Chapter 53: Population Ecology
53.1 Biological processes influence population density, dispersion, and demographics
A population is a group of individuals of a single species living in the same general area.
Population ecology is concerned with measuring changes in population size and composition.
A.
Density and Dispersion
Density is the number of individuals per unit area or volume, and dispersion is the pattern of spacing among individuals within the boundaries of the population.
Ecologists use the mark-recapture method to estimate the size of wildlife populations.
(See Figure 53.2)
Immigration: the influx of new individuals from other areas. Emigration: the movement of individuals out of a population and into other locations.
Both immigration and emigration can alter the density of many populations.
There can be varying patterns of dispersion. Dispersion patterns vary in a population’s range due to environmental patchiness. Dispersion patterns may range from clumped:
most common, individuals are aggregated in patches, to uniform: evenly spaced, to random: unpredictable spacing, as determined by various environmental or social factors.
B.
Demographics
Demography is the study of the vital statistics of populations and how they change overtime.
Age, structure, and sex ratio are important demographic features.
To summarize these vital statistics of a population, you make a life table, or an age- specific summary of the survival pattern of a population. Life tables tabulate mortality rates, survivorship from one age to the next. (See Table 53.1 for an example of a life table).
The best way to construct a life table is to use a cohort, a group of individuals of the
same age, from birth until all of the individuals are dead.
Survivorship curves, which plot the number in a cohort still alive at each age, can be classified into 3 general types (See Figure 53.6 for visual) of depending on the rates of mortality over the entire lifespan:
a. Type I curve; flat at the start, reflecting low death rates during early and middle life, then drops steeply as death rates increase among older age groups. Best example of a Type I curve:
Humans.
b. Type II curve: intermediate, with a constant death rate over the organism's life span.
Example: Belding's ground squirrels.
c. Type III curve: drops sharply at the start, reflecting very high death rates for the young, but flattens out as death rates decline for those few individuals that survive the early die-off.
Reproductive rates are measured using direct counts and mark recapture.
Reproductive table: fertility schedule, an age specific summary of the reproductive rates of a population. Constructed by measuring the reproductive output of a cohort from birth until death. Focuses on females, because they produce offspring.
53.2 The exponential model describes population growth in an idealized, unlimited environment
Ignoring immigration and emigration, a population’s growth rate (r) is the birth rate minus the death rate.
The per capita birth rate is the number of offspring produced per unit time by an average member of the population. The per capita death rate allows us to calculate the expected number of deaths per unit time in a population of any size.
Zero population growth occurs when the per capita birth rate and death rates are equal. (r=0).
A. Exponential Growth
The exponential growth equation represents a population’s potential growth in an unlimited environment, where rmax is the maximum possible growth rate and N is the number of individuals. The general equation for exponential growth is dN/dt = rinstN
The size of a population that is growing exponentially increases at a constant rate, resulting eventually in a J-shaped growth curve when population size is plotted over time (Figure 53.8). This model predicts that the larger the population, the faster it grows.
53.3 The logistic model describes how a population grows more slowly as it nears its
carrying capacity
A logistic model of population growth incorporates the concept of carrying capacity.
Exponential growth cannot be sustained indefinitely in a population. A more realistic model limits growth by incorporating carrying capacity (K), the maximum population size that can be sustained by available resources.
In the logistic population growth model, the per capita rate of increase approaches zero as the population size nears the carrying capacity.
The logistic model of population growth produces a sigmoid (S-shaped) growth curve when N (population size) is plotted over time. (See Figure 53.10 for Exponential vs.
Logistic)
53.4 Life history traits are products of natural selection
The traits that affect an organism’s schedule of reproduction and survival make up its life history.
Life Histories are highly diverse but exhibit patterns their variability. Natural selection has led to the evolution of diverse life history “strategies” that maximize lifetime reproductive success.
