New Neighbors

Interspecific competition slows range expansion and shapes range boundaries (2020) Legault et al., Proceedings of the National Academy of Sciences, https://doi.org/10.1073/pnas.2009701117

Image Credit: CISRO, CC BY 3.0

The Crux

Climate change has resulted in multifarious changes in the natural world, not the least of which being where one can find a given species. Because areas are growing warmer, some species are shifting their habitats to stay within the type of environment that they like. The thing about shifting habitats though is that a species that shifts is likely to run into/need to compete with another species that is already there. Competition affects the growth and dispersal of organisms, so it follows that this should have an effect on the ability of a given species to shift or expand its range. However, most studies do not take competition into account when predicting range expansion.

A classic example in the scientific literature that did take competition into account was that of the gray squirrel invasion of Britain. Gray squirrels invaded and subsequently displaced the native red squirrels, but competition appeared to have no influence. Instead, a pathogen appeared to be the likely cause of the contraction of the red squirrel range. This example, however, comes from an observational study of a single replicate. Today’s authors instead conducted a manipulative lab experiment to test for the effects of competition on range expansion.

What They Did

To test for an effect of competition on range expansion the authors used two species of flour beetle, the red flour beetle (Tribolium castaneum) and the confused flour beetle (Tribolium confusum). Using the red flour beetle as the focal species, the authors set up two different artificial landscapes: one where a red flour beetle population started in a plastic box connected to 15 other boxes that were empty (no competition treatment), and one where the red flour beetle population started in a plastic box connected to 15 other boxes, 8 of which held populations of the confused flour beetle (competition treatment). There were 15 replicates of each treatment (competition and no competition). The treatments were checked once per beetle generation (~6 weeks), allowing the authors to test for differences in range expansion over time.

Did You Know: Model Systems

The beetles used in this study are what’s known as a “model system”, meaning that there has been a large amount of work done using them to test a variety of questions. In particular, earlier work with both of these species has shown that they have very specific competition dynamics, which when combined with their short generation times, made them the perfect organisms to test the effects of competition on range expansion. Other examples of model systems include fruit flies, lab mice/rats, and Daphnia (water fleas). 

What They Found

There were clear and consistent differences in the abundances of the focal red flour beetle between the competition and no-competition treatments. These differences were not apparent in the first few generations, but by generation 8 the authors observed that very few red flour beetles were able to expand into the boxes occupied by the confused flour beetles, while the red flour beetles were able to expand into all of the boxes in the no competition treatment. This difference is due to the competition with the confused flour beetles, as A) the presence of competitors was the only difference between the treatments and B) previous work has shown the confused flour beetles lowers the abundances of the red flour beetle by eating their eggs. Interestingly, in the absence of competition the spread of the red flour beetle was relatively constant over time, but in the competition treatment the range expansion dynamics changed over longer time scales.

Results from the study. Panel A shows the mean population sizes of the red flour beetle over successive generations (colors from red to violet) as they spread from their starting box (box 1) to the other 15 in the absence of competition. Panel B shows the same thing, but in the presence of competitors. Panel B clearly shows that the red flour beetles are unable to expand into boxes already occupied by the confused flour beetles (Image credit: Legault et al. 2020)

Problems?

Not so much a problem per se, but more a consideration of the study system itself. These two species of flour beetle are very similar in many ways, thus competition between the two of them is quite strong. Think about if you and your sibling both wanted more dessert. There’s likely to be more of a fight between the two of you if you both want the last piece of cake, but less of a fight if one of you wanted cake and the other wanted cookies. So while this study was a very elegant test of the effects of competition on range expansion, it would be interesting to see how these dynamics change when competition isn’t as strong.

So What?

By experimentally testing range expansion in a controlled environment, the authors were able to show that not only does competition have an effect on range expansion, but that the dynamics of the range expansion itself differ when competitors are present. These results are likely to be valuable for future models of the spread of species ranges due to climate change. Additionally, these results are useful for those who are trying to control the spread of invasive species, especially as a form of biocontrol using a competitor to stop an invasive species.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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