In this food web, did humans matter more through forage or predators?

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Researchers questioned whether the debate over food webs being primarily regulated by predators (top-down effects) or primary plant productivity (bottom-up effects) may be becoming superfluous. Because most of the world's ecosystems are human dominated, they expanded this dichotomy to consider human effects in a terrestrial food web.

They studied a multiple human-use landscape in southwest Alberta, Canada, rather than the protected areas where previous terrestrial food-web studies had been conducted. They used structural equation models (SEMs) to assess the strength and direction of relationships among human density and distribution, wolf (Canis lupus), elk (Cervus elapahus) and domestic cattle (Bos taurus) density and distribution, and forage quality, quantity and utilization. Humans were measured using a density index, animals using resource selection functions, and forage at vegetation sampling plots. They evaluated relationships using temporal and spatial variation in human density, including day versus night and the two landscapes with the highest and lowest human density in the study area.

The researchers found that human effects mediated through forage took precedence over predator-mediated effects in the food web. In their parsimonious SEM, human occurrence was most strongly correlated with forage occurrence (β = 0.637, p<0.0001). Elk and cattle distributions were correlated with forage: elk by day β = 0.400, p<0.0001; elk by night β = 0.369, p<0.0001; cattle by day β = 0.403, p<0.0001; and cattle by night β = 0.436, p<0.0001. Elk or cattle distributions and wolf distribution were positively correlated both by day (elk: β = 0.293, p<0.0001; cattle: β = 0.303, p<0.0001) and by night (elk: β = 0.460, p<0.0001; cattle: β = 0.482, p<0.0001).

These results contrast with protected-area research suggesting that human effects in food webs are primarily mediated by predators. The researchers suggest instead that human influence on vegetation may strengthen bottom-up predominance and weaken top-down trophic cascades in ecosystems. They propose that human influences on ecosystems may usurp top-down and bottom-up effects.