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SECTION III Population Ecology 9 Population Distribution and Abundance

A population of saguaro cactus, Carnegiea gigantea, at Saguaro National Park near Tucson, Arizona. The natural distribution of the 9.4 Population density declines with saguaro cactus is restricted to the Sonoran Desert, extending from central Arizona through northwestern Mexico. Because saguaro cactus increasing organism size. 212 are very sensitive to freezing, the northern limit and highest elevations Concept 9.4 Review 213 occupied by the species appear limited by low temperatures. Applications: Rarity and Vulnerability to Extinction 214 Summary 216 CHAPTER CONCEPTS Key Terms 217 Review Questions 217

9.1 Environment limits the geographic distribution of species. 200 Concept 9.1 Review 204 LEARNING OUTCOMES After studying this section you should be able to do the following: 9.2 On small scales, individuals within populations are distributed in patterns 9.1 Define population, density, and abundance. that may be random, regular, or 9.2 Compare the characteristics of gray whale, monarch clumped. 204 butterfly, and Monterey pine populations. Concept 9.2 Review 208 9.3 Outline some of the reasons ecologists study populations. 9.3 On large scales, individuals within a population are clumped. 208 he distributions and dynamics of populations vary Investigating the Evidence 9: widely among species. While some populations are Clumped, Random, and Regular T small and have highly restricted distributions, other Distributions 209 populations number in the millions and may range over vast areas of the planet. Standing on a headland in central California Concept 9.3 Review 211 overlooking the Pacific Ocean, a small class of students spots a group of gray whales, Eschrichtius robustus, rising to the

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Chapter 9 Population Distribution and Abundance 199

from as far away as the Rocky Mountains of southern Canada. As the students watch the whales, the male monarch butterflies pursue and mate with the female monarch butterflies. After mating, the males die, while the females begin a migration that leads inland and north. The females stop to lay eggs on milkweeds they encounter along the way and eventually die; however, their offspring continue the migration. Monarch caterpillars grow quickly on their diet of milkweed and then transform to a pupa or chrysalis. The monarch butterflies that emerge from the chrysalises (or chrysalides) mate and, like the previous generation, fly northward and inland. By moving far- ther north and inland each generation, some of the monarch butterflies eventually reach the Rocky Mountains of southern Canada, far from where their ancestors fluttered around the (a) group of students on the pine-covered coastal headland. Then, as the autumn days grow shorter, the monarch butterflies begin their long flight back to the coastal grove of pines. This autumn generation, which numbers in the millions, flies southwest to its wintering grounds on the coast of central and southern California. Some of the monarchs might fly over 3,000 km. The monarch butterflies that survive the trip to the pine grove overwinter, hanging from particular roost trees in the thousands. They mate in the following spring and start the cycle all over again. Gray whales and monarch butterflies, as different as they may appear, lead parallel lives. The Monterey pines, Pinus radi- ata, covering the headland where the monarch butterflies over- winter and by which the gray whales pass twice each year are quite different. The Monterey pine population does not migrate (b) each generation and has a highly restricted distribution. The cur- rent natural range of the Monterey pine is limited to a few sites Figure 9.1 (a) A young gray whale, Eschrichtius robustus. During on the coast of central and northern California and to two islands their annual migration, gray whales migrate from subtropical waters off off the coast of western Mexico. These scattered populations are Baja California to the Arctic and back again, passing along the coast of California as they do so. (b) Along that same coast, monarch but- the remnants of a large, continuous population that extended for terflies, Danaus plexippus, winter in huge numbers, some having flown over 800 km along the California coast during the cooler cli- thousands of kilometers from the Rocky Mountains to reach the trees mate of the last glacial period. This history of the Monterey pine where they roost in winter. In contrast, the entire natural population of underscores a very important fact about species distributions: the Monterey pine, Pinus radiata, is restricted to five small areas along they are highly dynamic, especially over long periods of time. the California coast. With these three examples, we begin to consider the ecol- ogy of populations. Ecologists usually define a population surface and spouting water as they swim northward ( fig. 9.1a ). as a group of individuals of a single species inhabiting a spe- The whales are rounding the point of land on their way to cific area. A population of plants or animals might occupy a feeding grounds off the coasts of Alaska and Siberia. This mountaintop, a river basin, a coastal marsh, or an island, all particular group is made up of females and calves. The calves areas defined by natural boundaries. Just as often, the popu- were born during the previous winter along the coast of Baja lations studied by biologists occupy artificially defined areas California, the gray whale’s wintering grounds. Over the course such as a country, county, or national park. The areas inhab- of the spring, the entire population of over 20,000 gray whales ited by populations range in size from the few cubic centime- will round this same headland on their way to the Bering and ters occupied by the bacteria in a rotting apple to the millions Chukchi seas. Gray whales travel from one end of their range to of square kilometers occupied by a population of migratory the other twice each year, a distance of about 18,000 km. Home whales. A population studied by ecologists may consist of a to the gray whale encompasses a swath of seacoast extending highly localized group of individuals representing a fraction from southern Baja California to the coast of northeast Asia. of the total population of a species, or it may consist of all the The grove of pine trees on the headland where the individuals of a species across its entire range. students stand gazing at the whales is winter home to another Ecologists study populations for many reasons. Popu- long-distance traveler: monarch butterflies, Danaus plexippus lation studies hold the key to saving endangered species, ( fig. 9.1b ). The lazy flying of the bright orange and black mon- controlling pest populations, and managing fish and game arch butterflies gives no hint of their capacity to migrate. Some populations. They also offer clues to understanding and con- of the butterflies flew to the grove of pines the previous autumn trolling disease epidemics. Finally, the greatest environmental

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200 Section III Population Ecology

challenge to biological diversity and the integrity of the entire writings, Grinnell’s ideas of the niche centered around the biosphere is at its heart a population problem—the growth of influences of the physical environment, while Elton’s earliest the human population. concept included biological interactions as well as abiotic fac- All populations share several characteristics. The first is tors. However their thinking and emphasis may have differed, their d i s t r i b u t i o n . The distribution of a population includes the it is clear that the views of these two researchers had much size, shape, and location of the area it occupies. A population in common and that our present concept of the niche rests also has a characteristic pattern of spacing of the individuals squarely on their pioneering work. within it. It is also characterized by the number of individuals We can now point to a single paper authored by G. Evelyn within it and their d e n s i t y , which is the number of individuals Hutchinson (1957) as the agent that crystallized the niche con- per unit area. Additional characteristics of populations—their cept. In this seminal paper titled simply “Concluding Remarks,” age distributions, birth and death rates, immigration and emi- Hutchinson defined the niche as an n-dimensional hypervolume, gration rates, and rates of growth—are the subject of chapters where n equals the number of environmental factors important 10 and 11. In chapter 9 we focus on two population character- to the survival and reproduction by a species. Hutchinson called istics: distribution and a b u n d a n c e , the total number of indi- this hypervolume, which specifies the values of the n e n v i r o n - viduals, or biomass, of a species in a specified area. mental factors permitting a species to survive and reproduce, as the f u n d a m e n t a l n i c h e of the species. The fundamental niche defines the physical conditions under which a species might 9.1 Distribution Limits live, in the absence of interactions with other species. However, LEARNING OUTCOMES Hutchinson recognized that interactions such as competition may After studying this section you should be able to do the following: restrict the environments in which a species may live. He referred to these more restricted conditions as the r e a l i z e d n i c h e , which is 9.4 Define niche and distinguish between the fundamen- the actual niche of a species whose distribution is limited by biotic tal niche and the realized niche. interactions such as competition, predation, disease, and parasit- 9.5 Compare the perspectives of Grinnell, Elton, and ism. I n a s i n g l e w o r d , niche captures most of what we discussed Hutchinson on the nature of the niche. in section II and now turn to in section III. In this section, we 9.6 Discuss the factors limiting the distributions of plants consider how environment affects the growth, survival, reproduc- and animals on geographic and local spatial scales. tion, distribution, and abundance of species, a particularly timely topic as we face the ecological consequences of global warming. Environment limits the geographic distribution of species. A major theme in chapters 5, 6, and 7 is that populations have evolved physiological, anatomical, and behavioral character- Kangaroo Distributions and Climate istics that compensate for environmental variation. Organisms The Macropodidae includes the kangaroos and wallabies, compensate for temporal and spatial variation in the environ- which are some of the best known of the Australian animals. ment by regulating body temperature and water content and However, this group of large-footed mammals includes many by foraging in ways that maintain energy intake at relatively less-familiar species, including rat kangaroos and tree kan- high levels. However, there are limits on how much organisms garoos. While some species of macropods can be found in can compensate for environmental variation. nearly every part of Australia, no single species ranges across While there are few environments on earth without life, no the entire continent. All are confined to a limited number of single species can tolerate the full range of earth’s environments. climatic zones and biomes. For each species, some environments are too warm, too cold, too G. Caughley and his colleagues (1987) found a close saline, or unsuitable in other ways. As we saw in chapter 7, organ- relationship between climate and the distributions of the three isms take in energy at a limited rate. It appears that at some point, largest kangaroos in Australia (fig. 9.2). The eastern grey kan- the metabolic costs of compensating for environmental variation garoo, Macropus giganteus, is confined to the eastern third may take up too much of an organism’s energy budget. Partly of the continent. This portion of Australia includes several because of these energy constraints, the physical environment biomes (see chapter 2). Temperate forest grows in the south- places limits on the distributions of populations. The environ- east and tropical forests in the north. Mountains, with their mental limits of a species are related to its n i c h e . T h e w o r d niche varied climates, occupy the central part of the eastern grey has been in use a long time. Its earliest and most basic mean- kangaroo’s range (see figs. 2.13, 2.28, and 2.37). The climatic ing was that of a recessed place in a wall where one could set factor that distinguishes these varied biomes is little seasonal or display items. For about a century, however, ecologists have variation in precipitation or dominance by summer precipita- given a broader meaning to the word. To the ecologist, the niche tion. The western grey kangaroo, M. fuliginosus, lives mainly summarizes the environmental factors that influence the growth, in the southern and western regions of Australia, which survival, and reproduction of a species. In other words, a spe- coincides largely with the distribution of the Mediterranean cies’ niche consists of all the factors necessary for its existence— woodland and shrubland biome in Australia. The climati- approximately when, where, and how a species makes its living. cally distinctive feature of this biome is a predominance of The niche concept was developed independently by winter rainfall (see fig. 2.22). Meanwhile, the red kangaroo, Joseph Grinnell (1917, 1924) and Charles Elton (1927), who M. rufus, wanders the arid and semiarid interior of Austra- used the term niche in slightly different ways. In his early lia, areas dominated by savanna and desert (see figs. 2.16 and

