Mark Davis is the DeWitt Wallace Professor and Chair of Biology at Macalester College, St. Paul, MN. He is the author of Invasion Biology (Oxford U Press, 2009).
For 25 years, the American public has been inundated with horror stories involving non-native species. Think: snakehead, kudzu, Asian carp. This has largely been the result of selective communication from scientists and a media that too often have been more than eager to promote these stories without engaging in any critical analysis or research of their own. Usually provided with just a single perspective, the public largely accepted the idea that non-natives, as a group, are noxious and undesirable.
In fact, this is anything but true. Non-native species are just species. Like native species, some of them produce effects we like, some produce effects that we don’t like, and most are comparatively benign. Have some introduced species caused changes that most everyone would agree have been very harmful? Absolutely. By killing timber trees, gypsy moths and the emerald ash borer have caused, and continue to cause, enormous economic damage to the United States. Introduced pathogens that threaten human health are also clearly harmful species. At the same time, the danger posed by non-native species as a group often has been exaggerated and misrepresented. For example, ecologists and conservationists often describe non-native species as the world’s second greatest extinction threat, despite the fact that existing data shows this clearly not to be the case. It is true that introduced species can and have caused many extinctions in insular environments such as oceanic islands and freshwater lakes. But they have caused very few extinctions on continents or in marine systems. In fact, the primary regional biodiversity effect of introduced species is to increase species diversity. Due to the introduction of thousands of plant species, the United States has approximately 20 percent more wild plant species than it did 500 years ago.
While extinction threats should not be our only concern when it comes to non-native species, if we are going to label a species as “harmful,” that characterization needs to be based on good science. Efforts to vilify non-native species by misrepresenting their effects are ultimately counterproductive, as is the use of pejorative language to describe them: e.g., calling them “invaders” or “biological pollution.” Once harm is claimed, society is generally obligated to try to reduce or eliminate the harm. Also, if harm is misrepresented, scarce societal resources can end up being spent on needless and futile management programs. Good (or bad, depending upon your point of view) examples of such efforts include the persistent efforts by federal, state, and local agencies to eradicate or significantly reduce non-native herbs and shrubs in forest environments in the US.
Most of these efforts have been ecologically misguided since they typically have not addressed the underlying causes for the spread of these new species. Because many native species began to decline at about the same time that non-native species began to become abundant, ecologists and conservationists prematurely assumed that the non-native species were the cause for the decline of the native flora. Once good scientific studies were conducted on these systems, the data frequently revealed that other changes in the environment were the primary cause of the decline of the native species, and that the non-native species were simply better adapted to the new environment. A good example is garlic mustard, an introduced species usually described as an aggressive invader that chokes out native plant species. In fact, recent scientific studies of this species have concluded that garlic mustard is less an agent of change than a consequence of it. Because these non-native plants often have not been the primary drivers of native plant declines, efforts to reduce the abundance of the non-native species frequently do little to make the environment more suitable for the native plants. Besides their frequent long-term ineffectiveness, these management efforts often have involved extensive use of pesticides and/or mechanical removal, interventions that commonly produced considerable collateral damage, negatively affecting other species and the environment. Too often these eradication and control efforts have illustrated the adage, “a poor way to get what you want is to try to get rid of what you don’t want.”
Take a closer look at the non-native plant control efforts in eastern American forests that have focused on species such as garlic mustard and several species of non-native honeysuckle. These species are not threatening human health or causing any significant economic harm. Nor are these species threatening any native species with extinction. Nevertheless, many ecologists and conservationists have decided to characterize the ecological changes associated with the spread of the new plant species as harm. It is true that the relative abundance of species is changing in the US. Species that were once common in some areas are becoming less common while other species that were uncommon, or not even present, are becoming abundant. However, this change becomes harm only if someone declares it so.
Nature is dynamic. Organisms are opportunistic. Native birds and insects feed on the fruits, the leaves, and the nectar of non-native plant species. Non-native honeybees pollinate many native plants as well as providing an enormous ecological service by pollinating agricultural crops and orchards. After a while, the value distinction between native and non-native begins to blur. Few people would disagree that control measures should be undertaken to reduce the abundance of non-native species that threaten human health or cause great economic harm. But when there is no demonstrable harm – and instead simply ecological change – a much more sensible approach to non-native species is to learn to live with them. Like other species, we too need to learn to adapt to change. In many instances, rather than trying to manage nature, we would be much better off managing our reactions to it.
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