Effects of Urbanization on Native Bird Species in Three Southwestern US Cities

Urbanization presents novel challenges to native species by altering both the biotic and abiotic environment. Studies have attempted to make generalizations about how species with similar traits respond to urbanization, although existing results are idiosyncratic across cities and often fail to account for seasonality. Here, we present a comparative study in three US cities: Fresno, California; Tucson, Arizona; and Phoenix, Arizona. Using presence-absence data to define regional bird species pools and urban assemblages in non-breeding (winter) and breeding (spring) seasons, we tested whether urban avian assemblages were a random subset of regional assemblages on the basis of both traits and phylogeny, and whether urbanization was associated with homogenization among avian assemblages. We found evidence for non-random trait filtering into urban assemblages, including of diet guilds, migratory status, and primary habitat, but filtering differed across cities and seasons, being strongest for diet and in Fresno. There was no evidence for non-random phylogenetic-based filtering in urban avian assemblages. Dissimilarity in species and diet guild composition within each season was higher between cities than between regional species pools. These findings show the potential for biotic differentiation as opposed to homogenization as the outcome of environmental filtering processes operating on species traits across cities and seasons

Stable isotope values and trophic analysis of invasive three-spined stickleback in Upper Lake Constance points to significant piscivory

The three-spined stickleback Gasterosteus aculeatus was introduced into Lake Constance in the 1940s and occupied a limited range until late 2012. Since then the species has expanded from a solely littoral habitat in Upper Lake Constance, but now makes seasonal migrations into the pelagic zone. This behavioral change has been accompanied by a drastic increase in stickleback abundance. In order to integrate information about feeding of sticklebacks in Upper Lake Constance over two consecutive years, stomach content analysis was combined with seasonal stable isotope analysis on two types of tissue (muscle and liver). Isotope values were also obtained for zooplankton, whitefish larvae and eggs. We calculated the contribution of potential food sources for sticklebacks’ diet using a Bayesian mixing model (SIMMR). Furthermore, we determined stickleback trophic position, and δ15N and δ13C values were compared with those of other fish species of Lake Constance. The results of the Bayesian model as well as the stomach content analysis showed clear evidence of stickleback predation on fish eggs and larvae. Stickleback δ15N values were elevated during winter and comparable to those of piscivorous pike, while δ15N values of zooplankton were reduced, and those of whitefish larvae were similar to those of sticklebacks after accounting trophic fractionation of N isotopes. Trophic position calculations further identified sticklebacks as piscivorous, while the δ13C values of the liver and stomach content analysis suggests that a benthic-pelagic species pair may exist in Lake Constance. These findings support the hypotheses that sticklebacks in Lake Constance can display piscivorous feeding behaviour on sympatric fish species, most likely whitefish larvae and eggs

Mercury Exposure Associated with Altered Plasma Thyroid Hormones in the Declining Western Pond Turtle (<i>Emys marmorata</i>) from California Mountain Streams

Mercury (Hg) is a global threat to wildlife health that can impair many physiological processes. Mercury has well-documented endocrine activity; however, little work on the effects of Hg on the thyroid hormones triiodothyronine (T3) and thyroxine (T4) in aquatic wildlife exists despite the fact that it is a sensitive endpoint of contaminant exposure. An emerging body of evidence points to the toxicological susceptibility of aquatic reptiles to Hg exposure. We examined the endocrine disrupting potential of Hg in the western pond turtle (<i>Emys marmorata</i>), a long-lived reptile that is in decline throughout California and the Pacific Northwest. We measured total Hg (THg) concentrations in red blood cells (RBCs) and plasma T3 and T4 of turtles from several locations in California that have been impacted by historic gold mining. Across all turtles from all sites, the geometric mean and standard error THg concentration was 0.805 ± 0.025 μg/g dry weight. Sampling region and mass were the strongest determinants of RBC THg. Relationships between RBC THg and T3 and T4 were consistent with Hg-induced disruption of T4 deiodination, a mechanism of toxicity that may cause excess T4 levels and depressed concentrations of biologically active T3