Chapter 53 Population Ecology

You Must Know: How density, dispersion, and demographics can describe a population. The differences between exponential and logistic models of population growth. How density-dependent and density- independent factors can control population growth.

Life takes place in populations group of individuals of same species in same area at same time rely on same resources interact interbreed Population Ecology: What factors affect a population?

Introduction Population = group of individuals of a single species living in same general area Density: # individuals / area Dispersion: pattern of spacing between individuals

Determining population size and density: Count every individual Random sampling Mark-recapture method

WHITE BOARDS!

# marked / total population = (# recaptured and marked) / # recaptured WHITE BOARDS! # marked / total population = (# recaptured and marked) / # recaptured

# marked / total population = (# recaptured and marked) / # recaptured WHITE BOARDS! # marked / total population = (# recaptured and marked) / # recaptured

Patterns of Dispersal: Clumped – most common; near required resource Uniform – usually antagonistic interactions Random – unpredictable spacing, not common in nature

Clumped Pattern (most common) The most common pattern of dispersion is clumped, with the individuals aggregated in patches. Plants or fungi are often clumped where soil conditions and other environmental factors favor germination and growth. For example, mushrooms may be clumped on a rotting log. Many animals spend much of their time in a particular microenvironment that satisfies their requirements. Forest insects and salamanders, for instance, are frequently clumped under logs, where the humidity tends to be higher than in more exposed areas. Clumping of animals may also be associated with mating behavior. For example, mayflies often swarm in great numbers, a behavior that increases mating chances for these insects, which survive only a day or two as reproductive adults. Group living may also increase the effectiveness of certain predators; for example, a wolf pack is more likely than a single wolf to subdue a large prey animal, such as a moose

Uniform May result from direct interactions between individuals in the population  territoriality Clumped patterns A uniform, or evenly spaced, pattern of dispersion may result from direct interactions between individuals in the population. For example, some plants secrete chemicals that inhibit the germination and growth of nearby individuals that could compete for resources. Animals often exhibit uniform dispersion as a result of antagonistic social interactions, such as territoriality —the defense of a bounded physical space against encroachment by other individuals. Uniform patterns are not as common in populations as clumped patterns.

Demography: the study of vital statistics that affect population size Additions occur through birth, and subtractions occur through death. Life table : age-specific summary of the survival pattern of a population

Survivorship Curve: represent # individuals alive at each age What do these graphs tell about survival & strategy of a species? Type I: low death rate early in life (humans) Type II: constant death rate over lifespan (squirrels) Type III: high death rate early in life, but survivors then live long (oysters)

Reproductive strategies K-selected late reproduction few offspring invest a lot in raising offspring primates coconut r-selected early reproduction many offspring little parental care insects many plants K-selected r-selected

“Of course, long before you mature, most of you will be eaten.” Trade offs Number & size of offspring vs. Survival of offspring or parent r-selected K-selected “Of course, long before you mature, most of you will be eaten.”

Life strategies & survivorship curves 25 1000 100 Human (type I) Hydra (type II) Oyster (type III) 10 1 50 Percent of maximum life span 75 Survival per thousand K-selection A Type I curve is flat at the start, reflecting low death rates during early and middle life, then drops steeply as death rates increase among older age groups. Humans and many other large mammals that produce few offspring but provide them with good care often exhibit this kind of curve. In contrast, a Type III curve drops sharply at the start, reflecting very high death rates for the young, but then flattens out as death rates decline for those few individuals that have survived to a certain critical age. This type of curve is usually associated with organisms that produce very large numbers of offspring but provide little or no care, such as long–lived plants, many fishes, and marine invertebrates. An oyster, for example, may release millions of eggs, but most offspring die as larvae from predation or other causes. Those few that survive long enough to attach to a suitable substrate and begin growing a hard shell will probably survive for a relatively long time. Type II curves are intermediate, with a constant death rate over the organism’s life span. This kind of survivorship occurs in Belding’s ground squirrels and some other rodents, various invertebrates, some lizards, and some annual plants. r-selection

Change in Population Size dN/dt = B-D N = population size t = time Change in population size during time interval Births during time interval Deaths during time interval = -

Factors that limit population growth: Density-Dependent factors: population density matters i.e. Predation, disease, competition, territoriality, waste accumulation, physiological factors Density-Independent factors: population density not a factor i.e. Natural disasters: fire, flood, weather

Biotic & abiotic factors  Population fluctuations 1975-1980: peak in wolf numbers 1995: harsh winter weather (deep snow)

What do you notice about the population cycles of the showshoe hare and lynx?

