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Biological Invasions

"We must make no mistake; we are seeing one of the greatest convolutions
of the world's flora and fauna."  - Charles Elton, 1958

In his seminal book, Elton (1958) was speaking of the human-mediated biological invasions which have dramatically altered human history (Crosby 1986) and global biodiversity (Vitousek et al. 1997).  In fact, biological invasions are considered second only to habitat destruction in their effects on native biota and entire landscapes (Wilson 1992) and may even jeopardize endangered species (Glowka et al. 1994).  Habitat disturbance often makes systems more invasible (Orians 1986, Hobbs 1989, Hobbs and Huenneke 1992), and in turn, invaders alter disturbance regimes in natural systems (Mack and D’Antonio 1998).  This powerful synergism has led to substantial changes in whole ecosystems (Vitousek 1986), particularly in South Florida (Ewel 1986, Simberloff et al. 1997) and Hawaii (Moulton and Pimm 1986).  Arms races between natives and invaders continue to balance in favor of invaders, to the extent that we may be facing a global homogenization of our planetary biota in the near future (McKinney and Lockwood 1999).

"They change their clime, not their frame of mind, who rush across the sea."  - Horace

An "invader" is a species that easily crosses a barrier with or without the direct aid of humans, and establishes itself and expands its range on the other side (Ehrlich 1986).  In fact, the most successful invaders are ones able to cross major barriers because of their relationship with man (Elton 1958).  After arriving in new locations, invader populations often expand rapidly due to ecological release (Wilson 1961) and enter habitats beyond that of their native habitat (MacArthur and Wilson 1967).  Invaders may enjoy a lack of predators, a paucity of competitors, an overabundance of dietary or spatial resources, more favorable abiotic conditions, or a combination of these and other factors (Orians 1986, Shigesada and Kawasaki 1997).  Organisms that are good natural colonizers also tend to be good invaders, but the genetic, morphological, physiological, behavioral, and ecological attributes that make particular species good colonizers are elusive (Ehrlich 1986, Pimm 1989, Williamson 1996).

"All animals are equal, but some animals are more equal than others."
- George Orwell, Animal Farm, 1951

Although invasive species affect native species, natural patterns, and ecological processes through a variety of direct and indirect mechanisms occurring at genetic, individual, species, population, community, and ecosystem levels, the concept of environmental "impact" remains furtive with regards to invaders (Parker et al. 1999).  Extremely prolific, obviously detrimental species such as the zebra mussel (Dreissena polymorpha) win by attrition, causing extinctions of native species and completely altering the systems which they invade (Riccardi et al. 1998).  Exotic predators are often unequivocal in their effect, as in the brown tree snake (Boiga irregularis), which has nearly eliminated the avifauna of Guam (Rodda et al. 1999), and the brown trout in New Zealand (Salmo trutta), which has caused local extirpation and fragmentation of native fish populations (Townsend 1996).  Plants often embody the concept of "habitat," so the effects of prolific exotic plant species are very often far-reaching.  In Florida, whole ecosystems have been replaced by near monocultures of Brazilian pepper (Schinus terebinthifolius) and melaleuca (Melaleuca quinquenervia), which alter hydrology, microclimates, and fire regimes (Simberloff et al. 1997).  Although the role of competition in the extirpation of native species can sometimes be important (e.g. Riccardi et al. 1998), it is most often minimal (Simberloff and Boecklen 1991), depending on the identity of the species involved.  Invaders often compete with natives through subtle, more indirect pathways that are difficult to follow, but the effects of competition between invaders and surviving natives are often measurable, especially if background information is available on the native species or system (e.g. Moulton and Pimm 1983).  But the above scenarios of one invader affecting one native, while easier to explain and study, are probably the exception rather than the rule.  Invaders seem to beget further invasions, a result of established transportation corridors or pathways, increased vulnerability of recipient site habitats, or a combination of these and other factors.  Ultimately, multiple invaders confer negative effects on native systems that are much greater than the sum of the individual exotic species making up the community.  Such positive interactions, synergisms, or "invasional meltdowns" (Simberloff and Von Holle 1999) are powerful negative forces working against the conservation of native species, and can only lead to further homogenization of earth's biota.

