Mass-Balance Analyses of Boreal Forest Population Cycles: Merging Demographic and Ecosystem Approaches

Abstract

Using Ecopath, a trophic mass-balance modeling framework, we developed six models of a Canadian boreal forest food web centered around snowshoe hares, which have conspicuous 10-year population cycles. Detailed models of four phases of the cycle were parameterized with long-term population data for 12 vertebrate taxa. We also developed five other models that, instead of observed data, used parameter values derived from standard assumptions. Specifically, in the basic model, production was assumed to equal adult mortality, feeding rates were assumed to be allometric, and biomass was assumed to be constant. In the actual production, functional response, and biomass change models, each of these assumed values from the basic model was replaced individually by field data. Finally, constant biomass models included actual production by all species and functional responses of mammalian predators and revealed the proportion of herbivore production used by species at higher trophic levels. By comparing these models, we show that detailed information on densities and demographics was crucial to constructing models that captured dynamic aspects of the food web. These detailed models reinforced an emerging picture of the causes and consequences of the snowshoe hare cycle. The snowshoe hare decline and low phases were coincident with times when per capita production was relatively low and predation pressure high. At these times, ecotrophic efficiencies (EE) suggest there was little production that remained unconsumed by predators. The importance of both production and consumption implies that bottom-up and top-down factors interacted to cause the cycle. EEs of other herbivores (ground squirrels, red squirrels, small mammals, small birds, grouse) were generally low, suggesting weak top-down effects. Predation rates on these "alternative" prey, except ground squirrels, were highest when predators were abundant, not when hares were rare; consequently, any topdown effects reflected predator biomass and were not a function of diet composition or functional responses. Finally, several predators (lynx, coyotes, great-horned owls) showed clear bottom-up regulation, reproducing only when prey exceeded threshold densities. Taken altogether, these results demonstrate that ecosystem models parameterized by population data can describe the dynamics of nonequilibrial systems, but only when detailed information is available for the species modeled.

Terrestrial trophic dynamics in the Canadian Arctic

“…ECOPATH has been widely used in fisheries research, but there are almost no applications of this model to terrestrial ecosystems (Ruesink et al 2002). We have reached two tentative conclusions from our modeling of trophic flows at these arctic sites.…”

“…If lemming predators increase with a time lag, predator numbers after a lemming peak will not be in an equilibrium and their impact could be significantly higher than we have observed, which would further increase the EE value during the decline phase of the cycle. A similar scenario was modeled with data from cyclic snowshoe hare populations by Ruesink et al (2002).…”

Disentangling trophic relationships in a High Arctic tundra ecosystem through food web modeling

“…We used a mass balance ecosystem modeling software, Ecopath with Ecosim (EwE), that has been used extensively to study marine and aquatic food webs (Christensen et al 2008) as well as terrestrial ones (Ruesink et al 2002, Krebs et al 2003. Ecopath with Ecosim combines software for ecosystem mass balance analysis (Ecopath) with a dynamic modeling capability (Ecosim) to forecast species abundance in exploited ecosystems.…”

“…The phase of the cycle was assigned based on lemming abundance obtained by trapping (see the Appendix) following Bêty et al (2002). Intermediate years typically occurred ''post-peak'' (Gruyer et al 2008), which differs from other cycles in which intermediate years often occur before peaks (i.e., increase phase; Ruesink et al 2002, Gilg et al 2003. Consumption Q and production P were assessed both empirically and allometrically, using primarily the equations of Banse and Mosher (1980) and Nagy (1987) in the latter case.…”

Correlated cycles of snowshoe hares and Dall’s sheep lambs

“…A recently completed long-term experiment in the Kluane region of the southwest Yukon Territory, Canada, demonstrated that predation is the proximal cause of the decline of the hare population in each cycle (Krebs et al 2001b). However, the factors (likely food and predation) that contribute to the 10-year timing of the cycle (Ruesink et al 2002), the exact causes of the sustained low phase after the crash (Boonstra et al 1998;Hodges et al 1999), and the phenomena that synchronize the cycle across the boreal forest region of North America (Sinclair and Gosline 1997) have yet to be conclusively determined. It is clear that the hare cycle itself is an important driver of biotic interactions in the boreal forest (Krebs et al 2001b).…”