Browsing by Author "Meserve, PL"

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    A long-term study of vertebrate predator responses to an El Nino (ENSO) disturbance in western South America
    (1997) Jaksic, FM; Silva, SI; Meserve, PL; Gutierrez, JR
    We analyzed the putative effects of the El Nino Southern Oscillation (ENSO) of 1991-92 in a semi-arid locality of northern Chile. We obtained 30 months of pre ENSO data, followed by 36 months of peak and post ENSO data (total = 5.5 yr). The rainy winter of 1991 resulted in a three-fold increase in total seed bank (perennial and ephemerals pooled) and in ephemeral (but not perennial) herb cover. Seed and herbage eaters (rodents) irrupted to population levels ca 20 times higher during the breeding season of 1991 than the preceding wintering season. Diurnal carnivorous predators (hawks, owls, and foxes) showed a delayed response to the irruption, increasing from seven individuals sighted during the wintering season of 1991 to 13 during the wintering season of 1992. A seemingly counterclockwise trajectory of predator abundance versus prey levels suggested a pattern of prey-driven dynamics, but confidence intervals were likely broad. In this semiarid locality, it appears that ENSO effects did not cascade down from higher to lower trophic levels, but rather the opposite. In this bottom-up scenario, we predict that as primary productivity varies with rainfall, so should secondary (mammal prey densities), and tertiary productivity (vertebrate predators). Long-term monitoring of this terrestrial ecosystem is needed to test this prediction.
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    The interplay of biotic and abiotic factors in a semiarid Chilean mammal assemblage
    (1999) Meserve, PL; Milstead, WB; Gutiérrez, JR; Jaksic, FM
    Since early 1989, we have conducted a large-scale ecological manipulation in a semiarid thorn scrub community in north-central Chile. We have excluded vertebrate predators (raptors and mammalian carnivores), and larger small mammal herbivore/competitors (i.e., degus, Octodon degus) from replicated 0.56-ha plots, and monitored small mammal population and plant responses over more than ten years. Repeated measures ANOVAs on minimum number known alive (MNKA) estimates of small mammals for a six-year period (1990-1996) spanning an El Nino event in 1991-1992 showed strong responses of some species to predator exclusions (e.g., O. degus; Darwin's leaf-eared mouse, Phyllotis darwini; the chinchilla-rat, Abrocoma bennetti). However, responses varied in time with significant effects during pre-El Nino (1990-1992) and El Nino (1992-1994) periods (i.e., 0. degus), or pre-El Nino and post-El Nino (1994-1996) periods (P. darwini. A. bennetti). Other species showed no responses to predator exclusions (e.g., olivaceous field mouse, Akodon olivaceus; long-haired field mouse, Abrothrix longipilis; long-tailed rice rat, Oligoryzomys longi caudatus). Some effects of competitor (degu) exclusions were detected (e.g., A. bennetti during the El Nino and post-El Nino periods; O. longicatrrlntus during the El Nino). "Top-down," factors (i.e., biotic interactions) appear to have greater effects on "core species" (i.e., P. darwini, O. degus) which persist in the thorn scrub. Other species (e.g., A. longipilis, O. longicaudatus) are transitory residents or "opportunistic" with lesser effects of biotic interactions, and their populations may be controlled by source-sink dynamics. All species had strong, responses to the 1991-1992 El Nino indicating primary control by "bottom-up" factors.