Spatial heterogeneity and plant species richness at different spatial scales under rabbit grazing

Abstract

Herbivores influence spatial heterogeneity in soil resources and vegetation in ecosystems. Despite increasing recognition that spatial heterogeneity can drive species richness at different spatial scales, few studies have quantified the effect of grazing on spatial heterogeneity and species richness simultaneously. Here we document both these variables in a rabbit-grazed grassland. We measured mean values and spatial patterns of grazing intensity, rabbit droppings, plant height, plant biomass, soil water content, ammonia and nitrate in sites grazed by rabbits and in matched, ungrazed exclosures in a grassland in southern England. Plant species richness was recorded at spatial scales ranging between 0.0001 and 150 m(2). Grazing reduced plant height and plant biomass but increased levels of ammonia and nitrate in the soil. Spatial statistics revealed that rabbit-grazed sites consisted of a mixture of heavily grazed patches with low vegetation and nutrient-rich soils (lawns) surrounded by patches of high vegetation with nutrient-poor soils (tussocks). The mean patch size (range) in the grazed controls was 2.1 +/- 0.3 m for vegetation height, 3.8 +/- 1.8 m for soil water content and 2.8 +/- 0.9 m for ammonia. This is in line with the patch sizes of grazing (2.4 +/- 0.5 m) and dropping deposition (3.7 +/- 0.6 m) by rabbits. In contrast, patchiness in the ungrazed exclosures had a larger patch size and was not present for all variables. Rabbit grazing increased plant species richness at all spatial scales. Species richness was negatively correlated with plant height, but positively correlated to the coefficient of variation of plant height at all plot sizes. Species richness in large plots (<25 m(2)) was also correlated to patch size. This study indicates that the abundance of strong competitors and the nutrient availability in the soil, as well as the heterogeneity and spatial pattern of these factors may influence species richness, but the importance of these factors can differ across spatial scales.

Marsh rabbit mortalities tie pythons to the precipitous decline of mammals in the Everglades

“…As prey and predators at multiple trophic levels, nutrient cyclers and engineers of vegetation, mammals are an indispensable component of the GEE [38,39]. Our research clearly establishes pythons as a causal agent of marsh rabbit declines, a species we selected because of its theoretical resilience to predation pressure [16,17].…”

A unifying evolutionary theory for the biomass–diversity–fertility relationship

“…These include historic chance of colonisation (Ejrnaes et al 2006), environmental heterogeneity (Reynolds et al 2007), different spatial scales applying in different sections of the fertility spectrum (Anderson et al 2004), mycorrhizae (Klironomos et al 2000, decomposition (Mazzoleni et al 2007) and herbivory (Olofsson et al 2008). Whilst being important in understanding the biomass-diversity-fertility relationship, the interactions between these variables make interpreting field data difficult.…”

On the difference between “exclosures” and “enclosures” in ecology and the environment

“…Experimental exclosures (sensu Wagg, 1964) have been widely used as treatments to exclude (or statistically control for) the effects of predators, large herbivores, livestock, small mammals or birds on the species richness and recruitment in plant communities (Fraser and Madson, 2008;Jacobs and Naiman, 2008;Levick and Rogers, 2008;Negussie et al, 2008;Olofsson et al, 2008;Shitzer et al, 2008), on the abundance of other animals (Isaksson et al, 2007;Torre et al, 2007;Aerts et al, 2008;Huntzinger et al, 2008) and on processes such as sediment deposition, litter production, soil carbon sequestration and woody plant invasions (Descheemaeker et al, 2006a;2006b;Pei et al, 2008;Shrestha and Stahl, 2008;Yanoff and Muldavin, 2008).…”