Population Sampling
CHAPTER 8
Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.
8.1 Organisms May Be Unitary or Modular
A population is considered to be a group of individuals, but what constitutes an individual? For most of us, defining an individual would seem to be no problem. We are individuals, and so are dogs, cats, spiders, insects, fish, and so on throughout much of the animal kingdom. What defines us as individuals is our unitary nature. Form, development, growth, and longevity of unitary organisms are predictable and determinate from conception on. The zygote, formed through sexual reproduction, grows into a genetically unique organism (see Chapter 5). There is no question about recognizing an individual. This simplistic view of an individual breaks down, however, when the organism is modular rather than unitary.
In modular organisms, the zygote (the genetic individual) develops into a unit of construction, a module, which then produces further, similar modules. Most plants are modular in that they develop by branching, repeated units of structure. The fundamental unit of aboveground construction is the leaf with its axillary bud and associated internode of the stem. As the bud develops and grows, it produces further leaves, each bearing buds in their axils. The plant grows by accumulating these modules (Figure 8.1). The growth of the root system is also modular, growing through the process of branching and providing a continuous connection between above- and belowground modules. There are, however, a variety of growth forms produced by modular growth in plants; some plants spread their modules laterally as well as vertically. For example, some species produce specialized stems that either grow above the surface of the substrate, referred to as stolons (Figure 8.2a), or below the surface of the substrate, referred to as rhizomes (Figure 8.2b). These plants can produce new vertical stems and associated root systems from these laterally growing stolons or rhizomes. Similarly, some plants sprout new stems from the surface roots, which are called suckers (Figure 8.2c). In these laterally growing forms, the new modules may cover a considerable area and appear to be individuals. The plant produced by sexual reproduction, thus arising from a zygote, is a genetic individual, or genet. Modules produced asexually by the genet are ramets. These ramets are clones—genetically identical modules—and are collectively referred to as a clonal colony. The ramets may remain physically linked or they may separate. The connections between the modules may die or rot away, so that the products of the original zygote become physiologically independent units. These ramets can produce seeds (through sexual reproduction) and their own lateral extensions or ramets (asexual reproduction).
Whether living independently or physically linked to the original individual, all ramets are part of the same genetic individual. Thus, by producing ramets, the genet can cover a relatively large area and considerably extend its life. Some modules die, others live, and new ones appear.
Plants are the most obvious group of modular organisms; however, modular organisms include animals, such as corals, sponges, and bryozoans (Figure 8.3), as well as many protists and fungi.
Technically, to study populations of modular organisms, we must recognize two levels of population structure: the module (ramet) and the individual (genet). As such, characterizing the population structure of a modular species presents special problems. For practical purposes, ramets are often counted as—and function as—individual members of the population. Modern genetic techniques, however, have allowed ecologists to determine the structure of these populations in terms of genets and ramets, quantifying the patterns of genetic diversity (see this chapter, Field Studies: Filipe Alberto).
Field Studies Filipe Alberto
Department of Biological Sciences, University of Wisconsin–Milwaukee
Numerous plant species reproduce both sexually and asexually. For example, many grass species form dense mats of ramets through the growth of rhizomes or stolons, yet also produce new offspring by flowering and through seed production (new genets). For plant ecologists, this dual strategy of reproduction presents a critical problem in understanding the structure of plant populations. In theory, a field of grass occupied by a species exhibiting both reproductive strategies (sexual and asexual) could consist of only a single genetic individual (genet) and all of the apparent “individual” grass plants could be genetically identical ramets produced by a single parent plant. In contrast, the field could consist of a diversity of genets, each with an associated clonal colony of ramets that intermingle with ramets from adjacent clones.
Although impossible only a few decades ago, the development of new genetic technologies now enables ecologists to analyze the genetic structure of a population, and one of the “new generation” of ecologists that is engaged in this new field of molecular ecology is Filipe Alberto of the University of Wisconsin. The focus of Alberto’s research is to understand the population structure of marine plants and algae that inhabit the shallow waters of coastal environments. One of the species that has been the focus of Alberto’s work is the seagrass Cymodocea nodosa. C. nodosa is common throughout the Mediterranean and Atlantic coast of Africa, where it forms meadows in the shallow nearshore waters (see Figure 18.8). It colonizes disturbed areas where it plays an important role in the stabilization of sediments. The species is dioecious (having separate male and female individuals) reproducing both sexually as well as vegetatively (asexually) through the extension of rhizomes. It exhibits fast clonal growth, with extension rates up to 2 m per year.
In one study, Alberto and colleagues examined the genetic structure of a population of C. nodosa inhabiting Cádiz Bay along the southeastern coast of Spain. Within the bay, the researchers established a grid of 20 × 38 m with a grid spacing of 2 m, yielding a total of 220 sampling units (see Figure 1). For each sampling unit the researchers collected three to five shoots belonging to the same rhizome (genet) for genetic analysis. This sampling scheme allowed them to analyze the spatial pattern of relatedness (genetic similarity) among shoots at any point within the grid.
