Eduardo Ribeiro Cunha

Macrophyte Structural Complexity Influences Spider Assemblage Attributes in Wetlands

Macrophytes increase structural complexity in aquatic ecosystems and their emergent structures provide habitats for spiders. We sampled spiders in three species of macrophytes (Eichhornia azurea, Eichhornia crassipes and Limnobium laevigatum) and measured five traits indicative of structural complexity: horizontal structure, vertical structure, plant height, macrophyte richness and macrophyte biomass. We tested the hypothesis that spider density and diversity increase and guild composition changes along a gradient of structural complexity provided by macrophytes. Vertical and horizontal structure and macrophyte richness covaried with the attributes of spider assemblages. However, vertical structure was an important habitat-complexity trait because it covaried with all spider attributes investigated, positively affecting density and taxa richness and changing guild composition. Our findings suggest that the increase in macrophyte structural complexity, primarily through vertical structure, provides additional habitat and microhabitat variability. These forms of variability may increase the availability of prey and shelter and even reduce intraguild predation between spiders, thereby potentially increasing density and taxa richness. Moreover, an increase in vertical complexity provides available structures for web attachment, favoring web-building spiders and consequently affecting guild composition. Thus, habitat structure plays an important role in structuring spider assemblages and specific traits may mediate changes in particular attributes of spider assemblages.

Macrophyte species richness and composition are correlated with canopy openness and water depth in tropical floodplain lakes

Aquatic macrophytes colonize a variety of environments, and they play important roles in ecosystem function and in community structure. One key challenge for aquatic ecologists is to investigate the environmental factors that drive the richness and distribution of these plants. In this study, we assessed the importance of selected morphometric variables and of the canopy openness of the riparian vegetation to explain macrophyte species richness and composition. We sampled macrophytes along gradients of depth, littoral slope, distance from the connection with the river, and canopy openness in floodplain lakes connected with the Upper Paraná River. We used quadrats to survey a total of 40 sites in 11 lakes. We recorded 31 macrophyte species and found that macrophyte richness was best and positively correlated with canopy openness. This correlation suggests that canopy openness plays an important role as an environmental filter, regulating the amount of light availability for macrophytes. Macrophyte species composition was best correlated with depth, indicating that zonation of macrophyte species also occurs in these shallow lakes. These findings emphasize the importance of morphometric variables and canopy openness for structuring macrophyte assemblages. In addition, our results suggest that canopy openness and depth may be considered in management strategies aiming to recover macrophyte diversity and the processes mediated by aquatic macrophytes in small and shallow floodplain lakes.

Associations between Macrophyte Life Forms and Environmental and Morphometric Factors in a Large Sub-tropical Floodplain

Macrophyte assemblages are composed of species with different life forms and various ecological functions. Our aim was to investigate the potential environmental determinants of changes in the biomass of individual life forms and of the composition of the macrophyte assemblage in terms of life forms diversity. We sampled 23 waterbodies at low and high water levels in the Middle Paraná River floodplain. Macrophyte biomass samples were collected and classified in terms of life forms. We performed a redundancy analysis using the biomass of the various life forms to assess the importance of environmental variables to the composition of macrophyte life forms. Linear regressions were applied to investigate the environmental determinants of the biomasses of individual life forms. The degree of connectivity and the combination of depth, hydrology and nitrate were the main determinants of the composition in terms of life forms. The biomass of each individual life form was explained by different combinations of environmental variables, but the connectivity was the most important one. Our study shows that groups of species with similar life forms respond to environmental factors in particular ways, which might alter the biomass composition of life forms. Given that the ecosystem functioning depends on the functional characteristics of local communities, our findings about the relation between environmental changes and the community composition in terms of life forms (or functional composition) can be a helpful tool for predicting changes on ecosystem processes (such as nutrient cycling) against possible future scenarios.

Flying over water: how ''On bird species diversity'' influenced aquatic ecology

Habitat complexity has long been known to influence animal community structure by increasing the number of available habitats. Fifty years have passed since MacArthur brothers published the seminal paper “On bird species diversity”, which revolutionized studies of habitat structure. This paper first evidenced and quantified the relationship between species diversity (birds) and habitat structural complexity (the number of stratified layers of landscape vegetation). In this article, we aim to pay homage to R. H. MacArthur’s contribution and to briefly analyze the citation history and influence of “On bird species diversity”, focusing primarily on aquatic studies. We searched for all papers that cited “On bird species diversity” on Thomson Reuters (ISI—Web of Knowledge) and analyzed them for temporal citation trends. In addition, considering only aquatic papers, we explored whether and how habitat complexity was measured, as well as the ecological organization level, attributes of organisms, taxonomic groups and study design (observational or experimental). “On bird species diversity” citations increased over time, but this paper was less cited by limnologists compared to terrestrial and marine scientists. The majority of investigations in aquatic ecosystems quantified habitat complexity, but few used mathematical modeling. The high number of citations, which continues to increase, shows the great influence of “On bird species diversity” on ecological studies and typifies it as a classic in the ecological literature. However, the low citation frequency found in papers devoted to freshwater ecosystems indicates that limnologists in general neglect this original contribution in studies of habitat complexity.

Herbivory can mitigate, but not counteract, the positive effects of warming on the establishment of the invasive macrophyte Hydrilla verticillata

Hydrilla verticillata is a submerged, rooted macrophyte native to Asia and Australia, but currently attains broad distribution across all continents. Its success as an invasive species depends on the simultaneous influence of abiotic and biotic factors on different components of its performance. We conducted a factorial experiment to test the short-term responses of Hydrilla, present since 2005 in the upper Parana River (Brazil), to a native herbivore (apple snail Pomacea canaliculata) and increased water temperature, using two different spatial arrangements of macrophyte fragments (one simulating early establishment phase and other simulating late establishment phase). Temperature, herbivory and plant spatial arrangement individually, and in some cases through their interactions, caused changes in the growth likely indicating impacts for the ecological responses of Hydrilla´s establishment. Snail herbivory decreased plant growth thus exerting biotic resistance, while higher temperature increased Hydrilla´s invasiveness. According to our results and other pieces of evidence, invasions of Hydrilla might worsen under the future climate warming scenario, but herbivores might locally mitigate invasion speed or magnitude.