Seagrass resilience: Exploring recovery trajectories, succession and facilitation

Author Identifier

Samuel Billinghurst

Date of Award


Document Type



Edith Cowan University

Degree Name

Master of Science by Research


School of Science

First Supervisor

Kathryn McMahon

Second Supervisor

Paul Lavery


Globally seagrasses have been declining at approximately 7% annually since 1990, compromising their resilience and ecological function. Recovery from disturbance is a key component of any ecosystem’s resilience. A key mechanism of ecosystem recovery following large-scale disturbance is succession. Ecological succession refers to temporal changes in community composition, these transitions in community composition during succession are seen to be moving towards a distinct end point referred to as the climax community or, in ecological resilience language, the stable state. Stages can be identified during succession by differences in the community composition and are referred to as seres or seral stages. For seagrass meadows succession is influenced by the life history traits of species present, with colonising species potentially facilitating the recovery of later successional species. However, little is understood regarding succession and the mechanism driving it in seagrasses. This thesis aims to 1. identify whether tropical multi-species seagrass meadows follow a consistent successional pattern following disturbance and, if so, to identify the taxa that characterise the seral stages; and 2. test in an experimental setting whether a colonising seagrass species facilitate the survival and growth of later successional opportunistic seagrass species through the sediment trapping feedback loop. These aims were addressed in two separate Chapters 2 and 3.

The second chapter used a novel statistical approach to assess the recovery of benthic macrophyte species composition and abundance, following a major loss of seagrass that was credited to cyclone activity, to assess whether a successional pattern occurred and what species characterised the different seral stages. A successional pattern was identified along with three distinct seral stages. During this successional pattern there was a transition from colonising (early successional) to opportunistic (later-successional) species. The first sera had low percent cover (3-14%) community dominated by colonising seagrass species (Halophila group and H. uninervis) Green Branched Rhizomatous Algae (Caulerpa) and very low percent cover of ‘nonrooted’ Brown Algae and Green Plate Rhizoid Algae (Udotea). The second sera had high percentage cover (25-68% dominated by colonising seagrass species, particularly H. spinulosa and briefly by Green Branched Rhizomatous Algae (Caulerpa). The final sera had a moderate percent cover (12-28%) and was dominated by opportunistic seagrass species (C. serrulata and S. isoetifolium), ‘rooted’ and ‘non-rooted’ macroalgal species.

The third chapter tested via an experimental study whether the colonising species H. uninervis facilitated the survival and growth of the opportunistic species C. serrulata and whether this occurred through a sediment stabilising mechanism. The experiment successfully demonstrated that H. uninervis facilitated the survival of C. serrulata. Artificial Seagrass Units were also seen to facilitate the survival of C. serrulata indicating that the effect was a physical one rather than a biological or chemical effect, that is through physical sediment stabilisation. This was further supported by the observed effect of H. uninervis on sediment erosion; with Halodule tanks which were subjected to simulated hydrodynamic disturbance having reduced erosion. However, H. uninervis did not have a significant effect on the apical shoot growth or biomass ratios of C. serrulata.

This thesis is the first to use a novel and rigorous, multi-criteria approach to identify the presence of a successional pattern, the sera that make up this pattern and the species that are dominant in each sera. Additionally, this study is the first to explicitly demonstrate in an experimental setting that a colonising species facilitates the survival of later successional opportunistic species through the sediment trapping feedback loop. These findings provide: a blueprint of how similar seagrass ecosystems might recover from disturbance; what stage of recovery a system is in; and a pathway to guide and accelerate recovery through intervention. This significantly progresses the scientific knowledge surrounding seagrass recovery. This knowledge has implications for restoration, offering restoration practitioners the ability to intervene in successional patterns.



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