Relatedness (similarity in genotypes) of sampled shoots was determined through the use of microsatellites, tandem repeats of one to six nucleotides found along a strand of DNA (see Section 5.2). Tandem repeats occur in DNA when a pattern of two or more nucleotides is repeated and the repetitions are directly adjacent to each other (tandem); for example: ACACACACAC. The repeated sequence is often simple, consisting of two (as with the preceding example of AC), three, or four nucleotides (di-, tri-, and tetranucleotide repeats, respectively). Microsatellites are ideal for population studies because, first, they are typically abundant in all species, and secondly, they are highly polymorphic—that is, for a given microsatellite locus on the DNA, there are typically many different forms of the microsatellite—different alleles. Each sequence with a specific number of repeated nucleotides is designated as an allele. So, a locus with six repeats is one allele (ACACACACACAC or AC6), whereas the same locus in another individual that contains nine repeats is another (different) allele. For the analysis of relatedness in the C. nodosa population, Alberto used nine different microsatellites that he had identified in a previous genetic study of the species.
The researchers identified 41 different alleles (across the nine microsatellites) and a total of 83 different genotypes in the sample grid. The 83 different genotypes represent 83 genetically unique individuals that were produced through sexual reproduction. The number of different genotypes also corresponds to the number of clones (where clone is defined as a colony of ramets originating from the same genet) in the population. A map of the genotypes on the sampling grid is presented in Figure 1. The result shows an extremely skewed distribution of clone sizes (Figure 2), with a median clone dimension of 3.6 m. Despite the genetic richness of the meadow (83 different genotypes) and the resulting presence of many unique smaller clones, the meadow (sample grid) is spatially dominated by a few large clones.
To determine how the two strategies of reproduction (sexual and asexual) influence the genetic structure of the population across the landscape, Alberto undertook a spatial analysis of relatedness using the data presented in Figure 1. With a plant species that can reproduce asexually through the lateral extension of rhizomes, one would assume that there is a high probability that adjacent shoots are ramets from the same genet (they belong to the same clone). But how would the probability of two shoots belonging to the same clone change for greater and greater distance between them? To answer this question, Alberto calculated the probability of clonal identity for the different distance classes (recall that the sample points are on a grid of 2 m). The probability of clonal identity F r is defined as the probability that a randomly chosen pair of shoots separated by a distance r belong to the same clone (genetically identical).
The calculated values of F r for each distance class (0–2 m; 2–4; 4–6; 6–8; … ) is presented in Figure 3. The probability of clonal identity (F r) declines with increasing distance, from approximately 25 percent in the first distance class (0–2 m) to reach zero (meaning no pairs shared the same genotype) at a distance of 25–30 m. This distance (25–30 m) corresponds to the dimensions of the largest clones found in the populations (see Figure 3). The results indicate that in this population of seagrass, sampling shoots at an interval of 30 m or more will assure that the samples represent unique genotypes—genetically unique individuals—rather than ramets.
8.2 The Distribution of a Population Defines Its Spatial Location
The distribution of a population describes its spatial location, the area over which it occurs. Distribution is based on the presence and absence of individuals. If we assume that each red dot in Figure 8.4 represents an individual’s position within a population on the landscape, we can draw a line (shown in blue) defining the population distribution—a spatial boundary within which all individuals in the population reside. When the defined area encompasses all the individuals of a species, the distribution describes the population’s geographic range.
Population distribution is influenced by a number of factors. We introduced the concept of habitat—the place or environment where an organism lives—in Chapter 7 (Section 7.14). Each species has a range of abiotic environmental and resource conditions under which it can survive, grow, and reproduce. The primary factor influencing the distribution of a population is the occurrence of suitable environmental and resource conditions—habitat suitability. Red maple (Acer rubrum), for example, is the most widespread of all deciduous trees of eastern North America (Figure 8.5). The northern limit of its geographic range coincides with the area in southeastern Canada where minimum winter temperatures drop to −40°C. Its southern limit is the Gulf Coast and southern Florida. Dry conditions halt its westward range. Within this geographic range, the tree grows under a wide variety of soil types, soil moisture, acidity, and elevations—from wooded swamps to dry ridges. Thus, the red maple exhibits high tolerance to temperature and other environmental conditions. In turn, this high tolerance allows a widespread geographic range.
A species with a geographically widespread distribution, such as red maple, is referred to as ubiquitous. In contrast, a species with a distribution that is restricted to a particular locality or localized habitat is referred to as endemic. Many endemic species have specialized habitat requirements. For example, the shale-barren evening primrose (Oenothera argillicola; Figure 8.6) is a member of the evening primrose family (Onagraceae). This species is adapted to hot, shale-barren environments that form when certain types of shale form outcrops on south- to southwest-facing slopes of the Allegheny Mountains. Most members of this group of plants are listed as endangered or threatened because they are found in these specific habitats only from southern Pennsylvania through West Virginia to southern Virginia, where shale barrens are formed.
