specific question and the unique properties of the organisms.

Problems often arise when defining where in space a population is. For some organisms that are fixed to a certain place or move only short distances, we can know exactly where the individuals are, but not where the boundaries of the population are. In these cases decisions are often taken arbitrarily based on sampling considerations, or based on landscape boundaries (a recognizable patch, a woodlot, etc.). On the other hand, a population of animals migrating in flocks or herds, for instance, has clearly recognizable boundaries at any moment, but its place changes constantly.

The most variable term in the definition is the individual, even though it is the most tangible entity in ecology. In unitary organisms (most animals) there is usually no problem recognizing individuals, because they are both genetically and physiologically separate. Many species of plants and invertebrates, however, have vegetative propagation to produce new individuals, besides sexual reproduction. In such modular organisms new functional modules are formed (ramets) from a single genetically unique individual (the genet). If the ramets are physiologically independent, they can be counted as individuals, though not genetically distinct. This is just like internodes and branches of a plant, except that the connections between them are gone.

The term species may also pose problems, if taxonomic definitions of species vary (splitting one species or lumping two species), or if the biological species concept, requiring the possibility of mating between all individuals, is not applicable. This is the case when there is no sexual reproduction at all, only fragmentation (a form of vegetative propagation common in flatworms, for instance), or cleistogamy (sexual reproduction by self-pollination without crossing between individuals in many plants - wild barley, for instance).

The question "What determines structure and functioning of a population?" implies that we are interested in understanding which factors influence a population, and how it responds to these factors. In order to deal with these questions, we have to specify what properties populations have that can be described, and what kind of factors can influence them. There are many differences between populations - first of all between populations of different species, and even two populations of the same species can be very different. Nevertheless, there are properties that all populations possess, though in different shapes and proportions. These properties are:

a) dimensions - population density, abundance or size, based on the number of individuals (per unit area, or in the entire population);

b) composition - the proportions of individuals with different states (gender, age, developmental stage, or size);

c) dynamics - changes in time of the dimensions and composition of a population; this includes the rate of change of the dimensions (population growth rate), and changes in the distributions of individuals' states.

The properties of populations change all the time, and the rate of change can vary from very low to high. Even in populations that do not change in density, the individuals change and are replaced. They are born, grow and reproduce (or not), and die. These changes in the state of individuals can be summarized as rates of change, such as birth rate and mortality rate, or the proportion of individuals growing to a particular size in an interval of time, etc. These demographic processes are direct causes for the changes (or lack of it) in the population's dimensions, composition and dynamics.

Demographic processes are abstractions, derived from actual changes in the state of the individuals. They are calculated as the proportion of individuals in a particular stage who manage to get to the next stage. Each individual experiences a different rate of change, in accordance with its general condition, its past and its genotype. Vital rates are demographic processes averaged over groups of individuals (according to age, size, or stage). Vital rates include birth rate, survivorship, growth, and fertility or fecundity. The states of individuals and the rates that form the connections between them, determine the life-cycle of the population.

Two different populations of the same species differ first of all in their vital rates, since they express how individuals respond to environmental variation. For example, comparing a population of a particular animal species in a rich and a poor habitat regarding food availability, we will usually see that population growth rate in the rich place is higher than in the poorer place. This is because individuals survive better, grow faster, attain reproductive maturity faster, and produce more offspring when there is more food.

Environmental factors that affect demographic processes are very diverse and can be divided in many ways. Categories are not exclusive: some factors may appear in more than one category. The following division is based on the properties of the factors and their effects on the organisms.

a) Environmental conditions - states of the environment that affect organisms, but are not consumed by them: pH, temperature, air humidity (including their variation).

b) Resources - physical commodities (material or energy in any form, including other organisms), which are supplied by the environment and consumed by the organism; resources are at least as diverse as the organisms that require them.

c) Structures - availability of sites where individuals can perform their essential functions (shelter, nesting, foraging, mating and recruitment sites); sites can be viewed as a structural sort of resource.

d) Interactions with other organisms - all relationships with other organisms except consuming another as resource; interactions include (the passive side of) predation, herbivory and parasitism, and competition for resources and structures; besides negative interactions, they can be positive as well (facilitation, mutualism).

e) Internal regulation - effects of the population itself (or some of its members) on the way environmental factors affect the organisms. (Strictly speaking this is not an environmental factor, although for each member of a population the other members are part of its environment.) Processes affected this way are called density-dependent. Density-dependence can be positive or negative, depending on whether density of the population reduces or enlarges the effect of the environmental factor. Intraspecific competition is a negatively density-dependent process, and operates on resource acquisition.

