Size matters: Predator outbreaks threaten foundation species in small Marine Protected Areas

Abstract

The unanticipated impacts of consumers in fragmented habitats are frequently a challenge for ecosystem management. On Indo-Pacific coral reefs, crown-of-thorns sea stars (Acanthaster spp.) are coral predators whose outbreaks cause precipitous coral decline. Across large spatial scales, Acanthaster densities are lower in large no-take Marine Protected Areas (MPAs) and reefs subject to limited human exploitation. However, using a combination of observational and manipulative experiments, we found that Acanthaster densities within a network of small, no-take MPAs on reef flats in Fiji were ~2-3.4 times greater inside MPAs than in adjacent fished areas and ~2-2.5 times greater than the upper threshold density indicative of an outbreak. This appeared to result from selective Acanthaster migration to the coral-rich MPAs from fished areas that are coral-poor and dominated by macroalgae. Small MPAs can dramatically increase the cover of foundation species like corals, but may selectively attract coral predators like Acanthaster due to greater food densities within MPAs or because the MPAs are too small to support Acanthaster enemies. As coral cover increases, their chemical and visual cues may concentrate Acanthaster to outbreak densities that cause coral demise, compromising the value of small MPAs. An understanding of predator dynamics as a function of habitat type, size, and fragmentation needs to be incorporated into MPA design and management.

Fig 1. Mean Acanthaster density is ~2–3.4 times greater within Marine Protected Areas (MPAs) than adjacent fished areas.

(Top panel) Village and MPA locations along the coast of Viti Levu, Fiji. Dark gray sections represent the MPAs at each site. (Bottom panel) Violin plots depicting the mean ± SE Acanthaster density (large black dots and error bars), the frequency of plots with differing densities of Acanthaster (the enclosed areas), and each individual plot as a function of Acanthaster counted in that 15 X 15 m plot (small black dots) within MPAs (dark gray) and adjacent fished areas (white) at each village (n = 15 quadrats reef^-1 location^-1). Data for each pairwise comparison were analyzed using a generalized linear model (GLM) with a Poisson distribution (Votua) or quasi-GLM models (Namada and Vatu-o-lalai).

Fig 2. Acanthaster move selectively to Marine Protected Areas (MPAs).

Movement directions of initial (solid dots) and final (open dots) Acanthaster relocations from release points at MPA/non-MPA borders on each side of the MPA at each of the three villages. Arrows represent the resultant vector (R) for initial (black) and final (gray) relocations. * and * indicate a significant difference between Acanthaster movement towards MPAs (white region) rather than fished areas (shaded region) (Modified Hodges-Ajne test, p < 0.050).

Fig 3. Reef habitat differs immediately inside vs. outside MPAs.

Comparisons of benthic cover (mean % ± SE) 20 m inside (black) and 20 m outside (gray) of MPA borders perpendicular to the coastline at Namada, Vatu-o-lalai, and Votua villages (n = 20 transects border^-1 location^-1). The category “Other” includes dead coral, rock, rubble/sand, and uncommon benthic organisms (e.g., zooanthids, soft coral). Asterisks after p-values indicate comparisons analyzed with quasi-GLM models.

Fig 4. Acanthaster displacement is negatively correlated with local coral cover.

(a) Relationship between individual Acanthaster displacement between consecutive days (m day^-1) and coral cover (%) at each individual’s release location along MPA borders. (b) Relationship between coral cover (mean % ± SE) and Acanthaster displacement between consecutive days (m day^-1; mean ± SE) when pooled by MPA border. See Fig 1 for village site names. Coefficients of regression (R²) and p-values are indicated in the graph. Two data points with extreme Acanthaster displacement values (y₁ = 42.65 m, y₂ = 34.39 m) at low coral cover (x₁ = 0%, x₂ = 11.25%) were excluded from analyses after performing an outlier analysis (Jackknife distances) using JMP (Version 11.0.0).