Ialc Peace Fellowship Report When Will Fragmentation per Se Increase/decrease Biodiversity? a Species-area Relationship Approach

Fragmentation usually couples two separate processes, the loss of area and the decrease in the connectivity of the remaining patches. The well established pattern of species-area relationships (SPAR) clearly show that a loss of area will result with a decrease in biodiversity. However, the effect of fragmentation per se on biodiversity is less clear. In the literature, fragmentation per se has been shown to increase, decrease or not affect biodiversity. In the work I've done with Dr. Rosenzweig, we explored the affect of fragmentation per se on biodiversity of spiders in the fragmented landscapes of Southern Judea Lowlands (SJL), Israel. SJL lies in the transition zone between the Mediterranean ecosystem in the north, and the arid Negev desert in the south. Over a short gradient of 30 km, mean annual precipitation drops from 450 mm/y to 250 mm/y. At the Landscape scale, SJL is an agro-ecosystem with patches of different area embedded inside an agricultural matrix. We used a data-set comprising of more than 15,000 spider individuals from 200 species or morpho-species. The spiders were sampled in three 4×3.2 km landscapes along the sharp climatic gradient. In each landscape, I've sampled 12 patches of different area and a continuous area. For each landscape, we estimated the number of species in each patch using accumulation curves. We conducted a linear regression of log(species) against log(area) and extrapolated the SPAR to the combined total area of all 12 patches. We estimated the number of species in all 12 patches together, and explored if the value lay above, below or on the line of the regression. Additional analysis was conducted to separate the affect of fragmentation per se from area by selecting all combination of patches whose total area lies in a small range (±5%) around the area of a focal patch. We did it separately for different fragmentation levels (number of patches whose area is combined), ranging from 2 till 9. We calculated the mean Fisher's alpha for different fragmentation levels and compared the value to the Fisher's alpha of the focal patches. We found that in all three landscapes, the estimated number of species in all 12 patches lay above the expected species diversity of the SPAR. We also found that in SJL, an increase in Fisher's alpha correlates with an increase in the fragmentation levels. We suggest that in SJL, the turnover of species between patches overcompensates for the reduced number of species in each patch due to area loss. Finally, we developed a simple, SPAR-based-theory that may explain the contradicting affect of fragmentation per se on biodiversity.