#30: Global trends in species shifts caused by climate change
Dan Isaak
They’re on the march…
Hi Everyone,
Hope summer was a good for everyone, if a bit on the warm side. We’re presently ¾’s of the way through what’s likely to again rank in the top 10 of the Earth’s warmest years since consistent instrumental records were begun in the late 19th Century. And it may well prove to be the warmest year ever for the U.S. where large swaths of the country have experienced extended drought and 10’s of thousands of new maximum temperature records have been set (graphic 1). With that sobering prelude, it seems like a relevant time to begin the Climate-Aquatics Module in earnest. Everything through the previous 30 blogs has been mainly foreplay, since what we really care about are FISH, & figuring out how this whole climate change thing may affect them. And this is where it gets a bit more complicated, because now we have not only the physics of stream habitats and their response to climate forcing (Blog Modules 1 & 2), but we’re dropping a slippery fish that evolves and exhibits behavioral plasticity on top of that shifting habitat template. This is why I tell my physical scientist friends that biology is not rocket science, it’s way harder.
Though challenging, it’s also quite doable given the tools, techniques, and datasets available to us & we’ll step through what is known and unknown regarding climate effects on fish as we work through this module. Before getting too far into those details, however, it’s good to establish a mechanistic rationale for why we’d expect fish to be affected by climate change. Perhaps the primary reason is simply that they (& most other aquatic organisms) are ectotherms, meaning that their physiology and metabolic efficiency is dictated by temperatures in the surrounding environment (attached paper by Portner and Farrell 2008; graphic 2). As we’ve seen in several previous blogs, those temperatures have generally been increasing (blog #’s 10, 11, 13, 23). Accompanying temperature increases, other trends in stream environments are apparent such as runoff timing and flood frequency (Blog #17), declining summer baseflows (Blog #18), & alteration of sediment regimes (Blog #22). The combination of these trends creates a shifting set of habitat conditions, what I call a dynamic disequilibrium, that differs fundamentally from the dynamic equilibrium we’ve long assumed. Some fish species and populations in some areas may benefit from these changes whereas many others will not. Our challenge is understanding and predicting the biological effects, with enough resolution, and far enough in advance that it’s possible to make intelligent decisions about what to do and where to do it so that we’re managing and conserving efficiently this century.
Before starting on that topic next time, however, there’s an important paper I wanted to pass along. It’s a bit out of date, having been published in 2003, before we just passed through the Earth’s warmest decade in recent centuries. It’s a review paper by Parmesan and Yohe that does a meta-analysis of trends in species distributions and phenologies inferred from long-term monitoring records. Dozens of plant and animal taxa are included and some 1,700 individual species assessed. The conclusion a decade ago was that species weren’t waiting around for climate change to happen, they’ve been marching to its drumbeat for many decades now. Significantly higher proportions of species distributions have been shifting to cooler areas (at higher elevations or poleward) and advancing the timing of their phenologies than would be expected by random chance. The estimated global rates of distribution shifts are 6.1 kilometers/decade (or 6.1 meters/decade higher in steep areas; graphic 3) and phenologies are advancing 2.3 days/decade (graphic 4). If you read the paper, you’ll notice that not many of those 1,700 species were fish (& none were freshwater fish), so see what we’ve learned in the last decade throughout the rest of this Climate Aquatics biology module.
Until next time, best regards,
Dan