Habitat loss effects on plant - herbivore foodwebs

CAGNOLO, L.; VALLADARES, L.; SALVO, A.

San Carlos de Bariloche

Workshop; Workshop on Forest Fragmentation in South America; 2006

International Association of Landscape Ecology (IALE).

Food webs depict the general architecture of ecological systems, providing information on energy pathways and feeding habits of interacting species. High quality interaction webs can help understanding extinction effects as they allow to trace perturbation effects to indirectly interacting species. As habitat loss and fragmentation reduces species diversity, communities loose alternative routes of energy transfer and this effect could cascade-up and down the whole system (Memmott et al. 2005). The aim of this work was to construct plant-leafminer food webs in Chaco Serrano woodland patches and to analyze possible effects of habitat loss on network structure. Valid comparisons of food webs require the use of standard methodologies, therefore we applied the same sampling procedure in 19 Chaco Serrano woodland remnants varying in size from over 1000ha to 0.13ha. In every site we collected all mined leaves in five transects of 50m long and 2m wide in two occasions. The mined leaves were kept in plastic bags until emergence of adult insects. After identifying the miner species, we constructed quantitative interaction matrices of leafminer species on plant species for each site. Through regression analysis we evaluated woodland area effects on food web size, plant and miner species richness, number of links between plants and miners, connectance (proportion of realized links from total possible), miner/plant species rate, proportion of consumed plants within the whole plant community, diversity and evenness of trophic links (considering number and strength of interactions), and the slope of the cumulative distribution of links. Area reduction of Chaco Serrano woodland was related to food web reduction in terms of miner and plant species richness as well as number of links (all cases p<0.001). Neither the proportion of plant species consumed by miners nor the ratio between miner and plant species changed with area reduction (p>0.05). Connectance showed a negative relation to woodland area (p=0.001) indicating that plant leafminer communities in smaller woodlands are proportionally more connected. Other topological and quantitative parameters of the studied food webs did not change with habitat reduction. Our results indicate that woodland reduction had profound effects on plant-leaf miner food webs by reducing the number of species but increasing the connectance between them. At the same time, other topological and quantitative food web parameters seemed to be robust to this kind of perturbation.