Abstract
Recognition is growing that besides ungulates, small vertebrate and invertebrate herbivores are important drivers of grassland functioning. Even though soil microarthropods play key roles in several soil processes, effects of herbivores—especially those of smaller body size—on their communities are not well understood. Therefore, we progressively excluded large, medium and small vertebrate and invertebrate herbivores for three growing seasons using size-selective fences in two vegetation types in subalpine grasslands; short-grass and tall-grass vegetation generated by high and low historical levels of ungulate grazing. Herbivore exclusions generally had few effects on microarthropod communities, but exclusion of all herbivore groups resulted in decreased total springtail and Poduromorpha richness compared with exclusion of only ungulates and medium-sized mammals, regardless of vegetation type. The tall-grass vegetation had a higher total springtail richness and mesostigmatid mite abundance than the short-grass vegetation and a different oribatid mite community composition. Although several biotic and abiotic variables differed between the exclusion treatments and vegetation types, effects on soil microarthropods were best explained by differences in nutrient and fibre content of the previous year’s vegetation, a proxy for litter quality, and to a lesser extent soil temperature. After three growing seasons, smaller herbivores had a stronger impact on these functionally important soil microarthropod communities than large herbivores. Over longer time-scales, however, large grazers created two different vegetation types and thereby influenced microarthropod communities bottom-up, e.g. by altering resource quality. Hence, both short- and long-term consequences of herbivory affected the structure of the soil microarthropod community.
Highlights
Grazing is widely recognised as one of the main ecosystem drivers in grasslands [1]
Juvenile microarthropods were omitted from all analyses, as we focus on relative differences in microarthropod communities between vegetation types and herbivore exclusion treatments, not on absolute abundances of different microarthropod groups
The abundance of Monogynaspida was significantly higher in the tall- than in the shortgrass vegetation (Table 1, Fig. 2), while the abundance of none of the other soil microarthropod groups differed among vegetation types
Summary
A large body of literature has provided proof of the manifold influences of large grazers, mainly ungulates, on the structure and functioning of grasslands [2]. It is becoming increasingly clear that the functionally different smaller vertebrate [3, 4] and invertebrate [5] herbivores play important roles in the ecology of these ecosystems. Like large herbivores, mediumsized and small herbivores can lower plant abundance through consumption, lower or enhance plant productivity through changes in nutrient cycling, increase plant biomass through changes in competitive interactions among plants or alter plant diversity [3,4,5,6]. Smaller herbivores tend to return nutrients (faeces, urine) to the system more evenly distributed compared with large grazers [9]
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