Biology 311: Chapter 53: Population Ecology: Reading Guide

What three variables form the life history of a species?

A life history entails three main variables: when reproduction begins (the age at first reproduction or age at maturity), how often the organism reproduces, and how many off spring are produced per reproductive episode.

In the following graph, explain why the logistic model predicts a sigmoid (S-shaped) growth curve when the population density is plotted over time. Hint: The critical part of this answer concerns why growth slows as N approaches K.

See page 1198 in your text for the labeled figure. when there is not only a breeding population of substantial size, but also lots of available space and other resources in the environment. The population growth rate decreases dra-matically as N approaches K. For a population's growth rate to decrease, the birth rate, b, must decrease, the death rate, m, must increase, or both. Factors affecting these rates include the presence of disease, predation, and limited amounts of food and other re-sources.

What are two examples of conditions that might lead to exponential population growth in natural populations?

The J-shaped curve of exponential growth is characteristic of some populations that are introduced into a new environment or whose numbers have been drastically reduced by a catastrophic event and are rebounding.

Population statistics are often summarized in a life table. Study Table 53.1 in your text to get an idea of the information that can be gleaned from one. For example, in what two age periods is the death rate highest? When is the reproductive rate highest?

The highest death rate is in the 8-9 and 9-10 age periods. The highest reproductive rate is in the 3-4 and 4-5 age periods.

Compare and contrast these two terms: density-independent regulation: density-dependent regulation:

A birth rate or death rate that does not change with pop-ulation density is said to be density independent. A death rate that rises as population density increases or a birth rate that falls as population density increases is said to be density dependent.

Is your biology class a cohort? Explain.

A cohort is a group of individuals of the same age. Most college-level biology classes may include a variety of ages and therefore are not a cohort while high school biology classes typically teach a particular age and are therefore a cohort.

Explain how two critical factors influence whether a species will evolve toward semelparity or iteroparity.

A current hypothesis suggests that there are two critical factors: the survival rate of the off spring and the likelihood that the adult will survive to reproduce again. Where the survival rate of offspring is low, typically in highly variable or unpredictable environments, the pre-dicttion is that semelparity will be favored. Adults are also less likely to survive in such environments, so producing large numbers of offspring should increase the probability that at least some of those offspring will survive. Iteroparity may be favored in more dependa-ble environments, where adults are more likely to survive to breed again and where com-petition for resources may be intense. In such cases, a few relatively large, well-provisioned offspring should have a better chance of surviving until they can reproduce.

If the carrying capacity (or K) is 1,000 and N is 10, the term (K - N)/K is large. Explain why a large value for (K - N)/K predicts growth close to the maximum rate of increase for this population.

A large value for (K - N)/K predicts growth close to the maximum rate of increase for this population because there is lots of available space and other resources in the environment. The population growth rate decreases dramatically as N approaches K.

Work through Figure 53.2 in your text to determine a population size using the Mark-Re-capture Method. Do the math to make sure you get the same answer as the text. Note and understand what the letters of the formula mean. Next, try the following problem.

A population ecologist wished to determine the size of a population of white-footed deer mice, Peromyscus leucopus, in a 1-hectare field. Her first trapping yielded 80 mice, all of which were marked with a dab of purple hair dye on the back of the neck. Two weeks later, the trapping was repeated. This time 75 mice were trapped, out of which 48 of the mice were marked. Using the formula N = mn/x, what is the population of mice in the field? (Answer is at the end of this Reading Guide chapter.) N = (80) (75)/ 48 or 125 mice/hectare

What determines a population?

A population is a group of individuals of a single species living in the same general area.

Refer to Figure 53.13 to explain the effect of offspring care on parental survival in kestrels.

A study of European kestrels demonstrated a survival cost to parents that care for a large number of young. The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents.

Survivorship curves show patterns of survival. In general terms, survivorship curves can be classified into three types. Using the following figure, label and explain the three idealized survivorship patterns.