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Chapter 9 Population Distribution and Abundance 201

M. fuliginosus lives in The distribution of the tiger beetle Macropus giganteus lives southern Australia, where C. longilabris, across North America in eastern Australia, where winter rainfall dominates. suggests that it is confined to cool, precipitation varies little moist habitats. seasonally or falls mainly in summer. In the far north, C. longilabris lives throughout the boreal forests of North America.

Macropus giganteus M. fuliginosus

M. rufus lives in central and western Australia, where conditions are hot and dry. South of boreal forest, C. longilabris is confined to high mountain forests and meadows.

Figure 9.3 A tiger beetle, Cicindela longilabris, confined to cool M. rufus environments. Physiological studies conducted on populations indicated by yellow dots (data from Schultz, Quinlan, and Hadley 1992). Figure 9.2 Climate and the distributions of three kangaroo species (data from Caughley et al. 1987). on hot black beaches in New Zealand. In chapter 6, we com- pared the water loss rates of tiger beetles from desert grass- 2.19). Of the three species of large kangaroos, the red kanga- lands and riparian habitats in Arizona. Here, we consider the roo occupies the hottest and driest areas. distribution of a tiger beetle that inhabits the cold end of the The distributions of these three large kangaroo species range of environments occupied by tiger beetles. cover most of Australia. However, as you can see in figure 9.2, The tiger beetle Cicindela longilabris lives at higher none of these species lives in the northernmost region of Aus- latitudes and higher elevations than just about any other spe- tralia. Caughley and his colleagues explain that these northern cies of tiger beetle in North America. In the north, C. longi- areas are probably too hot for the eastern grey kangaroo, too labris is distributed from the Yukon Territory in northwestern wet for the red kangaroo, and too hot in summer and too dry in Canada to the maritime provinces of eastern Canada ( fig. 9.3 ). winter for the western grey kangaroo. However, they are also This northern band of beetle populations coincides with the careful to point out that these limited distributions may not be distribution of northern temperate forest and boreal forest in determined by climate directly. Instead, they suggest that cli- North America (see figs. 2.28 and 2.31). C. longilabris also mate often influences species distributions through factors such lives as far south as Arizona and New Mexico. However, these as food production, water supply, and habitat. Climate also southern populations are confined to high mountains, where affects the incidence of parasites, pathogens, and competitors. C. longilabris is associated with montane coniferous forests. Regardless of how the influences of climate are played As we saw in chapter 2, these high mountains have a climate out, the relationship between climate and the distributions of similar to that of boreal forest (see fig. 2.38). species can be stable over long periods of time. The distri- Ecologists suggest that during the last glacial period butions of the eastern grey, western grey, and red kangaroos C. longilabris lived far south of its present range limits. Then have been stable for at least a century. In the next example, we with climatic warming and the retreat of the glaciers, the tiger discuss a species of beetle that appears to have maintained a beetles followed their preferred climate northward and up in stable association with climate for 10,000 to 100,000 years. elevation into the mountains of western North America. As a consequence, the beetles in the southern part of this species’ range live in isolated mountaintop populations. This hypothesis A Tiger Beetle of Cold Climates is supported by the fossil records of many beetle species. Tiger beetles have entered our discussions several times. In Intrigued by the distribution and history of C. longilabris, chapter 5, we saw how one species regulates body temperatures Thomas Schultz, Michael Quinlan, and Neil Hadley (1992) set

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202 Section III Population Ecology

out to study the environmental physiology of widely separated eastward (Ehleringer and Clark 1988). Encelia californica, populations of the species. Populations separated for many the species with the least pubescent leaves, occupies a narrow thousands of years may have been exposed to significantly coastal zone that extends from southern California to north- different environmental regimes. If so, natural selection could ern Baja California (fig. 9.5). Inland, E. californica i s r e p l a c e d have produced significant physiological differences among by E. actoni, which has leaves that are slightly more pubes- populations. The researchers compared the physiological cent. Still farther to the east, E. actoni is in turn replaced by characteristics of beetles from populations of C. longilabris E. f r u t e s c e n s a n d E. farinosa. from Maine, Wisconsin, Colorado, and northern Arizona. These geographic limits to these species’ distributions Their measurements included water loss rates, metabolic correspond to variations in temperature and precipitation. rates, and body temperature preferences. The coastal environments where E. californica lives are all Schultz and his colleagues found that the metabolic rates relatively cool. However, average annual precipitation dif- of C. longilabris are higher and its preferred temperatures fers a great deal across the distribution of this species. Annual lower than those of most other tiger beetle species that have precipitation ranges from about 100 mm in the southern part been studied. These differences support the hypothesis that C. longilabris is adapted to the cool climates of boreal and C. longilabris living in This is virtually identical to montane forests. In addition, the researchers found that none the northern regions of the preferred temperature of of their measurements differed significantly among popula- Maine and Wisconsin C. longilabris living in the tions of C. longilabris. Figure 9.4 illustrates the remarkable have a preferred body southern Rocky Mountains of similarity in preferred body temperature shown by forag- temperature of 348C. Colorado and Arizona. ing C. longilabris from populations separated by as much as 3,000 km and, perhaps, by 10,000 years of history. These 40 results support the generalization that the physical environ- C) ment limits the distributions of species. It also suggests that 8 those limits may be stable for long periods of time. 30 Now, let’s consider how the physical environment may limit the distribution of plants. Our example is drawn from the 20 arid and semiarid regions of the American Southwest.

Distributions of Plants Along a Preferred temperature ( 10 Moisture-Temperature Gradient

In chapter 5, we discussed the influence of pubescence on 0 leaf temperature in plants of the genus Encelia. Variation Maine Wisconsin Colorado Arizona in leaf pubescence among Encelia species appears to cor- Figure 9.4 Uniform temperature preference across the extensive respond directly to the distributions of these species along geographic range of the tiger beetle, Cicindela longilabris (data from a moisture-temperature gradient from the California coast Schultz, Quinlan, and Hadley 1992).

E. californica is confined to a narrow E. actoni lives farther inland, E. farinosa and E. frutescens live still zone along the coast of California and where conditions are drier and farther inland in areas that are much Baja California, which is cool and moist warmer than the areas hotter. The geographic distributions of in the north and cool and dry in the south. inhabited by E. californica. these two species overlap a great deal.

E. californica E. actoni E. farinosa E. frutescens Figure 9.5 The distributions of four Encelia species in southwestern North America (data from Ehleringer and Clark 1988).

