Donald W. Sada

Effects of Environmental Heterogeneity and Disturbance on the Native and Non-native Flora of Desert Springs

Vegetation often is used as a decision variable for conservation and resource management. Because time and money are limited, it is useful to identify predictable relationships between measures of vegetation diversity or status, the physical environment, and disturbance; native and non-native plants may have different functional responses. Working towards development of effective, practical strategies for management and ecological restoration in the Spring Mountains, an isolated mountain range in the eastern Mojave Desert (Nevada, USA) that is a focus of regional conservation planning, we examined whether native and non-native assemblages of spring-associated perennial plants have predictable relationships with elevation, springbrook length, and various land uses. We also tested whether elevation, springbrook length, and overall disturbance were associated with the degree of predictability of local species presence and absence. Consistent with work in other systems, species richness and cover of native plants tended to decrease as intensity of disturbance increased, whereas species richness (but not cover) of non-native plants tended to peak with intermediate disturbance. Our results may suggest that invasions of non-native plants at springs in the Spring Mountains are relatively recent, and that rapid restoration and management actions may help protect ecological processes and viability of native plants. Ability to predict the order in which individual species are likely to be extirpated from or colonize springs was limited, perhaps reflecting considerable environmental heterogeneity among springs.

Demography and habitat use of the Badwater snail (Assiminea infima), with observations on its conservation status, Death Valley National Park, California, U.S.A.

Cultural use of spring-fed wetlands in Death Valley National Park, California has reduced populations of endemic macroinvertebrates. Studies were conducted during the spring and late autumn of 1994 to assess demography and habitat use by the Badwater snail (Assiminea infima), which is endemic to low-elevation, spring-fed habitats in Death Valley where its abundance is believed to be adversely affected by municipal diversions and habitat trampling by Park visitors. Effects on demography and habitat were examined at sites highly, lightly, and unaffected by these activities. Field experiments examined the response of its habitat and abundance to trampling.

Factors affecting spatiotemporal benthic macroinvertebrate diversity and secondary production in a semi-arid watershed

Spatial and temporal changes of the benthic macroinvertebrate (BMI) community composition and secondary production provide valuable information about the current ‘health’ of aquatic systems. We analyzed seasonal and inter-annual changes in BMI communities at five sites along an elevation and land use gradient in the Walker River, NV, USA. In addition, bimonthly collections of Baetis mayflies were made at two sites to assess the effect of nutrient enrichment on Baetis secondary production. BMI community composition changed spatially and temporally in response to the physicochemical environment. BMI diversity was positively correlated to substrate size and stream width but negatively correlated with stream temperature and organic pollutants. Cluster analysis based on BMI community composition revealed a similar community structure along the river, except at one site which was located below the outflow of a eutrophic reservoir. The BMI community in the river was dominated by collector–gatherers while shredders were absent through most of the downstream and impaired sites. Secondary production of Baetis was almost twice as high at the nutrient-enriched site (1.3 g dry mass m−2 yr−1) than at the undisturbed site (0.7 g m−2 yr−1), most likely the consequence of warmer water temperatures and superior food quality. Results from this study imply that a variety of anthropogenic activities have altered disparate reaches of the Walker River and that BMI community composition would change towards natural conditions with an increase in discharge and reduced water temperature and nutrients. This work also underscores the importance of considering biotic indices and secondary production to effectively assess stream ecosystem health and to design successful restoration strategies.

