Population Ecology Chapter 52

Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size. Demographics refers to the study of vital statistics especially birth and death rates.

Fig. 53-14 Figure 53.14 White rhinoceros mother and calf

Population, Density and Dispersion A population is a group of individuals of a single species living in the same general area Density is the number of individuals per unit area or volume Ex: the number of squirrels per square kilometer or The number of esterichia coli bacteria per milliliter in a test tube.

Determining Density In most cases, it is impractical or impossible to count all individuals in a population Researchers use different methods to estimate: One way is to count the number of individuals in a series of randomly located plots, calculate the average density in the samples, and extrapolate to estimate the population size in the entire area. Such estimates are accurate when there are many sample plots and a homogeneous (evenly dispersed) habitat.

The mark-recapture method Commonly used to estimate wildlife populations. Individuals are trapped and captured, marked with a tag, recorded, and then released. After a period of time has elapsed, traps are set again, and individuals are captured and identified. The second capture yields both marked and unmarked individuals. From this data, researchers estimate the total number of individuals in the population.

Density in a population depends on: Fig. 53-3 Density in a population depends on: Immigration is the influx of new individuals from other areas Emigration is the movement of individuals out of a population Births Deaths Births and immigration add individuals to a population. Deaths and emigration remove individuals from a population. Figure 53.3 Population dynamics Immigration Emigration

Dispersion is the pattern of spacing among individuals within the boundaries of the population.. They may be Clumped Uniform random

Patterns of Dispersion Dispersion is clumped when individuals aggregate in patches. Plants and fungi are often clumped where soil conditions favor germination and growth. Animals may clump in favorable microenvironments (such as isopods under a fallen log) or to facilitate mating interactions. Group living may increase the effectiveness of certain predators, such as a wolf pack.

Video: Flapping Geese (Clumped) Fig. 53-4a Figure 53.4a Patterns of dispersion within a population’s geographic range (a) Clumped dispersion in the intertidal zone Video: Flapping Geese (Clumped)

CLUMPED DISPERSION in a forest

Dispersion is uniform when individuals are evenly spaced. For example, some plants secrete chemicals that inhibit the germination and growth of nearby competitors. Animals often exhibit uniform dispersion as a result of territoriality, the defense of a bounded space against encroachment by others.

Fig. 53-4b Figure 53.4b Patterns of dispersion within a population’s geographic range (b) Uniform

In random dispersion, the position of each individual is independent of the others, and spacing is unpredictable. Random dispersion occurs where there is neither a strong attraction or repulsion among individuals in a population, or when key physical or chemical environmental factors are relatively homogeneously (evenly) distributed. For example, plants may grow where windblown seeds land.

Video: Prokaryotic Flagella (Salmonella typhimurium) (Random) Fig. 53-4c Figure 53.4c Patterns of dispersion within a population’s geographic range (c) Random Video: Prokaryotic Flagella (Salmonella typhimurium) (Random)

Demography Demography is the study of the vital statistics of populations and how they change over time. Of particular interest are birth rates and how they vary among individuals (specifically females) death rates. A life table is an age-specific summary of the survival pattern of a population.

A graphic way of representing the data Fig. 53-5 A graphic way of representing the data in a life table is a survivorship curve. 1,000 100 Number of survivors (log scale) Females 10 Males Figure 53.5 Survivorship curves for male and female Belding’s ground squirrels 1 2 4 6 8 10 Age (years)

Survivorship curves can be classified into three general types: A graphic way of representing the data in a life table is a survivorship curve. Survivorship curves can be classified into three general types: A Type I curve is relatively flat at the start, reflecting a low death rate in early and middle life, and drops steeply as death rates increase among older age groups. Humans and many other large mammals exhibit Type I survivorship curves.

The Type II curve is intermediate, with constant mortality over an organism’s life span. Many species of rodent, various invertebrates, and some annual plants show Type II survivorship curves. Prey species that are subject to predation.

A Type III curve drops off at the start, reflecting very high death rates early in life, then flattens out as death rates decline for the few individuals that survive to a critical age. Type III are usually organisms that produce large numbers of offspring but provide little or no parental care. Examples are many fishes, long-lived plants, and marine invertebrates. Also young that are subject to predation and severe environmental conditions

Idealized survivorship curves: Types I, II, and III Fig. 53-6 Idealized survivorship curves: Types I, II, and III 1,000 I 100 II Number of survivors (log scale) 10 Figure 53.6 Idealized survivorship curves: Types I, II, and III III 1 50 100 Percentage of maximum life span

Life history traits Natural selection favors traits that improve an organism’s chances of survival and reproductive success. In every species, there are trade-offs between survival and traits such as The age at which reproduction begins How often the organism reproduces How many offspring are produced during each reproductive cycle The traits that affect an organism’s schedule of reproduction and survival make up its life history.

Evolution and Life History Diversity Life histories are very diverse Species that exhibit semelparity, or big-bang reproduction, reproduce once and die Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly Highly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproduction

Fig. 53-7 Century Plants grow in arid climates with unpredictable rainfall. After several years it sends up a large flowering stalk, reproduces and then dies. semelparity Figure 53.7 An agave (Agave americana), an example of big-bang reproduction

Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce Dandelion

Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established Coconut palm

how much does an individual gain in reproductive success through one pattern versus the other? The critical factor is survival rate of the offspring. In highly variable or unpredictable environments, when the survival of offspring is low, semelparity (big-bang) reproduction is favored. In dependable environments where competition for resources is intense, iteroparity (repeated reproduction ) is favored.

