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Chapter 55
Population Ecology
CHAPTER OUTLINE
How Do Ecologists Study Populations?
How Do Ecological Conditions Affect Life
Histories?
What Factors Influence Population Densities?
How Do Spatially Variable Environments
Influence Population Dynamics?
How Can We Manage Populations?
Reindeer game
In 1944, in the midst of World War II, the U.S. Coast Guard established a LORAN—a LOng Range Aids to Navigation—station on the island of St. Matthews, a tiny otherwise unoccupied bit of tundra more than 300 kilometers from the nearest village in Alaska. As an emergency food supply for the 19 men assigned to the island, the Coast Guard brought 29 reindeer (Rangifer tarandus) by barge and released them. The reindeer thrived on the thick, lush mat of lichen that covered the island. Other than the humans, the reindeer faced no risk from predators; the only other island occupants were arctic foxes, one species of vole, and a few ground-nesting birds. As the war progressed and priorities changed, the men were removed from the island, but the reindeer remained behind.
In 1957, David Klein, at the time a U.S. Fish and Wildlife biologist, visited the island with a field assistant; together they counted over 1,350 reindeer, most of which appeared fat and healthy. He also noticed a handful of seriously overgrazed lichen mats. He didn’t return to the island until 1963, when he and three colleagues hitched a ride with a Coast Guard cutter; by that time, the reindeer numbered more than 6000 and the island was covered with the tracks and droppings of reindeer. Packed in at a density of 47 per square mile, these reindeer were distinctly smaller than the ones sighted only six years earlier. Punishing storms, record low temperatures, and tremendous snowfalls on the island were reported during the winter of 1963-1964; by August 1965 Klein heard alarming accounts from Coast Guard personnel visiting the island of massive reindeer death. He managed to arrange a return visit in summer 1966 with two colleagues and found the entire island littered with reindeer skeletons. The three scientists could locate only 42 living reindeer, 41 of which were adult females; the lone male appeared to sport deformed antlers. In a remarkably short period of time, the reindeer population had declined by over 99%. Lichens had essentially disappeared from the island, replaced almost entirely with sedges and grasses on which reindeer cannot subsist. With no prospects of maintaining a healthy breeding population, reindeer disappeared entirely from the island by 1980.
Introducing large hooved mammals to small islands is an inherently risky enterprise, as the experience on St. Matthews Island so graphically illustrates. But such introductions don’t always end in disaster. Reindeer populations introduced to the sub- Antarctic island of South Georgia almost a century ago have persisted and appear to be stable
Why would populations of a particular species in one place explode and crash but in another seemingly similar place remain stable over time? Understanding how and why populations change in size is more than just an academic pursuit of interest only to ecologists. Studying how populations change over time is critical for understanding why some species become pests in some places and not in others, for managing sustainable harvests of economically important species, and for designing plans for conserving endangered species.
IN THIS CHAPTER we examine the ways that ecologists study populations and investigate how an organism’s habits and behavior affect the dynamics of population growth. We will identify factors that influence population densities and determine impacts of environmental variation on population dynamics. Finally, we will show how an understanding of population dynamics is crucial in managing populations of special interest or importance to humans.
55.1 How Do Ecologists Study Populations?
A population consists of the individuals of a species within a given area at a particular time. Although much insight into the ecology of any particular organism can be gained by studying individuals, populations are important units for study because groups of individuals belonging to the same species that interact in time and space have ecological characteristics that individuals do not. At any given moment, an individual organism occupies only one spot in space, and is a particular age and size. The members of a population, however, are distributed over space, and differ in age and size. Thus, populations have density (number of individuals per unit of area or volume). Density is a function of the interplay between processes that add individuals to the population--birth rates and immigration (movement into the population)-- and processes that reduce the number of individuals in the population—death rates and emigration (movement out of the population). The study of the birth, death, and movement rates that create population dynamics (changes in population structure and density) is known as demography.
Populations also have a characteristic age structure, or distribution of individuals across age categories, as well as dispersion pattern, or spatial distribution of individuals in the environment. Collectively, age distribution and dispersion pattern describe population structure. Ecologists study population structure because the age and spatial distributions of individuals influence the stability of a population and affect the ways that populations of one species interact with populations of other species.
A wide range of human activities involve managing − and therefore understanding − population dynamics. Well before ecology became a distinct biological discipline, people managed populations of many organisms. Growing crops and raising livestock, for example, explicitly increases the population sizes of domesticated plants and animals. Game wardens, park managers, and conservation biologists aim to maintain stable populations of wildlife, fish, and threatened or endangered species; and physicians, veterinarians, and exterminators work to reduce the population growth of species whose presence is undesirable or detrimental.
Ecologists use a variety of approaches to count and keep track of individuals
To study populations, ecologists need to determine the number of individuals in a given area, the spatial distribution of those individuals, the rate at which other individuals enter the population by birth or immigration, and the rate at which individuals leave via death or emigration. How these measurements are made depends a great deal on the nature of the organism under study. For example, populations of animals can be more challenging to measure than populations of long-lived trees, because animal can move around and, to avoid double counting, individuals must be identified. However, even though the individuals in a redwood forest may be standing still, there may be so many individuals in the population that counting every single individual would be logistically very difficult.
In some species, individuals are large enough and populations are small enough that investigators can actually identify all of the individuals and count them in a full census.
The free-ranging elephant population of Samburu and Buffalo Springs National Reserves, Kenya, exceeding 760 individuals, was monitored for 21 months, with each elephant identified individually, primarily by unique and distinctive earmarkings (Figure 55.1A) . Orcas (“killer whales”) can be identified by distinctive spotting patterns; coat color patterns have been used to count and monitor populations of feral cats.
