The impact of ecosystem connectivity on coral reef resilience

Study Number:

10

Author:

P. J. Mumby & A. Hastings

Abstract:

• Ontogenetic dispersal of animals has been observed among many ecosystems, but its full ecological significance is poorly understood. By modeling the consequences of ontogenetic reef fish dispersal between Caribbean mangroves and adjacent coral reefs, we quantify the broader implications of ecosystem connectivity for ecosystem function and resilience to climate-driven disturbance (climate change).
• Mangrove-driven enrichment of parrotfish grazing on two coral reef habitats was calculated using empirical data. The consequences of increased grazing were then investigated using a spatial simulation of coral reef dynamics in shallow (depth 3–6 m) and mid-shelf forereefs (depth 7–15 m).
• The largest increase in grazing occurred in shallow reefs, but was found to have negligible consequences for coral population dynamics.
• In contrast, relatively weak increases in grazing on deeper reefs had profound consequences: reefs near mangroves were able to experience coral recovery under the most intense hurricane regimes of the Caribbean, whereas those lacking ecosystem connectivity had little capacity for recovery.
• This surprising result occurs because reefs exhibit multiple stable equilibria and mangrove enrichment of grazing in mid-shelf reefs coincides with a zone of system instability. A small increase in grazing shifted the reef beyond a bifurcation point, thereby enhancing resilience massively. A relatively large increase in grazing in shallow reefs had minimal ecosystem consequence because the grazing levels concerned were more than double the levels needed to exceed the corresponding bifurcation point for this habitat.
• Synthesis and applications, Caribbean mangroves are being deforested at a faster rate than rainforests, yet their protective role against hurricane damage extends not only shoreward to coastal environments but also seaward to increasing the resilience of offshore coral reefs. Specifically, ontogenetic mechanisms of ecosystem connectivity involving parrotfish may increase the probability that coral populations will recover from climate-induced changes in hurricane disturbance. Efforts to arrest mangrove deforestation and restore mangrove habitats are likely to increase the likelihood of recovery of corals on mid-depth (7–15 m) reefs after disturbance. In general, the ecosystem-level consequences of ontogenetic migration do not correspond necessarily to the magnitude of locally observed effects (i.e. the pattern of grazer enrichment exhibited the opposite pattern to that of its consequences for system resilience). Therefore, caution must be exercised when interpreting the functional significance of changes in species abundance for ecosystem process (e.g. grazing pressure and its implications for coral growth and survival). Impacts of shifting abundance or process are perhaps best appreciated using mechanistic ecosystem models.

Main Results and Conclusions:

• “Here, we hypothesize that the response of coral populations to increased grazing will be greater in habitats which benefit most strongly from mangrove-reef connectivity” (855).
• This study used a simulation model of coral reef dynamics “to investigate the ecosystem-level consequences of elevated parrotfish densities in reefs connected to mangrove ecosystems (Fig. 1)” (855).
• In mid-shelf reef ecosystems, one fish species in particular, Scarus iserti relied heavily on mangrove habitat: “Mangrove connectivity increased the biomass of Scarus iserti in Mesoamerica by 42%, but did not influence the density of any other parrotfish species in this habitat (Mumby et al. 2004). The resulting shift in biomass is responsible for a 50% increase in the grazing intensity of S. iserti in rich mangrove systems (0·14% h –1 to 0·21% h –1 in depauperate and rich systems, respectively, averaged across three atoll-sized systems in each treatment). Although S. iserti is one of the smallest species of parrotfish, typically reaching a total length of around 20 cm, it is also the most abundant. On average, S. iserti comprises 20% of the total parrotfish grazing intensity on reefs without mangrove connectivity (averaged from 30 reef sites in the Bahamas and Belize). If the contribution of S. iserti to the total grazing impact in mangrove-depauperate systems (i.e. 30% of the reef 6 months –1) is isolated (i.e. 20% of 30, giving six) and enriched by 50% (i.e. three units of grazing impact), the total effective grazing impact in mangrove-enriched mid-shelf reefs rises to 33% 6 months –1. In other words, mangrove connectivity increases the total grazing impact of parrotfish communities on mid-shelf reefs by ~10% (from 30% to 33% of the reef)” (855).
• In shallow reef ecosystems, S. guacamaia relied heavily on mangrove habitat: “The main impact of mangroves on shallow reefs is the support of adult S. guacamaia (Mumby et al. 2004; Dorenbosch et al. 2006)…As individual fish could not be harvested, the grazing impact of S. guacamaia was modelled by assuming that allometric scaling of bite size with body size held within genera. Bite rates and home ranges were determined by following eight individuals in Bonaire and Belize for a 2-min period. Observations of bite rate had a reasonably high precision [standard error (SE)/mean] of less than 20% (Andrew & Mapstone 1987). A home range of 1600 m2 (estimated conservatively) is larger than that of many other scarids (Mumby & Wabnitz 2002), and grazing by this species represents ~14% of the total grazing intensity measured for mangrove depauperate systems (0·041% h–1 of 0·302% h–1). Increasing the total grazing impact for shallow reefs (49% 6 months–1) by this proportion yields a new impact of 56% 6 months–1. Combining the contributions of both S. guacamaia and S. iserti to grazing in mangrove rich shallow reefs gives a total grazing impact of 57% 6 months–1, which is an overall enrichment of 16% (eight/49)” (858).
• Models showed mangroves acting as an incredible aide to coral reef recovery after natural phenomena such as hurricanes: “Under intense decadal hurricane disturbance, reefs with mangrove connectivity managed to achieve high levels of coral cover (> 50%), irrespective of the initial state of the reef (Fig. 4b). Moreover, this mangrove-based impact on resilience was greater than halving the frequency of hurricanes on a mangrove-depauperate reef. In contrast, reefs without mangrove connectivity had little potential for recovery and achieved much lower coral cover. For example, reefs starting at a relatively unhealthy cover of 10% showed little ability to improve after 50 years (Fig. 4b). Even when reefs started at a healthy 30% coral (by today’s standards), there was no net increase in cover” (859).

Works Cited:

Andrew, N.L. & Mapstone, B.D. (1987) Sampling and the description of spatial pattern in marine ecology. Oceanography and Marine Biology Annual Review, 25, 39–90.

Mumby, P.J., Edwards, A.J., Arias-Gonzalez, J.E., Lindeman, K.C., Blackwell, P.G., Gall, A., Gorczynska, M.I., Harborne, A.R., Pescod, C.L., Renken, H., Wabnitz, C.C.C. & Llewellyn, G. (2004) Mangroves enhance the biomass of coral reef fish communities in the Caribbean. Nature, 427, 533–536.

Mumby, P.J. & Wabnitz, C.C.C. (2002) Spatial patterns of aggression, territory size, and harem size in five sympatric Caribbean parrotfish species. Environmental Biology of Fishes, 63, 265–279.