1. Biological resilience is of heightened concern in increasingly anthropogenic landscapes. Quantification of faunal resilience across a wide range of spatial scales and geographical areas is necessary to understand factors influencing the rate and degree of recovery, especially in fragmented ecosystems. 2. We evaluated the recovery of a riverine fish assemblage from a major diesel oil pipeline spill and associated fish kill in 37 km of the Reedy River, South Carolina, U.S.A. The fish assemblage was monitored at four disturbed sites within the fish kill zone and one upstream, undisturbed reference site over a 112-month (9.3-year) period following the disturbance. We used non-metric multidimensional scaling (NMS) ordination to evaluate change in fish assemblage structure among sites and to determine the degree of recovery in assemblage structure. 3. NMS ordination of species relative abundance in two dimensions represented 93% of the total variation in fish assemblage structure among samples and illustrated recovery of the fish assemblage. Initial dissimilarity in assemblage structure was evident between the disturbed sites and the reference site, reflecting high mortality from the oil spill. The disturbed sites as a group increased in similarity to the reference assemblage with time, while the reference assemblage remained relatively stable. Strongest similarity in assemblage structure between the disturbed group and the reference group was achieved by October 2000 (52 months post-disturbance), indicating recovery from the oil spill. Remaining variation in assemblage structure was consistent with longitudinal site position and comparable to that of an undisturbed reference river, attributable to inherent longitudinal variation along the 37-kilometre river section. 4. Recovery rate among sites varied in relation to proximity and connectivity to recolonisation sources on a landscape scale. Recovery of the uppermost disturbed site was faster than the other disturbed sites because of its proximity to the undisturbed main stem fish assemblage, whereas the three most downstream sites were slower to recover largely because of isolation by anthropogenic barriers. These observations illustrate the influence of fragmentation on fish assemblage resilience at large spatial scales.
South Atlantic coastal plain streams are unique and understudied freshwater environments that provide crucial habitats for a wide range of aquatic taxa. We investigated patterns in fish assemblages across South Carolina's coastal plain, and developed statistical models to identify the dominant multiscale abiotic environmental factors that influence assemblage structure. Cluster and indicator species analyses of fish assemblage data collected at 208 wadeable streams indicated the presence of four predominant species associations that commonly occur in the South Atlantic coastal plain, which we termed the (1) fluvial, (2) Eastern Mudminnow, (3) centrarchid, and (4) nonfluvial assemblages. A random forest analysis indicated that geographic gradients and instream habitat variables associated with velocity, channel form, stream size, depth, and large wood generally played a greater role in distinguishing fish assemblages than catchment land cover. A follow‐up principal component analysis showed that these instream habitat variables showed weak relationships with current anthropogenic land‐cover conditions. We suggest several possible explanations for our findings, including (1) a sustained dynamic equilibrium among catchment, riparian, and instream conditions over time may encourage assemblage partitioning among differential instream habitats, (2) past landscape conditions may have a greater influence on current instream habitat conditions and fish assemblage structure than current landscape conditions, (3) weak relationships between catchment land cover and instream habitats may be common in low‐elevation regions, and (4) seasonal variation in hydrologic patterns may dominantly influence instream habitat conditions and fish assemblage structure. Our study adds essential information towards a better understanding of how South Atlantic coastal plain fish assemblages respond to environmental factors across multiple spatial scales. Such knowledge will help improve management and conservation strategies, as well as assist in the development of appropriate indicators for standardized evaluations of ecological integrity.
Bartram's Bass" Micropterus sp. cf. coosae is endemic to the upper Savannah River basin of the southeastern United States and is threatened by hybridization with invasive Alabama Bass Micropterus henshalli. Bartram's Bass have been functionally extirpated from reservoirs, and hybrid individuals have been detected in several tributaries. However, the extent of introgression in tributaries is currently unknown. Our objectives were to (1) assess the distribution of Bartram's Bass, native Largemouth Bass M. salmoides, invasive Alabama Bass, and their hybrids in streams of the upper Savannah River basin and (2) quantify effects of abiotic variables on the distribution of each species. We sampled 154 locations in 2017 and 2018 and assigned genetic identity using hydrolysis probes and microsatellites. We used conditional inference trees to quantify variables affecting the occurrence of each species and hybrids. We observed widespread hybridization across the basin. Pure Bartram's Bass were collected at 27% (42) of sites, among which only 12 sites contained pure Bartram's Bass and no other congeners. Thirty sites where pure Bartram's Bass were collected contained hybrids. In the montane Blue Ridge ecoregion, occurrence of pure Bartram's Bass was negatively affected by low levels of local-scale developed land cover. In the lower-relief Piedmont ecoregion, pure Bartram's Bass were positively associated with watershed-scale forest land cover and stream gradient. Distance from a reservoir was positively associated with occurrence of pure Bartram's Bass in both ecoregions. Pure Bartram's Bass are likely to occur with high probability in only 16% of nonimpounded stream segments; this represents a conservative estimate, and the true number is likely lower. However, future work
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