What Did Darwin Note About The Islands Animals Regarding Their Reaction To Human Intruders

Exotic (nonindigenous) species introductions correspond a major threat to both gild and the earth's biota. From an economic standpoint, the costs associated with species introductions have been high. During 1906–1991, monetary amercement resulting from the establishment of 79 exotic species in the Us approximated $97 billion (OTA 1993). Now, nevertheless, with the introduction of well-nigh 50,000 nonnative species into the United States, economic damages (which also include control costs) are approximated at $137 billion per twelvemonth (Pimentel et al. 2000). Beyond these economic considerations, many of the earth'due south ecosystems accept suffered astringent ecological damage—upon which no budgetary value can be placed—following the introduction of exotic species. This impairment, resulting from a multifariousness of mechanisms (eastward.chiliad., competition, predation, hybridization), has included restructuring of populations and communities, alteration of big-scale ecosystem processes, and loss of biodiversity (Lodge 1993a, 1993b, Williamson 1996, Vitousek et al. 1997, Pimentel et al. 2000). Owing to the negative impacts successful invaders can have on ecosystems, one can easily understand why human-based exotic species invasion is considered a leading threat to biodiversity (Vitousek et al. 1997, Sala et al. 2000) and well-nigh probable has contributed significantly to the recent increase in earth's extinction rate, which is equal in magnitude to prehuman periods of mass extinction (Lawton and May 1995, Pimm et al. 1995).

Because of the increase in biological invasions during the 20th century (Mills et al. 1993, Club 1993a, Williamson 1996), research aimed at describing, understanding, and predicting species invasions has increased. In fact, interest in the procedure of biological invasions has been so overwhelming recently that its study has developed into a new subfield within ecology (Guild 1993a), with its own theory and conceptual framework (e.g., Moyle and Light 1996, Williamson 1996). Only only how novel is this subfield, given that much of what we know today almost biological invasions—and accept as conventional wisdom—is similar to ideas consort past 19th-century ecologists? In this article, nosotros endeavour to answer this question by focusing on ideas presented by Charles Darwin in The Origin of Species. Toward this end, we demonstrate that Darwin knew about and appreciated biological invasions, and that the current conceptual framework underlying biological invasions is alike to insights inside Darwin'southward seminal text. Ultimately, in discussing these parallels, we seek to bestow more kudos upon the already honored Charles Darwin.

Sources of data: Darwin and modern-day ecologists

This essay is neither a comprehensive review of biological invasion theory nor a critical evaluation of the ideas (or paradigms) espoused by modernistic-day ecologists or Darwin. Such an undertaking would take been monumental, detracting from our central purpose. Thus, we include only plenty of the primary literature to demonstrate that Darwin's views on biological invasions, as taken from the second edition of On the Origin of Species Past Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1859; cited text, however, corresponds with Darwin 1996), parallel the thoughts and ideas espoused by contemporary ecologists and evolutionary biologists (regardless of whether they are correct). Although many splendid, comprehensive texts and papers have been published on biological invasions (see Lodge 1993b, Williamson 1996), most of the contemporary views presented herein derive from Williamson (1996). We selected Williamson's (1996) fix of ideas for two reasons. Offset, no other document has synthesized all phases of biological invasion (inflow and establishment, spread, equilibrium and effects, and implications) into a concise, organized, and cohesive framework (but run across Moyle and Light 1996). 2nd, although Williamson (1996) did not discover any unconditional rules that govern invasions, he nicely characterized the current conventional wisdom and its weaknesses.

The Origin of Species is not a text exclusively nigh biological invasions. Every bit such, ideas related to biological invasions were non presented in a cohesive, straightforward manner. Within this text, however, we found dozens of statements about characteristics of species and communities that influence invasion success, as well as data relating to why we need to be concerned about species introductions (i.e., a conceptual framework). Beneath, we contrast Darwin'south invasion model, as constructed from statements in The Origin of Species, with the first 10 points of Williamson's (1996) conceptual framework (Tabular array i).

A comparison of conceptual frameworks

Post-obit the arrangement of Williamson (1996), we grouped conceptual framework points in guild to delineate among the diverse stages of the biological invasion process. We beginning hash out factors concerning the arrival and establishment of a species in a new environment ("Arrival and Establishment"). We then proceed (chronologically) to discuss factors relating to its spread in that environment ("Spread"), as well as the potential effects an invader tin can accept on the invaded ecosystem one time information technology has become established ("Equilibrium and Furnishings"). We conclude our comparing of Darwin's and Williamson's ideas by discussing how an exploration of biological invasions can aid our understanding of ecological and evolutionary processes ("Implications").

