Threshold effects of habitat fragmentation on fish diversity at landscapes scales

Abstract

Habitat fragmentation involves habitat loss concomitant with changes in spatial configuration, confounding mechanistic drivers of biodiversity change associated with habitat disturbance. Studies attempting to isolate the effects of altered habitat configuration on associated communities have reported variable results. This variability may be explained in part by the fragmentation threshold hypothesis, which predicts that the effects of habitat configuration may only manifest at low levels of remnant habitat area. To separate the effects of habitat area and configuration on biodiversity, we surveyed fish communities in seagrass landscapes spanning a range of total seagrass area (2-74% cover within 16 000-m landscapes) and spatial configurations (1-75 discrete patches). We also measured variation in fine-scale seagrass variables, which are known to affect faunal community composition and may covary with landscape-scale features. We found that species richness decreased and the community structure shifted with increasing patch number within the landscape, but only when seagrass area was low (<25% cover). This pattern was driven by an absence of epibenthic species in low-seagrass-area, highly patchy landscapes. Additional tests corroborated that low movement rates among patches may underlie loss of vulnerable taxa. Fine-scale seagrass biomass was generally unimportant in predicting fish community composition. As such, we present empirical support for the fragmentation threshold hypothesis and we suggest that poor matrix quality and low dispersal ability for sensitive taxa in our system may explain why our results support the hypothesis, while previous empirical work has largely failed to match predictions.

Trait sensitivities to seagrass fragmentation across spatial scales shape benthic community structure

“…Our work extends previous studies documenting fragmentation effects on seagrass community composition which have generally focused on environmental effects on community structure at one or two scales (e.g. microhabitat and patch) (Hovel & Lipcius, ; Irlandi, ; Yeager et al, ) to reveal the complex, interacting processes by which fragmentation may affect ecological communities. Differential responses of fauna to fragmentation across scales based on species traits suggest that habitat fragmentation may alter community structure by shifting selective pressures on species traits during multiple community assembly stages.…”

“…All sites were separated by at least 3 m. Sampling sites were located across 21 seagrass “landscapes” which were defined by 200 m × 80 m rectangles (matching common bed size and shape within our system). These landscapes were previously selected to represent independent gradients in both total seagrass cover (260–11,764 m 2 ) and landscape patchiness (1–75 individual patches; Table ; Yeager et al, ). Sampling sites in the current study were haphazardly placed across all 21 landscapes (3–7 sites per landscape), but always located within seagrass itself, and not the unvegetated matrix.…”

Habitat Connectivity of Fish in Temperate Shallow-Water Seascapes

“…Fundamental to ecology is an understanding of assemblage composition and interactions. Recent efforts have shifted toward examining connectivity between habitats (and habitat patches) within the seascape mosaic, in an effort to broaden applicability to marine spatial planning and comprehensive management, (e.g., Gillanders et al, 2003;Unsworth et al, 2009;Yeager et al, 2011Yeager et al, , 2016Caldwell and Gergel, 2013).…”

Historical photographs of captures of recreational fishers indicate overexploitation of nearshore resources at an oceanic island

“…Although coastal fish communities are negatively affected by many factors (pollution, habitat fragmentation and climate change; Poulard & Blanchard, ; Lawrence & Hemingway, ; Yeager et al, ), historical photographs provide valuable information about the effects of over‐exploitation, through the decrease in the mean size of target species and the reduction of the abundance of bentho‐demersal top predators, like groupers, but also can document the process of succession of species (Tuya et al, ).…”