Posted by: oikosasa | May 21, 2013

Even the small ones are important!

Size is not all! Even small herbivores have effect on plant community, as shown in Salvador Rebollo and co-workers new Early View paper shows: “Disproportionate effects of non-colonial small herbivores on structure and diversity of grassland dominated by large herbivores”.

Here is a summary of the study by Rebollo:

Daniel G. Milchunas and Salvador Rebollo, two out of five authors of the article, along-side a large-plus-small herbivore exclosure in the shortgrass steppe.  We tested our hypotheses over a 14-year period in pastures grazed at moderate intensities by cattle and in two types of exclosures: for large (barbed-wire) and for large-plus-small herbivores (small-mesh hardware cloth).

Grasslands are grazed by a complex assemblage of herbivores that differ in body size, abundance, diet, and foraging strategies.  Grazing studies have most often examined effects of large herbivores, probably due to their greater amounts of plant consumption and economic importance.  Studies of small herbivores have focused on social, central-place foragers that reach high local densities and build conspicuous burrow systems, such as prairie dogs or European rabbits.  The role of more evenly dispersed small herbivores in structuring vegetation, especially in perennial grasslands, has been less studied.  What is the importance of these cryptic small herbivores?

Our research was conducted in the semiarid shortgrass steppe of the North American Great Plains, a grassland with a long evolutionary history of grazing by large generalist herbivores and one of the most tolerant ecosystems to grazing by these herbivores.  This ecosystem is considered marginal habitat for small herbivores (except for the social and colonial prairie dogs) due to the lack of overhead cover, the low seed-to-vegetation production ratios, and small seeds of the dominant plant species.  Peak biomass and consumption of rodents and rabbits was estimated to be a fraction (<8%) of that of large herbivores.  We hypothesized that 1) large generalist herbivores would affect more abundant plant species and proportions of litter (old fallen vegetation), bare ground, and vegetation cover through non-selective herbivory, and 2) small herbivores would affect cover and richness of less abundant species, through selective but limited consumption.

Photo 2

Vegetation in one of the large-plus-small herbivore exclosures. Exclusion of herbivores of both body sizes had complementary and additive effects that were linked to increased abundance of tall and decreased abundance of short plant species. Uncommon species as a group were not affected by the additional exclusion of small herbivores, but the tall annual Tragopodium dubious (compositae plant located in the front part of the photo), was an example of one uncommon species that did increase with small herbivore exclusion.

David Augustine conducts a prescribed burn in shortgrass steppe at the Central Plains Experimental Range.

David Augustine conducts a prescribed burn in shortgrass steppe at the Central Plains Experimental Range.

The study site was at the Central Plains Experimental Range (CPER) in northeastern Colorado, USA, one of the Long Term Ecological Research (LTER) grassland sites (Photos 1 and 2), as well as a Long-Term Agro-ecosystem Research (LTAR) network site.  We found that the exclusion of large herbivores affected litter and bare ground, and basal cover of abundant, common, and uncommon species.  Contrary to our hypothesis, additional exclusion of small herbivores did not affect uncommon components of the plant community, but had indirect effects on abundant species, decreased the cover of the dominant grass Bouteloua gracilis (blue grama) and total vegetation, and increased litter and species diversity.

Paul Stapp handles a thirteen-lined ground squirrel, one of the most common small mammal species in shortgrass steppe at the Central Plains Experimental Range.

Paul Stapp handles a thirteen-lined ground squirrel, one of the most common small mammal species in shortgrass steppe at the Central Plains Experimental Range.

Our findings show that small mammalian herbivores had disproportionately large effects on plant communities relative to their small consumption of biomass.  Grazing by the combination of large and small herbivores favored recovery of short grasses after extreme droughts, providing resilience to the shortgrass steppe and contributing to the long-term maintenance of vegetation basal cover.  Our study expands prior work about small herbivores and demonstrates that even in small-seeded perennial grasslands with a long history of intensive grazing by large herbivores, non-colonial small mammalian herbivores should be recognized as an important driver of grassland structure and diversity.  Therefore, the importance of small herbivores was greater than initially thought and their effects on plant communities, isolated or in interaction with large herbivores, should be part of an integrated theory of how about herbivores influence grassland diversity.

Photo 5

Justin Derner sorts yearling steers for grazing experiments (and provides comedy relief) at the Central Plains Experimental Range.


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