Ecological and genetic assessment of spatial structure among replicate contact zones between two topminnow species

Abstract

The spatial structure of contact zones is often described as disjunct, diffuse or mosaic and presumed to be related to underlying ecological gradients. However, the ecology of contact zones, how they are structured, and if that structure is predictable based on the strength and nature of ecological gradients is unknown. Large spatial scales and the unreplicated nature of many of the best studied contact zones has made it difficult to codify broader ecological patterns. Freshwater stream fish contact zones have the advantage of being potentially replicated with well defined boundaries and predictable linear gradients (river continuum concept). We sampled four replicate topminnow (Fundulus olivaceus and F. notatus) contact zones in Gulf of Mexico drainages. In each, we quantified contact zone spatial structure and the strength of ecological gradients (habitat, physicochemical variables and fish community functional traits). All three types of contact zone structure were represented. Systems with weaker gradients had diffuse contact zones, low species richness and were numerically dominated by generalist species. Rates of hybridization were also variable among systems. There was no hybridization detected in the mosaic zone while hybrids were found at most of the co-occurrence sites in the diffuse and disjunct zones. Overall, local ecology clearly influences contact zone structure and the two species interact in fundamentally different ways in these four systems.

The emblematic minnows of the North American Great Plains: A synthesis of threats and conservation opportunities

“…Sharpnose shiner has apparently been extirpated from the North and South Wichita rivers of the Red River basin (Wilde & Urbanczyk, ); we follow Cross, Mayden and Stewart () in treating this as a native population although introduction of individuals from the Brazos River basin as bait bucket fish has also been postulated (Hall, ; Miller, ). The Sabine shiner comprises three disjunct populations (Figure d), relatively stable populations in a number of rivers in the Gulf Coast lowlands, from the San Jacinto of south‐eastern Texas to the Sabine basin in south‐western Louisiana (Heins, ; Hubbs, ; Hubbs et al., ; Schaefer, Duvernell & Kreiser, ; Thomas et al., ; Williams & Bonner, ). The species has been recorded irregularly and apparently uncommonly in the White, Black and St. Francis drainages of north‐central Arkansas and south‐east Missouri (Bounds, ; Matthews & Harp, ; McAllister, Starnes, Raley & Robison, ; Robison & Beadles, ).…”

Variability in movement dynamics of topminnow (Fundulus notatusandF. olivaceus) populations

“…A primary motivation for conducting this movement study is that F. notatus and F. olivaceus have broadly overlapping geographic ranges, often co‐occurring in the same drainages where they encounter one another within zones of contact (Duvernell et al. 2007; Schaefer et al. 2011).…”

Hatch success and temperature-dependent development time in two broadly distributed topminnows (Fundulidae)

“…Reproductive phenology of the species overlaps, evidenced by low levels of hybridization observed throughout the range (Duvernell et al 2006;Vigueira et al 2008;Schaefer et al 2011). While hybridization rates are low, isolating mechanisms and ecological differences between the species have been elusive.…”

“…Both are abundant in coastal drainages along the northern Gulf of Mexico, USA (30°N latitude). F. notatus ranges north into Great Lakes drainages (43°N latitude), while F. olivaceus is only found as far north as the Ozark uplands (38°N latitude) (Schaefer and Walters 2010;Schaefer et al 2011) (Fig. S1).…”