Disentangling the effects of sampling scale and size on the shape of species abundance distributions.

Many authors have tried to explain the shape of the species abundance distribution (SAD). Some of them have suggested that sampling spatial scale is an important factor shaping SADs. These suggestions, however, did not consider the indirect and well-known effect of sample size, which increases as samples are combined to generate SADs at larger spatial scales. Here, we separate the effects of sample size and sampling scale on the shape of the SAD for three groups of organisms (trees, beetles and birds) sampled in the Brazilian Atlantic Forest. We compared the observed SADs at different sampling scales with simulated SADs having the same richness, relative abundances but comparable sample sizes, to show that the main effect shaping SADs is sample size and not sampling spatial scale. The effect of scale was minor and deviations between observed and simulated SADs were present only for beetles. For trees, the match between observed and simulated SADs was improved at all spatial scales when we accounted for conspecific aggregation, which was even more important than the sampling scale effect. We build on these results to propose a conceptual framework where observed SADs are shaped by three main factors, in decreasing order of importance: sample size, conspecific aggregation and beta diversity. Therefore, studies comparing SADs across sites or scales should use sampling and/or statistical approaches capable of disentangling these three effects on the shape of SADs.

Can plant DNA barcoding be implemented in species-rich tropical regions? A perspective from São Paulo State, Brazil.

DNA barcoding helps to identify species, especially when identification is based on parts of organisms or life stages such as seeds, pollen, wood, roots or juveniles. However, the implementation of this approach strongly depends on the existence of complete reference libraries of DNA sequences. If such a library is incomplete, DNA-based identification will be inefficient. Here, we assess if DNA barcoding can already be implemented in species-rich tropical regions. We focus on the tree flora of São Paulo state, Brazil, which contains more than 2000 tree species. Using new DNA sequence data and carefully assembled GenBank accessions, we assembled 12,113 sequences from ten different regions. The ITS, rbcL, psbA-trnH, matK and trnL regions were better represented within the available sequences for São Paulo tree flora. Currently, only 58% of the São Paulo tree flora currently have at least one barcoding sequence available. However, these species represent on average 89% of the trees in São Paulo state forests. Therefore, conservation-oriented and ecological studies can already benefit from DNA barcoding to obtain more accurate species identifications. We present which taxa remain underrepresented for the São Paulo tree flora and discuss the implications of this result for other species-rich tropical regions.

Canopy gap colonization in the Atlantic Montane Rain Forest

In the Atlantic Montane Rain Forest of South-eastern Brazil, a study was carried out to describe and evaluate canopy gap colonization. Gap composition by herb species was assessed through their soil coverage and woody species by measuring and identifying all individuals taller than one meter. Gap structure (gap size, number and diameter of treefalls), topographic position and surrounding vegetation were also measured. Two genera of Marantaceae were markedly frequent and abundant inside gaps. The more abundant and frequent woody species were shade tolerant. Species-rich families found inside gaps did not differ from the forest as a whole. Results revealed that direct and indirect influences of topography were important to determine gap composition of woody species. Evidently gap colonization had a considerable influence of topography and pre-established individuals besides variables of gap structure.

Na Floresta Pluvial Atlântica Montana do Sudeste Brasileiro, foi realizado um estudo para descrever e avaliar a colonização de clareiras. A composição de clareiras foi levantada através da cobertura do solo para as espécies herbáceas enquanto que todos os indivíduos lenhosos maiores que um metro de altura foram mensurados e identificados. Também foram coletadas informações sobre a estrutura das clareiras (área da clareira, número e diâmetro das quedas), posição topográfica e vegetação circundante. Dois gêneros de Marantaceae apresentaram considerável freqüência e abundância nas clareiras. As espécies lenhosas mais freqüentes e abundantes pertenceram ao grupo não-pioneiro e as famílias mais ricas encontradas nas clareiras não diferiram quando comparado à floresta como um todo. Como para as variáveis do estrato herbáceo e da vegetação circundante, os resultados revelaram que efeitos diretos e indiretos da topografia são importantes na determinação da composição interna de clareiras por espécies lenhosas. Estes resultados sugerem que a colonização de clareiras é influenciada não apenas pelas variáveis de estrutura, mas também pela posição topográfica e pelos indivíduos pré-estabelecidos.