Large predators and trophic cascades in the American West

Background/Question/Methods

The strength of top-down forces in terrestrial food webs is highly debated as there are few examples illustrating the role of large mammalian carnivores in structuring biotic and abiotic systems. We undertook field work in five national parks in the American West (Olympic, Yosemite, Yellowstone, Zion, and Wind Cave) to test the hypothesis that humans, through extirpation of large predators in the early 20^th century, caused a trophic cascade involving cervids and deciduous trees. We also undertook field measurements of deciduous trees in each park for assessing vegetation responses to predator–prey dynamics over time. For the selected deciduous tree species, we determined their age structure using tree rings from increment coring. We used regression analysis to establish an exponential equation between observed tree frequency (X_o) and establishment date during the ‘‘pre-treatment” period (i.e., predators present). Each fitted curve was extended to the present and represented expected tree frequencies (X_e) during the ‘‘post-treatment” period (i.e., when large predators had become absent or scarce). We assessed the general ‘‘strength” of the trophic cascade for a given park by calculating a log₁₀ response ratio of tree frequencies (i.e., log₁₀ (X_o/X_e)) for each decade after predators had become absent or scarce.

 Results/Conclusions

Age structure data for deciduous trees indicate major impacts to woody plant communities by cervids following the extirpation or displacement of large predators. Temporal patterns of tree recruitment (age structure) over a period of 160 years (1840–2000) illustrate (1) ongoing recruitment when large predators were present and (2) significant decreases (95% lower CI) in recruitment after large predators were extirpated/displaced. The time series of average log₁₀ response ratios of tree frequency, relative to when predators became extirpated/displaced, illustrates the decline in tree recruitment in our study areas generally began soon after the loss of large predators. In addition, the strength of the trophic cascade has become stronger (increasingly negative) over time regardless of whether or not a particular park undertook cervid culling efforts following the loss of large predators. The magnitude and consistency of vegetation impacts found within these five parks, in conjunction with other recent North American studies, indicate that the extirpation of large predators likely caused broad changes to ecosystem structure, processes and biodiversity.