Spatial distribution of oak mistletoe as it relates to habits of oak woodland frugivores.

Ethan A Wilson,Janis L Dickinson,Patrick J Sullivan,Erik Sotka

doi:10.1371/journal.pone.0111947

Abstract

This study addresses the underlying spatial distribution of oak mistletoe, Phoradendron villosum, a hemi-parasitic plant that provides a continuous supply of berries for frugivorous birds overwintering the oak savanna habitat of California's outer coast range. As the winter community of birds consuming oak mistletoe varies from group-living territorial species to birds that roam in flocks, we asked if mistletoe volume was spatially autocorrelated at the scale of persistent territories or whether the patterns predicted by long-term territory use by western bluebirds are overcome by seed dispersal by more mobile bird species. The abundance of mistletoe was mapped on trees within a 700 ha study site in Carmel Valley, California. Spatial autocorrelation of mistletoe volume was analyzed using the variogram method and spatial distribution of oak mistletoe trees was analyzed using Ripley's K and O-ring statistics. On a separate set of 45 trees, mistletoe volume was highly correlated with the volume of female, fruit-bearing plants, indicating that overall mistletoe volume is a good predictor of fruit availability. Variogram analysis showed that mistletoe volume was spatially autocorrelated up to approximately 250 m, a distance consistent with persistent territoriality of western bluebirds and philopatry of sons, which often breed next door to their parents and are more likely to remain home when their parents have abundant mistletoe. Using Ripley's K and O-ring analyses, we showed that mistletoe trees were aggregated for distances up to 558 m, but for distances between 558 to 724 m the O-ring analysis deviated from Ripley's K in showing repulsion rather than aggregation. While trees with mistletoe were aggregated at larger distances, mistletoe was spatially correlated at a smaller distance, consistent with what is expected based on persistent group territoriality of western bluebirds in winter and the extreme philopatry of their sons.

Highlights

The spatial distribution of resources is an important driver of social behavior in a range of animal species [1], [2], [3]
Spatial autocorrelation of mistletoe volume We examined the spatial autocorrelation of the continuous variable, mistletoe volume, in order to test for evidence consistent with the hypothesis that sons remaining on high mistletoe volume territories and establishing territories adjacent to their parents’ territories will have an increased chance of acquiring a high mistletoe volume territory themselves
Analysis of the variogram indicates that mistletoe volume is spatially autocorrelated for distances less than 250 meters

Summary

Introduction

The spatial distribution of resources is an important driver of social behavior in a range of animal species [1], [2], [3]. Where food plants and animal social systems are interdependent, they can become mutual engineers of each other’s demography, dispersal, distribution, and abundance. We examine the spatial distribution of a key winter resource, oak mistletoe (Phoradendron villosum), in light of the behavior of its winter frugivores, which vary from species that hold persistent year-round territories to those that flock and adopt a mobile lifestyle in winter. From the point of view of the plants, territorial frugivores may reduce opportunities for long distance dispersal of seeds [6]. We conduct spatial analysis at two different scales to look for signatures consistent with both types of seed dispersers

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Conclusion