Kevin E. McCluney

Riverine macrosystems ecology: sensitivity, resistance, and resilience of whole river basins with human alterations

Riverine macrosystems are described here as watershed-scale networks of connected and interacting riverine and upland habitat patches. Such systems are driven by variable responses of nutrients and organisms to a suite of global and regional factors (eg climate, human social systems) interacting with finer-scale variations in geology, topography, and human modifications. We hypothesize that spatial heterogeneity, connectivity, and asynchrony among these patches regulate ecological dynamics of whole networks, altering system sensitivity, resistance, and resilience. Long-distance connections between patches may be particularly important in riverine macrosystems, shaping fundamental system properties. Furthermore, the type, extent, intensity, and spatial configuration of human activities (eg land-use change, dam construction) influence watershed-wide ecological properties through effects on habitat heterogeneity and connectivity at multiple scales. Thus, riverine macrosystems are coupled social–ecological sy...

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

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 Earths 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.

Water availability directly determines per capita consumption at two trophic levels

Community ecology has long focused on energy and nutrients as currencies of species interactions. Evidence from physiological ecology and recent studies suggest that in terrestrial systems, water may influence animal behavior and global patterns of species richness. Despite these observations, water has received little attention as a currency directly influencing animal species interactions. Here, we show that the per capita interaction strength between predatory wolf spiders and their primary prey, field crickets, is strong (-0.266) when predators and prey are maintained in ambient dry conditions, but is near zero (0.001) when water is provided ad libitum. Moreover, crickets consume 31-fold more moist leaf material in ambient dry conditions, switching from old litter to moist green leaves when free water is scarce. Under dry conditions, animals may make foraging decisions based first on water needs, not energy or nutrients, suggesting strong and predictable effects of alterations in aridity on species interactions.

Dryland Riparian Ecosystems in the American Southwest: Sensitivity and Resilience to Climatic Extremes

Drylands make up over 40% of the earth’s land surface (Noy-Meir 1973; Reynolds and others 2007). In these arid and semiarid landscapes, surface and subsurface water flows create mesic riparian environments (Kingsford 2006; Stromberg and Tellman 2009). Dryland riparian areas sustain water-limited plants and animals that cannot withstand upland conditions year-round, thereby supporting regional diversity (Sabo and others 2005; Lite and others 2005). Dryland rivers are also characterized by occasional large, high-intensity floods that rework the bed geomorphology and increase riparian heterogeneity. This combined variation in water availability and flood disturbance creates spatiotemporally complex and unique landscapes (Soykan and Sabo 2009; Soykan and others 2012). The hydrology of desert riparian ecosystems has been extensively altered by human activities including stream diversion and groundwater extraction, and is expected to undergo further change in response to changing climates (Patten 1998). Globally, increasing temperatures associated with climate change are expected to intensify the water cycle, leading to greater rates of evapotranspiration, more water in the atmosphere, and more-frequent intense storms (Huntington 2006; IPCC 2007). More intense storms are likely to increase flood intensity over the next 50 years (Dominguez and others 2012). In the American Southwest in particular, warmer sea surface temperatures could spur stronger and more-frequent El Nino-associated winter storms (Garfin and Lenart 2007). Perhaps of greater importance are projected increases in aridity. Average precipitation is expected to increase at a global scale, but some regions, including the American Southwest, are expected to experience substantial decreases in average annual rainfall as well as increases in frequency and intensity of droughts (Seager and others 2007). As aridity increases, streams and riparian zones undergo declines in surface flow and depth to groundwater; there are declines, as well, in the extent of river segments with perennial (vs. intermittent or ephemeral) stream flow. Given that rivers and their riparian corridors function as keystone elements of arid and semiarid landscapes, climate change will have far reaching effects on regional biota (Stromberg and others 2010). Nevertheless, the extent of biotic change for riparian ecosystems in coming decades will be tempered by the fact that for desert rivers, climatic and hydrologic variation is the norm (Milly and Received 30 May 2012; accepted 13 September 2012

GREENFALL LINKS GROUNDWATER TO ABOVEGROUND FOOD WEBS IN DESERT RIVER FLOODPLAINS

Groundwater makes up nearly 99% of unfrozen freshwater worldwide and sustains riparian trees rooted in shallow aquifers, especially in arid and semiarid climates. The goal of this paper is to root animals in the regional water cycle by quantifying the significance of groundwater to riparian animals. We focused our efforts on the cricket, Gryllus alogus: a common primary consumer found in floodplain forests along the San Pedro River, in southeast Arizona, USA. Cottonwood trees make groundwater available to G. alogus as dislodged, groundwater-laden leaves (greenfall). We hypothesized that groundwater fluxes mediated by greenfall sustain G. allogus through the prolonged dry season and link these aboveground consumers to belowground aquifers. To test this hypothesis, we first characterized gradients in absolute humidity (air) and water stress in field-collected G. alogus. Absolute humidity declined with distance from river across wide stands of floodplain cottonwood forest during the dry season, but not durin...

