Exotic Species article--Tucson Audubon Society

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Tucson Audubon Society
Exotic Species

Exotic Species
by Bill Mannan
(Reprinted from the Vermilion Flycatcher, newsletter of Tucson Audubon)
The introduction and proliferation of non-native, or “exotic,” organisms can have disastrous effects on native biota via competition, predation, disease, or habitat alternation. On a global scale, the conservation problems caused by exotic species now rival “anthropogenic loss of habitat” in significance. The biota in southern Arizona, like that in most of the rest of the world, also suffers from problems caused by exotic species, and conservationists here are struggling to deal with these problems. The purpose of this article is to discuss why some introductions succeed and other don’t, to review the reasons why non-native organisms are introduced into new places, and to identify the conservation strategies (if any) that can reduce problems caused by the introductions.

WHY SOME INTRODUCTIONS SUCCEED AND SOME DON’T?
Perhaps the most remarkable fact about the introduction of non-native species, given the tremendous problems they have caused, is that most introductions do not succeed. But the lack of success makes sense when considering the challenges faced by all tiny populations, and evolutionary constraints. Non-native organisms are just that – they evolved in ecosystems that differ from the ones into which they are being introduced. They are, therefore, almost always at a disadvantage because their adaptations likely do not match the environmental challenges they face in their new homes. For example, the foraging strategies of the introduced animals may not be effective given novel sources of food, or their strategies for avoiding being eaten may not work given a new set of predators. Furthermore, most introductions, whether accidental or deliberate, consist of a small number of organisms. Small populations suffer from numerous problems that cause their local extinction, even if they are well suited to the environment. (Our concerns about endangered species illustrate these problems dramatically.) It should come as no surprise, therefore, that most introductions of non-native organism fail. Is this reason for ambivalence about the issue of introducing exotic species? Certainly not, because obviously some introduced organisms happen to fit their new environments even better than the native species. If they surpass the challenges faced by small populations, they can then explode into their new homes. The consequences of these “explosions” often are disastrous and far-reaching.

WHY ARE EXOTIC SPECIES INTRODUCED?
Given the problems that non-native organisms cause when they proliferate in new environments, it is reasonable to ask why they are ever introduced in the first place. The answer is that most recent introductions are accidental. Humans today occupy and easily move among nearly all places on the globe, and when they move they often unwittingly carry with them and deposit seeds or organisms into new places. For example, the brown tree snake was likely introduced onto the island of Guam in a shipment of military cargo during World War II. It has since proliferated and exterminated by predation many native species of birds and reptiles. But some of what are now considered to be the most “infamous” introductions were deliberate. What could possibly motivate anyone to intentionally introduce non-native plants and animals? There were three primary reasons: 1) to produce new populations to harvest, either for food or fur; 2) to fix a problem; and 3) what can only be categorized as whims.

Many of the exotic species that cause problems today were deliberately introduced in the late 1800s and early to mid-1900s in attempts to increase the number of animal populations available for food, fur, or sport hunting. Well known examples are the European hare in Australia and the carp and nutria in the United States. The European hare practically denuded the vegetation in parts of Australia as its population rapidly (i.e., 1000 miles in 16 years) spread across the countryside. The destruction of vegetation nearly devastated the livestock industry and likely negatively influenced many native species. The carp was introduced into the United States as a source of food (it was considered a delicacy in Europe and the east), and now occurs in thousands of lakes and rivers where it destroys aquatic habitat by muddying the water and competes with native fish. Other species introduced as potential game animals, such as the ring-necked pheasant and chuckar partridge, appear to be doing no real harm, but the environments into which they were introduced (e.g., farmlands) were already changed dramatically before the introductions were made. Of interest is that until the early1970s, the US Fish and Wildlife Service employed people whose job it was to seek new organisms to fill the empty “niches” in the United States. It is tempting to condemn the folks who made these introductions as being bereft of their senses, but it is important to remember that many animal populations in the United States had nearly been eliminated by market hunting during this period, and the science of ecology was nonexistent. Thus, actions that seemed reasonable at the time were not wise, but we recognize the lack of wisdom only in hindsight.

Sometimes, non-native organisms are introduced into new environments to “fix a problem.” The problem to be “fixed” is, more often than not, another exotic species. Two examples of this kind of introduction are the mongoose in Hawaii and mosquitofish in Arizona and elsewhere. The mongoose was introduced into Hawaii to control Norway rats. Unfortunately, the mongoose preferred to eat the eggs and nestlings of ground-nesting birds, and has caused considerable damage to the native Hawaiian avifauna. Mosquitofish were bred and distributed for introduction in Arizona and elsewhere to control mosquito larvae, but they have proliferated and now compete with and eat native fishes such as the topminnow. Non-native plants also have been introduced to “fix problems.” Lehmann’s lovegrass was introduced widely into the grasslands of southern Arizona during the 1970s to control erosion and increase production of forage for cattle. This exotic grass is now widespread throughout southern Arizona and competes with native grasses for space, nutrients, and water.

The third “reason” why non-native organisms were introduced into the United States includes the actions of people who had no real reason at all. Perhaps the most famous of these is the man in New York who introduced the European starling to his neighborhood in the 1918 because he wanted to live amongst all the species of birds that were mentioned in the works of Shakespeare. The starling spread rapidly across the United States and currently causes millions of dollars of damage to crops annually, and it competes aggressively for nest sites with native cavity-nesting birds.

CONSERVATION STRATEGIES
Once established, exotic species have proven to be notoriously difficult to eradicate. Successful eradication generally has been limited to small, aquatic environments such as isolated ponds or lakes, or reaches of streams where re-introductions can be controlled. Bullfrogs (an exotic to the western United States) and non-native fish have been successfully removed from some ponds and sections of streams in Arizona by draining the ponds and poisoning the streams, but the threat of re-invasion by exotic species into these areas is real and constant. Prescribed fire has been tried as a tool to reduce the spread of Lehmann’s lovegrass in Arizona, with limited success. The lack of ability to eradicate non-native species means that we will likely have to deal with their presence, and limit their impacts when possible. For example, erecting nest boxes for bluebirds in the eastern United States can reduce competition for nest sites with starlings, but this kind of conservation effort is limited in scope and requires long-term maintenance.

The inability to eradicate exotic species, once they are established, places a high priority on reducing the chance of future introductions. Current state and federal regulations generally prohibit intentionally introducing non-native organisms, but accidental introductions are difficult to control. Individuals can do their part, however, by landscaping with native species of plants, disposing of unwanted “pets” (e.g., exotic fish) in a manner that prohibits survival in the wild, and reducing any man-made structures or activities that promote the survival of non-native species. For more information about exotic species in Pima County, see The Sonoran Desert Conservation Plan (www.co.pima.az.us/cmo/sdcp/sdcp2/SOC/soc.html).

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This page was updated on 12/28/05