Interspecific Competition Influences the Niche of a Species.

Previously, we defined the ecological niche of a species as the range of physical and chemical conditions under which it can persist (survive and reproduce) and the array of essential resources it uses and drew the distinction between the concepts of fundamental and realized niche (Chapter 12, Section 12.6). The fundamental niche is the ecological niche in the absence of interactions with other species, whereas the realized niche is the portion of the fundamental niche that a species actually exploits as a result of interactions with other species. As preceding examples have illustrated, competition may force species to restrict their use of space, range of foods, or other resource-oriented activities. As a result, species do not always occupy that part of their fundamental niche where conditions yield the highest growth rate, reproductive rate, or fitness. The work of Jessica Gurevitch of the University of New York–Stony Brook illustrates this point well. Gurevitch examined the role of interspecific competition on the local distribution of Stipa neomexicana, a C3 perennial grass found in the semiarid grassland communities of southeastern Arizona. Stipa is found only on the dry ridge crests where grass cover is low, rather than in moister, low-lying areas below the ridge crests where grass cover is greater. In a series of experiments, Gurevitch removed neighboring plants from individual Stipa plants in ridge-crest, midslope, and lower-slope habitats. She compared the survival, growth, and reproduction of these plants with control individuals (whose neighboring plants were not removed). Her results clearly show that Stipa has a higher growth rate, produces more flowers per plant, and has higher rates of seedling survival in midslope and lower-slope habitats (Figure  13.13). But its population density in these habitats is limited by competition with more successful grass species. Thus, Stipa distribution (or realized niche) is limited to suboptimal habitats because of interspecific competition.

Interpreting Ecological Data

1. Q1. How does the influence of interspecific competition on seedling survival of Stipa differ between the ridge-crest and lower-slope habitats?

2. Q2. Experiment results show that Stipa individuals can effectively grow at the lower slope even under conditions of interspecific competition, as indicated by values of mean basal area in part (b). Based on the results in Figure 13.13, what part(s) of the Stipa life cycle are most heavily influenced by interspecies competition, and how would these limitations affect distribution of the species on the landscape?

Much of the evidence for competition comes from studies, such as the one just presented, demonstrating the contraction of a fundamental niche in the presence of a competitor. Conversely, when a species’ niche expands in response to the removal of a competitor, the result is termed competitive release . Competitive release may occur when a species invades an island that is free of potential competitors, moves into habitats it never occupied on a mainland, and becomes more abundant. Such expansion may also follow when a competing species is removed from a community, allowing remaining species to move into microhabitats they previously could not occupy. Such was the case with the distribution of cattails along the gradient of water depth discussed previously, where in the absence of competition from Typha latifoli, the distribution of Typha angustifolia expanded to areas above the shoreline (expressed as negative values of water depth; see Figure 12.13).

An example of competitive release in a lake ecosystem is presented by Daniel Bolnick and colleagues at the University of Texas. Bolnick and his colleagues tested for short-term changes in the feeding niche of the three-spine stickleback (Gasterosteus aculeatus) after experimentally manipulating the presence or absence of two interspecific competitors: juvenile cut-throat trout (Oncorhynchus clarki) and prickly sculpin (Cottus asper). Direct examination of stomach contents of sculpin and trout reveals overlap with stickleback diets. Sculpin are exclusively benthic feeders, whereas juvenile trout feed at the surface and in the water column. In contrast, stickleback feed in both microhabitats. The experiment consisted of 20 experimental enclosures (made of netting) in Blackwater Lake on northern Vancouver Island, British Columbia. Five replicate blocks of four enclosures each were distributed along the shoreline of the lake. Sticklebacks collected from similar habitats nearby were placed in the enclosures. The enclosures in each of the blocks were assigned to one of four treatments: (1) competition with sculpin and trout present, (2) release from sculpin with trout present, (3) release from trout with sculpin present, and (4) total release with no competitors. The experimental treatments were left undisturbed for 15 days, after which all sticklebacks were removed, and the researchers identified (to the lowest feasible taxonomic level) and counted prey in the stomach of each stickleback. The diversity of prey species in the diet of the sticklebacks in each treatment was used as a measure of niche breadth. Results of the experiment reveal no significant change in the niche breadth (diversity of prey consumed) for the stickleback population when released from competition from sculpin. When released from competition from juvenile cut-throat trout, however, the researchers observed a significant expansion of niche breadth for the stickleback population (Figure 13.14).

