Pollinator-plant species specificity as it relates to viable seed production and level of disturbance

Background/Question/Methods

Pollinator species densities are often altered as a result of anthropogenic disturbance and habitat fragmentation in tropical forests. Most tropical tree species are biotically pollinated and self-incompatible, and are thus dependent on mobile animals for reproduction. Little is known about how plant-insect pollination interactions change across human-altered landscapes. With the continuous conversion of tropical forest to agricultural land, it is important to identify which pollinators are most vulnerable to habitat change, and what the contribution of each pollinator species is to the reproductive success of tropical plants in order to make sound conservation decisions. We conducted a single visit experiment in two populations of the understory tree Miconia affinis in Panama, one population from highly conserved mature forest and one from a highly disturbed agricultural area. In each population, we randomly chose 30 trees and bagged five random inflorescences per tree. The bagged inflorescences were exposed to a single insect visitor, which was subsequently caught and identified. The fruits were allowed to develop to maturity and collected. We compared different pollinator species in terms of success setting fruits and number of viable seeds per fruit, and we estimated the effects of disturbance on these relationships.

Results/Conclusions

Our results indicate that pollinator species differed widely in their ability to set fruits and produce viable seeds. Despite a high diversity of bees visiting our study trees, five species were represented in high enough densities in both populations for comparison, ranging in size from 3.5- 12 mm.  Of these, two of the largest species, Melipona panamensis and Trigona fuscipennis (~12 and 7 mm, respectively) exhibited a significantly higher average fruit set in mature forest landscape. There were very few instances of these species pollinating trees in the agricultural site. Overall, we detected a lower average intertegular distance for the pollinator community in the highly disturbed agricultural site.  In addition, the larger pollinator species also produced higher proportions of viable seeds per fruit than the smaller-bodied species. This study suggests that plant-pollinator relationships may be degraded with increasing disturbance as smaller, less effective bees take on a higher pollination load.