Large predators limit herbivore densities in northern forest ecosystems: William J. Ripple & Robert L. Beschta (2012)

Abstract:

Wolves primarily prey on ungulates that are considerably larger and

possess formidable defenses in their antlers, hooves, and behavioral

responses to predator attacks. Thus, many wolf predation attempts are

unsuccessful. Photo: Cindy Goeddel (http://goeddelphotography.com/ )

"There is a lack of scientific consensus about how top-down and bottom-up forces interact to structure terrestrial ecosystems. This is especially true for systems with large carnivore and herbivore species where the effects of predation versus food limitation on herbivores are controversial. Uncertainty exists whether top-down forces driven by large carnivores are common, and if so, how their influences vary with predator guild composition and primary productivity. Based on data and information in 42 published studies from over a 50-year time span, we analyzed the composition of large predator guilds and prey densities across a productivity gradient in boreal and temperate forests of North America and Eurasia. We found that predation by large mammalian carnivores, especially sympatric gray wolves (Canis lupus) and bears (Ursus spp.), apparently limits densities of large mammalian herbivores. We found that cervid densities, measured in deer equivalents, averaged nearly six times greater in areas without wolves compared to areas with wolves. In areas with wolves, herbivore density increased only slightly with increasing productivity. These predator effects are consistent with the exploitation ecosystems hypothesis and appear to occur across a broad range of net primary productivities. Results are also consistent with theory on trophic cascades, suggesting widespread and topdown forcing by large carnivores on large herbivores in forest biomes across the northern hemisphere. These findings have important conservation implications involving not only the management of large carnivores but also that of large herbivores and plant communities."

Introduction:

Conceptual diagram of trophic cascades showing hypothesized
predator densities, herbivore densities, and plant damage for populations
across a productivity gradient in the presence (left column) and absence
(right column) of large predators in productive ecosystems with NPP>
0.7 kg/m2/year (based on Oksanen 1992; Oksanen et al. 1981)

"[...] Consistent with the GWH (Hairston et al. 1960) and the EEH (Oksanen 1992), we hypothesize a set of trophic interactions across a gradient of primary productivity for boreal and temperate ecosystems that are productive enough to support carnivores. In systems with intact and ecologically effective large carnivore guilds (i.e., tri-trophic cascades, left side of Fig. 1), we expect (1) carnivore densities to be resource-limited and positively correlated with primary productivity and (2) herbivores mainly predator-limited, such that herbivore density increases only slightly with increasing productivity. In this system, herbivore damage to palatable plants would be relatively low regardless of productivity. Although not rigorously tested herein, EEH predicts that when the removal of large predators occurs (i.e., truncated trophic cascades, right side of Fig. 1), herbivore density would be positively correlated with productivity and damage to palatable plants relatively high regardless of primary productivity.

 

Understanding the effects of large predator and herbivore populations upon native plant communities has potentially important conservation implications. If large predators reduce plant damage by altering cervid behavior and limiting their densities, the maintenance of large predators across landscapes could be a crucial option for sustaining the ecological integrity of ecosystems (Estes et al. 2011; Miller et al. 2001; Soulé et al. 2003; Terborgh and Estes 2010). Furthermore, in areas where large predators have been displaced or locally extirpated, their reintroduction may represent a particularly effective approach for passively restoring those ecosystems."

Conclusions:

Wolf densities (y) as a function of net primary productivity (x),
where y013.08x-3.49; SEE010.0, R200.35, p00.008, and n019. SEE
Standard error of the estimate

"On average, we found cervid densities in systems without wolves to be approximately six times greater than that of systems with wolves (2.6 vs. 15.5 DE/km2). Cervid densities in systems with wolves and bears ranged from 0.03 to 8.4 DE/km2 with density increasing slightly with NPP.
These results could serve as benchmarks, based on the productivity of a particular region, for designing and evaluating the management of non-migratory cervid populations, where the goal is to emulate the range of densities typically found when wolves and bears are present in northern forests. Additionally, the wolf densities presented herein (x ¼ 17:8=1000km2, range 2.3–40.2) could also be used as benchmarks for assessing wolf management goals across a range of productivities. In general, regions with higher productivity and intact habitat have the capability to support higher wolf densities than areas with lower productivity (Fig. 4)."

 

"Recent research suggests that conservation programs based on the presence of apex predators may lead to broader biodiversity benefits (Sergio et al. 2008). Thus, sites containing intact carnivore guilds and which retain ecological processes should be considered as priority areas for both research and conservation planning (Woodroffe and Ginsberg 2005). Additionally, repatriating large carnivores to portions of their former range may still be possible and could have positive ecological effects. For example, the reintroduction or recolonization of wolves, decades after extirpation, has shown to positively affect tree and shrub recruitment at some sites on ungulate winter ranges where cervids seasonally migrate (Beschta and Ripple 2007; Ripple and Beschta 2012). More research is needed to determine to what extent large predators structure ecosystems in areas with both migrating and nonmigrating large herbivores. The preservation or recovery of large predators may thus represent an important conservation need for helping to maintain the resiliency of northern forest ecosystems, especially in the face of a rapidly changing climate."

 
Large predators limit herbivore densities in northern forest ecosystems: William J. Ripple & Robert L. Beschta (2012) http://bit.ly/PAshaN Eur J Wildl Res (DOI 10.1007/s10344-012-0623-5) 20 February 2012 Springer-Verlag 2012 (Publicado en http://www.cof.orst.edu/ )