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Dilts T.E.,University of Nevada, Reno | Weisberg P.J.,University of Nevada, Reno | Yang J.,University of Nevada, Reno | Olson T.J.,University of Nevada, Reno | And 2 more authors.
Annals of the Association of American Geographers | Year: 2012

In arid regions of the world, the conversion of native vegetation to agriculture requires the construction of an irrigation infrastructure that can include networks of ditches, reservoirs, flood control modifications, and supplemental groundwater pumping. The infrastructure required for agricultural development has cumulative and indirect effects, which alter native plant communities, in parallel with the direct effects of land use conversion to irrigated crops. Our study quantified historical land cover change over a 150-year period for the Walker River Basin of Nevada and California by comparing direct and indirect impacts of irrigated agriculture at the scale of a 10,217 km 2 watershed. We used General Land Office survey notes to reconstruct land cover at the time of settlement (1860-1910) and compared the settlement-era distribution of land cover to the current distribution. Direct conversion of natural vegetation to agricultural land uses accounted for 59 percent of total land cover change. Changes among nonagricultural vegetation included shifts from more mesic types to more xeric types and shifts from herbaceous wet meadow vegetation to woody phreatophytes, suggesting a progressive xerification. The area of meadow and wetland has experienced the most dramatic decline, with a loss of 95 percent of its former area. Our results also show Fremont cottonwood, a key riparian tree species in this region, is an order of magnitude more widely distributed within the watershed today than at the time of settlement. In contrast, areas that had riparian gallery forest at the time of settlement have seen a decline in the size and number of forest patches. © 2012 Taylor and Francis Group, LLC.


Yang J.,University of Nevada, Reno | Dilts T.E.,University of Nevada, Reno | Condon L.A.,Otis Bay Ecological Consultants | Turner P.L.,Partnership for Conservation and Development | Weisberg P.J.,University of Nevada, Reno
Landscape Ecology | Year: 2011

Riparian vegetation is distinct from adjacent upland terrestrial vegetation and its distribution is affected by various environmental controls operating at the longitudinal scale (along the river) or transverse scale (perpendicular to the river). Although several studies have shown how the relative importance of transverse or longitudinal influences varies with the scale of observation, few have examined how the influences of the two scales vary with the level of ecological organization. We modeled vegetation-environment relationships at three hierarchically nested levels of ecological organization: species, plant community, and vegetation type. Our hierarchically structured analyses differentiated the spatial extent of riparian zones from adjacent upland vegetation, the distribution of plant community types within the riparian zone, and the distribution of plant species within community types. Longitudinal gradients associated with climate and elevation exerted stronger effects at the species level than at the community level. Transverse gradients related to lateral surface water flux and groundwater availability distinguished riparian and upland vegetation types, although longitudinal gradients of variation better predicted species composition within either riparian or upland communities. We concur with other studies of riparian landscape ecology that the relative predictive power of environmental controls for modeling patterns of biodiversity is confounded with the spatial extent of the study area and sampling scheme. A hierarchical approach to spatial modeling of vegetation-environment relationships will yield substantial insights on riparian landscape patterns. © 2010 Springer Science+Business Media B.V.


Weisberg P.J.,University of Nevada, Reno | Dilts T.E.,University of Nevada, Reno | Becker M.E.,University of Nevada, Reno | Young J.S.,Great Basin | And 3 more authors.
Acta Oecologica | Year: 2014