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Chapter 9 Population Distribution and Abundance 203

of its distribution to well over 400 mm in the northern part. Nonpubescent leaves of By comparison, E. actoni occupies environments that are E. frutescens absorb only slightly warmer but considerably drier. The rainfall in approximately 80% of areas occupied by E. frutescens and E. farinosa is similar to incident photosynthetically active radiation. the amount that falls in the areas occupied by E. actoni and E. californica. However, the environments of E. frutescens 100 and E. farinosa are much hotter. Variation in leaf pubescence does not correspond entirely 80 to the macroclimates inhabited by Encelia species. The leaves of E. frutescens are nearly as free of pubescence as the 60 Pubescent leaves of coastal species, E. californica. H o w e v e r , E. frutescens grows E. farinosa absorb side by side with E. farinosa in some of the hottest deserts in less than 40%. 40 the world. Because they are sparsely pubescent, the leaves of E. frutescens absorb a great deal more radiant energy than

Light absorbance (per cm) 20 the leaves of E. farinosa (f i g . 9.6). Under similar condi- tions, however, leaf temperatures of the two species are nearly identical. How does E. frutescens avoid overheating? 0 The leaves do not overheat because they transpire at a high 400 500 600 700 800 rate and are evaporatively cooled as a consequence. Wavelength (nm) Evaporative cooling solves one ecological puzzle but Figure 9.6 Light absorption by leaves of Encelia frutescens and appears to create another. Remember that these two shrubs live E. farinosa (data from Ehleringer and Clark 1988). in some of the hottest and driest deserts in the world. Where does E. frutescens get enough water to evaporatively cool Distributions of Barnacles Along its leaves? Although the distributions of E. frutescens a n d E. farinosa overlap a great deal on a geographic scale, these two an Intertidal Exposure Gradient species occupy distinctive microenvironments. As shown in The marine intertidal zone presents a steep gradient of physi- f i g u r e 9.7, E. farinosa grows mainly on upland slopes, while cal conditions from the shore seaward. As we saw in chapter 3, E. frutescens is largely confined to ephemeral stream channels, the organisms high in the intertidal zone are exposed by vir- or desert washes. Along washes, runoff infiltrates into the deep tually every tide while the organisms that live at lower levels soils increasing the availability of soil moisture to E. frutescens. in the intertidal zone are exposed by the lowest tides only. This example reminds us of a principle that we first considered Exposure to air differs at different levels within the inter- in chapter 5: organisms living in the same macroclimate may, tidal zone. Organisms that live in the intertidal zone have because of slight differences in local distribution, experience evolved different degrees of resistance to drying, a major substantially different microclimates. This is certainly true of factor contributing to zonation among intertidal organisms the two barnacle species we consider in the following example. (see fig. 3.16).

Despite low transpiration The rate of transpiration rates, pubescent leaves of by E. frutescens is E. farinosa remain sufficient so that its relatively cool because leaves evaporatively cool. they are highly reflective. H2O

E. farinosa grows E. farinosa mainly on slope habitats in shallow H O 2 H2O H2O soils that store E. frutescens limited water.

E. frutescens can maintain higher transpiration rates because it exploits the greater water available in deep soils along washes.

Figure 9.7 Temperature regulation and distributions of Encelia farinosa and E. frutescens across microenvironments.

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204 Section III Population Ecology

Balanus balanoides larvae settle Chthamalus stellatus larvae settle than Chthamalus ( fig. 9.9 ). Meanwhile, Balanus in the lower throughout intertidal zone but in middle and upper intertidal intertidal zone showed normal rates of mortality. Of the two survive to adulthood mainly in zones but survive to adulthood species, Balanus appears to be more vulnerable to desicca- middle to lower zones. mainly in upper intertidal zone. tion. Higher rates of desiccation may exclude this species of barnacle from the upper intertidal zone. Balanus Chthamalus Vulnerability to dessication, however, does not completely Larvae Adults Larvae Adult explain the pattern of intertidal zonation shown by Balanus Mean high spring tide and Chthamalus. What excludes Chthamalus from the lower intertidal zone? Though the larvae of this barnacle settle in the Mean high neap tide lower intertidal zone, the adults rarely survive there. Connell explored this question by transplanting adult Chthamalus to the lower intertidal zone and found that transplanted adults Mean tide level survive in the lower intertidal zone very well. If the physi- cal environment does not exclude Chthamalus f r o m t h e l o w e r intertidal zone, what does? It turns out that this species is excluded from the lower intertidal zone by competitive inter- Mean low neap tide actions with Balanus. We discuss the mechanisms by which this competitive exclusion is accomplished in chapter 13, Mean low spring tide which covers interspecific competition. Figure 9.8 Distributions of two barnacle species within the These barnacles remind us that the environment consists intertidal zone (data from Connell 1961b). of more than just physical and chemical factors. An organ- ism’s environment also includes biological factors. In many situations, biological factors may be as important or even Warm weather and calm seas produced much higher mortality more important than physical factors in determining the niche, 100 among Balanus balanoides than and therefore, the distribution and abundance of a species. among Chthamalus stellatus in Now that we have considered factors limiting the distribu- upper intertidal zone. tions of populations, let’s consider the patterns of distribution 80 Balanus of individuals within their habitat. Let’s begin by considering three basic patterns of distribution. Chthamalus 60 Concept 9.1 Review

Mortality (%) 40 1. How may a species respond to climate change? 2. How might biological and physical aspects of the envi- ronment interact to influence a species’ geographic 20 distribution?

0 12 9.2 Patterns on Small Scales Age (years) LEARNING OUTCOMES Figure 9.9 Barnacle mortality in the upper intertidal zone (data After studying this section you should be able to do the following: from Connell 1961b). 9.7 Define small and large scale from an ecological perspective. Barnacles, one of the most common intertidal organisms, 9.8 Describe random, regular, and clumped distributions. show distinctive patterns of zonation within the intertidal 9.9 Discuss the mechanisms producing changes in distri- zone. For example, Joseph Connell (1961a, 1961b) described bution, as stands of creosote bush age. how along the coast of Scotland, adult Chthamalus stellatus 9.10 Explain how interactions between individuals in are restricted to the upper levels of the intertidal zone, while a population are thought to influence distribution adult Balanus balanoides are limited to the middle and lower patterns in populations. levels ( fig. 9.8 ). What role does resistance to drying play in the intertidal zonation of these two species? Unusually calm and warm weather combined with very low tides gave Connell On small scales, individuals within populations are some insights into this question. In the spring of 1955, warm distributed in patterns that may be random, regular, or weather coincided with calm seas and very low tides. As a clumped. We have just considered how the environment lim- consequence, no water reached the upper intertidal zone occu- its the distributions of species. When you map the distribution pied by both species of barnacles. During this period, Balanus of a species such as the red kangaroo in Australia (see fig. 9.2 ), in the upper intertidal zone suffered much higher mortality or the zoned distribution of Chthamalus a n d Balanus in the

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intertidal zone (see fig. 9.8), the boundaries on your map indi- Regular patterns of distribution are produced when individuals cate the range of the species. In other words, your map shows avoid each other or claim exclusive use of a patch of landscape. where at least some individuals of the species live and where Neutral responses contribute to random distributions. they are absent. Knowing a species’ range, as defined by pres- The patterns created by social interactions may be rein- ence and absence, is useful, but it says nothing about how the forced or damped by the structure of the environment. An envi- individuals that make up the population are distributed in the ronment with patchy distributions of nutrients, nesting sites, areas where they are present. Are individuals randomly distrib- water, and so forth fosters clumped distribution patterns. An uted across the range? Are they regularly distributed? As we environment with a fairly uniform distribution of resources and shall see, the distribution pattern observed by an ecologist is frequent, random patterns of disturbance (or mixing) tends to strongly influenced by the scale at which a population is studied. reinforce random or regular distributions. Let’s now consider factors that influence the distributions of some species in nature. Scale, Distributions, and Mechanisms Ecologists refer frequently to large-scale and small-scale Distributions of Tropical Bee Colonies p h e n o m e n a . What is “large” or “small” depends on the size of Stephen Hubbell and Leslie Johnson (1977) recorded a dra- organism or other ecological phenomenon under study. For this matic example of how social interactions can produce and discussion, small scale refers to small distances over which there enforce regular spacing in a population. They studied competi- is little environmental change significant to the organism under tion and nest spacing in populations of stingless bees in the study. Large scale refers to areas over which there is substan- family Trigonidae. The bees they studied live in tropical dry tial environmental change. In this sense, large scale may refer forest in Costa Rica. Though these bees do not sting, rival col- to patterns over an entire continent or patterns along a mountain onies of some species fight fiercely over potential nesting sites. slope, where environmental gradients are steep. Let’s begin our Stingless bees are abundant in tropical and subtropi- discussion with patterns of distribution observed at small scales. cal environments, where they gather nectar and pollen from Three basic patterns of distribution are observed on small a wide variety of flowers. They generally nest in trees and scales: random, regular, or clumped. A random distribution live in colonies made up of hundreds to thousands of workers. is one in which individuals within a population have an equal Hubbell and Johnson observed that some species of stingless chance of living anywhere within an area. A regular distri- bees are highly aggressive to other members of their species bution is one in which individuals are uniformly spaced. In a from other colonies, while others are not. Aggressive species clumped distribution , individuals have a much higher prob- usually forage in groups and feed mainly on flowers that occur ability of being found in some areas than in others ( fig. 9.10 ). in high-density clumps. The nonaggressive species feed singly These three basic patterns of distribution are produced by or in small groups and on more widely distributed flowers. the kinds of interactions that take place between individuals Hubbell and Johnson studied several species of sting- within a population, by the structure of the physical environ- less bees to determine whether there is a relationship between ment, or by a combination of interactions and environmental aggressiveness and patterns of colony distribution. They pre- structure. Individuals within a population may attract each dicted that the colonies of aggressive species would show reg- other, repel e a c h o t h e r , o r ignore each other. Mutual attrac- ular distributions while those of nonaggressive species would tion creates clumped, or aggregated, patterns of distribution. show random or clumped distributions. They concentrated

An individual has an equal Individuals are uniformly Individuals live in areas probability of occurring spaced through the of high local abundance, anywhere in an area. environment. separated by areas of low abundance.

