Categorized | Climate Change

Beyond Climate Envelopes

Most portrayals of the effects of climate change on wildlife and ecosystems is that species will simply shift towards the poles or upslope to follow the climate conditions they have evolved with as the earth warms.  But species are much more complicated than that.

One of the biggest hurdles in climate studies involves the complexity of species and habitats.  No species or habitat exists on its own; rather, an unknown number of interactions is constantly happening, and in turn affecting the status of said species or habitats.  Competition for food and habitat and displacement of species are among those interactions.  Consequently, when one wants to study a species or habitat response to a certain factor, it is important to consider those interactions as much as possible, seeing that they play a significant role in the possible outcome.

The state of the art in climate-species models for many years has been the climate-envelope model. which describes the climate variables such as temperature, humidity, and precipitation, among others, which characterize the conditions under which a species is found.  Once the climate-envelope of a species or habitat has been modeled, you can then project where that climate-envelope will exist under future climate scenarios and you assume that is an approximation for where species will need to go to survive.  However, these types of models have been traditionally taken into account the effect of the modeled conditions on other species that ultimately will be interacting with the species or system of focus and affecting its future.

A new study, however, is one of the first to include species interactions and dispersal abilities under the modeled climate scenarios, therefore portraying a much more accurate view of what the future will likely hold for a species, habitat, or ecosystem.  This is particularly important in the estimation of species extinction rates, since new distributions of species due to climate change might lead to interactions never before seen, as well as to the loss of others, and species might find themselves under conditions that were nonexistent before.  When shifting its range to a more suitable climate, a species may encounter others that are strong competitors and that will prevent its establishment, and in the event of no other suitable climate – or better dispersal abilities – lead to its eventual extinction due not to lack of available habitat, but rather lack of available living conditions.

The main findings of the paper are that climate change will strongly affect communities where “species have narrow niches, low mean dispersal rates,” and large differences in dispersal ability between species.  The authors reached those findings by simulating a 40-species community undergoing 4⁰C change over 100 years.  During the simulations, species were exposed to different thermal ranges and were assigned different dispersal rates and temperature tolerances, and their predicted distribution shifts tracking climate change determined future competitive interactions.  Interactions could be with species better adapted to the novel climate in a new area (due to their assigned thermal range and temperature tolerances), with species that had been in the new area before the new species arrival, or could be new interactions in the original distribution area due to the loss of one of more species that could not track climate change.  Those interactions, together with loss of thermal range for survival, were in turn used to predict extinctions.

In natural communities species will always have different dispersal abilities.  The rates of dispersal played a vital role in the simulations, as they would determine which species would be able to track climate change, and which would have the probability of facing competition.  Without dispersal variation, the model predicted a 98% extinction rate when species did not disperse.  That figure is lower (74%) when species have low dispersal levels and cannot track their optimal thermal range, and is lowest (13%-24%) when species have a high dispersal level and are able to track their thermal optima.

Variation in dispersal rates determined contractions and expansions in distribution, and those in turn determined what interactions occurred and which were lost due to climate change.  Competition for suitable habitat within a species temperature tolerance was the main interaction modeled, and the authors found that without competition, actually very few species would become extinct, independently of their dispersal rates.  Competition usually increases extinctions risks due to reduced fitness (species get stressed and don’t do as well when competing with others) and abundance (species numbers get reduced due to direct or indirect mortality because of competition), and climate change increases those risks because species are under added stress.  More new competitive interactions with consequent extinctions were observed when species had different dispersal rates, because good and poor dispersers would have more probability of interacting.  The study also found that competition reduced range expansions and slowed climate tracking, but interestingly, under low dispersal rates, it was sometimes to some species advantage: because the ones that could not disperse well would become extinct, the remaining species could better track climate change under less competition.

Finally, the authors state that climate change is an important factor adding to competition – in the absence of the competition, the effects of the climate change are predicted to be less severe for many species.  This is an important study for the understanding of how climate change can affect species, which can provide better guidance to the process of climate adaptation and related conservation strategies.

This post was written by:

- who has written 12 posts on dotWild.

Astrid Caldas is the Climate Change & Wildlife Science Fellow for Defenders of Wildlife. Astrid provides scientific support for Defenders, including providing technical assistance for integrating climate adaptation into programs, doing synthetic research, and publishing papers and reports on climate and wildlife issues.

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