Thaisa Sala Michelan

Regeneration and Colonization of an Invasive Macrophyte Grass in Response to Desiccation

The emergent macrophyte Urochloa subquadripara, an exotic and invasive species, causes extensive damage to aquatic plant assemblages. Regeneration and colonization by fragments of U. subquadripara may be affected by desiccation and may differ according to the fragment portion (apical, intermediate or basal). We tested the hypotheses that the ability of U. subquadripara fragments to regenerate and colonize declines with increasing time of exposure to desiccation, and that apical portions regenerate and colonize more quickly than intermediate and basal ones. Apical, intermediate and basal portions were exposed to different desiccation periods and left to grow in a greenhouse. Sprout and root growth were negatively and significantly affected by the desiccation period; fragments exposed to intermediate levels of desiccation regenerated and colonized the sediment at rates similar to those of the control. In addition, apical fragments showed greater development and sediment colonization than intermediate and basal fragments. Thus, the data supported our two hypotheses. Our results show that U. subquadripara has a high regeneration potential, indicating that the use of water drawdown to control its spread may be ineffective, and new strategies are required.

The influence of “Homage to Santa Rosalia” on aquatic ecology: a scientometric approach

In 1959, G.E. Hutchinson provided a general explanation for the diversity of species in his paper “Homage to Santa Rosalia or why are there so many kinds of animals?” To assess the contribution of the ideas Hutchinson introduced in “The Homage” to aquatic ecology research, we performed a bibliometric evaluation of all the articles that cited this paper between 1960 and 2009. The articles were retrieved using the database from Thomson Reuters (ISI Web of Knowledge) in March 2009. For each paper, we first identified the studied environment (terrestrial, marine or freshwater) and whether the study was theoretical or empirical. For marine and freshwater studies, we recorded the journal where the article was published, the year of publication, the number of citations, the taxonomic group (e.g. fish, phytoplankton, zooplankton, macroinvertebrates or macrophytes), the habitat (e.g. wetlands, lakes, rivers, streams or ocean) and the main ideas addressed. A total of 1345 articles cited “The Homage”, and the number of citations increased significantly with time. Most of the articles that cited “The Homage” described research carried out in a terrestrial environment or were theoretical, and a lesser number of citations came from freshwater and marine papers, which used mainly fish and invertebrates as model organisms. In the aquatic sciences, most of the papers discussed the influences of competition and energy (productivity) on diversity. Our results indicate that “The Homage” can still be considered a “citation classic” and a breakthrough contribution, and that it is still having a great impact on different fields of ecology, including limnology and marine ecology.

The colonization success of an exotic Poaceae is related to native macrophyte richness, wind disturbance and riparian vegetation

Biotic and abiotic factors are filters that prevent invasions in aquatic and terrestrial ecosystems. In this investigation we tested the hypothesis that the success of a non-native Poaceae (Urochloa subquadripara) is positively correlated with the richness of native macrophytes and negatively correlated with wind disturbance (fetch) and presence of riparian vegetation on coarse spatial scales. Our samplings were carried out in a tropical reservoir (Rosana Reservoir, Brazil). We first compared competing models using the Akaike criterion to find the main combinations of explanatory variables (native macrophyte richness, fetch, and presence of riparian vegetation) associated with the success of U. subquadripara. Then, we applied multiple regressions to assess the coefficient of determination of the best models selected according to the Akaike criterion. The probability of occurrence of U. subquadripara increased significantly with increases in the number of native macrophyte species, but decreased with fetch and the presence of riparian vegetation. Stand width and maximum depth of occurrence (indicators of the success of this Poaceae) were also positively related with native richness and negatively with fetch and riparian vegetation. Our results supported our expectation that wave disturbance is an important variable explaining U. subquadripara success. Because the less exposed sites are also more favorable for colonization by natives, positive relationships between the success of non-native species and native diversity emerge at the coarse scale. Taken together, our results support the theory of “biotic acceptance”; that is, favorable sites are more prone to colonization by both native and non-native species.

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.

Competitive Effects Hinder the Recolonization of Native Species in Environments Densely Occupied by One Invasive Exotic Species

The responses of native plants to competition with invasive plants depend mainly on the density of the invasive plants and on the ability of the native plants to compete for resources. In this study, we tested the influence of the invasive exotic Urochloa arrecta (Poaceae) on the early colonization of two native species (Pontederia cordata and Leersia hexandra) of aquatic macrophytes. Our hypotheses were (i) the competitive effects of U. arrecta on the native species P. cordata and L. hexandra are density-dependent and that (ii) these species respond differently to competitive interactions with the invasive species. We conducted the experiments in a greenhouse and in the field, in a tropical reservoir. The biomass of U. arrecta (ranging from 206.2 to 447.1 g) was manipulated in the greenhouse in trays with different densities. After the establishment of the invasive species, we added P. cordata and L. hexandra propagules to each tray. In the field, a propagule of P. cordata was planted in 36 sites with different densities of U. arrecta. The biomass and length of the natives and the biomass of the invasive species were measured in the greenhouse and in the field experiments. The biomass and length of the native plants decreased with increasing biomass of the exotic species in both experiments, showing that the competition between U. arrecta and native species depends on the density of the exotic species. The root:shoot ratio of L. hexandra decreased with increasing U. arrecta biomass, but the opposite occurred for P. cordata. These results indicate that native species exhibit different strategies of biomass allocation when interacting with U. arrecta. The strong competitive effects of U. arrecta and the different responses of the native species help to explain the reduced diversity of native macrophytes observed in sites colonized by U. arrecta. The results also suggest that in a scenario of dominance of exotic species, recolonization by native macrophytes is unlike to occur naturally and without human interventions that reduce the biomass of the exotic species.