Overlapping trophic niches among co‐occurring amphipods from a cryptic species complex

Abstract

Ecological differences among species are usually associated with phenotypic differences that enable species to lessen interspecific competition. Many cryptic species co‐occur in communities, thus raising the question of their ecological equivalency. In the case of freshwater amphipods from the Hyalella azteca cryptic species complex, both ecological differences and overlaps have been reported among species that co‐occur in lakes. Since lakes are heterogeneous habitats that vary in space and time, it is possible that species minimise interspecific competition by occupying different trophic niches. We tested the hypothesis that amphipod species from the H. azteca complex co‐occur by feeding on different food resources in the littoral zone of boreal lakes. We compared the stable isotopes ratios (δ13C and δ15N) of amphipods from different species, and reconstructed their diets using mixing models. We showed that H. azteca amphipods from different species had isotopic compositions and habitat uses that greatly overlapped. Differences in isotopic composition often occurred, but they were also observed for amphipods of the same species sampled at different depths. This suggests that H. azteca amphipods fed mainly on resources that were available at the site where they occurred rather than distributed according to the occurrence of a specific resource. The overlap in diets suggests that H. azteca amphipods had overlapping trophic niches. Therefore, niche partitioning for food resources was not a process explaining the co‐occurrence of these species within these lakes. Competition for food resources was not likely an important mechanism to explain H. azteca species assemblages in communities. Each H. azteca species apparently fed on food items that were abundant in the littoral zone of lakes, meaning that competition for food might be avoided most of the time. If food scarcity ever happens, the period of scarcity is probably too short to allow the exclusion of one amphipod species by competitive exclusion because of the low differences in fitness between the species, allowing Hyalella cryptic species to co‐occur during the open‐water season. Neutral mechanisms and ecological drift might also influence assemblage dynamics.

Dinner with the roommates: trophic niche differentiation and competition in a mutualistic ant‐ant association

“…Here, we showed that their trophic niches also largely overlap, despite some evidence for niche partitioning. This resembles findings in a species complex of freshwater amphipods with overlapping isotopic signatures (Dionne et al ., 2017), and suggests that niche differences between co‐occurring species can be subtle, especially among closely related species. If species entirely overlap in their niche and fulfil similar functions in an ecosystem, they are considered ‘neutral species’ within (but not beyond) their functional group (McPeek, 2017).…”

“…If species entirely overlap in their niche and fulfil similar functions in an ecosystem, they are considered ‘neutral species’ within (but not beyond) their functional group (McPeek, 2017). Such neutral species can persist in the same community via random processes like ecological drift, or if competitive superiority is context‐dependent (Leibold & McPeek, 2006; Andersen, 2008; Dionne et al ., 2017; Gilbert & Levine, 2017). The weak, albeit significant, differentiation of dietary niches seems not sufficient to explain their co‐existence from a niche‐based view.…”

Geographic isolation facilitates the evolution of reproductive isolation and morphological divergence

“…We focused on the freshwater genus of talitrid amphipods, Hyalella (Smith 1874). The nominal species H. azteca (Gonzalez & Watling, ) has been found to contain extensive cryptic diversity (Dionne, Dufresne, & Nozais, ; Dionne, Vergilino, Dufresne, Charles, & Nozais, ; Vergilino, Dionne, Nozais, Dufresne, & Belzile, ; Witt & Hebert, ; Witt, Threloff, & Hebert, ). Also belonging to this genus are numerous morphologically distinct nominal species, each endemic to just a single locality (Baldinger, ; Baldinger, Shepard, & Threloff, ; Cole & Watkins, ; Stevenson & Peden, ; Witt et al., ).…”

“…azteca (Cole & Watkins, 1977;Stevenson & Peden, 1973;Witt et al, 2006), suggesting that reproductive isolation has allowed the two forms to coexist sympatrically without the endemic form going extinct due to introgression. This assertion is supported by al lack of evidence for niche partitioning among sympatrically occurring populations of Hyalella (Dionne et al, 2017). The presence of cryptic lineages, variation in the degree of geographic isolation among lineages, and the evidence of local adaptation in the narrowly distributed lineages makes Hyalella an ideal system for quantifying the factors that contribute to the evolution of reproductive isolation.…”

Trophic niche partitioning of dominant North‐Atlantic krill species, Meganyctiphanes norvegica, Thysanoessa inermis, and T. raschii

“…Where λ is the TL of the baseline used, i.e., TL = 1 for POM, δ 15 N consumer = δ 15 N of krill, δ 15 N base = δ 15 N of POM for each sampling date and Δ n is the estimated trophic discrimination factor for nitrogen. As trophic discrimination factors are undefined for krill and might be subject to uncertainty because of variability of the trophic discrimination factor with diet quality (Caut et al ), species (Philips et al ), season (Dionne et al ) and/or metabolic rate (McCutchan et al ), a trophic discrimination factor of 2‰ for nitrogen was chosen based on McCutchan et al () and Chew et al ().…”