Reproductive Strategies

Learn about reproductive strategies that enhance a species' survival.

     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.

 Reproductive Strategies

 r-selected

 K-selected

 mature rapidly

 mature slowly

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 youngsters

 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 new areas

maintain numbers in stable ecosystems

    We see that r-selected organisms 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.

bear with cubs     Elephants 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 elephant babies.

     Let's look at the impact of human activities on these two reproductive strategies.

Habitat Alteration vs Impacts on Individuals

R-selected species

     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 well.

doe with fawnK-selected species

     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 well.

     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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