Life history traits represent trade-offs between conflicting demands for limited time, energy, and nutrients. Limited resources mandate trade-offs between investments in reproduction and in survival.
Semelparous organisms reproduce a single time and then die, while iteroparous organisms reproduce repeatedly over several breeding.
a. Semelparity: a "one-shot" pattern of big-bang reproduction.
b. Iteroparity: in contrast to semelparity, repeated reproduction.
When survival between breeding season is low or if there is a large trade-off between fecundity and survival, semelparity is favored over iteroparity.
Clutch size and age at first reproduction involve trade-offs between current and future fecundity, fecundity and adult survival, or fecundity and survival of the offspring.
a. Fecundity = Birth rate
According to a proposed relationship between population density and life histories,
selection should favor traits that allow survival and reproduction with fewer resources in
populations that live at densities near carrying capacity (K).
a. Organisms that live near their carrying capacity are K-selective, selection for traits that are sensitive to population density and are favored at high densities, also called density-dependent selection.
b. If the numbers fluctuate then it is r-selective, selection for traits that maximize reproductive success in uncrowded environments, also called density-independent selection.
Population growth is limited by both density-dependent and density-independent factors,
53.5 Many factors that regulate population growth are density dependent A. Population Change and Population Density
Similar to the case of r-selection, a birth rate or death rate that does not change with population density is said to be density-independent.
Density-independent factors, such as climatic events and fires, reduce population size by a given fraction, regardless of its density.
The population size of many species, particularly small organisms such as insects, is limited by seasonally occurring density-independent factors.
In contrast, a death rate that increases with population density or a birth rate that falls with rising density is said to be density-dependent, similar to k selection.
Several density-dependent factors (intraspecific competition for limited resources, increased predation, stress due to crowding, or buildup of toxins) can cause population growth rates to decline.
A mix of density-dependent and independent factors probably limits the growth of most populations.
Populations that are generally stable are probably close to a carrying capacity determined by density-dependent limits, but their short-term fluctuations are density-independent.
B. Mechanisms of Density-Dependent Population Regulation
Without some type of negative feedback between population density and the rates of birth and death, a population would never stop growing.
Feedback provided by: Density-dependent regulation halts population growth through
mechanisms that reduce birth rates or increase death rates.
Mechanisms such as: disease, predation, territoriality, competition for resources (See Figure 53.18).
C. Population Dynamics
All populations show some fluctuations in size. Such population fluctuations from year to year or place to place, called population dynamics, are influenced by many factors and affect other species.
While many populations fluctuate at unpredictable intervals, others undergo regular boom and bust cycles.
Immigration and emigration are particularly important when a number of local populations are linked, forming a metapopulation.
53.6 The human population is no longer growing exponentially but is still increasing rapidly
Explosive human population growth and massive consumption of resources by developed nations are the primary causes of environmental degradation.
A. The Global Human Population
The human population has been growing almost exponentially for centuries but cannot do so indefinitely, however. Figure 53.22 illustrates the explosive growth of human population over the last four centuries.
Since the industrial revolution, human population growth has been sustained by such factors as improved nutrition, medical care, and sanitation, which have lowered death rates.
The movement from high birth and death rates toward low birth and death rates, which tends to accompany industrialization and improved living conditions, is called the demographic transition.
Reduced family size is the key to the demographic transition.
The age structure of the population is a major factor in different growth rates of different countries, age structure is the relative number of individuals of each age in the population. Commonly graphed as "pyramids" like those in Figure 53.24.
Infant mortality, the number of infant deaths per 1,000 live births, and life expectancy at
birth, the predicted average length of life at birth, vary widely in different countries.
B. Global Carrying Capacity
Carrying capacity is difficult to estimate, and no one has ever accurately predicted Earth's carrying capacity, and thus whether or not it is overpopulated, or when it will be.
The ecological footprint concept summarizes the aggregate land and water area required by each person, city, or nation to produce all of the resources it consumes and to absorb all the water it generates.