Microclimate variability impacts the coexistence of highland and lowland ectotherms

Abstract

Understanding differences in life-history outcomes under variable abiotic conditions is essential for understanding species coexistence. At middle elevations, a mosaic of available sets of abiotic conditions could allow highland and lowland species of the same ecological guild to overlap. Therefore, these sites are excellent to study the influence of abiotic conditions on life history and, thus, spatial overlap patterns of competing species. To test differences in life-history outcomes, we selected a pair of closely related lacertids, Iberolacerta horvathi and Podarcis muralis, with an overlapping geographical range but a contrasting elevational distribution. To assess how abiotic and biotic factors contribute to the realized niches of both species, we first built dynamic energy budget (DEB) models for each species based on available functional and life-history data. Then, we used a mechanistic modelling framework (NicheMapR) to simulate the microclimatic conditions at 15 study sites across an elevational gradient and performed whole life-cycle simulations for both species to compare egg development times, lifespans, reproductive years, mean yearly basking and foraging times and yearly fecundity in syntopy and allotopy along the elevational gradient. Our simulations show that the variability of abiotic conditions along an elevational gradient affects life-history traits of both species. We found strong effects of species and elevation on life-history outcomes such as longevity, activity and fecundity. We also observed the effects of syntopy/allotopy on egg development times, activity and reproductive output. In addition, we found a significant interplay between elevation and species impacting fecundity where occupying higher elevation habitats resulted in a more pronounced reduction in fecundity in P. muralis. Furthermore, using two different thermal preferences for spring and summer, we show that some physiological and reproductive traits change with seasonal changes in thermal preferences. Based on our simulations, we conclude that the intermediate elevations that harbour the majority of syntopic populations exhibit high environmental variability that is likely facilitating species coexistence. Since our model predictions support that the current elevational distribution of the species is not only affected by abiotic factors, this suggests that past historical contingencies might have also played a significant role. Our study provides a framework using mechanistic models to understand current distribution patterns of two interacting species by comparing life-history differences between species based on responses to changing abiotic conditions along an elevation gradient.

Keywords: Lacertidae; dynamic energy budget; ectotherms; elevation; life history; microclimate; syntopy.

FIGURE 1

Iberolacerta horvathi (denoted in red) and Podarcis muralis (denoted in yellow) were studied across an elevation range between 250 and 1250 m above sea level and across allotopic and syntopic sites (syntopy—denoted in blue) (a) distribution of study sites across elevation with relative densities represented with the shaded triangles based on previous data collected in the study site (Žagar, 2016). (b) Main hypothesis: (1) the current distribution is a matter of historical and dispersal factors, and range overlap is not explained by the environment, (2) middle elevations represent the least optimal conditions for both species, thus representing a shared area of unfavourable abiotic conditions, (3) middle elevations provide favourable and resource abundant microclimatic conditions allowing both species to sustain viable populations. (c) Map of the study area with study sites. (d) Study species (Photo credits: Miha Krofel).

FIGURE 2

Plots of OLS linear regression models of the effect of elevation (x axis) on chosen environmental variables at 15 different locations of known allotopy or syntopy. Each point represents a unique location. Each data point in (a, c, d) represents the average mean yearly temperature during our simulation (16 years). The points in (b) represent the total number of days without snow in the full simulation.

FIGURE 3

Plots of OLS linear regressions of the effect of elevation, species and location type on life‐history traits (blue—syntopy study sites, red—highland species Iberolacerta horvathi allotopy study sites, yellow—lowland species Podarcis muralis allotopy study sites) at 30 different microclimate–species trait combinations. Panels on the left represent simulation predictions when using spring preferred body temperatures and panels on the right when using summer preferred body temperatures. In the case of years reproducing, we avoided showing separate slopes by location type (syntopy/allotopy) since we found no significant effect. Yearly fecundity is expressed as mean yearly fecundity throughout the reproductive years of the animal.