Per Capita Rate of Increase If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate

Zero population growth occurs when the birth rate equals the death rate Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time: N t  rN where N = population size, t = time, and r = per capita rate of increase = birth – death

Population Growth Exponential population growth is a population increase under idealized conditions rate of reproduction is at its maximum, called the intrinsic rate of increase. Any species regardless of life history is capable of exponential growth if resources are unlimited

2,000 = 1.0N 1,500 = 0.5N Population size (N) 1,000 500 5 10 15 Fig. 53-10 2,000 Exponential curve dN = 1.0N dt 1,500 dN = 0.5N dt J shaped curve Population size (N) 1,000 500 Figure 53.10 Population growth predicted by the exponential model 5 10 15 Number of generations

Fig. 53-11 The J-shaped curve of exponential growth characterizes some rebounding populations such as the African Elephant 8,000 6,000 Elephant population 4,000 2,000 Figure 53.11 Exponential growth in the African elephant population of Kruger National Park, South Africa 1900 1920 1940 1960 1980 Year

The Logistic Growth Model (logical) Exponential growth cannot be sustained for long in any population. Limiting resources, food, water, space eventually limit population growth. Carrying capacity (K) is the maximum population size the environment can support.

In the logistic growth model, per capita rate of increase slows down or declines as carrying capacity is reached.

Fig. 53-13 Figure 53.13 How well do these populations fit the logistic growth model? 1,000 180 150 800 120 Number of Paramecium/mL 600 Number of Daphnia/50 mL 90 400 60 200 30 Figure 53.13 How well do these populations fit the logistic growth model? 5 10 15 20 40 60 80 100 120 140 160 Time (days) Time (days) (a) A Paramecium population in the lab (b) A Daphnia population in the lab

S shaped curve Exponential growth 2,000 = 1.0N 1,500 K = 1,500 Fig. 53-12 J shaped curve Exponential growth 2,000 dN = 1.0N dt 1,500 K = 1,500 S shaped curve Population size (N) Logistic growth 1,000 dN 1,500 – N = 1.0N dt 1,500 Figure 53.12 Population growth predicted by the logistic model 500 5 10 15 Number of generations

The Logistic Model and Life Histories Life history traits favored by natural selection may vary with population density and environmental conditions. K-selection, or 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

Density- Dependent factors that regulate population growth Factors that reduce birth rate or increase death rates are density dependent. Competition for resources such as food, space, water or essential nutrients intensifies as populations increase. Territoriality- available space for territory or nesting may be limited.

Disease- Increasing densities allow for easier transmission of disease. Swine flu is a perfect example Predation- As prey populations increase, predators may find prey more easily. Density-dependent regulation provides a negative feedback system that helps reduce birth rates and increase death rates or a population would grow exponentially!

Density Independent Factors When a birth or death rate does not change with regard to population density it is said to be density independent. Natural Disasters will cause an increase in death rate regardless of density. Weather and climate such as floods or Drought are density independent factors.

The human population Global human populations have grown almost continuously throughout history. But skyrocketed after the industrial revolution. Note the dip resulting from the plague in Europe during the 1300 s

7 6 5 4 3 2 1 Human population (billions) The Plague 8000 B.C.E. 4000 Fig. 53-22 7 6 5 4 Human population (billions) 3 2 The Plague Figure 53.22 Human population growth (data as of 2006) 1 8000 B.C.E. 4000 B.C.E. 3000 B.C.E. 2000 B.C.E. 1000 B.C.E. 1000 C.E. 2000 C.E.

The Global Human Population 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

Famine in china 1960s about 60 million died Fig. 53-23 2.2 2.0 1.8 1.6 1.4 2005 Annual percent increase 1.2 Famine in china 1960s about 60 million died Projected data 1.0 0.8 0.6 Figure 53.23 Annual percent increase in the global human population (data as of 2005) 0.4 0.2 1950 1975 2000 2025 2050 Year

Age Structure Pyramids One important demographic factor in present and future growth trends is a country’s age structure Age structure is the relative number of individuals at each age Each horizontal bar represents a specific age group dividing male and female sides

Wide bottom small top Developing nations rapid growth Fig. 53-25 Wide bottom small top Developing nations rapid growth Columnar structure industrialized nation slow growth Small bottom wider in the middle stable population Rapid growth Slow growth No growth Afghanistan United States Italy Male Female Age Male Female Age Male Female 85+ 85+ 80–84 80–84 75–79 75–79 70–74 70–74 65–69 65–69 60–64 60–64 55–59 55–59 50–54 50–54 45–49 45–49 40–44 40–44 35–39 35–39 30–34 30–34 25–29 25–29 20–24 20–24 Figure 53.25 Age-structure pyramids for the human population of three countries (data as of 2005) 15–19 15–19 10–14 10–14 5–9 5–9 0–4 0–4 10  8 6 4 2 2 4 6 8 10  8 6 4 2 2 4 6 8 8 6 4 2 2 4 6 8 Percent of population Percent of population Percent of population

Estimates of Carrying Capacity The carrying capacity of Earth for humans is uncertain The average estimate is 10–15 billion people A concept termed ecological footprint examines the total land and water area needed for all resources a person consumes in a population. Currently 1.7 hectares (app 4.2 acres) per person is considered sustainable. A typical person in the United States has a footprint of 10 hectares (almost 25 acres)