Russell A. Wright

Resilience and Resistance of a Lake Phosphorus Cycle Before and After Food Web Manipulation

Parameters of a phosphorus cycling model were estimated for two configurations of a lake ecosystem. The piscivore-dominated configuration had one more trophic level than the planktivore-dominated configuration. We derived four main conclusions from analysis of the model. (1) Results support the argument of DeAngelis et al. that turnover rate of a limiting nutrient is directly related to ecosystem resilience. (2) Results support the hypothesis of Pimm and Lawton that longer food chains are less resilient. (3) Inputs of phosphorus to the pelagic system derived from inshore feeding by fishes were a large flux, which is comparable to inputs from physical-chemical fluxes. (4) Algal (seston) standing crops, unlike all other compartments, were less sensitive to phosphorus inputs in the piscivore-dominated system. Consistent with the trophic cascade hypothesis, the piscivore-dominated system had higher herbivore standing crops and lower algal standing crops than the planktivore-dominated system. Changes in trophic structure that derive from trophic cascades can be viewed as changes in the phosphorus cycle driven by fishes.

Overwinter Growth and Survival of Largemouth Bass: Interactions among Size, Food, Origin, and Winter Severity

Abstract Winter severity (temperature, duration, and photocycle), geographic origin, food availability, and initial body size likely influence growth, survival, and, therefore, recruitment of age-0 largemouth bass Micropterus salmoides. We collected age-0 largemouth bass (70–160 mm total length) from low (33°N), intermediate (40°N), and high (45°N) latitudes throughout their natural range (origin), and we subjected all three groups of fish to three experimental winters that mimicked these latitudes (N = 9 largemouth bass per treatment). Within each winter and origin, one-half of the largemouth bass were fed fish prey, whereas the remaining one-half were starved. Winter strongly influenced survival; overall survival rates in the high-, intermediate-, and low-latitude winters were 34.9, 59.4, and 61.1%, respectively (χ2 test, P < 0.05). Largemouth bass from 33°N suffered high mortality in the high-latitude winter. Across all winters, more fed fish (64.5%) survived than did starved fish (38.1%) (χ2 test, P <...

From Star Charts to Stoneflies: Detecting Relationships in Continuous Bivariate Data

Within many ecological systems, relationships between controlling factors and associated response variables are complex. In many cases, the response should vary little when the controlling factor exerts strong effects. Conversely, when the effect of the controlling factor is weak or absent, the response may vary greatly with effects of other factors. Correlation or regression analyses often may not be appropriate for testing these relationships, because variance of the response changes with values of the controlling factor. We suggest using a technique from the astronomy literature, a two-dimensional Kolmogorov-Smirnov (2DKS) test, to detect relationships in bivariate data with these patterns of variance. This technique successfully identified simulated bivariate data composed of paired independent values as having nonsignificant relationships and simulated bivariate data in which mean and variance of y was constrained at high levels of x as having significant relationships. Using these simulations and examples from aquatic and terrestrial systems, we demonstrate that the 2DKS is a robust test for detecting nonrandom patterns in bivariate distributions that commonly arise in many ecological systems.

Non-Additive Impact of Blue Crabs and Spot on Their Prey Assemblages

We designed a field experiment to examine predator interactions-in par- ticular, the effect of each predator on the growth and survival of the other, and to examine the effects of predation on prey assemblages-in particular, predation effects by each pred- ator alone as well as together. Blue crabs (Callinectes sapidus) and the fish, spot (Leiostomus xanthurus), co-occur in Southeastern estuaries of the USA, and share many prey taxa and habitat types. We predicted that the blue crabs and spot would suffer both interference and exploitation competition when held together in enclosures. We also predicted that, as a consequence of the competitive interactions, their joint impact on prey assemblages would be different from that predicted based on the impact when held alone. We installed mesh enclosures in an existing earthen pond after allowing it to be filled with water from Bogue Sound, North Carolina, and to be colonized by the naturally occurring species assemblage. A factorial design was used to allow us to test for non- additive effects of the two predators on their prey assemblage. Contrary to our predictions, we found that spot survival was enhanced in the presence of blue crabs. This enhancement was probably effected by removal of the alga Enteromorpha intestinalis by the crabs. The alga was positively affected by spot. The positive effect of spot on Enteromorpha, and the negative effect of crabs, was probably responsible for differences in densities and distri- butions of prey taxa within the enclosures. Significant interaction terms in our analysis suggest that community response to either predator is not independent of the other. Our data suggest that we cannot expect to explain adequately the effects of multi-species predator assemblages on their prey by combining information obtained through single-predator experiments.

The effects of predation on the survival and size-distribution of estuarine fishes: an experimental approach

SynopsisThe effect of southern flounder,Paralichthys lethostigma, predation on the survival and size-distribution of spot,Leiostomus xanthurus, was investigated using feeding experiments in outdoor tanks and estuarine ponds. In the outdoor tank experiments, southern flounder, 143 mm average standard length (SL), fed for 48 h on spot (25–30 mm SL), over a range of densities (4–20 m−2). In these trials, flounder showed a Type II functional response to spot density. In the estuarine pond experiment, flounder density (2 or 4 flounders 70 m−2) contributed significantly to spot mortality relative to predator-free controls. The effect of flounder density on spot mortality was non-linear; in the 4-flounder treatment the daily instantaneous mortality due to flounder (0.0177) was 3.5 times that in the 2-flounder treatment (0.0051). The total spot mortality rate in the 4-flounder treatment (0.028) was similar to that observed from published field observations. Flounder also altered the size distribution of spot relative to predator-free controls. Predation by flounder resulted in fewer intermediate-sized spot at the end of the experiment when compared to predator-free controls. Flounder also significantly reduced survival and mean length of two killifishes,Cyprinodon variegatus andFundulus heteroclitus, that invaded the pond. Flounder predation was a significant size-structuring force on the prey fish assemblage in the pond. Results from both the outdoor tank experiment and the pond experiment indicate that flounder may represent a significant source of mortality for juvenile estuarine fishes.

