The joint effects of forest habitat area and fragmentation on Dung beetles

Habitat loss and habitat fragmentation usually occur together, at the same time and place. However, while there is consensus that habitat loss is the preeminent threat to biodiversity, the effects of fragmentation are contentious. Some argue that habitat fragmentation is not bad for biodiversity, and even that it is good. Generally, the studies that find no harm or positive outcomes of fragmentation invariably assume that it is independent of habitat loss. However, dissociating the effects of habitat fragmentation from habitat loss is questionable because the two are essentially coupled. Accordingly, we evaluated how forest area and fragmentation (via edge effects) influenced dung beetles per se, and through their effects on the abundance of mammals, using structural equation modeling (SEM). Dung beetles are very sensitive to forest habitat loss and fragmentation, and to changes in the abundance of mammals on which they depend for dung. Our study area was in the Tana River, Kenya, where forest fragments are depauperated of mammals except for two endemic species of monkeys. We mapped 12 forests, counted the resident monkeys, and sampled 113,955 beetles from 288 plots. Most of the 87 species of beetles found were small tunnellers. After implementing a fully latent Structural Regression SEM, the optimal model explained a significant 26% of the variance in abundance, and 89% of diversity. The main drivers of beetle abundance were positive, direct, effects of forest area and number of monkeys, and negative edge effects. The main drivers of diversity were the direct effects of the beetle abundance, indirect effects of forest area and abundance of mammals, and indirect negative edge effects. Thus forest area, fragmentation (via edge effects), and the number of monkeys jointly influenced the abundance and diversity of the beetles directly and indirectly.

生境丧失（habitat loss）与生境破碎化（habitat fragmentation）通常在同一时空下相伴发生。尽管学界已达成共识，认为生境丧失是对生物多样性（biodiversity）最突出的威胁，但生境破碎化的影响仍存在争议。部分学者主张生境破碎化对生物多样性非但无害，甚至有益。这类认为破碎化无危害或存在积极影响的研究，无一例外均假设破碎化与生境丧失相互独立。然而，由于二者本质上相互耦合，将生境破碎化的效应与生境丧失剥离开来的做法本身便值得商榷。为此，我们借助结构方程模型（structural equation modeling, SEM），分别评估了林区面积与破碎化（通过边缘效应（edge effects）体现）对蜣螂（dung beetles）的直接影响，以及其通过改变哺乳动物种群丰度对蜣螂产生的间接影响。蜣螂对森林生境丧失、破碎化，以及其赖以获取粪便资源的哺乳动物种群丰度变化均极为敏感。本研究区域位于肯尼亚塔纳河沿岸，其境内的森林片段中除两种特有种（endemic species）猴类外，其余哺乳动物类群均已极度匮乏。我们共测绘了12片林区，统计了栖息的猴群数量，并在288个样地中采集了共计113955头蜣螂。本次发现的87种蜣螂中，绝大多数为小型隧栖蜣螂。在构建完全潜变量结构回归型结构方程模型后，最优模型可显著解释蜣螂丰度26%的变异（具有统计学显著性），以及89%的物种多样性变异。蜣螂丰度的主要驱动因子为林区面积与猴类数量的正向直接效应，以及负向的边缘效应。而蜣螂物种多样性的主要驱动因子则包括蜣螂丰度的直接效应、林区面积与哺乳动物丰度的间接效应，以及负向的边缘间接效应。综上，林区面积、生境破碎化以及猴类数量，可通过直接与间接途径共同影响蜣螂的种群丰度与物种多样性。