Patterns Random Regular Clumped

Attraction between Neutral interactions Antagonistic interactions individuals or attraction Processes between individuals, and between individuals or of individuals to a between individuals and local depletion of resources local environment common resource

Figure 9.10 Random, regular, and clumped distributions.

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206 Section III Population Ecology

their studies on a 13 ha tract of tropical dry forest that con- battles over a nest tree that lasted for 2 weeks. Each dawn, 15 tained numerous nests of nine species of stingless bees. to 30 workers from two rival colonies arrived at the contested Though Hubbell and Johnson were interested in how bee nest site. The workers from the two rival colonies faced off behavior might affect colony distributions, they recognized that in two swarms and displayed and fought with each other. In the availability of potential nest sites for colonies could also the displays, pairs of bees faced each other, slowly flew verti- affect distributions. So, in one of the first steps in their study, cally to a height of about 3 m, and then grappled each other they mapped the distributions of trees suitable for nesting. to the ground. When the two bees hit the ground, they sepa- They found that potential nest trees were distributed randomly rated, faced off, and performed another aerial display. Bees did through the study area and that the number of potential nest not appear to be injured in these fights, which were apparently sites was much greater than the number of bee colonies. ritualized. The two swarms abandoned the battle at about 8 or Hubbell and Johnson were able to map the nests of five 9 a.m. each morning, only to re-form and begin again the next of the nine species of stingless bees accurately. The nests of day just after dawn. While this contest over an unoccupied nest four of these species were distributed regularly. As they had site produced no obvious mortality, fights over occupied nests predicted, all four species with regular nest distributions sometimes killed over 1,000 bees in a single battle. These trop- were highly aggressive to bees from other colonies of their ical bees space their colonies by engaging in pitched battles. own species. The fifth species, Trigona dorsalis, was not As we see next, plants space themselves by more subtle means. aggressive and its nests were randomly distributed over the study area. Figure 9.11 contrasts the random distribution of T. dorsalis with the regular distribution of one of the aggres- Distributions of Desert Shrubs sive species, T. fulviventris. Half a century ago, desert ecologists suggested that desert The researchers also studied the process by which the shrubs tend to be regularly spaced due to competition between aggressive species establish new colonies. In the process, the shrubs. You can see the patterns that inspired these early they made observations that provide insights into the mecha- ecologists by traveling across the seemingly endless expanses nisms that establish and maintain the regular nest distributions of the Mojave Desert in western North America. One of the of species such as T. fulviventris. This species and the other most common plants you will see is the creosote bush, Larrea aggressive species apparently mark prospective nest sites with tridentata, which dominates thousands of square kilometers a pheromone. P h e r o m o n e s are chemical substances secreted of this area. As you look out across landscapes dominated by by some animals for communication with other members of creosote bushes, it may appear that the spacing of these shrubs their species. The pheromone secreted by these stingless bees is regular ( fig. 9.12 ). In places, their spacing is so uniform they attracts and aggregates members of their colony to the prospec- appear to have been planted by some very careful gardener. As tive nest site; however, it also attracts workers from other nests. we shall see, however, visual impressions can be deceiving. If workers from two different colonies arrive at the pro- Quantitative sampling and statistical analysis of the dis- spective nest, they may fight for possession. Fights may be tributions of creosote bushes and other desert shrubs led to a escalated into protracted battles. Hubbell and Johnson observed controversy that took the better part of two decades to settle. In

Colonies of the stingless bee, The less interactive stingless Trigona fulviventris, which bee, T. dorsalis, is distributed interact aggressively, are randomly across the same distributed regularly across tract of forest. this tract of forest. m 0 100

Regular distribution Random distribution

T. fulviventris from rival colonies battled daily for 2 Figure 9.12 Observing the distribution of individuals in local weeks for possession of populations of creosote bush, Larrea tridentata, ecologists suggested this potential nest tree. that their spacing is regular. Notice on the left side of this photo how the spacing of creosote bushes in the distance suggest that they have Figure 9.11 Regular and random distributions of stingless bee been planted in regularly spaced rows. This view is similar to what colonies in the tropical dry forest are related to levels of aggression you would see if you looked between two rows of trees in a fruit (data from Hubbell and Johnson 1977). orchard.

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Chapter 9 Population Distribution and Abundance 207

Small shrubs establish in Mortality as the shrubs grow Competition enforces a high densities and produce reduces clumping and produces regular distribution a clumped distribution. a random distribution among among large shrubs. medium shrubs.

Young, small Medium Large shrubs shrubs shrubs

Clumped Random Regular

Figure 9.13 Change in creosote bush distributions with increasing shrub size.

short, when different teams of researchers quantified the distri- that takes place belowground. How can we study these below- butions of desert shrubs, some found the regular distributions ground interactions? Work by Jacques Brisson and James reported by earlier ecologists. Others found random or clumped Reynolds (1994) provides a quantitative picture of the below- distributions. Still others reported all three types of distributions. ground side of creosote bush distributions. These researchers Though we are generally accustomed to having one carefully excavated and mapped the distributions of 32 creo- answer to our questions, the answers to ecological questions sote bushes in the Chihuahuan Desert. They proposed that if are often more complex. Research by Donald Phillips and creosote bushes compete, their roots should grow in a way that James MacMahon (1981) showed that the distribution of creo- reduces overlap with the roots of nearby individuals. sote bushes changes as they grow. They mapped and analyzed The 32 excavated creosote bushes occupied a 4 m by 5 m the distributions of creosote bushes and several other shrubs at area on the Jornada Long Term Ecological Research site near nine sites in the Sonoran and Mojave deserts. Because earlier Las Cruces, New Mexico. Creosote bush was the only shrub researchers had suggested that creosote bush spacing changed within the study plot. Their roots penetrated to only 30 to with available moisture, they chose sites with different aver- 50 cm, the depth of a hardpan calcium carbonate deposition age precipitations. Precipitation at the study sites ranged from layer. Because they did not have to excavate to great depths, 80 to 220 mm, and average July temperature varied from 278 Brisson and Reynolds were able to map more root systems to 358 C. Phillips and MacMahon took care to pick sites with than previous researchers. Still, their excavation and mapping similar soils and with similar topography. They studied popu- of roots required 2 months of intense labor. lations growing on sandy to sandy loam soils with less than The complex pattern of root distributions uncovered con- 2% slope with no obvious surface runoff channels. firmed the researchers’ hypothesis: Creosote bush roots grow The results of this study indicate that the distribution of in a pattern that reduces overlap between the roots of adja- desert shrubs changes from clumped to random to regular cent plants (fig. 9.14). Notice that the root systems of creo- distribution patterns as they grow. The young shrubs tend to sote bushes overlap much less than they would if they had be clumped for three reasons: (1) because seeds germinate at circular distributions. Brisson and Reynolds concluded that a limited number of “safe sites,” (2) because seeds are not competitive interactions with neighboring shrubs influence dispersed far from the parent plant, or (3) because asexually the distribution of creosote bush roots. Their work suggests produced offspring are necessarily close to the parent plant. that creosote bushes compete for belowground resources. Phillips and MacMahon proposed that as the plants grow, After more than two decades of work on this single spe- some individuals in the clumps die, which reduces the degree cies of plant, desert ecologists have a much clearer understand- of clumping. Gradually, the distribution of shrubs becomes ing of the factors that influence the distribution of individuals more and more random. However, competition among the on a small scale. On small scales, the creosote bush may have remaining plants produces higher mortality among plants clumped, random, or regular distributions. Hubbell and John- with nearby neighbors, which thins the stand of shrubs still son (1977) showed that stingless bee colonies may also show further and eventually creates a regular distribution of shrubs. different patterns of distribution, depending on the level of This hypothetical process is summarized in figure 9.13. aggression between colonies. As we shall see in the following Phillips and MacMahon and other ecologists proposed that section, however, on larger scales, individuals have clumped desert shrubs compete for water and nutrients, a competition distributions.

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208 Section III Population Ecology

The root systems of 32 creosote If excavated shrubs had circular root The actual root systems bushes were mapped. systems, 20% of the area would were not circular and include extensive overlap of four or overlapped extensively in more shrubs (shaded area). only 4% of the area.

