A recent article in the New York Times Science section, “Invasive Species Aren’t Always Unwanted,” highlights an ongoing debate between people who are alarmed about human-mediated re-shuffling of species across the planet, and those who accept the re-shuffling of species as part of an ongoing, natural process of change in ecosystems.
In a previous Sense and Sustainability post, I discussed a 2015 legal battle about ballast water regulation and invasive species prevention. That post was written on the assumption that invasive species are bad and their spread should be prevented when possible. But the New York Times article reports on the other side of the invasion coin: that some “invasive” species are fine—sometimes even good—and that negative reactions to their spread could be based more on an ecological manifestation of xenophobia than on science.
Camels evolved in North America and slowly migrated into Asia and the Middle East over hundreds of thousands to millions of years. They are now extinct in North America. Should camels be considered invasive?
A prominent proponent of this view is Ken Thompson, who wrote a book titled “Where do camels belong: why invasive species aren’t all bad.” The title character of Thompson’s book is the camel. Camels first evolved in North America around 40 million years ago and then expanded their range some 10 million years later to Asia via a land bridge across the Bering Straight. Today, camels aren’t found in North America. They went extinct here about 8,000 years ago. Most people associate camels with the Middle East region or Asia, because that is where they are found today, almost all in captivity. But Thompson asks the reader, where do camels belong? His point is that animals move around and that sometimes the distribution of a species on the planet will change dramatically over time. Can camels be considered “native” to the Middle East when they evolved in North America? It depends on your frame of reference. If not, should humans somehow try to eradicate camels from the Middle East? Most people would find such an endeavor ludicrous.
The points laid out in both the article and book represent fundamental but often implicit policy judgments that are made before acting to prevent or respond to a species invasion. First, those on Thompson’s side highlight that there are natural processes that reshuffle organisms around the planet all the time. Second, they point out that humans today are the biggest drivers of change to ecosystems and that species invasions are just one of many ways we have altered virtually all ecosystem on the planet. And finally, the remind the reader that sometimes there are benefits when a new species arrives on the scene. These points, however, require more critical exploration because timescales, context, and values will dictate how we perceive invasive species. Unfortunately, while the goal of both the Times article and Thompson’s book is to draw the reader’s attention to complexities about invasive species, there are certain scientific nuances that are lost in both publications.
It is overly simplistic to conclude that invasive species should not be considered a conservation priority because species distributions have changed constantly throughout history. The concept of shifting distributions and shifting ecological compositions—while leading some to the aforementioned simplistic position—is, ironically, a highly nuanced and complex way to think about the Earth’s biodiversity. If species are always moving around on their own, why is it a problem when humans do it? Clearly there are lines that have to be drawn when considering species invasions. But where are the lines? And how can they be defined in a way that will improve environmental management?
Timescale and means of spread are crucial to understanding the environmental impact of species redistributions and, therefore, to deciding if there is an environmental threat that needs to be managed. Consider another species that is currently found somewhere it didn’t used to be: the red lionfish. This particular species of lionfish was only observed in the Western Pacific and coral triangle regions until the late 1980s when a small number of fish were released accidentally into the Atlantic Ocean near Miami, Florida. For a while, lionfish appeared to stay put. By 2000, however, that had changed, and lionfish had spread north to North Carolina and Bermuda. By 2004 they had colonized the Bahamas, and since then, they have spread throughout the Gulf of Mexico and the Caribbean Sea as far south as Brazil. The US Geological Survey has an interactive map showing lionfish sightings.
The rapid spread and ecological devastation caused by lionfish have led many scientists to call the lionfish invasion the worst marine fish invasion ever. Effects of the invasion are rippling up to human communities that rely on the harvest of fish species that are now being eaten by lionfish.
Unlike camels, translocation of lionfish from the Western Pacific to the Western Atlantic was completely facilitated by humans and happened over the course of decades instead of hundreds of thousands or millions of years. The impact of lionfish on their new ecosystem has been devastating, unlike—to our knowledge—the spread of camels into Asia. One study found that on a coral reef in the Bahamas, lionfish had reduced the total volume (biomass) of smaller prey fish by 32%. A measurement of the diversity of different species—something linked directly to ecosystem health—had decreased by 21%.
The New York Times article reports on a small eruption of the controversy about invasive species in 2011. Nature published one opinion expressed by Mark Davis and 18 coauthors who advocated a de-emphasis on whether or not a species is native or non-native and a shift to determining whether that species is causing harm or doing good. A response letter led by Dr. Daniel Simberloff and 141 other scientists criticized Davis’ portrayal in the original opinion piece.
A crucial point that is lost in the discussion of this controversy is the fact that by the time a species invades, it is often too late to do anything about it even if conservationists and managers want to. Barring the introduction of some form of control that does not yet exist (like genetic modifications similar to those proposed to control some mosquitoes), lionfish cannot and will not be eradicated from the Caribbean. Instead, scientists and managers are now trying to find a magic threshold to which lionfish populations can be culled to allow the existing reef ecosystems to rebound.
Not only is it almost impossible to eliminate a species once it becomes invasive, but it is still tremendously difficult to predict whether or not a species will actually become invasive when introduced. So, should the International Maritime Organization abandon all ballast water regulation because sometimes marine species shift their ranges naturally and therefore, it’s okay to move species around the planet? No. Should we stop checking for agricultural pests in shipments of produce to the US? Probably not. Risk and uncertainty mean that we rarely know when something will establish as invasive or not. Precaution must be used.
Ecosystems change, and sometimes that change is dramatic and rapid. Such changes can and often do occur in the absence of a species invasion. In their controversial Nature essay, Mark Davis and colleagues highlight that ecosystems are always in flux due to many forces, including “climate change, nitrogen eutrophication, increased urbanization and other land-use changes.” However, species redistributions due to human actions that can lead to ecosystem collapse, species extinctions, or extreme reductions in ecosystem function cannot be ignored just because there is also other anthropogenic-driven global change. To dismiss invasive species in this way is a form of environmental defeatism that will lead to inaction in the face of potentially manageable environmental threats.
Image courtesy of Flickr. Originally published by S&S on April 20, 2016.