Herbivore regulation of savanna vegetation: Structural complexity, diversity, and the complexity–diversity relationship

Large mammalian herbivores exert strong top-down control on plants, which
in turn influence most ecological processes. Accordingly, the decline,
displacement, or extinction of wild large herbivores in African savannas
is expected to alter the physical structure of vegetation, the diversity
of plant communities, and downstream ecosystem functions. However,
herbivore impacts on vegetation comprise both direct and indirect effects
and often depend on herbivore body size and plant type. Understanding how
herbivores affect savanna vegetation requires disaggregating the effects
of different herbivores and the responses of different plants, as well as
accounting for both the structural complexity and composition of plant
assemblages. We combined high-resolution Light Detection and Ranging
(LiDAR) with field measurements from size-selective herbivore exclosures
in Kenya to determine how herbivores affect the diversity and physical
structure of vegetation, how these impacts vary with body size and plant
type, and whether there are predictable associations between plant
diversity and structural complexity. Herbivores generally reduced the
diversity and abundance of both overstory and understory plants, though
the magnitude of these impacts varied substantially as a function of body
size and plant type: only megaherbivores (elephant and giraffe) affected
tree cover, whereas medium- and small-bodied herbivores had stronger
effects on herbaceous diversity and abundance. We also found evidence that
herbivores altered the strength and direction of interactions between
trees and herbaceous plants, with signatures of facilitation in the
presence of herbivores and of competition in their absence. While
megaherbivores uniquely affected tree structure, medium- and small-bodied
species had stronger (and complementary) effects on metrics of herbaceous
vegetation structure. Plant structural responses to herbivore exclusion
were species-specific: of five dominant tree species, just three exhibited
significant individual morphological variation across exclosure
treatments, and the size class of herbivores responsible for these effects
varied across species. Irrespective of exclosure treatment, more
species-rich plant communities were more structurally complex. We conclude
that the diversity and architecture of savanna vegetation depends on
consumptive and non-consumptive plant–herbivore interactions; the roles of
herbivore diversity, body size, and plant traits in mediating those
interactions; and a positive feedback between plant diversity and
structural complexity.

大型哺乳类食草动物对植物施加强烈的自上而下调控（top-down control），而植物又反过来影响大多数生态过程。因此，非洲稀树草原中野生大型食草动物的减少、迁移或灭绝，预计会改变植被的物理结构、植物群落的多样性以及下游生态系统功能。然而，食草动物对植被的影响既包括直接效应也包括间接效应，且通常取决于食草动物的体型和植物类型。要理解食草动物如何影响稀树草原植被，需区分不同食草动物的效应与不同植物的响应，同时考虑植物组合的结构复杂性和组成。我们将高分辨率光探测与测距技术（Light Detection and Ranging, LiDAR）与肯尼亚体型选择性食草动物排除区的野外测量相结合，以确定食草动物如何影响植被的多样性和物理结构、这些影响如何随体型和植物类型变化，以及植物多样性与结构复杂性之间是否存在可预测的关联。食草动物总体上降低了上层和下层植物的多样性与丰度，但这些影响的程度因体型和植物类型而存在显著差异：仅巨型食草动物（megaherbivores，如大象和长颈鹿）会影响树木覆盖度，而中小型食草动物则对草本植物的多样性和丰度有更强的影响。我们还发现证据表明，食草动物改变了树木与草本植物之间相互作用的强度和方向：存在食草动物时表现出促进效应（facilitation）的特征，而缺失时则表现出竞争特征。尽管巨型食草动物对树木结构有独特影响，但中小型物种对草本植被结构指标的影响更强（且具有互补性）。植物对食草动物排除的结构响应具有物种特异性：在五种优势树种中，仅三种在不同排除处理间表现出显著的个体形态变异，且导致这些效应的食草动物体型类别因物种而异。无论排除处理如何，物种更丰富的植物群落结构更复杂。我们得出结论：稀树草原植被的多样性和结构取决于消耗性与非消耗性的植物-食草动物相互作用；食草动物多样性、体型和植物性状在调节这些相互作用中的作用；以及植物多样性与结构复杂性之间的正反馈。