Inflow and establishment

Conceptual framework point (CFP) 1: Most arrivals at present are from human importations, but natural arrivals also are of interest. Although nigh contempo investigations of biological invasions have focused on those stemming from anthropogenic influences (Mills et al. 1993, Vitousek et al. 1997), Williamson (1996) reminds us that species introductions as well occur naturally. Guild (1993a, p. 133) likewise holds this view, having proposed "biological invasions are commonplace in nature, and should non, in general, be viewed as aberrant events" (also see Elton 1958, Ashton and Mitchell 1989, Carlton 1996). As farther support, Williamson and Brown (1986) demonstrated that more than 75% of exotic avian species entering Britain did so without homo involvement. Similarly, a recent hurricane in the Caribbean, which transported green iguanas (Iguana iguana) from Guadeloupe to Anguilla, has reincited the debate well-nigh the importance of natural dispersal equally a mechanism for agreement the biogeography of terrestrial animals (Censky et al. 1998).

Equally with contemporary scientists, Darwin grasped the regularity and potential significance of natural invasions. In fact, he used it to help account for the nonintuitive biogeographical distribution of species, while as well lending support for his theory of evolutionary modify. Darwin (1996) went to nifty lengths to describe how species invasions deriving from glaciation events, operating on evolutionary time scales (pp. 295–309), and "accidental means" (p. 290), operating on ecological time scales (eastward.g., plants, seeds, mussel or clam veligers transported on the human foot of a duck, pp. 290–293, 311–314; seeds carried in the digestive tract of fish and birds, pp. 292–293, 313; birds blown across the bounding main, pp. 292, 313; seeds drifting in oceanic currents, pp. 290–291), could help explain discrepancies among biogeographical patterns, every bit evidenced from the fossil record (Table one).

Of greater importance, at least on an ecological time scale, are human-assisted introductions, which have increased the charge per unit of biological invasions to the fastest in World's history (Club 1993a, 1993b, Williamson 1996, Vitousek et al. 1997). Carlton and Geller (1993) speculate that on whatsoever unmarried day, more than 3000 species may exist contained within the anchor water of oceangoing ships, potentially leading to the introduction of tens of thousands of species into foreign areas annually (Carlton 1996). Likewise, Mills et al. (1993) showed that, although exotic species introductions have occurred in the Not bad Lakes since the early 1800s, the charge per unit of introduction has increased since that time, with nearly i-3rd of invasions occurring during the last thirty years.

Although humankind'south role in species introductions is emphasized more in contempo literature than in The Origin of Species, Darwin did recognize that role. He wrote, for example, that "the rat and mouse…have been transported by human to many parts of the world, and now have a far wider range than any other rodent" (Darwin 1996, p. 116). Similarly, Darwin (1996, p. 318) credited humankind for the presence of terrestrial mammals on oceanic islands: "As yet I have non found a single instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by natives [emphasis added]) inhabiting an island situated above 300 miles from a continent or great continental island." Other examples regarding the man-assisted spread of exotic species ("domestic productions") too tin can be found in The Origin of Species (Table 1).

CFP two: Nigh invasions fail; only a limited number of taxa succeed (the "tens rule"). Contemporary theory suggests that invasion success is low, with the probability of success following a statistical rule, namely, the "tens rule" (Williamson and Dark-brown 1986, Williamson 1996). A broad generalization spanning a broad range of taxa (including both plants and animals), the tens dominion states that only x% of introduced (or feral) species volition become established (i.e., class cocky-sustaining populations, become naturalized), and of that ten%, only 10% will grow plenty in number to become nuisance or pest species (i.e., take a negative economic effect). Williamson (1992) has demonstrated that the rule has some roughness, in that values tend to range from five% to 20%. Lodge (1993b) settled upon a higher maximum value (i.e., 38%) for successful invasions, but suggested that this value may be biased high considering many studies failed to certificate unsuccessful invaders, focused only upon easily observed species, or had a narrow taxonomic basis. Although deviations from the rule exist (Club 1993a, Williamson 1996), as a predictor of invasion success, the tens rule has held true for a variety of organisms, including angiosperms, grasses, legumes, terrestrial vertebrates, fishes, mollusks, and plant pathogens (Lodge 1993a, 1993b; Mills et al. 1993; Williamson 1996, Fig. ii-2).

Darwin did not endeavour to derive a statistical rule to draw the probability of invasion success. All the same, he did recognize that not all nonindigenous species would successfully colonize their new environment. Theorizing that native species are more suitably adjusted to their environments than invaders, Darwin (1996, p. 65) wrote, "we tin run across that when a plant or creature is placed in a new country amongst new competitors, though the climate may exist exactly the same as in its former home,…the atmospheric condition of its life will generally exist changed in an essential way."

Darwin (1996, p. 65) also suggested that the simply means for an exotic species to establish itself successfully ("increase its average numbers in its new dwelling") would be to "modify it in a dissimilar way to what nosotros should have done in its native country; for we should have to give information technology some advantage over a different prepare of competitors or enemies." Thus, although we cannot be certain that Darwin would accept supported all facets of the tens rule, nosotros are confident that he would take accepted its main precept: Not all exotic species introductions will succeed.