(a) Excavated root systems (b) Hypothetical circular root systems (c) Actual root systems Figure 9.14 Creosote bush root distributions: hypothetical versus actual root overlap (data from Brisson and Reynolds 1994).

Concept 9.2 Review Bird Populations Across North America Terry Root (1988) mapped patterns of bird abundance across 1. Are the concepts of “small” versus “large” scale the North America using the “Christmas Bird Counts.” These bird same for all organisms? counts provide one of the few data sets extensive enough to study 2. How could you test the hypothesis that low overlap in distribution patterns across an entire continent. Christmas Bird root systems in creosote bush populations (see fig. 9.14) Counts, which began in 1900, involve annual counts of birds dur- is the result of ongoing competition? ing the Christmas season. The first Christmas Bird Count was 3. In the study of the distribution of stingless bee colonies attended by 27 observers, who counted birds in 26 localities—2 (see pp. 205–206), why were measurements of the num- in Canada and the remainder in 13 states of the United States. ber and distribution of potential nest trees necessary? In the 1985–86 season, 38,346 people participated in the Christ- mas Bird Count. The observers counted birds in 1,504 localities throughout the United States and most of Canada. The Christmas 9.3 Patterns on Large Scales Bird Count marked its centennial anniversary in the year 2000. It continues to produce a unique record of the distribution and pop- LEARNING OUTCOMES ulation densities of wintering birds across most of a continent. After studying this section you should be able to do the following: Root’s analysis centers around a series of maps that show patterns of distribution and population density for 346 species 9.11 Review evidence that wintering and breeding birds of birds that winter in the United States and Canada. Although are clumped at large scales. species as different as swans and sparrows are included, the 9 . 1 2 Explain the clumped distributions of trees along maps show a consistent pattern. At the continental scale, bird moisture gradients in terms of niches (Concept 9.1). populations show clumped distributions. Clumped patterns occur in species with widespread distributions, such as the On large scales, individuals within a population are American crow, Corvus brachyrhynchos, as well as in species clumped. We have considered how individuals within a with restricted distributions, such as the fish crow, C. ossi- population are distributed on a small scale: how bee colonies fragus. Though the winter distribution of the American crow are distributed within a few acres of forest and how shrubs are includes most of the continent, the bulk of individuals in this distributed within a small stand. Now let’s step back and ask population are concentrated in a few areas. These areas of high how individuals within a population are distributed on a larger density, or “hot spots,” appear as red patches in figure 9.15 a. scale over which there is significant environmental variation. For the American crow population, hot spots are concentrated For instance, how does the density of individuals vary across along river valleys, especially the Cumberland, Mississippi, the entire range of a species? Is population density fairly regu- Arkansas, Snake, and Rio Grande. Away from these hot spots larly distributed across the entire area occupied by a species, the winter abundance of American crows diminishes rapidly. or are there a few centers of high density surrounded by areas The fish crow population, though much more restricted in which the species is present but only in low densities? than that of the American crow, is also concentrated in a few

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Chapter 9 Population Distribution and Abundance 209

Investigating the Evidence 9

Clumped, Random, and Regular Distributions

LEARNING OUTCOMES Sample a b c d e f g h i j k After studying this section you should be able to do the following: Number of 62652573598 9.13 Describe how relative values of mean density and Species A variance in density correspond to random, regular, Number of 56655556455 and clumped distributions. Species B 9.14 Given a table of values, calculate density variance Number of 20 1 2 1 15 3 1 1 10 2 2 to mean ratios and interpret their meaning in terms Species C of distribution patterns. The distribution of individuals among the samples of species A, B, and C is quite different. For instance, each of the samples Imagine sampling a population of plants or animals to contained approximately the same number of individuals of determine the distribution of individuals across the habitat. species B. In contrast, the numbers of species C varied widely One of the most basic questions that you could ask is, “How among samples. Meanwhile, counts of species A showed a level are individuals in the population distributed across the study of variation somewhere in between variations in species B and area?” How might they be distributed? The three basic patterns C. The samples of species A, B, and C may give the impression that we’ve discussed in this section are clumped, random, and of random, regular, and clumped distributions. We can quan- regular distributions. The first step toward testing statistically tify our visual impressions by calculating the sample means between these three types of distributions is to sample the pop- and sample variances for the densities of species A, B, and C: ulation to estimate the mean (p. 18) and variance (pp. 88–89) in density of the population across the study area. The theoreti- Statistic Species A Species B Species C cal relationships between variance in density and mean density __ in clumped, random, and regular distributions are as follows: Sample mean, X 5.27 5.18 5.27 Sample variance, s2 5.22 0.36 44.42 s2 Distributions Relation of variance to mean __ 0.99 0.07 8.43 X Clumped Variance > Mean, or Variance/Mean > 1 Random Variance 5 Mean, or Variance/Mean 5 1 While the mean density calculated from the samples Regular Variance < Mean, or Variance/Mean < 1 was very similar for all three species, their variance in den- sity among samples was quite different. As a consequence, the s2 How do we connect these relationships between variance and ratios of sample variance to sample means, __ , were also differ- mean density with what we see on the ground? In a clumped X s2 distribution, many sample plots will contain few or no indi- ent. While __ for species A was nearly 1, this ratio was much X viduals while some will contain a large number. As a conse- less than 1 for species B, and much greater than 1 for species C. quence, the variance among sample plots will be high and the s2 variance in density will be greater than the mean. In contrast, These results show how the __ ratio can quantify the visual sample plots of a population with a regular distribution will all X include a similar number of individuals. As a result, the vari- impression of random, regular, and clumped distributions that ance in density across samples will be low when taken from a we formed when we inspected the samples of the three species. s2 population with a regular distribution; therefore, the variance Can we conclude from the __ ratios we calculated that species will be less than the mean. Meanwhile, in a randomly distrib- X uted population, the variance in density across the habitat will A has a random distribution, that species B has a regular distri- be approximately equal to the mean density. bution, and that C has a clumped distribution? While it is likely Consider the following samples of three different popu- that they do, in science we need to attach probabilities to such lations of herbaceous plants growing on a desert landscape. conclusions. To do that, we need to consider these samples of Each sample is the number counted in a randomly located species A, B, and C from a statistical perspective. We will look 1 m 2 area at the study site. at the statistics of these samples in chapter 10 (see p. 236).

C RITIQUING THE E VIDENCE 9 1. According to the results of Phillips and MacMahon, what is the approximate value of the ratio of variance in shrub densit y to mean shrub density (variance/mean) for young, medium-age, and older creosote bushes (see fig. 9.13)?

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210 Section III Population Ecology

High The American crow, which is Observers counted few very widely distributed, is red-eyed vireos along Low most abundant in a limited most census routes. number of “hot spots.” 250

200

150

Large numbers of red-eyed 100 vireos were encountered on Number of routes just a few census routes. 50

0 0 50 100 150 (a) Number of red-eyed vireos Figure 9.16 Red-eyed vireos, Vireo olivaceus, counted along census routes of the Breeding Bird Survey (data from Brown, Mehlman, and Stevens 1995).

Canada. For their analyses, they chose species of birds whose geographic ranges fall mainly or completely within the eastern and central regions of the United States, which are well cov- Within its restricted range, ered by study sites of the Breeding Bird Survey. the fish crow lives at high Like Root, Brown and his colleagues found that a relatively densities in three areas. small proportion of study sites yielded most of the records of each bird species. That is, most individuals were concentrated in a fairly small number of hot spots. For instance, the den- sities of red-eyed vireos are low in most places (fig. 9.16). Clumped distributions were documented repeatedly. When the numbers of birds across their ranges were totaled, generally (b) about 25% of the locations sampled supported over half of Figure 9.15 ( a ) Winter distribution of the American crow, Corvus each population. By combining the results of Root and Brown brachyrynchos. ( b ) Winter distribution of the fish crow, C. ossifragus and his colleagues, we can say confidently that at larger (data from Root 1988). scales, bird populations in North America show clumped pat- terns of distribution. In other words, most individuals within areas ( fig. 9.15 b ). Fish crows are restricted to areas of open a bird species live in a few hot spots, areas of unusually high water near the coast of the Gulf of Mexico and along the south- population density. ern half of the Atlantic coast of the United States. Within this Brown and his colleagues propose that these distributions restricted range, however, most fish crows are concentrated are clumped because the environment varies and individuals in a few hot spots—one on the Mississippi Delta, another on aggregate in areas where the environment is favorable. What Lake Seminole west of Tallahassee, Florida, and a third in the might be the patterns of distribution for populations distrib- everglades of southern Florida. Like the more widely distrib- uted along a known environmental gradient? Studies of plant uted American crow, the abundance of fish crows diminishes populations provide interesting insights. rapidly away from these centers of high density. Might bird populations have clumped distributions only on the wintering grounds? James H. Brown, David Mehlman, and Plant Distributions Along Moisture Gradients George Stevens (1995) analyzed large-scale patterns of abun- Decades ago, Robert Whittaker gathered information on the dance among birds across North America during the breeding distributions of woody plants along moisture gradients in season, the opposite season from that studied by Root. In their several mountain ranges across North America. As we saw in study these researchers used data from the Breeding Bird Sur- chapter 2 (see fig. 2.38) environmental conditions on moun- vey, which consists of standardized counts by amateur orni- tainsides change substantially with elevation. These steep thologists conducted each June at approximately 2,000 sites environmental gradients provide a compressed analog of the across the United States and Canada under the supervision continental-scale gradients to which the birds studied by Root of the Fish and Wildlife Services of the United States and and Brown and his colleagues were presumably responding.

