Fungi and deadwood diversity: A test of the area-heterogeneity trade-off hypothesis

Environmental heterogeneity is one of the most fundamental drivers of
species diversity. For decades, ecologists have suggested that
heterogeneity-diversity relationships are generally positive. But today, a
greater variety of heterogeneity-diversity relationships is discussed. In
this study, we contrasted two hypotheses for wood-inhabiting fungi: The
classical heterogeneity-diversity hypothesis, that predicts positive
relationships due to an increase in niche dimensionality with increasing
heterogeneity. And the more recently stated area-heterogeneity trade-off
hypothesis, that predicts a unimodal pattern due to an inherent trade-off
between the number of occupied niches and the effective area per species.
It allows positive and negative relationships only as special cases. We
sampled 3,715 deadwood objects on 135 plots along a forest structure
gradient in the Black Forest, Germany, and recorded 284 wood-inhabiting
fungal species. To assess heterogeneity of deadwood structures, we
calculated two multidimensional structural diversity indices: Structural
richness was used as a measure of available niche space, and structural
divergence as a measure of multivariate variance within that niche space.
Those indices were then related to species richness estimates for rare,
common, and dominant species, using the framework of Hill numbers. We
found a linear, positive effect of structural richness and a unimodal
effect for structural divergence on estimated species diversity.
Structural richness, but not structural divergence was strongly correlated
with the number of sampled deadwood objects. No clear differences between
the responses of rare, common, and dominant species to the two
heterogeneity gradients were found. We also estimated the mean abundance
as proxy for mean population size, which decreased significantly with
structural richness, but was non-significantly related to structural
divergence. Synthesis: In general, the results of this study suggest a
unimodal heterogeneity-diversity relationship for deadwood-inhabiting
fungi, and are thereby in line with the area-heterogeneity trade-off
hypothesis. Thus, the negative effect of heterogeneity should lead to
lower species richness and higher risk of stochastic extinctions at high
levels of heterogeneity. However, as deadwood amount and deadwood
diversity are often strongly correlated, we argue that the positive effect
of resource availability on species richness may mask the negative effect
of structural heterogeneity in some cases.

环境异质性（environmental heterogeneity）是驱动物种多样性（species diversity）形成的最核心因素之一。数十年来，生态学家普遍认为异质性-物种多样性关系通常呈正向关联。但如今，学界对该关系的讨论已呈现出更多样的类型。本研究针对木生真菌（wood-inhabiting fungi）对比了两类假说：一是经典异质性-多样性假说（classical heterogeneity-diversity hypothesis），该假说认为随着异质性提升，生态位维度（niche dimensionality）随之增加，因此二者呈正向关联；二是近年提出的面积-异质性权衡假说（area-heterogeneity trade-off hypothesis），该假说指出二者会呈现单峰模式（unimodal pattern），其核心源于占据的生态位数量与物种种群有效面积之间存在固有权衡，仅在特殊情况下才会出现正向或负向关联。我们在德国黑林山（Black Forest）沿森林结构梯度设置的135个样地（plot）中，共采集了3715件枯木（deadwood）样本，记录到284种木生真菌。为评估枯木结构的异质性，我们计算了两类多维结构多样性指数：以结构丰富度（structural richness）表征可用生态位空间，以结构分化度（structural divergence）衡量该生态位空间内的多元方差。随后基于希尔数（Hill numbers）分析框架，将这些指数与稀有种、常见种及优势种的物种丰富度估计值进行关联分析。研究发现，结构丰富度对估计的物种多样性呈线性正向影响，而结构分化度则表现出单峰效应。结构丰富度与采样枯木样本的数量显著相关，而结构分化度则无此关联。未发现稀有种、常见种与优势种对两类异质性梯度的响应存在显著差异。我们还以平均多度（mean abundance）作为种群平均大小的代理指标，发现其随结构丰富度提升显著下降，而与结构分化度无显著关联。综合分析表明：本研究结果显示，枯木栖息真菌的异质性-物种多样性关系呈单峰模式，这与面积-异质性权衡假说相符。据此，当异质性水平过高时，其负向效应会导致物种丰富度降低，同时提升随机灭绝（stochastic extinctions）风险。但由于枯木总量与枯木多样性通常存在较强相关性，我们认为资源可获得性（resource availability）对物种丰富度的正向效应，在部分场景中可能会掩盖结构异质性带来的负向影响。