Elemental stoichiometry of basal resources and benthic macroinvertebrates along a land use gradient in a Great Basin watershed

The effects of land use on the elemental stoichiometry of aquatic organisms have rarely been studied in semi-arid watersheds. In eight semi-arid sub-watersheds differing in land use, we determined which predictor variable(s) best explains the elemental variability in two basal food resources and benthic macroinvertebrates (BMI). The elemental composition of periphyton and seston was best explained by percentage of urban and agricultural areas, forested land and associated differences in SRP, DOC, and stream water N:P ratios. In contrast, consumer elemental stoichiometry was related to taxonomic identity and feeding mode. Elemental imbalances were higher for collector-gatherer than for scraper and collector-filterer. However, high spatial and temporal variability in the elemental composition of basal food resources obscured clear spatial patterns of imbalances between nutrient-poor upstream and nutrient-rich downstream sites. Results from this study suggest that land use can affect BMI due to alteration in stoichiometry of their food resources. However, taxonomy and allometry must be taken into account to better understand spatial and temporal changes in the elemental composition of BMI. Our results indicate the importance of considering multiple effects to accurately assess land use effects on producer and consumer stoichiometry, particularly the in highly variable Great Basin watersheds.

Effects of Recreational use on Branchiopod Egg and Ephippia Density, Black Rock Desert-High Rock Canyon Emigrant Trails National Conservation Area, Nevada, USA

Numerous plays playas occur on valley floors of endorehic basins in arid regions of the western USA. Their openness makes them attractive for hiking, vehicle travel, military, and other uses when dry. Branchiopod crustacean dormant egg banks survive in these systems and are a rich food resource for migrating birds. Brachniecta gigas Lynch, 1937, B. mackini Dexter, 1956, and Lepiduras lemmoni Holmes, 1895, and Moina cf. macrocopa (Straus, 1820) occur in the Black Rock Desert playa, Nevada, USA. We collected playa egg bank samples to determine effects of human use in three studies. We compared intact egg and ephippia density in virgin playa areas with: 1) a heavily used vehicle track, and 2) recreational camping and vehicle activity mitigated by dust abatement in Black Rock City (site of the Burning Man Festival). We also attempted to quantify changes in intact egg and ephippia density through repeated vehicle travel over a track on virgin playa. We found no observed decrease in intact egg or ephippia density attributed to incrementally increased vehicle travel over virgin playa, which may be attributed to strength of the playa substrate matrix. Differences in intact egg and ephippia density were substantially lower in heavily used vehicle tracks than in adjacent playa, but differences were not statistically significant. Density of intact eggs and ephippia in Black Rock City were lower following the festival, but differences were statistically significant only for eggs in camping areas and not roads. Weak effects observed on these roads may be attributed to dust abatement that maintained substrate density.

Climate Change and the Fate of Desert Springs

Springs are integral components of the unique web of life in desert ecosystems of the western United States. Many desert springs would not exist without local mountains to intercept and store water from rainfall and snowmelt, and many desert aquatic ecosystems would not exist without the springs, illustrating the connectivity between landscape processes (the realm of geoscientists) and ecosystem functioning (the realm of ecologists). On a human scale, early exploration, inhabitation, and survival in the arid and semiarid western United States would not have been feasible without springs. People living there today continue to value springs as dependable sources of water for irrigation, livestock, drinking, and recreational and economic uses (e.g., hot springs). Unfortunately, some desert springs may be less resistant to the effects of climate change than others. How can this resistance be quantified?

Spatial and Temporal Pattern in Length-Mass Regressions of Freshwater Gastropods in Nevada Spring Ecosystems

ABSTRACT We developed length-mass regressions for 16 aquatic gastropods collected from 27 springs in Nevada, United States. The relationship between shell length and body mass for all species was best described by fitting a power function. The mean value of the exponent b (2.40 ± 0.39) was lower than reported for other aquatic gastropods, particularly for smaller genera, such as Pyrgulopsis and Tryonia. Length-mass regressions of gastropods showed intraspecific variation, as well as spatiotemporal variation among the same species. Certain habitat characteristics, especially water temperature, may play a key role in determining growth rates and ultimately length-mass regressions in this study. Although the power term b showed high variability when the same species (Melanoides tuberculata, Pyrgulopsis lockensis, and Pyrgulopsis turbatrix) were collected from different springs, the power term was similar when the same species (Pyrgulopsis marcida) was collected from the same spring during different seasons and years. Therefore, we suggest that published equations should be used with caution and only when habitat characteristics and length ranges are similar compared to those in the literature.