DSpace Angular :: Browsing by Author "Jaksic, FM"

Browsing by Author "Jaksic, FM"

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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.
Demographic dynamics of a neotropical small rodent (Phyllotis darwini)
(2001) Lima, M; Julliard, R; Stenseth, NC; Jaksic, FM
1. The leaf-eared mouse (Phyllotis darwini) exhibits large numerical fluctuations associated with high- and low-rainfall years in semi-arid Chile. Using capture-mark-recapture (CMR) statistical modelling.. we provide a detailed description of the demographic variation in this species. We studied between-year and seasonal variation of demography, and tested for the relative importance of endogenous and exogenous factors as covariates of survival, recruitment, maturation and reproduction over a 12-year period.
Demography and population dynamics of the mouse opossum (Thylamys elegans) in semi-arid Chile
(2001) Lima, M; Stenseth, NC; Yoccoz, NG; Jaksic, FM
Here, we present, to the authors' knowledge for the very first time for a small marsupial, a thorough analysis of the demography and population dynamics of the mouse opossum (Thylamys elegans) in western South America. We test the relative importance of feedback structure and climatic factors (rainfall and the Southern Oscillation Index) in explaining the temporal variation in the demography of the mouse opossum. The demographic information was incorporated into a stage-structured population dynamics model and the model's predictions were compared with observed patterns. The mouse opossum's capture rates showed seasonal (within-year) and between-year variability, with individuals having higher capture rates during late summer and autumn and lower capture rates during winter and spring. There was also a strong between-year effect on capture probabilities. The reproductive (the fraction of reproductively active individuals) and recruitment rates showed a clear seasonal and a between-year pattern of variation with the peak of reproductive activity occuring during winter and early spring. In addition, the fraction of reproductive individuals was positively related to annual rainfall, while population density and annual rainfall positively influenced the recruitment rate. The survival rates were negatively related to annual rainfall. The average finite population growth rate during the study period was estimated to be 1.011 +/- 0.0019 from capture-recapture estimates. While the annual growth rate estimated from the seasonal linear matrix models was 1.026, the subadult and adult survival and maturation rates represent between 54% (winter) and 81% (summer) of the impact on the annual growth rate.
El Nino events, precipitation patterns, and rodent outbreaks are statistically associated in semiarid Chile
(1999) Lima, M; Marquet, PA; Jaksic, FM
In the last two decades, several researchers have noted rodent population outbreaks in semiarid South America, in association with unusually high precipitation that seemingly concurs with Ei Nino events. To date, no studies have been conducted to determine the statistical relationships between ENSO (El Nino Southern Oscillation) events. increased precipitation, and rodent irruptions. Here we show that: 1) there is a statistical association between ENSO events and increased precipitation in the semiarid region of northern Chile, 2) the occurrence or rodent outbreaks in that region is statistically related with the precipitation levels of the same year; 3) the multi-annual patterns of the total annual precipitation levels and population abundance of those rodents Juring the summer are positively associated. The putative chain of effects seems to start with unusually high rainfall brought by ENSO to semiarid environments. which thus respond with increased primary productivity (herbage and seeds), which then fuels the rodent outbreaks.
El Nino-southern oscillation-driven rainfall variability and delayed density dependence cause rodent outbreaks in western South America: Linking demography and population dynamics
(UNIV CHICAGO PRESS, 1999) Lima, M; Keymer, JE; Jaksic, FM
It is well known that some rodent populations display dramatic density fluctuations in semiarid regions of western South America after the unusual rainfall levels associated with El Nino-southern oscillation (ENSO) disturbances. These correlated phenomena have led some ecologists to believe that rodent outbreaks are determined solely by density-independent factors (e.g., rainfall regime). However, demographic studies have detected strong delayed density-dependent effects in one of the most irruptive rodent species, the leaf-eared mouse Phyllotis darwini. We tested the effects of rainfall and delayed density-dependent factors by constructing a structured model based on demographic data estimated from a capture-mark-recapture study of this species in Chile. A model including both rainfall and delayed density-dependent effects predicts the observed population dynamics rather accurately over a 10-yr period. Interestingly, small changes in model parameters result in large changes in model dynamics, which strongly suggests that local variations in demographic features are important in explaining the asynchronous pattern in outbreak occurrences. These findings suggest that inextricably intertwined endogenous and exogenous forces cause rodent outbreaks in western South America. The former are characterized by delayed nonlinear feedbacks, whereas the latter are characterized by the positive effects of the El Nino phases and the negative effects of the La Nina phases of the ENSO disturbance.
