Publication Date:
2012-07-06
Description:
Unimodal patterns of diversity along gradients of both disturbance and productivity rank amongst the most celebrated generalizations in ecology, known as the Intermediate Disturbance Hypothesis (IDH) and Productivity-Diversity Hypothesis. However, doubt about their generality has arisen from studies that have failed to confirm the predicted patterns. Models suggest the interactive effects of disturbance and productivity on diversity to be responsible for the variability of diversity patterns. I conducted a 2-factorial field experiment (7 disturbance frequencies, 3 nutrient levels) on a sub-tropical intertidal rocky shore to test the dependence of the IDH on productivity (i.e. nutrient level). Treatment responses varied between three distinct community types found at the two study sites. At one site, the turf-dominated communities showed no effect of disturbance on species richness, evenness, dry mass, biotic cover or community composition. Nutrient additions statistically decreased evenness, yielded trends of increased dry mass and biotic cover, and affected community structure. At the other site, where encrusting algae dominated, disturbance increased evenness and decreased biotic cover. Nutrient enrichment produced increased species richness and dry mass, a trend of increased evenness, and changed community structure. Under high enrichment, a third type of community of mixed turf-forming and encrusting algae was formed and disturbance and productivity interacted to produce the decreasing portion of the IDH curve. Background disturbances exerted by sand, waves and intertidal stress may have interfered with the disturbance treatment. Different mechanisms controlled species coexistence in different community types, some failing to fulfill all assumptions of the IDH. Generalizations about factors that affect diversity should therefore be based on mechanisms rather than patterns.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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