Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 111

ing one surviving offspring and staying alive itself, or by dying but producing two surviving
offspring).
Finally, note that birth, death, migration rates and are likely to be infl uenced by a variety of

environmental factors: physicochemical conditions (such as temperature, humidity, pH), resource
availability (light or nutrients for plants, food or nest sites for animals) and the presence of enemies
(predators, parasites or species that compete for the same resources).
Why are some species common and others rare? Why does a species occur at low density in
one place and at high density in another? Why does one species fl uctuate dramatically in density,
while another remains constant? These are the central questions of population dynamics. Their
answers depend on knowing how the environmental factors affect B , D , I and E and the conse-
quences of these effects for the mean and variance of population size.

λ

Population regulation and determination - density-dependent and density-independent
factors
Population regulation is the tendency of a population to decrease in size when it is above a par ticular
level, but to increase in size when below that level. This occurs as a result of density-dependent
processes that act on rates of birth and/or death and/or migration. A population may be regulated
at a certain density because the resources are suffi cient to support only a certain number of
animals: thus, the per capita death rate due to starvation may be higher when density is high than
when low. Similarly, a limit on potential nest sites may cause the per capita birth rate to be lower
when density is high. A density-dependent regulatory effect might equally apply because predators
or parasites cause a higher death rate in the affected population when it is at high density.
It is important to understand that while the processes discussed above may act in a density-
dependent way, they do not necessarily do so. In addition, physicochemical factors affect popula-
tions in a density-independent manner. For example, a blizzard can be expected to kill the same
proportion of baby reindeer whether the population is at high or low density.
In contrast to regulation , the actual density of individuals in a population is determined by the
combined effects of all the processes that affect it, whether they are dependent or independent of
density. Figure 5.1 illustrates this in a simple way for three hypothetical populations of a plant
species. The birth rate is density-dependent (because the number of seeds per individual is reduced
at high density as a result of competition for limited soil nutrients) but the death rate is density-
independent and depends on temperature, which differs in three locations. There are three equi-
librium populations ( N* 1 , N* 2 , N* 3 ), corresponding to the three death rates that, themselves,
correspond to the three sets of physical conditions. Density dependence is operating in all three
populations, but the density around which they are regulated differs. Thus, we must turn to the
determination of density if we want to understand why a particular population has a particular
density in a particular place, and not some other density.

Models of population dynamics
The development of population dynamics theory has primarily involved the formulation of general
equations that incorporate, as realistically as possible, the various processes that infl uence popula-
tion size. We need not go into detail (see Begon et al., 2006, for a comprehensive treatment), but
it will be useful to give a fl avor of the approach and to introduce some key parameters that will
crop up later in the chapter.
The speed (d N/ d t ) at which a population increases in size ( N ) as time ( t ) progresses can be
written as:

d N/ d t rN

=

where r is the population's intrinsic rate of natural increase . As long as r is greater than zero the
population will increase exponentially. (Note in passing that r is related to , described earlier, by

λ

r .)

However, real populations do not continue to increase indefi nitely in this way. We can incorporate
density dependence (resulting from competition for resources among population members) by
introducing the concept of the carrying capacity ( K ):

ln

= λ