Title

Plant-herbivore interactions in a changing climate and implications for seagrass resilience

Author Identifier

Caitlyn O'Dea

https://orcid.org/0000-0001-9637-2682

Date of Award

2023

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Master of Science (Environmental Management)

School

School of Science

First Supervisor

Kathryn McMahon

Second Supervisor

Paul Lavery

Abstract

Climate change is affecting herbivore distributions and abundance, resulting in intensified grazing pressure on foundation species such as seagrasses. This, coupled with other, rapid changes in estuarine ecosystems, may threaten the resilience of seagrass. While the overall resilience of seagrasses is generally well-studied, the timeframes of recovery have received comparatively little attention, particularly in temperate estuaries. This study investigated the recovery time of two colonising seagrasses (Halophila ovalis and Ruppia megacarpa) following various intensities of simulated grazing in two southwestern Australian estuaries. Simulated grazing treatments (25%, 50%, 75%, and 100% removal from 1 m2 plots) were applied in field experiments replicated at four locations in each estuary during summer. Seagrass cover was used as the main measure of recovery, and was monitored over 3 months following the simulated grazing while excluding swans. The results showed that Halophila ovalis recovered within 4–6 weeks from lower grazing intensities (25% and 50%), and within 7–19 weeks from higher grazing intensities (75% and 100%). Furthermore, the variability in recovery time among experimental locations increased with grazing intensity. Ruppia megacarpa recovered from 100% grazing within 2-9 weeks. The recovery time for R. megacarpa was faster than for other colonising species, but this cannot be assumed to always be the case, as recovery did not occur within the timeframe of the experiment in one location. The overall conclusion is that colonising seagrasses, represented by H. ovalis and R. megacarpa, can recover rapidly following simulated grazing and through multiple mechanisms. Vegetative growth, and particularly regrowth from the surrounding meadow, appears instrumental in recovery for both species at the scale of disturbance simulated. A novel finding is that recovery occurred via recruitment from fragments in both species. No recovery was observed by recruitment from seed germination. These results indicate that these species in these systems, are likely resilient to current and increased levels of grazing pressure. However, estuarine ecosystems will have to remain resilient to other pressures in a changing world. Estuaries worldwide are subjected to anthropogenic impacts and climate change pressure, which is not only affecting grazing dynamics, but also introducing altered physio-chemical conditions and feedbacks. These pressures may act synergistically and could alter the resilience of seagrass and make it difficult to predict their recovery capacity. Intervention strategies such as future-proofing existing populations or restoration of lost meadows, or a combination of both, may be required to prevent reductions or loss of seagrass. The study provides a baseline for understanding grazing pressure as a singular disturbance. Future work can examine how grazing and other potentially interacting pressures in a changing climate could impact seagrass recovery timescales or trajectories.

DOI

10.25958/cc7e-m121

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