John L. Sabo

RIPARIAN ZONES INCREASE REGIONAL SPECIES RICHNESS BY HARBORING DIFFERENT, NOT MORE, SPECIES

Riparian zones are habitats of critical conservation concern worldwide, as they are known to filter agricultural contaminants, buffer landscapes against erosion, and provide habitat for high numbers of species. Here we test the generality of the notion that riparian habitats harbor more species than adjacent upland habitats. Using previously pub- lished data collected from seven continents and including taxa ranging from Antarctic soil invertebrates to tropical rain forest lianas and primates, we show that riparian habitats do not harbor higher numbers of species, but rather support significantly different species pools altogether. In this way, riparian habitats increase regional ( g-) richness across the globe by .50%, on average. Thus conservation planners can easily increase the number of species protected in a regional portfolio by simply including a river within terrestrial biodiversity reserves. Our analysis also suggests numerous possible improvements for future studies of species richness gradients across riparian and upland habitats. First, ,15% of the studies in our analysis included estimates of more than one taxonomic group of interest. Second, within a given taxonomic group, studies employed variable methodologies and sampling areas in pursuit of richness and turnover estimates. Future analyses of species richness patterns in watersheds should aim to include a more comprehensive suite of taxonomic groups and should measure richness at multiple spatial scales.

Length–mass relationships for adult aquatic and terrestrial invertebrates in a California watershed

We collected 541 invertebrate specimens in riparian and upland habitats of the South Fork (SF) Eel River, California, USA, representing 12 orders and 55 families, and including adults of both aquatic and terrestrial origin. We fitted a power function to this data set at 4 taxonomic levels: 1) the entire pooled sample, 2) composite samples of aquatic and terrestrial taxa, 3) individual orders, and 4) individual families for which there were adequate data (n > 8; 24 families). Coefficients of determination (R2) for length–mass relationships generally increased with increasing taxonomic specificity. Composite samples of aquatic and terrestrial taxa had significantly different length–mass relationships, with dry mass increasing faster with length in terrestrial taxa. Differences in length–dry mass relationships between aquatic and terrestrial taxa appeared to result from significantly higher slopes for terrestrial taxa in length–width relationships, and significantly lower slopes in length–water content relationships. Our results suggest that the use of terrestrial regressions to estimate aquatic insect biomass flux in riparian habitats overestimates both the absolute magnitude of biomass flux as well as its relative importance to insect standing stocks. Development of unique regression algorithms for adult aquatic insect taxa increases the accuracy of aquatic insect biomass estimates in terrestrial habitats.

Physiology on a Landscape Scale: Plant-Animal Interactions

Abstract We explore in this paper how animals can be affected by variation in climate, topography, vegetation characteristics, and body size. We utilize new spatially explicit state-of-the-art models that incorporate principles from heat and mass transfer engineering, physiology, morphology, and behavior that have been modified to provide spatially explicit hypotheses using GIS. We demonstrate how temporal and spatial changes in microclimate resulting from differences in topography and vegetation cover alter animal energetics, and behavior. We explore the impacts of these energetic predictions on elk energetics in burned and unburned stands of conifer in winter in Yellowstone National Park, chuckwalla lizard distribution limits in North America, California Beechey Ground squirrel and Dusky Footed woodrat mass and energy requirements and activity patterns on the landscape, their predator prey interactions with a rattlesnake, Crotalus viridis, and shifts in that food web structure due to topographic and vegetative variation. We illustrate how different scales of data/observation provide different pieces of information that may collectively define the real distributions of a species. We then use sensitivity analyses of energetic models to evaluate hypotheses about the effects of changes in core temperature (fever) global climate (increased air temperature under a global warming scenario) and vegetation cover (deforestation) on winter survival of elk, the geographic distribution of chuckwallas and the activity overlap of predator and prey species within a subset of commonly observed species in a terrestrial food web. Variation in slope and aspect affect the spatial variance in solar radiation incident on the ground, hence ground surface temperature, at the same elevation, same hourly 2 m air temperatures, and wind speeds. We illustrate visually how spatial effects and landscape heterogeneity make statistical descriptions of animal responses problematic, since multiple distributions of their responses to climate, topography, and vegetation on the landscape can yield the same descriptive statistics, especially at high (30 m) resolution. This preliminary analysis suggests that the model has far-reaching implications for hypothesis testing in ecology at a variety of spatial and temporal scales.

