Spatiotemporal variability in flow determines the physical structures of habitat. During low flows, aquatic organisms can be exposed to reduced dissolved oxygen concentrations, increased water temperature, and desiccation, whereas at high flows, increased velocity and hydraulic forces on the streambed can be equally detrimental. These constraints create a mosaic of habitat that influences the distribution and abundance of aquatic biota. This mosaic can change due to stochastic events or those mediated by humans. Understanding how low and high flow conditions affect aquatic organisms is critical not only for advancing ecological knowledge but also for protecting imperiled aquatic species such as unionid mussels. The overall goal of this project was to examine how substrate and hydrologic conditions affect mussel habitat and to then use the resulting information combined with life‐history traits and shell morphology (i.e., sculpturing) to better understand how flow shapes mussel assemblage structure. Using quantile regression, we found that low values of relative shear stress (RSS), a measure of substrate stability, were associated with high mussel species richness and density. Change point analysis using threshold indicator taxa analysis (TITAN) indicated species‐specific preferences for varying levels of bed stability. These preferences were best explained by life‐history strategy and shell morphology based on the results of a principal component analysis. Using these results, we then present a conceptual model from which to derive expectations concerning taxonomic composition, life‐history strategy, and shell sculpture type based on the degree of substrate mobility using RSS and variability in RSS.
ABSTRACT1. The Leon River drainage, located in the Brazos River basin, has not been extensively surveyed for freshwater mussels (Family Unionidae). This is problematic given that three state-threatened species, Quadrula houstonensis, Quadrula mitchelli, and Truncilla macrodon, have historically occurred in this drainage and two are now candidates for protection under the US Endangered Species Act.2. Mussels were sampled qualitatively at 44 sites in the summer and fall of 2011 to determine whether these species were still extant in the Leon River. The distributions and abundances of species at present considered common were also examined. Shell length data were assessed to determine the overall viability of the mussel fauna within the Leon River drainage.3. In total, 2081 live mussels were collected representing 12 species, including the federal candidate species Quadrula houstonensis, but Lampsilis hydiana, Quadrula mitchelli and Truncilla macrodon were not collected. Overall mussel abundance and species richness was low and community composition was highly fragmented with riverine species largely occurring in the middle portion of the Leon River. There was evidence that population recruitment is occurring, but only for a few species.4. River impoundment, inadequate instream flows, and agricultural practices are probable causes of the changes in mussel species composition. Further studies are needed to evaluate the impacts of reservoir releases on mussel persistence within this basin and in areas where droughts and low stream flow are commonplace. More information is needed on how agricultural practices affect mussel communities; the information that is currently available does little in the way of identifying factors that can be managed at site or reach scales. Studies that address these knowledge gaps will help resource managers to design more effective strategies to protect mussel populations within and outside this basin.
Methods for monitoring bird nests might influence rates of nest predation, but the effects of various methods (e.g., visual markers and observer visitation rates) are often separately investigated among disparate avian taxa and geographic regions. Few investigators have explored the potential effects observers might have on nest success of grassland birds, despite concerns regarding population declines of these species in North America. We examined the possible effects of three monitoring techniques on daily nest survival of Lark Sparrows (Chondestes grammacus): (1) presence or absence of visible markers near nests, (2) observer visitation frequency, and (3) presence or absence of data loggers in nests. We monitored 113 Lark Sparrow nests during the 2009 breeding season. Of these nests, 88.5% failed due to predation, abandonment, weather, or unknown causes, yielding an overall nest success estimate of 9.8% based on daily survival estimation. Main effects of each monitoring technique appeared in top (ΔAICc <2) logistic exposure models. However, 95% confidence intervals around parameter estimates for each technique included zero, indicating no significant effects on daily nest survival. Our results suggest that the nest‐monitoring techniques we used had no effect on Lark Sparrow nest success and, if true, nest survival of other songbirds in arid grasslands of the Great Plains may also be unaffected by cautious nest monitoring. However, we cannot rule out the possibility that any effects of the various techniques in our study were masked by locally intense nest predation. Therefore, additional study is needed to determine if there may be observable variation in nest survival among various nest‐monitoring treatments in other areas of the southern Great Plains where nest predation is less frequent.
Human‐mediated threats have led to the rapid decline of species inhabiting freshwater ecosystems, and among the groups most affected are freshwater mussels of the family Unionidae. As a result, species translocation is increasingly used in conservation programmes, yet experimental evidence documenting the success of this strategy is limited. The goal of this study was to examine the effects of translocation on survival probability, shell growth and body condition of a state‐threatened (Quadrula houstonensis) and common (Amblema plicata) species of freshwater mussel in the San Saba River, located in a West Gulf Coastal Plain river system, USA. Survival probability estimated from a joint live and dead encounter model was high (> 0.85) and varied by treatment for both species. However, differences in survival probability between resident and transplant treatments were relatively small for A. plicata (0.01) and Q. houstonensis (0.12). Generalized additive mixed models of yearly proportional growth and linear mixed models of Fulton's K index for A. plicata varied by treatment and were lower in transplant treatments. Shell growth of Q. houstonensis was unaffected by translocation; whereas, Fulton's K was higher in the transplant treatment. Methods used to translocate mussels were important factors leading to high survival and limited impacts to shell growth and body condition in this study. Differences in shell growth rate between treatments are attributed to possible differences in habitat quality between sites. Our results demonstrate that A. plicata and Q. houstonensis are tolerant of translocation, despite the broad assumption that translocation is detrimental to mussels. Thus, there is a continuing need to study species' responses to translocation to test and improve the ecological soundness of this strategy, particularly because climate change and other human stressors will exacerbate the need to implement conservation measures such as translocation in future decades.
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