The geographic distribution of red maple in Figure 8.5 illustrates another important factor limiting the distribution of a population: geographic barriers. Although this tree species occupies several islands south of mainland Florida, the southern and eastern limits of its geographic range correspond to the Gulf of Mexico and Atlantic coastline. Although environmental conditions may be suitable for establishment and growth in other geographic regions of the world (such as Europe and Asia), the red maple is restricted in its ability to colonize those areas. Other barriers to dispersal (movement of individuals), such as mountain ranges or extensive areas of unsuitable habitat, may likewise restrict the spread and therefore the geographic range of a species (see Chapter 5, Section 5.8 and Figure 5.19 for an example of Plethodon salamanders). Later, we will explore another factor that can restrict the distribution of a population: interactions, such as competition and predation, with other species (Part Four).
Within the geographic range of a population, individuals are not distributed equally. Individuals occupy only those areas that can meet their requirements (suitable habitat). Because organisms respond to a variety of environmental factors, they can inhabit only those locations where all factors fall within their range of tolerance (see Chapter 7, Section 7.14). As a result, we can describe the distribution of a population at various spatial scales. For example, in Figure 8.7, the distribution of the moss Tetraphis pellucida is described at several different spatial scales, ranging from its geographic distribution at a global scale to the location of individuals within a single clump occupying the stump of a dead conifer tree. This species of moss can grow only in areas in which the temperature, humidity, and pH are suitable; different factors may be limiting at different spatial scales. At the continental scale, the suitability of climate (temperature and humidity) is the dominant factor. Within a particular area, distribution of the moss is limited to microclimates along stream banks, where conifer trees are abundant. Within a particular locality, it occupies the stumps of conifer trees where the pH is sufficiently acidic.
As a result of environmental heterogeneity, most populations are divided into subpopulations, each occupying suitable habitat patches of various shapes and sizes within the larger landscape of unsuitable habitat. In the example of Tetraphis presented in Figure 8.7, the distribution of individuals within a region is limited to stream banks, where temperature and humidity are within its range of tolerance, and stands of conifers are present to provide a substrate for growth. As a result, the population is divided into a group of spatially discrete local subpopulations, (Figure 8.8). Ecologists refer to the collective of local subpopulations as a metapopulation, a term coined in 1970 by the population ecologist Richard Levins of Harvard University. Although spatially separated, these local populations are connected through the movement of individuals among them (Section 8.7). A more detailed discussion of metapopulations is presented in Chapter 19 (Landscape Dynamics).
Ecologists typically study these local, or subpopulations, rather than the entire population of a species over its geographic range. For this reason, it is important when referring to a population to define explicitly its boundaries (spatial extent). For example, an ecological study might refer to the population of red maple trees in the Three Ridges Wilderness Area of the George Washington–Jefferson National Forest in Virginia or to the population of Tetraphis pellucida along the Oswagatchie River in in the Adirondack Mountains of New York.
8.3 Abundance Reflects Population Density and Distribution
Whereas distribution defines the spatial extent of a population, abundance defines its size—the number of individuals in the population. In Figure 8.4, the population abundance is the total number of red dots (individuals) within the blue line that defines the population distribution.
Abundance is a function of two factors: (1) the population density and (2) the area over which the population is distributed. Population density is the number of individuals per unit area (per square kilometer [km], hectare [ha], or square meter [m]), or per unit volume (per liter or m3). By placing a grid over the population distribution shown in Figure 8.4, as is done in Figure 8.9, we can calculate the density for any given grid cell by counting the number of red dots that fall within its boundary. Density measured simply as the number of individuals per unit area is referred to as crude density. The trouble with this measure is that individuals are typically not equally numerous over the geographic range of the population (see Section 8.2). Individuals do not occupy all the available space within the population’s distribution because not all areas are suitable. As a result, density can vary widely from location to location (as in Figure 8.9).
How individuals are distributed within a population—in other words, their spatial position relative to each other—has an important bearing on density. Individuals of a population may be distributed randomly, uniformly, or in clumps (aggregated; Figure 8.10). Individuals may be distributed randomly if each individual’s position is independent of those of the others. In contrast, individuals distributed uniformly are more or less evenly spaced. A uniform distribution usually results from some form of negative interaction among individuals, such as competition, which functions to maintain some minimum distance among members of the population (see Chapter 11). Uniform distributions are common in animal populations where individuals defend an area for their own exclusive use (territoriality) or in plant populations where severe competition exists for belowground resources such as water or nutrients (Figure 8.11; see also Figures 11.17 and 11.19).
The most common spatial distribution is clumped, in which individuals occur in groups. Clumping results from a variety of factors. For example, suitable habitat or other resources may be distributed as patches on the larger landscape. Some species form social groups, such as fish that move in schools or birds in flocks (see Figure 8.10). Plants that reproduce asexually form clumps, as ramets extend outward from the parent plant (see Figure 8.1). The distribution of humans is clumped because of social behavior, economics, and geography, reinforced by the growing development of urban areas during the past century. In the example presented in Figure 8.9, the individuals within the population are clumped; as a result, the density varies widely between grid cells.
