CH 08 Population & Carrying Capacity Section 01

A. POPULATION DYNAMICS AND CARRYING CAPACITY Three general patterns of population distribution occur in a habitat: clumping, uniform distribution and random dispersion. Most species live in clumps Uniform pattern distribution may occur where a resource such as water In random distribution members of a species are placed in seemingly random placement.

Distribution Patterns PLAY ANIMATION

Which pattern is this? Fig. 8-2c, p. 162

Which pattern is this? Fig. 8-2b, p. 162

Which pattern is this? Fig. 8-2a, p. 162

Four variables influence/govern population size: births, deaths, immigration, and emigration Populations increase through births and immigration Populations decrease through deaths and emigration Show ucla video on population dymanics

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. From this go to flash of population age structure. U.S.

A Population’s Age Structure Determines its Potential for Growth PLAY ANIMATION

Age Structure Diagram Generally, human populations in the developed world are “rectangular” and those of developing nations are “pyramidal.” Populations with a pyramid-shaped age structure will grow explosively. Populations with a rectangular age structure will grow much slower.

No population can increase its size indefinitely due to limited resources such as light, water, and nutrients and because of competitors or predators The biotic potential is the population’s capacity for growth The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources.

5. Environmental resistance consists of factors that limit population growth Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. CHANGE IN THE POPULATION Figure 8-4

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. PLAY ANIMATION Time (t) Fig. 8-3, p. 163

A population can grow rapidly with ample resources With few resource limitations, This population will have a fixed rate of growth that will take be a J-shaped growth curve This represents its intrinsic rate of increase (r) or biotic potential

Animation: Exponential Growth PLAY ANIMATION

This exponential growth is converted to logistic growth when the populations face environmental resistance In logistic growth, the growth rate levels off as population size reaches or nears carrying capacity The sigmoid (s-shaped) population growth curve shows that the population size is stable

As a population levels off, it often fluctuates slightly above and below the carrying capacity Overshooting an environment’s resources often is a result of a reproductive time lag The reproductive time lag can produce a dieback/crash of organisms unless the organisms can find new resources or move to an area with more resources If the carrying capacity of an area is exceeded, changes in the area itself can reduce future carrying capacity

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

9. Exceeding Carrying Capacity: Move, 9. 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.

Population overshoots carrying capacity Population Number of reindeer 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

Population Crash PLAY VIDEO

The density of a population may or may not affect how rapidly it can grow Population density: the number of individuals in a population found in a particular area or volume. Density-independent population controls affect a population’s size regardless of its density (abiotic factors: weather) Density-dependent factors population controls have a greater affect on the population as its density increases (biotic factors:disease)

Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. Stable: fluctuates slightly above and below carrying capacity because species are living under fairly constant environmental conditions Irruptive: populations explode and then crash to a more stable level which is characteristic of short-lived, rapidly reproducing species Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles. Irregular: erratic changes possibly due to chaos or drastic change.

Interaction between predators and their prey change in cycles Hypothesis of top-down control of prey by predators may not be only explanation for cycling Bottom-up control hypothesis is that plants consumed too rapidly by herbivores for replacements to keep up

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