Joel C. Trexler

Nontraditional Regression Analyses

Least-squares linear regression and multiple regression are among the most commonly used analytical techniques of ecologists. However, these techniques only address a portion of the possible applications of regression methods. We discuss two less commonly used regression analyses that could find wide application in ecology, logistic regression and LOWESS regression. Logistic regression is appropriate in cases where the dependent vari- able is categorical, dichotomous, or polychotomous. It can be used with continuous and/ or discrete independent variables. Logistic regression is motivated by the underlying bi- nomial or multinomial distribution of dichotomous and polychotomous dependent vari- ables and transforms the data to explicitly model these distributions. Locally weighted regression scatterplot smoothing or LOWESS regression is used to model the relationship between a dependent variable and independent variable when no single functional form will do. LOWESS regression is motivated by the assumption that neighboring values of the independent variable are the best indicators of the dependent variable in that range of

Sampling Aquatic Invertebrates from Marshes: Evaluating the Options

Designing an effective sampling program and accurately interpreting the results requires a knowledge of the sampling characteristics of the various devices which might be used, but such knowledge is lacking for invertebrate samplers that can be used in heavily vegetated wetlands. We evaluated the sampling characteristics of 8 invertebrate samplers in vegetated habitats by employing them side-by-side in the Florida Everglades. The samplers differed in the number of individuals captured, number of species captured, and the equitability of species abundances. A funnel trap, a D-frame sweep net, and a stovepipe collected more individuals, more taxa, and a more even distribution of individuals among taxa, than did a 1-m2 throw trap and Hester-Dendy artificial substrates. Three other samplers, a minnow trap, a benthic corer, and a plankton net, captured very few individuals. Most importantly, samplers differed consistently in the taxonomic composition of the invertebrates each captured. These differences argue for the use of several complementary methods in order to gain a complete representation of the invertebrate assemblage. We discuss issues involved in choosing samplers and recommend the use of 3: the funnel trap, the D-frame sweep net, and the 1-m2 throw trap, for studies of aquatic invertebrates in heavily vegetated wetlands such as the Everglades.

Sampling Fishes in Vegetated Habitats: Effects of Habitat Structure on Sampling Characteristics of the 1-m2 Throw Trap

Abstract Enclosure traps that quickly surround well-defined areas of habitat are perhaps the most widely used method for sampling fishes in vegetated habitats. However, relatively few data are available to evaluate the effects of habitat structure on sampling characteristics of enclosure traps. In this study, we determined how clearing efficiency and accuracy of 1-m2 throw traps varied across a range of environmental conditions in the Florida Everglades by sampling within enclosed areas of marsh habitat. Throw trap clearing efficiency and sampling accuracy did not differ among two widely separated locations and appeared to be unaffected by variation in water depth, canopy height, plant cover, plant stem density, and periphyton volume. Sampling accuracy averaged 63% of fishes present after correcting for clearing efficiencies. On average, 83% of the fishes present in a throw trap were recovered. Therefore, it appeared that about 17% of the missing fishes may have burrowed into the substrate or been discard...

Empirical assessment of fish introductions in a subtropical wetland: An evaluation of contrasting views

We summarized data from eight quantitative fish surveys conducted in southern Florida to evaluate the distribution and relative abundance of introduced fishes across a variety of habitats. These surveys encompassed marsh and canal habitats throughout most of the Everglades region, including the mangrove fringe of Florida Bay. Two studies provided systematically collected density information over a 20-year period, and documented the first local appearance of four introduced fishes based on their repeated absence in prior surveys. Those species displayed a pattern of rapid population growth followed by decline, then persistence at lower densities. Estuarine areas in the southern Everglades, characterized by natural tidal creeks surrounded by mangrove-dominated marshes, and canals held the largest introduced-fish populations. Introduced fishes were also common, at times exceeding 50% of the fish community, in solution holes that serve as dry-season refuges in short-hydroperiod rockland habitats of the eastern Everglades. Wet prairies and alligator ponds distant from canals generally held few individuals of introduced fishes. These patterns suggest that the introduced fishes in southern Florida at present may not be well-adapted to persist in freshwater marshes of the Everglades, possibly because of an interaction of periodic cold-temperature stress and hydrologic fluctuation. Our analyses indicated low densities of these fishes in central or northern Everglades wet-prairie communities, and, in the absence of experimental data, little evidence of biotic effects in this spatially extensive habitat. There is no guarantee that this condition will be maintained, especially under the cumulative effects of future invasions or environmental change.

