Estimated flows and biomass in a no-take coral reef from the eastern tropical Pacific through network analysis
In the southern Gulf of California, the Cabo Pulmo reef has been the focus of many studies because it is the northern-most coral reef in the eastern Pacific. It is a paragon of a well-managed marine protected area. Under the assumption that fishing mortality is negligible, we want to identify and quantify major energy flows in an ecosystem without human intervention and describe the ecosystem resources and their interactions among species, to provide a tool for ecosystem-based management. We built a trophodynamic model using Ecopath to perform network analysis. Based on fieldwork (October 2017 – May 2018) and literature review, we identified 57 functional groups comprising 51 consumers (including 15 top predators), five primary producers plus detritus, and cluster analysis of trait profiles. The connectance index (0.17) and the system omnivory index (0.22) are low, suggesting that consumers feed on a few discrete trophic levels. Biomass of primary producers (grazing food chain; 186.8 t km−2) provides 9,813 t km−2 y−1, whereas flow from detritus supply 344.9 t km−2 y−1. The transfer efficiency decreases as flows go up the food web, from 12% at TL II to 4% at TL X, and throughput cycled (including detritus) = 118.7 t km−2 y−1. In comparison with other coral reefs, we found that Cabo Pulmo complies with the attributes to resist disturbances, with an estimated total system throughput = 95,789 t km−2 y−1, a net system production = 38,535 t km−2 y−1, a large mean path length = 12.11, ascendency = 123,662 (52%) flowbits and overhead = 116,164 (48%) flowbits. The high quality of the ecosystem services provided by Cabo Pulmo and the scenic beauty appeals to developers. Although the system is resilient, unregulated human activities may impact the reef condition and decrease the residents' quality of life and that of all the people who make a living from the low impact activities currently in effect. The trophic web model presented here may help to improve the response capacity of the coalition of residents, authorities, diving companies, and NGO's to preserve the reef and be a key element to conserve the system by contributing to its best management.