#35: Part 3, Fish distribution shifts from climate change: Empirical evidence for range expansions
Dan Isaak
Are cold fish basking in balmy new habitats somewhere?
Hi Everyone,
So we’re addressing the same question this time as last (i.e., are fish distributions shifting as per the predictions from the bioclimatic models?) but now we’re looking at the other end of the thermal spectrum; to streams that were previously too cold for some species. Just as we’d predict species & local populations to be lost from warm habitats as climate change proceeds, some critters should also be expanding into new habitats elsewhere (graphic 1). This one’s a bit tougher to tease out a climate effect due to the global pandemic of non-native species invasions that provide ample examples of distributions expanding. In most cases, however, those invaders are just running willy-nilly through new environments rather than basking in newly unfrozen ones. So addressing the range expansion-climate change question most precisely requires looking at species that have been parts of stream ecosystems long enough to have more or less filled their ecological niches. The best candidates for this will usually be natives (or a non-native that’s been there a long time) & we’d want to focus our attention on population boundaries in cold places like the upstream distributional extent in a mountain stream or the northerly extent of a species’ range. As was the case with empirical evidence of range contractions last time (blog #34), there aren’t a lot of case histories documenting climate-related range expansions for fish species and I had to stretch to find a few.
The first study is an overview of the work that Milner and colleagues have been doing in Glacier Bay National Park the last several decades in Alaska (graphic 2). It’s something of a special case since huge chunks of ice aren’t usually sitting on top of most streams, but it does illustrate the general point in a brute force sort of way. As the glaciers have melted and retreated in GBNP over the last few hundred years, new streams and associated ecosystems have been developing as temperatures gradually increased. Long-term monitoring data over a 30+ year period documented the colonization of some of these streams by Dolly Varden, then coho salmon, followed by chum salmon, and coast range sculpin. Moreover, because the glacial retreat varies in age across many streams, the colonization sequence is at different stages throughout the GBNP & interested readers should check out some of Milner’s earlier papers for more of those details.
The second study is less than perfect, since rather than looking at fish, we’re looking at bugs that are typically considered fish food (no offense to the entomologists in the room). Moreover, these bugs disperse mainly by flight after emerging from streams and lakes where they spend their juvenile stages (but there’s just not a lot to choose from in this genre, so we’ll go with it). In the study by Hickling and colleagues, distributions of 37 Odonate species (dragonflies and damselflies) were assessed across Britain between two periods (1960-1970 and 1985-1995). Of the 37 species examined, 34 exhibited a northward range shift over this time, with the average shift being 74 kilometers (graphic 3). This study is a great example of the utility of historical survey data—a topic we’ll be exploring in much more detail in future blogs.
So I’ve no doubt missed a few, but the 4 studies highlighted in this blog and the previous one constitute a significant amount of the empirical evidence that exists for distribution shifts in freshwater organisms related to climate change. Given that freshwater ecosystems contain a disproportionate amount of biodiversity (something like 6% of all described species occur in freshwater environments that cover < 1% of the Earth’s surface), we have some work to do to catch the biological evidence up to the model predictions (blog #33) for these important ecosystems. That evidence can be accumulated relatively quickly using existing databases, by mimicking some of the study designs used previously in other disciplines, and tailoring the questions specifically to aquatic environments. But before plunging ahead, it’s good to think critically about how and where to look and what we’d expect to see if warming trends are truly causing widespread distribution shifts. By gathering ourselves first, we can develop and test some interesting hypotheses along the way, add scientific rigor to the work, and maximize what is learned from the global warming experiment that’s being run.
Until next time, best regards,
Dan
To access a .pdf of this blog with associated graphics and full citations for the articles described above, go here: http://www.fs.fed.us/rm/boise/AWAE/projects/stream_temp/stream_temperature_climate_aquatics_blog.html