Although diverse, survivorship curves can be classified into three general types (Figure 53.5). A Type I curve is flat at the start, reflecting low death rates during early and middle life, and then drops steeply as death rates increase among older age-groups. Many large mammals, including humans, produce few offspring but provide them with good care and 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 flattens out as death rates decline for those few individuals that survive the early period of die-off. 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 most marine invertebrates. An oyster, for example, may release millions of eggs, but most larvae hatched from fertilized eggs die from predation or other causes. Those few offspring that survive long enough to attach to a suitable substrate and begin growing a hard shell tend to 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 (see Figure 53.4) and some other rodents, various invertebrates, some lizards, and some annual plants. Many species fall somewhere between these basic types of survivorship or show more complex patterns. In birds, mortality is often high among the youngest individuals (as in a Type III curve) but is fairly constant among adults (as in a Type II curve). Some invertebrates, such as crabs, may show a "stair-stepped" curve, with brief periods of increased mortality during molts, followed by periods of lower mortality when their protective exoskeleton is hard.

Why do populations in nature rarely match the predictions of the logistic growth model? Use an example in your explanation.

Bacteria, Paramecium and yeasts often fit a logistic curve in the lab, because they are grown in a constant environment lacking predators and competing species, conditions that rarely occur in nature. The logistic model assumes that populations adjust instantaneously to growth and approach carrying capacity smoothly. In reality, there is often a delay before the negative effects of a population increase is realized. If food becomes limiting for a population, for instance, reproduction will decline eventually, but females may use their energy reserves to continue reproducing for a short time and can cause a population to overshoot its carrying capacity.

Explain how negative feedback plays an essential role in the unifying theme of regulation of populations. Does negative feedback play a role in both density-independent and density-dependent regulation?

Biology's unifying theme of feedback regulation (see Chapter 1) applies to population dynamics. Without some type of negative feedback between population density and the rates of birth and death, a population would never stop growing. Density regulation provides that feedback, halting population growth through mechanisms that reduce birth rates or increase death rates.

Explain the impact of immigration and emigration on population density. (To avoid confu-sion between these two terms, it might help to use this memory trick: immigration is the movement into a population, whereas emigration is the exiting of individuals from a pop-ulation.)

Density is not a static property but changes as individuals are added to or removed from a population (Figure 53.3). Additions occur through birth (which we define here to include all forms of reproduction) and immigration, the influx of new individuals from other areas. The factors that remove individuals from a population are death (mortality) and emigration, the movement of individuals out of a population and into other locations.

Look at Figures 53.21 and 53.22 in your text. If no population can grow indefinitely, how might density-dependent and density-independent factors affect human population growth?

Density-dependent factors such as disease and competition for resources will most likely affect human population growth. Density-independent factors such as natural disasters could cause catastrophic change in the human population.

What are six examples of limiting resources that can influence carrying capacity?

Energy, shelter, refuge from predators, nutrient availability, water, and suitable nesting sites can all be limiting factors.

Explain the importance of immigration and emigration in metapopulations.

Immigration and emigration are particularly important when a number of local populations are linked, forming a metapopulation. Local populations in a metapopulation can be thought of as occupying discrete patches of suitable habitat in a sea of otherwise unsuitable habitat. Such patches vary in size, quality, and isolation from other patches, factors that influence how many individuals move among the populations. Patches with many individuals can supply more emigrants to other patches. If one population becomes extinct, the patch it occupied can be recolonized by immigrants from another population. The metapopulation concept underscores the significance of immigration and emigration in the butterfly populations. It also helps ecologists understand population dynamics and gene flow in patchy habitats, providing a framework for the conservation of species living in a network of habitat fragments and reserves.

Explain the difference between semelparity (big-bang reproduction) and iteroparity (re-peated reproduction) as life history strategies.

In semelparity, organisms produce many offspring all at once to increase the probability that at least some of those offspring will survive. In contrast to semelparity is iteroparity (from the Latin iterare, "to repeat"), or repeated reproduction. In iteroparity, organisms produce relatively few but large offspring each time they reproduce, and they provision the offspring better.

Why do infant mortality and life expectancy vary so greatly between certain countries?

Infant mortality, the number of infant deaths per 1,000 live births, and life expectancy at birth, Although global life expectancy has been increasing since about 1950, it has recently dropped in a number of regions, including countries of the former Soviet Union and in sub Saharan Africa. In these regions, social upheaval, decaying infrastructure, and infectious diseases such as AIDS and tuberculosis are reducing life expectancy.

