Knowledge about the occurrence and strength of trophic cascades (indirect effects of predators on plants and abiotic processes) is vital to understand the forces that structure food webs. Much of the empirical information about trophic cascades derives from tractable systems that are variously small scale, aquatic, invertebrate and captive (Alston et al., 2019; Ford & Goheen, 2015;Piovia-Scott et al., 2017). Less is known about trophic cascades in terrestrial wildlife systems due in part to the difficulty and cost of measuring these large and uncontrolled systems in accordance with basic principles of sampling design, including control of variation, replication and randomisation (Allen et al., 2017; Ford & Goheen, 2015;Hayward et al., 2019).A textbook example of a trophic cascade in a terrestrial wildlife community is the interaction between wolves (Canis lupus), elk (Cervus canadensis) and woody deciduous plants (Populus spp., Salix spp.) in northern Yellowstone National Park (YNP). In this system, the trophic cascade hypothesis states that reintroduced wolves scared away and/or killed enough elk to allow plants to recover from decades of unchecked browsing. A main support for this hypothesis is time series data showing annual decreases in browsing and annual increases in plant height following wolf reintroduction

Impacts of climate change on multiple use management of Bureau of Land Management land in the Intermountain West, USA.

1 Understanding how wildland ecosystems respond to the loss and recovery of large 2 predators is vital to decipher the forces that structure food webs and to guide the practice 3 of ecosystem conservation, restoration, and rewilding. This is a major scientific challenge, 4 however, because these large-scale, uncontrolled systems are difficult (or impossible) to 5 sample properly. We show how a tradition of nonrandom sampling has confounded this 6 understanding in a textbook system (Yellowstone National Park) where carnivore [wolf 7 (Canis lupus)] recovery is often associated with a trophic cascade involving changes in 8 herbivore [elk (Cervus canadensis)] behavior and density that promote plant regeneration. 9 Long-term data indicate that a customary practice of sampling only the tallest young plants 10 overestimated regeneration of aspen (Populus tremuloides) by a factor of 3-44 compared to 11 random sampling. Our results demonstrate how seemingly minor departures from 12 principled sampling can generate substantial misunderstandings about the strength of 13

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Predation is commonly cited as a top-down effect that structures food webs, with the reintroduction of wolves to Yellowstone as perhaps the most famous example. However, despite two decades of research, there is still debate as to whether wolves (Canis lupus) have indirectly benefited aspen (Populus tremuloides) by reducing herbivory from elk (Cervus canadensis). As such, the purpose of this study was to investigate the role of top-down and bottom-up forces on aspen recruitment in northern Yellowstone. The UW-NPS grant funds were used to conduct a genetic analysis of 59 aspen stands in an effort to determine whether genetic variation is one control of aspen recruitment. During summer 2018, 122 leaves were collected and sent to the Mock Lab at Utah State University. The samples will undergo DNA extractions at 12 microsatellite loci, which will provide us with genotype and ploidy level, with the genotype at a resolution to distinguish clones. Concurrently, we measured aspen heights and browse rates, as well as soil moisture. The median aspen height in 2018 was 110 cm (SD=135), and the browse rate was 0.45. Stands varied in volumetric water content (VWC), ranging from a mean VWC of 2.7% to 45.2%.   Featured photo by Yellowstone National Park on Flickr. https://flic.kr/p/efXLV2