A variety of factors affect population growth.

Populations will grow as long as growth factors are available, but as the population gets larger, resources start to decline. The limiting factor, the resource that is most scarce, tends to determine carrying capacity. The effects that predators (a resistance factor) have on populations can vary widely, in part because predators, along with disease and competition, are density-dependent factors—their effects all increase as the prey’s population size goes up. Wolves and other predators are density-dependent factors affecting elk herd size. In the same way, elk are density-dependent factors on the ability of young aspens to grow after sprouting (aspen is an important winter food for elk; they eat the young shoots). On the other hand, some factors affect a population no matter how large or small it is, such as droughts, storms, and fire. These density-independent factors don’t necessarily regulate population size, but they can increase or decrease it. INFOGRAPHIC 9.4

density-dependent factors

Factors, such as predation or disease, whose impact on a population increases as population size goes up.

density-independent factors

Factors, such as a storm or an avalanche, whose impact on a population is not related to population size.

DENSITY-DEPENDENT AND DENSITY-INDEPENDENT FACTORS AFFECT POPULATION SIZE

Density-dependent factors exert more of an effect as population size increases. On the other hand, density-independent factors have the same effect regardless of population size.

James Balog/Aurora/Getty Images
David R: Frazier/Danita Delimont/Newscom
Karl Gehring/Denver Post via Getty Images
Debbie Noda/Modesto Bee/ZUMA/Newscom
Barrett Hedges/National Geographic/Getty Images
© Lars Thulin/Johné Images/Corbis

Identify the following as either density-dependent or density-independent factors for an elk population: a tick infestation, building a dam that floods a valley in elk habitat, drought, bison, and a blizzard.

• tick infestation: Density-dependent

• dam: Density-independent

• drought: Density-independent

• bison: Density-dependent

• a blizzard: Density-independent

KEY CONCEPT 9.6

some factors have an increased effect on population size when a population’s size is large (density-dependent), and others have an effect that is not related to population size (density-independent).

The biology of a species (which reflects its adaptations for growth, reproduction, and survival) also affects its populations’ growth potential. For instance, ecologists recognize a continuum of life-history strategies among species. Species whose members mature early, have high fecundity, and have relatively short life spans are known as r-selected species—so named because of their high rate of population increase (r). Yellowstone r-selected species, such as deer mice and spotted knapweed, are well adapted to exploit unpredictable environments and are able to increase quickly if resources suddenly become available.

life-history strategies

Biological characteristics of a species (for example, life span, fecundity, maturity rate) that influence how quickly a population can potentially increase in number.

r-selected species

Species that have a high biotic potential and that share other characteristics, such as short life span, early maturity, and high fecundity.

167

168

On the other hand, K-selected species, which in Yellowstone include bears, wolves, and slow-growing trees like spruce, are found at the other end of the continuum. Individuals in these species have longer life spans, are slow to mature, and have lower fecundity. Because of this, their reproductive rates are lower, but this means their population growth rates are responsive to environmental conditions; they decrease or increase slowly if resources decrease or increase in availability. This responsiveness tends to keep population sizes close to carrying capacity (K). INFOGRAPHIC 9.5

KEY CONCEPT 9.7

The life-history strategy of a species influences the growth potential of its populations. Population size of r-selected species can increase or decrease quickly in response to environmental changes. K-selected species are more likely to have stable population sizes close to carrying capacity but are less adaptable in the face of environmental change.

K-selected species

Species that have a low biotic potential and that share characteristics such as long life span, late maturity, and low fecundity; generally show logistic population growth.

LIFE-HISTORY STRATEGIES

Different species have different potentials for population growth, known as life-history strategies. A species’ biology may place it anywhere along a continuum between two extremes—the r- and K-selected species.

From left: Norbert Rosing/National Geographic Stock; © Wayne Lynch/AGE fotostock; © Juniors Bildarchiv GmbH/Alamy; Tom Reichner/Shutterstock; John W. Bova/Science Source; Taylor S. Kennedy/National Geographic Stock

Consider an aquatic ecosystem. Where would you place the following organisms on a life-history continuum: tuna, sperm whale, plankton, and jellyfish? (Hint: Identify the most extreme r and K species and then place the others in between those on the continuum.)

r ---- plankton ------ jellyfish ------ tuna ----- sperm whale ----K

Some species, like elk and deer, have characteristics of both r and K species; they fall somewhere in the middle of the continuum. They are large organisms that have one or two offspring per year and provide parental care (K characteristics), but their population sizes can increase rapidly if conditions are favorable for growth and survival (r-characteristics).

K-species and r-species often experience different types of population change. For instance, population sizes tend to be stable, especially for K-species, in undisturbed, mature areas. On the other hand, r-species with rapid reproductive potential sometimes have sudden, rapid population growth, characterized by occasional surges to very high population numbers, which may overshoot carrying capacity, followed by sudden crashes, especially in response to seasonal availability of food or temperature changes; their high rate of reproduction does not allow the population the time to adjust and produce fewer offspring as resources become scarce. When this occurs, the population that exceeds carrying capacity will drop below carrying capacity and then increase again; some populations will eventually level off close to carrying capacity, while others continue to overshoot and crash.

169

Predator and prey also respond to each other; predator populations increase as their prey population increases. But more predators eventually reduce the prey population. Fewer prey means less food and eventually decreases the number of predators, allowing the prey species to recover. In some cases, the fluctuations in population size are large enough to result in boom-and-bust cycles, with the peaks of the predator population size lagging behind those of its prey or food source. A classic example is that of the snowshoe hare and Canada lynx. Long-term data based on the fur trade and going back as far as the 1800s show that snowshoe hare populations undergo cyclic oscillations. According to recent studies by University of British Columbia researcher Charles Krebs, these mirror the size of predator populations, such as lynx and great horned owls. INFOGRAPHIC 9.6

boom-and-bust cycles

Fluctuations in population size that produce a very large population followed by a crash that lowers the population size drastically, followed again by an increase to a large size and a subsequent crash.

SOME POPULATIONS FLUCTUATE IN SIZE OVER TIME

Why don’t K-selected species usually go through overshoot and crash cycles?

Because they have lower reproductive rates as their population size approaches carrying capacity and resources become more scarce, individual reproductive success is likely to be reduced as well, reducing the chance that they will significantly overshoot carrying capacity.