Can the world's population sustain an ecological footprint that is currently the average American footprint? Justify your response.

No. The ecological footprint concept summarizes the aggregate land and water area required by each person, city, or nation to produce all the resources it consumes and to absorb all the waste it generates. One way to estimate the ecological footprint of the entire human population is to add up all the ecologically productive land on the planet and divide by the population. This calculation yields approximately 2 hectares (ha) per person (1 ha = 2.47 acres). Reserving some land for parks and conservation means reducing this allotment to 1.7 ha per person—the benchmark for comparing actual ecological footprints. Anyone who consumes resources that require more than 1.7 ha to produce is said to be using an unsustainable share of Earth's resources. A typical ecological footprint for a person in the United States is about 8 gha.

In which population statistics do demographers have a particular interest?

Of particular interest to demographers are birth rates and death rates. A useful way to summarize some of the vital statistics of a population is to make a life table.

What does it mean for a population to be in exponential population growth?

Population increase under ideal conditions in a population whose members all have access to abundant food and are free to reproduce at their physiological capacity is called exponential population growth, also known as geometric population growth. Under these conditions, the per capita rate of increase may assume the maximum rate for the species. The equation for exponential population growth is: dN/dt = rN 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.

The end of this concept attempts to bring together the ideas of life histories and growth models. This is done with the introduction of two new terms: K-selection and r-selection. Explain the ideas behind the creation of these two terms.

Selection for traits that are sensitive to population density and are favored at high densities is known as K-selection, or density-dependent selection. In contrast, selection for traits that maximize reproductive success in uncrowded environments (low densities) is called r-se-lection, or density-independent selection. These names follow from the variables of the logistic equation. K-selection is said to operate in populations living at a density near the limit imposed by their resources (the carrying capacity, K), where competition among indi-viduals is stronger. Mature trees growing in an old-growth forest are an example of K-selected organisms. In contrast, r-selection is said to maximize r, the per capita rate of increase, and occurs in environments in which population densities are well below carrying capacity or individuals face little competition.

What is carrying capacity?

The J-shaped curve of exponential growth is characteristic of some populations that are introduced into a new environment or whose numbers have been drastically reduced by a catastrophic event and are rebounding.

What is the difference between density and dispersion?

The density of a population is the number of individuals per unit area or volume. Dispersion is the pattern of spacing among individuals within the boundaries of the population.

Summarize human population growth since 1650.

The exponential growth model in Figure 53.7 approximates the human population explosion over the last four centuries (Figure 53.21). Ours is a singular case; no other population of large animals has likely ever sustained so much growth for so long. The human population increased relatively slowly until about 1650, at which time approximately 500 million people inhabited Earth. Our population doubled to 1 billion within the next two centuries, doubled again to 2 billion by 1930, and doubled again by 1975 to more than 4 billion. The global population is now more than 7.6 billion people and is increasing by about 80 million each year. Currently the population grows by more than 200,000 people each day, the equivalent of adding a city the size of Amarillo, Texas. At this rate, it takes only about four years to add the equivalent of another United States to the world population. Population ecologists predict a population of about 9.8 billion people on Earth by the year 2050. Although the global population is still growing, the rate of growth did begin to slow during the 1960s (Figure 53.22). The annual rate of increase in the global population peaked at 2.2% in 1962; by 2009, it had declined to 1.2%. Current models project a continued decline in the annual growth rate to roughly 0.5% by 2050, a rate that would still add 45 million more people per year if the population climbs to a projected 9 billion. The reduction in growth rate over the past four decades shows that the human population has departed from true exponential population dynamics due to diseases, including AIDS, and to voluntary population control.

In the natural world, many species show survivorship curves that are combinations of the standard curves. How would an open nesting songbird's survivorship curve appear if it was Type III for the first year and then Type II for the rest of its life span? Sketch this curve on the survivorship curve graph in question 10.

The first part of this curve would fall sharply, like the blue line in the graph, then angle over like the black line to the far right corner of the graph.

Study the graph with Figure 53.19 in your text, then explain why the best current theory for the population cycling of snowshoe hare and lynx populations is predator-prey interaction.