The logical relations between environmental factors and the demography and properties of a population are shown in Fig.1. A factor influences one or more demographic processes within the population. This implies that a change in the level of the factor causes a response of a vital rate (survival, growth, reproduction) so that it becomes higher or lower. This is another way of saying that the factor operates on individuals, and changes their ability to survive, grow and reproduce. Finally, the demographic processes shape the entire population's properties such as composition and density, and its dynamics.

Fig. 1. Relations between environmental factors, the demography and properties of a population, and internal regulation.

A simple example is a population of animals that consume parts of plants in their habitat. When there is a lot of plant growth, the animal population grows steadily. Another population, in a habitat with less plant growth, grows slower. This is, for instance, because the individual animals grow slower on the average (or more die or less become reproductive). This example does not include internal regulation, however.

The situation becomes a little more complicated if we include the internal regulation loop in Fig.1. A property of the population (its density, or the density of a particular group of individuals) influences the way an environmental factor works. In the herbivore population example, an increase in density (of adults, for instance) causes a change in another demographic process (growth of the smaller animals, for instance). This is the result of competition, by consumption of food by the adults, so that there is less available for the others. The slower growth of the younger animals, in turn, decreases a property of the population, its population growth rate. If this regulatory process of negative density dependence continues, there is not sufficien resource to support more individuals, so that the population stops growing and stays near a particular density.

The analysis of the example contains a explanation of a phenomenon (dependence of the growth of the animal population on plant growth) including an environmental factor (food supply) and a population property (population growth rate). The explanation uses a number of mechanisms involving the individuals of the population (growth of small animals as a function of food supply, and competition by adults decreasing the supply). These are the details mentioned before.

The mechanisms do not happen at the same level of organization as the phenomenon of population growth: the latter is observed by focusing on the properties of the population, while the mechanisms only come in view when we focus on individuals and follow them over a period of time. The time is the same as the one over which we measured population growth.

It can be argued that this explanation is not complete at all - there are still a lot of questions. For instance: how come the animals grow less if there is less plant growth? Do they eat less, or different things, with different digestibility? And, how do adults bother the smaller ones? Are they faster to get at the food, or do they push the smaller ones away? Answering these questions would add a lot of important information, and deepens the understanding of the population phenomenon. Answering them requires us to change our focus further, and study what individuals do (feeding and interfering). We can even ask questions about their digestion, down to the molecular level. The point is, that the more detailed the questions become, the further we get from the original focus, and the less we add to the explanation. The reason is, that we also are looking at a different time scale, because feeding behavior and nutritional physiology have typically shorter time spans than growth of individuals and their population.

We can also extend our study to higher levels of organization, which is relevant for understanding the context of the population phenomenon (the background). We can ask: How does the growth of the food plant vary in the landscape? However, further looking for details about the causes of differences in food plant growth may be irrelevant. Another set of questions that may help understanding the context of the population phenomenon (the dependence of population growth on food plant growth) is about its consequences. Again, we will have to change our focus and look at the animal community, to see if other animals (predators, competitors) are affected.

Begon, M., J. L. Harper and C. R. Townsend, 1990. Ecology - Individuals, Populations and Communities, Blackwell Scientific Publ., London, UK, 2nd edition (or newer).

Caswell, H. 1989. Matrix population models. Sinauer, Sunderland, USA.

Silvertown, J. 1982. Introduction to Plant Population Ecology., Longman, London, UK.

Stearns, S. C., 1992. The Evolution of Life Histories. Oxford Univ. Press, Oxford, UK.

* Allan, T. F. H. and Hoekstra, T. W. 1985. The confusion between scale-defined levels and conventional levels of organization in ecology. Journal of Vegetation Science 1(1): 5-12.