Shifting species interactions in terrestrial dryland ecosystems under altered water availability and climate change

Abstract

Species interactions play key roles in linking the responses of populations, communities, and ecosystems to environmental change. For instance, species interactions are an important determinant of the complexity of changes in trophic biomass with variation in resources. Water resources are a major driver of terrestrial ecology and climate change is expected to greatly alter the distribution of this critical resource. While previous studies have documented strong effects of global environmental change on species interactions in general, responses can vary from region to region. Dryland ecosystems occupy more than one-third of the Earth's land mass, are greatly affected by changes in water availability, and are predicted to be hotspots of climate change. Thus, it is imperative to understand the effects of environmental change on these globally significant ecosystems. Here, we review studies of the responses of population-level plant-plant, plant-herbivore, and predator-prey interactions to changes in water availability in dryland environments in order to develop new hypotheses and predictions to guide future research. To help explain patterns of interaction outcomes, we developed a conceptual model that views interaction outcomes as shifting between (1) competition and facilitation (plant-plant), (2) herbivory, neutralism, or mutualism (plant-herbivore), or (3) neutralism and predation (predator-prey), as water availability crosses physiological, behavioural, or population-density thresholds. We link our conceptual model to hypothetical scenarios of current and future water availability to make testable predictions about the influence of changes in water availability on species interactions. We also examine potential implications of our conceptual model for the relative importance of top-down effects and the linearity of patterns of change in trophic biomass with changes in water availability. Finally, we highlight key research needs and some possible broader impacts of our findings. Overall, we hope to stimulate and guide future research that links changes in water availability to patterns of species interactions and the dynamics of populations and communities in dryland ecosystems.

It is getting hotter in here: determining and projecting the impacts of global environmental change on drylands

“…Experiments and syntheses conducted in recent years have projected modifications and disruptions in species and multi-trophic interactions in drylands [61,68]. The complexities in the responses of organisms to global change challenge the establishment of general predictions.…”

“…The dryland biota in general is known to be well-adapted to infrequent, intense and unpredictable pulses of precipitation [4]. However, increasing temperatures and prolonged drought events associated with global change will pose physiological water balance challenges to a wide suite of organisms, including birds [60], reptiles [61] and insects [62], as transpiration increases. Expected changes in temperature and rainfall frequency may even promote extreme mortality events, as recently recorded in organisms as disparate as small birds [63] and mosses [64].…”

Animal water balance drives top-down effects in a riparian forest—implications for terrestrial trophic cascades

“…Water availability probably has both bottom-up and top-down effects on food webs [9], but evidence for top-down effects comes mostly from observational research [5,[10][11][12], limited field manipulations [13,14] or cage experiments [15]. This evidence suggests that consumers may meet water demands by consuming large amounts of moist food when environmental water sources (e.g.…”

“…Previous work has shown that water balance can influence consumption of moist food for particular consumers [9,15]. We hypothesized that this alteration of consumption would lead to changes in the strength of top-down effects in real food webs.…”

“…This evidence suggests that consumers may meet water demands by consuming large amounts of moist food when environmental water sources (e.g. saturated soil or open water) are unavailable [9,10,15]. In these situations, terrestrial food webs can be viewed as water webs, with water driving trophic interactions [15].…”