As with geographic distribution (see Section 8.2), the spatial distribution of individuals within the population can also be described at multiple spatial scales. For example, the distribution of the shrub Euclea divinorum, found in the savanna ecosystems of Southern Africa, is clumped (Figure 8.11a). The clumps of Euclea are associated with the canopy cover of another plant that occupies the savanna: trees of the genus Acacia (Figure 8.11b). The clumps, however, are uniformly spaced, reflecting the uniform distribution of Acacia trees on the landscape. The regular distribution of Acacia trees is a function of competition among neighboring individuals for water (see Section 11.10). In the example of Tetraphis presented in Figure 8.7, the spatial distribution of individuals is clumped at two different spatial scales. Populations are concentrated in long bands or strips along the stream banks, leaving the rest of the area unoccupied. Within these patches, individuals are further clumped in patches corresponding to the distribution of conifer stumps.
To account for patchiness, ecologists often refer to ecological density, which is the number of individuals per unit of available living space. For example, in a study of bobwhite quail (Colinus virginianus) in Wisconsin, biologists expressed density as the number of birds per mile of hedgerow (the birds’ preferred habitat), rather than as birds per hectare (Figure 8.12). Ecological densities are rarely estimated because determining what portion of a habitat represents living space is typically a difficult undertaking.
8.4 Determining Density Requires Sampling
Population size (abundance) is a function of population density and the area that is occupied (geographic distribution). In other words, population size = density × area. But how is density determined? When both the distribution (spatial extent) and abundance are small—as in the case of many rare or endangered species—a complete count may be possible. Likewise, in some habitats that are unusually open, such as antelope living on an open plain or waterfowl concentrated in a marsh, density may be determined by a direct count of all individuals. In most cases, however, density must be estimated by sampling the population.
A method of sampling used widely in the study of populations of plants and sessile (attached) animals involves quadrats, or sampling units (Figure 8.13). Researchers divide the area of study into subunits, in which they count animals or plants of concern in a prescribed manner, usually counting individuals in only a subset or sample of the subunits (as in Figure 8.9). From these data, they determine the mean density of the units sampled. Multiplying the mean value by the total area provides an estimate of population size (abundance). The accuracy of estimates of density derived from population sampling can be influenced by the manner in which individuals are spatially distributed within the population (Section 8.3). The estimate of density can also be influenced by the choice of boundaries or sample units. If a population is clumped—concentrated into small areas—and the population density is described in terms of individuals per square kilometer, the average number of individuals per unit area alone does not adequately represent the spatial variation in density that occurs within the population (Figure 8.14). In this case, it is important to report an estimate of variation or provide a confidence interval for the estimate of density. In cases where clumping is a result of habitat heterogeneity (habitat is clumped), ecologists may choose to use the index of ecological density for the specific areas (habitats) in which the species is found (for example, stream banks in Figure 8.7).
For mobile populations, animal ecologists must use other sampling methods. Capturing, marking, and recapturing individuals within a population—known generally as mark-recapture—is the most widely used technique to estimate animal populations (Figure 8.15). There are many variations of this technique, and entire books are devoted to various methods of application and statistical analysis. Nevertheless, the basic concept is simple.
Capture-recapture or mark-recapture methods are based on trapping, marking, and releasing a known number of marked animals (M) into the population (N). After giving the marked individuals an appropriate period of time to once again mix with the rest of the population, some individuals are again captured from the population (n). Some of the individuals caught in this second period will be carrying marks (recaptured, R), and others will not. If we assume that the ratio of marked to sampled individuals in the second sample (n/R) represents the ratio for the entire population (N/M), we can compute an estimate of the population using the following relationship:
NM=nrNM=nr
The only variable that we do not know in this relationship is N. We can solve for N by rearranging the equation as follows:
N=nMRN=nMR
For example, suppose that in sampling a population of rabbits, a biologist captures and tags 39 rabbits from the population. After their release, the ratio of the number of rabbits in the entire population (N) to the number of tagged or marked rabbits (M) is N/M. During the second sample period, the biologist captures 15 tagged rabbits (R) and 19 unmarked ones—a total of 34 (n). The estimate of population size, N, is calculated as
N=nMR=(34×39)15=88N=nMR=(34×39)15=88
This simplest method, the single mark–single recapture, is known as the Lincoln–Petersen index of relative population size.
As with any method of population estimation, the accuracy of the Lincoln–Peterson index depends on a number of assumptions. First, the method assumes that the sampling is random, that is, each individual in the population has an equal probability of being captured. Secondly, the marked individuals must distribute themselves randomly throughout the population so that the second sample will accurately represent the population. Last, the ratio of marked and unmarked individuals must not change between the sampling periods. This is especially important if the method of marking individuals influences their survival, as in the case of highly visible marks or tags that increase their visibility to predators.
