Non-native species are typically described as “invasive” species, clearly not a term of endearment. However, many biologists and conservationists are having a change of heart regarding these much-maligned “aliens” (another common descriptor): they’re not all bad - and some may even help native species survive and thrive, especially in biological communities under stress from habitat loss and a changing climate. A zero-tolerance approach to non-native species makes no sense when their effects are often neutral or positive.
From selected readings, more food for thought:
Non-native species that are resilient to climate change may provide suitable environmental conditions to native species that would otherwise be unable to maintain viable populations under future climate scenarios. Non-native species have already been found to help some native species survive in stressed ecosystems, such as in some parts of the Chesapeake Bay where retreating eelgrass beds were eventually replaced by non-native microalgae, which provided needed habitat for the local crab population. The authors acknowledge that introduced species have often caused serious problems in new habitats, but argue for a case-by-case approach to the intermingling of native and non-native species, urging conservationists and land managers to focus “much more on the functions of species, and much less on where they originated.” (Bulleri F, Eriksson BK, Queirós A, Airoldi L, Arenas F, Arvanitidis C, et al., 2018)
Classifying species according to some standard of belonging does nothing to advance our understanding of the factors that undermine or support biological communities. Native species can wreak havoc in their local habitat and non-natives can help struggling natives survive and thrive. An example of the former is the native mountain pine beetle, currently suspected of killing more trees than any other cause in North America. Tamarisk shrubs are an example of the latter. Introduced over a century ago from Eurasia and Africa into the US southwest, these shrubs use water at a rate comparable to their native counterparts and have become the preferred nesting habitat of the endangered southwestern willow flycatcher. (Davis, M.A., Chew, M.K., Hobbs, R.J., et al., 2011)
In a comprehensive study documenting bird foraging behavior on native and nonnative street trees throughout Los Angeles, the authors found that both native and non-native trees were being used by birds as food resources (e.g., berries and seeds), the non-native Chinese elm being the most preferred. Non-native trees attracted birds because they provided food resources outside of the typical timing of native trees, as found in other studies of bird foraging behavior. The authors suggest that having a variety of food resources available to birds throughout the year may become increasingly necessary in ecosystems lacking tree diversity, especially given the potential impact of climate change on the annual cycle of plants and animals. (Wood, E. M., and Esaian, S.., 2020).
Using examples from Northern and Central Europe, the authors illustrate how non-native tree species benefit urban forests. Such trees provide animals with food and shelter and contribute to ecosystem resilience by tolerating pests, disease and extreme weather conditions better than many native trees. The authors argue that opposition to exotic species “is often based on philosophical-political notions rooted in romanticism, rather than scientific results”. (Sjöman, H. , Morgenroth J., Sjöman J. D., Sæbø A., and Kowarik I.., 2016)
European starlings have gotten a bad rap in North America. Their impact on native species may not have been as negative as once thought. According to Breeding Bird Survey and Christmas Bird Count data, the presence of starlings does not actually explain native bird declines. Even if the initial impact of starlings harmed some native species, these species seem to have adjusted to the starlings’ presence. For example, tree swallows appear to have changed their nesting behavior in response to starlings. Long ago, starlings were designated an invasive alien that threatened native birds, but as ecosystems change, so does the impact of starlings on those ecosystems. ( Natalie Hofmeister, 2021)
A third of California's butterfly species have been reported as laying eggs or feeding on non-native plants. Many more use introduced plants as nectar sources. Some California butterflies have expanded their geographic ranges and/or extended their flight seasons by feeding on non-native plants. While some non-native plants are known to have a negative impact on native butterflies, this is not always the case. Conservationists should make selective use of non-native plants to boost populations of endangered butterflies in California. (Graves SD, Shapiro AM, 2003)
The Australian Casuarina tree appears to provide refuge for Japan’s native land snails by protecting them from predation by non-native rats. Although Casuarina trees have damaged native plants in some areas, their eradication could have unintended effects on other native species. Sometimes non-native species can be used to mitigate the negative impacts of other non-native species. In contrast to a zero-tolerance strategy of eradicating all non-native species, perhaps the non-native Casuarina trees should be monitored and managed to optimize their beneficial effects and minimize the negative effects. ( Chiba S., 2010)
“[Non-native] species have been responsible for the extinction of native plants and animals, degradation of rare and threatened ecosystems and ecological communities, crop failure and declining agricultural productivity, loss of cultivar and animal breed diversity and damage to property, infrastructure, native fisheries, tourism and outdoor recreation…[However] many alien species may be both problematic and useful. Furthermore, some species will be manageable, but others will be intractable and need to be recognized as such. Responses, including strategies for prevention and adaptive management, will therefore need to be flexible and pragmatic.” (Plenary of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, Sixth session, March 2018).
—
References:
Bulleri F, Eriksson BK, Queirós A, Airoldi L, Arenas F, Arvanitidis C, et al. (2018) Harnessing positive species interactions as a tool against climate-driven loss of coastal biodiversity. PLoS Biol 16(9): e2006852. https://doi.org/10.1371/journal.pbio.2006852
Chiba S (2010) Invasive non-native species' provision of refugia for endangered native species. Conservation Biology 24: 1141–1147. https://doi.org/10.1111/j.1523-1739.2010.01457.x
Davis, M.A., Chew, M.K., Hobbs, R.J., et al., (2011). Don’t judge species on their origins. Nature 474, 153–154. https://www.nature.com/articles/474153a
Essay: Are Starlings Really “Invasive Aliens”? By Natalie Hofmeister. Living Bird Magazine/The Cornell Lab. March 19, 2021 https://www.allaboutbirds.org/news/essay-are-starlings-really-invasive-aliens/
Graves SD, Shapiro AM (2003) Exotics as host plants of the California butterfly fauna. Biology Conservation 110: 413–433. https://doi.org/10.1016/S0006-3207(02)00233-1
Plenary of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, Sixth session, March 2018. Information on scoping for a thematic assessment of invasive alien species and their control. https://ipbes.net/sites/default/files/ipbes-6-inf-10_en.pdf
Sjöman, H. , Morgenroth J., Sjöman J. D., Sæbø A., and Kowarik I.. (2016) Diversification of the urban forest—can we afford to exclude exotic tree species? Urban Forestry and Urban Greening. Urban & Fischer 18:237–241. https://doi.org/10.1016/j.ufug.2016.06.011
Wood, E. M., and Esaian, S.. (2020). The importance of street trees to urban avifauna. Ecological Applications 30( 7):e02149. https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/eap.2149