November 2021 Director’s Corner
in·va·sive
/inˈvāsiv/
adjective
- (especially of plants or a disease) tending to spread prolifically and undesirably or harmfully.
To most people, the operative and memorable terms in the above definition are “undesirably” or “harmfully”. If we are considering one species at a time, we have time to reflect on if/how they are harmful, how/why they are undesirable. Then we can imagine pulling out one multiflora rose, catching a snakehead, removing a lone Phragmites. But then the number of individuals or species becomes obvious, and the regional size of the issue starts to overwhelm: there are already more than 200 known or possible invasive species in the Chesapeake Bay region. Do we need to manage all of them? Can we afford to? Do we know how?
A common entry point into navigating the issue is the impact of invasive species on global biodiversity, assuming that invasive species are directly related to decline. According to the World Conservation Union, invasive alien species are the second most significant threat to biodiversity, after habitat loss. As the movement of people and goods around the world has increased, the number of species introduced to areas outside their natural ranges has risen significantly; Seebens et al., 2017 found that over one third of all introductions in the past 200 years occurred after 1970. The rate of introductions continues to grow; Seebens et al., 2020 predicts that the number of established alien species will increase by 36% between 2005 and 2050. Given those numbers, how alarmed should we be? As an ecologist who came of age with the writings of Paul Ehrlich and E.O. Wilson, a foundational belief has been that biodiversity is always good and almost any artifact of human activities, including invasive species, were bad for its maintenance. However, ongoing science illuminated a more nuanced view when considered across spatial scales. Sax and Gaines (2003) assessed changes in species richness at both the global and local scales over the period of human activity, and showed that while global biodiversity was decreasing, evidence for declines in biodiversity at regional and local scales was mixed, with a lot of evidence that diversity might commonly be increasing at these finer spatial scales. In some instances, the differences were striking: on oceanic islands, there was no net change for birds, a doubling of plant species, and an almost four-fold increase in freshwater fishes, and on mainland areas there was no net change for birds, and a 20 percent increase for both plants and fishes, on average. If the general assumption that invasive species leads to declines in biodiversity can’t be applied at these smaller spatial scales, how should we be thinking about invasive species?
The next question is if the ability of any ecosystem to provide valued services is compromised by declines in diversity, or boosted by increases in diversity, whether native or not. The science provides us with a host of answers, and again, issues of scale, values, and tradeoffs abound. At a very local scale, measured in meters, the long-standing work of David Tilman and others shows that there is an undeniable link between ecological function and diversity, and the Millenium Ecosystem Assessment (now decades old) outlined the potential impacts at a global scale. But human activity most often happens at all the spatial scales between these two bookends, and this is where we often make intentional and wholesale tradeoffs of one function for another, as in the conversion of forest to agriculture, trading carbon storage for food production, and the issue may not be related to diversity but to the individual species and the land disturbance that goes along with it. So our next navigation point may be to consider the ecological functions that we desire at a community scale and the role of individual species in providing those.
So let me give you fair warning: this is where my blindspot will become obvious. If we value natural systems at the community scale and the ecosystem services that they provide, then we have something to be concerned about. According to the National Wildlife Federation, approximately 42 percent of threatened or endangered species are at risk due to invasive species. We are residents of a system that currently has thermal barriers limiting the establishment of invasive species, and that will become more suitable for alien species as the climate changes. So how do we approach that 200 species number? At the risk of oversimplifying, I am reminded by something that one of our speakers (Ryan Davis) at our last CRC Roundtable pointed out: that the scale at which most people experience their ecosystem is limited to car travel from home to work/errand spaces. So if we are to be able to thoughtfully and intentionally decide which of those 200 species we ought to be addressing, then perhaps a good first step would be to get out in those natural systems and think about what we value about them. And express gratitude for what they provide.
Have a thankful holiday.
Sax, D. F., and S. D. Gaines. 2003. Species diversity: From global decreases to local increases. Trends in Ecology & Evolution 18:561–566.
Seebens, H., Blackburn, T., Dyer, E. et al. No saturation in the accumulation of alien species worldwide. Nat Commun 8, 14435 (2017). https://doi.org/10.1038/ncomms14435
Seebens, Hanno, et al. “Projecting the continental accumulation of alien species through to 2050.” Global Change Biology 27.5 (2021): 970-982.