CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

17. Population Ecology

17.2. Reproductive Capacity

Sex ratios and age distributions within a population have a direct bearing on the rate of reproduction. A population’s reproductive capacity, or biotic potential, is the theoretical number of offspring that could be produced. Some species produce huge numbers of offspring, whereas others produce few per lifetime. In some cases, the reproductive capacity is generally much larger than the number of offspring needed simply to maintain the population. For example, a female carp may produce 1 million to 3 million eggs in her lifetime; this is her reproductive capacity. However, only two or three of these offspring ever develop into sexually mature adults. Therefore, her reproductive rate is two or three offspring per lifetime and is much smaller than her reproductive capacity.

In general, there are two strategies for assuring that there will be enough offspring that live to adulthood to ensure the continuation of the species. One strategy is to produce huge numbers of offspring but not provide any support for them. For example, an oyster may produce a million eggs a year, but not all of them are fertilized, and most that are fertilized die. An apple tree with thousands of flowers may produce only a few apples because the pollen that contains the sperm cells was not transferred to the female part of each flower in the process of pollination. Even after offspring are produced, mortality is usually high among the young. Most seeds that fall to the Earth do not grow, and most young animals die. Usually, however, enough survive to ensure the continuance of the species. Organisms that reproduce in this way spend large amounts of energy on the production of gametes and young, but no energy caring for the young. Thus, the probability that any individual will reach reproductive age is small.

The second way of approaching reproduction is to produce relatively fewer individuals but provide care and protection, which ensures a higher probability that the young will become reproductive adults. Humans generally produce a single offspring per pregnancy, but nearly all of them live. In effect, with this strategy, energy has been channeled into the care and protection of the young, rather than into the production of incredibly large numbers of potential young. Even though fewer young are produced by animals such as birds and mammals, their reproductive capacity still greatly exceeds the number required to replace the parents when they die.

17.2. CONCEPT REVIEW

5. In what way do the activities of species that produces few young differ from those that produce huge numbers of offspring?

6. How does reproductive capacity compare to the reproductive rate?