Abstract
Species interactions and abiotic factors are important determinants of
abundance and distribution, but accounting for biotic interactions is
complicated by the fact that interactions occur at the individual-level
at unknown spatial scales. Ignoring individual-level interactions can
yield incorrect conclusions about biotic interactions when analyzing
aggregated count data or presence-absence data. We present a
hierarchical species distribution model that includes a Markov point
process in which an individual’s location is dependent upon both abiotic
variables and the locations of individuals of another species. The model
can be regarded as a thinned point process in which encounter
probability is a function of the distance between individual activity
centers and survey locations. We applied the model to spatial
capture-recapture data on two ecologically similar songbird species –
hooded warbler (Setophaga citrina) and black-throated blue
warbler (Setophaga caerulescens) – that segregate over a climate
gradient in the southern Appalachian Mountains, USA. In spite of coarse
spatial segregation and many ecological similarities between the two
species, we found minimal evidence of spatial competition. There were
strong, and opposing effects of climate on spatial variation in
population densities, but spatial competition did not influence their
distributions. A small simulation study indicated that the model can
identify the distinct effects of environmental variation and biotic
interactions on co-occurring species distributions. Unlike previous
statistical models that attempt to infer competition from species-level
co-occurrence data, the framework proposed here can be used to
investigate how population-level patterns emerge from individual-level
processes, while also allowing for inference on the spatial scale of
biotic interactions. Our finding of minimal spatial competition between
black-throated blue warbler and hooded warbler adds to the growing body
of literature suggesting that, contrary to early theory from
biogeography, abiotic factors may be more important than competition at
low-latitude range margins.