13.11 Coexistence of Species Often Involves Partitioning Available Resources

All terrestrial plants require light, water, and essential nutrients such as nitrogen and phosphorus. Consequently, competition between various co-occurring species is common. The same is true for the variety of insect-feeding bird species inhabiting the canopy of a forest, large mammalian herbivores feeding on grasslands, and predatory fish species that make the coral reef their home. How is it that these diverse arrays of potential competitors can coexist in the same community? The competitive exclusion principle introduced in Section 13.5 suggests that if two species have identical resource requirements, then one species will eventually displace the other. But how different do two species have to be in their use of resources before competitive exclusion does not occur (or conversely, how similar can two species be in their resource requirements and still coexist)?

We have seen that the coexistence of competitors is associated with some degree of “niche differentiation”—differences in the range of resources used or environmental tolerances—in the species’ fundamental niches. Observations of similar species sharing the same habitat suggest that they coexist by partitioning available resources. Animals use different kinds and sizes of food, feed at different times, or forage in different areas. Plants require different proportions of nutrients or have different tolerances for light and shade. Each species exploits a portion of the resources unavailable to others, resulting in differences among co-occurring species that would not be expected purely as a result of chance.

Field studies provide many reports of apparent resource partitioning. One example involves three species of annual plants growing together on prairie soil abandoned one year after plowing. Each plant exploits a different part of the soil resource (Figure 13.15). Bristly foxtail (Setaria faberii) has a fibrous, shallow root system that draws on a variable supply of moisture. It recovers rapidly from drought, takes up water rapidly after a rain, and carries on a high rate of photosynthesis even when partially wilted. Indian mallow (Abutilon theophrasti) has a sparse, branched taproot extending to intermediate depths, where moisture is adequate during the early part of the growing season but is less available later on. The plant is able to carry on photosynthesis at low water availability (Section 6.10). The third species, smartweed (Polygonum pensylvanicum), has a taproot that is moderately branched in the upper soil layer and develops mostly below the rooting zone of other species, where it has a continuous supply of moisture.

Apparent resource partitioning is also common among related animal species that share the same habitat and draw on a similar resource base. Tamar Dayan, at Tel Aviv University, examined possible resource partitioning in a group of coexisting species of wild cats inhabiting the Middle East. Dayan and colleagues examined differences among species in the size of canine teeth, which are crucial to wild cats in capturing and killing their prey. For these cats, there is a general relationship between the size of canine and the prey species selected. Dayan found clear evidence of systematic differences in the size of the canine teeth, not only between male and female individuals within each of the species (sexual dimorphism) but also among the three coexisting cat species (Figure 13.16; see also Chapter 10). The pattern observed suggests an exceptional regularity in the spacing of species along the axis defined by the average size of canine teeth (x-axis in Figure 13.16). Dayan and colleagues hypothesize that intraspecific and interspecific competition for food has resulted in natural selection favoring the observed differences, thereby reducing the overlap in the types and sizes of prey that are taken.

The patterns of resource partitioning discussed previously are a direct result of differences among co-occurring species in specific physiological, morphological, or behavioral adaptations that allow individuals access to essential resources while at the same time function to reduce competition (see Chapter  5). Because the adaptations function to reduce competition, they are often regarded as a product of coevolutionary forces (see Chapter 12, Sections 12.3 and Section 12.6 for discussion and example of coevolution driven by competition). Although patterns of resource partitioning observed in nature are consistent with the hypothesis of phenotypic divergence arising from coevolution between competing species, it is difficult to prove that competition functioned as the agent of natural selection that resulted in the observed differences in resource use (observed differences in fundamental niches of the species). Differences among species may relate to adaptation for the ability to exploit a certain environment or range of resources independent of competition. Differences among species have evolved over a long period of time, and we have limited or no information about resources and potential competitors that may have influenced natural selection. This issue led Joseph Connell, an ecologist at the University of California–Santa Barbara, to refer to the hypothesis of resource portioning as a product of coevolution between competing species as the “ghosts of competition past.” Unable to directly observe the role of past competition on the evolution of characteristics, some of the strongest evidence supporting the role of “competition past” comes from studies examining differences in the characteristics of subpopulations of a species that face different competitive environments. A good example is the work of Peter Grant and Rosemary Grant, of Princeton University, involving two Darwin’s finches of the Galápagos Islands. The Grants studied the medium ground finch (Geospiza fortis) and the small ground finch (Geospiza fuliginosa), both of which feed on an overlapping array of seed sizes—for further discussion and illustrations, see Section 5.9. On the large island of Santa Cruz, where the two species of finch coexist, the distribution of beak sizes (phenotypes) of the two species does not overlap. Average beak size is significantly larger for G. fortis than for the smaller G. fuliginosa (Figure  13.17a). On the adjacent—and much smaller—islands of Los Hermanos and Daphne Major, the two species do not coexist, and the distributions of beak sizes for the two species are distinctively different from the patterns observed on Santa Cruz. The medium ground finch is allopatric (lives separately) on the island of Daphne Major, and the small ground finch is allopatric on Los Hermanos. Populations of each species on these two islands possess intermediate and overlapping distributions of beak sizes (Figures 13.17b and 13.17c). These patterns suggest that on islands where the two species coexist, competition for food results in natural selection favoring medium ground finch individuals with a large beak size that can effectively exploit larger seeds while also favoring small ground finch individuals that feed on smaller seeds. The outcome of this competition was a shift in feeding niches. When the shift involves features of the species’ morphology, behavior, or physiology, it is referred to as character displacement .

