L

Disturbances Frequent — Soon After a Disturbance Disturbance Large

Infrequent Long After Small

FIGURE 7.3 Graph of the intermediate disturbance hypothesis which suggests that the maximum diversity occurs when the disturbance level is moderate. (Adapted from Connell, J. H., 1978. Science. 199:1302-1310.)

Disturbances Frequent — Soon After a Disturbance Disturbance Large

Infrequent Long After Small

FIGURE 7.3 Graph of the intermediate disturbance hypothesis which suggests that the maximum diversity occurs when the disturbance level is moderate. (Adapted from Connell, J. H., 1978. Science. 199:1302-1310.)

species is reduced. This is a promising focus to take for understanding exotic invasions, especially due to the well-developed theory of disturbance in ecology (Clark, 1989; Connell, 1978; Levin and Paine, 1974; Petraitis et al., 1989; Pickett and White, 1985; Pickett et al., 1989; Reice, 2001; Sousa, 1984; Walker, 1999). This theory states that species can adapt to natural disturbances, and in some cases they even use the disturbance as an energy source. As examples, energy from disturbances can be used for accelerating nutrient cycling or dispersing propagules. Connell's (1978) study, which showed that the maximum diversity was found at intermediate levels of disturbance (Figure 7.3), became a benchmark in documenting the important role of disturbance in ecology. The hump-shaped pattern arises for several reasons. At low levels of disturbance, the most adapted species outcompete all of the other species (for example, those that are "K-selected," see Chapter 5), which lowers diversity. At high levels of disturbance, only a few species can adapt to the environmental conditions that change so often (for example, those that are "r-selected"), which also lowers diversity. The highest diversity occurs at intermediate levels of disturbance because some species adapted to the entire disturbance spectrum are supported. Energy theory provides an alternative explanation: the intermediate levels of disturbance provide the most energy subsidy to the ecosystem, while low levels of disturbances provide less energy subsidy and high levels of disturbance act as stress rather than subsidy (E. P. Odum et al., 1979). Disturbance theory has led ecologists to emphasize nonequilibrium concepts of ecosystems over the earlier ideas of more static "balance-of-nature" concepts. The theory of island biogeography (see Chapters 4 and 5) is an example of an equilibrium model for explaining species diversity. Under equilibrium conditions competitive exclusion can run its course, eliminating inferior competitors and selecting for the species best adapted to a site. However, under nonequilibrium conditions the environment changes frequently enough that competitive exclusion cannot run its course and thus more species are supported on the site. Nonequilibrium theory was first used by Hutchinson (1961)

to explain the "paradox of the plankton" or why so many species of phytoplankton are found to coexist in the epilimnion or upper layer of a lake. The epilimnion seemed to offer only one niche for phytoplankton since it was uniformly mixed with constant light intensity. Under these conditions the competitive exclusion principle (see Chapter 1) dictated that only one species of phytoplankton should be found at equilibrium. Yet many species are found there. Hutchinson solved this paradox by suggesting that environmental conditions (such as temperature and nutrient concentrations) actually change with sufficient frequency to preclude the onset of competitive equilibrium, thus allowing many species to coexist. Huston (1979) elaborated and generalized Hutchinson's nonequilibrium concept of species diversity in an important paper published one year after Connell's classic. Since the 1970s non-equilibrium and disturbance theory have become dominant in ecology (Chesson and Case, 1986; DeAngleis and Waterhouse, 1987; Reice, 1994; Wiens, 1984). The shift in emphasis from equilibrium to nonequilibrium perspectives is critically important in ecology, but it does not necessarily imply that the field is without order or predictability. Rather, as noted by Wu and Loucks (1996), "harmony is embedded in the patterns of fluctuation, and ecological persistence is 'order within disorder'."

It is not enough to simply correlate exotic invasion with disturbances caused by humans. Much research is needed for understanding how the various kinds of human disturbances act. Frequency, intensity, and duration have been found to be good descriptors of natural disturbances. Work is needed to quantitatively derive similar descriptors of human disturbances in relation to exotic invasions. For example, no simple relation was found between urbanization as a form of disturbance and degree of exotic invasion by Zinecker (1997) for riparian forest plant species in northern Virginia. The approach of Reeves et al. (1995) in developing "a new human-influenced disturbance regime" might be a good model for the disturbances that facilitate invasion of exotic species.

