Insect herbivory on mangrove leaves in Guadeloupe: effects on biomass and mineral content
Extensive defoliation of Avicennia germinans by Junonia evarete (Stoll) (Nymphalidae) and Hyblaea purea (Cramer) (Hyblaeidae) was noted following hurricanes Hugo (1989), Luis & Marilyn (1995). The proposed work is about evaluating the direct loss of organic matter and minerals by insect grazing during an inter-hurricane period. The physiological response of the plant was assessed on perforated leaves over a 3 week period. A. germinans in two contrasted sites exhibited a leaf area loss close to 4.5% which corresponds to 50 kg D.W. / ha. In a third site, the low consumption rate of about 0.83% could be related to a drop in the insect population following an earlier outbreack and full tree defoliation. Damage in R. mangle was exceptionally low (0.2%). Bulk minerals trapped in the folivore pathway was about 1.2 kg N and 60 g P / ha in A. germinans stands. Leaf grazing brings a significant accumulation of organic matter and minerals by importing compounds from other tissues of the plant. This reflects a sudden change in leaf metabolism and we may hypothesise scarring reaction and/or secondary compound accumulation. In any case, leaf grazing demands additional energy and minerals to maintain partially grazed leaves. Moreover, such leaves are susceptible to greater loss in the event of repeated insect damage, due to higher concentrations. From these results, it appears that physiological effects of insect grazing have to be considered not only as direct loss in living tissues but also as deep changes in internal fluxes of the carbon and minerals.
Keywordsmangrove ecosystems defoliation mineral cycling Avicennia germinans Rhizophora mangle Junonia evarete Hyblaea purea French West Indies
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