Vegetação de cerrado e conservação : relação entre teorias, influência da escala e variação espacial
Baldoni, Raquel Negrão
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A community can be defined purely as a group of individuals in a certain area? Since the beginning of ecology as a science, many authors have proposed theories to explain the structure of plant communities and therefore the spatial variation. However, although the accumulated knowledge has advanced to challenge the theories with each other, we have not reached many consensus within the community ecology. Likewise it has not been an easy task to establish physical or functional boundaries between the plant communities in the field, has also not been easy to establish boundaries between theories, approaches and influences of diferents areas of science, such as the biogeografia and evolution. Although some researchers have been working on attempts to establish an integrated theory was not only achieved little success and earned the antipathy of many trends or incited passions in scientific circles. On the one hand, the structure of the plant community may be the result of chance that accumulates over time and space, or by environmental heterogeneity due to specific ecological adaptations of different species. On the other hand, the gradient study indicates recognition of closed communities separated by transitional areas whose influence among communities, such highly dynamic areas, regulate their diversity, as endemic center would be essential for management and maintenance actions. Thus, the thresholds may reflect discontinuities on many different levels, but all kinds of limits can help to draw the line where there is success or failure in the prevalence of survival, growth and reproduction, sufficient to prevent the extinction of a species. In this context, the objective of this work is especially to propose a new old way to look at the structure of plant communities, trying to recognize the principles of the proposed theories from the scale of influence on spatial variation in community composition. Still, considering the current context of environmental changes caused by human activities, we will evaluate the effect of artificial transitions in the structure and diversity of communities. Furthermore, given the important contribution of ecotonal areas as endemic centers, we evaluate the contribution of the occurrence of threatened species in these areas for the Brazilian ecosystems conservation status. In this analysis, greater focus was given to the Cerrado, as the Brazilian Phytogeographic Domain presenting transition areas with almost all other ecosystems, with the exception of Pampa. Our results indicated a greater heterogeneity among the grasslands than in the savanna, but in thin scales both guard similarity close to 1, with a higher similarity between samples than expected by chance. This allows us to consider that the fine-scale continuum theory applies to structuring of communities and its spatial variation. In a coarser scale, communities vary as of continuum, in which the case studied in Itirapina Ecological Station, the continuum is represented by the part of Coutinho's gradient ("Grassland – Savanna - Forest") as Grassland-Forest. However, within a mosaic of ecotones and homogeneous types of vegetation, we find gradients nested with the refinement of the scale. In this case, the influence of the scale in approaches to the structuring of the Cerrado communities could be related to fractals, from the recognition of repeated linked units. Considering the intense fragmentation of the Cerrado vegetation, the invasibility of the communities and the biodiversity loss by invasive alien species, it is necessary to carefully evaluate the effect of artificial transitions in the structure of plant communities and establish 3 effective measures of management of invasive grasses for the focus of established invasion. Still, one must consider the ecosystem conservation status are significantly influenced by the heterogeneity that should be considered in priority actions for conservation.