Variations in Species Ranges

Douglas firs are widespread in western North America, whereas giant sequoias are restricted to a few groves in the southern Sierra Nevada of California. The desert pupfish is restricted to a single spring in Death Valley, California, whereas smallmouth bass live in most of the rivers and lakes in eastern North America. Why do the geographic ranges of species vary so much? The factors contributing to this varia tion include speciation processes, dispersal abilities, and interactions with other species. As you might suspect, not all of the factors that influence geographic ranges are important for all species.

speciation processes influence range sizes. As we saw in

Chapter 24, there are several ways in which a new species can originate. A species that arises by polyploidy inevitably begins with a very small range. Because many polyploid plant species have formed only recently and have not spread much beyond the site of their origin, many of these plants have small ranges. Similarly, species that arise through founder events begin their history with small ranges. In contrast, most species that arise via allopatric speciation begin with large ranges. Finally, as a species declines toward extinction, as may be happening to giant sequoias (Figure 54.15), its range shrinks until it vanishes when the last individual dies.

dispersal abilities restrict geographic ranges. As we also saw in Chapter 24, the dispersal abilities of different species vary greatly. The experiments with small arthropods living in mosses on rocks show that even narrow barriers may prevent some species from reaching and colonizing an area. The solitary spring that is home to the desert pupfish is isolated from other bodies of fresh water, so the fish cannot disperse. Thus, the absence of many species from an area is simply due to a failure to get there. Zebra mussels, for

Sequoiadendron giganteum

54.15 The Last Refuge The range of giant sequoias has progressively shrunk to a few remaining groves of trees scattered in the southern Sierra Nevada mountains of California.

Sequoiadendron giganteum

54.15 The Last Refuge The range of giant sequoias has progressively shrunk to a few remaining groves of trees scattered in the southern Sierra Nevada mountains of California.

example, were not found in North America before 1985 because they were unable to disperse across the Atlantic Ocean from Europe. Lack of suitable habitat was not the reason for their absence, as demonstrated by their dramatic population growth in North America once they were transported there by human activities. Once they reached North America, they were able to disperse rapidly because the larvae are free-swimming and the adults can attach to moving objects, such as boat hulls.

predators may restrict species' ranges. Predators may eliminate their prey in some places, but not in others. For example, chorus frogs (Pseudacris triseriata) are found in only some of the ponds on islands in Lake Superior. Three major predators—the larvae of a salamander, the nymphs of a large dragonfly, and dytiscid beetles—eat chorus frog tadpoles. An ecologist noticed that the tadpoles were common in ponds with beetles, but rare in ponds with salamander larvae and dragonfly nymphs. In laboratory experiments, he established that the salamander larvae could eat only small tadpoles, but that dragonfly nymphs could eat tadpoles of all sizes. Therefore, he hypothesized that dragonfly nymphs were responsible for eliminating chorus frogs from many ponds. He tested his hypothesis by manipulating densities of predators and prey in ponds. The results showed that dragonfly nymphs can eliminate chorus frogs from ponds that would otherwise be suitable for them (Figure 54.16).

competition may restrict species' ranges. How competitive interactions may restrict the ranges of species is illustrated by interactions between two species of barnacles, Balanus balanoides and Chthamalus stellatus, on rocky North Atlantic seashores. These barnacles have planktonic larvae, which settle between high and low tide levels on the shoreline and become sessile adults. Adult Chthamalus generally live higher in the intertidal zone than do adult Balanus, and there is little overlap between the two species. What keeps their ranges so distinct?

By experimentally removing one or the other species, researchers have shown that the vertical ranges of adults of both species are greater in the absence of the other species. Chthamalus larvae normally settle in large numbers in the Bal-anus zone. If Balanus are absent, young Chthamalus survive and grow well in the Balanus zone, but if Balanus are present, they smother, crush, or undercut the Chthamalus. Balanus larvae also settle in the Chthamalus zone, but the young Balanus grow slowly there because they lose water rapidly when exposed to air, so Chthamalus outcompete Balanus in that zone. The result of the competitive interaction between the two species is intertidal zonation, with Chthamalus growing above Balanus (Figure 54.17).

54.16 Predators Exclude Prey from Some Habitats The speed with which dragonfly nymphs can eliminate tadpoles of the chorus frog from a pond is illustrated by the results of experiments in which populations of predators and prey were manipulated.

54.16 Predators Exclude Prey from Some Habitats The speed with which dragonfly nymphs can eliminate tadpoles of the chorus frog from a pond is illustrated by the results of experiments in which populations of predators and prey were manipulated.

Chthamalus

Balanus can live over a broad range of depths but is more sensitive to desiccation (drying out) than Chthamalus.

Chthamalus is more resistant to desiccation, but is outcompeted by Balanus lower in the intertidal zone.

Spring high tide

Neap high tide

Mean tidal level

Neap low tide

Spring low tide

Balanus can live over a broad range of depths but is more sensitive to desiccation (drying out) than Chthamalus.

Chthamalus is more resistant to desiccation, but is outcompeted by Balanus lower in the intertidal zone.

Spring high tide

Neap high tide

Mean tidal level

Neap low tide

Spring low tide

Chthamalus Balanus Competition

54.17 Competition Restricts the Intertidal Ranges of Barnacles Interspecific competition between Balanus and Chthamalus makes the zone each species occupies smaller than the zone it could occupy in the absence of the other species. The width of the red and gold bars is proportional to the density of the populations.

54.17 Competition Restricts the Intertidal Ranges of Barnacles Interspecific competition between Balanus and Chthamalus makes the zone each species occupies smaller than the zone it could occupy in the absence of the other species. The width of the red and gold bars is proportional to the density of the populations.

Sessile animals such as barnacles and many plants compete for space, but most mobile animals compete for food. As an example of how competition can restrict the ranges of such species, consider the distribution of two species of wasps in California. These wasps lay their eggs on scale insects, and the larvae that hatch from those eggs burrow into, eat, and kill the scale insects. Both wasps were introduced to control outbreaks of scale insects that were damaging citrus orchards. The Mediterranean wasp Aphytis chrysomphali was introduced to southern California around 1900, but it failed to control the scale insects. Therefore, a close relative from China, A. lingnanensis, was introduced in 1948. A. lingnanen-sis, which has a higher reproductive rate, increased rapidly. Within a decade it had not only reduced population densities of the scale insects, but had also displaced A. chrysomphali from most of its range in California.

For many centuries, people have tried to reduce populations of species they consider undesirable, such as scale insects, and maintain populations of desirable species. Efforts to control and manage populations of organisms are more likely to be successful if they are based on knowledge of how populations grow and are regulated. Let's see how such information can be used to manage populations.

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