Association Between the Success of an Invasive Macrophyte, Environmental Variables and Abundance of a Competing Native Macrophyte

The success of invasive species depends on the overcoming of abiotic and biotic filters. Abiotic variables likely have greater relative importance over invasion at broad spatial scales, while biotic interactions are more important at fine spatial scales. In this study, we tested the hypotheses that (i) the abundance of the invasive Hydrilla verticillata is more correlated with abiotic factors than with competing native species at broad spatial grain; and that (ii) H. verticillata abundance is more correlated with competing native species than with abiotic factors at fine spatial grain. Here, we considered spatial scale as the grain size (i.e., the extent of sampling unit) assuming broad spatial scales as a large area encompassing the entire patches of macrophytes, and fine spatial scales as a small area inside one macrophyte patch. We collected the abundance of hydrilla and the competing native species along with environmental variables in a large subtropical reservoir. To evaluate how the relative importance of the abiotic factors and the competing native species vary between spatial grains we used Bayesian Generalized Linear Models. At broad grain, the abundance of the competing native species, maximum fetch (positive correlation), turbidity and conductivity (negative correlation) were the most important factors to explain the hydrilla abundance. At fine grain, alkalinity, total organic matter of the sediment and the abundance of a competitive native species (all negative correlations) were the most important variables. Our results indicate a greater importance of abiotic factors at broader grains while competitive interactions seem to be important only in the finer spatial grains. Environmental heterogeneity may explain the positive correlation between native and invasive abundances at broad grain, while the negative correlation at fine grain suggests the effect of competition. In synthesis, we show that the abiotic factors that explain the invasion success of a submerged invasive macrophyte are the same in two spatial grains, but the importance of biotic interactions changed with grain. Thus, our data suggest that models that attempt to explain the success of invasive plants, should consider spatial scales

Inorganic carbon shortage may limit the development of submersed macrophytes in habitats of the Paran\ue1 River basin

AIM: In this investigation we carried out an experiment to assess whether the growth of Egeria najas and E. densa (two rooted submersed Hydrocharitacea, native to South America) are limited by inorganic carbon or not; METHODS: We measured the early plant growth in 3 L microcosms with alkalinities of 100 µM.L-1 and 500 µM.L-1. These alkalinites represent extremes which are typical of most waters in the Upper Paraná River basin and they represent low and high inorganic carbon (mainly bicarbonate) availabilities, respectively; RESULTS: The elongation of E. densa, and the relative growth rates and root development of both species were significantly higher when they grew in the high alkalinity treatment; CONCLUSIONS: Our results strongly suggest that in several habitats and during certain periods of the year, inorganic carbon shortage may limit the growth of both species. In this sense, phosphorus and nitrogen may assume a secondary role as controlling factors of these plants, because they assimilate these nutrients from sediment, where they are usually found in high concentrations. Thus, controlling eutrofication as a strategy to reduce their biomass may not be successful

A simple method to estimate spatial complexity in aquatic plants

We used a computerized approach to measure spatial complexity for the structural habitat provided by eight aquatic plant species collected from backwater lagoons located in the Upper Paraná River floodplain, Brazil. The plant species were: Cabomba furcata Schult. and Schult.f., Eichhornia azurea (Sw.) Kunth (stems and roots), Eichhornia crassipes (Mart.) Solms, Egeria najas Planchon, Heteranthera cf. zosterifolia, Potamogeton cf pusillus, Utricularia foliosa L., and Nymphaea amazonum Mart. and Zucc. The upper 0.5 m length of the terminal stems was quantified for complexity. Mean frequency and length of the interstices were significantly different among plant species. Spatial complexity varied (F = 17.30; p < 0.0001) among the different plant species with E. azurea roots and U. foliosa exhibiting the highest levels and E. azurea and N. amazonum stems the lowest. These unique spatial complexities suggested that aquatic plants possessed a differential contribution to the habitat heterogeneity in Upper Paraná River lagoons

Effects of limnological and morphometric factors upon Zmin, Zmax and width of Egeria spp stands in a tropical reservoir

In this work, regression analyses were used to test the effects of fetch, littoral slope and Secchi disk depth upon the stand width (Wi) and maximum (Zmax) and minimum (Zmin) colonization depths of Egeria spp in a large tropical reservoir (Rosana Reservoir). Littoral slope was the only variable correlated with Wi, which was larger in locations with lower slopes. The higher Wi values (up to 260m) were found in locations where slopes were lower than 0.05 m.m-1. Zmin and Zmax were correlated positively with fetch. Nevertheless, different relationships were found at lower and higher fetch values observed in the arms and in the main body of the reservoir, respectively. At lower values, Secchi disk depth was the main variable explaining Egeria Zmax. On the other hand, in locations where fetch was longer (i.e. in the main axis of the reservoir), it was the only variable that explained Zmax significantly. Despite the great variability, the results indicated that the measured limnological (Secchi disk) and morphometric (fetch and slope) variables were important determinants of Egeria spp colonization in the Rosana Reservoir

Biological interactions in the co-occurrence of Eichhornia azurea and free-floating macrophytes = Interações biológicas na co-ocorrência de Eichhornia azurea e macrófitas flutuantes livres

The dynamic of plant communities is strongly influenced by biologicalinteractions. We studied the effects of Eichhornia azurea on the growth of the free-floating macrophytes Eichhornia crassipes, Salvinia minima, Salvinia auriculata and Limnobium laevigatum. The experiments were carried out in three lakes of the upper Paraná river floodplain. We used 0.5 m side squares where these species grew for 11 days (S. auriculata and L. laevigatum) and 17 days (E. crassipes and S. minima), in the presence and absence of E. azurea. There were no significant differences between the relative growth rates of macrophytes growing in the absence and presence of E. azurea. The relative interaction index showed the existence of competition between E. azurea and L. laevigatum, and neutrality with the other three species. There was no evidence of facilitation related to the improvement of physicochemistry characteristics that could enhance macrophytes growth. However, we do not discard a ‘mechanical’ facilitation between E. azurea, which offers protection against windand waves, and the other species of macrophytes.A dinâmica das comunidades vegetais é fortemente influenciada pelasinterações positivas e negativas que desempenham papel importante na manutenção e diversidade das espécies, coexistência e produtividade das comunidades vegetais. Foram estudados os efeitos da macrófita enraizada Eichhornia azurea sobre as espécies de macrófitas flutuantes Eichhornia crassipes, Salvinia minima, Salvinia auriculata e Limnobium laevigatum, em três ressacos da planície de inundação do alto rio Paraná, Brasil. Para os experimentos in situ foram utilizados quadrados de 0,5 m de lado, onde estas espécies cresceram durante 11 dias (S. auriculata e L. laevigatum) e 17 dias(E. crassipes e S. minima), em presença e ausência de E. azurea. Não houve diferenças significativas entre a taxa de crescimento relativo das macrófitas flutuantes crescendo na ausência e na presença de E. azurea. O índice de interação relativa demonstrou a existência de competição entre esta espécie e L. laevigatum e neutralidade na relação com as demais espécies. Não houve, porém, indícios da ocorrência de um mecanismo facilitador relacionado a mudanças nas características físicas e químicas, ocasionadas por E. azurea, que estimulasse o crescimento das espécies flutuantes. Porém, não descartamos a ocorrência de facilitação ‘mecânica’ entre E. azurea, a qual oferece proteção contra as ondas e o vento, e as espécies flutuantes livres