DOI: 10.5061/dryad.r7sqv9sfz
title: Invasive rodent responses to experimental and natural hurricanes with implications for global climate change

description: Hurricanes cause dramatic changes to forests by opening the canopy and
 depositing debris onto the forest floor. How invasive rodent populations
 respond to hurricanes is not well understood, but shifts in rodent
 abundance and foraging may result from scarce fruit and seed resources
 that follow hurricanes. We conducted studies in a wet tropical forest in
 Puerto Rico to better understand how experimental (Canopy Trimming
 Experiment) and natural (Hurricane Maria) hurricane effects alter
 populations of invasive rodents (Rattus rattus [rats] and Mus musculus
 [mice]) and their foraging behaviors. To monitor rodent populations, we
 used tracking tunnels (inked and baited cards inside tunnels enabling
 identification of animal visitors’ footprints) within experimental
 hurricane plots (arborist trimmed in 2014) and reference plots (closed
 canopy forest). To assess shifts in rodent foraging, we compared seed
 removal of two tree species (Guarea guidonia and Prestoea acuminata)
 between vertebrate-excluded and free-access treatments in the same
 experimental and reference plots, and did so 3 months before and 9 months
 after Hurricane Maria (2017). Trail cameras were used to identify animals
 responsible for seed removal. Rat incidences generated from tracking
 tunnel surveys indicated that rat populations were not significantly
 affected by experimental or natural hurricanes. Before Hurricane Maria
 there were no mice in the forest interior, yet mice were present in forest
 plots closest to the road after the hurricane, and their forest invasion
 coincided with increased grass cover resulting from open forest canopy.
 Seed removal of Guarea and Prestoea across all plots was rat dominated
 (75%-100% rat-removed) and was significantly less after than before
 Hurricane Maria. However, following Hurricane Maria, the experimental
 hurricane treatment plots of 2014 had 3.6 times greater seed removal by
 invasive rats than did the reference plots, which may have resulted from
 rats selecting post-hurricane forest patches with greater understory cover
 for foraging. Invasive rodents are resistant to hurricane disturbance in
 this forest. Predictions of increased hurricane frequency from expected
 climate change should result in forest with more frequent periods of
 grassy understories and mouse presence, as well as with heightened rat
 foraging for fruit and seed in pre-existing areas of disturbance.
description: Each dataset was based on observations. 1)
 Tracking tunnels: To assess rodent incidence,
 which is a reflection of abundance and activity, we used tracking tunnels
 in each plot. Tracking tunnels are inked and baited cards
 placed in tunnels enabling foot prints of vertebrate visitors to be
 identified to genus (Shiels and Ramírez de Arellano 2018, Madden et al.
 2019, Shiels and Ramírez de Arellano 2019). In each of the 12 CTE plots, two tracking
 tunnels that were 60 cm long with 10 × 10 cm openings were randomly placed
 approximately 10 m apart from each other; one tunnel was placed on the
 ground, and the other tunnel was secured by plastic straps on a branch or
 liana in the lower canopy at an average height of 135 cm (SE = 5 cm). The
 rodent tracks (rat or mouse) were identified on the tracking cards after 2
 days. These are presence/absence data measured at each
 tunnel. 2) Seed removal trials: Trials were conducted to determine whether
 rats removed native seeds from the forest floor. Two common tree species, muskwood
 (<em style="font-family:'Times New Roman' , serif;font-size:12pt;">Guarea guidonia</em>, Meliaceae) and sierra palm (<em style="font-family:'Times New Roman' , serif;font-size:12pt;">Prestoea acuminata</em>, Arecaceae), were chosen based on their
 fruit and seed availability during the trial period (June-July 2017 and
 2018). The seeds (6 per station for
 <em>Guarea</em>, 3 per station for
 <em>Prestoea</em>) were placed on the ground without any
 caging surrounding them and then they were visited to count the number of
 seeds in each treatment over time for 7 days, in the Summers of 2017 and
 2018. 3) Canopy openness: Canopy openness was determined 9 months
 before and 9 months after Hurricane Maria by taking hemispherical pictures
 using a digital camera (Nikon 4500, Nikon Inc, Tokyo, Japan) with a
 fish-eye lens (Nikon FC-E8) positioned at 1 m height above ground using a
 tripod, at five locations (i.e., the four corners and the center) in each
 20×20 m measurement area of the
 plot. The photos were taken on the following
 dates: 7-16 December 2016 (pre-hurricane), and 1-3
 July 2018 (post-hurricane). Each picture was analyzed for percent
 canopy openness using Gap Light Analyzer software (version 2; Simon Fraser
 University, Cary Institute of Ecosystem Studies), setting thresholds at 82
 for all 2016 pictures and 152 for all 2018 pictures; the higher threshold
 was needed in 2018 to prevent the greater light levels from obscuring the
 remaining vegetation. Percent canopy openness was based on the
 average of the five pictures per plot. 4)
 Percent grass cover: Grass cover (percent ground
 coverage &lt; 1 m height) was visually estimated to the nearest 1% in
 September or early October of each year since the onset of the experiment
 within five permanent 1 × 3.5 m subplots within each of the 12 plots (see
 Shiels et al. 2010). We obtained an annual average of the five
 subplots per plot to use in our analysis (<em style="font-family:'Times New Roman' , serif;font-size:12pt;">n</em> = 3 for canopy trimmed plots;
 <em style="font-family:'Times New Roman' , serif;font-size:12pt;">n </em>= 9 for reference plots) for 2012 to
 2019.