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Chapter 9 Population Distribution and Abundance 211

Let’s look at the distributions of some tree species along On this mountainside, moisture gradients in two of the mountain ranges studied by Table mountain pine table mountain pines Whittaker. Robert Whittaker and William Niering (1965) Red maple are most abundant on studied the distribution of plants along moisture and eleva- Hemlock drier upper slope. tion gradients in the Santa Catalina Mountains of southern Arizona. These mountains rise out of the Sonoran Desert near Tucson, Arizona, like a green island in a tan desert sea. Veg- High etation typical of the Sonoran Desert, including the saguaro cactus and creosote bush, grow in the surrounding desert and on the lower slopes of the mountains. However, the summit of the mountains is topped by a mixed conifer forest. Forests also extend down the flanks of the Santa Catalinas in moist, shady canyons. Red maples are There is a moisture gradient from the moist canyon Middle most abundant bottoms up the dry southwest-facing slopes. Whittaker and at midslope. Niering found that along this gradient the Mexican pin- yon pine, Pinus cembroides, is at its peak abundance on Position on slope the uppermost and driest part of the southwest-facing slope (fig. 9.17). Along the same slope, Arizona madrone, Arbutus arizonica, reaches its peak abundance at middle elevations. Finally, Douglas firs, Pseudotsuga menziesii, a r e r e s t r i c t e d to the moist canyon bottom. Mexican pinyon pines, Arizona Low madrone, and Douglas fir are all clumped along this moisture gradient, but each reaches peak abundance at different 0 0.2 0.4 0.6 0.8 positions on the slope. These positions appear to reflect the Proportion of population Hemlocks are different environmental requirements of each species. most abundant Whittaker (1956) recorded analogous tree distributions on moist valley along moisture gradients in the Great Smoky Mountains of east- bottom. ern North America. Again, the gradient was from a moist valley bottom to a drier southwest-facing slope. Along this moisture Figure 9.18 Abundance of three tree species on a moisture gradient in the Great Smoky Mountains, Tennessee (data from gradient, hemlock, Tsuga canadensis, was concentrated in the Whittaker 1956). moist valley bottom and its density decreased rapidly upslope (fig. 9.18). Meanwhile red maple, Acer rubrum, g r e w a t h i g h - est densities in the middle section of the slope, while table mountain pine, Pinus pungens, was concentrated on the driest upper sections. As in the Santa Catalina Mountains of Arizona, these tree distributions in the Great Smoky Mountains reflect the moisture requirements of each tree species. Mexican pinyon pine High The distribution of trees along moisture gradients seems Arizona madrone to resemble the clumped distributional patterns of birds Douglas fir across the North American continent but on a smaller scale. All species of trees discussed here showed a highly clumped On this mountainside, distribution along moisture gradients, and their densities Mexican pinyon pines Arizona madrones are most abundant on decreased substantially toward the edges of their distributions. are most abundant drier upper slope. In other words, like birds, tree populations are concentrated at midslope. Middle in hot spots. As we shall see in the next Concept, population density is influenced by organism size. Position on slope Concept 9.3 Review

Douglas firs are most abundant 1. What factors might be responsible for the aggregation in moist canyon of American crows in winter (see fig. 9.15 )? Low bottom. 2. Why might the winter aggregations of crows occur 0 mainly along river valleys? 0.0 0.2 0.4 0.6 0.8 3. What does the position of pines along moisture gradi- Proportion of population ents in both the Santa Catalina Mountains of Arizona Figure 9.17 Abundances of three tree species on a moisture (see fig. 9.17) and the Great Smoky Mountains of Ten- gradient in the Santa Catalina Mountains, Arizona (data from nessee (see fig. 9.18) suggest about pine water relations? Whittaker and Niering 1965).

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212 Section III Population Ecology

9.4 Organism Size Average population density of herbivorous mammals decreases and Population Density with increasing body size.

LEARNING OUTCOMES 104 After studying this section you should be able to do the following:

3 9.15 Draw a scatter plot of the general relationship ) 10 2 between organism size and population density. 9.16 Describe differences among animal groups, for 102 example, mammals versus aquatic invertebrates, in their relationships between size and population 10 density.

1

Population density declines with increasing organism Herbivore density (per km size. If you estimate the densities of organisms in their natu- ral environments, you will find great ranges. While bacterial 10–1 populations in soils or water can exceed 10 9 per cubic centi- 2 4 6 meter and phytoplankton densities often exceed 106 per cubic 10 10 10 meter, populations of large mammals and birds can average Body mass (g) considerably less than one individual per square kilometer. Figure 9.19 Body size and population density of herbivorous What factors produce this variation in population density? mammals (data from Damuth 1981). The densities of a wide variety of organisms are highly corre- lated with body size. In general, densities of animal and plant animals. Their analysis included terrestrial invertebrates, populations decrease with increasing size. aquatic invertebrates, mammals, birds, and poikilothermic While it makes common sense that small animals and vertebrates. They included animals representing a great range plants generally live at higher population densities than larger in size and population density. Animal mass ranged from ones, quantifying the relationship between body size and 10 2 11 to about 10 2.3 kg, while population density ranged from population density provides valuable information. First, quan- less than 1 per square kilometer to nearly 10 12 per square tification translates a general qualitative notion into a more kilo-meter. When Peters and Wassenberg plotted animal mass precise quantitative relationship. For example, you might want against average density, they, like Damuth, found that popula- to know how much population density declines with increased tion density decreased with increased body size. body size. Second, measuring the relationship between body If you look closely at the data in figure 9.20, however, it size and population density for a wide variety of species is clear that there are differences among the animal groups. reveals different relationships for different groups of organ- First, aquatic invertebrates of a given body size tend to have isms. Differences in the relationship between size and popula- higher population densities, usually one or two orders of mag- tion density can be seen among major groups of animals. nitude higher, than terrestrial invertebrates of similar size. Second, mammals tend to have higher population densities Animal Size and Population Density than birds of similar size. Peters and Wassenberg suggest that John Damuth (1981) produced one of the first clear demon- it may be appropriate to analyze aquatic invertebrates and strations of the relationship between body size and population birds separately from the other groups of animals. density. He focused his analysis on herbivorous mammals. The general relationship between animal size and popula- The size of herbivorous mammals included in the analysis tion density has held up under careful scrutiny and reanalysis. ranged from small rodents, with a mass of about 10 g, to large Plant ecologists have found a qualitatively similar relation- herbivores such as rhinoceros, with a mass well over 10 6 g. ship in plant populations, as we see next. Meanwhile, average population density ranged from about 1 individual (10 2 1 ) per 10 km2 to about 10,000 (104 ) per 1 km2 , Plant Size and Population Density which spans approximately five “orders of magnitude,” or James White (1985) pointed out that plant ecologists have powers of 10, in population density. As figure 9.19 shows, been studying the relationship between plant size and popu- Damuth found that the population density of 307 species of lation density since early in the twentieth century. He sug- herbivorous mammals decreases, from species to species, gests that the relationship between size and density is one of with increased body size. The regression line (p. 188) in the the most fundamental aspects of population biology. White graph shows the average decrease in population density with summarized the relationship between size and density for a increased body size. large number of plant species spanning a wide range of plant Building on Damuth’s analysis, Robert Peters and Karen growth forms ( fig. 9.21 ). Wassenberg (1983) explored the relationship between body The pattern in figure 9.21 illustrates that as in animals, size and average population density for a wider variety of plant population density decreases with increasing plant size.

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Chapter 9 Population Distribution and Abundance 213

Overall, average Many aquatic population density invertebrates live decreases with at higher increasing body size population across a wide spectrum densities than other of animal groups. 1012 animals of comparable size. ) 2 108

Aquatic 104 invertebrates Terrestrial invertebrates Mammals tend to live Mammals at higher population Animal density (per km 1.0 Birds densities than birds. Vertebrate poikilotherms 10–4 10–8 10–6 10–3 1.0 103 Body mass (kg) Figure 9.20 Animal size and population density (data from Peters and Wassenberg 1983).

