An introduction to life histories and density dependence   Leave a comment

Biologists and conservationists alike are interested in the life history of the plant or animal species that they study. Life history is the schedule of important events and processes in the life of an organism – when it matures, when and how often it reproduces, and how its likelihood of survival might change throughout its lifespan. Different kinds of plants and animals have evolved different life history strategies that reflect the particular challenges they face in their environments. For example, the plants that we call weeds live in habitats that are frequently disturbed, like gardens or a field that gets tilled every year. The plants that are successful in such an environment grow quickly and reproduce early. They invest so much in reproduction that many garden weed species do not live for more than one summer. On the other hand, trees grow, survive, and reproduce in forests where there is little disturbance, and competing trees are well established. Young trees grow slowly, taking many years to mature, waiting for an opportunity, like a newly formed gap in the canopy, before reaching their full size and investing in reproduction. A tree’s life history strategy is to ensure its survival from one year to the next – maximizing the success of its offspring by spreading reproduction out over many years. A weed’s life history strategy is to invest resources in reproduction, sacrificing a long life for lots of young in the short term.

Snapping turtle nesting on the shoulder of a small highway. (Photo: C. Davy)

 
 
 
Animals show a similar range of life history strategies, and turtles are much more like a mighty oak than a garden weed. Like trees, most of a turtle’s offspring don’t survive very long, but when a few do reach maturity, they can live for a very long time. Because turtles are adapted to spread their reproductive effort out over a long time, it takes many years for the average mother turtle to produce enough nests so that she will be able to replace herself in the next generation. 
 

Snapping turtle female laying eggs. Turtles dig a nest chamber in the ground using their hind feet, and then deposit the eggs into the nest chamber before scooping earth back on top of the eggs to cover the nest. (Photo: C. Davy)

 

 

 

Density dependence is an important concept in the ecology of plant and animal populations and it is what tends to keep populations stable. Density is the number of individuals of a species for a given unit of area. If there are 200 pike in a 20 hectare lake, then the density is 10 pike per hectare. When the number of individuals in a habitat is very high, there is more competition for less resources, and predators and parasites increase. In this situation there are fewer births and lower survival because each animal has less resources to put into avoiding predators, growth, and reproduction. However, when density is low, each individual can find food more easily because there is less competition and there can also be fewer parasites and predators. The few individuals that are around survive better, grow faster, and produce more offspring so the population increases. In a stable environment these complementary aspects of density dependence act to keep population size fairly constant – if the population gets low, individuals reproduce more and it recovers, but if the population is too high, reproduction goes down, survival is lower, and the number of animals is reduced.

This tiny hatchling was found feeding on mosquito larvae in a small pool. It had had already survived its first harsh Algonquin winter. If it is a female it will take 16-19 more years before it lays its first nest. (Photo: M. Keevil)

 
All populations are not affected the same way by density dependence. How strongly a population responds to changes in density depends on both the environment and the life history of the species in question. If a new factor – such as an unusually harsh winter or an increase in human hunting – causes a loss in numbers, some animal populations, especially those that are more like weeds in their life histories, return quickly to their former levels. On the other hand, animals such as turtles that take a long time to grow and reproduce take much, much longer to recover. Even if competition is reduced and survival is high once more, it still takes a long time for a young turtle to reach maturity and contribute to population recovery.
 
 — Matt Keevil
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Posted February 22, 2012 by thinkingturtles in Uncategorized

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