CFP three: Invasion (or propagule) pressure level is an important variable, so invasions are often to accessible habitats by transportable species. Because the probabilities of finding a mate (for those invaders that rely on sexual reproduction) and escaping elimination by pressures associated with weather condition, predators, and parasites increase with propagule pressure (i.e., the number of individuals that invade habitat; Williamson 1996), species that tin hands cross geographical barriers; have resistant, highly dispersive seed or larval stages; or are highly fecund are expected to exist successful invaders. Indeed, much empirical evidence supports this contention, which helps to explain the loftier invasion success of bird (Williamson 1996), fish (Mills et al. 1993), insect (Simberloff 1989), and establish (Bazzaz 1986) species.

Darwin (1996, p. 310) also recognized the importance of dispersal, resistance to desiccation, and fecundity to the successful institution of exotics (Tabular array 1): "This power in fresh-h2o productions of ranging widely…I think, in most cases tin be explained by their having become fitted, in a manner highly useful to them, for curt and frequent migrations." Darwin (1996, p. 314) afterward wrote that "the wide distribution of fresh-water plants and of the lower animals…I believe mainly depends on the broad dispersal of their seeds and eggs." Examples of organisms likely to be successful invaders that were mentioned by Darwin (1996) include aquatic plants (e.g., Helosciadium, Nelumbium luteum, Potamogeton; pp. 290, 313) and invertebrate animals (e.g., the clam Ancylus, a Colymbetes "h2o-beetle," p. 312), every bit well as terrestrial animals that take evolved the ability to wing (due east.m., birds, bats; pp. 292, 319) (Tabular array 1). In fact, Darwin (1996) suggested that the scarcity of terrestrial mammals (excepting bats, because of their ability to wing; p. 319) and amphibians (i.east., "frogs, toads, and newts"; p. 318) on "so many oceanic islands cannot be accounted for by their physical atmospheric condition" just by their "smashing difficulty in their transportal beyond the sea" (p. 318). Through the apply of empirical observation and experimentation, Darwin attributed the invasion success of these species to adaptations that let them either to survive harsh conditions (e.g., prolonged exposure to salt water, air, or digestive juices within the stomachs of fish and birds; pp. 290–293, 312–313) or to be transported by natural processes (e.yard., wind and water currents) across swell distances and for long periods of time (pp. 290–293, 312–313). That Darwin'due south stereotypical successful invader was more r-selected (i.e., highly fecund, high charge per unit of per-capita population growth (r), tolerance for a wide range of conditions, capability of rapid dispersal) than One thousand selected (i.e., depression fecundity, depression r, poor dispersal and colonizing abilities) meshes well with the current conventional wisdom (Lodge 1993a, 1993b, Williamson 1996).

CFP 4: All communities are invasible, perhaps some more than others. One generalization that has go crystallized in our reading of the invasion literature is that all habitats can exist invaded, including protected nature preserves and national parks (Elton 1958, Moyle et al. 1986, Moyle and Low-cal 1996, Williamson 1996, Lonsdale 1999, Stohlgren et al. 1999). In a review of plant invasions in Usa national parks and preserves, Vitousek et al. (1997) demonstrated that exotic vascular plants incorporate 5% to 25% of the flora in many of these protected areas, whereas nonnatives contribute l% to 70% of the flora in Hawaiian reserves. Likewise, in a written report of 23 nature preserves worldwide, Usher (1988) documented that each contained at least one exotic vertebrate and several invasive tracheophyte species. In fact, 18% of terrestrial vertebrates and 30% of the vascular plant species across Usher's report preserves were nonindigenous. Interestingly, Usher (1988) and Lonsdale (1999) both demonstrated that the susceptibility of these "protected" environments to plant invasion was positively related to the number of visitors to the park (i.e., humans serve as a mechanism to increment ship of seeds or fragments, and ultimately propagule pressure).

Even though all communities (and ecosystems) appear susceptible to invasion, Williamson (1996) suggests that disturbed habitats are more decumbent to successful establishment of exotics than are pristine ones. In the start text specifically dedicated to biological invasions, The Ecology of Invasions by Animals and Plants, Elton (1958) also suggested that anthropogenic disturbance increases the likelihood of exotic institution. Elton specifically identified simplified habitats (e.g., monocultures produced by humans) every bit beingness highly vulnerable to exotic invaders, and that human population growth and expansion into new territories, "resulting in unnatural disturbance," take led to an increment in biological invasions of belatedly. Although theoretical and empirical investigations support the notion that disturbed areas are more than probable to be invaded than nondisturbed ones (e.g., Orians 1986, Pimm and Hyman 1987, Carlton 1996, Moyle and Light 1996), the true role disturbance plays in biological invasion success remains unresolved (Ashton and Mitchell 1989, Simberloff 1989, Lodge 1993a, 1993b, Williamson 1996, Stohlgren et al. 1999).

