2013-2015 deer mouse monitoring annual report: Channel Islands National Park

Coonan, Timothy

In spring 2013 through spring 2015 staff at Channel Islands National Park conducted population monitoring of deer mice (Peromyscus maniculatus) on three islands, completing 23 years of deer mouse monitoring at the park. Population monitoring was conducted using capture-mark-recapture methods at Santa Barbara Island (2 sites), Anacapa Island (1 site) and San Miguel Island (3 sites). Each site is a 10 by 10 grid of small box traps and is trapped for 3 consecutive nights in the spring and in the fall. Captured mice are marked with eartags, sexed and aged. We used the number of unique individuals caught on a grid as an index of mouse abundance, since that metric is highly correlated with density estimated from programs CAPTURE, MARK and DENSITY. Mouse abundance over time showed extensive fluctuation and evidence of seasonal variability, with fall numbers being generally higher than spring numbers, due to annual summer recruitment following winter mortality. There was evidence of multi-year cycles on Santa Barbara and San Miguel islands. However, when island foxes (Urocyon littoralis) were absent from the wild on San Miguel during a period of captive breeding (2000-2004), deer mouse cycles appeared to be more annual in nature, and abundance was generally higher than before or after the captive breeding period. On San Miguel, deer mice are still at population lows, and they have stayed at low numbers throughout the current drought (2011-2015). High island fox density may be suppressing mouse numbers, preventing them from increasing. In contrast, on Santa Barbara Island, mouse populations have begun to increase from lows in 2012-2014, and if they follow the typical 3-4 year cycle, may be increase to high numbers in 2016. Santa Barbara lacks the generalist island fox, and barn owls (Tyto alba), a specialist predator, typically decline as mice decline. Mice on East Anacapa exhibited typical spring lows and fall highs, with little evidence of multi-year cycles. Mouse weight may be an indication of body condition and thus habitat quality. Comparing spring male weights over time showed that Santa Barbara mouse weights appeared to decrease during the drought, but San Miguel weights increased in the latter part of the recent drought. As an indicator of habitat quality, there was little difference between the two Santa Barbara grids, one of which is in grassland, and the other in coreopsis scrub. The three San Miguel grids showed no difference in mouse weights during the drought, but some consistent differences prior to the drought. In non-drought years, the Nidever Canyon gird, in lupine scrub habitat, had higher weights than the Airstrip grid (Isocoma scrub), and the Willow Canyon grid (grassland) had lower weights. Thus, there may be habitat differences during years of normal or above-normal precipitation, though it appears that during a drought all habitats decline in quality.

2013-2015 deer mouse monitoring annual report: Channel Islands National Park

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