Christopher L. Higgins

Elements of metacommunity structure of Paraguayan bats: multiple gradients require analysis of multiple ordination axes

Techniques to evaluate elements of metacommunity structure (EMS; coherence, species turnover and range boundary clumping) have been available for several years. Such approaches are capable of determining which idealized pattern of species distribution best describes distributions in a metacommunity. Nonetheless, this approach rarely is employed and such aspects of metacommunity structure remain poorly understood. We expanded an extant method to better investigate metacommunity structure for systems that respond to multiple environmental gradients. We used data obtained from 26 sites throughout Paraguay as a model system to demonstrate application of this methodology. Using presence–absence data for bats, we evaluated coherence, species turnover and boundary clumping to distinguish among six idealized patterns of species distribution. Analyses were conducted for all bats as well as for each of three feeding ensembles (aerial insectivores, frugivores and molossid insectivores). For each group of bats, analyses were conducted separately for primary and secondary axes of ordination as defined by reciprocal averaging. The Paraguayan bat metacommunity evinced Clementsian distributions for primary and secondary ordination axes. Patterns of species distribution for aerial insectivores were dependent on ordination axis, showing Gleasonian distributions when ordinated according to the primary axis and Clementsian distributions when ordinated according to the secondary axis. Distribution patterns for frugivores and molossid insectivores were best described as random. Analysis of metacommunities using multiple ordination axes can provide a more complete picture of environmental variables that mold patterns of species distribution. Moreover, analysis of EMS along defined gradients (e.g., latitude, elevation and depth) or based on alternative ordination techniques may complement insights based on reciprocal averaging because the fundamental questions addressed in analyses are contingent on the ordination technique that is employed.

Cross-Scale Responses of Biodiversity to Hurricane and Anthropogenic Disturbance in a Tropical Forest

A bstractIn studies of biodiversity, considerations of scale—the spatial or temporal domain to which data provide inference—are important because of the non-arithmetic manner in which species richness increases with area (and total abundance) and because fine-scale mechanisms (for example, recruitment, growth, and mortality of species) can interact with broad scale patterns (for example, habitat patch configuration) to influence dynamics in space and time. The key to understanding these dynamics is to consider patterns of environmental heterogeneity, including patterns produced by natural and anthropogenic disturbance. We studied how spatial variation in three aspects of biodiversity of terrestrial gastropods (species richness, species diversity, and nestedness) on the 16-ha Luquillo Forest Dynamics Plot (LFDP) in a tropical forest of Puerto Rico was affected by disturbance caused by Hurricanes Hugo and Georges, as well as by patterns of historic land use. Hurricane-induced changes in spatial organization of species richness differed from those for species diversity. The gamma components of species richness changed after the hurricanes and were significantly different between Hurricanes Hugo and Georges. Alpha and two beta components of species richness, one related to turnover among sites within areas of similar land use and one related to variation among areas of different land use, varied randomly over time after both hurricanes. In contrast, gamma components of species diversity decreased in indistinguishable manners after both hurricanes, whereas the rates of change in the alpha component of species diversity differed between hurricanes. Beta components of diversity related to turnover among sites declined after both hurricanes in a consistent fashion. Those related to turnover among areas with different historic land uses varied stochastically. The immediate effect of hurricanes was to reduce nestedness of gastropod assemblages. Thereafter, nestedness increased during post-hurricane secondary succession, and did so in the same way, regardless of patterns of historic land use. The rates of change in degree of nestedness during secondary succession were different after each hurricane as a result of differences in the severity and extent of the hurricane-induced damage. Our analyses quantified temporal changes in the spatial organization of biodiversity of gastropod assemblages during forest recovery from hurricane-induced damage in areas that had experienced different patterns of historic human land use, and documented the dependence of biodiversity on spatial scale. We hypothesize that cross-scale interactions, likely those between the local demographics of species at the fine scale and the landscape configuration of patches at the broad scale, play a dominant role in affecting critical transfer processes, such as dispersal, and its interrelationship with aspects of biodiversity. Cross-scale interactions have significant implications for the conservation of biodiversity, as the greatest threats to biodiversity arise from habitat modification and fragmentation associated with disturbance arising from human activities.

