Date of Award

Winter 12-16-2016

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Zoology

Advisor

Hamish Greig

Second Committee Member

Jacquelyn Gill

Third Committee Member

Brian McGill

Abstract

Climate change has altered disturbance frequency and intensity in many ecosystems, and predictions show that these trends are likely to continue. There are two main mechanisms by which disturbance frequency affects community structure. If disturbances depress community or population-level abundances each time they occur, then repeated disturbances over time are more likely to cause stronger effects than a single disturbance (Lande 1993). In addition, when communities recover from a perturbation, their structures change and develop over time (Clements 1916, Gleason 1917). By extension, the changes in community structure and taxon traits that occur during succession could also lead to altered vulnerability of an assemblage to disturbance (Neutel et al. 2007). This thesis examines the importance of disturbance frequency in determining community structure, and successional state in determining community structure and vulnerability.

In streams, past experiments indicate that flooding and flood frequency have a strong role in structuring communities. However, this knowledge comes from systems often sampled at inconsistent times in recovery, so our understanding of the effects of repeated disturbances is confounded by differences in successional processes operating in the time since the last disturbance. I used in-situ stream mesocosms to examine the effect of disturbance frequency on community structure. Five disturbance frequencies were applied to replicate stream mesocosms, with the last disturbance occurring on the same day across all treatments. After a recovery period, communities were sampled to detect compositional changes. Several metrics showed that community structure changed along a gradient of disturbance frequency, and family-level richness decreased with more disturbances. Individual taxa showed differential vulnerability to disturbance, but any strong negative responses were masked by rapid recovery of the most dominant taxon, chironomid midges, so overall abundance was unchanged. Thus, the cumulative effects of environmental filtering from repeated disturbances—not just successional state—affect community structure. Thus, the timing of past disturbances should be considered when anticipating responses to future perturbations.

Following these perturbations, ecological communities change in structure over successional time. However, ecologists have not related these compositional changes to community vulnerability to disturbance. Data are especially lacking on the responses of mobile individuals and higher trophic levels. This experiment used temporary pond mesocosms to examine whether disturbance vulnerability changes as communities age. Systems of varying ages were either disturbed or retained as undisturbed controls to 1) observe how community structure changed over successional time, 2) relate structural changes to community vulnerability to disturbance, and 3) determine how these responses differed across mobility classes and trophic guilds. I observed changes in community structure over time and with disturbance, but no metrics of whole-community structure reflected an interaction. However, one subset of the community—mobile predators—responded more strongly to disturbance in older communities than mid-aged or young communities, consistent with these taxa actively leaving disturbed mesocosms. This result shows that often overlooked taxa—mobile taxa and predators—may be the most vulnerable to disturbance. Conservation implications include a need to examine the strength of disturbance cues and the availability of landscape-level refugia to anticipate whole-community disturbance responses

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