Just as individuals live within a population, populations
exist within communities. A community refers to all the populations
that interact with each other in a given environment and geographical
area. The specific role and way of life of each population is called
a niche. When populations have overlapping niches,
a variety of types of interaction may occur, including competition,
symbiosis, predation, and other food relationships. Communities are
shaped over time by ecological succession.
Each population in a community plays a unique role in
the community. This role, the population’s niche, ranges
from where the members of a population live, what they eat, when they
sleep, how they reproduce, and every other characteristic that defines
a population’s lifestyle within a community. You can think of the
niche as a sort of node in the network of interactions that make
up a community. Wherever the niches of two populations overlap, interaction
When two populations share some aspect of a niche, such
as a nesting site or a food source, competition results. There are
two basic outcomes of competition between populations:
- One population will be a more effective
competitor. The population that is more effective will eventually
“win” and drive the second, less effective population from their
niche. With the niche freed, the winning population will grow to
the carrying capacity of the niche.
- The two populations will evolve into less competitive
niches. If two populations compete on even terms, it may
be beneficial for both populations to modify their niches so that
the populations’ niches overlap less or not at all. In these cases,
natural selection will favor individuals in both populations that
have non-overlapping niches, and over time the two populations will
evolve into different niches.
Symbiosis refers to an intimate association between organisms
called symbionts. The symbiotic relationship may or
may not be beneficial to the organisms involved. There are three kinds
of symbiosis: parasitism, commensalism,
and mutualism. Each type of symbiosis describes a different
relationship of benefits between the two symbionts. A tapeworm is
a parasite that lacks a digestive tract and therefore infects a
host and steals predigested food; parasites benefit while their
hosts suffer. In commensalism, one species benefits and the other
remains unaffected. Barnacles and whales live in a commensal relationship.
Finally, in mutualism, both species benefit from the presence of
and interactions with each other. Lichens, which consist of a fungus
and alga that provide for each other, respectively, moisture and
food through photosynthesis, are a good example of a mutualist relationship.
Predation refers to one organism eating another.
Predation does not only refer to carnivores. Just
as an eagle eating a rodent is a form of predation, so is a rodent
munching on some grass. In fact, predation doesn’t always result
in the death of the prey. An antelope that gets eaten by a lion
will die, but a tree that loses a few leaves to a hungry giraffe
will go right on living.
Carrying capacity shifts in a periodic manner based on
the cycles of predation. When the population of rabbits increases,
the population of coyotes that eat the rabbits will also increase,
as there’s more food for the coyotes. However, at some point, there
will be so many coyotes eating so many rabbits that the rabbit population
will fall in number. The coyotes’ great success in eating rabbits
has eliminated their food source, and as the rabbit population declines,
so will the coyote population. But as the coyote population dwindles, the
lack of predators allows the rabbit population to grow again, and
so the cycle continues.
Evolution Caused by Predation
The change in a population due to a shift in environment
is one of the engines of evolution. Imagine the rabbits and their
predators, the coyotes. As the coyotes increase in number, the rabbit
population ceases to grow, and many rabbits are caught and eaten.
As the coyotes increase in number, the carrying capacity of the
rabbit population shrinks. But it is important to notice that not all rabbits
are caught by the coyotes. The faster rabbits escape capture by
the coyotes far more often than the slower rabbits. Fast
rabbits survive and breed and have offspring, while slower rabbits
get eaten. The next generation of rabbits will therefore be faster
because they are descended from faster parents—this is directional
selection in action. The population of increasingly fast rabbits
means that the coyotes must be faster in order to catch the rabbits.
More fast coyotes catch rabbits and live to reproduce, creating
a next generation of faster coyotes. When two populations affect
their mutual evolution in this manner it is called coevolution.
It is arguable that predation is actually helpful to the
prey population. Since predators want to capture prey with the least
possible effort, the weakest members of the prey population are
usually targeted. In this way, the predators often remove from the
gene pool of a population those prey animals that have the weakest
and least fit alleles.