Why are tropical mountain passes “low” for some species? Genetic and stable-isotope tests for differentiation, migration and expansion in elevational generalist songbirds

Blood Variation Implicates Respiratory Limits on Elevational Ranges of Andean Birds

AbstractThe extent to which species ranges reflect intrinsic physiological tolerances is a major question in evolutionary ecology. To date, consensus has been hindered by the limited tractability of experimental approaches across most of the tree of life. Here, we apply a macrophysiological approach to understand how hematological traits related to oxygen transport shape elevational ranges in a tropical biodiversity hot spot. Along Andean elevational gradients, we measured traits that affect blood oxygen-carrying capacity-total and cellular hemoglobin concentration and hematocrit, the volume percentage of red blood cells-for 2,355 individuals of 136 bird species. We used these data to evaluate the influence of hematological traits on elevational ranges. First, we asked whether the sensitivity of hematological traits to changes in elevation is predictive of elevational range breadth. Second, we asked whether variance in hematological traits changed as a function of distance to the nearest elevational range limit. We found that birds showing greater hematological sensitivity had broader elevational ranges, consistent with the idea that a greater acclimatization capacity facilitates elevational range expansion. We further found reduced variation in hematological traits in birds sampled near their elevational range limits and at high absolute elevations, patterns consistent with intensified natural selection, reduced effective population size, or compensatory changes in other cardiorespiratory traits. Our findings suggest that constraints on hematological sensitivity and local genetic adaptation to oxygen availability promote the evolution of the narrow elevational ranges that underpin tropical montane biodiversity.

Morphological and behavioural differences facilitate tropical butterfly persistence in variable environments

The thermal biology of ectotherms largely determines their abundance and distributions. In general, tropical species inhabiting warm and stable thermal environments tend to have low tolerance to cold and variable environments, which may restrict their expansion into temperate climates. However, the distribution of some tropical species does extend into cooler areas such as tropical borders and high elevation tropical mountains. Behavioural and morphological differences may therefore play important roles in facilitating tropical species to cope with cold and variable climates at tropical edges. We used field-validated biophysical models to estimate body temperatures of butterflies across elevational gradients at three sites in southern China and assessed the contribution of behavioural and morphological differences in facilitating their persistence in tropical and temperate climates. We investigated the effects of temperature on the activity of 4,844 individuals of 144 butterfly species along thermal gradients and tested whether species of different climatic affinities-tropical and widespread (distributed in both temperate and tropical regions)-differed in their thermoregulatory strategies (i.e. basking). In addition, we tested whether thermally related morphology or the strength of solar radiation (when butterflies were recorded) was related to such differences. We found that activities of tropical species were restricted (low abundance) at low air temperatures compared to widespread species. Active tropical species were also more likely to bask at cooler body temperatures than widespread species. Heat gain from behavioural thermoregulation was higher for tropical species (when accounting for species abundance), and heat gain correlated with larger thorax widths but not with measured solar radiation. Our results indicate that physiological intolerance to cold temperatures in tropical species may be compensated through behavioural and morphological responses in thermoregulation in variable subtropical environments. Increasing climatic variability with climate change may render tropical species more vulnerable to cold weather extremes compared to widespread species that are more physiologically suited to variable environments.

Contrasting drivers of diversity in hosts and parasites across the tropical Andes

