Lethal and non-lethal effects of predators and temporal trends
in activity
We summarized direct cause-specific mortalities of tree voles by
recording when a dead tree vole was observed in the presence of a
predator on the nest platform. We interpreted these events as the
strongest causal evidence of direct mortality of tree voles by a
specific nest predator. To assess the potential response of nest
predators to increases in tree voles in year 1, we assessed trends in
seasonal and multi-annual detections of species at monitored nest
platforms. We presented trend results as a multi-annual graph of monthly
occupancy (proportion of monitored nest platforms occupied in a given
month) and interpreted seasonal and annual peaks in detections for each
taxa.
To examine non-lethal effects of nest predators, in addition to the
previous predator detection analysis of detection/non-detection data, we
summarized an index of tree vole activity at nest platforms (count of #
detections per week) 12-weeks before and 12-weeks after a predator was
detected and compared this among our three nest predators. We excluded
zeros (weeks in which no tree vole was detected at a given nest
platform) to avoid zero-inflating our count data.
Predators can adjust their temporal activity patterns to coincide with
those of their main prey (Forsman, Anthony, Meslow, & Zabel, 2004). To
examine whether temporal activity of predators coincided with those of
their prey we created two density plots: 1) diel patterns of tree voles,
flying squirrels, owls, weasels, and digging birds, and 2) diel patterns
depicting high and low likelihood of predation (for details on
assignment of low versus high likelihood, see Appendix S1) of tree voles
by weasels.