Coastal largemouth bass (Micropterus salmoides) movement in response to changing salinity

Estuaries are productive, heterogeneous, and dynamic systems that support a diverse array of fishes. However, our understanding of how presumably stenohaline fishes persist in such transitional systems is limited, particularly for most fishes in tidal freshwater areas. We conducted a laboratory experiment and field investigation along an upstream–downstream salinity gradient in the Mobile–Tensaw River Delta, Alabama, USA, to test the hypothesis that age-0 largemouth bass (Micropterus salmoides), an economically and ecologically important freshwater species that uses low-salinity habitats in many North American estuaries, move to avoid seasonal salinity increases. To do so, we quantified changes in otolith microchemistry (e.g., Sr to Ca ratios) along the major growth axis of otoliths in both field-collected and laboratory-reared individuals. Our experiment revealed a 21-day lag time between initial salinity changes and Sr:Caotolith saturation but that Sr:Caotolith in field-collected fish reflect changes in...

Evaluating How Local‐ and Regional‐Scale Processes Interact to Regulate Growth of Age‐0 Largemouth Bass

Abstract Regional- and local-scale processes may interact to influence early growth and survival, thereby governing cohort strength. During summer through fall 1994–1996, we assessed how precipitation (a regional-scale process) and prey availability (a local-scale process) influenced piscivory and growth of age-0 largemouth bass Micropterus salmoides in five Ohio reservoirs (190–1,145 ha). We expected early growth to vary with the abundance and relative sizes of age-0 gizzard shad Dorosoma cepedianum. We collected age-0 largemouth bass and prey fishes every 3 weeks in each reservoir. In 1994, May precipitation was low (total = 4 cm), resulting in low mean daily reservoir discharge (x 5 reservoirs = 3.6 m3/s). In four reservoirs, stable water levels may have led to successful largemouth bass reproduction and perhaps an early hatch. As such, age-0 largemouth bass in these systems were abundant, consumed gizzard shad, and reached large sizes by fall (15.3 g). In 1995 and 1996, high precipitation (total > 12 ...

Sample Preparation Techniques for Determination of Fish Energy Density via Bomb Calorimetry: An Evaluation Using Largemouth Bass

Abstract We evaluated three homogenization and subsampling techniques for preparing fish tissue samples for bomb calorimetry to identify differences in efficiency for estimating fish energy density. We compared (1) drying the whole fish and homogenizing the dried fish tissue, (2) homogenization prior to drying and then drying the subsample of fish tissue, and (3) homogenization after autoclaving to soften the hard structures and then drying a subsample of the homogenized fish tissue. Sample drying time and energy density estimates were compared among techniques across a size range (wet mass = 32–1,080 g) of largemouth bass Micropterus salmoides. Both of the subsampling techniques reduced drying time by about 40% relative to drying the whole fish. All three methods provided statistically similar estimates of largemouth bass energy densities. The autoclave process was most efficient, minimizing both sample preparation time and drying time. Variance of energy density estimates was greater for both subsamplin...

Evaluating the potential for predatory control of gizzard shad by largemouth bass in small impoundments: a bioenergetics approach.

Abstract Gizzard shad Dorosoma cepedianum is an important fish species that can have strong effects on aquatic communities throughout the southeastern and midwestern USA. Gizzard shad often become overabundant, competitively reducing growth and recruitment of sport fishes. Using field data and bioenergetics simulations, we evaluated the potential for largemouth bass Micropterus salmoides to control the abundance of gizzard shad in small impoundments. We sampled 1,313 largemouth bass stomachs, of which 66% contained prey, from four ponds during 1998 and 1999. Large (≥350-mm) largemouth bass consumed the greatest biomass of gizzard shad, eating gizzard shad throughout most of the year. Gizzard shad were more abundant in the stomach contents of largemouth bass less than 350 mm long during late summer and early fall than during winter and early spring. Variation in the mortality of predatory largemouth bass (21% versus 31%), as estimated from weighted catch-curves, resulted in only slight variation (6–7%) in ...

Energetic Adaptations along a Broad Latitudinal Gradient: Implications for Widely Distributed Assemblages

Abstract Most community-based models in ecology assume that all individuals within a species respond similarly to environmental conditions and thereby exert identical effects as consumers or prey. Rather, individuals differ among systems, with important implications for population demographics and community interactions. For widely distributed assemblages made up of poikilotherms with high first-year mortality, species-specific differences in growth reaction norms as affected by both temperature and genotype will influence biotic interactions. For a broadly distributed fish assemblage, first-year growth does not vary with latitude for a planktivorous prey species, but declines with increasing latitude for a terminal piscivore. Size-based competitive interactions between these species are likely to be more intense at high latitudes, as they spend an extended time sharing resources during early life. Such patterns probably are pervasive and must be considered when seeking to understand species interactions. Improving our knowledge of how temperature and local adaptations affect size-based interactions should enhance our ability to manage and conserve widespread assemblages.