Similar contemporary ecologists, Darwin recognized that no organisation is impervious to invasion (Table i). Using outcomes of man colonization attempts to support this contention, Darwin (1996, p. 69) wrote that

No country can exist named in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of them could anyhow be improved; for in all the countries, the native take been so far conquered by the naturalised productions, that they have allowed foreigners to have firm possession of the land. And as foreigners have thus everywhere beaten some of the natives, we may safely conclude that the natives might take been modified with advantage[s], so every bit to have better resisted such intruders.

In subsequently references that now business concern plants and animals, Darwin (1996) wrote, "The endemic productions of New Zealand, for instance, are perfect i compared to another; just they are now rapidly yielding before the advancing legions of plants and animals introduced from Europe. Natural option will not produce accented perfection, nor do we always encounter, as far as we can judge, with this high standard under nature" (p. 164). "Hardly an island can be named on which our smaller quadrupeds accept not become naturalised and greatly multiplied" (p. 319).

Clearly, these statements support our hypothesis that Darwin recognized that all communities—even those with perfectly coevolved species complexes—are nevertheless vulnerable to invasion by nonindigenous plants and animals.

CFP 5: The a priori obvious is often irrelevant to invasion success. Amidst factors to consider: the intrinsic rate of natural increase (r), abundance in native habitat, taxonomic isolation, climatic and habitat matching, vacant niches. The concluding aspect of the "inflow and establishment" stage of Williamson's (1996) conceptual framework concerns our power to predict invasion success. In general, Williamson (1996, p. 77) suggests that our predictive ability is express, and that "an invader tin be any sort of species going into any sort of habitat." His evaluation of both species (e.g., r, genetic construction, modes of reproduction) and community (e.chiliad., species richness, climate, vacant niches) characteristics indicates that generalities encompassing broad taxonomic ranges are non possible, given the large number of exceptions. In fact, Williamson (1996) suggests that only propagule pressure and previous invasion success in other habitats are consequent predictors of invasion success (only encounter Ehrlich 1989).

Even though disagreement exists amid ecologists regarding predictors of biological invasion success (Ashton and Mitchell 1989, Ehrlich 1989, Simberloff 1989, Williamson 1996, Stohlgren et al. 1999), a close match exists betwixt Darwin and contemporary ecologists regarding species and community attributes that promote successful introductions. For instance, relative to species characteristics, Darwin recognized that species with previous invasion success will likely exist successful invaders in the future; "Information technology is also natural that the dominant, varying, and far-spreading species, which already have invaded to a certain extent the territories of other species, should be those which would have the all-time hazard of spreading nonetheless farther" (Darwin 1996, p. 263). The notion that previous invasion success and the geographical range of species may be indicative of future invasion success is supported by the primary literature (e.g., Moulton and Pimm 1986, Williamson 1996). In his give-and-take of the success of naturalized plants throughout Asia and Due north America, Darwin (1996, p. 55) argued that "the geometrical ratio of increase…but explains the extraordinarily rapid increase and wide diffusion of naturalised productions in their new homes." And, despite being ignorant of Mendelian genetics, Darwin'southward (1996, p. 98) ability to recognize that "the more diversified in construction the descendants from any one species tin be rendered, the more than places they will exist enabled to seize on, and the more than their modified progeny will be increased" conspicuously portrays the wisdom of this thinker.

Darwin likewise recognized that characteristics of an invaded habitat can influence invasion success. Every bit gleaned from his detailed discussions of natural selection, Darwin (1996, p. 65) manifestly believed that biotic interactions (i.eastward., competition and predation) would be major regulators of invasion success:

Nosotros tin see that when a plant or animal is placed in a new country amongst new competitors, though the climate may be exactly the aforementioned as in its old home, nonetheless the conditions of its life will mostly exist inverse in an essential manner. If we wished to increase its average numbers in its new home, we should accept to alter it in a unlike way to what we should take done in its native country; for we should have to give it some advantage over a different set of competitors or enemies.

Attributable to the potential importance of competition to biological invasion success, Darwin (1996, pp. 94–95) speculated that successful invaders volition be species that can accept advantage of open up niches and that are diversified in structure (i.e., morphologically or behaviorally different) relative to ethnic species:

The truth of the principle, that the greatest amount of life can be supported by great diversification of construction, is seen under many natural circumstances…. Information technology is seen, that where they come into the closest competition with each other, the advantages of diversification of structure, with the accompanying differences of habit and constitution, make up one's mind that the inhabitants, which thus jostle each other virtually closely, shall, every bit a full general dominion, belong to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through homo's agency in foreign lands. It might accept been expected that the plants which have succeeded in becoming naturalised in any land would generally have been closely allied to the indigenes… It might also, perhaps have been expected that the naturalised plants would accept belonged to a few groups more peculiarly adjusted to sure stations in their new homes. But the case is very unlike…. These naturalised plants are of a highly diversified nature. They differ, moreover, to a big extent from the indigenes [emphasis added].