Geographic turnover in community composition is created and maintained by eco-evolutionary forces that limit the ranges of species. One such force may be antagonistic interactions among hosts and parasites, but its general importance is unknown. Understanding the processes that underpin turnover requires distinguishing the contributions of key abiotic and biotic drivers over a range of spatial and temporal scales. Here, we address these challenges using flexible, nonlinear models to identify the factors that underlie richness (alpha diversity) and turnover (beta diversity) patterns of interacting host and parasite communities in a global biodiversity hot spot. We sampled 18 communities in the Peruvian Andes, encompassing ∼1,350 bird species and ∼400 hemosporidian parasite lineages, and spanning broad ranges of elevation, climate, primary productivity, and species richness. Turnover in both parasite and host communities was most strongly predicted by variation in precipitation, but secondary predictors differed between parasites and hosts, and between contemporary and phylogenetic timescales. Host communities shaped parasite diversity patterns, but there was little evidence for reciprocal effects. The results for parasite communities contradicted the prevailing view that biotic interactions filter communities at local scales while environmental filtering and dispersal barriers shape regional communities. Rather, subtle differences in precipitation had strong, fine-scale effects on parasite turnover while host-community effects only manifested at broad scales. We used these models to map bird and parasite turnover onto the ecological gradients of the Andean landscape, illustrating beta-diversity hot spots and their mechanistic underpinnings.

Little genetic structure in a Bornean endemic small mammal across a steep ecological gradient

Janzen's influential "mountain passes are higher in the tropics" hypothesis predicts restricted gene flow and genetic isolation among populations spanning elevational gradients in the tropics. Few studies have tested this prediction, and studies that focus on population genetic structure in Southeast Asia are particularly underrepresented in the literature. Here, we test the hypothesis that mountain treeshrews (Tupaia montana) exhibit limited dispersal across their broad elevational range which spans ~2,300 m on two peaks in Kinabalu National Park (KNP) in Borneo: Mt Tambuyukon (MT) and Mt Kinabalu (MK). We sampled 83 individuals across elevations on both peaks and performed population genomics analyses on mitogenomes and single nucleotide polymorphisms from 4,106 ultraconserved element loci. We detected weak genetic structure and infer gene flow both across elevations and between peaks. We found higher genetic differentiation on MT than MK despite its lower elevation and associated environmental variation. This implies that, contrary to our hypothesis, genetic structure in this system is not primarily shaped by elevation. We propose that this pattern may instead be the result of historical processes and limited upslope gene flow on MT. Importantly, our results serve as a foundational estimate of genetic diversity and population structure from which to track potential future effects of climate change on mountain treeshrews in KNP, an important conservation stronghold for the mountain treeshrew and other montane species.

Panmixia across elevation in thermally sensitive Andean dung beetles

Janzen's seasonality hypothesis predicts that organisms inhabiting environments with limited climatic variability will evolve a reduced thermal tolerance breadth compared with organisms experiencing greater climatic variability. In turn, narrow tolerance breadth may select against dispersal across strong temperature gradients, such as those found across elevation. This can result in narrow elevational ranges and generate a pattern of isolation by environment or neutral genetic differentiation correlated with environmental variables that are independent of geographic distance. We tested for signatures of isolation by environment across elevation using genome-wide SNP data from five species of Andean dung beetles (subfamily Scarabaeinae) with well-characterized, narrow thermal physiologies, and narrow elevational distributions. Contrary to our expectations, we found no evidence of population genetic structure associated with elevation and little signal of isolation by environment. Further, elevational ranges for four of five species appear to be at equilibrium and show no decay of genetic diversity at range limits. Taken together, these results suggest physiological constraints on dispersal may primarily operate outside of a stable realized niche and point to a lower bound on the spatial scale of local adaptation.

Museum specimens of terrestrial vertebrates are sensitive indicators of environmental change in the Anthropocene

Natural history museums and the specimen collections they curate are vital scientific infrastructure, a fact as true today as it was when biologists began collecting and preserving specimens over 200 years ago. The importance of museum specimens in studies of taxonomy, systematics, ecology and evolutionary biology is evidenced by a rich and abundant literature, yet creative and novel uses of specimens are constantly broadening the impact of natural history collections on biodiversity science and global sustainability. Excellent examples of the critical importance of specimens come from their use in documenting the consequences of environmental change, which is particularly relevant considering the alarming rate at which we now modify our planet in the Anthropocene. In this review, we highlight the important role of bird, mammal and amphibian specimens in documenting the Anthropocene and provide examples that underscore the need for continued collection of museum specimens.This article is part of the theme issue 'Biological collections for understanding biodiversity in the Anthropocene'.