The effects of hydration on growth of the house cricket, Acheta domesticus

Abstract Maintenance of biochemical gradients, membrane fluidity, and sustained periods of activity are key physiological and behavioral functions of water for animals living in desiccating environments. Water stress may reduce the organisms ability to maintain these functions and as such, may reduce an organisms growth. However, few studies have examined this potential effect. The effects of altered hydration state of the house cricket, Acheta domesticus L. (Orthoptera: Gryllidae) on individual growth were studied under laboratory conditions. Crickets were permitted access to water for three different durations each day, resulting in significant differences in hydration state (32% greater hydration for maximum than minimum duration of water availability). Growth was 59% and 72% greater in dry mass and length, respectively, between the lowest and highest hydration state treatments. These findings may be representative for a variety of animal species and environments and could have important ecological implications.

Water as a trophic currency in dryland food webs

Water is essential for life on Earth, yet little is known about how water acts as a trophic currency, a unit of value in determining species interactions in terrestrial food webs. We tested the relative importance of groundwater and surface water in riparian food webs by manipulating their availability in dryland floodplains. Primary consumers (crickets) increased in abundance in response to added surface water and groundwater (contained in moist leaves), and predators (spiders and lizards) increased in abundance in response to added surface water, in spite of the presence of a river, an abundant water source. Moreover, the relative magnitude of organism responses to added water was greatest at the most arid site and lowest at the least arid site, mirroring cricket recruitment, which was greatest at the least arid site and lowest at the most arid site. These results suggest that water may be a key currency in terrestrial dryland food webs, which has important implications for predicting ecosystem response...

Tracing water sources of terrestrial animal populations with stable isotopes: laboratory tests with crickets and spiders.

Fluxes of carbon, nitrogen, and water between ecosystem components and organisms have great impacts across levels of biological organization. Although much progress has been made in tracing carbon and nitrogen, difficulty remains in tracing water sources from the ecosystem to animals and among animals (the “water web”). Naturally occurring, non-radioactive isotopes of hydrogen and oxygen in water provide a potential method for tracing water sources. However, using this approach for terrestrial animals is complicated by a change in water isotopes within the body due to differences in activity of heavy and light isotopes during cuticular and transpiratory water losses. Here we present a technique to use stable water isotopes to estimate the mean mix of water sources in a population by sampling a group of sympatric animals over time. Strong correlations between H and O isotopes in the body water of animals collected over time provide linear patterns of enrichment that can be used to predict a mean mix of water sources useful in standard mixing models to determine relative source contribution. Multiple temperature and humidity treatment levels do not greatly alter these relationships, thus having little effect on our ability to estimate this population-level mix of water sources. We show evidence for the validity of using multiple samples of animal body water, collected across time, to estimate the isotopic mix of water sources in a population and more accurately trace water sources. The ability to use isotopes to document patterns of animal water use should be a great asset to biologists globally, especially those studying drylands, droughts, streamside areas, irrigated landscapes, and the effects of climate change.

Sensitivity and Tolerance of Riparian Arthropod Communities to Altered Water Resources along a Drying River

Background Rivers around the world are drying with increasing frequency, but little is known about effects on terrestrial animal communities. Previous research along the San Pedro River in southeastern AZ, USA, suggests that changes in the availability of water resources associated with river drying lead to changes in predator abundance, community composition, diversity, and abundance of particular taxa of arthropods, but these observations have not yet been tested manipulatively. Methods and Results In this study, we constructed artificial pools in the stream bed adjacent to a drying section of the San Pedro River and maintained them as the river dried. We compared pitfall trapped arthropods near artificial pools to adjacent control sites where surface waters temporarily dried. Assemblage composition changed differentially at multiple taxonomic levels, resulting in different assemblages at pools than at control sites, with multiple taxa and richness of carabid beetle genera increasing at pools but not at controls that dried. On the other hand, predator biomass, particularly wolf spiders, and diversity of orders and families were consistently higher at control sites that dried. These results suggest an important role for colonization dynamics of pools, as well as the ability of certain taxa, particularly burrowing wolf spiders, to withstand periods of temporary drying. Conclusions Overall, we found some agreement between this manipulative study of water resources and a previous analysis of river drying that showed shifts in composition, changes in diversity, and declines in abundance of certain taxa (e.g. carabid beetles). However, colonization dynamics of pools, as well as compensatory strategies of predatory wolf spiders seem to have led to patterns that do not match previous research, with control sites maintaining high diversity, despite drying. Tolerance of river drying by some species may allow persistence of substantial diversity in the face of short-term drying. The long-term effects of drying remain to be investigated.

Implications of animal water balance for terrestrial food webs

Recent research has documented shifts in per capita trophic interactions and food webs in response to changes in environmental moisture, from the top-down (consumers to plants), rather than solely bottom-up (plants to consumers). These responses may be predictable from effects of physiological, behavioral, and ecological traits on animal water balance, although predictions could be modified by energy or nutrient requirements, the risk of predation, population-level responses, and bottom-up effects. Relatively little work has explicitly explored food web effects of changes in animal water balance, despite the likelihood of widespread relevance, including during periodic droughts in mesic locations, where taxa may lack adaptations for water conservation. More research is needed, particularly in light of climate change and hydrological alteration.