Theoretical work has shown that reduced phenotypic heterogeneity leads to population instability and can increase extinction potential, yet few examples exist of natural populations that illustrate how varying levels expressed diversity may influence population persistence, particularly during periods of stochastic environmental fluctuation. In this study, we assess levels of expressed variation and genetic diversity among demographically independent populations of tidewater goby (Eucyclogobius newberryi), show that reductions in both factors typically coincide, and describe how low levels of diversity contribute to the extinction risk of these isolated populations. We illustrate that, for this annual species, continuous reproduction is a safeguard against reproductive failure by any one population segment, as natural, stochastically driven salinity increases frequently result in high mortality among juvenile individuals. Several study populations deviated from the natural pattern of year-round reproduction typical for the species, rendering those with severely truncated reproductive periods vulnerable to extinction in the event of environmental fluctuation. In contrast, demographically diverse populations are more likely to persist through such periods through the continuous presence of adults with broader physiological tolerance to abrupt salinity changes. Notably, we found a significant correlation between genetic diversity and demographic variation in the study populations, which could be the result of population stressors that restrict both of these diversity measures simultaneously, or suggestive of a causative relationship between these population characteristics. These findings demonstrate the importance of biocomplexity at the population level, and assert that the maintenance of diversity contributes to population resilience and conservation of this endangered species.

Urban and agricultural development has resulted in drastically modified riverine corridors that are often considered to be detrimental to the recovery of anadromous salmonid populations. Although mitigation features (e.g., large wood and shallow-water areas) are frequently incorporated in flood control infrastructure to offset the impacts of streambank stabilization, little is known regarding their effectiveness and the habitat characteristics associated with enhanced nearshore rearing conditions in large rivers. We evaluated two measures of habitat use by emigrating juvenile Chinook Salmon Oncorhynchus tshawytscha relative to different shoreline types (rock revetment [riprap], mitigated, and natural) and analyzed associations between environmental characteristics and habitat occupancy using a large number of presence/absence samples in the lower Sacramento River, California. We found both measures of habitat use to be significantly higher at natural shorelines and those including mitigation features than at shorelines consisting predominantly of rock revetment. A predictive logistic regression model suggested that the density of woody material and inundated terrestrial vegetation, depth, and substrate type significantly affected habitat occupancy. Despite a moderate predictive capability (62% of correctly classified records in a leave-one-out simulation), the model was useful in identifying habitat characteristics associated with significantly increased habitat use in this large, low-gradient river, most notably the presence of instream cover (wood or vegetation), gently sloping streambanks, fine substrate, and variable nearshore current velocity. Conversely, habitat occupancy by juvenile Chinook Salmon diminished with large, rocky substrate and increased depth, characteristics favored by introduced predatory Smallmouth Bass Micropterus dolomieu. This study illustrates the value of incorporating mitigation features and identifies characteristics that enhance habitat use by emigrating juvenile Chinook Salmon.Urban and agricultural development along large, salmon-bearing streams of the Pacific Northwest has resulted in the need for and construction of extensive flood control features, such as levees and weirs, to protect valuable agricultural lands, public infrastructure, and private property. As a consequence, nearshore riverine habitat, which is

This study documents predation by the endangered tidewater goby, Eucyclogobius newberryi, upon the invasive New Zealand mudsnail, Potamopyrgus antipodarum, in Big Lagoon, California, USA. To estimate the prevalence of NZ mudsnails in the diet of tidewater goby, the gastric contents of 411 individuals, collected monthly from April 2009 to August 2010, were examined. NZ mudsnails were found in the digestive tract of tidewater goby that ranged in size from 14 to 52 mm total length, corresponding to post-settlement and nearly maximal sizes of this species. Unlike other native species which are unable to extract nutrition from these snails, tidewater goby fully digest this hard-shelled prey, as evidenced by the presence of shell fragments and complete absence of intact shells in the hind gut. The number of ingested NZ mudsnail ranged from 1 to 27 (mean 4.4), and ranged in length from 0.39 to 4.0 mm. The average size of ingested snails increased with fish length (r 2 = 0.42, P \ 0.001). NZ mudsnails were found in over 80% of individuals during the summer and fall of 2009, when the estimated population size of tidewater goby in Big Lagoon was greater than three million. This study documents the first instance of a native and endangered species that preys upon and utilizes the NZ mudsnail as a food source, and suggests that tidewater goby can exert substantial predation pressure upon NZ mudsnails and take advantage of these readily available novel prey items.

Extinction and colonization dynamics are critical to understanding the evolution and conservation of metapopulations. However, traditional field studies of extinction-colonization are potentially fraught with detection bias and have rarely been validated. Here, we provide a comparison of molecular and field-based approaches for assessment of the extinction-colonization dynamics of tidewater goby (Eucyclogobius newberryi) in northern California. Our analysis of temporal genetic variation across 14 northern California tidewater goby populations failed to recover genetic change expected with extinction-colonization cycles. Similarly, analysis of site occupancy data from field studies (94 sites) indicated that extinction and colonization are very infrequent for our study populations. Comparison of the approaches indicated field data were subject to imperfect detection, and falsely implied extinction-colonization cycles in several instances. For northern California populations of tidewater goby, we interpret the strong genetic differentiation between populations and high degree of within-site temporal stability as consistent with a model of drift in the absence of migration, at least over the past 20-30 years. Our findings show that tidewater goby exhibit different population structures across their geographic range (extinction-colonization dynamics in the south vs. drift in isolation in the north). For northern populations, natural dispersal is too infrequent to be considered a viable approach for recolonizing extirpated populations, suggesting that species recovery will likely depend on artificial translocation in this region. More broadly, this work illustrates that temporal genetic analysis can be used in combination with field data to strengthen inference of extinction-colonization dynamics or as a stand-alone tool when field data are lacking.