This study empirically examines predictions concerning boundary permeability as a function of patch contrast (i.e., the magnitude of difference in measures across patch interfaces), light level, and faunal mobility and density, as well as relative rates of emigration and immigration. I assessed permeability using invertebrate transfer in a macroalgal-seagrass-sand landscape, with particular emphasis on the caridean shrimp Thor floridanus. Proportional emigration was estimated by staining algal fauna in situ using neutral red (a live immersion stain) and then later collecting all animals in the algal clumps and in the surrounding sand or seagrass. Permeability was in part a function ofboundary contrast in this milieu; macroalgal patches were more of a closed system in sand than in seagrass. Contrary to predictions, proportional emigration across both the algal-seagrass and algal-sand boundaries was inversely related to faunal density. Another unexpected result was that the rate of change in emigration with increasing density was unaffected by differential contrast among habitat types. The observed enhanced permeability for more motile fauna was consistent with predictions. A related finding was thatpermeability was greater at night than during the day, probably as a function ofincreased nocturnal vagility of benthic fauna. Emigration was not in equilibriumwith immigration at the temporal scale of this study (hours) for the majority ofspecies tested. Emigration-immigration disequilibria should be most likely whenthe temporal scale is small, and in such cases caution should be exercised whenextrapolating from one transfer parameter to the other.

Our objective was to gain an understanding of the influences of habitat context and seasonal and interannual factors on arthropod assemblage structure in a wetland environment. We hypothesized that river and pond riparian habitats in the wetland would have greater diversity and abundance than core wetland habitat, and that these differences would be driven by aquatic subsidy via emerging aquatic insects. We also hypothesized that diversity and abundance of terrestrial fauna would decline through the dry summer. We sampled the study wetland, in Yosemite National Park, California, USA, through the growing seasons of 2013 and 2014; a large wildfire (> 100,000 ha) burned the entire study site during late summer of 2013. Assemblage structure was strongly influenced by habitat context, season, and year. Diversity and abundance were high at the river riparian sites, but these results were driven by a diverse and abundant terrestrial fauna, rather than by large numbers of emerging aquatic insects. Faunal assemblages became increasingly depauperate through the summer, likely due to drying of wetland habitat in this hot Mediterranean-type climate. Fire probably had a strong influence on faunal assemblages and vegetation structure, but we cannot rule out interannual variability independent of the fire.

We used two-dimensional hydrodynamic models for the assessment of water diversion effects on benthic macroinvertebrates and associated habitat in a montane stream in Yosemite National Park, Sierra Nevada Mountains, CA, USA. We sampled the macroinvertebrate assemblage via Surber sampling, recorded detailed measurements of bed topography and flow, and coupled a two-dimensional hydrodynamic model with macroinvertebrate indicators to assess habitat across a range of low flows in 2010 and representative past years. We also made zero flow approximations to assess response of fauna to extreme conditions. The fauna of this montane reach had a higher percentage of Ephemeroptera, Plecoptera, and Trichoptera (%EPT) than might be expected given the relatively low faunal diversity of the study reach. The modeled responses of wetted area and area-weighted macroinvertebrate metrics to decreasing discharge indicated precipitous declines in metrics as flows approached zero. Changes in area-weighted metrics closely approximated patterns observed for wetted area, i.e., area-weighted invertebrate metrics contributed relatively little additional information above that yielded by wetted area alone. Loss of habitat area in this montane stream appears to be a greater threat than reductions in velocity and depth or changes in substrate, and the modeled patterns observed across years support this conclusion. Our models suggest that step function losses of wetted area may begin when discharge in the Merced falls to 0.02 m3/s; proportionally reducing diversions when this threshold is reached will likely reduce impacts in low flow years.

Wilderness stream crossings used by mules, horses, and hikers are localized disturbances that may affect habitat immediately downstream, but the potential influence of fords on streams has received little investigation, particularly in terms of possible effects on fauna. Our overall null hypothesis was absence of below-above differences for either benthic macroinvertebrate assemblages or habitat characteristics at such fords. We further sought to determine (1) whether any such differences were present prior to annual use, suggesting longer-term effects, and (2) whether differences were present in late season, after annual use. We examined macroinvertebrates and habitat immediately below and above 2 fords crossing subalpine streams in Yosemite National Park in the Sierra Nevada (California, USA) in earlyand late season and over 2 years. There were both longer-term below-above differences, as well as differences thatbecame apparent in late season, both of which were indicative of below-ford effects. Below fords there was evidence,either as main effects or interactions, of higher silt, sand, and gravel cover; a thicker periphyton layer; a greater Hilsenhoffbiotic index; a higher proportion of tolerant taxa; higher chironomid midge and total densities; and greater speciesrichness, largely a function of chironomid richness. There was also a lower expected number of species, a smaller proportionof sensitive taxa and predators, and lower densities of some sensitive Ephemeroptera (mayflies) and Plecoptera (stoneflies) below fords. Both hikers and stock may contribute to the apparent effects, but management interventions targeting stock may be particularly achievable. Among other approaches, simply halting stock strings briefly before reaching fords should reduce the volume of urine and feces directly entering streams, and handlers can expedite crossings if watering is not necessary.

Pack stock are often used in mountain environments and are grazed in uplands and wetlands, particularly subalpine wet meadows. Effects of pack stock on wetlandinvertebrates are unknown. Sequoia National Park, (Sierra Nevada, USA), was an ideal location for the study of lasting stock impacts on fauna, because a) there was an 18-year database of stock usage, b) there were meadows with little grazing that could be contrasted with grazed meadows,c) there is a long winter with no stock use, and d) the start of grazing for each meadow is controlled, so we could sample after greenup but just before stock arrived. Wecould thus address persistent conditions produced by many years of stock use in isolation from any potential short termimpacts. We sampled terrestrial arthropods in paired “grazed” and “ungrazed” meadows across the Park andcollected associated vegetation data. We found some negative effects of grazing on vegetation structure, but few lasting negative or positive effects of long-term stock grazing on arthropods in these wetlands. Although it appears that pack stock do not cause lasting damage to thisarthropod assemblage, the extent of impact at the height of the grazing season remains unknown.

Invasive plants can eliminate native flora and ultimately have negative indirect effects on fauna and the functional ecology of ecosystems, but understanding of these cascading effects on arthropod assemblagesis poor. Desert spring habitats are small, isolated landscape elements that are literal oases for flora and fauna and support high diversity assemblages; invasive palms can colonize desert springs and formmonocultures. In an effort to understand effects of these invasive trees on higher terrestrial trophic levels at springs, we contrasted assemblage structure of terrestrial arthropods in native vegetation versus invasive palm habitat. We sampled arthropods in paired palm and native habitat at 10 springs in Death ValleyNational Park, California, USA, during both spring and fall growing seasons using suction sampling. The invading palms Phoenix dactylifera L. and Washingtonia filifera (Linden ex André) supported a mean of onlyone-sixth of the arthropod abundance, one-third of the species richness, and half the family richness of native habitat. Almost all orders were less abundant in palms, and most families and species were eitherabsent or virtually absent in palm habitat. Both live and dead palm leaves were depauperate, and season and block effects were minimal. Impacts of these visually striking invasives clearly go beyond monopolization of ground cover, and invading palms appear capable of reshaping the functional ecology of desertsprings. If control efforts are undertaken, we recommend complete removal of palms, because killed but standing ‘‘ghost palms” and associated thatch persist for many years and will continue to provide poorarthropod habitat and prevent native plant establishment.