Simulations of Long-Term Community Dynamics in Coral Reefs - How Perturbations Shape Trajectories

Abstract

Tropical coral reefs feature extraordinary biodiversity and high productivity rates in oligotrophic waters. Due to increasing frequencies of perturbations -anthropogenic and natural -many reefs are under threat. Such perturbations often have devastating effects on these unique ecosystems and especially if they occur simultaneously and amplify each other's impact, they might trigger a phase shift and create irreversible conditions. We developed a generic, spatially explicit, individual-based model in which competition drives the dynamics of a virtual benthic reef community -comprised of scleractinian corals and algae -under different environmental settings. Higher system properties, like population dynamics or community composition arise through self-organization as emergent properties. The model was parameterized for a typical coral reef site at Zanzibar, Tanzania and features coral bleaching and physical disturbance regimes as major sources of perturbations. Our results show that various types and modes (intensities and frequencies) of perturbations create diverse outcomes and that the switch from high diversity to single species dominance can be evoked by small changes in a key parameter. Here we extend the understanding of coral reef resilience and the identification of key processes, drivers and respective thresholds, responsible for changes in local situations. One future goal is to provide a tool which may aid decision making processes in management of coral reefs.

A spatially explicit and mechanistic model for exploring coral reef dynamics

“…In two subsequent developments of the model, Ortiz et al (2014) reproduced accurate recovery rates and final community composition of six coral taxa at fourteen reefs in the Great Barrier Reef, and Bozec et al (2015) reproduced the cover of seven coral species and the rugosity in reefs in Cozumel (Mexico). With a similar model, Kubicek et al (2012) generated time series of major coral taxa cover at Chumbe Island (Tanzania) similar to real data. Further, Kayal et al (2018) accurately reproduced colony density distributions of three coral species in four different sites in Moorea, French Polynesia, using integral-projection models.…”

“…We implemented the effects of hydrodynamic variation, thermal disturbances, and changes in grazing pressure, but reefs are also affected by other disturbances, and some of these have been implemented in previous models — including ocean acidification (e.g., Anthony et al, 2011; Madin et al, 2012b), predation by Acanthaster planci (e.g., Hogeweg and Hesper, 1990; Van der Laanm and Bradbury, 1990), disease (e.g., Brandt and McManus, 2009), destructive fishing (e.g., Kubicek et al, 2012), and pollution (e.g., Wolanski et al 2004, Melbourne-Thomas et al 2011, Kennedy et al 2013). We are currently not able to model the species-specific effects of these disturbances on coral assemblages because it is not clear what traits are relevant, nor how these relate to ecological processes and responses.…”

Use of Bayesian analysis with individual‐based modeling to project outcomes of coral restoration

“…CORE was least robust to uncertainty in probability of disturbance. Previous IBM models integrated competition and disturbance to predict short‐ (5 years, Tam & Ang ; Tam & Ang ) and long‐term (50–100 years, Kubicek et al ) community structure. However, only Sleeman et al () explored IBM in the context of coral restoration, specifically the effects of initial transplant density and arrangement on higher‐level properties (e.g.…”

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“…Coral reef health, evoked by even small changes in a key parameter. 58 As an example of the complexity of these interactions, a COTS out break on coral reefs, is in itself believed to be sensitive to the increased human activities causing ocean pollution, but can also be contributed to excessive fishing removing its key predators. 10 Unfortunately, there are no studies relating the apparent trend in COTS to water pollution or fisheries in the area.…”