Quantifying temporal variability in the metacommunity structure of stream fishes: the influence of non-native species and environmental drivers

Most studies characterize metacommunities based on a single snapshot of the spatial structure, which may be inadequate for taxa with high migratory behavior (e.g., fish). Here, we applied elements of metacommunity structure to examine variations in the spatial distributions of stream fishes over time and to explore possible structuring mechanisms. Although the major environmental gradients influencing species distributions remained largely the same in time, the best-fit pattern of metacommunity structure varied according to sampling occasion and whether or not we included non-native species in the analyses. Quasi-Clementsian and Clementsian structures were the predominant best-fit structures, indicating the importance of species turnover among sites and the existence of more or less discrete community boundaries. The environmental gradient most correlated with metacommunity structure was defined by altitude, area of artificial ponds in the catchment, and dissolved oxygen content. Our results suggest that the best-fit metacommunity structure of the native species can change in time in this catchment due to seasonal changes in distribution patterns. However, the distribution of non-native species throughout the landscape homogenizes the temporal variability in metacommunity structure of native species. Further studies are necessary from other regions to examine best-fit metacommunity structures of stream fishes within relatively short environmental gradients.

Modeling Stream Fish Assemblages with Niche Apportionment Models: Patterns, Processes, and Scale Dependence

Understanding patterns in terms of the mechanistic processes that produce them is the essence of ecology. However, many studies merely document nonrandom patterns of species coexistence without providing insight into the structuring mechanisms and their scale dependence. Moreover, few studies address the mechanisms driving functional diversity. The objectives of this study were to document empirical patterns of species coexistence within stream fish assemblages across a broad geographic region, addressing both the functional and taxonomic organization of those assemblages, and to determine whether the structuring mechanisms are scale dependent. We gathered data from a report published by the Texas Parks and Wildlife Department that provided empirical species abundance distributions for 62 assemblages from 7 ecoregions and 18 river basins. For each assemblage, we simulated expected species abundances using five previously published models of niche apportionment. These models generate relative abundances in proportion to niche breadth and are constrained to maintain observed species richness, but they do not include information on species identity. These models included four stochastic variants of a geometric series (dominance preemption, random fraction, broken stick, and dominance decay) in which species abundances relate directly to niche breadth and one model (random assortment) in which abundances are independent of niche partitioning altogether. To assess scale dependence, we repeated the analyses after pooling assemblages by ecoregion, river basin, and the entire state. The patterns of niche apportionment for assemblages pooled according to river basin are more consistent with local assemblages than with assemblages pooled according to ecoregions or the entire state. These results suggest that niche apportionment plays a crucial role in structuring stream fish assemblages and that functional groups divide niche space more evenly than do species.

Spatiotemporal variation in functional and taxonomic organization of stream-fish assemblages in central Texas

Stream ecosystems across the world are dynamic and complex. Biogeographic and historical constraints cause many of these systems to be unique and create difficulty in predicting compositional changes in fish assemblages. However, each system appears to comprise similar functional groupings despite phylogenetic differences. I hypothesized that assemblages within a river would be more similar to one another, regardless of season, than assemblages from other localities based on taxonomic organization; however, assemblages from different rivers but within the same season would be more similar to one another based on functional organization. I assessed functional and taxonomic organizations of fish assemblages from three tributaries of the Colorado River in Texas (Pedernales River, San Saba River, and South Llano River) across all four seasons. Direct ordination methods resulted in assemblages from the same river, regardless of season, clustering together. More specifically, the functional and taxonomic organizations of assemblages from the South Llano River, regardless of season, were noticeably different than that from both the Pedernales River and the San Saba River, at least with respect to the first ordination axis. Current velocity, habitat availability, and temperature were influential in structuring the functional organization of assemblages in central Texas, while current velocity, substrate availability, and stream width were influential in structuring the taxonomic organization.

Development of a Morphological Index of the Nutritional Status of Juvenile Largemouth Bass

Abstract We describe the morphological changes associated with starvation in larval largemouth bass Micropterus salmoides and develop bivariate and multivariate morphological indices of nutritional status. We obtained hatchery-reared largemouth bass, raised them until completion of fin development, and divided them into two randomized experimental groups of fed and unfed fishes. Fed fishes were provided with newly hatched brine shrimp twice daily. We quantified morphological changes in body shape using 23 morphometric characters. After only 3 d of food deprivation, we were able to detect statistically significant differences in morphology between the fed and unfed fish using multivariate analysis. The magnitude of the mean difference increased over time. An unexpected result suggested that a simple, bivariate ratio of standard length to body depth at the anus was almost as efficient and robust at classifying fed and unfed largemouth bass from an independent data set as a multivariate index based on all 23...

Multiple hypotheses testing of fish incidence patterns in an urbanized ecosystem

Ecological and evolutionary theories have focused traditionally on natural processes with little attempt to incorporate anthropogenic influences despite the fact that humans are such an integral part of virtually all ecosystems. A series of alternate models that incorporated anthropogenic factors and traditional ecological mechanisms of invasion to account for fish incidence patterns in urban lakes was tested. The models were based on fish biology, human intervention, and habitat characteristics. However, the only models to account for empirical patterns were those that included fish invasiveness, which incorporated species-specific information about overall tolerance and fecundity. This suggests that species-specific characteristics are more important in general distributional patterns than human-mediated dispersal. Better information of illegal stocking activities is needed to improve human-mediated models, and more insight into basic life history of ubiquitous species is needed to truly understand underlying mechanisms of biotic homogenization.

