Chapter 51

Chapter 51

The Logistic Model and Life Histories Life history traits favored by natural selection may vary with population density and environmental conditions. K-selection = density-dependent selection, selects for life history traits that are sensitive to population density. r-selection = or density-independent selection, selects for life history traits that maximize reproduction. Many factors that regulate population growth are density dependent

There are two general questions about regulation of population growth: What environmental factors stop a population from growing indefinitely? Why do some populations show radical fluctuations in size over time, while others remain stable? Population Change and Population Density In density-independent populations, birth rate and death rate do not change with population density. In density-dependent populations, birth rates fall and death rates rise with population density.

Density-Dependent Population Regulation Density-dependent birth and death rates are an example of negative feedback that regulates population growth. They are affected by many factors, such as competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors. In crowded populations, increasing population density intensifies competition for resources and results in a lower birth rate. Percentage of juveniles producing lambs Decreased reproduction at high population densities

100 80 60 40 20 0 200 300 400 500 Population size 600 Territoriality

In many vertebrates and some invertebrates, competition for territory may limit density. Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries. Territoriality (a) Cheetah marking its territory (b) Gannets Disease, Predation, & Toxic Wastes

Population density can influence the health and survival of organisms. In dense populations, pathogens can spread more rapidly. As a prey population builds up, predators may feed preferentially on that species. Accumulation of toxic wastes can contribute to densitydependent regulation of population size. For some populations, intrinsic (physiological) factors appear to regulate population size. Population Dynamics The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size. Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time. Weather can affect population size over time.

Changes in predation pressure can drive population fluctuations 2,500 50 Moose 40 2,000 30 1,500 20

1,000 10 500 0 1955 1965 1975 1985 Year

1995 0 2005 Number of moose Number of wolves Wolves Population Cycles: Scientific Inquiry Some populations undergo regular boom-and-bust cycles. Lynx populations follow the 10 year boom-and-bust cycle of hare populations. Three hypotheses have been proposed to explain

the hares 10-year interval. Snowshoe hare 120 9 Lynx 80 6 40 3

0 0 1850 1875 1900 Year 1925 Number of lynx (thousands) Number of hares (thousands)

160 Hypothesis 1: The hares population cycle follows a cycle of winter food supply. If this hypothesis is correct, then the cycles should stop if the food supply is increased. Additional food was provided experimentally to a hare population, and the whole population increased in size but continued to cycle. No hares appeared to have died of starvation. Hypothesis 2: The hares population cycle is driven by pressure from other predators. In a study conducted by field ecologists, 90% of the hares were killed by predators. These data support this second hypothesis.

Hypothesis 3: The hares population cycle is linked to sunspot cycles. Sunspot activity affects light quality, which in turn affects the quality of the hares food. There is good correlation between sunspot activity and hare population size. The results of all these experiments suggest that both predation and sunspot activity regulate hare numbers and that food availability plays a less important role. Immigration, Emigration, and Metapopulations Metapopulations are groups of populations linked by immigration and emigration. High levels of immigration combined with higher survival can result in greater stability in

populations. The human population is no longer growing exponentially but is still rapidly No population increasing can grow indefinitely, and humans are no exception. The human population increased relatively slowly until about 1650 and then began to grow exponentially. Though the global population is still growing, the rate of growth began to slow during the 1960s. Most of the current global population growth is

concentrated in developing countries. Human population growth 6 5 4 3 The Plague 2 1 0 8000 B.C.E. 4000 3000

2000 1000 B.C.E. B.C.E. B.C.E. B.C.E. 0 1000 C.E. 2000 C.E. Human population (billions) 7 Regional Patterns of Population Change

To maintain population stability, a regional human population can exist in one of two configurations: Zero population growth = High birth rate High death rate Zero population growth = Low birth rate Low death rate The demographic transition is the move from the first state toward the second state. Age Structure One important demographic factor in present and future growth trends is a countrys age structure. Age structure is the relative number of individuals at each age. Age structure diagrams can predict a populations growth trends.

They can illuminate social conditions and help us plan for the future. Age-structure pyramids for the human population of three countries Rapid growth Afghanistan Male Female 10 8 6 4 2 0 2 4 Percent of population Age 6

8 Slow growth United States Male Female 85+ 8084 7579 7074 6569 6064 5559 5054 4549 4044

3539 3034 2529 2024 1519 1014 59 04 10 8 6 4 2 0 2 4 Percent of population Age 6 No growth Italy

Male Female 85+ 8084 7579 7074 6569 6064 5559 5054 4549 4044 3539 3034 2529 2024

1519 1014 59 04 8 8 6 4 2 0 2 4 Percent of population 6 8 Estimates of Earths Carrying Capacity How many humans can the biosphere support? The carrying capacity of Earth for humans is uncertain.

The average estimate is 1015 billion. Limits on Human Population Size The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation. It is one measure of how close we are to the carrying capacity of Earth. Countries vary greatly in footprint size and available ecological capacity. Our carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastes. Population size (N) Review: Population Growth Curve

K = carrying capacity KN dN = rmax N dt K Number of generations You should now be able to: 1. Define and distinguish between the following sets of terms: density and dispersion; clumped dispersion, uniform dispersion, and random dispersion; life table and reproductive table; Type I, Type II, and Type III survivorship curves; semelparity and iteroparity; r-selected

populations and K-selected populations. 2. Explain how ecologists may estimate the density of a species. 3. Explain how limited resources and trade-offs may affect life histories. 4. Compare the exponential and logistic models of population growth. 5. Explain how density-dependent and densityindependent factors may affect population growth. 6. Explain how biotic and abiotic factors may work together to control a populations growth.

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