One of the important features of any population is its size. Population size has a direct bearing on the ability of a given population to survive. Very small populations are the most likely to become extinct. Random events or natural disturbances may endanger A few individuals more than they endanger many, and inbreeding can also be a negative factor in population survival if few potential mates are available. Not only does inbreeding often lead directly to a lowering of vigor by direct genetic effects, but the reduced levels of variability that result from inbreeding are likely to detract from the population's ability to adjust to changing conditions. If an entire species consists of only one or a few small populations, that species is likely to become extinct, especially if it occurs in areas that have been or are being changed radically.
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In addition to population size, population density is also extremely important.
If the individuals that make up the population are widely spaced, they may
rarely, if ever, encounter one another, and the reproductive capabilities-and
therefore the future-of the population may be very limited, even if the absolute
numbers of individuals over a wide area are relatively high. A related measure
is dispersion, the way in which the individuals of a population are arranged.
They may be randomly spaced, evenly spaced, or clumped. Each of these patterns
reflects the interactions between a given population and its environment,
including the other species that are present. Individuals may be evenly spaced
or clumped regardless of how abundant they are in a given habitat, and the
patterns may not be at all obvious. For example, the creosote bush (Lama divaricata)
is often dominant over wide areas in the deserts of Mexico and the southwestern
United States. Within these populations, the individuals are evenly spaced,
probably because the shrubs secrete chemicals that retard the establishment
of other individuals near established ones; however, they still form more
or less continuous populations. On the other hand, in tropical rain forests,
any given plant species may be represented by only a few individuals per hectare:
when measurements are taken that reveal the overall pattern, however, these
scattered individuals may be shown to be evenly spaced, clumped, or randomly
distributed.
Clumped distributions are frequent in nature because individual kinds of animals,
plants, and microorganisms tend to group in their particular habitats. These
habitats may be defined, for example, by soil types, moisture, and certain
kinds of host trees, but in any case they generally are not themselves randomly
distributed. As a result, the organisms associated with these environments
exhibit clumped distributions.
Although the individuals in a population may be evenly spaced or randomly
dispersed, clumped distribution patterns are the most frequent in nature.