"Yet, if we wield the sword of extermination as we advance,
we have no reason to repine the havoc committed."
- Charles Lyell, Principles of Geology 1932

As human population increases and development continues unfettered, loss of habitat and fragmentation of remaining habitat will continue to be the primary causes of the world-wide biodiversity crisis (Wilson 1992).  Fragmentation results in a patchwork of native habitat "islands" embedded in a matrix of variously disturbed sites.  Diamond (1975) applied the theory of island biogeography (MacArthur and Wilson 1967) to the concept of fragmentation, pitting the merits of a single large nature reserve against the pitfalls of several small nature reserves as two ends of a continuum, spawning the "SLOSS debate" (Simberloff and Abele 1976, 1982).  Whereas large reserves are clearly more beneficial when a species require extensive core habitat, metapopulation theory predicts that for many species, several small reserves are no less viable than are large uninterrupted tracts of land (Hanski 1999).  In fact, in many studies of fragmentation, habitat subdivision is confounded with habitat loss, the latter obviously having much greater influence on species diversity than the former (Fahrig 1997).  But while the biogeographical aspects of this problem have been debated energetically, the physical changes that occur in edge-rich remnant patches have been under-appreciated (Saunders et al. 1991).  A decrease in habitat patch size brings an increase in the edge-to-interior ratio and increases the relative influence of edge-effects (Ranney et al. 1981), such as increases in the variability of abiotic factors (Murica 1995) and significantly increased vulnerability to biological invasions (Ewel 1986, Hobbs 1989).  So, although intermediate levels of disturbance have long been thought to maximize species richness and benefit community stability (Connell 1978), with fragmentation, habitats often become so small as to be composed mainly (sometimes entirely) of edge, and doomed to harsher conditions and continued invasions (Hobbs and Huenneke 1992).

"We learn from history that we do not learn from history." - G. W. F. Hegel

Just as habitat disturbance may open systems to invasions, in turn, invaders alter disturbance regimes, exacerbate the effects of fragmentation, and even dominate whole systems (Mack and D’Antonio 1998).  Synergism between these processes often initiates a vicious cycle of invasion, habitat decline, more invasions, and so on.  Exotic agricultural species are particularly devastating because humans manage whole landscapes for their optimal production.  Habitat loss and fragmentation, proliferation of additional exotic species, and decline of native species often occur as a result of poor agricultural practices such as overgrazing (Hobbs and Huenneke 1992).  Even native, managed species can become "pests" and devastate habitat when, despite their need for control, they are allowed to proliferate unchecked as would exotics, as white-tailed deer have been allowed to do in many areas because of public sentiment for animal rights (Anderson 1997).  But unintentional invaders can be just as damaging when they occur in concert with habitat alteration and fragmentation.  A noteworthy example is cheatgrass (Bromus tectorum) in the western United States, for which a positive feedback loop has been established where fragmentation occurs, cheatgrass invades and alters the fire regime, which exacerbates fragmentation, increasing cheatgrass dominance, and so on (Knick and Rotenberry 1997).  In another study, the outcome of competition between native and exotic ants in California was influenced by proximity to an urban area, and thus, the degree of fragmentation (Suarez et al. 1998).  Studies of these types of synergisms only arrived in the literature in about the last 10 years, but appear to be increasing in number.

"It is ironic to me to hear people of European ancestry accuse other organisms
of being invasive exotics, displacing native species."  - J. L. Hudson 1997

South Florida contains more introduced plants and animals than anywhere in the continental United States (Ewel 1986, Simberloff et al. 1997). Whole regions are now dominated by highly allelopathic invasive plants such as Australian pine (Casuarina sp.), Brazilian pepper, and melaleuca, which render habitat inhospitable for most native species, and the cycle continues. In fact, the complete take-over of a site by invasive plants is often as damaging as direct alteration by development since, not only has native habitat essentially disappeared, but the novel habitat represents a significant source of the invasive species in question. As a result, in Florida, biomass and food web relationships of whole ecosystems are built upon foundations of exotic species. One often finds non-native or feral predators consuming exotic prey species in habitats composed largely of exotic plants. This level of disruption occurs mainly on developed or otherwise highly disturbed sites (e.g. road corridors, canal banks, and residential sites), but many seemingly "natural" habitats are being invaded as a result of less obvious human influences (e.g. hydrological alterations which assist in the spread of exotics in the Everglades).

