Human-mediated source-sink dynamics in a forest carnivore

Background/Question/Methods

The American marten is a forest carnivore species commonly presumed to rely on patches of mature, conifer-dominated forest for its continuing persistence. This assumption underlies legal forest management guidelines in Canada, so its validity has important economic and social ramifications. My colleagues and I tested the source-sink hypothesis through field studies of marten behaviour and demography in 4 forest landscapes in northern Ontario over 7 years. Over 400 marten individuals were equipped with radio-transmitters and their fates monitored over the next several years, as well as patterns of movement and habitat use. Functional relationships derived from these data were linked via a fully parameterized population viability model incorporating patterns of forest succession over time, predator-prey dynamics fitted to local forest conditions, and measured rates of fur-trapping by humans.

Results/Conclusions

Our results indicated that marten populations grew significantly faster and reached higher densities in mature forest than in regenerating forests, reducing the risk of decline due to environmental and demographic stochasticity. Observed differences in marten population dynamics between landscapes dominated by regenerating versus mature forests were mirrored by significant differences in the abundance of coarse woody debris, predation efficiency, body condition, survival, dispersal distance, and genetic structure. This evidence suggests that marten fitness depends on forest maturity, mediated largely through impacts on predation efficiency. At low levels of fur trapping mortality, our models predict low probability of marten populations becoming threatened, regardless of forest successional status. At higher, but realistic, levels of fur-trapping mortality, marten in regenerating forest landscapes may experience appreciable increase in risk of threat whereas landscapes dominated by mature forests are less affected. In other words, fur-harvesting acting in combination with forestry can induce source-sink dynamics, with significant effects on marten population viability.