THE LIVING WORLD

Unit Eight. The Living Environment

 

35. Populations and Communities

 

35.6. Life History Adaptations

 

Populations of many species, including annual plants, some insects, and most bacteria, can have very fast rates of growth when not limited by dwindling environmental resources. Habitats with more available resources than the population requires favor very rapid reproduction rates, which often approximate the exponential growth model discussed earlier.

Populations of most animals have much slower rates of growth with numbers limited by available resources. Growth slows as available resources become limiting, producing a sigmoid growth curve approximating the logistic growth model discussed earlier. Habitats with limited resources lead to more intense competition for resources, and favor individuals that can survive and successfully reproduce more efficiently. The number of individuals that can survive at this limit is the carrying capacity of the environment, or K.

The complete life cycle of an organism constitutes its life history. Life histories are very diverse, with different organisms having different adaptations in response to their environments. Some life history adaptations of a population favor very rapid growth in a habitat with abundant resources, or in unpredictable or volatile environments in which organisms have to take advantage of the resources when they are available. In these situations, reproducing early, producing many small offspring that mature quickly, and engaging in other aspects of “big bang” reproduction are favored. Using the terms of the exponential model, these adaptations, all favoring a high rate of increase r, are called r-selected adaptations. Examples of organisms displaying r-selected life history adaptations include dandelions, aphids, mice, and cockroaches (figure 35.14).

 

 

Figure 35.14. The consequences of exponential growth.

All organisms have the potential to produce populations larger than those that actually occur in nature. The German cockroach (Blatella germanica), a major household pest, produces 80 young every six months. If every cockroach that hatched survived for three generations, kitchens might look like this theoretical culinary nightmare concocted by the Smithsonian Museum of Natural History.

 

Other life history adaptations favor survival in an environment in which individuals are competing for limited resources. These features include reproducing late, having small numbers of large offspring that mature slowly and receive intensive parental care, and other aspects of “carrying capacity” reproduction. In terms of the logistic model, these adaptations, all favoring reproduction near the carrying capacity of the environment K, are called A-selected adaptations. Examples of organisms displaying K-selected life history adaptations include coconut palms, whooping cranes, and whales.

The r/K concept of life histories often provides a powerful way to examine more closely related organisms living in different types of habitats. In general, populations living in rapidly changing habitats tend to exhibit r-selected adaptations, whereas populations of closely related organisms living in more stable and competitive habitats exhibit more K-selected adaptations. Most natural populations show life history adaptations that exist along a continuum, ranging from completely r-selected traits to completely K-selected traits. Table 35.1 outlines the adaptations at the extreme ends of the continuum.

 

TABLE 35.1. r-SELECTED AND K -SELECTED LIFE HISTORY ADAPTATIONS

 

Adaptation

r-Selected Populations

K-Selected Populations

Age at first reproduction

Early

Late

Homeostatic capability

Limited

Often extensive

Life span

Short

Long

Maturation time

Short

Long

Mortality rate

Often high

Usually low

Number of offspring produced per reproductive episode

Many

Few

Number of reproductions per lifetime

Usually one

Often several

Parental care

None

Often extensive

Size of offspring or eggs

Small

Large

 

Key Learning Outcome 35.6. Some life history adaptations favor near-exponential growth, while others favor the more competitive logistic growth.