"This introduction into Queensland was made only after a careful analysis
of the pros and cons, and, according to the behaviour of the toad up to the present,
there appears to be no reason for the assumption that we have made an error in judgment."
- Reginald Mungomery, 1936, on the cane toad (Bufo marinus) in Australia.

Biological invasions provide unique opportunities for empirically testing the assumptions of many ecological theories (Williamson 1996).  However, invaders usually become established and confer negative effects on native species before anyone has had the opportunity to study the dynamics of the invasion process or the temporal development of negative interactions.  At that point, only removal studies can provide empirical answers.  By definition, removal studies are performed from the exact opposite viewpoint of that which occurs in invasions, by measuring the "recovery" of already negatively affected populations of natives.  The magnitude of their recovery is then used as a surrogate to estimate the negative effect of the invader.  However, the members of surviving native populations probably contain a preponderance of individuals that have successfully adapted to their new situation, thereby representing "ghosts of invasions past," if I may twist the phrase from Connell (1981).  In a removal experiment, recovery of the native might occur because the native population is actually more resilient than were their unencumbered ancestors and, in turn, making the magnitude of the original impact seem much larger than was really the case.  That said, removal is a valid and valuable approach, and is clearly the only option in most cases, as serious ethical ramifications accompany intentional introductions.  This is just one example of the difficulties  often faced while attempting to control variables and approximate reality in studies of ecology in general, and biological invasions in particular.  We have come a long way, yet we have far to go. 

"An ounce of prevention is worth a pound of cure." - Proverb

Control, management, and eradication programs for exotic pest species are often highly controversial, and arguments come from many competing interests, depending on the suite of native and exotic species involved and the nature of the affected habitats.  The problems arising from the use of mechanical and chemical methods are very different than the problems arising from the use of biological control agents.  All three methodologies receive criticism, usually for good reason, as many mistakes have been made.  Techniques that work for one exotic species may not work for others, and use of the wrong method can even exacerbate the problem in some situations.  The literature is packed with examples of well intentioned but badly planned bio-control programs that ended in the release of yet another pest, and half-hearted eradication programs that ended in failure only after an incredible amount of money was spent.  As in medicine, first and foremost we must do no harm.

"A new ecology is emerging, one we don't yet understand,
but one that will debase the marvelously rich diversity of life on earth
unless we manage it well.  That is the challenge of our feral future."
- Tim Low, Feral Future, 1999

On one hand, invasive species management is an "applied" genre of ecology, in which workers are usually trying hard to solve day to day problems in the field using techniques borrowed mainly from the agricultural sciences.  "Basic" research in the ecology and evolution of invaders, on the other hand, generally involves experimental manipulations and theoretical examinations of spatial spread, population dynamics, ecological impacts, and the prediction of future trends, often without focusing on a solution to a particular problem.  As with many disciplines of biology, research and/or teaching in one genre is carried out largely in isolation from the valuable findings and philosophies of the other.  Ironically, the unfortunate retreat from the study and funding of basic natural history surveys, in deference to esoteric mathematical models, sexy experimental designs, and product-oriented research, has compromised the collection of the very data that form the foundation of such studies.  The impressive All-Taxon Biological Inventory (ATBI) being conducted in the Great Smoky Mountains National Park and other federal areas suggests a reversal in this trend.  In the near future, we must form links between the applied and theoretical fields and award natural history surveys the importance they deserve, so we do not miss opportunities to halt or limit the spread, proliferation, and negative impacts that biological invasions have on native biota.  But this fight should not be founded purely on a sense of isolationism, egocentrism, or even patriotism.  Rather, our enthusiasm should be driven by the fear of global homogenization, an irreversible loss of the biotic uniqueness and integrity once present on each and every continent.  Our natural heritage is at stake.

"What the eye doesn't see, the heart doesn't grieve over."  - Proverb

Probably the largest obstacle faced by exotic species researchers and managers is the fact that people are largely unaware that a problem exists, or at least, are unaware that the consequences could be so dire.  There are so many exotic species these days, the public cannot be expected to know which species are just plain non-indigenous, and which are potentially invasive pests.  Effective educational programs will be essential..... 

This "living document," consisting largely of excerpts from Campbell (2000) and quotes found in Low (1999), was last updated by Todd Campbell on on 04/19/2010 .  Please direct any questions, comments, or additional sources of information that might be helpful to Todd Campbell at lizardman@utk.edu.

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