Henry R. Mushinsky

Ontogeny of Water Snake Foraging Ecology

Using an index of relative importance we analyzed the stomach contents of over 300 water snakes (Nerodia spp.). Ontogenetic changes in prey consumption are most striking in Nerodia erythrogaster (number with food in gut = 44) and N. fasciata (N = 72). Prey of these two species changes from fish to frogs as the snakes exceed a snout-vent length of 50 cm. Nerodia rhombifera (N = 159) and N. cyclopion (N = 65) primarily eat fish throughout their life. However, with maturity and increased body size both species change portions of their diets. Nerodia rhombifera preys upon larger fish which occupy deeper, open-water habitats, when the snakes exceed 80 cm. Nerodia cy- clopion eats a larger proportion of centrarchid fish as its body size increases. Small prey are found in the stomachs of most size-classes of all four snake species. Regression analysis indicates that all four species eat larger prey as they mature. However, the largest individuals are females, and in two of the four species the large females eat a different array of prey than smaller nonspecific males. The size sexual dimorphism does not reduce the overlap in the diets of the two species that eat anurans as adults.

Broad‐scale latitudinal patterns of genetic diversity among native European and introduced house sparrow (Passer domesticus) populations

Introduced species offer unique opportunities to study evolution in new environments, and some provide opportunities for understanding the mechanisms underlying macroecological patterns. We sought to determine how introduction history impacted genetic diversity and differentiation of the house sparrow (Passer domesticus), one of the most broadly distributed bird species. We screened eight microsatellite loci in 316 individuals from 16 locations in the native and introduced ranges. Significant population structure occurred between native than introduced house sparrows. Introduced house sparrows were distinguished into one North American group and a highly differentiated Kenyan group. Genetic differentiation estimates identified a high magnitude of differentiation between Kenya and all other populations, but demonstrated that European and North American samples were differentiated too. Our results support previous claims that introduced North American populations likely had few source populations, and indicate house sparrows established populations after introduction. Genetic diversity also differed among native, introduced North American, and Kenyan populations with Kenyan birds being least diverse. In some cases, house sparrow populations appeared to maintain or recover genetic diversity relatively rapidly after range expansion (<50 years; Mexico and Panama), but in others (Kenya) the effect of introduction persisted over the same period. In both native and introduced populations, genetic diversity exhibited large‐scale geographic patterns, increasing towards the equator. Such patterns of genetic diversity are concordant with two previously described models of genetic diversity, the latitudinal model and the species diversity model.

Epigenetic Variation May Compensate for Decreased Genetic Variation with Introductions: A Case Study Using House Sparrows (Passer domesticus) on Two Continents

Epigenetic mechanisms impact several phenotypic traits and may be important for ecology and evolution. The introduced house sparrow (Passer domesticus) exhibits extensive phenotypic variation among and within populations. We screened methylation in populations from Kenya and Florida to determine if methylation varied among populations, varied with introduction history (Kenyan invasion <50 years old, Florida invasion ~150 years old), and could potentially compensate for decrease genetic variation with introductions. While recent literature has speculated on the importance of epigenetic effects for biological invasions, this is the first such study among wild vertebrates. Methylation was more frequent in Nairobi, and outlier loci suggest that populations may be differentiated. Methylation diversity was similar between populations, in spite of known lower genetic diversity in Nairobi, which suggests that epigenetic variation may compensate for decreased genetic diversity as a source of phenotypic variation during introduction. Our results suggest that methylation differences may be common among house sparrows, but research is needed to discern whether methylation impacts phenotypic variation.

Foraging Ecology and Prey Size in the Mangrove Water Snake, Nerodia fasciata compressicauda

We report on the foraging ecology of a population of mangrove water snakes, Nerodiafasciata compressicauda, from central Florida. Quantitative estimates of available prey allowed us to evaluate prey selection ontogenetically. Large snakes were more discriminating among prey sizes than were small snakes. To estimate snake gape, we used the general formula for calculating the circumference of an ellipse whose major and minor axes corresponded to jaw length and jaw width. The computed gape index was used to evaluate size of ingested prey relative to size of the predator. Large snakes ingest smaller prey relative to computed gape size. Lastly, we consider several alternative explanations for our findings; we conclude that a major benefit of modifications of the snake skull is the reduction in time spent foraging and hence exposure to potential predators.