Helminth parasite assemblages in two cyprinids with different life history strategies

Parasitic organisms can affect ecosystems by driving population dynamics of the hosts and influencing community interactions. The life history of the host can determine the relationship with its parasites. Reproductive effort and age of the host are two life history aspects often used to explain parasitic infection. In this study, we examined helminth parasite assemblages in two cyprinids with contrasting reproductive strategies, Cyprinella venusta (crevice spawners) and Notropis volucellus (broadcast spawners), in the Paluxy River (Texas) from May 2014 through October 2015. Host reproduction was measured using the gonadosomatic index, and standard length was used as an estimate of age. Parasite infection was measured using total number of helminths, parasite richness, Shannon’s diversity, and Simpson’s diversity. Our results revealed significant differences in parasite number and diversity between the two species, but not between males and females within species. Additionally, our results showed that standard length was a better predictor of parasitic infection than the gonadosomatic index. The relationship between host size and parasitic infection was expected; however, the lack of a relationship between gonadosomatic indices and parasitic infection was surprising. In conclusion, standard length was a better predictor of parasitic infection than the gonadosomatic index, and as such multiple species and life history traits should be considered when investigating host–parasite relationships.

Taxonomic and functional organization in metacommunity structure of stream-fish assemblages among and within river basins in Texas

Metacommunities are spatially structured communities linked by dispersal. They provide a connection between local community assembly and regional-level processes. One of the more widely used methods to address questions related to metacommunity concepts is those that characterize elements of metacommunity structure by quantifying aspects of coherence, species turnover, and boundary clumping. In this study, we used this approach to study the spatial ecology of freshwater fishes in Texas. Stream-fish assemblages in Texas provide an excellent opportunity to examine the patterns of metacommunity structure due to the number of drainages that empty directly into the Gulf of Mexico, which minimizes the likelihood of dispersal between basins while allowing for longitudinal movement within basins. We used fisheries data published by the Texas Parks and Wildlife Department and from the North American Water Quality Association, which consisted of 94 sampling localities distributed across 18 river basins and 11 ecoregions. To examine within-basin patterns, we focused only on the Brazos, Colorado, and Trinity rivers because of the number of sites within each of these basins. From a taxonomic standpoint, we consistently observed Clementsian patterns regardless of whether it was among or within river basins, whereas we mostly observed Gleasonian patterns from a functional perspective. Only one functional group was found at all sites in each of the three main river basins, which consisted of invertivores with an equilibrium life-history strategy. Various bioclimatic variables were significantly correlated with metacommunity structure, but these correlations differed between taxonomic and functional organization and differed depending on which river basin was considered. The results of this study support previous findings that species composition and functional traits relate to environmental gradients, but further our understanding by providing additional evidence that species sorting processes are the dominant structuring mechanisms.

Environmental correlates of phylogenetic structure in Mexican bat communities

Contemporary examination of community structure requires analyzing multiple dimensions of biodiversity to provide a more holistic understanding of the ecological and historical factors involved in the formation and maintenance of local communities. We used empirical data on bat communities across Mexico to investigate patterns of taxonomic and phylogenetic diversities. More specifically, we 1) characterized phylogenetic structure using multiple measures of diversity and identified environmental factors that serve as potential drivers of the underlying structure, 2) determined whether any bat community showed evidence of phylogenetic overdispersion or underdispersion and characterized how taxonomic structure differed among overdispersed, randomly dispersed, and underdispersed sites, and 3) examined the relationship between phylogenetic diversity and size of the regional species pool. We used 3 indices of phylogenetic diversity (Faiths index [FI], mean nearest neighbor distance [MNND], and mean pairwise distance [MPD]) to quantify patterns of evolutionary relatedness in bat communities across Mexico. To determine whether a particular community was overdispersed, randomly dispersed, or underdispersed, we used randomization procedures with latitudinal and longitudinal constraints to define the regional species pool for each bat community. We used regression trees to investigate which climatic factors explained variation for each measure of phylogenetic diversity. Precipitation and longitude were correlated with FI, temperature was correlated with MNND, and temperature along with latitude and longitude were correlated with MPD. We found that just over one-half of the communities were underdispersed, just under one-half were randomly dispersed, and only 1 was overdispersed. Each measure of phylogenetic diversity was differently affected by the size of the regional species pool.