As in animals, plant population can live at very high densities. As the trees grow, density density decreases with increasing declines progressively until the mature trees live at low den- plant size across a wide range of sities. We discuss this process, which is called self-thinning, plant growth forms. in chapter 13. Thus, the size-density relationship changes dynamically within plant populations and differs significantly Duckweed, Lemna, one of The coastal redwood, Sequoia the smallest flowering sempervirens, one of the largest between populations of plants that reach different sizes at plants, lives at very high trees, lives at one of the lowest maturity. Despite differences in the underlying processes, the population densities. population densities. data summarized in figure 9.21 indicate a predictable relation- ship between plant size and population density. The value of such an empirical relationship, whether Annual herbs 106 for plants or animals, is that it provides a standard against which we can compare measured densities and gives an idea )

2 Clonal perennial herbs of expected population densities in nature. For example, sup- 104 pose you go out into the field and measure the population density of some species of animal. How would you know 2 10 Trees if the densities you encounter were unusually high, low, or about average for an animal of the particular size and taxon? 1 Without an empirical relationship such as that shown in Plant density (per m f i g u r e s 9.20 and 9.21 or a list of species densities, it would be impossible to make such an assessment. One question that we 10 –2 might attempt to answer with a population study is whether a species is rare. As we shall see in the following Applications 10 –4 10 –2 1 102 104 106 108 section, rarity is a more complex consideration than it might Plant mass (g) seem at face value. Figure 9.21 Plant size and population density (data from White 1985). Concept 9.4 Review

However, the biological details underlying the size-density 1. What are some advantages of Damuth’s strict focus on relationship shown by plants are quite different from those herbivorous mammals in his analysis of the relation- underlying the size-density patterns shown by animals. The ship between body size and population density (see different points in figures 9.19 and 9.20 represent different fig. 9.19 )? species of animals. A single species of tree, however, can span 2. How might energy and nutrient relations explain a very large range of sizes and densities during its life cycle. the lower population densities of birds compared to Even the largest trees, such as the giant sequoia, Sequoia comparable-sized mammals (see fig. 9.20 )? gigantea, start life as small seedlings. These tiny seedlings

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214 Section III Population Ecology

not independent. Instead, there is a strong positive correlation between the two variables for most groups of organisms. In other A p p l i c a t i o n s words, species abundant in the places where they occur are gen- Rarity and Vulnerability to Extinction erally widely distributed within a region, continent, or ocean, while species living at low population densities generally have LEARNING OUTCOMES small, restricted distributions. The positive relationship between After studying this section you should be able to do the following: range and population density was first brought to the atten- tion of ecologists by Ilka Hanski (Hanski 1982) and James H. 9.17 List the seven forms of rarity described by Brown (Brown 1984). Kevin Gaston (Gaston 1996; Gaston et Rabinowitz. al. 2000) points out that in the two decades since the early work 9.18 Detail five examples of organisms showing each by Hanski and Brown, ecologists have found a positive relation- form of rarity described by Rabinowitz. ship between range and population density for many groups, 9.19 Explain the relationship between the forms of rarity including plants, grasshoppers, scale insects, hoverflies, bum- and the vulnerability of species to extinction. blebees, moths, beetles, butterflies, birds, frogs, and mammals. Several mechanisms have been proposed to explain the positive Viewed on a long-term, geological timescale, populations relationship between local abundance and range size. Many of come and go and extinction seems to be the inevitable punc- the explanations focus on breadth of environmental tolerances tuation mark at the end of a species’ history. However, some and differences in population dynamics. However, as Gaston populations seem to be more vulnerable to extinction than and his colleagues point out (Gaston et al. 2000), there is still others. What makes some populations likely to disappear, no consensus on the most likely explanations. while others persist through geological ages? At the heart of Most species are uncommon; seven combinations of the matter are patterns of distribution and abundance. Rare range, tolerance, and population size each create a kind of species are often vulnerable to extinction, whereas abundant rarity. As a consequence, Rabinowitz referred to “seven forms species are seldom so. In order to understand and, perhaps, of rarity.” Let’s look at species that represent the two extremes prevent extinction, we need to understand the various forms of Rabinowitz’s seven forms of rarity. The first two discus- of rarity, especially in this time of rapid climate change. sions concern species that are rare according to only one attri- bute. These are species that, before they become extinct, may seem fairly secure. The final discussion concerns the very rar- Seven Forms of Rarity and One of Abundance est species, which show all three attributes of rarity. Though Deborah Rabinowitz (1981) devised a classification of com- these rarest species are the most vulnerable to extinction, rar- monness a n d rarity, based on combinations of three fac- ity in any form appears to increase vulnerability to extinction. tors: (1) the geographic range of a species ( extensive v e r s u s restricted ), (2) habitat tolerance (broad v e r s u s narrow ) , a n d Rarity I: Extensive Range, Broad Habitat (3) local population size ( large v e r s u s small ). Habitat tolerance Tolerance, Small Local Populations is related to the range of conditions in which a species can live. It is easy to understand how people were drawn to the original For instance, some plant species can tolerate a broad range of practice of falconry. The sight and sound of a peregrine soil texture, pH, and organic matter content, whereas other falcon, Falco peregrinus, in full dive at over 200 km per hour plant species are confined to a single soil type. As we shall must have been one of the great experiences of a lifetime see, tigers have broad habitat tolerance; however, within the (fig. 9.23). The peregrine, which has a geographic range that tiger’s historical range in Asia lives the snow leopard, which is circles the Northern Hemisphere and broad habitat tolerance, confined to a narrow range of conditions in the high mountains is uncommon throughout its range. Apparently, this one attri- of the Tibetan Plateau. Small geographic range, narrow habitat bute of rarity was enough to make the peregrine vulnerable tolerance, and low population density are attributes of rarity. to extinction. The falcon’s feeding on prey containing high As shown in figure 9.22, there are eight possible combi- concentrations of DDT, which produced thin eggshells and nations of these factors, seven of which include at least one nesting failure, was enough to drive the peregrine to the brink attribute of rarity. The most abundant species and those least of extinction. Peregrine falcons were saved from extinction by threatened by extinction have extensive geographic ranges, control of the use of DDT, strict regulation of the capture of broad habitat tolerances, and large local populations at least the birds, captive breeding, and reintroduction of the birds to somewhere within their range. Some of these species, such areas where local populations had become extinct. as starlings, Norway rats, and house sparrows, are associated The range of the tiger, Panthera tigris, once extended with humans and are considered pests. However, many species from Turkey to eastern Siberia, Java, and Bali and included of small mammals, birds, and invertebrates not associated environments ranging from boreal forest to tropical rain with humans, such as the deer mouse, Peromyscus manicu- forest. The tigers in this far-flung population varied enough latus, or the marine zooplankton, Calanus finmarchicus, also from place to place in size and coloration that many local fall into this most common category. populations were described as separate subspecies, includ- Ecologists exploring the relationship between size of geo- ing the Siberian, Bengal, and Javanese tigers. Like peregrine graphic range and population size have found that they are falcons, tigers had an extensive geographic range and broad

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Chapter 9 Population Distribution and Abundance 215

Most Species such as these common Species show no aspects of rarity; House sparrow Dandelion they are among the most Extensive geographic range common in the biosphere. Passer domesticus Broad habitat tolerance Large local population

Galápagos medium Monterey Restricted geographic range ground fnch pine Broad habitat tolerance Large local population

California grey whale Fremont All of these species Extensive geographic range cottonwood show one aspect of Narrow habitat tolerance rarity, which gives them Large local population some vulnerability to extinction. Tiger Grass fern Extensive geographic range Asplenium Broad habitat tolerance septentrionale Small local population

Restricted geographic range Fish crow Haleakala Narrow habitat tolerance silversword Large local population

With two aspects of Tasmanian devil Welwitschia Restricted geographic range rarity, these three Broad habitat tolerance groups of species are Small local population even more vulnerable to extinction.

Extensive geographic range Northern spotted owl Pacifc Narrow habitat tolerance yew Small local population

Species such as these Restricted geographic range Mountain gorilla Pritchardia monroi are the rarest in the Narrow habitat tolerance biosphere and are the Small local population No photo available most vulnerable to extinction. Rarest Text on white highlights aspects of rarity.

Figure 9.22 Rarity and vulnerability to extinction.

habitat tolerance but low population density. Over the cen- turies, relentless pursuit by hunters reduced the tiger’s range from nearly half of the largest continent on earth to a series of tiny, fragmented populations. Many local populations have become extinct and others, such as the magnificent Siberian tiger, teeter on the verge of extinction in the wild. These populations may survive only through captive breed- ing programs in zoos. The next example shows that narrow habitat tolerance can also lead to extinction.