NUMERICAL RESPONSE OF LIZARDS TO AQUATIC INSECTS AND SHORT-TERM CONSEQUENCES FOR TERRESTRIAL PREY

Spatial subsidies, or inputs of resources from more productive donor habitats, can cause numerical responses in consumer populations via behavioral and demographic mechanisms. In addition, subsidies may have indirect effects on the in situ prey of these consumers. These indirect effects can be either negative (e.g., apparent competition) or positive (e.g., via diet shifts) depending on the relative strength of the predators functional and numerical responses to prey subsidies. Here we report a numerical response by a lizard (Western fence lizard, Sceloporus occidentalis) to experimental reductions in the flux of river-derived insects. Initially, equal densities of lizards declined significantly faster in plots in which aquatic insect abundance was reduced by nearly 50% (season average) relative to controls. Abundance and biomass of terrestrial arthropods declined significantly between the start and end of the experiment across treatments. Despite consistently lower lizard abundance in plots with reduced subsidy levels, however, relative declines in the abundance and biomass of in situ terrestrial arthropods (all taxa combined) were not significantly different between reduced- and ambient-subsidy plots. Relative declines in spider biomass differed significantly between treatments and were higher in reduced-subsidy than ambient-subsidy plots, but only over one of three 3-wk sampling intervals. Thus, over the biologically active summer season, aquatic subsidies exerted brief positive or no significant indirect effects on the in situ prey of riparian lizards. These results suggest that, although aquatic insect prey may determine the spatial distribution and local abundance of riparian predators, the effects of increased predator density on in situ prey may be offset by higher per capita predation by these consumers on in situ prey in subsidy-poor relative to subsidy-rich habitats.

The role of discharge variation in scaling of drainage area and food chain length in rivers

River Food Chains Food chain length, or the number of organisms involved in any eat-or-be-eaten relationship, is a key property of aquatic habitats that regulates nutrient cycling, energy flow, and carbon exchange between aquatic ecosystems and the atmosphere. Investigating river food webs in North American watersheds, Sabo et al. (p. 965, published online 14 October) found discharge variation governed the link between ecosystem size (watershed area) and food chain length. River-drying truncated food chain length because intermittent streams have much higher discharge variation than perennial rivers. These results could be important for the management of rivers as drought and climate change may increase the frequency of river drying and discharge variability. The lengths of river food chains are indirectly related to drainage area and directly affected by flow and discharge. Food chain length (FCL) is a fundamental component of food web structure. Studies in a variety of ecosystems suggest that FCL is determined by energy supply, environmental stability, and/or ecosystem size, but the nature of the relationship between environmental stability and FCL, and the mechanism linking ecosystem size to FCL, remain unclear. Here we show that FCL increases with drainage area and decreases with hydrologic variability and intermittency across 36 North American rivers. Our analysis further suggests that hydrologic variability is the mechanism underlying the correlation between ecosystem size and FCL in rivers. Ecosystem size lengthens river food chains by integrating and attenuating discharge variation through stream networks, thereby enhancing environmental stability in larger river systems.

Reclaiming freshwater sustainability in the Cadillac Desert

Increasing human appropriation of freshwater resources presents a tangible limit to the sustainability of cities, agriculture, and ecosystems in the western United States. Marc Reisner tackles this theme in his 1986 classic Cadillac Desert: The American West and Its Disappearing Water. Reisners analysis paints a portrait of region-wide hydrologic dysfunction in the western United States, suggesting that the storage capacity of reservoirs will be impaired by sediment infilling, croplands will be rendered infertile by salt, and water scarcity will pit growing desert cities against agribusiness in the face of dwindling water resources. Here we evaluate these claims using the best available data and scientific tools. Our analysis provides strong scientific support for many of Reisners claims, except the notion that reservoir storage is imminently threatened by sediment. More broadly, we estimate that the equivalent of nearly 76% of streamflow in the Cadillac Desert region is currently appropriated by humans, and this figure could rise to nearly 86% under a doubling of the regions population. Thus, Reisners incisive journalism led him to the same conclusions as those rendered by copious data, modern scientific tools, and the application of a more genuine scientific method. We close with a prospectus for reclaiming freshwater sustainability in the Cadillac Desert, including a suite of recommendations for reducing region-wide human appropriation of streamflow to a target level of 60%.