For work with most animals, ecologists find that a measure of relative density or abundance is sufficient. Methods involve observations relating to the presence of organisms rather than to direct counts of individuals. Techniques include counts of vocalizations, such as recording the number of drumming ruffed grouse heard along a trail, counts of animal scat seen along a length of road traveled, or counts of animal tracks, such as may be left by a number of opossums crossing a certain dusty road. If these observations have some relatively constant relationship to total population size, the data can be converted to the number of individuals seen per kilometer or heard per hour. Such counts, called indices of abundance, cannot function alone as estimates of actual density. However, a series of such index figures collected from the same area over a period of years depicts trends in abundance. Counts obtained from different areas during the same year provide a comparison of abundance between different habitats. Most population data on birds and mammals are based on indices of relative abundance rather than on direct counts.
8.5 Measures of Population Structure Include Age, Developmental Stage, and Size
Abundance describes the number of individuals in the population but provides no information on their characteristics—that is, how individuals within the population may differ from one another. Unless each generation reproduces and dies in a single season, not overlapping the next generation (such as annual plants and many insects), the population will have an age structure: the number or proportion of individuals in different age classes. Because reproduction is restricted to certain age classes and mortality is most prominent in others, the relative proportions of each age group bear on how quickly or slowly populations grow (see Chapter 9).
Populations can be divided into three ecologically important age classes or stages: prereproductive, reproductive, and postreproductive. We might divide humans into young people, working adults, and senior citizens. How long individuals remain in each stage depends largely on the organism’s life history (see Chapter 10). Among annual species, the length of the prereproductive stage has little influence on the rate of population growth (see Chapter 9). In organisms with variable generation times, the length of the prereproductive period has a pronounced effect on the population’s rate of growth. The populations of short-lived organisms often increase rapidly, with a short span between generations. Populations of long-lived organisms, such as elephants and whales, increase slowly and have a long span between generations.
Determining a population’s age structure requires some means of obtaining the ages of its members. For humans, this task is not a problem, but it is for wild populations. Age data for wild animals can be obtained in several ways, and the method varies with the species (Figure 8.16). The most accurate, but most difficult, method is to mark young individuals in a population and follow their survival through time (see discussion of life table, Chapter 9). This method requires a large number of marked individuals and a lot of time. For this reason, biologists may use other, less-accurate methods. These methods include examining a representative sample of individual carcasses to determine their ages at death. A biologist might look for the wear and replacement of teeth in deer and other ungulates, growth rings in the cementum of the teeth of carnivores and ungulates, or annual growth rings in the horns of mountain sheep. Among birds, observations of plumage changes and wear in both living and dead individuals can separate juveniles from subadults (in some species) and adults. Aging of fish is most commonly accomplished by counting rings deposited annually (annuli) on hard parts including scales, otoliths (ear bones), and spines.
Studying the age structure of plant populations can prove even more difficult. The major difficulty lies in determining the age of plants and whether the plants are genetic individuals (genets) or ramets (Section 8.1).
The approximate ages of trees in which growth is seasonal can be determined by counting annual growth rings (Figure 8.17), a procedure called dendrochronology. But given the time and expense necessary to collect and analyze samples of tree rings, forest ecologists have tried to employ size (diameter of the trunk at breast height, or dbh) as an indicator of age on the assumption that diameter increases with age—the greater the diameter, the older the tree. Such assumptions, it was discovered, were valid for dominant canopy trees; but with their growth suppressed by lack of light, moisture, or nutrients, smaller understory trees, seedlings, and saplings add little to their diameters. Although their diameters suggest youth, small trees are often the same age as large individuals in the canopy.
Attempts to age nonwoody plants have met with less success. The most accurate method of determining the age structure of short-lived herbaceous plants is to mark individual seedlings and follow them through their lifetimes. The results of recent studies, however, suggest that annual growth rings form in the root tissues (secondary root xylem) of many perennial herbaceous plants and can be used successfully in the analysis of age structure in this group of plants. However, the use of size or developmental stage classes is often more appropriate than age in describing the structure of plant (and some animal) populations. As we shall see in Chapter 9, size or developmental stage often provides a better indicator of patterns of mortality and reproduction necessary for predicting patterns of population dynamics.
Once the age structure of a population has been determined, it can be represented graphically in the form of an age pyramid. Age pyramids (Figure 8.18) are snapshots of the age structure of a population at some period in time, providing a picture of the relative sizes of different age groups in the population. As we shall see, the age structure of a population is a product of the age-specific patterns of mortality and reproduction (Chapter 9). In many plant populations, the distribution of age classes is often highly skewed (Figure 8.19). In forests, for example, dominant overstory trees can inhibit the establishment of seedlings and growth and survival of juvenile trees. One or two age classes dominate the site until they die or are removed, allowing trees in young age classes access to resources such as light, water, and nutrients so they can grow and develop.
8.6 Sex Ratios in Populations May Shift with Age
Populations of sexually reproducing organisms in theory tend toward a 1:1 sex ratio (the proportion of males to females). The primary sex ratio (the ratio at conception) also tends to be 1:1. This statement may not be universally true, and it is, of course, difficult to confirm.
