Jessica E. Rettig

SCALE-DEPENDENT ASSOCIATIONS AMONG FISH PREDATION, LITTORAL HABITAT, AND DISTRIBUTIONS OF CRAYFISH SPECIES

To predict how species establish and disperse within novel communities, the spatial scale at which competition, predation, and habitat interact must be understood. We explored how these factors affect the distribution and abundance of the exotic crayfishes Orconectes rusticus and O. propinquus and the native O. virilis at both the site-specific and whole-lake scales in northern Wisconsin lakes. During summer 1990, we quantified crayfish, fish predators, and fish diets in cobble and macrophyte sites in Trout Lake, com- paring resulting patterns to those in 21 lakes surveyed during summer 1987. Within and across lakes, fish abundance was unrelated to habitat. Within Trout Lake, O. rusticus and O. propinquus were common in both cobble and macrophyte. Orconectes virilis was re- stricted to macrophyte, probably due to strong displacement by the invaders in cobble. Across lakes, O. rusticus increased where habitat was more than 16.7% cobble, O. pro- pinquus was generally rare, and O. virilis abundance was unrelated to cobble. Crayfish were generally small in cobble and large in macrophyte, perhaps because of habitat-specific, size-selective fish predation or because large crayfish leave cobble when it no longer pro- vides refuge. Orconectes virilis, the largest of three congeners, may have a size refuge in macrophyte but not in cobble. Across lakes, O. rusticus was only abundant when fish biomass was low; O. virilis abundance varied positively with fish. Effects of fish predation and habitat on the ability of invaders Orconectes rusticus and O. propinquus to establish and replace O. virilis appear to be scale dependent. At local (site-specific) scales, cobble likely interacts with selective predation for O. virilis to allow the invaders to establish and replace the native. At the lake-wide scale, high cobble facilitates invaders but predation may curb their successful dispersal and establishment at new sites. Models of community assembly and invasions need to incorporate scale dependencies in habitat availability and biotic interactions to effectively assess the invasion potential of novel species.

Interactions between adult and larval bluegill sunfish: positive and negative effects

Adult fish may affect the growth and survival of conspecific larvae through a variety of pathways, including negative interactions via competition for shared limiting resources or via predation (i.e., cannibalism), and positive interactions due to the consumption of larval predators and via resource enhancement (i.e., presence of adults increases availability of larval prey). To examine the overall effect of adult bluegill sunfish (Lepomis macrochirus) on larval bluegill, we conducted a field experiment in which we manipulated adult densities and quantified larval growth and survival, prey abundance, invertebrate predator abundance, and cannibalism. The presence of adult bluegill had a negative effect on final larval mass. This response was consistent with competition for zooplankton prey. Adult bluegill reduced the abundance of large zooplankton (e.g., Chaoborus and Daphnia), which were the dominant prey of bluegill larvae in the absence of adults. Larvae in the no-adult treatment also had significantly more prey in their stomachs compared to larvae in the presence of adults. Larval survival was maximized at intermediate adult densities and the overall production of larvae peaked at intermediate adult densities. The higher larval survival at intermediate adult densities is attributed to a reduction in invertebrate predators in treatments with adult bluegill; invertebrate predators experienced an 80% reduction in the presence of adult fish. Decreased larval survival at the highest adult density was not due to resource limitation and may be due to cannibalism, which was not directly observed in our study, but has been observed in other studies.

Changes in a Turtle Community from a Northern Indiana Lake: A Long-Term Study1

Abstract Knowledge of the long-term dynamics of freshwater turtle communities is important if we are to understand fully the impacts of human-induced changes in their aquatic and terrestrial habitats. We present data on a turtle community that has been monitored intermittently for more than 20 years and regularly for more than 10 years (1992–2003). The composition of the community has shifted with a decrease in the dominance of Painted Turtles (Chrysemys picta). This shift reflects a decline in the number of C. picta over the past decade, whereas numbers of other species have remained relatively constant. Adult survivorship of C. picta has declined in the past decade. The proportion of C. picta with watercraft propeller damage has increased, whereas propeller damage has remained constant for the two other species for which we have data, the Northern Map Turtle (Graptemys geographica) and the Red-Eared Slider (Trachemys scripta). Our study suggests that the turtle community in Dewart Lake has shifted from 1992–2003. It seems likely that this shift has occurred because of a decline in C. picta, possibly as a result of increased use of the lake by humans.

Microhabitat Selection by Northern Cricket Frogs (Acris crepitans) along a West-Central Missouri Creek: Field and Experimental Observations

Abstract We studied microhabitat use and selection by Northern Cricket Frogs (Acris crepitans) along a stream in west-central Missouri. Field observations of microhabitat use indicate that cricket frogs preferentially use moist (muddy) substrates with some proximity to shelter items (e.g., rocks) and the water. Laboratory choice experiments support the importance of substrate moisture and shelter presence. Temperature may also play a role in site selection independent of substrate selection.

