Population ecology is a science that deals with measuring
changes in population size and composition and identifying the factors
that cause these changes. A population is define as a group of one
species of organisms occupying the same general area, using the same resources,
and acted upon by the same environmental factors. Populations cannot grow
indefinitely, many populations will become stable over a period of time
while others will show sharp increases followed by similar decreases. Population
characteristics that afford study are its density and the spacing
of its individuals. Population density is the number of individuals per
unit area or volume. Population dispersion is the pattern of spacing among
the inin the parameters of the geographical boundaries of the population.
Measuring Density:
Due to the impracticality of capturing or counting each individual
in a given area, ecologists use a variety of methods to determine the density
of various populations of organisms. Some of the sampling techniques used
are as follows: Counting the number of nests or burrows in a given area,
examining the number of tracks, examination of solid waste products left
behind, and an actual capture method called mark - recapture. In the mark-recapture
method, animals are trapped within their boundaries, marked with a long
lasting sign, and released. At a later time some of the animals will be
recaptured along with other that are not marked. The data is then placed
into the following formula to calculate the population's density
(number marked) x ( total catch the second time)
N=______________________________________________
number of marked recaptures
This method assumes individuals have the same probability
of being captured as unmarked individuals. This assumption is not always
valid.
Patterns of Dispersion:
Local densities, within a population's range, may vary substantially
due to differences in the limiting factors present. There are three general
patterns of dispersion in relationship to other individuals: clumped,
uniform,
and random.
Clumped. is a pattern when individuals are aggregated
in patches. This style is caused by a heterogeneous environment with resources
concentrated in patches. Mating or social behavior of the individuals may
also contribute to this type of dispersion.
Uniform. is a pattern of equally spaced individuals.
Competition between individuals may set up zones or territories for feeding,
nesting or breeding.
Random. is a spacing pattern based on total unpredictability.
This form of dispersal is highly uncommon in nature. If it does it usually
results from the absence of a strong competition among individuals.
While the above patterns apply to individuals within a population,
keep in mind that populations within a species show dispersal patterns.
Biogeography
is the study of factors that influence the distribution of a species over
its range.
Demography:
Demography is the study of the vital statistics affecting
population size. This branch of science deals with the affects that immigration
and emigration have on a given population. Other than new organisms leaving
or entering the population birth and death rates are also studied.
Age Structure and Sex Ratio. Many populations have
overlapping generations where individuals of more than one generation coexist.
This situation produces an age structure in most populations. Every age
group has a characteristic birth and death rate. A standard rule of thumb
sets a high mortality for the lower and upper age groups and a low mortality
rate for the intermediate age group. Generation time refers to the
average time span between the birth of an individual and the birth of their
offspring. Body size has a major effect on generation time. Mice,which
are small,can produce a litter of individuals ever 21 days. Their offspring
usually are ready to reproduce in 3 to 4 weeks after birth. On the other
hand, the Elephant takes 2 years to produce an offspring and several more
years before the offspring can successfully reproduce. The sex ratio
plays an important role in the development of the population. Female number
plays a dominant role in a population since they are the ones to produce
the offspring. Males may mate with several females, diminishing them to
a subordinate reproductive role.
Life Tables and Survivorship Curves. Life tables describe
how birth rates and death rates vary with age over time corresponding to
maximum life span. A variety of information about change in population
size can be obtained from life tables. In a particular species of bird
fewer than 20% of the original females survive to an age of three years,
and a three year old female has a 44% chance of surviving to age 4. Mortality
rates for females are constant through the middle years but higher in young
and older birds. The birth rate is highest for 4 year olds and lower in
very young and very old females. Survivorship curves plot the numbers in
a cohort still alive at each age. There are three types of survivorship
curves: Type I curves flat during early and middle life and drops
suddenly as death rates increase among the older individuals. Type II
curves are intermediate with mortality being more constant over the life
span. Type III curves show very high death rates for the young followed
by lower death rates after the individuals have survived to a certain critical
age.
Density-Dependent and Density-Independent Factors:
Density Dependent Factors. Increasing population size
reduces available resources and this limit ultimately limits population
growth. In restricting population growth, a density-dependent factor intensifies
as the population size increases, affecting each individual more strongly.
population growth declines because the death rate increases, birth rate
decreases or both. There is a reduction in the food supply which restricts
reproduction resulting in less offspring. The competition for space
to
establish territories is a behavioral mechanism that may restrict population
growth. Predators concentrate in areas where there is a high concentration
of organisms. As long as the natural resources are available in sufficient
quantity the population will remain constant. As the population decreases
so do the predators. The accumulation of toxic wastes may also limit
the size of a population. Intrinsic factors may play a role in limiting
a populations size. High densities may cause
stress syndromes resulting
in hormonal changes that may delay the onset of reproduction. Immune disorders
are also reported to be related to stress in high densely populated areas.
Density-Independent Factors. Weather, climate, and
natural disasters such as freezes, seasonal changes, hurricanes, and fires
are examples. These factors are unrelated to population size and affect
everyone in the population regardless of population size.