Modelling patterns of coexistence of three congeneric headwater fishes
1. Mechanisms driving patterns of occurrence and co-occurrence among North
American freshwater fishes are poorly understood. In particular, the influence of
biotic interactions on coexistence among stream reaches and their effects on regional
species distribution patterns is not well understood for congeneric headwater
2. Occupancy models provide a useful framework for examining patterns of co-occurrence
while also accounting for imperfect detection. Occupancy models may
be extended to test for evidence that a dominant species influences the occurrence
of a subordinate species and thus evaluate support for the hypothesis that
species interactions drive patterns of coexistence.
3. We examined patterns of occurrence and co-occurrence at the stream-reach scale
among three species of darters (Percidae: Etheostomatinae) that occupy headwater
streams within a Gulf Coastal Plain drainage in the south-eastern U.S.A. We
assessed species occurrences at 97 sites in first- to third-order streams on one
occasion each and used data from four sub-reaches sampled with equal effort at
each site to estimate species-specific detection probabilities. Following sampling,
a suite of habitat variables was collected at three equidistant points along each
of the three transects established within a sub-reach. Coarse (stream-segment,
catchment, network) scale variables were also incorporated using geospatial data.
Single-species and two-species occupancy models were used to examine patterns
of occupancy and coexistence.
4. The occupancy of each species was influenced by distinct habitat variables.
Goldstripe darters (Etheostoma parvipinne) were constrained by a stream size gradient,
groundwater input appeared to influence the occurrence of Yazoo darters
(Etheostoma raneyi), and local habitat heterogeneity (e.g. variation in depth
and current velocity) appeared to influence the occupancy of redspot darters
5. We found no evidence that the presence of one species influenced the occurrence
of another within a stream-reach based on two-species occupancy models.
Rather, species co-occurrences were best explained as independent occurrences
within a stream-reach according to species-specific habitat associations.