In most mammalian populations, including humans, the secondary sex ratio (the ratio at birth) is often weighted toward males, but the population shifts toward females in the older age groups. Generally, males have a shorter life span than females do. The shorter life expectancy of males can be a result of both physiological and behavioral factors. For example, in many animal species, rivalries among males occur for dominant positions in social hierarchies or for the acquisition of mates (see Section 10.11). Among birds, males tend to outnumber females because of increased mortality of nesting females, which are more susceptible to predation and attack (Figure 8.21).
8.7 Individuals Move within the Population
At some stage in their lives, most organisms are mobile to some degree. The movement of individuals directly influences their local density. The movement of individuals in space is referred to as dispersal, although the term dispersal most often refers to the more specific movement of individuals away from one another. When individuals move out of a subpopulation, it is referred to as emigration. When an individual moves from another location into a subpopulation, it is called immigration. The movement of individuals among subpopulations within the larger geographic distribution is a key process in the dynamics of metapopulations and in maintaining the flow of genes between these subpopulations (see Chapters 5 and 19).
Many organisms, especially plants, depend on passive means of dispersal involving gravity, wind, water, and animals. The distance these organisms travel depends on the agents of dispersal. Seeds of most plants fall near the parent, and their density falls off quickly with distance (Figure 8.22). Heavier seeds, such as the acorns of oaks (Quercus spp.), have a much shorter dispersal range than do the lighter wind-carried seeds of maples (Acer spp.), birch (Betula spp.), milkweed (Asclepiadaceae), and dandelions (Taxaxacum officinale). Some plants, such as cherries and viburnums (Viburnum spp.), depend on active carriers such as particular birds and mammals to disperse their seeds by eating the fruits and carrying the seeds to some distant point. These seeds pass through the animals’ digestive tract and are deposited in their feces. Other plants possess seeds armed with spines and hooks that catch on the fur of mammals, the feathers of birds, and the clothing of humans. In the example of the clumped distribution of E. divinorum shrubs (see Figure 8.11), birds disperse seeds of this species. The birds feed on the fruits and deposit the seeds in their feces as they perch atop the Acacia trees. In this way, the seeds are dispersed across the landscape, and the clumped distribution of the E. divinorum is associated with the use of Acacia trees as bird perches.
For mobile animals, dispersal is active, but many others depend on a passive means of transport, such as wind and moving water. Wind carries the young of some species of spiders, larval gypsy moths, and cysts of brine shrimp (Artemia salina). In streams, the larval forms of some invertebrates disperse downstream in the current to suitable habitats. In the oceans, the dispersal of many organisms is tied to the movement of currents and tides.
Dispersal among mobile animals may involve young and adults, males and females; there is no hard-and-fast rule about who disperses. The major dispersers among birds are usually the young. Among rodents, such as deer mice (Peromyscus maniculatus) and meadow voles (Microtus pennsylvanicus), subadult males and females make up most of the dispersing individuals. Crowding, temperature change, quality and abundance of food, and photoperiod all have been implicated in stimulating dispersal in various animal species (Chapter 11, Section 11.8).
Often, the dispersing individuals are seeking vacant habitat to occupy. As a result, the distance they travel depends partly on the density of surrounding subpopulations and the availability of suitable unoccupied areas. The dispersal of individuals is a key feature in the dynamics of metapopulations (see Figure 8.8), where colonization involves the movement of individuals from occupied habitat patches (existing local populations) to unoccupied habitat patches to form new local populations. The role of dispersal and colonization in metapopulation dynamics is discussed further in Chapter 19 (Section 19.7).
Unlike the one-way movement of animals in the processes of emigration and immigration, migration is a round trip. The repeated return trips may be daily or seasonal. Zooplankton in the oceans, for example, move down to lower depths by day and move up to the surface by night. Their movement appears to be related to a number of factors including predator avoidance. Bats leave their daytime roosting places in caves and trees, travel to their feeding grounds, and return by daybreak. Other migrations are seasonal, either short range or long range. Earthworms annually make a vertical migration deeper into the soil to spend the winter below the freezing depths and move back to the upper soil when it warms in spring. Elk (Cervus canadensis) move down from their high mountain summer ranges to lowland winter ranges. On a larger scale, caribou (Rangifer tarandus) move from the summer calving range in the arctic tundra to the boreal forests for the winter, where lichens are their major food source. Gray whales (Eschrichtius robustus) move down from the food-rich arctic waters in summer to their warm wintering waters of the California coast, where they give birth to young (Figure 8.23). Similarly, humpback whales (Megaptera novaeangliae) migrate from northern oceans to the central Pacific off the Hawaiian Islands. Perhaps the most familiar of all are long-range and short-range migrations of waterfowl, shorebirds, and neotropical migrants in spring to their nesting grounds and in fall to their wintering grounds (see Chapter 11, Field Studies: T. Scott Sillett).
Another type of migration involves only one return trip. Such migrations occur among Pacific salmon (Oncorhynchus spp.) that spawn in freshwater streams. The young hatch and grow in the headwaters of freshwater coastal streams and rivers and travel downstream and out to sea, where they reach sexual maturity. At this stage, they return to the home stream to spawn (reproduce) and then die.