QUANTIFYING PERIODIC, STOCHASTIC, AND CATASTROPHIC ENVIRONMENTAL VARIATION

Environmental variation plays a central role in regulating processes at all levels of ecological organization. Environmental data (e.g., temperature, rainfall, stream discharge, water chemistry) are typically easy to collect in large quantity, a requirement for many data-hungry time series tools. Unfortunately, these data are very rarely used effectively in ecology. Here we address this problem by outlining a suite of tools that can be used to quantify periodic, stochastic, and catastrophic variation in environmental conditions. We illustrate the application of these tools using long-term records of average daily discharge in 105 streams and rivers maintained by the U.S. Geological Survey on the NWIS (National Water Information System) web site. Specifically, we apply Fourier analysis to estimate the periodic (seasonal) and stochastic (interannual) components of variation in discharge. We then estimate the temporal autocorrelation structure of stochastic variation (i.e., noise color) in daily flows for ea...

The effects of land use changes on streams and rivers in mediterranean climates

We reviewed the literature on the effects of land use changes on mediterranean river ecosystems (med-rivers) to provide a foundation and directions for future research on catchment management during times of rapid human population growth and climate change. Seasonal human demand for water in mediterranean climate regions (med-regions) is high, leading to intense competition for water with riverine communities often containing many endemic species. The responses of river communities to human alterations of land use, vegetation, hydrological, and hydrochemical conditions are similar in mediterranean and other climatic regions. High variation in hydrological regimes in med-regions, however, tends to exacerbate the magnitude of these responses. For example, land use changes promote longer dry season flows, concentrating contaminants, allowing the accumulation of detritus, algae, and plants, and fostering higher temperatures and lower dissolved oxygen levels, all of which may extirpate sensitive native species. Exotic species often thrive in med-rivers altered by human activity, further homogenizing river communities worldwide. We recommend that future research rigorously evaluate the effects of management and restoration practices on river ecosystems, delineate the cause–effect pathways leading from human perturbations to stream biological communities, and incorporate analyses of the effects of scale, land use heterogeneity, and high temporal hydrological variability on stream communities.

Acute stress rapidly decreases plasma testosterone in a free-ranging male songbird: Potential site of action and mechanism

We used a free-ranging, seasonally breeding adult male songbird, the rufous-winged sparrow, Aimophila carpalis, to investigate the effects of acute stress-induced by capture followed by restraint, on the hypothalamo-pituitary-testicular axis. Intra- and interindividual comparisons revealed that males decreased their plasma testosterone (T) by 37-52% in response to acute stress. The decrease occurred within 15 min of capture and persisted for at least another 15 min. Within 15 min, the decrease in plasma T was not associated with a reduction in plasma luteinizing hormone (LH). Thirty minutes after capture and restraint, the decrease in plasma T either was likewise not associated with decreased plasma LH (intraindividual comparison) or concurred with a reduction in plasma LH (interindividual comparison). These observations indicate that effects of stress may have been mediated at the pituitary gland and also directly at the testicular levels. To address this question, we measured the hormonal response to an injection of the glutamate receptor agonist N-methyl-d,l-aspartate (NMA) to stimulate to stimulate the release of gonadotropin-releasing hormone (GnRH) or of GnRH to stimulate the release of LH. Treatment with NMA did not change plasma LH, presumably because the birds were in breeding condition and already secreting GnRH at a maximum rate. Administration of GnRH increased plasma LH equally in birds that were or were not stressed before the treatment. An injection of purified ovine LH (oLH) increased plasma T equally in birds that were or were not acutely stressed before the hormone injection. Thus, the observed acute stress-induced decrease in plasma T was apparently not mediated by decreased responsiveness of the pituitary gland to GnRH or of the testes to LH. Decreased plasma T following stress may involve a direct impairment of the testicular endocrine function.

The added complications of climate change: understanding and managing biodiversity and ecosystems

Ecosystems around the world are already threatened by land-use and land-cover change, extraction of natural resources, biological disturbances, and pollution. These environmental stressors have been the primary source of ecosystem degradation to date, and climate change is now exacerbating some of their effects. Ecosystems already under stress are likely to have more rapid and acute reactions to climate change; it is therefore useful to understand how multiple stresses will interact, especially as the magnitude of climate change increases. Understanding these interactions could be critically important in the design of climate adaptation strategies, especially because actions taken by other sectors (eg energy, agriculture, transportation) to address climate change may create new ecosystem stresses.