Temporal and Spatial Dynamics of Larval and Juvenile Fish Abundance in a Temperate Floodplain River

We studied the dynamics of the young-of-the-year (YOY) fish community in a portion of the floodplain and adjacent channel of the Tallahatchie River, Mississippi, by quantifying temporal and spatial variation in abundances of larval and juvenile fishes. Multivariate comparisons of relative abundance data indicated that assemblage composition differed over time and among sampling locations. A spring assemblage consisting of larval gizzard shad (Dorosoma cepedianum), larval crappie (Pomoxis spp.), and larval darters (Etheostoma and Percina spp.) dominated samples from mid-April to early June. Composition of the spring assemblage did not differ among habitats although more larval fishes were caught in warmer, more lentic habitats. This assemblage was replaced in late June by a group consisting of larval and juvenile sunfishes (Lepomis spp.), YOY mosquitofish (Gambusia afinis), and juvenile shiners (Cyprinella and Notropis spp.). The relative abundance of fishes in this assemblage differed among habitats; larval and juvenile sunfish were most abundant in habitats exhibiting low flow rates and high temperatures, and juvenile shiners were most abundant in moderate to high flow habitats. High larval fish abundance in floodplains stems, in part, from the presence of larvae of species that inhabit both the channel and the floodplain as adults.

Behavioural allometry and interdemic variation in sexual behaviour of the sailfin molly, Poecilia latipinna (Pisces: Poeciliidae)

Sailfin mollies, Poecilia latipinna, exhibit remarkable levels of intraspecific variation in reproductive behaviour. Larger males exhibit higher rates of courtship and lowered rates of gonoporal nibbling and gonopodial thrusting (forced copulation attempts). Larger males have relatively longer and higher dorsal fins than smaller males. The dorsal fin is a prominent component of the courtship display. Variation in fin measurements, behaviour patterns, and body size of mature males is continuous, and fin shape and behaviour patterns are allometrically related to body size. The allometric pattern is displayed by individual traits as well as by the morphological or behavioural traits in ensemble. Eight populations of mollies from markedly distinct habitats exhibited similar consistent levels of intrademic variation in the size of mature males. Detailed studies on three populations showed that dorsal fin shape could be described by the same regression relationship in all populations, and indicted that a males shape was determined by his absolute body size. By contrast, there was some variation among populations in the relation of behaviour patterns to male body size. The pattern of this interdemic variation indicated that a males behaviour patterns were influenced by his relative size in a population. Successful inseminations following forced copulations were rare. The average size of successful males was smaller than the average size of unsuccessful males in all three populations. These results indicated that successful insemination through forced copulation was, like behaviour patterns generally, more a function of the relative size of the male, than his absolute size.

Evaluating active and passive sampling methods to quantify crayfish density in a freshwater wetland

Abstract We evaluated the sampling efficacy of 1-m2 throw traps (active sampler) and baited minnow traps (passive sampler) across an experimental density gradient (1, 2, 4, 6, 8, 10, and 15/m2) of the slough crayfish (Procambarus fallax) in 2 trials with different crayfish populations. In both trials, throw-trap density estimates were highly correlated with actual crayfish density (r2 = 0.96). The form of the relationships between density estimates and stocked densities was consistent between trials, and indicated that throw traps captured a similar proportion of the stocked crayfish regardless of the stocked density. When we adjusted the relationships to account for clearing efficiency (proportion of captured animals actually recovered from the trap), the slopes of the regressions were not significantly different from 1 in either trial. Size distributions and sex ratios of crayfish collected by the throw traps accurately reflected those of the stocked populations. Baited minnow traps performed inconsistently between the 2 trials. Catch-per-unit-effort (CPUE) and density were significantly correlated only in Trial 2 (r2 = 0.82). The slope of the regression in Trial 2 (0.621) was significantly <1, and the intercept was positive and nearly significant (p = 0.074), indicating that minnow traps captured increasingly smaller proportions of the stocked crayfish as the stocked density increased (i.e., differences between CPUE values underestimated actual differences between stocked densities along the gradient). Minnow traps were biased toward capturing large male crayfish, but the form of the relationships between CPUE and density did not improve when large-male CPUE was used in the regressions. Our results suggest that 1-m2 throw traps provide better estimates than baited minnow traps of crayfish densities in shallow vegetated habitats.