As well as it may work for orcas and elephants, individual recognition is an impractical goal for most organisms. Animals and plants that are difficult to distinguish from one another need to be tagged in some way. No single form of marking works for all species. Plants don’t move so they can be marked by tags tied to their branches or by stakes in the ground nearby. Birds are marked by colored bands on their legs (Figure 55.1B), and butterflies and beetles with small dabs of colored paint in different patterns. Honey bees can be monitored by using fully automatic radio-frequency identification (RFID) technology—the same technology used for tracking supermarket purchases (Figure 55.1D), with a reader at the hive entrance registering movements of the marked bees. Small mammals can be marked by bleaching or dyeing their fur in strategic places.
Marking individuals is only one part of estimating the size of a population. If the population is small enough, all individuals can be counted, but for most organisms populations are too large or individuals too small, similar in appearance, or mobile to conduct a complete census. Thus, measuring population sizes often involves taking samples and using statistical methods to make an estimate.
Population densities can be estimated from samples
Ecologists usually measure the densities of organisms in terrestrial environments as the number of individuals per unit of area or the number per unit of volume (a per-volume measurement is used when organisms reside in soil, air, or water). In most field studies of animal populations, counting every individual is not a practicable option, so total population density is extrapolated from representative samples.
Estimating population densities is easiest for sedentary organisms. Investigators need only count the number of individuals in a sample of representative locations and extrapolate the counts to the entire area. Individuals may be counted within a particular marked and measured area, called a quadrat, sited randomly and repeatedly within a population. Another approach, often used to estimate the size of plant populations, is a linear transect—a survey along a line sited or drawn across the population (often designated with a string marked at regular intervals. Any individual population that touches the line is counted. By making repeated censuses, either within quadrats or along transects, investigators can make estimates of the size and composition of a population.
Counting mobile organisms is more difficult because individuals move into and out of census areas. Estimating the number of individuals in a population often involves capturing, marking, and then releasing a number of individuals. Later, after the marked individuals have had time to mix with the unmarked individuals in the population, another sample of individuals is taken. In its simplest form, the proportion of individuals in the new marked sample can be used to estimate the size of the population using the formula
N = M x C
R
where
N = Estimate of total number in the population
M = Number of individuals captured and marked on the first visit
C = Total number of animals captured on the return visit
R = Number of individuals captured on the first visit that are recaptured on the second visit
This mark-recapture method is based on the assumption that, with two samples, the likelihood of capturing marked and unmarked individuals is the same, meaning that we assume the proportion of marked individuals recaptured in a second visit is the same as the proportion of captured individuals in the total population.
C = R
N M
The mark-recapture method also assumes that the marked individuals randomly mix with the unmarked individuals after they are released and that marked and unmarked individuals are equally likely to survive and be captured on a subsequent sampling visit.
For some species, however, these assumptions do not always apply. Some animals learn to avoid traps or leave the sampling area and thus are less likely to be recaptured than unmarked individuals, and some become “trap-happy” (some mice, for example, re-enter live traps repeatedly in order to snack on the peanut butter bait). In some cases the act of marking may reduce the survival probability of the individual due to the stress of handling or to inadvertent alterations of appearance that male the marked individual more conspicuous to predators. Ecologists have developed statistical techniques to correct for these errors and improve the accuracy of population estimates.
Populations have age structure and dispersion patterns
In trying to understand how populations change in size, it isn’t enough just to count the number of individuals because not all individuals contribute equally to population growth. The age structure of a population—the distribution of individuals across all age groups—has a profound effect on population growth. Populations with a large proportion of young individuals have a greater potential to grow than populations dominated by older individuals beyond their peak reproductive years. For some species, reproduction may be the province of only a tiny fraction of the entire population for only a tiny fraction of the total life cycle. In some species of the tiny flies commonly called no-see-ums (ceratopogonids), male fertility drops precipitously only 8 hours after emergence from the pupa. For example, adults of the “one-hour midge” (Clunio maritimus) mate, lay eggs, and die within about an hour after two weeks of larval development. In contrast, some vertebrates are capable of reproducing for years. In one long-term study, the age structure of the population of elephants in Kidepo Valley National Park in Uganda underwent significant change from 1967-2000. Relative to 1967, the 2000 population is significantly skewed toward females, particularly those over 25 years of age.. Such skew, produced by years of differential mortality due to poaching for ivory and drought, indicates that the rate of population growth may be increasing. African elephants become fertile around the age of ten and the 2000 age structure, with the relatively larger percentage of female elephants of reproductive age, portends a population increase.
Dispersion patterns offer other important clues about population growth. Dispersion refers to the distribution of individuals in space within a population. Dispersion determines patterns of interaction among individuals and can have major impacts on population growth. For example, aggressive interactions and territoriality may space males of many species far apart, reducing the number of reproducing males in a given area. Knowing dispersion patterns is important in devising sampling protocols for estimating population sizes.
Ecologists recognize three basic dispersion patterns (Figure 55.3). A random pattern occurs when there is an equal probability of an individual occupying a point in space, when the location of an individual is in no way influenced by the presence of another. A regular pattern occurs when the presence of one individual reduces the probability of others occurring near that point. A clumped dispersion pattern occurs when the presence of one individual increases the probability that others will occur near that point in space.
Several statistical tests can be used to identify and characterize dispersion patterns. Keep in mind that dispersion patterns vary depending on the scale of measurement. The size of the sampling unit can profoundly influence the apparent distribution. Gall wasps on the leaves of chestnut trees are randomly distributed within each tree but clumped when counted on all trees combined in the entire population.