Search for: All records

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] andMus 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 guidoniaandPrestoea 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 ofGuareaandPrestoeaacross 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 preexisting areas of disturbance.

Despite frequent occurrences of invasive rats (Rattusspp.) on islands, their known effects on forests are limited. Where invasive rats have been studied, they generally have significant negative impacts on native plants, birds, and other animals. This study aimed to determine invasive rat distribution and effects on native plant populations via short‐term seed removal trials in tropical rain forest habitats in the Luquillo Experimental Forest, Puerto Rico. To address the first objective, we used tracking tunnels (inked and baited cards inside tunnels enabling animal visitors’ footprints to be identified) placed on the ground and in the lower canopy within disturbed (treefall gaps, hurricane plots, stream edges) and undisturbed (continuous forest) habitats. We found that rats are present in all habitats tested. Secondly, we compared seed removal of four native tree species (Guarea guidonia,Buchenavia capitata, Tetragastris balsamifera,andPrestoea acuminata) between vertebrate‐excluded and free‐access treatments in the same disturbed and undisturbed habitats. Trail cameras were used to identify animals responsible for seed contact and removal. Black rats (Rattus rattus) were responsible for 65.1% of the interactions with seeds, of which 28.6% were confirmed seed removals. Two plant species had significantly more seeds removed in disturbed (gaps) than undisturbed forest.Prestoea acuminatahad the lowest seed removal (9% in 10 days), whereas all other species had >30% removal. Black rats are likely influencing fates of seeds on the forest floor, and possibly forest community composition, through dispersal or predation. Further understanding of rat–plant interactions may be useful for formulating conservation strategies.

Tropical forest understory regeneration occurs rapidly after disturbance with compositional trajectories that depend on species availability and environmental conditions. To predict future tropical forest regeneration dynamics, we need a deeper understanding of how pulse disturbance events, like hurricanes, interact with environmental variability to affect understory demography and composition. We examined fern and sapling mortality, recruitment, and community composition in relation to solar radiation and soil moisture using 17 years of forest dynamics data (2003–2019) from the Canopy Trimming Experiment in the Luquillo Experimental Forest, Puerto Rico. Solar radiation increased 150% and soil moisture increased 40% following canopy trimming of experimental plots relative to control plots. All plots were disturbed in 2017 by Hurricanes Irma and Maria, so experimentally trimmed plots presented the opportunity to study the effects of multiple hurricanes, while control plots isolated the effects of a single natural hurricane. Recruitment rates maximized at 0.14 individuals/plot/month for ferns and 0.20 stems/plot/month for saplings. Recruitment and mortality were distributed more evenly over the 17 years of monitoring in experimentally trimmed plots than in control plots; however, following Hurricane Maria demographic rates substantially increased in control plots only. In experimentally trimmed plots, the largest community compositional shifts occurred as a result of the trimming events, and compositional changes were greatest for control plots after Hurricane Maria in 2017. Pioneer tree and fern species increased in abundance in response to both simulated and natural hurricanes. Following Hurricane Maria, two dominant pioneer species,Cyathea arboreaandCecropia schreberiana, recruited abundantly, but only in control plots. In trimmed plots, increased solar radiation and soil moisture shifted understory species composition steadily toward pioneer and secondary‐successional species, with soil moisture interacting strongly with canopy trimming. Thus, both solar radiation and soil moisture are environmental drivers affecting pioneer species recruitment following disturbance, which interact with canopy opening following hurricanes. Our results suggest that if hurricane disturbances increase in frequency and severity, as suggested by climate change predictions, the understory regeneration of late‐successional species, such asManilkara bidentataandSloanea berteroana, which prefer deeper shade and slightly drier soil microsites, may become imperiled.

Invasive species pose a major threat to biodiversity on islands. While successes have been achieved using traditional removal methods, such as toxicants aimed at rodents, these approaches have limitations and various off-target effects on island ecosystems. Gene drive technologies designed to eliminate a population provide an alternative approach, but the potential for drive-bearing individuals to escape from the target release area and impact populations elsewhere is a major concern. Here we propose the “Locally Fixed Alleles” approach as a novel means for localizing elimination by a drive to an island population that exhibits significant genetic isolation from neighboring populations. Our approach is based on the assumption that in small island populations of rodents, genetic drift will lead to alleles at multiple genomic loci becoming fixed. In contrast, multiple alleles are likely to be maintained in larger populations on mainlands. Utilizing the high degree of genetic specificity achievable using homing drives, for example based on the CRISPR/Cas9 system, our approach aims at employing one or more locally fixed alleles as the target for a gene drive on a particular island. Using mathematical modeling, we explore the feasibility of this approach and the degree of localization that can be achieved. We show that across a wide range of parameter values, escape of the drive to a neighboring population in which the target allele is not fixed will at most lead to modest transient suppression of the non-target population. While the main focus of this paper is on elimination of a rodent pest from an island, we also discuss the utility of the locally fixed allele approach for the goals of population suppression or population replacement. Our analysis also provides a threshold condition for the ability of a gene drive to invade a partially resistant population.