Unit 4: Ecosystems // Section 5: Population Dynamics
Every organism in an ecosystem divides its energy among three competing goals: growing, surviving, and reproducing. Ecologists refer to an organism's allocation of energy among these three ends throughout its lifetime as its life history strategy. There are tradeoffs between these functions: for example, an organism that spends much of its energy on reproduction early in life will have lower growth and survival rates, and thus a lower reproductive level later in life. An optimal life history strategy maximizes the organism's contribution to population growth.
Understanding how the environment shapes organisms' life histories is a major question in ecology. Compare the conditions for survival in an unstable area, such as a flood plain near a river that frequently overflows its banks, to those in a stable environment, such as a remote old-growth forest. On the flood plain, there is a higher chance of being killed early in life, so the organisms that mature and reproduce earlier will be most likely to survive and add to population growth. Producing many offspring increases the chance that some will survive. Conversely, organisms in the forest will mature later and have lower early reproductive rates. This allows them to put more energy into growth and competition for resources.
Ecologists refer to organisms at the first of these two extremes (those adapted to unstable environments) as r-selected. These organisms live in settings where population levels are well below the maximum number that the environment can support—the carrying capacity—so their numbers are growing exponentially at the maximum rate at which that population can increase if resources are not limited (often abbreviated as r). The other extreme, organisms adapted to stable environments, are termed K-selected because they live in environments in which the number of individuals is at or near the environment's carrying capacity (often abbreviated as K).
Organisms that are r-selected tend to be small, short-lived, and opportunistic, and to grow through irregular boom-and-bust population cycles. They include many insects, annual plants, bacteria, and larger species such as frogs and rats. Species considered pests typically are r-selected organisms that are capable of rapid growth when environmental conditions are favorable. In contrast, K-selected species are typically larger, grow more slowly, have fewer offspring and spend more time parenting them. Examples include large mammals, birds, and long-lived plants such as redwood trees. K-selected species are more prone to extinction than r-selected species because they mature later in life and have fewer offspring with longer gestation times. Table 3 contrasts the reproductive characteristics of an r-selected mammal, the Norway rat, to those of a K-selected mammal, the African elephant.
|Feature||Norway rat (r-selected)||African elephant (K-selected)|
|Reaches sexual or reproductive maturity||3-4 months||10-12 years|
|Average gestation period||22-24 days||22 months|
|Time to weaning||3-4 weeks||48-108 months|
|Breeding interval (female)||Up to 7 times per year||Every 4 to 9 years|
|Offspring per litter||2-14 (average 8)||1 average, 2 high|
Many organisms fall between these two extremes and have some characteristics of both types. As we will see below, ecosystems tend to be dominated by r-selected species in their early stages with the balance gradually shifting toward K-selected species.
In a growing population, survival and reproduction rates will not stay constant over time. Eventually resource limitations will reduce one or both of these variables. Populations grow fastest when they are near zero and the species is uncrowded. A simple mathematical model of population growth implies that the maximum population growth rate occurs when the population size (N) is at one-half of the environment's carrying capacity, K (i.e., at N = K/2).
In theory, if a population is harvested at exactly its natural rate of growth, the population will not change in size, and the harvest (yield) can be sustained at that level. In practice, however, it can be very hard to estimate population sizes and growth rates in the wild accurately enough to achieve this maximum sustainable yield. (For more on over-harvesting, see Unit 9, "Biodiversity Decline.")