r-k Selection | Density Dependent Selection скачать в хорошем качестве

r-k Selection | Density Dependent Selection

Species have different reproductive patterns that can help enhance their survival. Species with a capacity for a high rate of population increase (r) are called r-selected species . These species have many, usually small, offspring and give them little or no parental care or protection. They overcome typically massive losses of offspring by producing so many off- spring that a few will likely survive to reproduce many more offspring to begin this reproductive pattern again. Examples include algae, bacteria, rodents, frogs, turtles, annual plants (such as dandelions), and most insects. Such species tend to be opportunists. They reproduce and disperse rapidly when conditions are favorable or when a disturbance opens up a new habitat or niche for invasion. Environmental changes caused by disturbances, such as fires, clear-cutting, and volcanic eruptions, can allow opportunist species to gain a foot- hold. However, once established, their populations may crash because of unfavourable changes in environmental conditions or invasion by more competitive species. This helps to explain why most opportunist species go hrough irregular and unstable boom-and-bust cycles in their population sizes. At the other extreme are competitor or K-selected species (Figure 5-14). They tend to reproduce later in life and have a small number of offspring with fairly long life spans. Typically, for K-selected mammals, the offspring develop inside their mothers (where they are safe), are born fairly large, mature slowly, and are cared for and protected by one or both parents, and in some cases by living in herds or groups, until they reach reproductive age. This reproductive pattern results in a few big and strong individuals that can compete for re- sources and reproduce a few young to begin the cycle again.