Although every organism struggles
to survive, individual survival is not enough to ensure continuation
of the species. Individuals must reproduce.
Reproduction takes extra energy, and so it would seem that
those organisms best suited to capture energy in the environment
are those most likely to leave numbers of descendants. Survival
(and reproduction) of the fittest helps to keep animals and plants
strong and adapted to their environments.
Scientists have identified two
strategies for leaving living descendants. These strategies are
called the r-strategy and the k-strategy. Some organisms,
animals and plants, use one strategy or the other: some seem
to tend towards one but are closer to a sort of mid-point. Let's
compare these strategies.
Remember that R stands
for Rapid so that you will remember which strategy is
which. R-Selected parents rapidly produce many
descendents, but do not provide care for them.
short-lived: most die before they reproduce
tend to live long lives:
low juvenile mortality rate
compete well for resources
have many offspring - tend to overproduce
have few offspring at a time
invest little in individual
care for their young
most pest species are r-selected
most endangered species are K-selected
population not regulated by density:
boom and bust population figures
population stabilizes near carrying capacity
opportunistic -- invade
maintain numbers in stable ecosystems
We see that
have many babies, but most of these youngsters never become adults.
Frogs are a good example of r-selected organisms. Frogs lay many
eggs and leave them in the water to hatch into tadpoles. Some
of the eggs get eaten, and many of the little tadpoles are eaten,
too, by dragon-fly larvae and fishes and fishing birds. When
the tadpoles become frogs, many animals are waiting on shore
to eat them: raccoons, foxes, snakes, cats, and many other small
predators. If one frog from a hundred eggs lives to be a parent,
his/her survival is really outstanding. But frogs go on because
they lay so many eggs.
are examples of K-selected animals. Female elephants have babies
about three years apart, and they have only one each time. The
whole group looks after the youngsters, and protects them through
childhood and adolescence. By reproducing at a rate that holds
their numbers close to constant, elephants are able to survive
in stable ecosystems. Because they ensure the survival of a good
percentage of their young, elephants do not need to produce many
Let's look at the impact of
human activities on these two reproductive strategies.
Habitat Alteration vs Impacts on Individuals
Human activities have had a
powerful impact on the environment. Sometimes this helps a species,
sometimes the effects are destructive. For example, despite
their fecundity, frogs are disappearing from many environments:
scientists are not sure why, although water pollution is
thought to be a factor. Destroying or poisoning the habitat has
a wide-ranging effect. In some cases, however, the environment
is altered in ways that enlarge the region a species inhabits.
Certainly our houses can provide niches for mice and cockroaches,
and we have enlarged the populations of our pet animals beyond
the numbers that we would expect to find living in a similar
amount of space in the wild. Our agriculture, too, has increased
the numbers of animals that we want to raise, sometimes at the
expense of wild species. Our chicken ranches, for instance, support
huge numbers of chickens by importing food. If, for some reason,
all humans were simply to disappear and all animals were turned
loose, there would be quite a reduction in the populations of
many of our farm animals.
The impact of affecting individuals directly may not be so
destructive as destroying the habitat. Insects, which are mostly
r-selected reproducers, have survived and prospered despite our
vigorous spraying of biocidal poisons. Insects breed
rapidly and mature quickly, and are present in such numbers that
a few survivors are likely to emerge from any catastrophe.
These survivors can breed rapidly and reestablish the population.
When conditions are changing, r-selected organisms can do very
Contrast this with the plight
of some of our large, K-selected animals. They have evolved to
live within the carrying capacity
of their environments, and when that environment changes, they
and the environment suffer. For example, the elephants in Africa
are losing habitat as increasing human populations bring more
land under cultivation. In some places elephants are crowded
onto an insufficient land area. The hungry animals are destroying
trees and changing that environment in long-term ways. The ecosystem
is out of balance, and the number of the animals who live in
it needs to be readjusted.
K-selected individuals, because they
are born only periodically, also affect the environment if they
die prematurely. A few hunters with powerful rifles can have
an impact on a population: diseases can have a devastating effect.
As numbers of animals drop, the loss of individuals becomes more
and more significant. We see this with endangered animals: tigers,
rhinos, and some of the whales. K-selected organisms have no
mechanism by which they can suddenly increase their numbers.
This is why small, rapidly maturing animals tend to be the survivors
after global catastrophes.
These breeding strategies set
out a basic reproductive plan. In themselves, they may not be
enough to ensure that species stay within the carrying capacity
of their habitat. Other mechanisms, such as territoriality or
dominance hierarchies, may be factors in population control as
It is interesting to look at
how humans deal with the challenge of carrying capacity. If resources
are abundant, our species can reproduce rapidly: under difficult
conditions many of our children die young. Humans have evolved
social mechanisms for keeping population growth in check. Such
mechanisms have included infanticide, not allowing widows to
remarry, postponing the marriage age, and preventing adults from
reproducing. During the Middle Ages, for example, 20% of the
adults were in monasteries and convents, and so childless. Modern
medicine and hygienic practices have fostered rapid population
increases. Our adults are living longer, and many more of our
children survive into adulthood. As populations cannot increase
indefinitely, new social mechanisms will have to be developed
to stabilize our numbers.
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© 1998, 2003. Elizabeth Anne
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