Relatively less attention has been given to the factor of increased dispersal by humans as the cause of exotic invasions, although it is usually acknowledged as being important. In fact, invasions can occur in systems that are not necessarily disturbed by humans, as long as an invader can reach the system. The invasion of isolated oceanic islands, such as the introduction of goats by explorers in the 1700s, is an example of this situation. However, increased disturbance and dispersal usually occur simultaneously, making it difficult to separate the two factors in most case studies. Increased dispersal of species from one biogeographic province to another is occurring due to increased rates of travel and trade within the global economy. Total amounts of dispersal are seldom known because only successful introductions are recorded (Simberloff, 1981, 1989; Welcomme, 1984). Ship ballast, as a form of increased dispersal for aquatic organisms, is a good example of a well-studied mechanism (Carlton, 1985; Williams, 1988) and has potential for regulation (National Research Council [NRC], 1996a). New syntheses of dispersal by exotic organisms must be based on detailed species-specific studies, such as Carlton's (1993) work on zebra mussels (Dreissena polymorpha), which are only now starting to accumulate in the literature. Studies of the dispersal of native species (Bullock et al., 2002; Clobert et al., 2001; Gunn and Dennis, 1976; Howe and Smallwood, 1982; van der Pijl, 1972; Wolfenbarger, 1975) can be models for the syntheses, but there will probably be new elements of preadaptation that help explain increased dispersal rates by exotic organisms.

The final component of the equation given earlier in this section is the concept of new systems with mixes of natives and exotics. Mooney and Drake (1989) emphasize the idea that these are "new," which is a different perspective than one gets from reading most literature on exotics. Rather than thinking of these as natural systems that have been degraded by the introduction of exotic species, they can be seen as new systems that have been reorganized from the old "natural" systems. The value of this perspective is that it allows thinking to be freed from biases to consider new forms of organization (see Chapter 9).

Humans are creating a tremendous number of new habitats that in turn create opportunities for new mixes of species. Cohen and Carlton (1998) describe the San Francisco Bay and Delta ecosystem as having perhaps the highest exotic species diversity of any estuary because the bay is a focal point for transport and, therefore, increased dispersal and because of extensive human disturbance (Nichols et al., 1986; Pestrong, 1974). In another example, Ewel (1986a) describes the new soil conditions of South Florida as being an important factor in the exotic invasion of terrestrial systems in the following quote:

Substrate modification, such as rock plowing, diking, strip-mining, and bedding, has created soils and topographic features heretofore unknown to Florida. These human-created soils, or anthrosols, are likely to support new ecosystems in which exotic species play dominant roles. The Hole-in-the-Doughnut in Everglades National Park exemplifies this situation. Despite efforts by the National Park Service to restore native vegetation to this rock plowed land, a peppertree/wax myrtle/saltbush ecosystem persists there.

The story of invasion of Gatun Lake in Panama by cichlid fish species (Swartzmann and Zaret, 1983; Zaret, 1975; Zaret and Paine, 1973) offers another view of new systems. This is an example that is often used to illustrate the severity of changes that exotic introductions can have on an ecosystem. In this case the cichlid is a voracious predator that was introduced into the lake. Changes in the lake's food web over time, which included dramatic reductions in native species and a simplification of the structure of the food web, were documented (Figure 7.4). While this is often used as an example of how much change an exotic can make in a native food web, in fact it may be better explained as an example of a reorganization of a new system because Gatun Lake is a reservoir formed as part of the Panama Canal rather than a natural lake. The original natural system was a river that was subsequently turned into a reservoir when the canal was built. This change in hydrology must have played a significant role in the changes in the food web that Zaret and Paine described. This interpretation is not intended to diminish the importance of Gatun Lake as an example of exotic invasion but rather to highlight the context of the example as a reorganized new system rather than a degraded natural system.

