Plant community dynamics in shortgrass steppe with grazing relaxation and imposition by large and small herbivores

Abstract

Semiarid rangelands often respond slowly to rest/relaxation of grazing pressure by large herbivores, and the effects of grazing are most often inferred from this direction of study because the imposition of grazing onto previous ungrazed/lightly grazed areas occurred prior to the age of scientific studies. These rangelands host a diversity of small and large herbivores, but grazing studies most often concern effects of the large generalists. Here, the effects of herbivore body size on plant species richness and dominant species, and imposition and relaxation of grazing by large herbivores were studied by opening half of exclosures established in 1939 and building new exclosures to large herbivores, and to small-plus-large herbivores. Plant richness using sensitive species-area sampling was studied in a dry and a wet year, about 62 years after initiating the long-term experiment and about 6-10 years after initiating the altered designs. Convergence of the newly opened to large herbivore grazing treatment to the long-term grazed treatment occurred within 10 years, but convergence of the newly excluded to large herbivore treatment to the long-term excluded treatment had only partly occurred. This indicated that recovery from grazing is slow relative to imposition of grazing by large herbivores, but effects of the additional exclusion of small-plus-large herbivores occurred relatively rapidly. These results were mirrored by trajectories of convergence of the dominant species, and this is discussed with respect to implications for state-and-transition models. Short-term exclusion of small-plus-large herbivores resulted in greater richness than even long-term exclusion of only large herbivores, even though quantities consumed by small herbivores are much less than by large. Grazing effects on plant richness were large in the wet year, but the very dry year suppressed richness on all treatments. When sampling effort and area are the same, the numbers and attributes of species unique to a treatment are indicators of rareness of the richness and traits selected for by the treatment. More unique species were sampled in the small-plus-large herbivore exclosures when comparing body size, and the long-term large herbivore exclosures when comparing time of exclosure. Unique species encountered during sampling the ungrazed treatments were generally forbs, exotic and/or weedy invasive species, and often tall, annual species.

Thresholds and gradients in a semi‐arid grassland: long‐term grazing treatments induce slow, continuous and reversible vegetation change

“…Several studies in other rangelands have also observed grazing-related changes that are reversible, but only over relatively long (e.g. decadal) time-scales (Fuhlendorf, Briske & Smeins 2001;Valone et al 2002;Milchunas 2011).…”

“…However, studies from several different rangelands have emphasized that grazing‐induced vegetation changes can occur gradually over time‐scales of decades or longer, and are frequently reversible at those time‐scales (Fuhlendorf, Briske & Smeins ; Valone et al . ; Milchunas ; Bestelmeyer et al . ).…”

“…This distinction is critical because the ecological mechanisms and management implications of a state transition generally differ dramatically from those of a phase shift. While considerable attention has been devoted to understanding the theoretical underpinnings of state transitions in rangelands (van de Koppel, Rietkerk & Weissing 1997;Briske, Fuhlendorf & Smeins 2003, 2005 and identifying thresholds at which such transitions can occur (Bagchi et al 2013;Kachergis, Rocca & Fern andez-Gim enez 2014), few studies have evaluated long-term vegetation dynamics associated with experimentally sustained and reversed management treatments (Milchunas 2011).…”

“…In some cases, empirical evidence supports the idea that grazing induces transitions to alternative states (Courtois, Perryman & Hussein 2004;Briske, Fuhlendorf & Smeins 2005;Miller et al 2011). However, studies from several different rangelands have emphasized that grazinginduced vegetation changes can occur gradually over time-scales of decades or longer, and are frequently reversible at those time-scales (Fuhlendorf, Briske & Smeins 2001;Valone et al 2002;Milchunas 2011;Bestelmeyer et al 2013). Differences in plant community responses to grazing are likely related to evolutionary history of grazing (Milchunas, Sala & Lauenroth 1988), landscape and management context, and the spatial and temporal scales of different studies.…”

Disproportionate effects of non‐colonial small herbivores on structure and diversity of grassland dominated by large herbivores

“…Based on data collected during the International Biological Program (IBP), their biomass consumption per unit area is relatively low, estimated to be ˜3% of aboveground net primary production (ANPP) compared with ˜40% by large herbivores at moderate cattle grazing intensity (Lauenroth and Milchunas 1991). Short‐term exclusion of large‐plus‐small herbivores resulted in greater richness of rare species than even long‐term exclusion of only large herbivores, indicating that selective consumers of a small proportion of ANPP might promote faster and greater effects on plant diversity than a generalist that consumes a much higher proportion (Bakker et al 2006, Milchunas 2011). Based on only two early years of data, effects of small herbivores on species richness observed in a very wet year were generally not evident in the following drought year with low ANPP, when drought‐ and grazing‐tolerant species predominated in all treatments (Milchunas 2011).…”