Our findings suggest that contrary to what we expected, the effect of increased water summer temperature was limited to a few species only and with the same relation for generalist and cold specialist species occupancy probabilities. We also found that the predicted decreasing hydroperiod and connectivity, especially considering topographic distance between ponds, resulted in reduced occupancy probabilities of species. Such relation influenced a greater number of studied species than the increased summer water temperatures. Current predictions of the effect of climate change on the distribution of species based principally on temperature variation minimized its effects on species occupancy probabilities.
4 − 1) Temperature was not the primary driver of biodiversity distribution
Temperature drives alpine species distribution. Its increase is influencing species densities (Alatalo et al., 2017) and increasing the risk of species local extinction. This last result has been demonstrated in all mountain ecosystems as terrestrial (plants) (Jump et al., 2009), streams (Stoneflies, Lednia) (Green et al., 2022) and ponds (Arctic fairy shrimp, Branchinecta paludosa) (Lindholm et al., 2015). Therefore, temperature increases are causing continued displacement of communities in mountain terrestrial ecosystems (Lenoir et al., 2008) as well as aquatic environments (Li et al., 2016; Sáinz-Bariáin et al., 2016). Consequently, temperature is the primary driver used to describe the effects of climate change on alpine species distribution (Adhikari et al., 2023; Engler et al., 2011; Tovar et al., 2022).
We observed that increasing the summer accumulated warm limited the occupancy probabilities of up to 30 percent of the 30 alpine pond species studied when minimum temperature had positive effects on species occupancy of 40 percent. Temperature increase had effects only in a part of studied species occupancy probabilities contrary to our expectations, but in line with the main results of a global meta-analysis including both terrestrial and marine fauna (Parmesan & Yohe, 2003).
When we showed an effect of temperature increase on species occupancy probabilities, summer temperature accumulated warm and minimum temperature effects were different. We found that summer accumulated warm increases decreased amphibian and macrophytes species occupancy probabilities when a threshold was reached but had no effects on Odonata’. Amphibian and macrophytes species occupancy probabilities began with an increase when water summer accumulated warm increased. This well studied phase (Cross & Zuber, 1972; Liu et al., 2022; McMaster & Wilhelm, 1997) corresponds to the triggering of their development and growth. As we showed, after a threshold occupancy probabilities of species decreased. This is because their development stops and if temperature accumulated continues to increase it can reach their lethal point (Abbasi et al., 2023; Wahid et al., 2007). The unexpected absence of effects of summer accumulated warm increases on Odonata species occupancy probabilities could be due to their tropical origin (Pritchard, 1989). Odonata exhibit thermoregulatory plasticity despite being ectothermic: color change, regulate hemolymph circulation in the thorax and abdomen, or employ "wing-whirring" behavior to thermoregulation (May, 1976; Polcyn, 1994; Sternberg, 1997). Our results suggest that cold specialist species of Odonata had the same capacity as other Odonata to colonize a broad spectrum of thermal habitats.
We also found that part of species occupancy probabilities from the three different studied groups were increased by summer minimum temperature increase. This is because in high mountains, low temperatures stress species, reducing their growth and survival (Cabrera, 1996; Larcher et al., 2010). Cold thermal specialist species are adapted to high mountain extreme colds. For example, cold adapted plants can express some particular forms/transcripts of proteins (e.g., rubisco) to increase “carbon assimilation rate supporting the photochemical mechanism of photosynthetic acclimation to cold” (Jurczyk et al., 2016). Unexpectedly and contrary to our predictions, cold specialists and thermal generalists studied species (40%) occupancy probabilities both increased with temperature increase. Low temperature increase reduces low temperatures stress for all species allowing them to colonize new alpine ponds. As we detected, temperature exerts a notable influence on species distribution, yet it is not the sole determinant of occupancy probabilities for all species in response to climate change.
4 − 2) Hydroperiod constrained more species occupancy probabilities than temperature
Hydroperiod reduction can be a major disturbance of aquatic biodiversity communities (Greig et al., 2013; Leigh & Datry, 2017; Stubbington et al., 2019). In lotic and lentic communities, absence of water leads to lower density and numbers of aquatic taxa at a given site than when water is present (Leigh & Datry, 2017; Rosset et al., 2017; Wissinger et al., 2009). The few studies conducted in alpine lentic and lotic freshwaters identified the reduction of density and diversity of Macroinvertebrates, bryophytes and high soil moisture vascular plant species with hydroperiod decrease (Doretto et al., 2020; He et al., 2016; Sandvik & Odland, 2014). Hydroperiod decrease can lead to changes in species distribution (Tolonen et al., 2019) and local extinction of species, for example amphibians, Bryophytes and aquatic vascular plants (Carlson et al., 2020; He et al., 2016; Sandvik & Odland, 2014).
