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.

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Detritus is a central feature in marine, freshwater, and terrestrial ecosystems. Despite the ubiquity of detritus, ecologists have largely ignored its role in influencing food web structure. We used a meta‐analytic approach to ask three questions about how detritus affects food web structure in a wide variety of ecosystems. First, what is the effect strength of detritus on primary producers, detritivores, herbivores, and predators? Second, what functional role does detritus serve for consumers (energetic, habitat, or both)? Third, how does the effect of detritus on consumers vary between aquatic and terrestrial ecosystems? We found that detritus has strong positive effects on primary producers and consumers in a wide range of ecosystems types. Detritus has a positive direct effect on detritivores by providing both an energetic resource and habitat (refuge from predators). Detritus has equally strong positive effects on herbivores and predators, driven by a positive direct effect of habitat. Detritus has positive effects on consumers in both aquatic and terrestrial ecosystems with 1.7 times stronger effects in terrestrial ecosystems. These results suggest that detritus has strong effects on food‐web structure in a variety of ecosystem types. Even the portion of the food web that is linked most strongly to living plant tissue as its primary energy source is strongly positively affected.

[1] Watersheds are drained by river networks, which route materials and energy from headwaters to terminal water bodies. River networks likewise perfuse the terrestrial portion of watershed ecosystems and reroute some of these materials upslope via material exchange between rivers and land. Here we develop a model of resource exchange between rivers and watersheds to predict the spatial extent of material and nutrient fluxes from aquatic portions of watershed ecosystems. The model is based on a geomorphic template that includes river network structure, topography, and channel sinuosity as well as important biological attributes (productivity and dispersal ability). Analysis of this model suggests that the geomorphic template strongly influences the spatial extent of resource flows in watershed ecosystems. The geomorphic template also predicts the location of areas of concentrated resource exchange, typically at ridge crests, in meander bends, and tributary junctions. We contend that these areas represent hotspots of foraging activity for terrestrial consumers, especially those at the reach scale (meander bends). More generally, our model suggests that the spatial extent of aquatic resource flow equal in magnitude to 20% or greater of terrestrial production may encompass as much as 20%-50% of terrestrial portions of watersheds. Resource flow from rivers to terrestrial ecosystems is not merely an edge effect. Instead, the river network may reroute a substantial flux of materials into watershed ecosystems.Citation: Sabo, J. L., and E. M. Hagen (2012), A network theory for resource exchange between rivers and their watersheds, Water Resour. Res., 48, W04515,

Relative contributions of allochthonous inputs and autochthonous production vary depending on terrestrial land use and biome. Terrestrially derived organic matter and in-stream primary production were measured in 12 headwater streams along an agricultural land-use gradient. Streams were examined to see how carbon (C) supply shifts from forested streams receiving primarily terrestrially derived C to agricultural streams, which may rely primarily on C derived from algal productivity. We measured allochthonous input, chlorophyll a concentration, and periphyton biomass in each stream, and whole-stream metabolism in six streams. Our results suggest a threshold between moderate-and heavy-agriculture land uses in which terrestrially derived C is replaced by in-stream algal productivity as the primary C source for aquatic consumers. A shift from allochthonous to autochthonous production was not evident in all heavy-agriculture streams, and only occurred in heavy-agriculture streams not impacted by livestock grazing. We then compared our findings to rates of allochthonous input and GPP in streams with minimal human influences in multiple biomes to assess how land-use practices influence C sources to stream ecosystems. The proportion of C derived from allochthonous versus autochthonous sources to heavyagriculture streams was most similar to grassland and desert streams, while C sources to forested, light-, and moderate-agriculture streams were more similar to deciduous and montane coniferous forest streams. We show that C source to streams is dependent on land use, terrestrial biome, and degradation of in-stream conditions. Further, we suggest that within a biome there seems to be a compensation such that total C input is nearly equal whether it is from allochthonous or autochthonous sources.Electronic supplementary material The online version of this article (