The preceding example suggests that the competing species on the island of Santa Cruz exhibit character displacement as a result of coevolutionary forces—that is, divergence in phenotypic traits relating to the exploitation of a shared and limited resource. However, until recently, the process of character displacement had never been documented by direct observational data. The first direct evidence of character displacement is provided by the work of Peter and Rosemary Grant on the population of G. fortis inhabiting the small island of Daphne Major.

Before 1982, G. fortis (medium ground finch) was the only species of ground finch inhabiting the island of Daphne Major. The situation changed in 1982 when a new competitor species emigrated from the larger adjacent islands—the large ground finch, Geospiza magnirostris (see Section 5.9 and Figure  5.20). G. magnirostris is a potential competitor on the island as a result of diet overlap with G. fortis. G. magnirostris feeds primarily on seeds of the herbaceous forb, Jamaican feverplant (Tribulus cistoides). The seeds are contained within a hard seed coat and exposed when a finch cracks or tears away the woody outer coating. Large-beaked members of the G. fortis population also feed on these seeds; in fact, during the 1976–1977 drought, the survival of the population depended on this seed resource (see Section 5.6 for a discussion of natural selection in this population).

Initially, the population of G. magnirostris on Daphne Major was too small in relation to the food supply to have anything but a small competitive effect on G. fortis. From 1982 to 2003, however, the population increased. Then little rain fell on the island during 2003 and 2004, and populations of both finch species declined dramatically as a result of declining food resources. During this period, G. magnirostris depleted the supply of large seeds from the Jamaican feverplant, causing the G. fortis population to depend on the smaller seed resources on the island. The result of this shift in resource availability because of competition from G. magnirostris was that during 2004 and 2005, G. fortis experienced strong directional selection against individuals with large beaks. The resulting decrease in the average beak size of the G. fortis population provides a clear example of the coevolutionary process of character displacement.

13.12 Competition Is a Complex Interaction Involving Biotic and Abiotic Factors

Demonstrating interspecific competition in laboratory “bottles” or the greenhouse is one thing; demonstrating competition under natural conditions in the field is another. In the field, researchers (1) have little control over the environment, (2) have difficulty knowing whether the populations are at or below carrying capacity, and (3) lack full knowledge of the life history requirements or the subtle differences between the species.

In the previous sections, we reviewed an array of studies examining the role of competition in the field. Perhaps the most common are removal experiments, in which one of the potential competitors is removed and the response of the remaining species is monitored. These experiments might appear straightforward, yielding clear evidence of competitive influences. But removing individuals may have direct and indirect effects on the environment that are not intended or understood by the investigators and that can influence the response of the remaining species. For example, removing (neighboring) plants from a location may increase light reaching the soil surface, soil temperatures, and evaporation. The result may be reduced soil moisture and increased rates of decomposition, influencing the abundance of belowground resources. These sometimes “hidden treatment effects” can hinder the interpretation of experimental results.

As we have seen in previous sections, competition is a complex interaction that seldom involves the interaction between two species for a single limiting resource. Interaction between species involves a variety of environmental factors that directly influence survival, growth, and reproduction; these factors vary in both time and space. The outcome of competition between two species for a specific resource under one set of environmental conditions (temperature, salinity, pH, etc.) may differ markedly from the outcome under a different set of environmental conditions. As we shall see in the following chapters, competition is only one of many interactions occurring between species—interactions that ultimately influence population dynamics and community structure.

