Science of Water : Concepts and Applications

Water Ecology 169
Population density, the number of organisms per unit of area or volume, may change dramatically.
For example, if a dam were closed off in a river midway through salmon spawning season, with no
provision allowed for fi sh movement upstream (a fi sh ladder), it would drastically decrease the density
of spawning salmon upstream. Along with the swift and sometimes unpredictable consequences of
change, it can be diffi cult to draw exact boundaries between various populations. Density is the
characteristic of populations that has always been of greatest interest to man. Are there enough deer
to hunt? Are there too many pests?
The population density or level of a species depends on natality, mortality, immigration, and
emigration. Changes in population density are the result of both births and deaths. The birth rate
of a population is called natality and the death rate mortality. In aquatic populations, two factors
other than natality and mortality can affect density. For example, in a run of returning salmon to
their spawning grounds, the density could vary as more salmon migrate in or as others leave the
run for their own spawning grounds. The arrival of new salmon to a population from other places
is termed immigration (ingress). The departure of salmon from a population is called emigration
(egress). Thus, natality and immigration increase population density, whereas mortality and emigration
decrease it. The net increase in population is the difference between these two sets of factors.
The distribution of a population is the area in which that population can be found. Each organism
occupies only those areas that can provide for its requirements, resulting in an irregular distribution.
How a particular population is distributed within a given area has considerable infl uence on density.
As shown in Figure 6.12, organisms in nature may be distributed in three ways.
In a random distribution, there is an equal probability of an organism occupying any point in
space, and “each individual is independent of the others” (Smith, 1974).
In a regular or uniform distribution, in turn, organisms are spaced more evenly; they are not
distributed by chance. Animals compete with one another and effectively defend a specifi c territory,
excluding other individuals of the same species. Here, the competition between individuals can be
quite severe and antagonistic to the point where the spacing generated is quite even.
The most common distribution is the contagious or clumped distribution where organisms are
found in groups; this may refl ect the heterogeneity of the habitat.
Organisms that exhibit a contagious or clumped distribution may develop social hierarchies
to live together more effectively. Animals within the same species have evolved many symbolic
aggressive displays that carry meanings that are not only mutually understood but also prevent
injury or death within the same species.
√ Important Point: Distribution is usually mapped for species of plants and animals. For
example, bird guides contain distribution maps showing where each species of bird lives and
reproduces.
The size of animal populations is constantly changing due to natality, mortality, emigration,
and immigration. As mentioned, the population size will increase if the natality and immigration
rates are high, whereas it will decrease if the mortality and emigration rates are high. Each population
has an upper limit on size, often called the carrying capacity. Carrying capacity is the optimum
number of species’ individuals that can survive in a specifi c area over time. Stated differently,
Random Uniform Clumped
FIGURE 6.12 Basic patterns of distribution. (Adapted from Odum, E.P., Fundamentals of Ecology,
Saunders College Publishing, Philadelphia, 1971, p. 205.)