Abstract
Changes in primary productivity have the potential to substantially alter food webs, with positive outcomes for some species and negative outcomes for others. Understanding the environmental context and species traits that give rise to these divergent outcomes is a major challenge to the generality of both theoretical and applied ecology. In aquatic systems, nutrient-mediated eutrophication has led to major declines in species diversity, motivating us to seek terrestrial analogues using a large-mammal system across 598 000 km2 of the Canadian boreal forest. These forests are undergoing some of the most rapid rates of land-use change on Earth and are home to declining caribou (Rangifer tarandus caribou) populations. Using satellite-derived estimates of primary productivity, coupled with estimates of moose (Alces alces) and wolf (Canis lupus) abundance, we used path analyses to discriminate among hypotheses explaining how habitat alteration can affect caribou population growth. Hypotheses included food limitation, resource dominance by moose over caribou, and apparent competition with predators shared between moose and caribou. Results support apparent competition and yield estimates of wolf densities (1.8 individuals 1000 km−2) above which caribou populations decline. Our multi-trophic analysis provides insight into the cascading effects of habitat alteration from forest cutting that destabilize terrestrial predator–prey dynamics. Finally, the path analysis highlights why conservation actions directed at the proximate cause of caribou decline have been more successful in the near term than those directed further along the trophic chain.
Highlights
Processes that increase the energy or flow of nutrients through an ecosystem can substantially alter the structure and composition of food webs [1]
We evaluated support for alternate hypotheses that potentially affected caribou population growth rates (λ): (i) habitat alteration—where resource extraction has removed forage, leading to food limitation and lower growth rates in caribou, or (ii) habitat alteration that facilitated predator vagility, which can lead to increased foraging efficiency of wolves on caribou [41]; (iii) resource dominance—where higher productivity led to a numerical increase in moose, and lower caribou λ because caribou are a weaker competitor for a shared and limiting resource [9,43]; (iv) apparent competition—where productivity is positively linked to moose and wolf abundance [44], leading to reduced caribou λ [45]; (v) increased productivity—where both moose and caribou benefit from increased forage [29,33]
The uncertainty of putative outcomes in ecological communities will increase with food webs that are complex, if they are governed by indirect interactions such as apparent competition
Summary
Processes that increase the energy or flow of nutrients through an ecosystem can substantially alter the structure and composition of food webs [1]. We evaluated support for alternate hypotheses that potentially affected caribou population growth rates (λ): (i) habitat alteration—where resource extraction has removed forage, leading to food limitation and lower growth rates in caribou (sensu [24]), or (ii) habitat alteration that facilitated predator vagility, which can lead to increased foraging efficiency of wolves on caribou [41]; (iii) resource dominance—where higher productivity led to a numerical increase in moose, and lower caribou λ because caribou are a weaker competitor for a shared and limiting resource [9,43]; (iv) apparent competition—where productivity is positively linked to moose and wolf abundance [44], leading to reduced caribou λ [45]; (v) increased productivity—where both moose and caribou benefit from increased forage [29,33] To tackle these hypotheses, we considered landscape-scale variation in primary productivity, moose and wolf densities, and caribou λ across a 598 000-km study area. This design allowed us to validate a previously determined threshold for wolf abundance leading to dynamically stable caribou populations [46], which has informed recovery policies for woodland caribou [47]
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