Ecological traits and species vulnerability

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

San Carlos de Bariloche

Workshop; Workshop on Forest Fragmentation in South America; 2006

International Association of Landscape Ecology (IALE).

Habitat fragmentation, or the transformation of a large habitat into a number of smaller isolated patches, involves a reduction of the space available for some species. A positive relationship between habitat area and species richness has been widely demonstrated, but not all species show a uniform response. Differential susceptibility to habitat fragmentation is expected to depend on ecological traits like trophic position, habitat or food specialization, and population density (Tscharntke & Brandl, 2004). Here we explore these aspects for plant, leaf-mining insects and their parasitoids, in a fragmented landscape from Central Argentina, where human activities have led to a dramatic reduction in native Chaco Serrano Woodland, leaving isolated patches within a predominantly agricultural matrix. In 19 remnants of Chaco Serrano Woodland (0.13 ha to over 15,000 ha), floristic composition and plant abundance were recorded in two 500 m2 plots. All mined leaves in 5 transects (50m long, 2m wide) were collected at each site, from which adult insects were reared and identified. Species-area relationships were evaluated through log-lineal regressions and their slopes were compared (ANCOVA) considering the following traits: -         Trophic level and population size (all groups) -         Feeding specialization (leafminers) -         Origin and life-form (plants) Significant species-area relationships were observed for all groups. Greater species loss (steeper regression slopes) were shown by i) parasitoids in comparison with lower trophic levels, ii) rare than abundant species, iii) specialized leafminers as opposed to those with wider host ranges, although differences were statistically significant only when abundance was considered. Species loss showed similar rates among functional plant groups. Our results support the idea that larger habitats should be particularly rich in species with small population size, which would suffer the highest extinction risks as woodland remnants become smaller. This finding is relevant from a conservation perspective and suggests that large refuges would be necessary to increase the chances of survival of such rare species. Given the predominance of generalists in the upper trophic level (parasitoids) whereas their leaf-mining hosts are mainly specialists, conflictive predictions arise for this system  from the trophic level and specialization hipotheses, which could explain the weak relationship between trophic level and habitat loss effects here observed.