Reconsidering The Value of Non-Native Species

Conservation biologists are not so sure non-native species, such as the tamarisk plants shown here, are as bad as they were once considered to be. (Credit: James Steinberg / Photo Researchers, Inc.)

Invasive species don’t get much love. Most often, they are portrayed as the bad guy in ecosystems, where their misdeeds include outcompeting native species, spreading disease, and damaging agricultural crops. However, some scientists argue that this characterization is not as black-and-white as many in the scientific community once believed it to be.

Before we delve too far into the topic, lets discuss some terminology. An introduced species, also called an alien or non-native species, is any organism that was brought to an ecosystem as the result of human actions. In some cases, the introduced species may die out or is able to coexist with native species. In other cases, the introduced species may pose a threat to the stability of an ecosystem by preying on or outcompeting native species for resources. Additionally, introduced species may cause economic damage, particularly in terms of lost revenue resulting from the damage to agricultural crops. When an introduced species causes ecosystem instability or economic harm, it is referred to as an invasive species.

Though the introduction of non-native species to some ecosystems was not intentional, that is not always the case. For example, starlings and house sparrows were first introduced into the United States in the 1890s after an eccentric drug manufacturer named Eugene Scheiffelin from the Bronx decided that all of the songbirds mentioned in Shakespeare’s plays should be imported from England into New York. In the case of the starlings, what initially began as a population of 100 birds released into Central Park has since turned into a population of over 200 million, which can be found across the entire continent of North America, and even up into Alaska. Other species of plants and animals have been intentionally introduced for a variety of reasons including uses for agricultural, recreational, and ornamental purposes.

Though many decry the spread of colony-collapse disorder that is affecting honeybee colonies across North America, and researchers are rushing to find its cause and cure, nobody talks about the fact that honeybees are in fact a non-native species to North America. English settlers introduced honeybees to the New World in the 1600s. Since that time, additional colonies of bees have been introduced from Europe, Asia, and Africa. However, no one argues that since their introduction, honeybees have taken on a significant role as pollinators to a variety of plants, including many important crop species. The loss of honeybees at this point could have a devastating effect on crop production that depends on them, such as the Californian almond industry.

Clearly, not all non-native species are perceived of as being bad. But when should a non-native species be considered to be good, and when should it be considered to be bad? Can such a distinction be made? Three scientists recently published a review article in the journal Conservation Biology that calls into question the practice of thinking that non-native species are inherently bad. The article, written by Martin Schlaepfer (State University of New York-Syracuse), Dov Sax (Brown University), and Julian Olden (University of Washington), suggests that non-native species can, and do, contribute to conservation objectives. These contributions include:

  • providing habitat or food resources to rare or endangered species,
  • serving as functional substitutes for extinct species, and
  • providing desirable ecosystem functions.

According to the authors, Non-native species might contribute to achieving conservation goals in the future because they may be more likely than native species to persist and provide ecosystem services in areas where climate and land use are changing rapidly and because they may evolve into new and endemic taxa.

One example of an often-maligned non-native (and invasive) species that is providing ecosystem services is the tamarisk, or saltcedar, tree. This tree is of particular concern in the southwestern United States, where it is commonly found alongside rivers. Concerns about this species include its ability to outcompete local plant species and its ability to draw a significant amount of water from the water table. Conservation biologists worry that if left unchecked, the tamarisk tree could replace native species, thus affecting those species that depend on native plants as sources of food or shelter. However, recent research suggests that the plants ability to draw down the water table have been exaggerated. In addition, these trees now serve as habitat for the southwestern willow flycatcher. Eradicating tamarisk trees now would result in reduced habitat available for this endangered bird species.

Research conducted by scientists at Princeton University found that in some cases, non-native species take over the roles once held by native species. The scientists compared the health of plant populations on New Zealand’s North Island, where most native vertebrate pollinators have gone extinct, and the health of plant populations on the remote Little Barrier Island, just off the coast of New Zealand, where native species thrive. In their study, published in the journal Proceedings of the Royal Society B, the scientists found that on the North Island, non-native species, such as rats, have taken on the role as pollinators once held by the now-extinct native species. These results indicate that just removing the non-native species could cause significant harm to the North Island plant population. Instead, in such cases, conservation biologists need to take into consideration the role that non-native species have in an ecosystem before taking action to eradicate them.

According to David Wilcove, co-author of the New Zealand study, “our findings show that eliminating an invasive species for the benefit of native species could actually harm an ecosystem, a surprising dynamic that could frustrate ecosystem restoration efforts.”

Clearly, non-native species are not always the evil-doers that they are often portrayed to be. In fact, the longer a non-native species is in an ecosystem, the more it may become entangled in the ecosystems functions. Over time, a non-native species may even prove to be a benefit to the ecosystem in which it was once reviled.

According to the authors of the Conservation Biology article, “We predict the proportion of non-native species that are viewed as benign or even desirable will slowly increase over time as their potential contributions to society and to achieving conservation objectives become well recognized and realized.”

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