The Role of Temperature on the Behavioral and Ecological Associations of Sympatric Water Snakes

Several aspects of the thermal ecology of five sympatric water snake species were explored quantitatively. Body temperatures of captured snakes are compared to ambient temperatures at the capture site and evaluated in terms of daily and seasonal activity patterns. Three species of water snakes, Nerodia rhombifera, N. cyclopion and N. fasciata, account for 86% of this snake community. Each species has a relatively distinctive usage of the arboreal and aquatic habitats and each has a characteristic activity pattern. A fourth congener, Nerodia erythrogaster is uniformly nocturnal (77% of census data) and aquatic (88%). The fifth species, Regina grahamii was seasonally asynchronous with Nerodia spp. and was the most arboreal member of the community (84%). N. cyclopion was the only species active in all months of the year. During the cool months (Nov.-March) it was primarily a diurnal perching snake (70-90% of census), and during the summer N. cyclopion was nocturnal and aquatic (82100%). The trend toward becoming nocturnally active in the summer was characteristic of all Nerodia spp. N. rhombifera, however, became much more arboreal during the summer months. N. fasciata was uniform in habitat usage for most of the year (60% aquatic) but did not become nocturnal until June a month or two after congeners. Observed year to year variations in mean body temperature of each species reflected climatic fluctuations over the four years of study. The data indicate that only one species, N. cyclopion, makes seasonal adjustments of daily activity periods and arboreal tendencies that maintain fairly uniform body temperatures even through the cool months. When the thermal axis of the fundamental niche is considered in light of available data on the food, time, and space axes, the influence of temperature on this ectothermic community appears to be significant. In summary, we maintain that the underlying mechanism for the asynchronous daily and seasonal patterns of habitat use is specific differences in thermal niches. The two most ecologically similar species do not appear to be in competition for thermal niche space, but rather partition the thermal resource to reduce interference competition.

HABITAT FRAGMENTATION AND THE ABUNDANCES OF VERTEBRATES IN THE FLORIDA SCRUB

Based on presence/absence of species, we proposed previously that large fragments of the Florida sand pine scrub habitat do not possess greater conservation value for vertebrates than do archipelagos of small and medium-sized fragments. We reexamined the values of fragments of different sizes, based this time on abundances of species. We asked whether abundances tend to decrease with decreasing scrub size or are more strongly related to other scrub attribute(s). We censused vertebrates with trap arrays and by direct observation, and we measured 13 potentially important environmental attributes related to area, isolation, and habitat structure in 16 scrubs distributed along the Lake Wales Ridge of central Florida. We cor- related abundance of species with the 13 attributes. We calculated the evenness of relative abundance distributions and grouped scrubs based on the shapes of relative abundance distributions. We determined whether or not members of different groups of scrubs pos- sessed different values for any attribute. Finally, we examined both densities of individuals and dispersions of species among large, medium, and small scrubs. Abundance was related to many of the attributes. Abundances of 11 of 18 species were correlated positively with area. Percentage of variation explained by area generally was low, and several of the rare species apparently maintain relatively large populations in small scrubs. Abundances of five species were correlated negatively with measures of isolation. Abundances of three species were correlated both positively and negatively with measures of habitat structure. Environmental attributes related to isolation and habitat structure were correlated most strongly with both evenness and groupings of relative abun- dance distributions. Total density of individuals (i.e., numbers of captures per trap array) could not be shown to differ among scrubs of different sizes. Individually, the species that reached their highest densities in relatively small scrubs often were the ones considered to be rare. Spatial distributions of species tended to become more homogeneous as area decreased, but the strength of this effect varied among species, perhaps accounting, at least in part, for the different relationships between abundance and area among species. Our results could lead to broad generalizations that seem to be relevant to scrub con- servation. For example, because most species are relatively abundant in large scrubs, large scrubs should be selected for reserves over small ones. We suggest, however, that our results actually best serve as a caution about the use of generalizations, based on data from one or a few species, for conservation planning in general. Our results clearly show that the benefits gained for certain species from focusing on the preservation of larger scrub frag- ments could be offset by harm done to other species, especially rare species. We conclude, therefore, that selection of a system of scrub reserves should be based, in large part, on knowledge of the biologies of as many of the resident organisms as possible. We suggest, however, that any reserve selection procedure for the scrub habitat probably is outmoded, and that ecologists should not even engage in debate about the conservation value of small scrub fragments. When a habitat has declined as precipitously as scrub has, and truly large remaining fragments are, at best, extremely rare, then smaller fragments are likely to be of considerable value, no matter how they compare to larger fragments.

