Century‐scale effects of invasive deer and rodents on the dynamics of forests growing on soils of contrasting fertility

Abstract

Understanding the long-term impacts of invasive mammalian browsers and granivores in mixed forests is difficult due to the many processes potentially affecting the demography of long-lived trees. We constructed individual-based spatially explicit simulation models of two mixed conifer-angiosperm forests, growing on soils of contrasting phosphorus (P) availability, to investigate how browsing by invasive red deer (Cervus elaphus scoticus) and granivory by invasive rodents (primarily house mouse Mus musculus) might alter forest dynamics. Models were parameterized with field data. Seedling growth and survival rates were estimated inside and outside deer exclosures. Seed predation rates were estimated at high and low rodent densities in mast and non-mast seeding years. For the alluvial terrace forest, which grew on P-rich soil, our model contained 15 tree species dominated by angiosperms; our model of the P-poor marine terrace forest contained seven species dominated by conifers. The two forest models were used to explore four 500-year scenarios: deer and rodents present, deer present and rodents absent, deer absent and rodents present, and deer and rodents absent.Our field studies revealed that the highest probabilities of seed predation by rodents occurred for two canopy species in mast years and in neighborhoods of high species-specific basal area, and that deer browsing had the greatest negative effects on the growth and survival rates of angiosperm seedlings. Our simulation models predicted that the presence of deer and rodents would reduce the abundances of canopy codominants, and hence total basal area, in the alluvial terrace forest. The presence of deer increased the dominance of conifers in both forests, but effects of deer and rodents were much stronger in the alluvial terrace forest. Our study revealed two emergent properties. First, rodent-and deer-induced reductions in seedling abundances did not always translate into reduced sapling and adult tree abundances. Second, when deer changed the abundance of a canopy dominant, other species were affected by altered interspecific competition.We conclude that the effects of invasive deer and rodents will be greatest in forests growing on P-rich soils. Exclusion of deer is predicted to have greater long-term effects on forest dynamics than exclusion of rodents.

Species‐ and site‐specific impacts of an invasive herbivore on tree survival in mixed forests

“…Fitted values for average leaf lifespan of the five tree species were consistent with published values for evergreen, native New Zealand tree species including W. racemosa (Richardson et al. ), and fitted growth efficiencies make biological sense as S. digitata is fast‐growing relative to the other species in Table (Forsyth et al ). Independent data were used to compare predicted against observed mortality for five tree species in three new areas with paired possum treatment/nontreatment sites.…”

Predator Free 2050: A flawed conservation policy displaces higher priorities and better, evidence‐based alternatives

“…Consider also that, while introduced predators threaten indigenous biodiversity they are not the largest impact on biodiversity in all places. The biodiversity values of some NZ ecosystems are impacted more by other introduced animals, particularly grazers and browsers, than the predators being targeted (e.g., Cruz, Thomson, Parkes, Gruner, & Forsyth, 2017;Forsyth et al, 2015). Also, in other environments and at some scales, lower intensity predator suppression, habitat protection and restoration, and enhanced refugia from predators may be more efficient at protecting and recovering biodiversity (Doherty, Dickman, Nimmo, & Ritchie, 2015;Doherty & Ritchie, 2017;Ruffell & Didham, 2017).…”

Timber harvest as the predominant disturbance regime in northeastern U.S. forests: effects of harvest intensification

“…We implemented the six harvest scenarios in SORTIE-ND (Coates et al 2003, Uriarte et al 2009, Forsyth et al 2015, a spatially explicit individual-tree forest stand model that tracks the recruitment, growth, and mortality of all individual seedlings, saplings, and adult trees over time. The model has been parameterized from FIA data for the 50 most common tree species in the eastern United States Murphy 2016a, b, 2017).…”