Freshwater species in the Northwest are coping with a lot these days. Not only is climate change altering streamflows and increasing water temperatures, but another challenge is at play – native species are competing with invasive species who may be benefitting from some of the changes that a warming climate brings.
Though climate change and invasive species pose major threats to native biodiversity, little is known about the complex ways in which they interact to cause further stress to native species. To address this knowledge gap, former NW CASC fellow Donovan Bell and NW CASC researchers Dr. Clint Muhlfeld and Dr. Ryan Kovach, along with colleagues, used long-term monitoring data from a diverse range of freshwater ecosystems across the northern Rocky Mountains of Montana to examine how two iconic Northwest fish species – bull trout and cutthroat trout – have been affected by climate change and invasive trout species in the past, and how they’ll likely be affected into the future.
Trout are a group of coldwater fishes that have developed species-specific adaptations to water temperature and flow, making them especially sensitive to changes in their environments. Throughout much of the last century, invasive brook trout, brown trout and rainbow trout were introduced for sportfishing across the interior west and have proven to negatively affect native trout populations through competition, predation and hybridization.
In this study, researchers used almost 22,000 surveys from the last 30 years to quantify the effects of climate change on the past and future distributions of native bull trout and cutthroat trout and these three invasive trout species. This study is unique in that it used empirical data – data based on observation or experience – to model the combined effects of climate change and invasive species over a large geography and across a large time scale (species distribution models like those used in this study are common, though efforts to validate model results with past data are rare).
The results of this study showed region-wide declines in both bull trout and cutthroat trout over the last three decades and continued declines of both species into the future, driven by a combination of climate-induced reductions in habitat and expansions of invasive species. Though this isn’t necessarily surprising, what is surprising is that the factors responsible for the declines of these two species are quite different. This study showed that declines in bull trout are mostly related to habitat changes like decreasing summer streamflow and increasing stream temperatures — not invasive species. Conversely, declines in cutthroat trout are strongly tied to interactions with invasive species, especially rainbow trout. Climate-induced changes in cutthroat trout habitat play a smaller role than they do for bull trout.
For invasive brown trout and rainbow, however, climate change is creating more favorable conditions that may allow them to thrive into the future. Though they will be negatively impacted by reduced streamflow, other climate-induced habitat changes – including increased water temperature – will allow them to occupy more habitat into the future than if climate change wasn’t occurring. Invasive brook trout will not fare as well in a warming climate and will likely face declines as stream temperatures increase.
This study shows how climate change can affect ecologically similar, co-occurring species through distinct pathways. In the case of native bull trout and cutthroat trout, they will require tailored management and climate adaptation strategies to help them cope with the differing threats they face. For example, efforts to protect cutthroat trout into the future may be more successful if focused on suppressing and removing invasive species, whereas conserving bull trout may be more effective if focused on protecting and restoring cold-water habitats across landscapes.
The findings of this study have significant implications for how we manage native species under climate change. Acknowledging species-specific sensitivities to climate change and its interactions with other stressors, like invasive species, will be critical for effective climate adaptation planning now and in the future.
This paper has been picked up by many news outlets since its publication in December 2021. Read the paper here and check out a Washington Post article about this NW CASC-funded research.