Short-term thermal anomalies are associated with increased use of localized water resources by a montane bat community

Technical Summary: Short-term Thermal Anomalies and Montane Bat Resource Use

Problem Statement
While environmental variability is a known driver of ecological processes, the specific influence of short-term thermal anomalies—deviations in ambient temperature ($T_a$) from long-term seasonal expectations—on how animal communities utilize spatially restricted resources remains poorly understood. Climate change is projected to increase the frequency and intensity of such extremes rather than merely raising average temperatures. In montane ecosystems, where organisms face high metabolic and water demands due to marked seasonal and short-term climatic variation, these anomalies may force significant behavioral adjustments. Specifically, it is unclear how anomalously warm or cold conditions alter the use of localized water sources, which serve as critical resource patches for volant insectivores like bats that experience high water turnover during flight.

Methodology
The study utilized a montane bat community in La Malinche National Park, central Mexico, as a model system to evaluate the association between thermal anomalies and the use of localized water resources.

• Data Collection: Bat capture data were compiled from standardized monthly surveys conducted over two periods (2014–2015 and 2023–2024) at artificial ponds. Bats were captured using mist nets during new moon periods. Ambient temperature ($T_a$) data were sourced from the nearest regional meteorological station.
• Thermal Anomaly Definition: To decouple short-term anomalies from predictable seasonal cycles, the study calculated thermal anomalies as the deviation of the observed monthly minimum $T_a$ from the long-term nightly monthly mean. These values were standardized (z-scored).
• Statistical Modeling:
  • Primary Model: A negative binomial generalized linear mixed-effects model (GLMM) using the glmmTMB package was employed to analyze capture counts, treating them as indices of resource use. The model included standardized thermal anomaly as a fixed effect, species identity as a random intercept, and the logarithm of sampling effort (number of ponds) as an offset.
  • Phylogenetic Controls: To account for phylogenetic non-independence, two additional models were fitted: a frequentist phylogenetic GLMM (Poisson distribution) using the pglmm function, and a Bayesian phylogenetic GLMM (negative binomial distribution) using the brms package. The Bayesian model allowed for the estimation of species-specific random slopes for thermal responses.
  • Model Diagnostics: Fit and distributional assumptions were validated using simulated residuals (DHARMa package).

Key Results

• Positive Association: The negative binomial GLMM revealed a statistically significant positive association between thermal anomalies and pond-based capture activity ($\beta = 0.33 \pm 0.14$, $p = 0.018$). This corresponds to an approximate 39% increase in expected capture activity for every one standard deviation increase in thermal anomaly.
• Phylogenetic Consistency: Both the frequentist phylogenetic model ($\beta = 0.44$, $p < 0.001$) and the Bayesian phylogenetic model (posterior mean $\beta = 0.33$) yielded qualitatively similar positive associations, confirming that the relationship holds even when accounting for evolutionary relatedness.
• Interspecific Variation: While the community-wide trend was positive, substantial interspecific variation was observed. The Bayesian analysis indicated that the probability of a positive response was highest for Eptesicus fuscus and Myotis californicus, whereas responses for less frequently captured species were more uncertain.
• Seasonal Decoupling: The anomaly transformation successfully removed seasonal structure from the temperature predictor (ANOVA: $p = 1$), confirming that the observed effects were driven by short-term deviations rather than regular seasonal cycles.

Significance and Claims
The paper claims that short-term climatic anomalies are associated with shifts in the use of localized water sources by free-ranging animal communities, independent of normal seasonal patterns. The author posits that anomalously warm conditions likely increase evaporative water loss and potentially insect activity, thereby driving higher visitation rates to water sources.

The study suggests that localized water sources may function as "ecological focal points" whose importance fluctuates with short-term climatic variability. In montane ecosystems, where water availability is often spatially restricted, these anomalies may concentrate animal activity around specific resources, potentially reshaping interspecific interactions and community dynamics. The author emphasizes that these findings highlight the need to consider short-term thermal variability, rather than just average temperature changes, when predicting ecological responses to climate change.

Limitations Noted by the Author
The paper explicitly states that its observational design precludes causal inference. The author notes several limitations:

1. Capture activity serves as an index of resource use rather than a direct measure of abundance or occupancy.
2. Temperature data represent regional conditions rather than the specific microclimate experienced by bats at the ponds.
3. Unmeasured variables, such as changes in pond vegetation, water depth, or prey availability, could contribute to temporal variation.
4. Direct physiological measurements were not obtained to confirm the mechanisms (e.g., evaporative water loss) linking temperature to behavior.

Consequently, the paper frames its conclusions as associations suggesting that short-term anomalies alter resource use patterns, rather than definitive proof of physiological mechanisms or population-level impacts.