Detecting spatial ontogenetic niche shifts in complex dendritic ecological networks

Abstract/Summary

Ontogenetic niche shifts (ONS) are important drivers of population and community dynamics, but they can be difficult to identify for species with prolonged larval or juvenile stages, or species that inhabit continuous habitats. Most studies of ontogenetic niche shifts focus on single transitions among discrete habitat patches at local scales. However, for species with long larval or juvenile periods, affinity for particular locations within connected habitat networks may differ among cohorts. The resulting spatial patterns of distribution can result from a combination of landscape-scale habitat structure, position of a habitat patch within a network, and local habitat characteristics – all of which may interact and change as individuals grow. We estimated such spatial ONS for spring salamanders (Gyrinophilus porphyriticus), which have a larval period that can last four years or more. Using mixture models to identify larval cohorts from size-frequency data, we fit occupancy models for each age class using two measures of the branching structure of stream networks, three measures of stream network position. Larval salamander cohorts did indeed have different responses to the position of a site within the network, and the strength of these responses depended on the basin-wide spatial structure of the stream network. The isolation of a site had a stronger effect on occupancy in watersheds with more isolated headwater streams, while the catchment area, which is associated with gradients in stream habitat, had a stronger effect on occupancy in watersheds with more paired headwater streams. Our results show that considering the spatial structure of habitat networks can provide new insights on ontogenetic niche shifts in long-lived species.

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