8.8 Population Distribution and Density Change in Both Time and Space
Dispersal has the effect of shifting the spatial distribution of individuals and consequently the localized patterns of population density. Emigration may cause density in some areas to decline, whereas immigration into other areas increases the density of subpopulations or even establishes new subpopulations in habitats that were previously unoccupied.
In some instances, dispersal can result in the shift or expansion of a species’ geographic range. The role of dispersal in range expansion is particularly evident in populations that have been introduced to a region where they did not previously exist. A wide variety of species have been introduced, either intentionally or unintentionally, into regions outside their geographic distribution. As the initial population becomes established, individuals disperse into areas of suitable habitat, expanding their geographic distribution as the population grows. A map showing the spread of the gypsy moth (Lymantria dispar) in the eastern United States after its introduction in 1869 is shown in Figure 8.24. The story of the introduction of this species is presented in the following section (see this chapter, Ecological Issues & Applications).
In other cases, the range expansion of a population has been associated with temporal changes in environmental conditions, shifting the spatial distribution of suitable habitats. Such is the case of the shift in the distribution of tree populations in eastern North America as climate has changed during the past 20,000 years (see Section 18.9, Figure 18.25). Examples of predicted changes in the distribution of plant and animal populations resulting from future human-induced changes in Earth’s climate are discussed later in Chapter 27 . Although the movement of individuals within the population results in a changing pattern of distribution and density through time, the primary factors driving the dynamics of population abundance are the demographic processes of birth and death. The processes of birth and death, and the resulting changes in population structure, are the focus of our attention in the following chapter.
Ecological Issues & Applications Humans Aid in the Dispersal of Many Species, Expanding Their Geographic Range
Dispersal is a key feature of the life histories of all species, and a diversity of mechanisms have evolved to allow plant and animal species to move across the landscape and seascape. In plants, seeds and spores can be dispersed by wind, water, or through active dispersal by animals (see Section 15.14). In animals, the dispersal of fertilized eggs, particularly in aquatic environments, can result in the dispersal of offspring across significant distances. But dispersal typically involves the movement—either active or passive—of individuals, both juvenile and adult. In recent centuries, however, a new source of long-distance dispersal has led to the redistribution of species at a global scale: dispersal by humans.
Humans are increasingly moving about the world. As they do so, they may either accidentally or intentionally introduce plants and animals to places where they have never occurred (outside their geographic range). Although many species fail to survive in their new environments, others flourish. Freed from the constraints of their native competitors, predators and parasites, they successfully establish themselves and spread. These nonnative (nonindigenous) plants and animals are referred to as invasive species .
Sometimes these introductions are harmless, but often the introduced organisms negatively affect native species and ecosystems. In the past few centuries, many plants and animals, especially insects, have been introduced accidentally by accompanying imported agricultural and forest products. The seeds of weed species are unintentionally included in shipments of imported crop seeds or on the bodies of domestic animals. Or seed-carrying soil from other countries is often loaded onto ships as ballast and then dumped in another country in exchange for cargo. Major forest insect pests such as the Asian longhorned beetle (Anoplophors galbripennis) are hitchhikers on wooden shipping containers and pallets.
Humans have also introduced nonnative plants intentionally for ornamental and agricultural purposes. Most of these introduced plants do not become established and reproduce, but some do. On the North American continent, the ornamental perennial herb purple loosestrife (Lythrum salicaria; Figure 8.25a), originally introduced from Europe in the mid-1800s, has eliminated native wetland plants to the detriment of wetland wildlife. The Australian paperbark tree (Melaleuca quinquenervia; Figure 8.25b), introduced as an ornamental plant in Florida, is displacing cypress, sawgrass, and other native species in the Florida Everglades, drawing down water and fostering more frequent or intense fires. The most notorious plant invader in the United States is cheatgrass (Bromus tectorum; Figure 8.25c), a winter annual accidentally introduced from Europe into Colorado in the 1800s. It arrived in the form of packing material and possibly crop seeds. It spread explosively across overgrazed rangeland and winter wheat fields in the Pacific Northwest and the Intermountain Region. By 1930 it became the dominant grass, replacing native vegetation. Cheatgrass is highly flammable, and densely growing populations provide ample fuel that increases fire intensity and often decreases the time intervals between fires (fire frequency). One of the most widely spread invasive plants in North America is kudzu (Pueraria montana), a species of vine native to Asia. This plant was originally introduced to the United States as an ornamental vine at the Philadelphia Centennial Exposition of 1876. By the early part of the 20th century, kudzu was being enthusiastically promoted as a fodder crop, and rooted cuttings were sold to farmers through the mail. In the 1930s and 1940s, kudzu was propagated and promoted by the Soil Conservation Service as a means of holding soil on the swiftly eroding gullies of the deforested southern landscape, especially in the Piedmont regions of Alabama, Georgia, and Mississippi. By the 1950s, however, kudzu was recognized as a pest and removed from the list of species acceptable for use under the Agricultural Conservation Program, and in 1998 it was listed by the United States Congress as a Federal Noxious Weed. Although it spreads slowly, kudzu completely covers all other vegetation, blanketing trees with a dense canopy through which little light can penetrate (Figure 8.26). Estimates of kudzu infestation in the southeastern United States vary greatly, from as low as 2 million to as high as 7 million acres.
