Species co-occurrence patterns among cavity nesting birds across seasons and in a landscapes with varying habitat complexity

Abstract

Species co-occurrence patterns inform the selection of surrogate species in conservation planning because co-occurrence is often a non-random process. Probabilistic models can determine whether a species co-occurrence is significant (i.e., non-random) where significant positive co-occurrence indicates potential mutualisms and negative co-occurrence indicates competition (1). One challenge is that co-occurrence is dynamic over time (2), especially in successional habitats or seasonally with migratory species.

In complex landscapes, habitat quality may be related to the composition and complexity of available cover types (3) because species may benefit from exploiting resources in multiple cover types. This is known as landscape complementation, when non-substitutable resources, such as food and nest sites, are acquired in different cover types to meet requirements across the annual cycle. Species that rely on landscape complementation may be ideal surrogate species because their varied habitat requirements may lead to co-occurrences with a diverse set of species. This project focuses on Red-headed Woodpeckers (Melanerpes erythrocephalus, hereafter RHWO), a species of conservation concern, at an active military base in Virginia, USA where prescribed fire and timber harvests are common. RHWO has experienced a 68% range-wide population loss over the last 45 years and is designated as near threatened by the IUCN. Our objectives were to (1) determine if RHWO prefer areas with multiple rather than one homogenous cover type and (2) if they are effective surrogates for cavity nesting species. We compared RHWO abundance in sites with varying amounts of closed canopy forest, savanna and wetland. We also assessed species co-occurrence in the nonbreeding and breeding seasons as well as in homogenous (closed canopy forest) and heterogeneous (mix of closed canopy, savanna and wetland) habitats.

Our results indicate that sites with a mix of cover types have higher RHWO abundance in both summer and winter. We also found more significant positive species associations among cavity nesting birds in the winter than in the breeding season, and in heterogeneous habitats. Specifically, 25% of species co-occurrences in winter were non-random compared with only 7% in the breeding season. None of the co-occurrence patterns were significant in closed canopy forests whereas 5-9% were significant in sites that had a mix of closed canopy forest and wetland or closed canopy and savanna habitat. White-breasted nuthatch (Sitta carolinensis), a weak excavator, was positively associated with more species than any other, and RHWO had the second most significant positive species co-occurrences. Our preliminary results indicate that managing for a mosaic of suitable habitats is important for sustaining declining RHWO populations across the annual cycle, and that this species may be a viable surrogate for other cavity nesting species.