There are many different factors that can drive a plant population toward extinction, but anthropogenic threats (eg. habitat loss, invasive species) are the most common and often the most destructive. Unfortunately, mitigating the impact of those threats to reduce a population’s risk of extinction is really complicated.
Threats commonly have both direct and indirect effects, creating interconnected cascading or web-like relationships among many different factors that ultimately all impact a plant population’s risk of extinction. Threats tend to come together. Thus, even in a single population, land stewards are almost always trying to manage multiple threats simultaneously. Threats occur at multiple spatial scales, from local threats like herbivory to the global threat of climate change. And, there are examples where mitigating local threats only has a limited ability to decrease the long-term risk of extinction without the larger scale threats also being addressed. Lastly, threats can influence a plant population’s risk of extinction in interactive or synergistic ways. Meaning that the net impact of two or more threats is more (or less) than would be expected based on the individual impacts of each threat (i.e. they are more than the sum of their parts).
Despite all this complexity, the extinction risk of a population can be characterized one main thing — its growth dynamics (eg. birth and death rates). So, what we need to ask to know a population’s risk of extinction is whether that population is, in the long run, growing or declining. While conceptually, characterizing extinction risk is much simpler to understand than all the complexities of the threats that are ultimately creating that risk, collecting enough data to detect long-term population growth or decline, and enough data to link a specific threat to that trend, is tough. Doing so requires a lot data, collected over a long period of time from many populations. Thus, it is understandable that those sorts of studies rarely happen for plant species.
The study, “Count population viability analysis finds that interacting local and regional threats affect the viability of a rare plant,” had a unique opportunity to use a long-term dataset to assess the individual and interactive effects of multiple threats that occur across different spatial scales at numerous populations of a rare plant species. The Plants of Concern program is a citizen science initiative that tracks the population sizes of, and the types and levels of threats to, rare plant species in a large region south and west of Lake Michigan in the United States.
This study focused on Eurybia furcata, a rare woodland Aster found in six Midwestern states, but with the bulk of its global population in the Plants of Concern region in Southern Wisconsin and Northern Illinois. In that region, E. furcata is threatened by two local threats, displacement by invasive species (primarily the shrub invasive Buckthorn, Rhamnus cathartica) and mammalian herbivory (primarily by White-tailed Deer, Odocoileus virginianus). Also, due to severe dispersal limitations, it is expected to be impacted by the large-scale threat of climate change. This study quantified and compared the impacts of these three threats on E. furcata using count-based population projection models to estimate population size and the probability of extinction 50 years into the future under different threat levels and historic or expected future climatic conditions.
First, the impacts of the two local threats, invasive species and herbivory, were partially confounded. This was because they often co-occurred in populations across the region and because of statistical interactions between them, that when combined, meant the two effects could not completely be separated analytically. They are treated as a single, local threat unit for much of the study. As one might expect, the projection models using data from populations with higher levels of local threats had smaller population sizes and higher risks of extinction. But, the absolute differences were small (eg. an increase from 39% to 46% chance of extinction from little or no local threats to higher local threat levels, respectively).
Second, the climate models showed a negative impact of climate change on E. furcata’s predicted future population sizes. The models under historic climatic conditions produced an average of 236% more individuals than the models under future climatic conditions. Despite this change in population size, climate change alone was not predicted to increase E. furcata’s overall risk of extinction. Therefore, local threats and climate change independently are expected to have only minor to moderate negative impacts on E. furcata within the next 50 years.
The interaction, however, was astonishing, and shows that E. furcata could be much more at-risk than either the local threat or climate models would predict. When modelled separately, populations under little to no pressure from local threats were also not predicted to be impacted by climate change (Figure Panel A). Whereas populations under higher pressure from local threats were hugely impacted by climate change. The models under historic climatic conditions produced an average of 11,813% more individuals than the models under future climatic conditions (Figure Panel B). Thus, the synergistic effect of all three threats is predicted to drive this species slowly, but steadily toward extinction.
Interestingly, the synergistic effect between the threats may also be key to preventing this at-risk species from going extinct. The direction of the synergism suggests that with local scale management, control of invasive species and herbivores, the singular stress of climate change will only decrease population sizes, not drive populations totally extinct. And so, there is hope that with local threat management, possibly combined with occasional population augmentation to increase size and genetic diversity, this species can be maintained in place, without having to disperse or be moved by human intervention to track its climate.
These findings are described in article Count population viability analysis finds that interacting local and regional threats affect the viability of a rare plant, recently published in the journal Ecological Indicators. This work was conducted by Holly L. Bernardo as part of her dissertation research at Washington University in St. Louis, Pati Vitt, Rachel Goad, and Susanne Masi while at the Chicago Botanic Garden, and Tiffany M. Knight from the Institute of Biology/Geobotany and Botanical Garden at Martin-Luther-University, the Department of Community Ecology at UFZ and the Helmholtz Centre for Environmental Research and the German Centre for Integrative Biodiversity Research (iDiv) in Germany.