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Accueil du site → Doctorat → États-Unis → 2016 → Dynamics of Native and Invasive Non-native Plant Species in Desert Sand Dunes of Eureka Valley, Death Valley National Park

University of California, Davis (2016)

Dynamics of Native and Invasive Non-native Plant Species in Desert Sand Dunes of Eureka Valley, Death Valley National Park

Chow, Elaine K.

Titre : Dynamics of Native and Invasive Non-native Plant Species in Desert Sand Dunes of Eureka Valley, Death Valley National Park

Auteur : Chow, Elaine K.

Université de soutenance : University of California, Davis,

Grade : Doctor of Philosophy (PhD) 2016

The dynamics of native and non-native invasive plant species in desert sand dune environments have not been well explored. This is partially due to the isolation of inland dunes. For the same reason, endemism is likely to occur. Thus, invasions of desert dunes pose a large threat to endemic taxa that are confined to these terrestrial islands. One of the most floristically species-rich sand dune ecosystems of western North America is in Eureka Valley, Death Valley National Park. Park managers and conservationists fear that populations of two of the three dune endemics may be vulnerable to the invasion of Salsola gobicola. Furthermore, one of those two vulnerable species, Oenothera californica ssp. eurekensis, is currently listed as federally endangered. Despite a scarcity of reliable monitoring data, the U.S. Fish & Wildlife Service (U.S.F.W.S.) recently proposed to delist the subspecies because the original threat to the population, off-road vehicle usage, was removed. However, solid observational or experimental data on the biological threat of S. gobicola on the dune endemic and other native populations are lacking. The potential impacts of S. gobicola were explored through target-neighborhood approaches (one observational and one experimental) to examine the influences of inter- and intraspecific competitive effects as well as effects of neighbor density in the non-native species and two native species, Crypthantha micrantha and O. californica ssp. eurekensis. The observational field study suggested competition between two abundant populations of annuals, S. gobicola and C. micrantha. Comparisons of natural density gradients of neighbors of S. gobicola, C. micrantha, all species excluding conspecifics, and all species including both study species of interest indicated the latter two variables had the most influence on performance of S. gobicola and C. micrantha targets, respectively. This implies diffuse competition may play a stronger role than inter- and intraspecific competition among the native and non-native annual species observed. A manipulative pot study was conducted with S. gobicola and the perennial, O. californica ssp. eurekensis to directly determine the effects of inter- and intraspecific neighbors, neighbor density and stage class (first- and second-year) of perennial plants on target plant performance of both species. The manipulative study revealed the strongest competitors were the older perennial targets and neighbors in both intra- and interspecific competition. The response to neighbor density on the native species differed between studies ; negative and positive effects were observed in the field study and no effect was observed in the manipulative study. However, both studies indicated that S. gobicola was, at best, competitively similar to the competitive effects of the native species. These outcomes on the competitive effects of the invasive species on a common desert annual and a rare endemic species may dispel the bias that invasive species are stronger competitors than rare and native species. To address the scarcity of reliable population data on O. californica ssp. eurekensis for both park managers and the U.S.F.W.S., a monitoring study was implemented in the field. The method of distance-sampling was used for its repeatability, efficiency, accuracy and precision in estimating abundance of rare species. The four-year data set revealed large variability in the spatio-temporal distribution and abundance of the endangered species at multiple scales. Thus, more years of monitoring appears necessary to determine population trends and stability Nevertheless, these more reliable population estimates and the findings from the competition studies can better inform management strategies of both invasive and sensitive native species. Furthermore, such observational and manipulative studies can improve our understanding of invasions in unique and protected ecosystems like the sand dunes in Eureka Valley.

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Page publiée le 12 septembre 2017