Reindeer games
I
n 1944, in the midst of World War II, the U.S. Coast
together they counted more than 1,350 reindeer, most of
Guard established a LORAN (Long-Range Aids to Navi
which appeared fat and healthy. However, they also rio
gation) tracking station on the tiny island of St. Matthew
ticed areas of seriously overgrazed lichen mats. Klein did
in Alaska, an isolated and otherwise unoccupied patch of
not return to the island until 1963, when he and three col
tundra more than 300 km from the nearest village. As an
leagues hitched a ride with a Coast Guard cutter. By that
emergency food supply for the 19 men assigned to the
time, there were more than 6,000 reindeer packed in at a
island, the Coast Guard brought in 29 reindeer (Rangifer
density of 47 per square mile. The island was covered with
tarandus) by barge and released them.
reindeer tracks and droppings, and the animals were dis
The rei ndeer thrived on the thick, lush mat of lichens
tinctly smaller than the ones sighted 6 years earlier.
that covered the 128-square-mile island . Other than the
The winter of 1963-1964 brought punishing storms,
men, the island had no reindeer predators; its only other
record low temperatures, and tremendous snowfalls to St.
animal occupants were Arctic foxes, one species of vole,
Matthew Island. In August 1965, Coast Guard personnel re
and a few ground-nesting birds. As the war wound down
ported massive reindeer deaths. Klein and two colleagues
and the men left the island, the reindeer were left behind in
arranged a return visit in the summer of 1966, at which
an environment of plentiful food and no natural predators.
time they found the island littered with reindeer skeletons.
In 1957, David Klein, at the time a U.s. Fish and Wild life biologist, visited St. Matthew with a field assistant;
The scientists could locate only 42 living reindeer, 41 of which were adult females; the lone male appeared to have deformed antlers. In a remarkably short period, the reindeer population had de clined by over 99 percent. Lichens had essentially disappeared from the island, replaced almost entirely by sedges and grasses, on which reindeer cannot sub sist. By 1980, the reindeer had entirely disappeared from the island. Introducing large hoofed mammqls to small islands is an inherently risky enterprise, as the experience on St. Matthew graphically illustrates. But
Reindeer Pause Part of the 8t. Matthew reindeer herd is seen In this photograph taken in 1963, shortly before a particularly severe winter destroyed most of this ,isolaF ed population. The herd had grown expo-' nentially for almost 40 years since being introduced to the island as a food source for soldiers during World War II.
CHAPTER OUTLINE 55.1 55.2
How Do Ecologists Study Populations?
55.3 55.4
What Factors Limit Population Densities?
55.5
How Can Populations Be Managed Scientifically?
How Do Environmental Conditions Affect Life Histories? How Does Habitat Variation Affect Population Dynamics?
55.1 How Do Ecologists Study Populations?
Predators and Population Cycles Wolves are the principal non-human predators of reindeer, moose, and other large grazing mammals in northern forests and tundra. There were no wolves on St. Matthew Island, a fact largely responsible for the reindeer population's ability to grow exponentially.
such introductions do not always end in disaster. Rein deer 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 simi lar place remain stable over time? Understanding how and why populations change in size is more than an aca demic pursuit. Ecologists study how populations change over time because that knowledge is critical for under standing why some species become pests in some places and not in others, for managing sustainable har vests of economically important species, and for design ing plans for conserving endangered species.
IN THIS CHAPTER we will examine how ecologists study populations and investigate how reproductive capac ity and environmental resources affect the dynamics of pop ulation growth. We will identify factors that limit population
Well before ecology became a distinct biological discipline, peo ple engaged in population management. Whenever we grow crops or raise livestock, we are explicitly increasing populations of domesticated plants and animals. Pest control strategies aim to reduce populations of organisms whose presence we consider W1-, desirable. Game wardens, park managers, and conservation biol ogists aim to maintain stable populations of fish, wildlife, and threatened or endangered species. All of these activities require an understanding of population dynamics: the patterns and processes of change in populations. The study of population dy namics also allows us to understand the changes in populations we make inadvertently in the course of other human activities as when the Coast Guard introduced reindeer to St. Matthew. A population consists of the individuals of a species that in teract with one another within a given area at a particular time. Populations are important units for study because groups of in dividuals that interact in time and space have ecological char acteristics that individuals do not. At any given moment, an in dividual organism occupies only one point in space and is a particular age and size. The members of a population, hO'wever, are distributed over space and vary in age and size. Population density is the number of individuals per unit of area or vollUDe. Density is a property of all populations and is a function of the processes that add individuals to the popula tion (births and irrunigration, or movement of individuals into the population) and the processes that reduce the number of in dividuals in the population (deaths and emigration, or move ment of individuals out of the population). Populations also have a characteristic age structure, or dis tribution of individuals across age categories, and a character istic dispersion pattern, or spatial distribution of individuals in the environment. These characteristics, which are constantly changing due to births, deaths, and movement, influence the stability of populations and affect the ways in which popula tions of one species interact with populations of other species. Thus, to study populations, ecologists need to count the indi viduals in a given area, determine their ages, and calculate the rates at which individuals enter and leave the population. The study of these processes is known as demography.
densities and determine the effects of environmental vari ation on population dynamics. Finally, we will show how an understanding of population dynamics is applied to man aging populations of importance to humans.
Ecologists use a variety of approaches to count and track individuals How individuals are counted depends on the nature of the or ganism under study. Populations of animals, for example, can