Image Credit: Christa Rohrbach, CC BY-NC-SA 2.0, Image Cropped

Modelling temperature-driven changes in species associations across freshwater communities (2021) Perrin et al., Global Change Biology, https://doi.org/10.1111/gcb.15888

The Crux

Species interact in many complex ways within an ecosystem. One species may prey on another, which can keep that prey species at a low density. Two species might compete for resources, keeping each other’s populations in check. These interactions are part and parcel of any ecosystem. But what happens when the climate changes in that ecosystem? If one species does better under warmer conditions, perhaps that could lead to that species completely wiping out a prey species, or even a competitor?

You’d think that monitoring this would be easy, given the extensive environmental data and species occurrence data we’re able to come up with in the digital age. Yet the problem is that when a species population drops as the climate changes, it’s very hard to tell whether that’s a direct result of the climate itself, or whether that species is being negatively affected by another species which has benefited from the change. That’s what we wanted to try and figure out.

What We Did

We were lucky enough to have access to data from the 1995 Nordic Freshwater Survey, an enormous undertaking which charted the presence or absence of over 20 species of fish across Norway, Sweden, and Finland. The same survey took a bunch of environmental data as well, which meant we had access to everything from lake size and chemistry to air temperature and distance to the nearest road.

We then modelled these environmental variables against the presence or absence of the fish. This is standard practice in ecological modelling. However in any model, once environmental variables have been taken into account, there are still patterns of occurrence left over which can’t be explained by those variables. Here, we compared those patterns to patterns of species associations (whether or not different species were present in the same lake) to get an idea of which species could be influencing the presence (or absence) of others. And, since these lakes occur over a range of different temperatures, we were able to estimate how those associations changed as the climate changed!

Did You Know: Fresh-Faced Freshwater

This part of the world still had glaciers heaving across it until roughly 10,000 years ago, which means the native fish population has only been in place since then. After the glaciers melted, many fish made their way into Sweden and Finland through the Baltic Sea, which was back then a massive freshwater lake. But along the Norwegian west coast, the only fish which could colonise the lakes naturally were fish from the sea that could tolerate both salt and freshwater, meaning this area is very low on species.

What We Found

Salmonid species like the Arctic charr and brown trout are very well adapted for cold waters, and are often preyed on extensively by more warm-tolerant fishes like the northern pike and European perch. So it’s not a surprise that these fishes were negatively associated with each other – meaning that if pike were present in a lake, Arctic charr were much less likely to be there. It is worrying though that as temperatures increased, that association between some species became MORE negative, meaning for instance that as temperatures increase the presence of pike in a lake could easily result in the extinction of brown trout.

This is in line with previous studies that showed these associations happening on local scales, within a single river system or lake. But results on this large a scale mean that as temperatures rise over the coming decades, we could see cold-water species pushed further and further towards the planet’s poles.

Problems

Since we had to use presence-absence data, it’s hard to know whether or not we picked up on some species associations. A negative effect of one species on another could be as simple as one fish moving to a different part of the lake, or evolving a smaller body size. Yet because the data we worked with simply shows whether or not a fish species is present, the only negative effect we were able to pick up on was complete extinction, making for a less nuanced model.

So What?

Knowing where rising temperatures are likely to cause problems for a species enables us to pinpoint exactly which areas require our attention. Species like the pike aren’t present in lakes further north in the Nordic region, so illegal introductions will be particularly devastating as the climate changes. Using these models, we can figure out in which lakes extinctions are likely to happen, and make sure invasive species never get in there in the first place.

From a modelling perspective, it’s encouraging to know that large-scale models can reflect more practical, small-scale studies, and gives us trust in using larger scale data to predict larger-scale trends.

---

Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology who thinks the Arctic charr is really very gorgeous and deserves your respect and your home number. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.