Boom-and-bust cycles Predator-prey interactions Eg. lynx and snowshoe hare on 10-year cycle

Introduced species Non-native species kudzu transplanted populations grow exponentially in new area out-compete native species loss of natural controls lack of predators, parasites, competitors reduce diversity examples African honeybee gypsy moth zebra mussel purple loosestrife gypsy moth kudzu

Population Growth Models

Exponential population growth: ideal conditions, population grows rapidly

Exponential growth rate Characteristic of populations without limiting factors introduced to a new environment or rebounding from a catastrophe Whooping crane coming back from near extinction African elephant protected from hunting The J–shaped curve of exponential growth is characteristic of some populations that are introduced into a new or unfilled environment or whose numbers have been drastically reduced by a catastrophic event and are rebounding. The graph illustrates the exponential population growth that occurred in the population of elephants in Kruger National Park, South Africa, after they were protected from hunting. After approximately 60 years of exponential growth, the large number of elephants had caused enough damage to the park vegetation that a collapse in the elephant food supply was likely, leading to an end to population growth through starvation. To protect other species and the park ecosystem before that happened, park managers began limiting the elephant population by using birth control and exporting elephants to other countries.

Exponential Growth Equation dN/dt = change in population r = growth rate of pop. N = population size

Exponential Growth Problem Sample Problem: A certain population of mice is growing exponentially. The growth rate of the population (r) is 1.3 and the current population size (N) is 2,500 individuals. How many mice are added to the population each year?

Zebra mussel reduces diversity ~2 months ecological & economic damage reduces diversity loss of food & nesting sites for animals economic damage

Logistic rate of growth Can populations continue to grow exponentially? Of course not! no natural controls K = carrying capacity Decrease rate of growth as N reaches K effect of natural controls What happens as N approaches K?

Carrying capacity Time (years) 1915 1925 1935 1945 10 8 6 4 2 Number of breeding male fur seals (thousands) Maximum population size that environment can support with no degradation of habitat varies with changes in resources 500 400 300 200 100 20 10 30 50 40 60 Time (days) Number of cladocerans (per 200 ml) What’s going on with the plankton?

Changes in Carrying Capacity Population cycles predator – prey interactions At what population level is the carrying capacity? K K

Unlimited resources are rare Logistic model: incorporates carrying capacity (K) K = maximum stable population which can be sustained by environment

Logistic Growth Equation dN/dt = change in population r = growth rate of pop. N = population size K = carrying capacity

Logistic Growth Problem Sample Problem: If a population has a carrying capacity (K) of 900, and the growth rate (r) is 1.1, what is the population growth when the population (N) is 425?

WHITE BOARDS! r is increasing r = 0.045

WHITE BOARDS! 172 frogs

WHITE BOARDS! dN/dt = (b-d)N (1134-1492)1 = (b – 0.395) 1492 So b = 0.155 b = 0.155

WHITE BOARDS! 8.5 turkeys/acre 110 turkeys/year 890 turkeys

WHITE BOARDS! 3200 dandelions

WHITE BOARDS! Last one 1371 dandelions

Life History: traits that affect an organism’s schedule of reproduction and survival 3 Variables: Age of sexual maturation How often organism reproduces # offspring during each event Note: These traits are evolutionary outcomes, not conscious decisions by organisms

Semelparity Big-bang reproduction Many offspring produced at once Individual often dies afterwards Less stable environments Agave Plant

Iteroparity Repeated reproduction Few, but large offspring More stable environments Lizard Critical factors: survival rate of offspring and repeated reproduction when resources are limited

K-selection r-selection K-selection: pop. close to carrying capacity r-selection: maximize reproductive success K-selection r-selection Live around K Exponential growth High prenatal care Little or no care Low birth numbers High birth numbers Good survival of young Poor survival of young Density-dependent Density independent ie. Humans ie. cockroaches

Zero Population Growth

Human Population Growth 2 configurations for a stable human population (zero population growth): High birth / high death Low birth / low death Demographic transition: occurs when population goes from A  B

Age-Structure Diagrams

Global Carrying Capacity Current world population (2015) = 7.3 billion Estimated carrying capacity = 10-15 billion? Ecological footprint: total land + water area needed for all the resources a person consumes in a pop. 1.7 hectares (ha)/person is sustainable Typical person in U.S. = 10 ha footprint Limitations? Consequences? Solutions?

Map of ecological footprint of countries in the world (proportional sizes shown)