Effects of Salinity on Survivorship of Wood Frog (Rana sylvatica) Tadpoles

ABSTRACT We examined the survivorship of wood frog (Rana sylvatica) tadpoles when exposed continuously to various concentrations of sodium chloride, a common component of road salt. A high environmental concentration of NaC1 (5.6 g L−1) resulted in decreased survivorship within a short period of time compared to controls; lower concentrations of NaC1 also decreased survivorship, but the effect took longer to appear. These results indicate that environmental salinization, as results from road salt run-off, will have negative consequences for the wood frog.

Effects of potential predator and competitor cues and sibship on wood frog (Rana sylvatica) embryos

Chemical cues emitted from predators or competitors are often important for animals living in aquatic ecosystems as they allow potential prey to assess predation risk and make appropriate risk-sensitive responses. In our experiment, we examined if wood frog (Rana sylvatica) embryos exposed to potential predator and competitor cues would alter their time to hatching, size at hatching, or survivorship. Eggs from four different sibships were subjected to a variety of cues including dragonfly larvae (potential tadpole predator), mosquitofish (Gambusia affinis; a non-native potential egg and tadpole predator), and overwintered tadpoles of Rana sp. (potential competitors). We found no significant effects of any of the cues. However, we did find significant variation in mean time to hatching and mean hatchling size among sibships. Our results suggest that wood frog embryos may have limited ability to respond to some cues at the embryonic stage, at least for the concentrations and conditions used in this experiment. Our results do indicate genetic or parental effects can affect embryonic wood frog development rate and hatchling size.

Effects of Roundup formulations, nutrient addition, and Western mosquitofish (Gambusia affinis) on aquatic communities.

Aquatic communities can be affected by herbicides, nutrient addition, and non-native fish species. We conducted a mesocosm experiment to examine the direct and interactive effects of three stressors: (1) Roundup formulations (Roundup Weed and Grass Killer® and Roundup Poison Ivy and Tough Brush Killer Plus®), (2) nutrient addition, and (3) the presence of the non-native Western mosquitofish (Gambusia affinis), on experimental pond communities. Roundup formulations had the most widespread effects on the zooplankton community, but effects varied between formulations and among taxa. The only significant effect of nutrient addition was a lowering of Daphnia abundance in the nutrient addition treatments. The abundances of Daphnia, mid-sized cladocerans, and total zooplankton were lowered by mosquitofish, but no other taxa showed significant mosquitofish effects. We found several two-way and three-way interactions among the stressors, but these varied among zooplankton taxa. Chlorophyll a levels were higher with nutrient addition but were not significantly affected by Roundup formulation or mosquitofish. Our results suggest toxicity of Roundup formulations varies among taxa, and Roundup formulations differ in their toxicity to zooplankton, but with no cascading effects on primary producers. In addition, interactions among stressors affected the zooplankton community.

Possible Behavioral Avoidance of UV-B Radiation and Sunlight in Wood Frog (Lithobates sylvaticus) Tadpoles

Little is known of the ability of amphibian larvae to behaviorally avoid exposure to UV radiation. We examined whether wood frog (Lithobates sylvaticus) tadpoles avoid UV-B radiation and, more generally, sunlight. We found that wood frog tadpoles were located more often under a UV-B blocking filter (only after 60 min) or under shade than would be expected if their behavior was random. Our observations suggest that wood frog tadpoles may minimize exposure to UV-B, either by directly avoiding UV-B or coincidentally by choosing shade.

Substrate preference of eastern red-backed salamanders, Plethodon cinereus : A comparison of deciduous and coniferous substrates

Forest tree composition can affect the distribution and abundance of terrestrial salamanders. We examined the effect of leaf litter type (deciduous or coniferous) and soil type (deciduous and coniferous) on red-backed salamander ( Plethodon cinereus ) substrate choice using a series of laboratory experiments. Salamanders preferred deciduous leaves over coniferous pine needles and the deciduous soil/deciduous leaf litter combination over all others. In addition, our results suggest that leaf litter type may be more important than soil type in influencing salamander substrate preference in our study. Our results suggest that behavioural avoidance may partially explain differences in P. cinereus abundances in deciduous and coniferous forest habitats.

Foraging Behavior of Male and Female Mosquitofish (Gambusia affinis) in Single- and Mixed-Sex Groups

ABSTRACT Female mosquitofish (Gambusia affinis) experience high levels of sexual harassment from males. We examined feeding rates among groups of solitary male mosquitofish, solitary female mosquitofish, and groups of males and females to examine how such harassment may affect feeding rates of males and females. Females feed more than males in general. Both males and females showed a reduction in strike rate when in mixed-sex groups compared to single-sex groups. Our results suggest that both male and female G. affinis suffer reductions in foraging when in mixed-sex groups, presumably as a consequence of the harassment of females by males.