Dispersal and spatial distribution of the desert mistletoe, Phoradendron californicum, at multiple scales: Patterns, processes and mechanisms

Abstract

Describing processes that lead to the distribution of parasites in space is important for understanding disease transmission and spread. Similarly, describing plant distribution patterns is important to understanding ecological processes. Indeed, distinguishing between dispersal and establishment limitation is central in plant ecology. Mistletoes allow doing both because they are plant parasites with clearly defined spatial distributions among hosts. Most mistletoes are dispersed by birds that consume mistletoe berries and defecate seeds onto host trees in a mutualistic relationship. I studied desert mistletoes, Phoradendron californicum (Viscaceae), which in the Sonoran desert, parasitize legume trees and are dispersed by Phainopepla nitens (phainopeplas). I examined patterns of spatial distribution and dispersal of P. californicum and the processes and mechanisms underlying these patterns at multiple scales. By counting mistletoes and deposited mistletoe-seeds, and watching phainopepla behavior, I found that mistletoes were aggregated within host trees, and that seed deposition was highest in tall and infected hosts. Likewise, phainopeplas perched preferentially in these trees, creating a positive feedback in which highly infected trees received many seeds and were likely to become reinfected. However, phainopeplas spent less time in trees than it takes for a seed to pass through their guts, which suggested interhost seed dispersal. I conducted a mistletoe removal experiment that confirmed a high degree of inter-host seed dispersal. These observations suggested that mistletoes would be aggregated at scales larger than individual trees. By mapping mistletoes and defecated seeds within a 4-hectare plot, I found that mistletoes were spatially correlated to at least 145 meters. Sampling at larger scales indicated that mistletoe prevalence was spatially correlated to approximately 1500 m and at scales larger than 4000 m. I also found that seed deposition increased with mistletoe prevalence in local neighborhoods. In conclusion, mistletoes are dispersal limited plants and are spatially correlated at several scales. Desert mistletoes are aggregated within hosts and their prevalence is spatially correlated at <1500 m and >4000 m. At the individual and local scales, their distribution is shaped by where birds defecate, which is influenced by host and neighborhood characteristics. At larger scales, their distribution may be primarily influenced by abiotic effects.