Bryophyte richness of soil islands on rocky outcrops is not driven by island size or habitat heterogeneity

ABSTRACT The species-area relationship (SAR) is one of the oldest and most studied ecological models, having even served as the foundation of the Theory of Island Biogeography. Nevertheless, the relative importance of habitat heterogeneity to SAR remains poorly understood. Our aim was to test the relative importance of habitat heterogeneity to the SAR of bryophyte assemblages of soil islands of rocky outcrops in the semi-arid region of Brazil. We randomly selected 15 to 20 soil islands on each of four outcrops for a total of 59 soil islands, and calculated the area, mean depth, and number of substrates for each. We used Generalized Linear Models (GLM) to test the SAR with two models, one using species richness and another using life-form richness as the dependent variables. We found no positive relationship between area and habitat heterogeneity, nor any evidence of a SAR, such as a positive relationship between area and species or life-form richness neither between habitat heterogeneity and species richness. However, our findings did show that life-form richness is related to bryophyte species richness on the soil islands. We conclude by suggesting that not only can microclimate influence bryophyte richness, but opportunistic colonization by bryophytes is also important.

摘要：种-面积关系（species-area relationship, SAR）是最古老且研究最为广泛的生态学模型之一，甚至曾作为岛屿生物地理学理论的核心基础。尽管如此，生境异质性对种-面积关系的相对重要性仍有待深入阐明。本研究旨在检验生境异质性对巴西半干旱区域岩石露头土壤岛苔藓植物群落的种-面积关系的相对重要性。我们在4处露头中各随机选取15至20个土壤岛，共计59个土壤岛，并计算了每个土壤岛的面积、平均深度以及基质数量。我们采用广义线性模型（Generalized Linear Models, GLM）构建两类模型以检验种-面积关系：其一以物种丰富度为因变量，其二以生活型丰富度为因变量。研究结果显示，面积与生境异质性之间不存在正相关关系，同时既未发现面积与物种丰富度或生活型丰富度之间存在与种-面积关系相关的正相关，也未发现生境异质性与物种丰富度之间存在关联。但本研究确实发现，土壤岛的苔藓植物生活型丰富度与物种丰富度存在关联。最后我们提出，微气候不仅会影响苔藓植物的物种丰富度，苔藓植物的机会性定植也同样具有重要意义。