Richard E. Strauss

Reliability Estimates for Ivlev's Electivity Index, the Forage Ratio, and a Proposed Linear Index of Food Selection

Abstract Ivlevs electivity index and the forage ratio, two commonly used measures of food selection, are significantly biased when the sizes of the prey samples from the gut of the predator and the habitat are unequal. Approximate confidence-interval expressions are derived for these indices. A stochastic (Monte Carlo) model was used to validate these expressions and to explore the statistical properties of the indices. The statistical reliability of each index is shown to be a function of the absolute and relative sample sizes and the relative abundances of the prey species in the environment. A linear index of food selection is proposed which avoids most of the statistical and mathematical inadequacies of these indices. Regardless of the index used, however, inadequate habitat sampling, differential availability of prey to the predator, and differential digestion of prey may be significant sources of error in the interpretation of food selection data.

MULTIVARIATE ANALYSIS OF SCALE-DEPENDENT ASSOCIATIONS BETWEEN BATS AND LANDSCAPE STRUCTURE

The assessment of biotic responses to habitat disturbance and fragmentation generally has been limited to analyses at a single spatial scale. Furthermore, methods to compare responses between scales have lacked the ability to discriminate among patterns related to the identity, strength, or direction of associations of biotic variables with landscape attributes. We present an examination of the relationship of population- and community- level characteristics of phyllostomid bats with habitat features that were measured at mul- tiple spatial scales in Atlantic rain forest of eastern Paraguay. We used a matrix of partial correlations between each biotic response variable (i.e., species abundance, species richness, and evenness) and a suite of landscape characteristics to represent the multifaceted asso- ciations of bats with spatial structure. Correlation matrices can correspond based on either the strength (i.e., magnitude) or direction (i.e., sign) of association. Therefore, a simulation model independently evaluated correspondence in the magnitude and sign of correlations among scales, and results were combined via a meta-analysis to provide an overall test of significance. Our approach detected both species-specific differences in response to land- scape structure and scale dependence in those responses. This matrix-simulation approach has broad applicability to ecological situations in which multiple intercorrelated factors contribute to patterns in space or time.

COMPARATIVE PATTERNS OF GROWTH AND DEVELOPMENT IN CRICETINE RODENTS AND THE EVOLUTION OF ONTOGENY

The quantitative description of growth curves for morphometric traits provides a basis for assessing the ontogenetic patterns underlying differences in morphological structure, as demonstrated with comparisons among neotomine‐peromyscine rodents. Morphometric differences among contemporary rodent species are shown to result from relatively simple changes in relative growth rates and timing. Quantitative ontogenetic studies add a dynamic component to the assessment of morphological similarity, thus providing a more robust procedure for detecting homoplasy than static comparison of adult morphology. Applying the principles of phylogenetic systematics to studies of developmental timing among closely related taxa may be a useful and informative complement to studies based on molecular similarity or static comparison of adult morphology. Interspecific and intraspecific differences in allometric scaling of anatomical structures may reflect differences in growth patterns among the taxa compared; caution is warranted in inferring patterns of genetic correlation from data on phenotypic scaling.

EVALUATION OF THE PRINCIPAL-COMPONENT AND EXPECTATION-MAXIMIZATION METHODS FOR ESTIMATING MISSING DATA IN MORPHOMETRIC STUDIES

Abstract Vertebrate skeletons, particularly fossils, commonly have damaged, distorted, or missing structures. Because multivariate morphometric methods require complete data matrices, there are two possible solutions: to omit the specimens or characters having missing values, or to estimate missing values from the remainder of the data. Omission of specimens or characters reduces the data available for analysis, and thus the power to detect patterns or differences. Univariate and bivariate-regression methods are known to reduce the total variance of the data, and thus are not considered here. We compared the two most common multivariate methods: expectation-maximization (EM), which uses the covariance matrix directly, and principal-component (PC) estimation, based on regression of characters on principal components. Performance was evaluated by computer simulation of randomly introduced missing data in constructed data sets of known structure, and in several complete fossil (Pterodactylus skeleton) and recent (Alligator skeleton, Canis skull) data sets. The EM and PC methods displayed consistent and similar patterns of behavior for varying combinations of specimens and characters and across a broad range of amounts of missing data. Reliability was greatest for moderate numbers of characters (6–12) and larger sample sizes. For fewer characters the maximum amount of missing data that can be predicted increases substantially, but with a decrease in reliability. Both methods produce accurate estimates of missing values, but EM estimates are more precise. EM also outperforms the PC method in the maximum proportion of missing values that can be reliably estimated (almost 50% for small numbers of characters).

On Allometry and Relative Growth in Evolutionary Studies

In his critique of contemporary morphometry, Blackstone (1987) asserts that the conceptual, pattern-oriented framework of allometry ignores developmental processes and is therefore inappropriate for studies in evolutionary morphology. His intent is to emphasize the limitations of allometric studies in the understanding of evolutionary processes. In doing so, however, Blackstone focuses on a descriptive approach to allometry and obscures several fundamental issues. In particular, he claims that information on rates of growth in units of chronological time is necessary to deduce patterns of evolutionary change in developmental processes, and that such information is ignored in allometric analyses. He traces this disregard to a historical departure from the study of underlying processes of growth to the purely descriptive study of size and shape. In contrast, I will argue first that chronological time does not have theoretical or operational priority over other estimates of biological age, such as body size, for the description and comparison of growth patterns; and second that the apparent historical trend he describes corresponds instead to a logical progression from the study of growth rates of individual traits per chronological time, to growth rates of the same traits in terms of more robust estimates of biological time.