Here, we found that most (90%) studied species occupancy probabilities decreased when summer drying increased, corroborating previous studies on fishes and amphibians (Ogston et al., 2016; Walls et al., 2013). This was notably the case for species from the three biological groups we studied. From Odonata, amphibians and macrophytes, some species are known to be summer drying-resistant. For example, larvae of Coenagrion hastulatum can resist desiccation and Somatochlora alpestris is able to grow without free water both by burrowing into the mud or damp peat (Heidemann & Seidenbusch, 2002; Kury & Wildermuth, 2013). It is also the case for amphiphytes, plants species which ability to produce terrestrial and aquatic growth forms, or aquatic and aerial leaves (heterophylly phenotypic plasticity) allow them to survive short-term drying (De Wilde et al., 2017; Wells & Pigliucci, 2000; Zelnik et al., 2021): Caltha palustris (Dorotovičová, 2013), Juncus sp. and Carex sp. (Casanova, 1997). However, like all studied species, drying-resistant species occupancy probabilities decreased with summer drying increased and this is probably because thresholds in drying-resistant exist, as found in rivers and streams for example (Stubbington & Datry, 2013).
We found here that when the number of water-freezed days increased, most (83%) studied species occupancy probabilities decreased. This was the case for species from two biological groups studied (Odonata and macrophytes). The low occupancy probabilities of these species associated to a low number of water-freezed days could be explain by the exposure to extreme air temperature, winter insufficient-feeding resources or low nutrients when photoperiod is sufficient to species breaking diapause and the energetic cost to adapt to frequent water state changes (Bale & Hayward, 2010). In fact, we showed for Odonata and macrophytes that species occupancy probabilities increased with freezing days up to a threshold, aligning with findings from ice enclosure stress tolerant aquatic invertebrates and vascular plants (Green et al., 2022; McAllen, 1997; Renman, 1989). In fact, alpine freshwater species are adapted to a water-freezing state and it helps them eliminate species coming from warmer habitats, potentially competitors or predators (Carbonell et al., 2024; Theissinger et al., 2013). Nevertheless, when the threshold was reached the occupancy probabilities of species decreased because long-term freezing still represents a possible lethal freezing risk for these species (Boudot et al., 2014; Hotaling et al., 2021; Rehm et al., 2021). Amphibians were the sole group examined whose species occupancy probabilities were unaffected by the potential long duration of these stresses and this is probably because they are adapted to survive to long-term freezing as found in different studies (Costanzo & Lee, 2013; Storey, 1999; Yokum et al., 2023).
Drying decreased the occupancy probabilities of a higher number of studied species (summer hydroperiod: 90%; winter drying duration: 83%) than summer water temperature increase (summer temperature warm: 30%; minimum temperature: 40%). To enhance our comprehension of climate change effects on freshwater species distribution, improving knowledge of hydroperiods is necessary. Because of the lack of long-term hydroperiod records, for example community compositions are used to try to predict the hydroperiod of wetlands or water-bodies (Gaiser et al., 1998; Lillie, 2003). It is urgent to set up long-term monitoring of hydroperiods in alpine ponds to lay the foundations to develop a model that predicts hydroperiods for known ones.
4 − 3) Isolation constrains species occupancy probabilities
Theoretical models suggest that limiting connectivity will reduce colonization or recolonization and increase local extinctions in source–sink systems (Macarthur & Wilson, 1967). Connectivity decrease should in turn affect metacommunity dynamics (sensu Leibold et al., 2004). Aquatic species are already concerned by these isolation threats altering their movement and survival (Serrano et al., 2020). In lotic and lentic ecosystems, dried hydrologic connections act as barriers to species displacement, for example for fish (Baber et al., 2002; Jaeger et al., 2014; Perkin & Gido, 2012) or macroinvertebrates (Bae & Park, 2016; Gauthier et al., 2021; Sarremejane et al., 2021). In lentic patchy environments, drying impacts suitable habitat (patch) surface and connectivity between them, for example for turtles (Kindlmann & Burel, 2008; Serrano et al., 2020). To recolonize suitable patches, species need to be able to migrate to connected ones (patches not dried) (Macarthur & Wilson, 1967). Decreases of connectivity affect persistence and turn-over of species and ultimately lead to changes in their occupancy probabilities (Serrano et al., 2020).