Further highlighting the potential office of competition and open up niches to invasion success, Darwin (1996) hypothesized that the probability for invasion would exist greater in small, freshwater systems than in big, continental systems: "As the number of kinds is modest, compared with those on the state, the contest will probably be less severe betwixt aquatic than between terrestrial species; consequently an intruder from the waters of a foreign state, would have a better hazard of seizing on a place, than in the instance of terrestrial colonists" (p. 314).

Although the importance of vacant niches and species variety to the success of biological invasions remains a contentious result (Stohlgren et al. 1999), many gimmicky ecologists have used reasoning analogous to that of Darwin to explain patterns of invasion success. For example, Moyle (1986) used vacant niche theory to help explain why exotic fishes have been more successful at invading western reservoirs (those with low species diverseness) than eastern reservoirs (those with loftier diversity) in the United States. Likewise, our understanding of the success of many exotic fish species in the Corking Lakes appears to be linked to the decimation of ecologically similar native species (Pimm and Hyman 1987, Moyle and Calorie-free 1996). Farther, contemporary ecologists still use Darwin's (1996) reasoning that "[on] a small island, the race for life will take been less astringent" (p. 88) to explicate why pocket-sized islands of low variety accept been more prone to successful invasion than large, speciose continents (Elton 1958, Orians 1986, Pimm 1989). In fact, Moyle and Light (1996) developed a full general empirical dominion for freshwater invasions, which suggests that systems with disrupted native assemblages (i.e., those with open niches) are virtually prone to invasion (besides see Simberloff 1981, Lodge 1993a).

Nosotros conclude the discussion of CFP five by noting what scientists, erstwhile and new, have to say virtually our ability to predict invasion success. Darwin (1996, p. 65) stated that "it is skilful thus to try in our imagination to give any form some advantage over another. Probably in no single instance should nosotros know what to do, so as to succeed. It will convince us of our ignorance on the common relations of all organic beings; a confidence every bit necessary, as information technology seems hard to learn."

He added (p. 307) that "I am far from supposing that all difficulties are removed…in regard to the range and affinities of the allied species which live in the northern and southern temperate zones…. I do not pretend to indicate the exact lines and means of migration [i.e., invasion], or the reason why certain species and non others have migrated."

Ironically, almost 140 years later on ecologists are drawing the same conclusions equally Darwin. Society (1993a, p. 135) suggested that "because all patterns are characterized past large variance and exceptions, nosotros cannot with whatsoever confidence predict the outcome of any particular introduction. For successful prediction, every potential invader and target community must be intensively studied." Similarly, Ehrlich (1989, p. 326) stated that "ecologists tin can brand some powerful and wide-ranging predictions almost invasions…. On the other paw, ecologists cannot accurately predict the results of a unmarried invasion or introduction event." Clearly, our inability to forecast species reflects the complex nature of ecological interactions (Ehrlich 1989).

Spread

CFP 6: Spread can be at whatever speed in any direction. Although Williamson (1996) appears pessimistic about our ability to predict invasion success, he feels that our capacity to predict rates of spread post-obit establishment may be greater. In full general, Williamson (1996) suggests that, although the rate and management of spread tin can be of any speed or direction, knowledge of the intrinsic rates of increase and improvidence may aid our predictive capabilities. Admittedly, Darwin'south discussions of rates of spread are limited. Even so, he did admit "the fact that many species, naturalised through man'south agency, have spread with amazing rapidity over new countries…[and] that most species would thus spread" (p. 325). Darwin (1996) also mentioned attributes that very likely govern the charge per unit of spread and "procedure of diffusion," such as the geometrical charge per unit of increase (pp. 54–55, 263), competitive abilities (pp. 87–88, 263), ecological and evolutionary history of the species (pp. 263-264), dispersal mechanisms (pp. 310–315), biotic interactions (i.e., "the weather condition of life"; pp. 263, 324), and climatic and geographic features (pp. 263, 306–307) (meet Table 1).

Equilibrium and furnishings

CFP vii: Near invaders produce pocket-sized consequences (tens dominion). Although exotic invaders can cause extensive ecological, evolutionary, and economic damage, every bit an axiom of CFP 2 (i.e., the tens rule), Williamson (1996) suggests that about invaders (about 99%) will have little ecological or economic impact. Darwin did non attempt to quantify the number of invasions that would succeed; notwithstanding, nosotros plant evidence to propose that he realized that successful invasions would not ever take major effects on the invaded community: "Probably no region is as nevertheless fully stocked, for at the Greatcoat of Skilful Promise, where more species of plants are crowded together than in whatsoever other quarter of the world, some foreign plants have get naturalised, without causing, as far as we know, the extinction of any natives" (Darwin 1996, p. 91).

