Abstract
Although there is considerable evidence that large mammalian herbivores influence ecosystem‐level processes, studies have reported such widely varying results that generalizations have remained elusive. Here, we use an 18‐year‐old exclosure experiment—stratified across a landscape heterogeneous with respect to soil texture, moisture and herbivore activity—to understand the variable effects of tule elk (Cervus canadensis nannodes), a native reintroduced herbivore, on soil properties along the coast of northern California. Elk significantly increased soil bulk density and created a compacted layer at shallow soil depth, while decreasing infiltration rate and pH. The effects of elk on bulk density, penetration resistance, and pH varied with soil type, being least pronounced in coarse, sandy loams, and greatest in loose sand. The effects of elk on nutrient availability varied along gradients of soil texture and moisture. In coarser soils, elk decreased ammonium availability, but increased it in finer soils. Elk also decreased soil moisture content, in part through their positive effect on bulk density, and this effect was most pronounced in coarser soils. Through decreasing soil moisture content, elk also decreased nitrate availability in coarser soils. At greater levels of elk activity (as measured by dung deposition), the elk effect on bulk density was amplified, and this had a corresponding negative effect on nitrate and phosphate availability. Our study has demonstrated that a better understanding of spatial variation in the effects of herbivores on ecosystems can emerge by evaluating their influences across gradients of soil texture, soil moisture, and herbivore activity. These data enabled us to evaluate several frameworks that have been developed to understand the variable effects of herbivores on ecosystems, which is a significant step in reconciling the many competing ideas put forth to explain the context‐dependent effects of large herbivores on grazed ecosystems.
Highlights
Through their activities as consumers, disturbance agents, and fertilizers, large herbivores can be major drivers of ecosystem-level processes throughout many regions of the world (Ruess & McNaughton, 1987; Hobbs & Thompson, 1996; Verchot, Groffman, & Frank, 2002; Binkley, Singer, Kaye, & Rochelle, 2003, see review by Forbes et al, 2019)
Our research addressed the following questions: (a) Does a large, native, mammalian herbivore influence nutrient availability and physical characteristics of the soil, and do these effects vary across a landscape that is heterogeneous in soil texture and moisture? and (b) Is the magnitude of an herbivore's effect on soil characteristics influenced by the intensity with which it uses an area? Our experiment provides a robust approach to assess the generality of Schrama, Ciska Veen, et al (2013) framework as well as the importance of intensity of herbivore use, by explicitly addressing herbivore effects on ecosystem processes across gradients of herbivore activity, soil textures and moisture levels, and vegetation
We analyzed bulk density, infiltration rate, penetration resistance, pH, nitrate, ammonium, and phosphate using linear mixed models in JMP 13 Pro (SAS Institute), with elk, soil formation (Kehoe 138, Kehoe 139, Sirdrak, mixed K/S soil) and their interaction as fixed effects, and sample nested within plot pair (1–12) and plot pair nested within soil formation as random effects
Summary
Through their activities as consumers, disturbance agents, and fertilizers, large herbivores can be major drivers of ecosystem-level processes throughout many regions of the world (Ruess & McNaughton, 1987; Hobbs & Thompson, 1996; Verchot, Groffman, & Frank, 2002; Binkley, Singer, Kaye, & Rochelle, 2003, see review by Forbes et al, 2019) They can influence a range of soil properties, including nutrient availability and mineralization rates (Bardgett & Wardle, 2003; Hobbs & Thompson, 1996; McNaughton, Banyikwa, & McNaughton, 1997; Pastor, Dewey, Naiman, McInnes, & Cohen, 1993; Ritchie, Tilman, & Knops, 1998), bulk density (AbdelMagid, Schuman, & Hart, 1987; Gass & Binkley, 2011; Steffens, Kölbl, Totsche, & Kögel-Knabner, 2008), infiltration rates (Daryanto, Eldridge, & Wang, 2013; Thrash, 1997), moisture levels (Gass & Binkley, 2011), salinity (Buckeridge & Jefferies, 2007; Chaneton & Lavado, 1996; Lavado & Alconada, 1994), pH (Binkley et al, 2003; Hatton & Smart, 1984), temperature (Bakker, Olff, Olff, Boekhoff, Gleichman, & Berendse, 2004; van der Wal, van Lieshout, & Loonen, 2001), erosion (Ford & Grace, 1998), and microbial communities (Bardgett, Wardle, & Yeates, 1998; Frank, Gehring, Machut, & Phillips, 2003; Murray, Frank, & Gehring, 2010). Addressing these questions will further our understanding of how herbivores influence ecosystem-level processes and what factors are important to include within a predictive framework
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