Food web structure and climate effects on the dynamics of small mammals and owls in semi-arid Chile
(2002) Lima, M; Stenseth, NC; Jaksic, FM
Population dynamics of small mammals and predators in semi-arid Chile is positively correlated with rainfall associated with incursions of El Nino (El Nino Southern Oscillation: ENSO). However, the causal relationships between small mammal fluctuations, predator oscillations, and climatic disturbances are poorly understood. Here, we report time series models for three species of small mammal prey and two species of owl predators. The large differences in population fluctuations between the three small mammal species are related to differences in their respective feedback structures. The analyses reveal that per capita growth rate of the leaf-cared mouse is a decreasing function of log density and of log barn owl abundance together with a positive rainfall effect. In turn, per capita population growth rate (R-function) of the barn owl is a negative function of log barn owl abundance and a positive function leaf-eared mouse abundance, suggesting a predator-prey interaction. The dramatic population fluctuations exhibited by leaf-cared mouse (Phyllotis darwini) are caused by climate effects coupled with a complex food web architecture.
Great horned and barn owls prey differentially according to the age size of a rodent in northcentral Chile
(1995) Castro, SA; Jaksic, FM
Based on cranial measurements we estimated the size- and age-class (juvenile or adult) of leaf-eared mice (Phyllotis darwini) consumed by great horned (Bubo virginianus) and barn owls (Tyto alba) during 1990-92 in a semiarid locality of northcentral Chile. We compared this information to that obtained from mice live-trapped at the same place and period. The frequency distributions of body weights of leaf-eared mice consumed by the two owl species was not statistically different, despite the four-fold difference in body size and different hunting mode of the two owls. In two out of three breeding seasons the owls preyed more frequently on adult mice. In the remaining breeding season and the three wintering seasons they preyed upon adults and juveniles in about the same proportion as they were live-trapped in the field. The higher predation on adult individuals (reproductive) was associated to a greater mobility of adult versus juvenile mice, which may render the former more vulnerable to owl detection.
Nutritional ecology and digestive response to dietary shift in the large South American fox, Pseudalopex culpaeus
(2005) Silva, SI; Jaksic, FM; Bozinovic, F
We tested the role of dietary shifts (from rodents to fruits and to mixed diets) oil the nutritional ecology of the culpeo fox Psendalopex culpaeus, a native canid of South America. We studied the effects of food quality oil digestive processes. nutrition, and mass balance, and the implications of diet quality for fox Survival. We observed at the end of the nutritional trials that body mass differed significantly between the three diet groups (fruits, rats and mixed diets), while percentage of body mass change differed significantly only ill the fruit diet treatment. Foxes fed with fruits consumed more food to meet their dietary and metabolic needs. Across diets, dry-matter as well as energy digestibility increased significantly with diet quality. Also, mean retention time was negatively and significantly correlated with dry-matter intake. We put forth that mixed diet may yield higher assimilation efficiencies and hence higher nutrient intakes than those predicted from the ingestion and assimilation of pure diets (i.e,. only rats, only fruits). We hypothesize that during periods of low availability of mammalian prey, a mixed diet should yield a positive energy/mass balance for the fox. We Conclude that temporal and spatial variation in nutrient, energy, and water contents of prey available in a given habitat could have all important effect on fox nutrition, energy use, and mass balance. Finally, we postulate that P.culpaeus Could not survive on fruits only past seven days.
