Temperature, invaders and patchy habitat interact to limit the distribution of a vulnerable freshwater fish

Abstract

Interacting global-change drivers such as invasive species and climate warming are likely to have major and potentially unexpected influences on aquatic ecosystems. In river networks, modified water temperature combined with patchy physical conditions will likely cause shifts in the amount and distribution of suitable habitat, with influential invasive species further altering habitat availability. We examined how distributions of a thermally sensitive galaxiid fish native to the alpine rivers of New Zealand, Galaxias paucispondylus, were influenced by these drivers using spatially extensive presence-absence electrofishing surveys of 46 sites spread over four subcatchments. A unimodal response to water temperature and an interaction with substratum size meant G. paucispondylus were limited to streams with average summer water temperatures between 10.6 and 13.8°C and were absent when average substratum sizes were <36 mm, regardless of temperature. In addition, non-native trout >150 mm long excluded G. paucispondylus, but were only found in streams with average summer water temperatures <10.6°C. These influences of trout likely strengthened the unimodal temperature response of G. paucispondylus and led to a very small G. paucispondylus realized niche. When predicted temperature increases were applied to catchment models, G. paucispondylus distributions were patchy and variable across subcatchments. Moreover, local physical characteristics of river networks were particularly important because of the nonlinear and interactive influences of temperature and substratum size on the outcome of species interactions. Therefore, substratum sizes, water temperature and a non-native predator combined to influence the distribution of this thermally sensitive fish, illustrating how the effects of climate warming will likely be strongly contextdependent and interactive.

Could spatial heterogeneity in flow disturbance drive temporal stability of native–invasive species co‐occurrence in riverscapes?

“…Our work also highlights that identifying mechanisms enhancing the stability of ecosystems, such as interactions between native and invasive species, is critical to do at spatial scales relevant for land management (Wilcox et al., 2017). This is especially important when widespread homogenisation of riverscapes may be increasing temporal variability in native–invasive interactions, making assemblages more vulnerable to additional stressors such as climate change (Boddy & McIntosh, 2017). Our findings suggest that maintaining riverscape‐scale heterogeneity, by underpinning the buffering of assemblage stability, will probably enhance ecosystem resilience.…”

Current knowledge and potential impacts of climate change on New Zealand’s biological heritage

“…In contrast, more specialised salmonids are expected to show major southwards shifts, primarily due to reproduction failure, resulting in possible extirpation of Northland populations (Scott & Poynter 1991). This process may release habitat for galaxiids that will be free of salmonid competition and predation in warmer waters, but it may have the opposite effect in cooler waters (Boddy & McIntosh 2017). In contrast, many specialised indigenous species are high-altitude coldtolerant species which may be severely affected by increasing temperature (Jenkins et al 2011), while low-altitude systems may be strongly impacted by changes in hydrology and sealevel rise (Schallenberg et al 2003).…”

Climate‐change impacts exacerbate conservation threats in island systems: New Zealand as a case study

“…In contrast, some effects of climate-driven habitat shifts are predictable, based on knowledge of well-studied invaders like trout (Figure 4b). For example, native alpine galaxiid fish are restricted by warm summer temperatures at one extreme of their temperature range, and by introduced predatory trout at the other extreme (Boddy and McIntosh 2017). As such, rising alpine stream temperatures will shift the distribution of suitable alpine habitat for both galaxiids and their predators, creating complex shifts in the spatial distribution of usable habitat for galaxiids.…”

“…In the case of the brown mudfish, a tornado that knocked down a swath of trees in a forest (Figure 4c) greatly increased suitable mudfish habitat, thereby enhancing metapopulation persistence in the face of drought (White et al 2016). Similarly, small increases in stream temperature restrict the habitat of nonnative trout, releasing native alpine galaxiids from predation and increasing the amount of suitable galaxiid habitat (Boddy and McIntosh 2017). However, because the thermal ranges of these fish may be relatively small, larger increases in stream temperature (eg 5°C) can negatively affect both introduced and native species (Boddy and McIntosh 2017).…”

“…Similarly, small increases in stream temperature restrict the habitat of nonnative trout, releasing native alpine galaxiids from predation and increasing the amount of suitable galaxiid habitat (Boddy and McIntosh 2017). However, because the thermal ranges of these fish may be relatively small, larger increases in stream temperature (eg 5°C) can negatively affect both introduced and native species (Boddy and McIntosh 2017).…”