SPATIAL SCALE AND ABUNDANCE PATTERNS OF LARGE FISH COMMUNITIES IN FRESHWATER MARSHES OF THE FLORIDA EVERGLADES

Anthropogenic habitat alterations and water-management practices have imposed an artificial spatial scale onto the once contiguous freshwater marshes of the Florida Everglades. To gain insight into how these changes may affect biotic communities, we examined whether variation in the abundance and community structure of large fishes (SL > 8 cm) in Everglades marshes varied more at regional or intraregional scales, and whether this variation was related to hydroperiod, water depth, floating mat volume, and vegetation density. From October 1997 to October 2002, we used an airboat electrofisher to sample large fishes at sites within three regions of the Everglades. Each of these regions is subject to unique water-management schedules. Dry-down events (water depth <10 cm) occurred at several sites during spring in 1999, 2000, 2001, and 2002. The 2001 dry-down event was the most severe and widespread. Abundance of several fishes decreased significantly through time, and the number of days post-dry-down covaried significantly with abundance for several species. Processes operating at the regional scale appear to play important roles in regulating large fishes. The most pronounced patterns in abundance and community structure occurred at the regional scale, and the effect size for region was greater than the effect size for sites nested within region for abundance of all species combined, all predators combined, and each of the seven most abundant species. Non-metric multi-dimensional scaling revealed distinct groupings of sites corresponding to the three regions. We also found significant variation in community structure through time that correlated with the number of days post-dry-down. Our results suggest that hydroperiod and water management at the regional scale influence large fish communities of Everglades marshes.

Phenotypic plasticity in the sailfin molly, Poecilia latipinna (pisces : poeciliidae). I : Field experiments

Sailfin mollies (Poecilia latipinna) display marked interdemic variation in body size. We employed “common‐garden” experiments in field enclosures to explore the potential role of environmental factors in determining the interdemic phenotypic variation in growth rate, age at maturity, and size at maturity. The largest single, consistent source of variation for all traits was family identity within populations. Environmental effects acted predominantly through family x environment interactions. There was little evidence for any intrinsic variation among populations once family heterogeneity had been accounted for. In general, when statistically significant differences existed, fish raised in a saltwater pond grew faster than their broodmates raised in a freshwater pond. Both males and females tended to mature at a smaller size and later in the freshwater pond than in the saltwater pond. The effects of the environmental conditions differed among the three years in which we performed these studies. In only one year was there a substantial difference between fish raised under the two environmental conditions. These results indicate that direct environmental effects are not strong enough to account for the differences in body size among natural populations and that intrinsic differences among natural populations are due to different frequency distributions of genotypes that are present in all populations.

Size-selective predation of sailfin mollies by two species of heron

Two species of herons (Aves: Ardeidae), the great egret and the snowy egret, fed preferentially on relatively large individual sailfin mollies (Pisces: Poeciliidae). The level of selectivity by great egrets decreased when plant cover was added to the experimental conditions. We documented the preference of snowy egrets for large mollies in all combinations of two prey sex ratios and two prey size distributions with 50% plant cover. Snowy egrets chose females preferentially to males when females were rare, but not when they were commmon. This preference may be caused by the larger overall size of females when gravid in comparison to males of the same length and/or by the behavior of females when they are in a minority relative to males. Selective predation by wading birds is likely to contribute to patterns of size variation among some populations of sailfin mollies but not to the patterns of female-skewed sex ratios in adults