Chapter 8 Population Ecology

POPULATION DYNAMICS AND CARRYING CAPACITY Most populations live in clumps although other patterns occur based on resource distribution. Figure 8-2

Changes in Population Size: Entrances and Exits Populations increase through births and immigration Populations decrease through deaths and emigration

Age Structure: Young Populations Can Grow Fast How fast a population grows or declines depends on its age structure. Prereproductive age: not mature enough to reproduce. Reproductive age: those capable of reproduction. Postreproductive age: those too old to reproduce.

Limits on Population Growth: Biotic Potential vs Limits on Population Growth: Biotic Potential vs. Environmental Resistance No population can increase its size indefinitely. The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

Environmental Resistance Carrying capacity (K) Population size (N) Biotic Potential Exponential Growth Figure 8.3 Natural capital: no population can continue to increase in size indefinitely. Exponential growth (lower part of the curve) occurs when resources are not limited and a population can grow at its intrinsic rate of increase (r) or biotic potential. Such exponential growth is converted to logistic growth, in which the growth rate decreases as the population becomes larger and faces environmental resistance. Over time, the population size stabilizes at or near the carrying capacity (K) of its environment, which results in a sigmoid (S-shaped) population growth curve. Depending on resource availability, the size of a population often fluctuates around its carrying capacity, although a population may temporarily exceed its carrying capacity and suffer a sharp decline or crash in its numbers. Time (t) Fig. 8-3, p. 163

Number of sheep (millions) Overshoot Carrying capacity Number of sheep (millions) Figure 8.4 Boom and bust: logistic growth of a sheep population on the island of Tasmania between 1800 and 1925. After sheep were introduced in 1800, their population grew exponentially thanks to an ample food supply. By 1855, they had overshot the land’s carrying capacity. Their numbers then stabilized and fluctuated around a carrying capacity of about 1.6 million sheep. Year Fig. 8-4, p. 164

Population overshoots carrying capacity Number of reindeer Population Crashes Number of reindeer Figure 8.6 Exponential growth, overshoot, and population crash of reindeer introduced to the small Bering Sea island of St. Paul. When 26 reindeer (24 of them female) were introduced in 1910, lichens, mosses, and other food sources were plentiful. By 1935, the herd size had soared to 2,000, overshooting the island’s carrying capacity. This led to a population crash, with the herd size plummeting to only 8 reindeer by 1950. Carrying capacity Year Fig. 8-6, p. 165

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Over time species may increase their carrying capacity by developing adaptations. Some species maintain their carrying capacity by migrating to other areas. So far, technological, social, and other cultural changes have extended the earth’s carrying capacity for humans.

Types of Population Change Curves in Nature Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. Stable: fluctuates slightly above and below carrying capacity. Irruptive: populations explode and then crash to a more stable level. Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles. Irregular: erratic changes possibly due to chaos or drastic change.

Types of Population Change Curves in Nature Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources. Figure 8-7

Population size (thousands) Hare Lynx Population size (thousands) Figure 8.7 Population cycles for the snowshoe hare and Canadian lynx. At one time scientists believed these curves provided circumstantial evidence that these predator and prey populations regulated one another. More recent research suggests that the periodic swings in the hare population are caused by a combination of top-down population control—predation by lynx and other predators—and bottom-up population control. In the latter, changes in the availability of the food supply for hares help determine hare population size, which in turn helps determine the lynx population size. (Data from D. A. MacLulich) Year Fig. 8-7, p. 166

Case Study: Exploding White-Tailed Deer Populations in the United States Since the 1930s the white-tailed deer population has exploded in the United States. Nearly extinct prior to their protection in 1920’s. Today 25-30 million white-tailed deer in U.S. pose human interaction problems. Deer-vehicle collisions (1.5 million per year). Transmit disease (Lyme disease in deer ticks).

REPRODUCTIVE PATTERNS Some species reproduce without having sex (asexual). Offspring are exact genetic copies (clones). Others reproduce by having sex (sexual). Genetic material is mixture of two individuals. Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly. Major advantages: genetic diversity, offspring protection.

Sexual Reproduction: Courtship Courtship rituals consume time and energy, can transmit disease, and can inflict injury on males of some species as they compete for sexual partners. Figure 8-8

Carrying capacity K K species; experience K selection Number of individuals Figure 8.9 Positions of r-selected and K-selected species on the sigmoid (S-shaped) population growth curve. r species; experience r selection Time Fig. 8-9, p. 168

Little or no parental care and protection of offspring r-Selected Species Cockroach Dandelion Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species Figure 8.10 Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes. Fig. 8-10a, p. 168

Fewer, larger offspring High parental care and protection of offspring K-Selected Species Elephant Saguaro Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete Late successional species Figure 8.10 Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes. Fig. 8-10b, p. 168

Survivorship Curves: Short to Long Lives The way to represent the age structure of a population is with a survivorship curve. Late loss population live to an old age. Constant loss population die at all ages. Most members of early loss population, die at young ages.

Survivorship Curves: Short to Long Lives The populations of different species vary in how long individual members typically live. Figure 8-11

Percentage surviving (log scale) Late loss Constant loss Percentage surviving (log scale) Figure 8.11 When does death come? Survivorship curves for populations of different species, show the percentages of the members of a population surviving at different ages. Most members of a late loss population (such as elephants, rhinoceroses, and humans) live to an old age. Members of a constant loss population (such as many songbirds) die at all ages. In an early loss population (such as annual plants and many bony fish species), most members die at a young age. These generalized survivorship curves only approximate the realities of nature. Early loss Age Fig. 8-11, p. 169