Ecological Issues & Applications Is Range Expansion of Coyote a Result of Competitive Release from Wolves?

Before European settlement, two species of wild dog (genus Canis) were among the most abundant large carnivores occupying the North American continent. The gray wolf, Canis lupus, once ranged from the Atlantic to the Pacific coast and from Alaska to northern Mexico (Figure 13.18). It occurred in virtually all North American habitats (grasslands, eastern deciduous forest, northern conifer forest, southwest desert, etc.). In contrast, the coyote (Canis latrans) had a much more restricted distribution to the prairie grassland and desert habitats of the Great Plains and desert region of the southwest and Mexico (Figure 13.19). Since European settlement of the continent, however, the fate of these two species has taken different paths.

As early as 1630, the Massachusetts Bay Colony paid an average month’s salary for any wolf that was killed. Bounties like this continued until the last wolf in the Northeast was killed around 1897. The fate of the wolf population in other areas of its range was similar. Settlers moving westward depleted the populations of bison, deer, elk, and moose on which the wolves preyed. Wolves then turned to attacking sheep and cattle, and to protect livestock, ranchers and government agencies began an eradication campaign. Bounty programs initiated in the 19th century continued as late as 1965. Wolves were trapped, shot, dug from their dens, and hunted with dogs. Poisoned animal carcasses were left out for wolves, a practice that also killed eagles, ravens, foxes, bears, and other animals that fed on the tainted carrion. By the time wolves were protected by the Endangered Species Act of 1973, only a few hundred remained in extreme northeastern Minnesota and a small number on Isle Royale, Michigan.

In contrast to the gray wolf, the coyote did not originally occur in eastern North America, and with the westward expansion of settlement into the Great Plains, the coyote was perceived as less of a threat to farmers and ranchers. By the turn of the 20th century, it began to take advantage of newly open habitat that agriculture and logging had created, and its distribution expanded eastward. There were two main waves of colonization, northern and southern (Figure 13.19). The northern wave occurred first—coyote were reported in Michigan in about 1900, in southern Ontario by 1919, and in northern New York in the late 1930s. Most of the southeast was not colonized until the 1960s. Whereas the gray wolf population has been virtually eliminated in the continental United States, the range of the coyote has expanded to cover most of the areas once occupied by wolves, and coyote now occupy virtually every habitat in eastern North America (compare Figures 13.18 and 13.19) from forests, wooded areas, grassland, and agricultural land to suburban areas.

The concurrent expansion of the coyote with the decline of the wolf population in North America has caused ecologists to question whether the two occurrences are linked in some way. In North American ecosystems where gray wolves occur, interactions with other large carnivores are common, with competition being most intense with species having a similar ecology. Interference competition (see Section 13.1) occurs between the wolves and coyotes, with wolves limiting coyote access to resources by direct aggression. Field studies in regions where wolves and coyotes overlap indicate that coyotes are excluded from wolf territories and that wolves will go out of their way to kill coyotes. One of the leading hypotheses put forward to explain the dramatic range expansion of the coyote is that the eradication of the gray wolf from its former range may have reduced the competitive pressures limiting coyotes to their former range: range expansion is a result of “competitive release” (see Section 13.10). Now as a result of recent conservation efforts, ecologists are able to test this hypothesis directly.

Thanks to conservation efforts, the gray wolf is beginning to make a comeback. The wolf’s comeback within the United States is as a result of its listing under the Endangered Species Act, which provided protection from unregulated killing and resulted in increased scientific research, along with reintroduction and management programs. As of 2013 about 2200 wolves live in Minnesota, 8 on Lake Superior’s Isle Royale, about 650 in Michigan’s Upper Peninsula, and at least 800 in Wisconsin. In the northern Rocky Mountains, the U.S. Fish and Wildlife Service reintroduced gray wolves into Yellowstone National Park and U.S. Forest Service lands in central Idaho in 1995 and 1996. The reintroduction was successful, and as of 2013 there were at least 1650 wolves in the northern Rocky Mountains of Montana, Idaho, and Wyoming. These reintroductions of wolves into areas now occupied by coyotes have enabled ecologists to directly examine the role of competition on the populations of the two carnivores and test the hypothesis that the range expansion of the coyote in the United States is in part the result of competitive release from wolves.