The most damaging introduced insect pest of the eastern United States hardwood forests is the gypsy moth (Lymantria dispar). The gypsy moth is found mainly in the temperate regions of the world, including central and southern Europe, northern Africa, central and southern Asia, and Japan. Leopold Trouvelot, a French astronomer with an interest in insects, originally introduced the species into Medford, Massachusetts in 1869. As part of an effort to begin a commercial silk industry, Trouvelot wanted to develop a strain of silk moth that was resistant to disease. However, several gypsy moth caterpillars escaped from Trouvelot’s home and established themselves in the surrounding areas. Some 20 years later, the first outbreak of gypsy moths occurred, and despite all control efforts since that time, the gypsy moth has persisted and extended its range (see Figure 8.24). In the United States, the gypsy moth has rapidly moved north to Canada, west to Wisconsin, and south to North Carolina. Gypsy moth caterpillars defoliate millions of acres of trees annually in the United States (Figure 8.27). In the forests of eastern North America, annual losses to European gypsy moths are estimated at $868 million, and the Asian strain that has invaded the Pacific Northwest has already necessitated a $20 million eradication campaign.
The problem that invasive species present is not restricted to terrestrial environments. More than 139 nonindigenous aquatic species that affect native plant and animal species have invaded the Great Lakes by way of global shipping. Most notorious is the zebra mussel (Dreissena polymorpha; Figure 8.28) native to the lakes of southern Russia. The species was introduced from the ballast of ships traversing the St. Lawrence Seaway. Since it first appeared in 1988, the zebra mussel has spread to most eastern river systems (Figure 8.29). In addition to their impact on wildlife, zebra mussels colonize water intake pipes, severely restricting the water flow to power plants or other municipal or private facilities.
The San Francisco Bay Area is occupied by 96 nonnative invertebrates, from sponges to crustaceans. Exotic fish, introduced purposefully or accidentally, have been responsible for 68 percent of fish extinctions in North America during the past 100 years and for the population decline of 70 percent of the fish species listed as endangered.
Summary
Unitary and Modular Organisms 8.1
A population is a group of individuals of the same species living in a defined area. Populations are characterized by distribution, abundance, density, and age structure. Most animal populations are made up of unitary individuals with a definitive growth form and longevity. In most plant populations, however, organisms are modular. These plant populations may consist of sexually produced parent plants and asexually produced stems arising from roots. A similar population structure occurs in animal species that exhibit modular growth.
Distribution 8.2
The distribution of a population describes its spatial location, or the area over which it occurs. The distribution of a population is influenced by the occurrence of suitable environmental conditions. Within the geographic range of a population, individuals are not distributed equally throughout the area. Therefore, the distribution of individuals within the population can be described as a range of different spatial scales.
Individuals within a population are distributed in space. If the spacing of each individual is independent of the others, then the individuals are distributed randomly; if they are evenly distributed, with a similar distance among individuals, it is a uniform distribution. In most cases, individuals are grouped together in a clumped or aggregated distribution.
Abundance 8.3
Abundance is defined as the number of individuals in a population. Abundance is a function of two factors: (1) the population density and (2) the area over which the population is distributed. Population density is the number of individuals per unit area or volume. Because landscapes are not homogeneous, not all of the area is suitable habitat. The number of organisms in available living space is the true or ecological density.
Sampling Populations 8.4
Determination of density and dispersion requires careful sampling and appropriate statistical analysis of the data. For sessile organisms, researchers often use sample plots. For mobile organisms, researchers use capture-recapture techniques or determine relative abundance using indicators of animal presence, such as tracks or feces.
Age, Stage, and Size Structure 8.5
The number or proportion of individuals within each age class defines the age structure of a population. Individuals making up the population are often divided into three ecological periods: prereproductive, reproductive, and postreproductive. Populations can also be characterized by the number of individuals in defined classes of size or stage of development.
Sex Ratios 8.6
Sexually reproducing populations have a sex ratio that tends to be 1:1 at conception and birth but often shifts as a function of sex-related differences in mortality.
Dispersal 8.7
At some stage of their life cycles, most individuals are mobile. For some organisms, such as plants, dispersal is passive and dependent on various dispersal mechanisms. For mobile organisms, dispersal can occur for a variety of reasons, including the search for mates and unoccupied habitat. For some species, dispersal is a systematic process of movement between areas in a process called migration.
Population Dynamics 8.8
Dispersal has the effect of shifting the spatial distribution of individuals and as a result the localized patterns of population density. Although the movement of individuals within the population results in a changing pattern of distribution and density through time, the primary factors driving the dynamics of population abundance are the demographic processes of birth and death.
Invasive Species Ecological Issues & Applications
Humans have either accidentally or intentionally introduced plant and animal species to places outside their geographic range. Sometimes these introductions are harmless, but often the introduced organisms negatively affect the populations of native species and ecosystems.
