Aim As global air temperatures continue to rise in response to climate change, environmental conditions for many freshwater fish species will change. Warming air temperatures may lead to warming lake temperatures, and subsequently, the availability of suitable thermal habitat space. Our objectives are to identify the responses of three fish species from three thermal guilds to climate change in Ontario and consequently, the potential for novel competitive interactions between two top predators. We focus on lakes in Ontario because it is a dynamic region that encapsulates the northern and southern range extents of warm and cold-water fish species.Location Ontario, Canada.Methods Using lake morphology, water chemistry, climate and fish occurrence data for smallmouth bass (warmwater predator), walleye (coolwater predator) and cisco (cold-water forage fish), we modelled the occurrence rates of three fish in 2050 and 2070 under 126 scenarios of climate change. We also calculated the percentage change in co-occurrence of walleye and smallmouth bass in 2050 and 2070.Results Smallmouth bass occurrence rates were predicted to increase bỹ 306% (ranging between 55 and 422%) by 2070 relative to their current distributions. Walleye were projected to decline by 22% (À42 to a +6% change) and cisco by 26% (À7 to À47%) by 2070. By 2070, walleye-smallmouth bass co-occurrence was predicted to increase by 11%, with walleye in central and northern Ontario at greatest vulnerability due to increased competition with smallmouth bass.Main conclusions These results highlight three unique responses to climate change: range expansion, northward range shift, and range contraction for warmwater, coolwater and cold-water fish species, respectively. Alterations in distributions of these three ecologically important fish species may lead to shifts in fish community structure and novel species interactions in Ontario lakes, exacerbating the vulnerability of native coolwater predators to climate change.

Aim The combined effects of multiple environmental stressors continue to threaten global biodiversity, yet predicting how biotic interactions between native and invasive species may change across a landscape in a multiple stressor environment is relatively understudied. We aim to identify how the invasion of smallmouth bass (Micropterus dolomeiu) may influence native walleye (Sander vitreus) populations across Ontario lakes at the landscape scale in a changing climate. Location Ontario, Canada. Methods Using a database that included the abundance and occurrence of over 130 fish species, lake chemistry and lake morphology for 722 lakes, a redundancy analysis was conducted to identify environmental conditions preferred by walleye and smallmouth bass. Multiple linear regression models were then developed to identify the relationship between walleye and multiple stressors (including climate change and biotic interactions with invasive species). Using future scenarios of climate change, we were then able to project future walleye–smallmouth bass co‐occurrences. Results Smallmouth bass were found to prefer different environmental conditions than walleye; however, when walleye and smallmouth bass were found in the same lakes, walleye abundance was reduced almost threefold. Multiple regression models further suggested that there are fewer walleye in lakes with smallmouth bass. Subsequently, we predicted that under future scenarios of climate change the overlapping co‐occurrence of walleye and smallmouth bass may increase by 86–332% by the year 2070. Main conclusions We illustrate the importance of including multiple environmental stressors in statistical models when attempting to understand how native species will be impacted by invasive species and climate change. While independently climate change is anticipated to lower walleye abundances across Ontario, this change is expected to be exacerbated by invasions of warmwater predators.

Habitat alteration and climate change are two important environmental stressors posing increasing threats to woodland caribou, Rangifer tarandus caribou, in Ontario. Our first objective was to identify the importance of linear features, habitat, and climate on the occurrence of woodland caribou during the winter season using over 30 years of records . Our second objective was to forecast the impacts of climate change on the future occurrence and range of woodland caribou. Woodland caribou occurrence and environmental data collected during 1980 to 2012 were obtained from the Ontario Ministry of Natural Resources (OMNR). Logistic regression models were used to identify the importance of linear features, habitat, and climate on woodland caribou. We then forecast future caribou occurrences using 126 future climate projections. Woodland caribou preferred coniferous forests and mixed forests that tended to be associated with increased lichen coverage, and regions with colder winters. Woodland caribou also avoided anthropogenically disturbed regions, such as areas associated with high road density or developed areas. Caribou range extent was projected to contract by 57.2-100% by 2050 and 58.9-100% by 2070. Furthermore, all 126 climate change scenarios forecast a range loss of at least 55% for woodland caribou in Ontario by 2050. We project complete loss of woodland caribou in Ontario if winter temperatures increase by more than 5.6°C by 2070. We found that woodland caribou in Ontario are sensitive to changes in climate and forecasted that an average of 95% of Ontario's native woodland caribou could become extirpated by 2070. The greatest extirpations were projected to occur in the northernmost regions of Ontario as well as northeastern Ontario, while regions in western Ontario were projected to have the lowest rates of extirpation. This underscores the importance of mitigating greenhouse gases as a means to protect this iconic species.