Modelling food webs and assessing trophic impacts of invertebrate grazing on seagrass systems in Maqueda Channel, Caramoan Peninsula, Philippines
Date of Publication
Doctor of Philosophy in Biology
College of Science
This study aims to contribute in the understanding of diverse pathways for the transfer of energy from the primary producers to higher order consumers in seagrass systems. The food web components and links between the primary producer and the consumer trophic levels in two seagrass systems at Maqueda Channel, Caramoan Peninsula, Philippines were characterized by constructing trophic models using the Ecopath with Ecosim (EwE) software. The models were then subjected to elasticity analysis to explore impacts of grazing in seagrass systems. An in situ experiment was conducted to validate hypotheses derived from the models and elasticity analysis. Results revealed the low Ecotrophic Efficiency (EE) of seagrasses, suggesting that they are not fully consumed. T. gratilla, herbivorous gastropods, and polychaetes were the important grazers in the seagrass systems studied. A hypothetical increase of the biomass of grazers T. gratilla had a negative effect on biomass values of seagrasses, while detritus and phytoplankton would have positive impacts on most benthic groups. Moreover, it is projected that an increase in the biomass of grazers will lead to the consumption of biomass that otherwise will be exported from the seagrass systems as indicated by the decreased sum of all exports. On the contrary, reduction of the biomass of grazers decreased the EE of seagrasses and increase of the flow to detritus. Thus, if grazers will be harvested for example, the biomass of the seagrasses primarily enters the detrital pathway. In situ inclusion/exclusion experiment showed that sea urchins in inclusion cages consumed 70% to 85% of seagrass biomass within 30 days or 2.3% or 2.8% daily. Biomass of Cymodocea serrulata, Enhalus acoroides, and Halodule uninervis significantly decreased in the inclusion cages while those in the exclusion cages remained stable suggesting full utilization of these seagrasses. E. acoroides on the other hand were underexploited and their excess production went to detritus. Repeated measures ANOVA revealed that throughout the experimental period, shoot biomass significantly decreased in the inclusion cages but seagrass root biomass remained constant in the exclusion and inclusion cages. Thus, herbivore pressure was significant in inclusion cages than in exclusion cages and on seagrass shoots than on seagrass roots. The findings of the current study contributes to the ongoing reevaluation of the role of herbivory and detrivory in the energetics of seagrass habitats. Clearly, when in the steady-state conditions, the Maqueda Channel seagrass systems contribute significantly their seagrass production to the detrital pool. This result of the model analysis was supported by the elasticity analysis and in situ experiment which also showed that when grazers become abundant, much of this production proceeds to herbivory pathway. Constrained by the uncertainty of the data to integrate information needed, this research was still able to provide robust models to derive insights on energy dynamics of seagrass structure. Analysis of data from this study showed that interactions are strong between the functional groups at the lower trophic levels in seagrass systems and that both top-down and bottom-up factors control seagrass ecosystem structure and function.
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Clores, M. A. (2015). Modelling food webs and assessing trophic impacts of invertebrate grazing on seagrass systems in Maqueda Channel, Caramoan Peninsula, Philippines. Retrieved from https://animorepository.dlsu.edu.ph/etd_doctoral/1182