In addition, based on Darwin'south discussions of the importance of the "weather of life" to invasion success, even suitable climate matches between the invader and the invaded habitat do not guarantee invasion success: "That climate acts in main role indirectly past favouring other species, nosotros may clearly see in the prodigious number of plants in our gardens which can perfectly well endure our climate, but which never become naturalised, for they cannot compete with our native plants nor resist destruction by our native animals" (1996, p. 58). Thus, for an exotic invader to succeed, some major "advantage" over its competitors and predators would exist required (Darwin 1996, p. 65).

CFP 8: The consequences of invasions tin can be severe, ranging from depressed populations to individual extinctions to ecosystem restructuring, and the causal mechanisms driving these changes can be various. Conceptual framework point 2 suggests that about invasions have little impact on the biotic and abiotic components of the invaded ecosystem. Withal, of those systems that are afflicted, the consequences tin be severe, including alteration of population demographics (Moyle and Light 1996, Ayres et al. 1999, McKinney and Lockwood 1999, Parker et al. 1999), community system (e.g., species composition and abundance, food-web structure; Zaret and Paine 1973, Schulz and Yurista 1998, Parker et al. 1999), and ecosystem function (Lodge 1993b, McKinney and Lockwood 1999, Parker et al. 1999), as well as species extirpation or extinction (Witte et al. 1992, Fritts and Rodda 1998, Ayres et al. 1999).

For example, in Lake Victoria approximately 200 of 300 Haplochromis cichlid species—of which 99% were endemic—were driven to extinction primarily past the introduction of piscivorous Nile perch (Lates niloticus) during the early on 1950s (Witte et al. 1992). In the Flathead catchment, Montana, post-obit the intentional introduction of opossum shrimp (Mysis relicta) into Flathead Lake in 1949 to improve the nonindigenous kokanee salmon (Oncorhynchus nerka) fishery, kokanee salmon declined in abundance considering their preferred casualty (cladoceran zooplankton) was, ironically, decimated by M. relicta (via both predation and competition) (Spencer et al. 1991). The reduction in kokanee salmon so triggered reductions in eagle, bear, gull, and duck visitations to the lake, ultimately resulting in reduced tourism to the area (Spencer et al. 1991). In Hawaii's Volcanoes National Park, invasion of Myrica faya, a nitrogen-fixing tree, altered nutrient cycling in that ecosystem, ultimately changing the "rules of existence" for many species (Vitousek et al. 1997). Specifically, Vitousek and Walker (1989) found that M. faya increased the availability of inorganic nitrogen, which is typically a limiting nutrient in this ecosystem, by more than fourfold. This, in turn, allowed other exotic species adapted to high food conditions to dominate these habitats.

A diverse array of mechanisms drives these changes, including vertical food concatenation effects (i.e., predation; Zaret and Paine 1973, Fritts and Rodda 1998, Schulz and Yurista 1998), horizontal food concatenation effects (i.due east., competition, both interference and exploitative; Moyle and Light 1996, Gild 1993a), and hybridization (Philipp 1991, Rhymer and Simberloff 1996, Ryman et al. 1995, Ayres et al. 1999, Parker et al. 1999). Because these are simply a few of numerous examples of negative impacts of biological invasions, we agree with Vitousek and colleagues (1997) that biological invasions are a "significant component of environmental change."

By contrasting a role of a heath that "had never been touched by the hand of human" (p. 59) with some other part that "had been enclosed 20-five years previously and planted with [exotic] Scotch fir" (p. 60), Darwin (1996) clearly demonstrates his sensation of how "potent" species introductions tin can exist. Specifically, he observed that "the alter in the native vegetation of the planted function of the heath was most remarkable…. Not simply the proportional numbers of the heath-plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the [unplanted] heath. The effect on the insects must take been still greater, for six insectivorous birds were very common in the plantations, which were not to exist seen on the [unplanted] heath" (p. threescore).

In another case, Darwin (1996) relates how important grazing cattle (an introduced herbivore) can be in structuring communities. In areas protected from grazing by enclosures, "it became thickly clothed with vigorously growing young firs. Even so the [unprotected] heath was so extremely barren and so all-encompassing that no one would ever have imagined that cattle would accept so closely and effectually searched it for food" (p. 60). Farther espousing the potential importance biological invasions can have on communities, Darwin (1996, p. 307) wrote, "In many islands the native productions are nearly equalled or even outnumbered past the naturalised; and if the natives have not been really exterminated, their numbers accept been greatly reduced, and this is the showtime stage towards extinction." Further, Darwin (1996, p. 315) stated that "there is reason to believe that the naturalised plants and animals take nearly or quite exterminated many native productions." From these examples and others (Tabular array i), nosotros clearly tin see that Darwin recognized that biological invasions can have dramatic effects on invaded communities.

