Niche flexibility and trout–galaxiid co-occurrence in a hydrologically diverse riverine landscape

Gorges partition diversity within New Zealand flathead Galaxias populations

Understanding the landscape factors governing population connectivity in riverine ecosystems represents an ongoing challenge for freshwater biologists. We used DNA sequence analysis to test the hypothesis that major geomorphological features underpin freshwater-limited fish diversity in a tectonically dynamic region of New Zealand. Phylogeographic analysis of 101 Galaxias depressiceps cytochrome b sequences, incorporating 55 localities from southern New Zealand, revealed 26 haplotypes, with only one shared among rivers. We detect strong hierarchical genetic differentiation both among and within river systems. Genetic structuring is particularly pronounced across the Taieri River system (63 individuals from 35 sites, 18 haplotypes), with 92% of variation partitioned among locations. Distinctive within-river genetic clusters are invariably associated with major subcatchment units, typically isolated by substantial gorges. The anomalous distribution of a single lineage across a major drainage divide is consistent with local, tectonically driven headwater capture. We conclude that major landscape features such as gorges can strongly partition riverine fish diversity and constrain freshwater biodiversity.

Habitat size influences community stability

Mechanisms linked to demographic, biogeographic, and food-web processes thought to underpin community stability could be affected by habitat size, but the effects of habitat size on community stability remain relatively unknown. We investigated whether those habitat-size-dependent properties influenced community instability and vulnerability to perturbations caused by disturbance. This is particularly important given that human exploitation is contracting ecosystems, and abiotic perturbations are becoming more severe and frequent. We used a perturbation experiment in which 10 streams, spanning three orders of magnitude in habitat size, were subjected to simulated bed movement akin to a major flood disturbance event. We measured the resistance, resilience, and variability of basal resources, and population and community-level responses across the stream habitat-size gradient immediately before, and at 0.5, 5, 10, 20, and 40 d post-disturbance. Resistance to disturbance consistently increased with stream size in all response variables. In contrast, resilience was significantly higher in smaller streams for some response variables. However, this higher resilience of small ecosystems was insufficient to compensate for their lower resistance, and communities of smaller streams were thus more variable over time than those of larger streams. Compensatory dynamics of populations, especially for predators, stabilized some aspects of communities, but these mechanisms were unrelated to habitat size. Together, our results provide compelling evidence for the links between habitat size and community stability, and should motivate ecologists and managers to consider how changes in the size of habitats will alter the vulnerability of ecosystems to perturbations caused by environmental disturbance.

First study of food webs in a large glacial river: the trophic role of invasive trout

Abstract The aim of this study was to determine the food webs structure of a large Patagonian river in two river sections (Upstream and Midstream) and to evaluate isotopic overlap between native and introduced species. We used stable isotope analyses of δ15N and δ13C and stomach content. The Upstream section had a more complex food webs structure with a greater richness of macroinvertebrates and fish species than Midstream. Upstream basal resources were dominated by filamentous algae. Lake Trout were found to have a higher trophic position than all other fish species in that area although, the most abundant fish species, were Rainbow Trout. Depending on the life stage, Rainbow Trout shifted from prey to competitor/predator. In the Midstream section, the base of the food webs was dominated by coarse particulate organic matter, and adult Rainbow Trout had the highest trophic level. Isotopic values changed among macroinvertebrates and fish for both areas. The two most abundant native and invasive species — Puyen and Rainbow Trout — showed an isotopic separation in Midstream but did not in Upstream areas. The presence of invasive fish that occupy top trophic levels can have a significant impact on native fish populations that have great ecological importance in the region.