Kim Berger and Eric Gese of Utah State University used data collected on wolf and coyote distribution and abundance to test the hypothesis that interference competition with wolves limits the distribution and abundance of coyotes in two regions of the Northern Rocky Mountains in which wolves have been recently reintroduced. From August 2001 to August 2004, the two researchers gathered data on cause-specific mortality and survival rates of coyotes captured at wolf-free and wolf-abundant sites in Grand Teton National Park (GTNP), and data on population densities of both species at three study areas across the Greater Yellowstone Ecosystem (GYE), to determine whether competition with wolves is sufficient to reduce coyote densities in these areas.

Berger and Gese found that although coyotes were the numerically dominant predator, across the GYE, densities varied spatially and temporally as a function of wolf abundance. Mean coyote densities were 33 percent lower at wolf-abundant sites in GTNP, and densities declined 39 percent in Yellowstone National Park following wolf reintroduction. A strong negative relationship between coyote and wolf densities (Figure 13.20), both within and across study sites, supports the hypothesis that competition with wolves limits coyote populations. Overall mortality of coyotes resulting from wolf predation was low but differed significantly for resident and transient individuals. Resident coyotes were members of packs that defended well-defined territories, whereas transients were associated with larger areas that encompassed the home ranges of several resident packs but were not associated with a particular pack or territory. Wolves were responsible for 56 percent of transient coyote deaths. In addition, dispersal rates of transient coyotes captured at wolf-abundant sites were 117 percent higher than for transients captured in wolf-free areas.

The work by Jerod Merkle and colleagues at the Yellowstone Wolf Project (Yellowstone Center for Resources, Yellowstone National Park) provides a detailed picture of the nature of competitive interactions between wolves and coyotes in areas where wolves have been reintroduced. In a series of field studies, the researchers examined interference competition between gray wolves and coyotes in Yellowstone National Park using radio-collared wolves (Figure 13.21). Merkle and colleagues documented 337 wolf–coyote interactions from 1995 to 2007. The majority (75 percent) of interactions occurred at the sites of wolf-killed ungulate carcasses (elk, buffalo, moose, mule deer, etc.) with coyotes attempting to scavenge. Wolves initiated the majority of encounters (85 percent), generally outnumbered coyotes (39 percent), and dominated (91 percent) most interactions. Wolves typically (79 percent) chased coyotes without physical contact; however, 7 percent of encounters resulted in a coyote death. Interactions decreased over time, suggesting coyote adaptation or a decline in coyote density. The results clearly show that wolves dominate interactions with coyotes.

Although data are limited to the few regions in which wolf populations have been successfully introduced, when combined with the results of studies of wolf–coyote interactions and population studies for regions of North America where these two species naturally co-occur (regions of Minnesota and Canada), a consistent picture emerges that the dramatic range expansion of coyote over the past century is as a result, at least in part, of the decline of wolf populations throughout most of its former range.

Summary

Interspecific Competition 13.1

In interspecific competition, individuals of two or more species share a resource in short supply, thus reducing the fitness of both. As with intraspecific competition, competition between species can involve either exploitation or interference. Six types of interactions account for most instances of interspecific competition: (1) consumption, (2) preemption, (3) overgrowth, (4) chemical interaction, (5) territorial, and (6) encounter.

Competition Model 13.2–13.3

The Lotka–Volterra equations describe four possible outcomes of interspecific competition. species 1 may outcompete species 2; species 2 may outcompete species 1. Both of these outcomes represent competitive exclusion. The other two outcomes involve coexistence. One is unstable equilibrium, in which the species that was most abundant at the outset usually outcompetes the other. A final possible outcome is stable equilibrium, in which two species coexist but at a lower population level than if each existed without the other.

Experimental Tests 13.4

Laboratory experiments with species interactions support the Lotka–Volterra model.

Competitive Exclusion 13.5

Experiment results led to the formulation of the competitive exclusion principle—two species with exactly the same ecological requirements cannot coexist. This principle has stimulated critical examinations of competitive relationships outside the laboratory, especially of how species coexist and how resources are partitioned.

Nonresource Factors 13.6

Environmental factors such as temperature, soil or water pH, relative humidity, and salinity directly influence physiological processes related to growth and reproduction but are not consumable resources that species compete over. By differentially influencing species within a community, these nonresource factors can influence the outcome of competition.

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