CFP 9: Genetic factors may decide invasion success; genetic factors affect events at the initial invasion; evolution may occur subsequently invasion. Although the recent invasion literature has focused primarily on the importance of abiotic and biotic interactions in governing invasion success, a small, only growing, component has focused on the role of the genetic construction of invaders (east.k., Philipp 1991, Ryman et al. 1995, Rhymer and Simberloff 1996, Ayres et al. 1999, Parker et al. 1999). These investigations take been largely theoretical, exploring the potential significance of a diversity of genetic attributes (e.g., ploidy, heterozygosity, asexual versus sexual reproduction) to the success of an invader.

Although a few empirical studies have suggested that heterozygosity is important to the invasion success of plants (e.chiliad., Martins and Jain 1979), the role of genetics is still largely unknown. Williamson (1996), however, suggests that genetic factors most likely are of import, given that a slight alteration in a species genome tin influence its fettle. In back up of this notion, Ayres et al. (1999) demonstrated that an exotic congener caused the local extinction of Spartina foliosa (a native cordgrass) via hybridization. Similarly, Philipp (1991) demonstrated that introduced Florida largemouth bass (Micropterus salmoides floridanus) readily hybridizes with its northern subspecies (Thousand. salmoides salmoides), which, in turn, can negatively affect growth and overwinter survival of the northern subspecies. Ryman and colleagues (1995) also provide numerous examples of how hybridization of cultured fish with wild fish can atomic number 82 to the loss of coadapted gene complexes and genetic integrity. As such, we concord with Ryman and colleagues (1995) that the genetic effects of introduced species is a crucial uncertainty that warrants further report.

Evidently, nosotros cannot argue that Darwin had any thoughts on the importance of the genetic construction of organisms to invasion success because he was non knowledgeable about Mendelian genetics. However, upon examination of his ideas regarding species that succeed in the game of natural selection, nosotros can meet that he recognized the potential importance of heritability (admitting in a somewhat Lamarckian sense) and variability to the time to come success of organisms. "We accept, likewise, seen that it is the most flourishing or dominant species of the larger genera which on an average vary most…. The larger genera thus tend to become larger; and throughout nature the forms of life which are now dominant tend to become all the same more dominant by leaving many modified and dominant descendents" (Darwin, 1996, p. 50), he wrote.

Later, Darwin (1996, p. 98) stated that "equally a general rule, the more diversified in structure the descendants from whatever one species can exist rendered, the more places they will exist enabled to seize on, and the more their modified progeny volition exist increased." Clearly, Darwin was speaking of phenotypic attributes in this instance. His ideas, nevertheless, certainly parallel our thoughts on the importance of genetic variability and morphological plasticity to the success of a species. If only Darwin had managed to pick upwards a copy of Gregor Mendel's Experiments in Plant Hybridization….

Implications

CFP x: Invasions are informative nigh the structure of communities and the strength of interactions, and vice versa. This CFP focuses on the necessity of understanding the dynamics of biological invasions and communities because of insights they can provide about community organisation, processes that regulate invasion success, ecological risks associated with a new invader (including organisms introduced as biological controls), and evolution. Our recent history is checkered by accidental and intentional introductions that caused dramatic ecological and economic harm (east.g., zebra mussels Dreissena polymorpha, Nile perch, opossum shrimp, M. faya, chocolate-brown tree snakes Boiga irregularis), many of which could have been avoided had we enough foresight to empathize the dynamics of the invader and invaded community. We have already mentioned the ecosystem-level furnishings experienced in the Flathead catchment attributable to the intentional introduction of mysids. Similarly, the planned introduction of a predatory snail, Euglandina rosea, into Hawaii every bit a biological command for another nonindigenous snail species (Achatina fulica) led to the elimination of all native Partula snails, and is expected to eliminate all endemic tree snails (Williamson 1996). For reasons such as these, we can empathize why fishery ecologists are warning against the intentional introduction of stocked fishes throughout Europe and Northward America (Moyle et al. 1986, Philipp 1991, Ryman et al. 1995).

Although the scientific community is simply now beginning to recognize that biological invasions can lend insight into ecological and evolutionary processes (eastward.g., Simberloff 1989, Club 1993a, 1993b), Darwin (1996, p. 95) was cognizant of this even before environmental became an organized discipline: "Past considering the nature of the plants or animals which take struggled successfully with the indigenes of any country, and have there go naturalised, we may gain some crude idea in what manner some of the natives would have to be modified, in gild to gain an advantage over the other natives."

