The effects of herbivores and their interactions with nutrient availability on primary production and plant community composition in grassland systems is expected to vary with herbivore type. Although nutrient additions are known to affect plant species diversity and primary productivity, the role of herbivores in mediating the strength of these effects also remains unclear. Herbivores may alter plant responses to nutrient additions in several ways. First, herbivores can alter the plant community response to nutrient additions by either selectively feeding on particular groups of species (e.g. grasses versus forbs) or by generally opening up space, allowing for species turnover and immigration. Second, feeding by herbivores may reduce the production response to nutrient additions if the plants cannot compensate for tissue lost to herbivory. As the functional effects of vertebrate and invertebrate herbivores on plant community composition and production may vary, the interactive effects of vertebrate versus invertebrate herbivores with nutrient additions may also vary. Here we are experimentally assessing the independent and interactive effects of removing vertebrate and invertebrate herbivores on aboveground biomass and plant community composition in native tallgrass prairie. Further, we are examining whether the removal of vertebrate and invertebrate herbivores interacts with nutrient availability. By doing this, we address three related questions: 1) what is the relative strength of the effects of invertebrate versus vertebrate herbivory in a grassland system; 2) how does herbivory (invertebrate and/or vertebrate) affect the relative abundances of grasses and forbs, the two dominant plant functional types within the ecosystem; and 3) what are the consequences of these changes in composition for aboveground net primary productivity, an important ecosystem function?
To examine the effects of invertebrate and vertebrate herbivores and their interactions with nutrient availability on grassland plant community composition and aboveground biomass.
Location of Sampling Stations: Uplands of watershed 2C (behind the Nutrient Network plots and adjacent to the Phosphorus plots
Frequency of Sampling:
To determine the effects of nutrient availability and herbivore removal on grassland productivity and diversity, plant species composition and end-of-season above-ground biomass are sampled yearly.
We are examining the effects of invertebrate and vertebrate herbivores and their interactions with nutrient availability on grassland plant community composition and aboveground biomass in the uplands of watershed 2C at Konza Prairie (behind the Nutrient Network plots and adjacent to the Phosphorus plots). To address the relative and interactive effects of bottom-up and top-down control over grassland communities, nutrient additions, vertebrate herbivore removals, and invertebrate herbivore removals were crossed in a fully factorial design. Plots are 2 x 2 m in area and treatment replicates (N=3) are arrayed in three blocks. Plots within each block are separated by 1 m aisles, while blocks are separated by 2 m aisles.
The nutrient treatments involve the addition of relatively high levels of nitrogen, phosphorous, and potassium plus other micronutrients, each applied at a rate of 10 g m-2 yr-1. Nitrogen is added in the form of time-release urea; phosphorous is added in the form of calcium phosphate; and potassium is added in the form of potassium sulfate. The micronutrient treatment involved the addition of Scott’s Micromax fertilizer, which contains calcium (6 g m-2), magnesium (3 g m-2), sulfur (12 g m-2), boron (0.1 g m-2), copper (1 g m-2), iron (17 g m-2), manganese (2.5 g m-2), molybdenum (0.05 g m-2), and zinc (1 g m-2). The micronutrient treatment was only applied in 2009 to prevent the build-up of these elements in the soil, some of which are toxic to plants at high levels. These nutrient additions occur once yearly at the start of the growing season.
The vertebrate herbivore removal (caged) treatment involves surrounding the entire plot with a fence. This fence consists of a 1 m high fine mesh with a 0.3 m skirt stapled to the ground, to discourage burrowing under fences. The herbivore removal treatment excludes medium- to large- vertebrate herbivores from the plots, but does not prevent access by invertebrate herbivores. Ortho Bug-B-Gone insecticide is applied to the invertebrate herbivore removal (insecticide) plots every two weeks throughout the growing season to ensure complete exclusion of invertebrate herbivores. An equal quantity of water is added to the insect control plots.
To determine the effects of nutrient availability and herbivore removal on grassland productivity and diversity, plant species composition and end-of-season above-ground biomass are sampled yearly. Plant species composition is measured in a permanent 1 m2 subunit within each of the experimental plots twice per growing season, once in spring (mid-May to early-June) to determine the abundance of early season forbs and C3 grasses, and once in fall (late-August to early-September) to determine the abundance of late season forbs and C4 grasses. Percent cover is determined for each species to the nearest 1% using a modified Daubenmire method. Two permanent 1 m2 subunits within each experimental plot are dedicated to destructive biomass sampling. The aboveground standing crop is sampled once per growing season, at peak biomass. One 0.1 m2 strip is clipped in each of the two destructive sampling subunit in each plot and the location of strips is moved each year to prevent resampling. Biomass is separated (2009: to the three dominant grasses, one dominant forb, other grasses, other forbs, woody, and previous year’s dead; 2010-2012: by species; 2013-current: by functional group), dried at 60 °C, and weighed.
Light attenuation curves were developed for each plot in 2009. Light availability was measured from ground level to above the canopy at 0.1m intervals four times during the growing season (May, June, July, August) using a PAR sensor. Root biomass was measured in 2010 by taking three soil cores spaced evenly around each plot (radius=1.25”, depth=15 cm), sieving the roots out of the soil, washing the roots, and drying the roots at 60 °C.
For additional metadata information see: http://lter.konza.ksu.edu/sites/default/files/DC.pdf
For additional methods information see: http://lter.konza.ksu.edu/sites/default/files/MM.pdf