Leaf vein fraction influences the Péclet effect and 18O enrichment in leaf water

The process of evaporation results in the fractionation of water isotopes such that the lighter ¹⁶O isotope preferentially escapes the gas phase leaving the heavier ¹⁸O isotope to accumulate at the sites of evaporation. This applies to transpiration from a leaf with the degree of fractionation dependent on a number of environmental and physiological factors that are well understood. Nevertheless, the ¹⁸O enrichment of bulk leaf water is often less than that predicted for the sites of evaporation. The advection of less enriched water in the transpiration stream has been suggested to limit the back diffusion of enriched evaporative site water (Péclet effect); however, evidence for this effect has been varied. In sampling across a range of species with different vein densities and saturated water contents, we demonstrate the importance of accounting for the relative ‘pool’ sizes of the vascular and mesophyll water for the interpretation of a Péclet effect. Further, we provide strong evidence for a Péclet signal within the xylem that if unaccounted for can lead to confounding of the estimated enrichment within the mesophyll water. This has important implications for understanding variation in the effective path length of the mesophyll and hence potentially the δ¹⁸O of organic matter.     

Scale‐dependent Orientation in Movement Paths: A Case Study of an African Viper

Decisions relating to the orientation of movement by animals and how this translates into movement patterns can occur at multiple spatial scales simultaneously, but this interaction is poorly understood for many groups of animals. Using the tracks left by moving snakes in their sandy habitat, we studied the movement paths of the African snake Bitis schneideri (Namaqua dwarf adder) for evidence of broad‐scale directional persistence and short‐range avoidance of exposure. Although snakes clearly displayed directional persistence, they preferentially moved to nearby shrubs, thereby minimizing exposure to solar and thermal radiation and/or predation. Thus, snakes made decisions relating to orientation at a minimum of two scales, the interaction of which resulted in snakes moving ≈17% (mean straightness index = 0.85) further than the simple broad‐scale straight‐line distance. We assert that the actual path chosen by moving snakes represents a trade‐off of various costs and risks that include risk of predation, exposure to the elements, time and energy expenditure. Our study highlights the need for cognizance of the possibility of the scale dependence of orientation and movement in studies of snake movement, and adds to a growing literature demonstrating previously unrecognized behavioural complexity in snakes.     

Translating niche features: Modelling differential exposure of Argentine reptiles to global climate change

Global climate change affects the distributions of ectotherms and may be the cause of several conservation problems, such as great displacement of climatic suitable spaces for species and, consequently, important reductions of the extent of liveable places, threatening the existence of many of them. Species exposure (and hence vulnerability) to global climate change is linked to features of their climatic niches (such as the relative position of the inhabited localities of each species in the climatic space), and therefore to characteristics of their geographic ranges (such as the extent of the distributions or altitudinal range inhabited by the species). In order to analyze the pattern of response of Argentine reptiles to global climate change, we ran phylogenetic generalized least squares models using species exposure to global climate change as a response variable, and (i) niche properties (breadth and position of the species in the climate space) and (ii) general features of the distribution of species (maximum latitude, altitudinal range, maximum elevation, distributional range and proximity to the most important dispersal barrier) as predictors. Our results suggest that the best way to explain climate change exposure is by combining breadth and position of climatic niche of the species or combining geographic features that are indicators of both niche characteristics. Our best model shows that in our study area, species with the narrowest distributional ranges that also inhabit the highest elevations are the most exposed to the effects of global climate change. In this sense, reptile species from Yungas, Puna and Andes ecoregions could be especially vulnerable to the effects of climate change. We believe that these types of models may represent an interesting tool for determining species and places particularly threatened by the effects of global climate change, which should be strongly considered in conservation planning.     

Introductions do not compensate for functional and phylogenetic losses following extinctions in insular bird assemblages

The ratio of species extinctions to introductions has been comparable for many insular assemblages, suggesting that introductions could have ‘compensated’ for extinctions. However, the capacity for introduced species to replace ecological roles and evolutionary history lost following extinction is unclear. We investigated changes in bird functional and phylogenetic diversity in the wake of extinctions and introductions across a sample of 32 islands worldwide. We found that extinct and introduced species have comparable functional and phylogenetic alpha diversity. However, this was distributed at different positions in functional space and in the phylogeny, indicating a ‘false compensation’. Introduced and extinct species did not have equivalent functional roles nor belong to similar lineages. This makes it unlikely that novel island biotas composed of introduced taxa will be able to maintain ecological roles and represent the evolutionary histories of pre‐disturbance assemblages and highlights the importance of evaluating changes in alpha and beta diversity concurrently.