基于结构方程模型分析荒漠啮齿动物优势种对不同放牧干扰的响应

Response of dominant desert rodent species to grazing disturbances: a structural equation modeling analysis

放牧被认为是草地生态系统的主要干扰之一。许多研究表明,放牧影响了啮齿动物的个体条件、繁殖、种群动态、群落组成及群落多样性等多个方面。由于受传统统计学方法的限制,这些研究只是单独的对放牧作用进行假设检验,而没有将多个假设放在同一个框架内进行验证。2006—2011年,采用标志重捕法对内蒙古阿拉善荒漠区不同放牧生境中啮齿动物优势种的数量进行研究并使用结构方程建模的方法将可能影响荒漠啮齿动物优势种的环境因子(气候条件、土壤硬度、植物隐蔽、植物生物量等19个指标)与啮齿动物优势种种群动态结合后进行多假设检验,旨在找出不同放牧干扰下制约优势啮齿动物环境因子的发生途径。结果表明:气候条件对五趾跳鼠和三趾跳鼠有直接不利影响,隐蔽对于两足活动的五趾跳鼠有负效应,而对四足活动的子午沙鼠有正效应。植物生物量对五趾跳鼠有正效应,但对子午沙鼠有负效应。土壤硬度的增加间接的有利于五趾跳鼠,对子午沙鼠有直接的正效应。因此荒漠啮齿动物优势种种群动态是多个过程共同作用的结果,气候条件是驱动啮齿动物种群动态的根本原因,而放牧会因为改变植物隐蔽而影响啮齿动物在群落中的组成。

Livestock grazing is one of the primary causes of disturbances to grassland ecosystems. Ecosystems pay dramatic ecological costs in terms of their composition, structure, and function because of overgrazing. These costs included decreases in plant biomass, reductions in species richness and the number of high quality forage species, and increases in the numbers of toxic species and the percentage of bare ground, as well as changes to the physical and chemical properties of their soil. Livestock grazing has an effect not only on plants and soil, but also on small mammals. Many studies have suggested that grazing affects rodents in terms of their individual conditions, breeding, population dynamics, community structure, and species diversity. These effects could be further complicated by a background of climate change. However, because of the limits of traditional statistical methods, most studies only test individual hypotheses for grazing impacts on rodents, and so few studies test multi-hypothesis frameworks. A study was conducted from 2006 to 2011 on sites experiencing grazing exclusion, rotational-grazing, and over-grazing to investigate the effects of grazing on the population densities of dominant rodent species in Alashan, Inner Mongolia, China. Rodent species were identified and monitored using a live-trapping method. We trapped nine species including Dipus sagitta, Stylodipus andrewsi, Allactaga sibirica, Phodopus roborovskii, Cricetulus barabensis, Allocricetulus eversmanni, Meriones meridianus, M. unguiculatus, and Spermophilus alaschanicus. D. sagitta, A. sibirica, and M. meridianus were found to be dominant in their local rodent communities. In addition, climate data and soil compaction, as well as the coverage, height, density, and biomass of vegetation were measured in the study areas. In this study, data on dominant rodent species population densities were combined with habitat factors to perform a multi-hypothesis test using structural equation modeling. We expected to find which habitat factors had the greatest effects on the rodent population. The results indicated that climate conditions had direct negative effects on the population densities of D. sagitta and A. sibirica. The vegetation shield had negative effects on the population density of A. sibirica, but positive effects on that of M. meridianus. Increasing soil compaction could indirectly promote population density of A. sibirica, but directly increased the population density of M. meridianus. The vegetation shield had the greatest direct effect on dominant rodents, but soil compaction had the most indirect effects. These results suggest that the population dynamics of dominance in desert rodent species have been driven by many processes, and that climate has been a key factor in these processes. However, the population dynamics of dominance in rodent communities has been primarily regulated by changes in grazing due to changing vegetation cover.