Aquatic insect β-diversity among small mountain headwater streams and the role of multiple mechanisms maintaining community structure
Date
2014
Authors
Harrington, Rachel Anne, author
Poff, N. LeRoy, advisor
Kondratieff, Boris, committee member
Ghalambor, Cameron, committee member
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Abstract
Despite lower local richness (α-diversity), individual headwater tributaries often retain highly distinct aquatic insect communities (β-diversity) within the overall river network. This trend is presumed especially true among high-elevation streams; where high β-diversity is driven by the effects of steep topography and harsh climatic conditions limiting dispersal between isolated mountaintop "islands." However, inference has been predominantly drawn from observed trends along single-thread channels (higher-elevation headwaters through lower-elevation mainstems); and the increased size and hydrologic connectivity accompanying lower-elevation mainstems provide potential alternative explanations for this pattern. Controlling for habitat size, I sampled aquatic insect communities in 24 headwater streams from three adjacent river drainages spanning 2000-3500 m in elevation. I measured β-diversity among streams within each drainage (community turnover- β across elevation) and β-diversity across drainages (community dissimilarity- β within elevation "zones"). Turnover- β across elevation was consistently high and displayed no trend. Additionally, dissimilarity-β across drainages was not significantly different between high-elevation and low-elevation zones. These results provide the first evidence that β-diversity among low-elevation headwater communities is equivalent to communities at high-elevations. Evidence suggests that high β-diversity among small headwater streams is attributed to low habitat connectivity and/or to high habitat heterogeneity, resulting from their isolated position within the dendritic network and strong responses to the surrounding environment. In order to disentangle the role of multiple mechanisms maintaining β-diversity, I utilized the unique landscape of mountain ranges, exhibiting steep gradients of spatial distance, local environmental conditions, and disturbance regimes. I characterized all 24 sites using explanatory variables categorized into spatial predictors (describing geographic location), environmental predictors (describing local habitat), and flow regime predictors (describing potential disturbances overtime). Using a series of redundancy analyses (RDA) I tested the ability of each categorized predictor group to significantly explain variation in community structure among those sites within a drainage and among those sites within an elevation zone. Further, original communities were partitioned into unique assemblages distinguished by the presence/absence of key ecological traits. Using interpretation of potential underlying mechanistic processes, I tested a priori hypotheses regarding the change in relationship between trait-partitioned assemblages. Results determined that although environmental predictors best explained community turnover-β within drainages, they were unable to explain community dissimilarity-β within any elevation zone, where habitat heterogeneity is presumably lower and inter-site network distance is higher. Additionally, dissimilarity-β among high-elevation communities was only explained by spatial predictors, supporting previous hypotheses that these communities are isolated by distance, while community dissimilarity-β among low-elevation sites was only explained by flow regime predictors. Overall, these findings suggest that despite consistent patterns in β-diversity, the relative role of mechanisms maintaining this diversity is context dependent, presenting important implications regarding the successful conservation of these dynamic systems.
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Subject
beta-diversity
benthic macroinvertebrates
disturbance
elevation
stream ecology
dispersal