Márcio José Silveira

Epiphyton or Macrophyte: Which Primary Producer Attracts the Snail Hebetancylus moricandi?

Abstract: Relationships between snails, epiphyton, and macrophytes are widely studied because epiphytes decrease light for macrophytes, and snails may benefit the latter when they consume epiphytes. Thus, organic compounds released by macrophytes that attract snails could be an evolutionarily advantageous mechanism. This hypothesis was tested with three species of submerged macrophytes (natives: Egeria najas and Cabomba furcata; exotic: Hydrilla verticillata), which were maintained in microcosms in the presence of ancylid snails. However, the hypothesis of limpet attraction by macrophytes was rejected. Instead, epiphyton attached to E. najas attracted more snails than that attached to the other species. This attraction could be explained by chemical signals (organic compounds), released by certain species of algae that are detected by snails.

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 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.

Effects of the density of the invasive macrophyte Hydrilla verticillata and root competition on growth of one native macrophyte in different sediment fertilities

The occurrence of non-native species at high densities may generate competition for resources and possibly exclude native species in various environments. We evaluated the effects of increased densities of the non-native invasive macrophyte Hydrilla verticillata on the growth of the native species Egeria najas in different sediment types and with only root interactions or root + shoot interactions. We tested the hypothesis that the effect of the invasive on the native species is density dependent and that it is greater when competition for light and nutrients occurs (root + shoot interactions). The results of these experiments demonstrated that increased density of the invasive species H. verticillata significantly decreased the growth of the native species independent of sediment type (sand or mud sediments). When plants competed for water and sediment resources (root + shoot interactions), the native species was more impacted by the invasive than when they competed only for water resources (only shoots interacting). Our results show that E. najas is probably unable to colonize sites highly colonized by hydrilla, and this applies to both sand and mud sediments. This outcome suggests that H. verticillata is a threat for E. najas and likely other native submerged species in South America.

The potential invasiveness of an aquatic macrophyte reflects founder effects from native niche

Biological invasions are important drivers of biodiversity loss. Hence, predicting invasion risks considering the invasion history of a species might help drive meaningful conservation strategies. We investigated the niche dynamics occurring during the invasion of Hydrilla verticillata in North and South America. Because of founder effects, we hypothesised that occupied invaded area niches across North and South American are a subset of the occupied niche in the native range. Moreover, according to the invasion history, we expected that the South American niche is more similar to the North American one than the native niche. We built ecological niche models to predict the potential distribution of hydrilla from native and invaded occurrence records, as well as analysing niche overlap, equivalency, and similarity between the native and two invaded areas. Although the models predicted spatially congruent suitable sites, the ones based on native occurrence records predicted larger geographical ranges for hydrilla across South America than those based on the records of its North American invasions. The environmental conditions the species occupied in both invaded areas are modestly overlapped with the native area (Schoener’s D < 0.6), with native and South American niches showing the highest overlap and significant equivalency. Contrary to our prediction, the invaded North and South American niches presented the smallest niche overlap. Our findings suggest that founder effects triggered deep shifts in hydrilla’s occupation abilities across invaded areas, but do not support successive invasion events. Hydrilla’s Grinnellian niche was maintained throughout the invaded areas, and its potential invasion across South America is massive, regardless of its origin.