What Factors Affect Diversity and Species Composition of Endangered Tumbesian Dry Forests in Southern Ecuador?

Abstract

This paper reports a study on species richness and composition of Tumbesian dry forest communities. We tested two alternative hypotheses about species assemblage processes in tropical dry forests: (1) species assemblage is determined by the filtering effect of environmental conditions and (2) species assemblage is determined by facilitative processes along the gradient of water availability, and thus, species richness and evenness increase as water becomes limited. In addition, we also explored the effect of climate and soil conditions on species composition in tropical dry forests. Species composition was sampled in 109 plots in terms of cover and tree diameter at breast height. Climatic, edaphic, topographic and anthropogenic degradation variables were obtained for each plot. We used generalized linear models and canonical correspondence analyses to evaluate the effect of environmental variables on species composition, richness and evenness. Water availability negatively affected richness and significantly determined the species assemblage. Species richness increased from ridges to valleys and evenness increased at higher altitudes. Soil characteristics showed no effect on richness and evenness but soil moisture, nitrogen concentration and soil temperature explained significant fractions of species composition. Although timber extraction and livestock in our study area were of low intensity, it negatively affected richness but had only a minor effect on species composition. Our results suggest that species composition in these endangered tropical dry forests may be at least partially explained by the stress-gradient hypothesis, with higher species richness at drier conditions probably induced by facilitation processes. Presentamos un estudio sobre la riqueza y la composición de las comunidades del bosque seco Tumbesino. Probamos dos hipótesis sobre el ensamble de especies en los bosques secos tropicales: (1) el ensamble es determinado por el efecto de filtrado de las condiciones ambientales, los sitios más benignos serían más ricos en especies, y (2) la agrupación de especies es determinada por los procesos de facilitación en el gradiente de disponibilidad de agua (Hipótesis de la Gradiente de Estrés). Además, se exploró el efecto del clima y condiciones del suelo sobre la composición de especies en estos bosques. La composición de especies fue muestreada en términos de densidad y diámetro del árbol a la altura del pecho en 109 parcelas. Para cada parcela obtuvimos variables climáticas, edáficas, topográficas y de degradación antropogénica. Utilizamos análisis multivariantes para evaluar el efecto de las condiciones ambientales sobre la composición de especies y sobre la riqueza y equitatividad. La disponibilidad de agua afectó negativamente a la riqueza y determinó de manera significativa el ensamble de especies. La riqueza se incrementó de las crestas a los valles y la equitatividad fue mayor a mayor altitud. La humedad del suelo, la concentración de nitrógeno y la temperatura del suelo explicaron fracciones significativas de la composición de especies. Nuestros resultados sugieren que la composición de especies en estos bosques puede ser, parcialmente explicada por la hipótesis de gradiente de estrés, con una mayor riqueza de especies en las condiciones más secas, probablemente inducida por los procesos de facilitación. TABLE S1. Individuals density for plot and groups in terms of taxonomic affiliation. TABLE S2. CCA models with the main matrix being the densities dataset and the basal area dataset. TABLE S3. Breakdown of average dissimilarity between the groups segregated in the cluster analysis into contributions from each species by means of a Similarity Percentage procedure. FIGURE S1. Study area and plots location (black triangle) in the Loja province, Southern Ecuador. Please note: Wiley-Blackwell is not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.