Invasive rat space use on tropical islands: Implications for bait broadcast

Abstract

Invasive rats on oceanic islands impact a large number of native species. Control programmes, and in many cases complete eradication, are used to alleviate these impacts. Basic data on rodent biology facilitate the design of control or eradication programmes, and is particularly required for programmes on tropical islands where such data are missing. Here we test for interactive effects of habitat and season that may alter black rat (Rattus rattus) space use dynamics and inform rodent management on two tropical islands. Five years of summer and winter trapping data were analysed using spatially explicit capture-recapture to calculate rat space-use and overlap, coupled with spool and line experiments ground-truthing microhabitat use. Variation in individual rat space use is primarily driven by sex and bottom-up trophic effects of seasonal rainfall on food resources, but is altered by island-specific contexts. In the absence of other introduced mammals, rats tend to have stable range overlap throughout the year but home range sizes fluctuate seasonally with rat density. The presence of other introduced mammals causes predictable greater seasonal fluctuations in rat space-use, putatively a behavioural adjustment to feral cats (Felis catus) diet-switching to rats from seasonal influxes of their alternative seabird prey. We identify winter as the recommended treatment period on both islands and discuss bait broadcast strategies. ZusammenfassungInvasive Ratten beeinträchtigen auf ozeanischen Inseln eine große Anzahl von einheimischen Arten. Kontrollprogramme und in vielen Fällen die vollständige Ausrottung werden genutzt, um diese Beeinträchtigungen zu mildern. Grundlegende Daten zur Kleinsäugerbiologie erleichtern die Planung von Kontroll-und Ausrottungsprogrammen. Sie werden besonders für Programme auf tropischen Inseln benötigt, wo solche Daten fehlen. Hier untersuchen wir die Interaktionen zwischen Habitat und Jahreszeit, die die Raumnutzung von Hausratten (Rattus rattus) verändern und dem Kleinsäugermanagement zugrunde gelegt werden können. Sommer-und Winterfallenfänge aus fünf Jahren mit räumlich explizitem Fang/Wiederfang wurden analysiert, um Raumnutzung und Überlappung der Ratten zu berechnen. Mit der Fadenspulen-Methode wurden Vergleichsmessungen zur Mikrohabitatnutzung durchgeführt. Die Variation der Raumnutzung durch Rattenindividuen wird in erster Linie vom Geschlecht und den durch saisonale Regenfälle bedingten Nahrungsressourcen bestimmt; sie wird aber durch inselspezifische Zusammenhänge modifiziert. (D. Ringler).Wenn keine anderen eingeführten Säuger vorhanden sind, ist die Überlappung der Territorien über das Jahr hinweg tendenziell stabil, aber die Größe der Aktionsräume schwankt saisonal mit der Rattendichte. Die Anwesenheit anderer eingeführter Säuger ist mit vorhersagbaren, größeren saisonalen Schwankungen der Raumnutzung durch die Ratten verbunden-vermutlich eine verhaltensbedingte Anpassung an den saisonal bedingten Nahrungswechsel verwilderter Hauskatzen (Felis catus)v on Seevögeln, ihrer alter...

Best practice guidelines for rat eradication on tropical islands

“…Thus, wet tropical islands have similarities to temperate islands in that productivity and conditions are predictable (Ringler et al, 2014). On many dry tropical islands productivity is closely tied to unpredictable rainfall.…”

“…Obtain sufficient inter and intra-seasonal information on local climate and its impact upon rodent phenology and other key factors, such as non-target bait competitors, and use it to inform the optimal timing of the eradication operation. Rat populations should be targeted for eradication at a time when natural food resources are declining in availability or are at their lowest levels, a period that likely coincides with the driest period on the island (Ringler et al, 2014). On tropical islands the timing of this period of low productivity may vary from year to year and in some years not occur at all (Russell et al, 2011).…”

Trophic roles of black rats and seabird impacts on tropical islands: Mesopredator release or hyperpredation?

“…Eradication of apex predators on both islands is therefore unlikely to cause numerical release of rats. However, cats' predation pressure plays an additional top-down role regulating the morphology (decrease in body size) and movements of rats (decrease in home range size) on Juan de Nova (Ringler et al, 2014;Russell et al, 2011a) (Fig. 6).…”

Invasive Rat Research and Management on Tropical Islands: A Case Study in the Iles Eparses

“…Bottom-up processes also affect behavioural traits of rats in the Iles Eparses. Home ranges are larger in poor quality habitat associated with low rat densities, when intraspecific competition is low (Ringler et al 2014). The home range size difference is about double between the most contrasting habitats of grassland and forest (home range radius ranges from 35 m to 70 m) (Ringler et al 2014).…”

“…Home ranges are larger in poor quality habitat associated with low rat densities, when intraspecific competition is low (Ringler et al 2014). The home range size difference is about double between the most contrasting habitats of grassland and forest (home range radius ranges from 35 m to 70 m) (Ringler et al 2014). On cat-free islands, seasonal variations of home ranges are either directly associated to food pulses (i.e., foraging area decreases when food is abundant) or indirectly through seasonal variations of density levels (i.e., intraspecific competition).…”