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Date Permissions Signed


Date of Award

Spring 2018

Document Type

Masters Thesis

Degree Name

Master of Science (MS)



First Advisor

Hooper, David U., 1961-

Second Advisor

Miner, Benjamin G., 1972-

Third Advisor

Peterson, Merrill A., 1965-


The role of plant diversity in reducing invasions has generated decades of debate. Diverse communities might be more resistant to invasion because the communities contain resident species that are functionally similar to the invader (limiting similarity/sampling effect), or the residents use the range of available resources more effectively (complementarity) than single species. However, the resistance of diverse communities to invasion appears to decline with increasing spatial and temporal scale, in a phenomenon called the “invasion paradox.” I addressed two groups of hypotheses related to this paradox, broadly that: (1) functional diversity and functional identity resist invasion initially, via complementarity or limiting similarity; and (2) disturbance and propagule pressure interact to weaken the effects of functional diversity and identity on invader success through time. Using long-term data from experimental assemblages in a serpentine grassland of California, I examined the abundance of a high impact invader, yellow starthistle (Centaurea solstitialis), in response to functional diversity (functional dispersion), functional dissimilarity, extent of pocket gopher disturbance, and propagule pressure. Neither diversity, nor dissimilarity, nor disturbance significantly impacted the success of C. solstitialis at any point in the study. Instead, propagule pressure was the single most important predictor of C. solstitialis abundance. These findings suggest that continued dispersal may eventually outweigh community resistance when considering a longer-term view of invasion. Thus, conservation efforts focused on preventing the introduction and establishment of invasive species should recognize that even diverse communities are susceptible to invasion over time.




species invasion, community assembly, invasion paradox, complementarity, limiting similarity, disturbance, propagule pressure, dispersal modeling, serpentine grassland, Centaurea solstitialis



Western Washington University

OCLC Number


Subject – LCSH

Grassland ecology; Plant diversity--Environmental aspects; Introduced organisms--Environmental aspects; Grasses--Variation--Environmental aspects; Yellow starthistle--Propagation--Environmental aspects; Yellow starthistle--Biological control--Environmental aspects




masters theses




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