Download Reproductive Strategies

Reproductive Strategies
Why is it that some animals, such as humans, tamanduas and elephants, live a long time and have few babies; while
others such as mice, Madagascar hissing cockroaches and some fish live a short time and have many babies? Why is it
that some animals, such as cicadas and salmon, live a long time, then have a burst of babies and die? These different
patterns of living are called reproductive strategies and life history patterns.
A population is an inbreeding group of individuals. Populations have various defining characteristics, including: species
of organism, time (historical), habitat, their numbers, density, distribution in space (dispersion), age
structure/demographics and niche. A population size of zero is unique in that a population reduced to zero is said to be
extinct. A population size of zero is also unique in that subsequent recovery (increase in size) is not possible.
K- and r- strategists
Different species vary in the amount of time and energy they use to raise their offspring. There are two extremes.
Species like humans and other large mammals have small numbers of offspring but invest large amounts of energy in
parental care, and most offspring survive. These are K-strategists. K-strategists are good competitors. Population sizes of
K-strategists are usually close to the carrying-capacity. In stable ecosystems natural selection favours species that can
maximize use of natural resources and have a high probability of survival so K-strategists outcompete r-strategists.
In contrast, invertebrates and fish use lots of energy in the production of vast numbers of eggs but none in raising the
young after hatching. They lay their eggs and leave them forever. They are r-strategists. The r-strategists reproduce
quickly, are able to colonize new habitats rapidly and make opportunistic use of short-lived resources. Due to their fast
reproductive and growth rates, they may exceed the carrying capacity, with a population crash as a result. They
predominate in unstable environments.
Test Yourself
1. Explain whether K- or r-strategists would be more likely to be regulated by density-independent limiting factors
e.g. the weather.
2. Describe and explain the shape of the survivorship curve for K-selected species.