Population dynamics of three Neotropical small mammals: Time series models and the role of delayed density-dependence in population irruptions
(1999) Lima, M; Jaksic, FM
It is widely believed that only precipitation levels (through increased primary production) determine irruptions of small mammals in semi-arid areas of western South America. Nevertheless, density-dependent factors may also drive population fluctuations. To test statistically these putative effects we analysed 11 years of population records on three sympatric species of small mammals at two different habitat types in north central Chile. We applied the classical diagnostic tools of time series analysis (the autocorrelation function: ACF) to the observed time series of three neotropical small mammals. We also used simple linear autoregressive time series models to reconstruct the endogenous dynamics of these populations. The analysis strongly suggests that population fluctuations of the three species have an important density-dependent component, with the most irruptive species (Phyllotis darwini, Waterhouse 1837) displaying stronger second order population feedbacks than the other two (AKodon olivaceus, Waterhouse 1837 and Thylamys elegans, Waterhouse 1839). The latter two species showed direct density-dependent feedbacks. We hypothesize that the frequent population outbreaks of I? darwini land perhaps of other species) in semi-arid regions of western South America, may be the result of population-level (direct density-dependence) and community-level processes (delayed density-dependence), interacting with exogenous perturbations (rainfall and associated primary production).
Population rate of change in the leaf-eared mouse: The role of density-dependence, seasonality and rainfall
(1999) Lima, M; Jaksic, FM
We analysed statistically the influence of density-dependent regulation, seasonality and precipitation on the realized population rate of change of the Neotropical rodent Phyllotis darwini (Waterhouse 1837) at an intraannual rime scale. We used four years of continuous live trapping at a semiarid locality of north central Chile. Results showed that density-dependence, seasonal effects and precipitation were important factors influencing population growth rates in this species. An empirical population model including a sine function fur seasonal effects, a linear form for density-dependence, and precipitation was fitted to the full data set and to the data set with the first year removed (after an outbreak). The empirical model explained 33% and 48% of the variance in population growth. The natural rate of population increase, estimated from the empirical model, was r(max) = 2.51 or 5.06 years(-1). These estimates indicate a great potential for population increase and may explain the capability of this species to undergo large irruptions. We propose that merging empirical and theoretical modelling with field research is the most promising avenue to understand the outbreaks experienced by some rodent species in western South America.
Population variability among three small mammal species in the semiarid Neotropics: The role of density-dependent and density-independent factors
(1998) Lima, M; Jaksic, FM
We addressed the role of density-dependent (direct and delayed) and density-independent (precipitation) factors in shaping the dynamics of fluctuating populations of three small mammal species. Using a stepwise regression procedure, we tested the effects of nonlagged population density (log(10)N(t-1)), lagged population density (log(10)N(t-2)), and annual precipitation on the per capita rate of population change of Phyllotis darwini, Akodon olivaceus, and Thylamys elegans in two habitat types of a semiarid region of Chile. The most irruptive species (P. darwini) showed direct and delayed density-dependent effects in equatorial subpopulation, and only direct density-dependence in polar subpopulation. The per capita rates of population change of A. olivaceus showed direct density-dependent and precipitation effects in both habitats types, while T. elegans showed direct density-dependence and precipitation effects in the equatorial subpopulation but only a marginal effect of direct density-dependence in the polar subpopulation. The presence of delayed density-dependent strongly suggests the importance of biological interactions in shaping the dramatic irruptions exhibited by P. darwini.
Prey size matters at the upper tail of the distribution
(1999) Santibañez, DP; Jaksic, FM
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.
Vocalization as a response to capture in the central Chilean lizard Liolaemus chiliensis (Tropiduridae)
(2001) Carothers, JH; Groth, JG; Jaksic, FM
Among 11 Liolaemus lizard species examined in central and south-central Chile only L. chiliensis responded to capture and handling by vocalizing. Vocalizations sounded more like a squeak than a hiss, and volume was sufficient to be heard at a distance of similar to3 m. The vocalization is probably produced by the larynx and may be an antipredator response. Similarities of L. chiliensis to the anguid lizard Elgaria multicarinata in California are discussed.

Browsing by Author "Jaksic, FM"

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