Four decades of phenology in an alpine amphibian: trends, stasis, and climatic drivers

Abstract

Strong phenological shifts in response to changes in climatic conditions have been reported for many species, including amphibians, which are expected to breed earlier. Phenological shifts in breeding are observed in a wide number of amphibian populations, but less is known about populations living at high elevations, which are predicted to be more sensitive to climate change than lowland populations. The goal of this study is to assess the main factors determining the timing of breeding in an alpine population of the common toad (Bufo bufo) and to describe the observed shifts in its breeding phenology. We modelled the effect of environmental variables on the start and peak dates of the breeding season using 39 years of individual-based data. In addition, we investigated the effect of the lunar cycle, as well as the individual variation in breeding phenology. Finally, to assess the individual heterogeneity in the timing of breeding, we calculated the repeatability of the timing of arrival at the breeding site. Breeding advanced to earlier dates in the first years of the study but the trend continued only until the mid 1990s, and stabilised afterwards. Overall, toads are now breeding on average around 30 days earlier than at the start of the study period. High temperatures and low snow cover in winter and spring, as well as reduced spring precipitation were all associated with earlier breeding. Additionally, we found evidence of males arriving on average before females at the breeding site but no clear and strong effect of the lunar cycle. We only found weak evidence of among-individual variation in shifts in the breeding phenology, as well as a low repeatability of arrival timing. Our findings show that the observed changes in breeding phenology are strongly associated with the environmental conditions. These results contribute to filling a knowledge gap on the effects of climate change on alpine amphibian populations. Moreover, we show that changes in phenology, especially in the mountains, can be hard to predict as local microclimatic conditions do not necessarily reflect the observed global climatic trends.

Interplay of abiotic, biotic, and individual factors in shaping individual growth and demography in a high-elevation common toad population

“…However, current effects of climate change on this population are not as strong and predictable as expected. For instance, winter and spring temperatures are not constantly increasing, nor is snow cover constantly decreasing (Lenzi et al, 2023), perhaps due to topologically driven micro-climatic conditions (Scherrer & Körner, 2011). If indeed females slowly move more consistently towards an annual breeding, there might be consequences for the population, as many more individuals will be produced.…”

Effects of temperature and precipitation changes on shifts in breeding phenology of an endangered toad

“…Phenology has a key role in amphibian lives, chiefly in species living in temperate regions where many activity cycles are determined by seasonality 17 , 18 . For instance, climatic features such as temperature and precipitation play a central role in determining breeding migrations and changes in phenology 19 , especially for species that rely on ephemeral waterbodies for reproduction and larval development 20 , 21 .…”

“…Furthermore, adults must time above-ground movements with meteorological conditions to avoid desiccation risk. Breeding phenology can also vary among individuals from the same population due to several factors, such as sex and size 18 , 22 . Indeed, in many species males migrate earlier than females to the breeding sites, in order to compete for the best spots in the pond where they will wait for the females 23 .…”

“…Indeed, in many species males migrate earlier than females to the breeding sites, in order to compete for the best spots in the pond where they will wait for the females 23 . Studies on amphibian responses to climate change have shown variability in phenological shifts across species and populations, with both earlier spring breeding 18 , 24 – 28 and delays in seasonal migrations 29 , 30 . This means that the direction, timing, and strength of these changes can depend on specific environmental conditions experienced at the local scale, along with other important factors such as endogenous rhythms 19 , 31 – 34 .…”