As predicted and in coherence with previous investigations we showed that decrease of connectivity decreased occupancy probabilities of most studied species. To analyze the effects of connectivity on species occupancy probabilities we used different structural metrics linked with the patch spatial distribution in the landscape (patch number, patch sizes, and inter-patch distances) (Tischendorf & Fahrig, 2001; With & Crist, 1995). We evidenced that number and patch sizes around a studied patch had effects on occupancy probabilities of less species (few species) than inter-patch topographic distances (83%). To face rapid climate change effects such as isolation caused by drying, we reinforced the previous result that corridors or chains of stepping-stones (short inter-patch topographic distances) are more crucial for sustaining most species populations than dense networks (Hodgson et al., 2012). Inter-patch topographic distances had the same effects on all species of all studied groups (Odonata, Amphibia, macrophytes). As for Fahrig (2017), one of our results was that the effects of inter-patch topographic distance on the occupancy probabilities were also the same for generalist and specialist species. We showed for these three groups' that species occupancy probabilities increased before decreasing when a threshold was reached. These results illustrate the importance of maintaining spatial heterogeneity of patches (inter-patch distance), like it was demonstrated by different authors (Gauze, 1934; Huffaker, 1958). It allows maintaining the persistence of prey and predator systems with separate prey refuges and dividing food resources in different habitats. It also can reduce the predator and parasitoid dispersal efficiency and decrease covariance of competing species (Roland, 1993). In addition, the distance between patches occupied by a matrix of terrestrial habitat is necessary for species (Duelli, 1997), for example for the maturation at different stages of studied species (Odonata and Amphibia). When the topographic inter-patch distances increased, a threshold was reached probably when it exceeded dispersal abilities of species (Macarthur & Wilson, 1967; Makoto & Wilson, 2019).
The effect of inter-patch topographic distance increase was neutral for the occupancy probabilities of few species. In fact, species with longer distance dispersal as Aeshna juncea, Aeshna cyanea and Libellula quadrimaculata are less sensitive to structural connectivity (Pearson & Dawson, 2005). Aeshna juncea was the only specialist species with a neutral effect of inter patch topographic distance on its occupancy probabilities. Nevertheless, we demonstrated that Aeshna juncea needs the presence of high areas of ponds in 10,000 meters buffers to increase its occupancy probabilities. This could be due to its necessity and flying ability to pass valleys to persist in a mosaic of alpine pond areas and valleys. Because we did not find similar studies, it will be particularly interesting to investigate genetic structuration of this species at a larger scale than we did, to better understand our results.
For Bufo bufo and Pyrrhosoma nymphula the densities of ponds and tributaries close (100 meters buffer) to the occupied pond were important to maintain their persistence. In order to increase their occupancy probabilities, they need to be isolated from the other ponds but be able to move to tributaries if conditions are unsuitable in their current living patch.
To improve knowledge about the effects of climate change on current and future distribution of species, inter-patch topographical distances need to be included in distribution models. Researches could begin with actual not widely known distribution of patch habitat. Nevertheless, one of the future challenges is to enhance their distribution and hydroperiod to limit biases. In fact, small decreases in hydroperiods lead to large decreases of connectivity between habitats in freshwater ecosystems (Baber et al., 2002; Malish et al., 2023; Stanley et al., 1997). Our results and future research will enable us to reinforce existing patch connectivity localizing where chains of patches require restoration or completion, thereby enhancing species resilience to ongoing climate change.
4–4) Mitigation of climate change benefit primary threatened specialist species
Consistent with prior research, we predicted that temperature increase would increase the occupancy probabilities of thermal generalist species, whereas cold specialist species occupancy probabilities would decrease (Lindholm et al., 2015; Pallarés et al., 2020; Rosset & Oertli, 2011). Contrary to what we expected, our results showed that temperature increase, hydroperiod decrease and connectivity decrease had the same effects on thermal specialist and generalist species occupancy probabilities. We found similar results in studies of the effect of climate change on presence-absence distribution of amphibians and Insects. In fact, Shadle and al. (2023) compared experimentally climate change effects on habitat specialist wood frogs (Lithobates sylvaticus) and more generalist spring peepers (Pseudacris crucifer) (Shadle et al., 2023). They demonstrated that warming accelerates the duration to metamorphosis, while drying leads to diminished body size at metamorphosis in both species. Other authors demonstrated that the Insects distribution trends over time were not significantly affected by species' range size across Europe (Engelhardt et al., 2022). Finally, we found that the main difference between specialist and generalist species is not on the effect of temperature increase, drying and isolation on their occupancy probabilities, but more on the proportion of species from each group concerned by these effects. Indeed, the impacts of temperature increase were observed to affect the occupancy probabilities of more specialist species compared to generalists (71% versus 50%). We found similar results in connectivity effects (93% versus 75%). Consequently, the mitigation of climate change effects will be beneficial to a greater number of specialist species than generalists.