One way to think of the new systems is as examples of alternative stable states. In this concept if a system is perturbed beyond some threshold of resilience, the system may change through succession to a new organization or stable state and not

FIGURE 7.4 Comparison of food webs in Gatun Lake, Panama, with and without an exotic fish predator. (A) Tarpon atlanticus. (B) Chlidonias niger. (C) Several species of herons and kingfishers. (D) Gobiomorus dormitor. (E) Melaniris chagresi. (F) Characinidae, including four common species. (G) Poeciliidae, including two common species; on exclusively herbivorous, Poecilia mexicana, and one exclusively insectivorous, Gambusia nicaraguagensis. (H) Cichla-soma maculicauda. (I) Zooplankton. (J) Terrestrial insects. (K) Nannophtoplankton. (L) Filamentous green algae. (M) Adult Cichla ocellaris. (N) young Cichla. (From Zaret, T.M. and R. T. Paine. 1973. Science. 182:449-455. With permission.)

FIGURE 7.4 Comparison of food webs in Gatun Lake, Panama, with and without an exotic fish predator. (A) Tarpon atlanticus. (B) Chlidonias niger. (C) Several species of herons and kingfishers. (D) Gobiomorus dormitor. (E) Melaniris chagresi. (F) Characinidae, including four common species. (G) Poeciliidae, including two common species; on exclusively herbivorous, Poecilia mexicana, and one exclusively insectivorous, Gambusia nicaraguagensis. (H) Cichla-soma maculicauda. (I) Zooplankton. (J) Terrestrial insects. (K) Nannophtoplankton. (L) Filamentous green algae. (M) Adult Cichla ocellaris. (N) young Cichla. (From Zaret, T.M. and R. T. Paine. 1973. Science. 182:449-455. With permission.)

revert back to the old organization (Holling, 1973; May, 1977). Thus, some form of disturbance may push a natural system into a new domain of stability with an entirely new set of species (Figure 7.5). Alternative stable states have been discussed for a number of ecosystems including coral reefs (Done, 1992; Hughes, 1994; Knowlton, 1992), grazing systems (Augustine et al., 1998; Dublin et al., 1990; Laycock, 1991; Rietkerk and Van de Koppel, 1997), mud flats (Van de Koppel et al., 2001), and lakes (Blindow et al., 1993; Scheffer and Jeppesen, 1998). The concept remains controversial but seems to be generally applicable (Carpenter, 2001; Law and Morton, 1993; Sutherland, 1974). Introduction of exotic species can be thought of as an impact that causes the system to change from one stable state to a new one with a reorganized ecosystem structure and function. For example, the invasion of zebra mussels into the Great Lakes has been suggested to cause a shift from a pelagic stable state to a benthic stable state because of the zebra mussels' ability to strip sediments and algae from the water column through suspension feeding (Kay and Regier, 1999; MacIsaac, 1996). With increased dispersal by humans many new mixes

Domain c Original Sys

Domain c Original Sys

FIGURE 7.5 Theory of alternative stable states in ecology. An ecosystem can be pushed between alternative domains by major disturbances. (Adopted from Bradbury, R.H. et al. 1984. Australasian Science. 14(11-12):323-325.)

Possible Domain of a New System

FIGURE 7.5 Theory of alternative stable states in ecology. An ecosystem can be pushed between alternative domains by major disturbances. (Adopted from Bradbury, R.H. et al. 1984. Australasian Science. 14(11-12):323-325.)

of species may come together on a site and allow for the creation of new alternative stable states. Perhaps the number of possible alternative stable states is much greater with the accelerated seeding rates of human introductions as compared with what is possible under the old natural conditions. In a sense, genetics is a limiting factor to ecosystem development in natural systems and may be overcome by exotic invasions that add species to the system.

The new order created by exotic invasions is both a challenge and a stimulus for learning about ecology. Marston Bates (1961) made this connection in a relatively early reference:

The animals and plants that have been accidentally or purposefully introduced into various parts of the world in the past offer many opportunities for study that have hardly been utilized. They can, in a way, be considered as gigantic, though unplanned, experiments in ecology, geography, and evolution, and surely we can learn much from them.

This was also stated by Allee et al. (1949) in their classic text on animal ecology:

The concept of biotic barriers may be tested by introducing animals and plants from foreign associations and observing the results. In most instances such tests have not been performed consciously. With the advent of modern transportation, many organisms are inadvertently introduced into ancient balanced communities. These unwitting experiments may be studied with profit.

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