ARS | Publication request: Food Abundance Determines Distribution and Density of A Frugivorous Bird Across Seasons

Submitted to: Oikos
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 31, 2010
Publication Date: January 11, 2011
Citation: Crampton, L.H., Longland, W.S., Murphy, D.D., Sedinger, J.S. 2011. Food abundance determines distribution and density of a frugivorous bird across seasons. Oikos. 120:65-76.

Interpretive Summary: Although it may seem clear that availability of food strongly influences where a particular species of animal occurs and how many of that species can occur in a given area, the importance of food abundance relative to many other factors that can influence animal distributions is seldom evaluated using rigorous scientific methodology. We conducted a three-year study of a fruit-eating bird, the phainopepla, that occurs in patchily-distributed desert woodlands with either acacia or mesquite trees and eats almost exclusively the berries of parasitic mistletoe plants that grow on these trees. Phainopeplas disperse mistletoe seeds to new branches and to new trees that were previously uninfected by mistletoe, and therefore have major effects of populations of both desert mistletoe and their tree hosts. We conducted a study to determine the relative influence of food availability versus various habitat features (e.g., type of trees, average tree height, size of woodlands) on which specific desert woodlands support phainopeplas and in what numbers. Mistletoe berry (i.e., food) abundance affected phainopepla distribution most strongly, but woodland isolation (i.e., distance to the next nearest woodland patch) and tree height had lesser effects. The birds were likely to occur in less isolated woodlands with tall trees after food abundance was taken into consideration.

Technical Abstract: Although food abundance is a principal determinant of distribution and abundance of many animals, most previous studies have not quantitatively assessed its importance relative to other factors that can determine species distributions. We estimated frugivorous phainopepla (Phainopepla nitens) occupancy and density, food density, and vegetation structure on transects in fragmented mesquite and acacia woodlands over three years. Initial occupancy in winter 2002 was high (0.87+0.047), and primarily positively correlated with food abundance and woodland area (Akaike weights wi = 0.998 and 0.750 respectively). Woodland area more strongly influenced occupancy where food was scarcer. Phainopepla density in both seasons was strongly positively correlated with food abundance, especially in the 2002 drought when density was higher (wi = 1.0 for food and year). Extinction probability (patches vacated) was low (0.078+0.040), and principally influenced by phainopepla density (wi = 0.968) and tree height (wi = 0.749). Colonization probability was low (0.15+0.034) and determined by vegetation structure (wi = 1 .0). These results suggest that for an animal occupying a highly fragmented landscape, distribution and density at the habitat patch scale are driven by food abundance and affected to lesser degrees by habitat fragmentation and vegetation structure.