The Art of SEM and Ecosystem Maintenance
Climatic fluctuations associated with the El Niño Southern Oscillation (ENSO) can have consequences for people living near coastal wetlands who depend on natural capital for their livelihoods. Changes in primary ENSO indicators, such as temperature and precipitation, directly link to changes in ecosystem integrity and resilience. Whilst the impact of sudden climatic ‘shock’ events is significant, the cumulative impact of responses to ENSO from ecosystem and biogeochemical processes in combination with responses from changes to anthropogenic stressors is often assumed or overlooked. Yet these can have important repercussions for ecosystem dynamics and services.
Our aim is to determine the effect of ENSO on artisanal fishery productivity (catch weight) – a key ecosystem service associated with mangroves. We hypothesise that the direct effect on catch weight from macroclimate responses to ENSO will be compounded by indirect effects caused by responses to ENSO from ecosystem processes such as salinity, dissolved oxygen and nutrients. Using two structural equation models (SEM), we quantified the direct and indirect effects of the ENSO on fisheries associated with the Ciénega Grande de Santa Marta (CGSM), an extensive (~800 km2) lagoon system, fringed by mangrove, on the northern Colombian Caribbean coast.
Structural equation models (SEM) provide a framework to incorporate multiple direct and indirect causal pathways using multiple predictor and response variables. SEMs differ from other multivariate analysis techniques in the use of an a priori or theoretical model to which all subsequent models are compared. The development of a SEM is an iterative process with models refined and re-specified in order to achieve the best ‘fit’. Our a priori model incorporated existing knowledge and understanding of mangrove fisheries gathered from interviews with fishermen and Rangers employed by Parques Nacionale in the CGSM, with current research in order to indicate probable directional pathways.
The impacts of ENSO events on coastal wetlands vary at a regional scale according to geographical location, hydrology and geomorphology, yet mangrove systems are functionally similar across their global range. As such, vulnerability assessments for these important coastal wetlands need to incorporate the cumulative effects of interactions between multiple ecosystem processes and macroclimate responses to ENSO.