Transgenic cotton over-producing spinach sucrose phosphate synthase showed enhanced leaf sucrose synthesis and improved fiber quality under controlled environmental conditions

Prior data indicated that enhanced availability of sucrose, a major product of photosynthesis in source leaves and the carbon source for secondary wall cellulose synthesis in fiber sinks, might improve fiber quality under abiotic stress conditions. To test this hypothesis, a family of transgenic cotton plants (Gossypiumhirsutum cv. Coker 312 elite) was produced that over-expressed spinach sucrose-phosphate synthase (SPS) because of its role in regulation of sucrose synthesis in photosynthetic and heterotrophic tissues. A family of 12 independent transgenic lines was characterized in terms of foreign gene insertion, expression of spinach SPS, production of spinach SPS protein, and development of enhanced extractable Vmax SPS activity in leaf and fiber. Lines with the highest Vmax SPS activity were further characterized in terms of carbon partitioning and fiber quality compared to wild-type and transgenic null controls. Leaves of transgenic SPS over-expressing lines showed higher sucrose:starch ratio and partitioning of 14C to sucrose in preference to starch. In two growth chamber experiments with cool nights, ambient CO2 concentration, and limited light below the canopy, the transgenic line with the highest SPS activity in leaf and fiber had higher fiber micronaire and maturity ratio associated with greater thickness of the cellulosic secondary wall.

Heterochronic variation in the developmental timing of cranial ossifications in poeciliid fishes (Cyprinodontiformes)

The quantitative description of ossification sequences and other developmental events with respect to body size provides a basis for assessing the ontogenetic patterns underlying differences in morphological structure. To the extent that such sequences are evolutionarily conservative, they may also provide a basis for phylogenetic inference. Ossification profiles were examined in five species of poeciliid fishes of the genera Poecilia, Xiphophorus, and Poeciliopsis, selected to represent three lineages of varying evolutionary distinctiveness. Although ossification sequences are highly correlated among species, numerous small timing differences and reversals are evident, differences that are accentuated more among than within genera. The timing profiles by themselves, when used to construct hypothetical phylogenetic trees, contain sufficient historical information to recover evolutionary relationships consistent with conventional systematic criteria based primarily on adult osteology and external morphology. Character‐state changes mapped onto the resulting trees can be interpreted directly as heterochronic accelerations and retardations of ossification.

Discriminating Groups of Organisms

A common problem in morphometric studies is to determine whether, and in what ways, two or more previously established groups of organisms differ. Discrimination of predefined groups is a very different problem than trying to characterize the patterns of morphological variation among individuals, and so the kinds of morphometric tools used for these two kinds of questions differ. In this paper I review the basic procedures used for discriminating groups of organisms based on morphological characteristics – measures of size and shape. A critical reading of morphometric discrimination studies of various kinds of organisms in recent years suggests that a review of procedures is warranted, particularly with regard to the kinds of assumptions being made. I will discuss the main concepts and methods used in problems of discrimination, first using conventional morphometric characters (measured distances between putatively homologous landmarks), and then using landmarks directly with geometric morphometric approaches.

Genetic diversity, population subdivision, and gene flow in Morelet's crocodile (Crocodylus moreletii) from Belize, central America

Abstract The lack of information surrounding natural history and ecology of the endangered Morelets crocodile (Crocodylus moreletii) has prompted a baseline study of the population genetics for this species. Nine microsatellite loci have been used to estimate genetic structure within and gene flow patterns among crocodiles (using a recently described maximum likelihood approach) from seven localities in north-central Belize. Individuals from the seven localities grouped into four apparent populations. Within localities, a high degree of genetic heterogeneity was observed. Among all localities, some subdivision was present (FST = 0.062; RST = 0.100). Furthermore, among the apparent populations, we found a significant correlation between geographic distance and genetic subdivision. Our findings suggest a relatively high level of migration among populations (Nm = 5.15) and are consistent with an isolation-by-distance model of gene flow. Two contiguous subpopulations in particular, New River and New River Lagoon, may form an important source for genetic variation for smaller populations throughout the region. These data will allow us to test hypotheses of relatedness among C. moreletii for other drainages in Belize and will be useful in optimizing future management programs for C. moreletii.

Predation and life-history variation in Poecilia reliculata (Cyprinodontiformes: Poeciliidae)

SynopsisReznick and Endler investigated natural variation in life-history traits of populations of Trinidad guppies exposed to one of three intensities of predation: (i) high predation directed primarily at adults, (ii) moderate predation directed primarily at juveniles, and (iii) low predation. They were able to document significant interpopulational differences in life-history traits associated with this differential predation on a trait-by-trait basis. However, the present extended multivariate analysis indicates that (1) life-history traits do not differ significantly between populations exposed to moderate versus low predation, although both differ greatly from high-predation populations; (2) life-history variation is strongly unifactorial; and (3) despite the importance of predation effects, approximately 17% of the variation in life-history variables cannot be accounted for by predation intensity. Residual variation has no obvious geographical patterns, but instead seems to reflect local environmental variability. Life-history differences between predation regimes are consistent with residual patterns of variation within regimes, suggesting that local variation provides the raw material for extrapolation in response to predation, but also that it influences the direction of correlated change in life-history traits.