The lynx population is dependent upon the hare population, as noted by the lag in the lynx population compared to the snowshoe hare population. Two hypotheses have been proposed to explain the data collected: the hares decline because of food shortage, or they are preyed upon and overexploited by other predators besides the lynx. Studies in dictate that the hares do not have a food shortage in a 10-year cycle, but other predators including coyotes, hawks, and owls prey upon the hares. When the hare population declines, there is less food for the lynx which are in competition with the other predators.

Explain the population dynamics resulting from both biotic and abiotic factors that account for the fluctuations in the moose population on Isle Royale over the last 50 years.

The moose population has experienced two major increases and collapses during the last 50 years. The first collapse was driven by a peak in the number of wolves on Isle Royale (density-dependent regulation), whereas the second collapse coincided with harsh winter weather (density-independent factor).

First, label the dispersion pattern shown by each population in the following figure. Second, and most important, what do the dispersion patterns tell us about the individuals in the population and their interactions?

The most common pattern of dispersion is clumped, in which individuals are aggregated in patches. Plants and fungi are often clumped where soil conditions and other environmental factors favor germination and growth. Mushrooms, for instance, may be clumped within and on top of a rotting log. Insects and salamanders may be clumped under the same log because of the higher humidity there. Clumping of animals may also be associated with mating behavior. Mayflies, which survive only a day or two as mating adults, often swarm in great numbers, a behavior that increases their chance of mating. Sea stars group together in tide pools, where food is readily available and where they can breed successfully (Figure 53.3a). Forming groups may also increase the effectiveness of preda-tion or defense; for example, a wolf pack is more likely than a single wolf to subdue a moose, and a flock of birds is more likely than a single bird to warn of a potential attack. A uniform, or evenly spaced, pattern of dispersion may result from direct interactions be-tween individuals in the population. 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 (Figure 53.3b). Uniform patterns are rarer than clumped patterns. In random dispersion (unpredictable spacing), the position of each individual in a population is independent of other individuals. The random pattern of dispersion occurs in the absence of strong attractions or repulsions among individuals or where key physical or chemical factors are relatively constant across the study area. Plants established by windblown seeds, such as dandelions, may be randomly distributed in a fairly uniform habitat (Figure 53.3c). Random patterns are not as common in nature as one might expect; most populations show at least a tendency toward a clumped distribution.

What is demographic transition? In demographic transition, which falls first, birth or 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. Death rates fall before birth rates, often by many years (150 years in Sweden).

What pieces of data are needed to mathematically determine population density?

The number of individuals and the unit area or volume are required to mathematically determine density.

In the following graph, explain why the line with the value of 1.0 shows a steeper slope that reaches exponential growth more quickly than does the line with the value of 0.5. On this graph, add a third line that approximates a population with an exponential value of 1.25.

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 53.7). Although the maximum rate of increase is constant, the population accumulates more new individuals per unit of time when it is large than when it is small; thus, the curves in Figure 53.7 get progressively steeper over time. This occurs because population growth depends on N as well as r and larger populations experience more births (and deaths) than small ones growing at the same per capita rate. It is also clear from Figure 53.7 that a population with a higher maximum rate of increase (dN/dt = 1.0N) will grow faster than one with a lower rate of in crease (dN/dt = 0.5N). The line for an r = 1.25 would be between the blue line and the y-axis, rising even steeper than the blue r = 1.0 value.

On what is the life history of an organism based?

The traits that affect an organism's schedule of reproduction and survival make up its life history.

What is the advantage to using per capita birth and death rates rather than just the raw numbers of births and deaths?

Using per capita birth rates allows for a comparison between populations; using just the number of births does not allow for comparisons. For example, a population of 10,000 would have a lot more births than one of 1,000 even if its per capita birth rate was much lower.

Study Figure 53.6 in your text, then decide which breeding female turtle laid the eggs in nest #74. Justify your response.

When the genetic profile of the eggshell sample #74 was compared to the breeding female #109, there was a match in the STR markers at 14 different loci, identifying the female that laid the eggs of that particular nest.

What are two ways a stable population with zero population growth can be achieved?

Zero population growth can be achieved with a high birth rate and a high death rate or a low birth rate and low death rate.

In the logistic population growth model, the per capita rate of increase approaches zero as the_______ is reached.

carrying capacity