Effects of habitat fragmentation on insect pollinators, plant pollination and seed output in a dry subtropical forest of northwestern Argentina

In the dry subtropical forest of Tucuman Province, Argentina, I assessed effects of habitat fragmentation on (1) the insect pollinator fauna by monitoring visits to flowers of two dominant leguminous trees and by sampling bees with yellow pan traps, and on (2) pollination and seed production levels by determining stigmatic pollen load sizes, number of pollen tubes at the base of the style, and number of fruits per flower (fruit set) and seeds per fruit (seed set) in 16 plant species representing a variety of growth forms, and pollination and breeding systems. The experimental design involved five study sites each with continuous forest, a large ($\u3e$2 ha) forest fragment, and a small ($\u3c$1 ha) fragment. Comparisons in pollinator abundance and diversity between fragments and continuous forest were replicated across 4 sites. Comparisons in pollination levels and fruit and seed set were replicated across 1-4 sites depending on the plant species. The exotic honeybee (Apis mellifera) dominated the flower-visiting fauna associated with the two leguminous species. Frequency and taxon richness of native flower-visitors to both plant species declined with decreasing fragment size, but frequency of honeybee visits increased, so neither species experienced significant effects of fragmentation on total visitor frequency. Similarly, in pan trap samples the relative frequency of honeybees increased with decreasing fragment size, whereas native bees increased in number and species diversity with increasing size of forest patch and as spring progressed. Fragmentation-related declines in pollination and seed output (fruit set x seed set) prevailed in the subset of the Chaco flora I studied. Among species, fragmented plant populations recorded an $\sim$20% median decrease in pollination and seed output levels. However, mechanisms behind these declines differed across species. In three self-incompatible species, decreases in the number of pollen tubes with fragmentation could be related to declines in pollination quality rather than in the number of pollen grains deposited on the stigmas. Correlative evidence showed that fragmentation-related declines in pollination could limit seed output in some plant species, but an increase in seed abortion due to inbreeding depression could also affect fragmented plant populations. Comparisons among self-compatible and self-incompatible species did not show any differential effect of fragmentation with breeding system. Overall, the results of this study show that fragmentation may exert chronic, negative effects on pollinators, pollination, and seed output with potential effects on plant regeneration, forest dynamics, and total species diversity. These results also indicate that fragmentation may alter community and population processes through subtle and complex mechanisms