Measuring the success of wildlife community restoration

It is difficult to measure the success of wildlife community restoration when the restoration goal concerns a relatively broad geographic area, rather than a particular piece of land, since many restored sites need to be compared to many reference sites simultaneously. A review of the methods used to measure success in previous restoration efforts indicated the potential value of multimetric methods to make the comparison. We designed a new method that retains some of the advantages of multimetric methods but also removes some of the associated problems. The new method was applied to data from 30 restored sites (phosphate-mined land) and 30 reference sites in central Florida (USA), and it showed the difference in wildlife composition between restored and reference sites to be large, relative to the maximum possible difference. Alternative methods were applied to the same data. Multivariate methods were unable to measure success adequately. Percentage similarity analysis yielded results seemingly comparabl...

Fire increases variance in genetic characteristics of Florida Sand Skink (Plestiodon reynoldsi) local populations

Fire is a complex event that maintains many ecological systems. The Florida Sand Skink (Plestiodon reynoldsi) is precinctive to Florida Scrub, a habitat that is maintained by infrequent fire. We characterize the effect of fire on genetic diversity and genetic differentiation at eight microsatellite loci in the Florida Sand Skink (n = 470) collected from 30 replicate sites over three ‘time since last fire’ categories at the Archbold Biological Station. Long unburned sites had greater allelic richness and expected heterozygosity than either recently or intermediately burned sites. More recently, burned sites had greater standard deviations of allelic richness and private allelic richness. Expected heterozygosity positively correlated with ‘time since fire’ (r = 0.36, P = 0.05) and abundance (r = 0.53, P = 0.002). There was a significant spatial component to genetic differentiation, and results indicate individuals rarely disperse >1 km. Genetic differentiation was positively correlated with geographic distance in long unburned units (r = 0.59, P = 0.04), yet this relationship was disrupted by fire in recently (r = 0.00, 1) and intermediately (r = −0.81, 0.05) burned areas. Simulations indicate that demographic changes to a local population could have generated the observed differences among ‘time since fire’ categories. Our findings indicate that infrequent fire may be beneficial to the Florida Sand Skink and that local populations begin to recover from changes attributable to the fire after 10 years. Too frequent fires may reduce genetic diversity because it may take multiple generations for local populations to recover.

ESTIMATES OF MINIMUM PATCH SIZE DEPEND ON THE METHOD OF ESTIMATION AND THE CONDITION OF THE HABITAT

Minimum patch size for a viable population can be estimated in several ways. The density-area method estimates minimum patch size as the smallest area in which no new individuals are encountered as one extends the arbitrary boundaries of a study area outward. The density-area method eliminates the assumption of no variation in density with size of habitat area that accompanies other methods, but it is untested in situations in which habitat loss has confined populations to small areas. We used a variant of the density area method to study the minimum patch size for the gopher tortoise (Gopherus polyphemus) in Florida, USA, where this keystone species is being confined to ever smaller habitat fragments. The variant was based on the premise that individuals within populations are likely to occur at unusually high densities when confined to small areas, and it estimated minimum patch size as the smallest area beyond which density plateaus. The data for our study came from detailed surveys of 38 populations of the tortoise. For all 38 populations, the areas occupied were determined empirically, and for 19 of them, duplicate surveys were undertaken about a decade apart. We found that a consistent inverse density area relationship was present over smaller areas. The minimum patch size estimated from the density-area relationship was at least 100 ha, which is substantially larger than previous estimates. The relative abundance of juveniles was inversely related to population density for sites with relatively poor habitat quality, indicating that the estimated minimum patch size could represent an extinction threshold. We concluded that a negative density area relationship may be an inevitable consequence of excessive habitat loss. We also concluded that any detrimental effects of an inverse density area relationship may be exacerbated by the deterioration in habitat quality that often accompanies habitat loss. Finally, we concluded that the value of any estimate of minimum patch size as a conservation tool is compromised by excessive habitat loss.

Population Biology of the Rare Florida Scrub Lizard in Fragmented Habitat

We completed a 3-yr demographic study of the rare Florida scrub lizard, Sceloporus woodi Stejneger, in a small habitat fragment. Censuses were conducted at 2–7 d intervals, with all hatchlings marked and monitored for survival. Field and laboratory observations were used to estimate fecundity. Survival and fecundity data were combined to estimate population growth rate. Sensitivity of the population growth rate to changes in survival and fecundity was examined by both retrospective and prospective analyses.  Survival rates of cohorts declined throughout the study. The decline in survival rates resulted principally from the low survival rates of reproductive females and resulted, in turn, in a negative population growth rate. Subsequent visits to the study site indicated that the population did not decline to extinction, but the factors that affected survival rates are not known. We present some evidence for the potential importance of predation by snakes.