Species Composition Changes in Response to Grazing

Article: Quantitative effects of grazing on vegetation and soils over a global range of environment Authors: Milchunas, D.G. and Lauenroth, W.K. Published: Ecological Monographs, 63(4). 1993. pp. 327-366 This is the second post dealing directly with the article under consideration.  For methodological details, please see the previous post.  We have seen the relationship between ecosystem productivity and grazing; there exists a strong and statistically significant relationship between aboveground net primary productivity (ANPP) and evolutionary history of an ecosystem. Yet changes in ANPP occur over the medium term; changes in species composition can occur over shorter time periods:  “…we may consider species composition a fast, ANPP an intermediate, and soil nutrient pool a slow response variable.” Thus, in the context of the multivariate regression models developed for this study, species composition was analyzed to answer the question: “Are there ecosystem-environmental and grazing variables that explain the variability in responses of dominant species to grazing?”  Changes in species composition were measured using dissimilarity values that ranged “from 0 (no differences between ungrazed and grazed sites) to 1 (completely different ungrazed and grazed sites).” Regression analysis from the available data revealed strong and statistically significant relationships for several key variables:

Precipitation alone explained 24% of the variation in species dissimilarity between ungrazed and grazed sites. Variables entering the model restricted to only grasslands were again ANPP, consumption, and evolutionary history of grazing. ANPP alone explained 40% of the grasslands variance compared to 54% for the entire model. Although level of consumption was consistently both significant and a large contributor to R, the number of years of protection did not significantly enter into any of the above models.  Increasing values of all independent variables led to larger differences in species composition between grazed and ungrazed sites.

What emerges is a portrait of those environmental and management variables that are most sensitive to species composition when sites are grazed or ungrazed:

The first three models (all communities, grasslands-plus-shrub- lands, and grasslands alone) consistently produced a sensitivity ranking of ANPP > evolutionary history of grazing > consumption, with ANPP increasing in effect on species dissimilarity as community types were narrowed.

A bullet point summary of the implications of these results is provided below:

• Grazing has a greater effect on species composition in humid (highly productive) environments, partly because plant canopies are taller and thus have a greater structural effect due to grazing

• Environments with a longer evolutionary history of grazing exhibit greater differences in plant communities when grazed

• Sites with higher consumption (i.e. utilization) of available biomass by grazing animals showed greater changes in species composition when grazed versus ungrazed

In addition to general changes in species composition, the authors analyzed changes in absolute abundance of the single most dominant species.  Their results were as follows:

Changes in absolute abundance of the single most dominant species with grazing was most strongly related to evolutionary history of grazing in the all-communities model, and, together with consumption, precipitation, low temperature, and years of protection, explained approximately half of the variation.  Similar to the models for species dissimilarity, a majority of the explained variance could be attributed to ecosystem-environmental variables rather than intensity (consumption) or duration (years of protection) of grazing.

Species dominance reacted somewhat differently to grazing than overall species composition, especially when factoring in evolutionary history.  The dominant species tended to change more dramatically when evolutionary history was short:

Increases in the dominant species occurred under conditions of low consumption-short evolutionary history, and large decreases occurred under conditions of high consumption-short evolutionary history regardless of the level of ANPP. Sensitivity to differences in ANPP or consumption were relatively small when evolutionary history was long.  Under conditions of short evolutionary history, changes in the other variables had large influence on the dominant species compared with relatively small influence at long evolutionary history

In addition to just looking at the single most dominant species, the authors also looked at the two most dominant species.  These results were also interesting:

For a total of 274 sites, 126 sites (46%) displayed decreases in both of the most abundant species with grazing, 125 sites (46%) displayed one species decreasing and the other increasing, and 23 sites (8%) displayed increases in both species. The single most dominant species decreased in 68% of the sites that had one of the two most abundant species decreasing and the other increasing.  Less than 1% of grassland sites displayed increases in both dominants compared with 9% of shrubland sites, suggesting dominants in the two types of communities may be responding differently to grazing.

Changes in species composition may or may not occur under grazing pressure, but changes in plant growth form and canopy are frequently observed, where there is “…selection for low-growing, prostrate growth forms…”.  Existing species may simply change their growth form, or this can result via an actual change in species composition.  While often considered a grazing avoidance mechanism, this also creates nutrient dense grazing lawns that can increase the grazing efficiency of ungulates. Although species composition can change over a relatively short time frame in response to grazing, this may not translate into an increase or decrease in primary productivity:

Primary productivity does not necessarily change when species composition changes, and can increase or decrease depending upon the replacement species, life-history traits, and the manner in which the continuing grazing pressure or stress affects water and light resources and nutrient cycling rates.

The response of shrublands to grazing is given some attention throughout this article.  In part, the authors are attempting to address the perception that shrub invasions increase with grazing. Average consumption for shrubland sites and other sites combined was greater than in grassland sites, although aboveground net primary production (ANPP) was generally lower in shrublands or other sites compared with grasslands.  Grazing of areas with a shrub component is commonly reported to lead to increased dominance by shrubs and the conversion of grasslands to less-desirable shrublands.  Why then are we grazing shrublands at a relatively greater intensity than grasslands? Changes in the dominant species in shrub ecosystems were also evaluated, again attempting to detect a discernable relationship between grazing and increased shrub prevalence: On average, species dissimilarity of grazed vs. ungrazed shrublands was less than for grasslands, and increases in dominants in grazed areas were 9 times more likely in shrublands compared with grasslands.  This would suggest that shrublands are inherently more sensitive to grazing, but environmental conditions and ecosystem attributes of shrublands are those under which relatively less impact of grazing occur. Careful monitoring of both management variables and changes in species composition could help to ameliorate shrub recruitment and successional changes as shrub-susceptible ecotypes are grazed.  In addition, more attention must be paid to utilization levels in shrub landscapes; reductions in utilization may reduce shrub invasion.  The use of GIS technology can help to identify areas on the landscape where shrubs are just beginning to invade.