Similarly, he fifty-fifty suggested that knowledge of mechanisms that allow successful colonization of an exotic species may shed insight into the biogeographical distribution and evolutionary success of species: "I practise not pretend to bespeak the exact lines and means of migration, or the reason why certain species and not others have migrated; why certain species have been modified and accept given rise to new groups of forms, and others take remained unaltered. We cannot hope to explicate such facts, until nosotros can say why one species and non some other becomes naturalised by man'south bureau in a foreign country" (p. 307).

The Origin of Species: More than a text most natural selection

Certainly, one can contend that Charles Darwin'due south The Origin of Species has been the single nigh of import contribution to modern biology. Nigh 140 years since its first publication, Darwin's theory of evolution (with subsequent modifications made past geneticists) is still revered equally the almost scientifically plausible explanation for speciation, and it has fix the foundation for the enormous evolution-based enquiry effort that pervades biological science, paleontology, and anthropology. Owing to its impact on how we interpret historical and modernistic day biology (eastward.one thousand., phenotypic and genotypic variation, functional morphology, species diversity, establish and beast behavior), one can easily understand why Futuyma (1995, p. v) considers development to be the "single most pervasive theme in biology, the unifying theme of the entire science."

Simply The Origin of Species is and then much more than a seminal text on development. Historians and biologists alike have argued that this work provided the basis of mod-24-hour interval ecology (McIntosh 1985), and a cursory read of The Origin of Species will reveal the truthful roots of many ecological theories and phenomena (e.g., competitive exclusion, limiting similarity, grapheme deportation, predation, sexual pick, kin selection, isle biogeography). In fact, Darwin's piece of work has been so influential—mainly those elements relating to evolution—that its touch on extends beyond scientific discipline itself, permeating many aspects of lodge, including politics, education, and faith (Futuyma 1995, Gould 1999).

Yet, despite all the recognition Darwin's seminal text has received, we still may not have attributed to him all the credit he deserves. For as we have just demonstrated, Darwin spoke authoritatively about the growing subfield of biological invasions. In fact, much of the current conventional wisdom regarding biological invasions was consort in The Origin of Species. Like many gimmicky ecologists, Darwin was aware that exploration of the biological invasion process is important considering it tin provide insight into how communities are structured, equally well as assist u.s. understand the biogeographical distribution and evolution of species. Similarly, he recognized that attributes of both the invader and the invaded community tin can be of import to understanding the success of an exotic species in a new environment. Darwin even drew the same conclusions (namely, that our ability to predict the success of invasions volition remain limited because of the complexity of ecosystems) that many ecologists now take as the current conventional wisdom (e.k., Ehrlich 1989, Guild 1993a).

Given that Darwin's conceptualization of biological invasion success really does not differ much from the present conventional wisdom (regardless of the correctness of ideas, whether old or new), we, like others (Williamson 1996, p. i, Stohlgren et al. 1999, p. 25), feel that Darwin's insights into biological invasions should be recognized. Quite possibly, had some of Darwin'south observations on biological invasions been better noted, we might not be experiencing the severity and variety of issues that nosotros currently face.

Acknowledgments

We give thanks D. Culver, 1000. Kershner, K. Greenstone, J. Holomuzki, R. Stein, T. Waite, and two bearding reviewers for their comments on previous drafts of this manuscript. Support for this work was provided past (1) a Federal Aid in Sport Fish Restoration Project F-69-P (to R. A. Stein), administered jointly by the The states Fish and Wildlife Service and Ohio Department of Natural Resource–Division of Wildlife, (2) the Section of Evolution, Ecology and Organismal Biology at The Ohio State University (OSU), and (3) a Presidential Fellowship awarded to S. A. Ludsin by OSU. Final preparation of this manuscript was assisted by support from a Great Lakes Institute for Environmental Research Post-doctoral Fellowship at the University of Windsor awarded to S. A. Ludsin and a Natural Sciences and Engineering Inquiry Council of Canada Strategic Grant (to P. F. Sale and others).

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Writer notes

Stuart A. Ludsin (e-mail: ludsin@uwindsor.ca) is a Great Lakes Institute for Environmental Research Postdoctoral Swain in the Department of Biological Sciences at the University of Windsor, Windsor, Ontario, Canada N9B3P4. Ludsin is interested in understanding mechanisms that regulate fish population and community dynamics, as well equally in applying this knowledge to conservation and management bug, particularly every bit they relate to the effects of anthropogenic perturbation on aquatic ecosystems.

Andrea D. Wolfe (e-mail service: wolfe.205@osu.edu) is an associate professor in the Department of Evolution, Ecology, and Organismal Biology at The Ohio State University, Columbus, Ohio 43210. Wolfe'due south enquiry focuses on the biosystematics of Penstemon and related genera of tribe Cheloneae (Scrophulariaceae) and the molecular evolution of nuclear and plastid genes in found lineages containing nonphotosynthetic members.

© 2001 American Establish of Biological Sciences

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