Rarity II: Extensive Range, Large Populations, Narrow Habitat Tolerance When Europeans arrived in North America, they encountered Figure 9.23 The peregrine falcon, Falco peregrinus, is found one of the most numerous birds on earth, the passenger pigeon. throughout the Northern Hemisphere but lives at low population The range of the passenger pigeon extended from the eastern densities throughout its range. shores of the present-day United States to the Midwest, and

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216 Section III Population Ecology

its population size numbered in the billions. However, the bird vulnerable. Of the 171 bird species and subspecies known had one attribute of rarity: It had a narrow requirement for its to have become extinct since 1600, 155 species have been nesting sites. The passenger pigeon nested in huge aggrega- restricted to islands. Of the 70 species and subspecies of birds tions in virgin forests. As virgin forests were cut, its range known to have lived on the Hawaiian Islands, 24 are now diminished and market hunters easily located and exploited extinct and 30 are considered in danger of extinction. its remaining nesting sites, finishing off the remainder of the Organisms on continents that are restricted to small areas, population. By 1914, when the last passenger pigeon died in have narrow habitat tolerance, and small population size captivity, one of the formerly most numerous bird species on are also vulnerable to extinction. Examples of populations earth was extinct. Extensive range and high population den- in such circumstances are common. More than 20 species sity alone, do not guarantee immunity from extinction. of plants and animals are confined to about 200 km 2 of The rivers in the same region inhabited by the passenger mixed wetlands and upland desert in California called pigeon harbored an abundant, widely distributed but narrowly Ash Meadows. The Ash Meadows stick-leaf, Mentzelia tolerant fish, the harelip sucker, Lagochila lacera. This fish l e u c o p h y l l a , inhabits an area of about 2.5 km 2 and has a total was found in streams across most of the east-central United population size of fewer than 100 individuals. Another plant, States and was abundant enough that early ichthyologists cited the Ash Meadows milk vetch, Astragalus phoenix, has a total it as one of the most common and most valuable food fishes population of fewer than 600 individuals. Human alteration in the region. However, the harelip sucker, like the passenger of Ash Meadows appears to have caused the extinction of pigeon, had narrow habitat requirements. It was restricted to at least one native species, the Ash Meadows killifish, large pools with rocky bottoms in clear, medium-size streams Empetrichthys merriami. about 15 to 30 m wide. This habitat was eliminated by the silt- Amazingly, there are species with ranges even more ing of rivers that followed deforestation and by the erosion of restricted than those of Ash Meadows, California. In 1980, poorly managed agricultural lands. The last individuals of this the total population of the Virginia round-leaf birch, Betula species collected by ichthyologists came from the Maumee uber, was limited to 20 individuals in Smyth County, Virginia. River in northwestern Ohio in 1893. Until recently the total habitat of the Socorro isopod, Thermo-sphaeroma thermophilum, of Socorro, New Mexico, Extreme Rarity: Restricted Range, Narrow was limited to a spring pool and outflow with a surface area of Habitat Tolerance, Small Populations a few square meters. Meanwhile, a palm species, Pritchardia Species that combine small geographic ranges with narrow monroi, which is found only on the island of Maui in the habitat tolerances and low population densities are the rarest Hawaiian Islands, has a total population in nature of exactly of the rare. This group includes species such as the mountain one individual! gorilla, the giant panda, and the California condor. Species Examples such as these fill books listing endangered showing this extreme form of rarity are clearly the most species. In nearly all cases, the key to a species’ survival is vulnerable to extinction. Many island species have these attri- increased distribution and abundance. This is often a key goal butes, so it is not surprising that island species are especially of programs aimed at preserving endangered species. Summary Ecologists define a population as a group of individuals of and moisture limits the distributions of certain desert plants, a single species inhabiting an area delimited by natural or such as shrubs in the genus Encelia. However, differences human-imposed boundaries. Population studies hold the key in the physical environment only partially explain the to solving practical problems such as saving endangered spe- distributions of barnacles within the marine intertidal zone, cies, controlling pest populations, or managing fish and game a reminder that biological factors constitute an important part populations. All populations share a number of characteris- of an organism’s environment. tics. Chapter 9 focused on two population characteristics: dis- On small scales, individuals within populations are tribution and abundance. distributed in patterns that may be random, regular, or While there are few environments on earth without life, clumped. Patterns of distribution can be produced by the no single species can tolerate the full range of earth’s envi- social interactions within populations, by the structure of the ronments. Because all species find some environments too physical environment, or by a combination of the two. Social warm, too cold, too saline, and so forth, environment limits organisms tend to be clumped; territorial organisms tend the geographic distribution of species. The environmental to be regularly spaced. An environment in which resources limits of a species are related to its niche. For instance, there are patchy also fosters clumped distributions. Aggressive is a close relationship between climate and the distributions species of stingless bees live in regularly distributed colonies, of the three largest kangaroos in Australia. The tiger beetle whereas the colonies of nonaggressive species are randomly Cicindela longilabris is limited to cool boreal and mountain distributed. The distribution of creosote bushes changes as environments. Large- and small-scale variation in temperature they grow.

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On large scales, individuals within a population are largest trees start life as small seedlings that can live at very clumped. In North America, populations of both wintering high population densities. As trees grow, their population den- and breeding birds are concentrated in a few hot spots of high sity declines progressively until the mature trees live at low population density. Clumped distributions are also shown by densities. plant populations living along steep environmental gradients Rare species are more vulnerable to extinction than are on mountainsides. common species. Rarity of species can be expressed as a Population density declines with increasing organism combination of extensive versus restricted geographic range, size. In general, animal population density declines with broad versus narrow habitat tolerance, and large versus small increasing body size. This negative relationship holds for population size. The most abundant species and those least animals as varied as terrestrial invertebrates, aquatic inver- threatened by extinction combine large geographic ranges, tebrates, birds, poikilothermic vertebrates, and herbivorous wide habitat tolerance, and high local population density. All mammals. Plant population density also decreases with other combinations of geographic range, habitat tolerance, increasing plant size. However, the biological details under- and population size include one or more attributes of rarity. lying the size-density relationship shown by plants are quite Populations that combine restricted geographic range with different from those underlying the size-density patterns narrow habitat tolerance and small population size are shown by animals. A single species of tree can span a very the rarest of the rare and are usually the organisms most large range of sizes and densities during its life cycle. The vulnerable to extinction. Key Terms abundance 200 fundamental niche 200 population 199 regular distribution 205 clumped distribution 205 large-scale phenomena 205 random distribution 205 small-scale phenomena 205 density 200 niche 200 realized niche 200 distribution 200 pheromone 206 Review Questions 1 . W h a t c o n f i n e s Encelia farinosa to upland slopes in the Mojave 6. Suppose that in the near future, the fish crow population in Desert? Why is it uncommon along desert washes, where it would North America declines because of habitat destruction. Now have access to much more water? What may allow E. frutescens that you have reviewed the large-scale distribution and abun- to persist along desert washes whereas E. farinosa cannot? dance of the fish crow (see fig. 9.15 b ), devise a conservation 2. Spruce trees, members of the genus Picea, occur throughout plan for the species that includes establishing protected refuges the boreal forest and on mountains farther south. For example, for the species. Where would you locate the refuges? How spruce grow in the Rocky Mountains south from the heart of many refuges would you recommend? boreal forest all the way to the deserts of the southern United 7. Use the empirical relationship between size and population den- States and Mexico. How do you think they would be distributed sity observed in the studies by Damuth (1981) (see fig. 9.19 ) in the mountains that rise from the southern deserts? In par- and Peters and Wassenberg (1983) (see fig. 9.20) to answer ticular, how do altitude and aspect (see chapter 5) affect their the following: For a given body size, which generally has the distributions in the southern part of their range? Would spruce higher population density, birds or mammals? On average, populations be broken up into small local populations in the which lives at lower population densities, terrestrial or aquatic southern or the northern part of the range? Why? invertebrates? Does an herbivorous mammal twice the size of 3. What kinds of interactions within an animal population lead to another have on average one-half the population density of the clumped distributions? What kinds of interactions foster a regu- smaller species? Less than half? More than half? lar distribution? What kinds of interactions would you expect to 8. Outline Rabinowitz’s classification (1981) of rarity, which she find within an animal population distributed in a random pattern? based on size of geographic range, breadth of habitat tolerance, 4. How might the structure of the environment; for example, the and population size. In her scheme, which combination of attri- distributions of different soil types and soil moisture, affect the butes makes a species least vulnerable to extinction? Which patterns of distribution in plant populations? How should inter- combination makes a species the most vulnerable? actions among plants affect their distributions? 9. Can the analyses by Damuth (1981) and by Peters and Was- 5. Suppose one plant reproduces almost entirely from seeds, and senberg (1983) be combined with that of Rabinowitz (1981) that its seeds are dispersed by wind, and a second plant repro- to make predictions about the relationship of animal size to duces asexually, mainly by budding from runners. How should its relative rarity? What two attributes of rarity, as defined by these two different reproductive modes affect local patterns of Rabinowitz, are not included in the analyses by Damuth and by distribution seen in populations of the two species? Peters and Wassenberg?

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