Abstract Body size strongly influences natural history and ecological traits. Although many squamates exhibit reduced body size with increasing latitude and elevation, cases of localized mainland dwarfism are rare. We investigated the ecology of a dwarf population of Phrynosoma hernandesi (greater short‐horned lizard) of the San Luis Valley, Colorado. Here, adults are >30% smaller than surrounding non‐dwarf populations and 10%–22% smaller than those at the northernmost extent of the species' range, where body size reaches its minimum. We quantified distribution, population demographics, diet, thermal characteristics, and behavior and compared these patterns to anecdotal and published data from non‐dwarf populations across the species' range. Dwarf lizards were widely distributed across the San Luis Valley, with the highest densities in stabilized dune habitats. Differences exhibited by the dwarf population were most pronounced in females, particularly in association with population demographics, reproduction, and diet. Population demographics were female‐biased, and temporal variation was associated with changes in female abundance. Dwarf females produced the smallest neonates reported across the species' geographic range while maintaining litter sizes within the range reported for high‐latitude populations. Across populations, diet composition inferred from scat content was dominated by Formicidae, Coleoptera, and pebbles. Dwarf lizard diets showed greater diet diversity than those of non‐dwarf populations, with variation among ontogenetic groups and higher prey consumption by gravid females. Thermal observations suggested that dwarf lizards maintained body temperatures comparable to non‐dwarf lizards, although sample sizes were limited. We also documented behaviors including blood‐squirting and novel use of bison and cattle dung as refugia. Overall, ecological characteristics of the dwarf population fall within the known range of variation for the species and most closely resemble those of high‐latitude populations where body size is reduced. These findings provide a baseline natural history and ecological characterization of a unique mainland dwarf population and highlight the need for targeted studies to evaluate the mechanisms underlying dwarfism in the San Luis Valley.
Abstract The rapid increase in human population and the transition of non‐timber forest products (NTFPs) collection from subsistence to commercial purposes lead to the overexploitation of forests. This study introduces a novel model, based on ordinary differential equations, that accounts for climatic variability alongside NTFP harvesting. It is a tool to investigate the synergistic effects of such perturbations on the survivability of tree populations. The methodology enables a derivation of a formula to calculate the maximum harvesting rate that may be implemented without jeopardizing tree population viability— the NTFP harvesting rate threshold , critical for devising sustainable NTFP harvesting strategies and, therefore, aiding the design of harvesting policies. The framework integrates NTFP harvesting data of Khaya senegalensis populations as well as temperature and precipitation from two different climatic regions of Benin, Africa. Under optimal temperature and precipitation, the results indicate that the NTFP harvesting rates should not exceed 35% to avert the extinction of the Khaya population. A harvesting rate that ensures tree survivability and a linear dependence of the net growth rate on temperature and precipitation were considered. In this case, the study indicates that concurrent increases in these climatic variables, within the trees' ecologically admissible range, result in an increased tree abundance, thereby enhancing the viability of the Khaya population. Nonetheless, for typical fluctuations of the climatic factors, variations in rainfall levels influence more significantly the survivability of the tree population than comparable fluctuations in temperature. However, assuming a nonlinear dependence of the net population growth rate on temperature, extreme temperature fluctuations significantly influence population survivability as they approach the trees' ecologically admissible limits.
Abstract Between season occupancy dynamics, such as colonization and extinction events, can be used to elucidate important relationships between species and their breeding habitat. We used 13 years of forest‐breeding bird data within national parks in the mid‐Atlantic region of the United states to examine relationships between local habitat variables and occupancy dynamics of a near threatened species, the wood thrush ( Hylocichla mustelina ). We used a multi‐season dynamic occupancy modeling approach to test hypotheses for how environmental covariates influenced patterns in local colonization, extinction, and initial occupancy. Wood thrush occupancy dynamics were strongly associated with six of the eight hypothesized variables, indicating a high sensitivity to local forest habitat conditions. Wood thrush colonization was driven primarily by forest cover, with sites with greater forest cover having a higher probability of colonization. Wood thrush extinction was influenced by tree age, shrub density, volume of coarse woody debris, and most notably, forest condition measured using the normalized difference vegetation index. Wood thrush were >15 times more likely to become locally extinct at sites with low forest condition compared to sites with high forest condition. Our findings provide insight into not only what environmental factors influence occupancy, but also, what factors contribute to this declining species' persistence in forest patches.
Tick dynamics and tick infection prevalence are influenced directly by climate conditions (e.g., temperature, precipitation) and indirectly by mast seeding patterns of trees that influence the abundance of tick hosts such as small mammals. Tick and tick-borne disease dynamic studies have been done primarily at local scales, which represent a small fraction of the geographical range of ticks across the United States; it is not yet known how synchronous tick dynamics are at a macrosystems scale. Here, we explored patterns and potential drivers of spatiotemporal synchrony of tick abundance dynamics for the lone-star tick, Amblyomma americanum, on a subcontinental scale, and test whether climate variables are associated with A. americanum nymph abundance and pathogen prevalence. We used 8 years of tick drag and tick pathogen data from the National Ecological Observatory Network (NEON), with a focus on seven distinct eco-regions ("domains") across the NEON network of sites, spanning over 2000 km. Multiple regression on distance matrices (MRM) analysis found that intraspecific spatial synchrony in temporal patterns of nymph abundance declined with increasing distance between NEON sites. A. americanum nymph dynamics were influenced by lags in July temperature differences, triangle T 3 (difference in mean July temperatures in year t - 3 minus t - 4) and triangle T 4 (difference in mean July temperatures in year t - 4 minus t - 5), and July temperatures and June precipitation were most strongly associated with the proportion of infected A. americanum nymphs. Understanding the synchrony and drivers of tick dynamics and tick infection prevalence across large spatial scales-knowing whether tick populations and tick-borne disease risk are high throughout their range, or not-could influence public messaging about ticks and the risk of tick-borne diseases.
Theory predicts that plant defense against herbivores should be greatest towards the equator due to the strong selective pressure of more populous and diverse herbivore communities; yet recent work suggests variation in plant defense is more complex, predicting divergent patterns of investment across defense types and underlying environmental drivers. To address gaps in previous research, we measure multiple defense types in the same species and consider both biotic and abiotic variables hypothesized to underlie latitudinal gradients. Using the tree species trembling aspen (Populus tremuloides), we combine field surveys of biotic interactions and defense traits with climate data and metabolomic analysis of plant chemistry across a continental scale latitudinal gradient. While some metrics of defense investment were greater at lower latitudes, we found that the environmental variables underlying variation in defense traits differed across defense types, with direct defense (i.e., chemistry) decreasing along a precipitation gradient and indirect defense (i.e., extrafloral nectaries) increasing with temperature and growing season length. Incorporating proxies for predation and herbivory pressure further suggested variation in defensive traits is driven by an interplay between biotic and abiotic variables. Together, our findings reinforce that, even within a single species, different defense strategies can correlate with distinct environmental variables across latitude, consistent with complexity in the selective mechanisms shaping defense variation across space. These results illustrate the importance of considering the full scope of defense strategies and environmental pressures when testing classic hypotheses of plant defense.
