Niche width collapse in a resilient top predator following ecosystem fragmentation

Abstract

Much research has focused on identifying species that are susceptible to extinction following ecosystem fragmentation, yet even those species that persist in fragmented habitats may have fundamentally different ecological roles than conspecifics in unimpacted areas. Shifts in trophic role induced by fragmentation, especially of abundant top predators, could have transcendent impacts on food web architecture and stability, as well as ecosystem function. Here we use a novel measure of trophic niche width, based on stable isotope ratios, to assess effects of aquatic ecosystem fragmentation on trophic ecology of a resilient, dominant, top predator. We demonstrate collapse in trophic niche width of the predator in fragmented systems, a phenomenon related to significant reductions in diversity of potential prey taxa. Collapsed niche width reflects a homogenization of energy flow pathways to top predators, likely serving to destabilize remnant food webs and render apparently resilient top predators more susceptible to extinction through time.

Figure 1

Each symbol represents an individual snapper and the lines represent the convex hull area used as a measure of niche width. White triangles are individuals from an unfragmented site, grey triangles from a partially fragmented site, and black triangles from a highly fragmented site (Cross Harbour, Sucking Fish, and Marsh Harbour, respectively, in Table 1).

Figure 2

Niche width plotted as a function of percent fragmentation, with niche width estimated as convex hull area (TA) encompassing 13 individuals in each population. Each symbol represents the estimated niche width of a gray snapper population in one of 11 tidal creek systems, with triangles corresponding to the three sites depicted in Fig. 1.

Figure 3

Stable isotope ratio data depicting resident taxa from food webs in one highly fragmented (Cross Harbour) and one unfragmented (Cherokee) tidal creek. Each point on the graph represents the mean value of 2–13 individuals of an individual species, with error bars around the mean omitted for simplicity. Diamonds = fish, squares = crustaceans, triangles = mollusks, and circles = other invertebrate taxa. The enlarged black diamond identifies the mean position of gray snapper in each food web.

Figure 4

Conceptual depictions of the ‘sink’ food webs (i.e. all energy flow pathways that culminate in grey snapper) in a typical fragmented and unfragmented creek system, as based on > 3500 individual isotope samples, hundreds of stomach content analyses, and quantitative floral and faunal surveys. Identity of intermediate consumers are not given to provide for a more generalized view of food web architecture.