Bowler et al. (2020) Impacts of predator-mediated interactions along a climatic gradient on the population dynamics of an alpine bird. Proceedings of the Royal Society B, 287, https://doi.org/10.1098/rspb.2020.2653.

The Crux

Whether or not a species will survive in an area can usually be broken down into two broad categories: how suitable the environmental characteristics of that area are (temperature, elevation, rainfall), and how it interacts with the other species found nearby. Early ecological theory predicted that in harsh environments, how a species interacts with other species wouldn’t matter as much, and would only come into play when the area was easier for the species to inhabit.

Yet more modern work often contradicts this theory. For instance, the Alternative Prey Hypothesis (APH) suggests that in areas where there are relatively few species as a result of harsh climates, interactions between those few species will be relatively strong. For example, if a prey species declines one year, then its usual predator must find an alternative prey species. This creates an indirect interaction between the two prey species, which is particularly strong in harsh environments where there aren’t other species around.

Today’s researchers wanted to figure out which of the two theories were more accurate in a sub-Arctic ecosystem, in which foxes prey on either rodents or ptarmigans (massive pigeon-looking birds, pronounced “tar-mi-gan”). The foxes will usually opt for rodent prey, but when rodent population grow low, the foxes will switch to prey on the ptarmigans as well.

What They Did

This dataset was collected in a large part by citizen/community scientists, who (with guidance from researchers) surveyed straight line transects throughout Norway from 2007 to 2017. The transects were on average 3.7km long, and involved two individuals walking along the transect and counting ptarmigans, as well as the distance from the transect that the ptarmigans were sighted (more on why this is important later on). They also noted whether or not rodent species were present in the area.

Once this data had been collected, the scientists involved estimated the effects of a range of different values on ptarmigan populations, including those of temperature and rodent presence.

Did You Know: Observation Error

One of the big problems with any study is that no matter how experienced an observer is, there will always be a few animals or plants that slip under the radar. Statistical modelling takers this into account though, and in the case of this study, the distance of ptarmigan populations from the line along which the observes walked was used to calculate how likely an observer was to miss seeing a ptarmigan. As an example, if the observer saw as many ptarmigans on the line itself as they did 100 metres from the line, that observer’s error could be considered quite low. Likewise, if along another transect an observer recorded many ptarmigans close to the line, but few further away, observer error would be higher.

What They Found

As temperatures decreased and conditions became harsher, ptarmigan population growth was lower, which made sense. However in harsher climates, the effect of rodent populations on ptarmigans increased markedly. This implies that in these areas, the increased presence of rodents leads to foxes switching to rodents as a main food source, and ptarmigan populations increasing as a result. Additionally, if there was a high number of rodents in one year, the resulting (assumed) increase in predator populations meant that the next year ptarmigan populations were low.

Problems

I have two issues here. One is purely semantic, and regards the constant use of the term species interactions, as opposed to species associations. The model doesn’t actually suggest interactions between the species, rather an association between population levels. I get that this paper rests on the assumption of the indirect interaction between the two species, but the lack of data on the predators themselves (my second issue) makes that link a little tenuous. Admittedly, fox data would have been much harder to come by, but it would have made for a great addition to the paper (though the scientists involved acknowledge this themselves).

So What?

This is a great study of how the harshness of the climate can affect species interactions. Research like this is particularly important for sub-Arctic and Arctic species, as many of these areas are probably going to become substantially less harsh as temperatures warm over the coming decade. An idea of how fluctuations in population numbers are likely to change would therefore be a great advantage for population managers in the future.

Dr. Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who is done with his PhD and about to put ‘Dr.’ ahead of absolutely everything. You can read more about the Dr.’s research and the rest of the Ecology for the Masses writers here, see more of the Dr.’s work at Ecology for the Masses here, or follow the Dr. on Twitter here.

Title Image Credit: Tim Bowman, USFWS