What Will You Do When You Get There: A perspective on species range shifts
[Originally posted on PLOS Ecology Field Reports, August 17, 2015]
By Jens Hegg
The first question out of people’s mouths when they learn you are substituting your present home for another is, “What will you be doing there?” Fitting into the milieu is important, having a place and role in the grand scheme of your surroundings is important.
After reading some recent articles from the PLOS ONE Ecological Impacts of Climate Change Collection, I sometimes wonder if we spend enough time studying this question when it comes the movement of species in the face of climate change.
Even a cursory look through the ecological literature surrounding climate change makes clear that the topic of range shifts is an extremely active area of research. There are examples of current or predicted range shifts attributed to climate change from every continent and just about any taxa you could name, plants, insects, mammals, and beyond. It makes sense that this is a hot topic, humans rely on the ecosystem the way it is currently organized and drastic changes could disrupt livelihoods and locales we care deeply about personally and culturally. In reading these articles I’m struck though that we aren’t asking the same questions we ask ourselves in new environments, “what will you do when you get there?” and “who do you know there?”
Several recent articles in PLOS have approached this problem in similar ways but come to fairly different conclusions about what policies could be implemented to deal with changes in where species move with climate change. Reading them got me thinking more deeply about how the ways we frame our studies of range shifts inform what we know, and how the limits of our knowledge on the subject might affect what happens if we decide drastic action needs to be taken to mitigate the effects of climate change.
In their recent article Pushing the Pace of Tree Species Migration in PLOS ONE, Lazarus and McGill modeled the speed at which we might expect northern temperate tree species to move northward in response to climate change. Their work is based on the knowledge from climate models that the habitable zones for various tree species are expected to move northward with climate change. Since trees are immobile individually their northward spread will be dictated by their ability to colonize new habitat, which will vary by the distance they are able to broadcast their offspring, and the fragmentation of the landscape through which they must pass.
Lazarus and McGill employ a simple but convincing model that predicts the spread of trees into suitable environments using a t-distribution to model species which are capable of broadcasting seeds widely, as well as those who’s apples don’t fall far from the tree, as it were. The model uses a randomly assigned grid of 10 meter square cells which can be populated by a tree or blocked as disturbed landscape. They ran the model for a 1000 year simulation using “natural” tree population based upon known ecological parameters for different tree species, as well as with “assisted” population growth where trees were “planted” at random ahead of the current range of the trees. The disturbance cells were also modeled both as random disturbances and with auto-correlation, which simulates the autocorrelated arrangement of human disturbance (in other words, we humans tend to build things near other human things, not at random across the landscape). The big questions in all of this modeling were: how does dispersal affect the speed that trees can keep up with a changing climate?; how does habitat fragmentation affect this rate of movement; and is it possible to speed this process up with assisted planting?
Their findings are somewhat alarming, that even the fastest spreading trees will not be able to keep pace with the speed of northward range changes accompanying climate change. They did, however, conclude that a large scale planting effort could allow these species to keep up with the expected rate of climactic shifts.
Lowland Grassland Communities
Harris et al., in their 2015 article Noah’s Ark Conservation Will Not Preserve Threatened Ecological Communities under Climate Change, asked similar questions about range shifts but came to very different conclusions. Studying threatened lowland grassland communities in Tasmania, Australia, they asked the question, “how should we manage these grasslands as their range decreases with climate change?”
Using multiple climate change scenarios they found that the area climactically suitable for these grassland communities is likely to shrink dramatically by the middle of the century. Further, because several of the dominant species ranges will shrink at a lesser rate, they infer that the likelihood of invasion is high since the community structure may be degraded in many of the current grassland areas. Management, they conclude, must not rely on protected areas which may not remain within the climactically suitable range. Further, management will have to respond differently under different local conditions. The particularly interesting conclusion is that in many areas managers must let go of the idea that these communities can be retained intact, instead they should be managed for resilience and diversity regardless of the species that ultimately dominate under the new regime. This, in their estimation, is the best way to preserve the diversity that will remain.
These two conclusions couldn’t be more different. On one side we’re told that range shifts should be augmented in order to keep pace with climate change. On the other we’re told that managing for resilience can be the best strategy, even if some species can’t remain. Clearly there are many complicating factors in comparing these two studies, but the conclusions are general enough to make one think. All the evidence points toward at least some degree of range shift for many species as a result of climate change. Whether species can keep up with this change is a worthy question, but it seems that many studies are focussed on the movement and not the end result.
When looking at a species on its own it’s easy to focus on the shift in it’s range over time as the most important factor, but that species occurs within a milieu of species whose ranges may be shifting at different rates. If we predict a species will spread to a new location we need to ask, “what will it do there?” How will it fit into a changing ecosystem, potentially invading an ecosystem which has itself lost elements of it’s trophic structure to northward range shifts. The literature currently is heavy on predictions of movement based on climatic variation, many of which implicitly assume that conditions will be favorable when these species arrive, based solely on abiotic factors.
This is the assumption at the root of calls for assisted migration, that species can be assisted in shifting their range and that the resulting movement will be a net positive for the ecosystem. Clearly this isn’t a new debate. The idea of assisted migration drew a pointed response from Ricciardi and Simberloff in 2009. One of the problems is that modeling ecosystems in flux is no easy task. It’s very difficult to know how these new ecosystems will arrange themselves and random effects can potentially have large consequences. Still, I wonder if a shift in mindset might help answer these questions.
Implications for Study Design
When designing studies I think ecology could gain by explicitly trying to answer, “What will you do when you get there?” For example, what role might this species play in it’s new range given what we know about other predicted range shifts into and out-of that environment? Perhaps asking the question, “Who do you know there?” might help in answering these questions as well. For example, given what we know of the environment into which these species will spread what similar species exist, which biotic interactions will be the same and which different?
Of course this discussion of biotic and abiotic control is an old one, with many entrenched views and no concrete resolution in sight. Still, if there is one thing ecologists agree on it is that every system is different and that ecology is complex. While the abiotic changes that climate change will bring are likely to have big impacts, I can’t help but think that we need to make explicit attempts at understanding what ecosystems will look like, and how they will be organized, as these range shifts occur. Only then can we know for a given system whether drastic attempts such as assisted migration will be useful, or if managing for a resilient system in spite of change is the most reasonable goal.
Jens Hegg is a PhD candidate in the Water Resources program at the University of Idaho, focusing on the ecology of migration. His research spans multiple fields including the role of phenotypic plasticity and environment in juvenile salmon migration decisions, migratory ecology of tropical catfish, geologic prediction of strontium isotopes as an aquatic tracer, and interdisciplinary work in data sonification and virtual reality as a tool for data analysis and scientific communication. More about him can be found on his blog A Fish In School. Twitter @AFishInSchool