Ecological niche theory implies that more heterogeneous habitats have the potential to support greater biodiversity. Positive heterogeneity-diversity relationships have been found for most studies investigating animal taxa, although negative relationships also occur and the scale dependence of heterogeneity-diversity relationships is little known. We investigated multi-scale, heterogeneity-diversity relationships for bird communities in a semi-arid riparian landscape, using airborne LiDAR data to derive key measures of structural habitat complexity. Habitat heterogeneity-diversity relationships were generally positive, although the overall strength of relationships varied across avian life history guilds (R2 range: 0.03-0.41). Best predicted were the species richness indices of cavity nesters, habitat generalists, woodland specialists, and foliage foragers. Heterogeneity-diversity relationships were also strongly scale-dependent, with strongest associations at the 200-m scale (4ha) and weakest associations at the 50-m scale (0.25ha). Our results underscore the value of LiDAR data for fine-grained quantification of habitat structure, as well as the need for biodiversity studies to incorporate variation among life-history guilds and to simultaneously consider multiple guild functional types (e.g. nesting, foraging, habitat). Results suggest that certain life-history guilds (foliage foragers, cavity nesters, woodland specialists) are more susceptible than others (ground foragers, ground nesters, low nesters) to experiencing declines in local species richness if functional elements of habitat heterogeneity are lost. Positive heterogeneity-diversity relationships imply that riparian conservation efforts need to not only provide high-quality riparian habitat locally, but also to provide habitat heterogeneity across multiple scales. © 2014 Elsevier Masson SAS.


Weisberg P.J.,University of Nevada, Reno | Mortenson S.G.,Otis Bay Ecological Consultants | Dilts T.E.,University of Nevada, Reno
Restoration Ecology | Year: 2013

Much riparian restoration focuses on establishment of gallery forests, with relatively limited effort to restore herbaceous wetlands as key components of riparian landscape mosaics. Multiple reasons for this include inherent cultural or esthetic preferences, greater availability of scientific knowledge to support riparian forest restoration, and choices of ecological indicators commonly used for monitoring and assessment. Yet riparian herbaceous wetlands have declined dramatically as a result of river regulation and agricultural development, leading to losses of important habitats and ecosystem services that differ from those provided by gallery forests. As an alternative to a single-minded focus on tree establishment, we advocate restoration of diverse and dynamic habitat mosaics in the context of natural variability of flow and sediment regimes. Landscape context should inform active restoration activities at the local scale, such that riparian forests are not planted in ecologically inappropriate sites. Models are needed to match life history requirements of particular wetland herbaceous plant species with details of flow and sediment transport regimes. We emphasize the importance of herbaceous wetlands as a critical and often overlooked component of riparian ecosystems, and the need for both passive and active restoration of fluvial marshes, sloughs, wet meadows, alkali meadows, off-channel ephemeral ponds, and other critical floodplain communities associated with herbaceous plant dominance. © 2012 Society for Ecological Restoration.


Young J.S.,Great Basin | Ammon E.M.,Great Basin | Weisberg P.J.,University of Nevada, Reno | Dilts T.E.,University of Nevada, Reno | And 3 more authors.
Ecological Indicators | Year: 2013

The use of a bird community index that characterizes ecosystem integrity is very attractive to conservation planners and habitat managers, particularly in the absence of any single focal species. In riparian areas of the western USA, several attempts at arriving at a community index signifying a functioning riparian bird community have been made previously, mostly resorting to expert opinions or national conservation rankings for species weights. Because extensive local and regional bird monitoring data were available for Nevada, we were able to develop three different indices that were derived empirically, rather than from expert opinion. We formally examined the use of three species weighting schemes in comparison with simple species richness, using different definitions of riparian species assemblage size, for the purpose of predicting community response to changes in vegetation structure from riparian restoration. For the three indices, species were weighted according to the following criteria: (1) the degree of riparian habitat specialization based on regional data, (2) the relative conservation ranking of landbird species, and (3) the degree to which a species is under-represented compared to the regional species pool for riparian areas. To evaluate the usefulness of these indices for habitat restoration planning and monitoring, we modeled them using habitat variables that are expected to respond to riparian restoration efforts, using data from 64 sampling sites in the Walker River Basin in Nevada and California. We found that none of the species-weighting schemes performed any better as an index for evaluating overall habitat condition than using species richness alone as a community index. Based on our findings, the use of a fairly complete list of 30-35 riparian specialists appears to be the best indicator group for predicting the response of bird communities to the restoration of riparian vegetation. © 2013 Published by Elsevier Ltd.

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