Disturbance-mediated changes in nectar availability alter pollinator population and foraging dynamics

Background/Question/Methods

Mutualistic interactions play an integral role in the assembly of ecological communities and are essential to the functioning of some ecosystems. However, mutualistic interactions have been shown to vary with abiotic and biotic disturbances such a habitat fragmentation and the introduction of non-native species. We focus on how biotic disturbance, specifically livestock grazing, influences a plant-pollinator mutualism, using as a focal system the interaction between Speyeria mormonia (Lepidoptera: Nymphalidae), a montane butterfly, and its nectar plants. We contrasted areas with and without seasonal grazing, using mark-realease-recapture to estimate the number of butterflies flying on each day of the flight season, the emergence of butterflies through time, the daily residence rate of the butterflies, and changes in age structure of the population through time. We also monitored fine-scale patterns of visitation of a preferred nectar plant, and sampled pollen from butterflies to determine pollinator efficiency. 

Results/Conclusions

 Livestock grazing reduced nectar availability throughout the butterfly flight season and subsequently impacted pollinator population and foraging dynamics. Butterfly densities did not vary spatially between grazed and ungrazed areas or temporally after seasonal grazing, and patterns of butterfly emergence were not significantly different between sites. However, we found evidence of increased movement by butterflies out of grazed areas. Specifically, butterfly daily residence rate was lower and the age structure was altered due to dispersal out of grazed areas. Additionally, butterflies changed their fine-scale foraging behavior in response to grazing. The probability and rate of butterfly visitation to a preferred nectar plant was higher in grazed areas compared to ungrazed areas before seasonal grazing, but depressed after grazing. However, the diversity of pollen grains on butterflies was not significantly different between grazed and ungrazed areas, suggesting butterflies have equivalent pollinator efficiency. We show that the interaction dynamics of mutualists were altered by disturbance even if the abundance of only one of the players (the plants) was altered. This study contributes to understanding the propagation of disturbance through biological systems by studying the influence of disturbance-mediated changes in resource availability on the interactions between mutualists.