Julio R. Gutiérrez

El Niño effects on the dynamics of terrestrial ecosystems

New studies are showing that the El Niño Southern Oscillation (ENSO) has major implications for the functioning of different ecosystems, ranging from deserts to tropical rain forests. ENSO-induced pulses of enhanced plant productivity can cascade upward through the food web invoking unforeseen feedbacks, and can cause open dryland ecosystems to shift to permanent woodlands. These insights suggest that the predicted change in extreme climatic events resulting from global warming could profoundly alter biodiversity and ecosystem functioning in many regions of the world. Our increasing ability to predict El Niño effects can be used to enhance management strategies for the restoration of degraded ecosystems.

Extreme climatic events shape arid and semiarid ecosystems

6 Climatic changes associated with the El Nino Southern Oscillation (ENSO) can have a dramatic impact on ter- restrial ecosystems worldwide, but especially on arid and semiarid systems, where productivity is strongly lim- ited by precipitation. Nearly two decades of research, including both short-term experiments and long-term studies conducted on three continents, reveal that the initial, extraordinary increases in primary productivity percolate up through entire food webs, attenuating the relative importance of top-down control by predators, providing key resources that are stored to fuel future production, and altering disturbance regimes for months or years after ENSO conditions have passed. Moreover, the ecological changes associated with ENSO events have important implications for agroecosystems, ecosystem restoration, wildlife conservation, and the spread of disease. Here we present the main ideas and results of a recent symposium on the effects of ENSO in dry ecosystems, which was convened as part of the First Alexander von Humboldt International Conference on the El Nino Phenomenon and its Global Impact (Guayaquil, Ecuador, 16-20 May 2005).

Thirteen Years of Shifting Top-Down and Bottom-Up Control

Abstract For 13 years, we studied the role of biotic interactions, including predation, herbivory, and interspecific competition, in a semiarid thorn scrub community in north-central Chile. Using a large-scale field manipulation, we monitored changes in small mammals, plants, and vertebrate predators. We documented important “top-down” predation effects on some small-mammal species, and small-mammal effects on some plant species resulting from experimental exclusions. However, periodic El Niño Southern Oscillation (ENSO) events caused several high rainfall episodes during this interval, resulting in large “bottom-up” increases in both plants and animals. Therefore, we suggest that instead of exclusive top-down or bottom-up control, this system undergoes shifting control, with relatively greater importance of biotic interactions in wet years and of resource limitation in dry ones. Because intervals between ENSO events are long and responses are slow, long-term studies are essential for understanding such effects in semiarid or arid systems.

A long-term study of vertebrate predator responses to an El Nino (ENSO) disturbance in western South America

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.

Chihuahuan Desert annuals: importance of water and nitrogen

We examined the effects of water supplementation and nitrogen amendment on biomass, cover, and density of annual plants on a termite-free and a termite-present area in the Chihuahuan Desert. Soil moisture was higher in the termite than in the termite-free plots, and in the watered than in the unwatered plots during the spring and summer. There were no differences in soil moisture among plots during the winter. Soil nitrogen was higher in the termite-free than in the termite plots. There were no differences in total plant biomass produced in termite and termite-free areas. There were significant differences in relative abundances of species among treatments. natural rainfall was sufficient for maximum spring-annual biomass development on all plots except for the termite-free unfertilized, unwatered plots. These were the driest plots but had high soil nitrogen. Most of the herbaceous species responded to the water amendments by lengthening growing seasons, increasing density, or increasing biomass. When there was sufficient water for most of the spring annuals, high soil nitrogen levels favored increased densities and biomasses of Descurainia pinnata and Lepidium lasiocarpum. The absence of C4 summer annuals in the high-nitrogen plots suggests that relatively high soil nitrogen adversely affected the summer annuals. Termite-free watered plots had higher soil moisture than the termite-unwatered plots, but summer annuals were relatively abundant on the latter. Water amendments had a greater effect on the species abundances in the termite-free area than in the one with termites. In the area with termites, nitrogen amendments had a greater effect on species abundances. Species diversity and richness were affected by fertility as was species composition. This study demonstrates that we must understand patterns of soil nitrogen availability and processes affecting nitrogen availability in addition to water availability, in order to understand productivity and species composition of Chihuahuan Desert annual plants.

Effects of predation risk on space use by small mammals : a field experiment with a Neotropical rodent

Predation is often implicated as the most important factor determining differential microhabitat use by small mammals, particularly in desert ecosystems. This generalization, however, is based primarily on observational and correlational approaches and only a few field experimental studies. In a large-scale, long-term experimental manipulation of predators in semiarid north-central Chile, we studied the effects of excluding vertebrate predators on the space use of a small mammal species. We used three different techniques to determine space use under and away from shrubs by the diurnal, herbivorous rodent Octodon degus. These included smoked tiles, fluorescent pigment tracking, and tabulation of numbers of runways among shrubs. Results show that O. degus used spaces away from shrubs more often, made more (and less straight) runways between shrubs and had smaller daily home ranges in grids where predators were excluded. Thus, besides the well known predator effects of density depression and survival reduction of their prey, our study demonstrates experimentally that prey respond behaviorally to perceived predation risks.

Heterogeneous responses of small mammals to an El Niño southern oscillation event in northcentral semiarid Chile and the importance of ecological scale

A prolonged El Nino Southern Oscillation (ENSO) event during 1991–1992 with three times the normal annual 85-mm rainfall was accompanied by major changes in numbers of small mammals at a semiarid Mediterranean site in northcentral Chile. Several demographic patterns were evident. Akodon olivaceas , an omnivore, had a rapid increase in population size of more than an order of magnitude. Phyllotis darwini , a granivore-herbivore, showed somewhat delayed, smaller increases superimposed on annual oscillations. Octodon degus , an herbivore, showed a delayed response with larger increases and extended breeding in 1992–1993. Finally, Oligoryzomys longicaudatus , a granivore, experienced increases during both a dry (1990, ppt = 32 mm) and a wet year (1992). Other species such as Abrothrix longipilis, Abrocoma bennetti , and Thylamys elegans had smaller, delayed demographic responses. Reproductive rates for the first three species were higher due to the ENSO event only in O. degus males and P. darwini as a quadratic function of time. Survival rates of all four principal species were significantly greater during the 1991–1992 ENSO. Finally, average movement between captures was lower during ENSO years, suggesting behavioral changes. Explanations for these patterns include rainfall-related increases in food-resource levels ( A. olivaceus and P. darwini ), the importance of source-sink processes in vagile species from more mesic, adjacent habitats ( O. longicaudatus ), and delayed responses to extrinsic events in species with long gestation ( O. degus, A. bennetti ). The heterogeneity of species responses suggests different capabilities for small mammals to respond to an extrinsic, large-scale event, and it emphasizes the importance of long-term studies in semiarid systems.

Spatial distribution of soil nutrients and ephemeral plants underneath and outside the canopy of Porlieria chilensis shrubs (Zygophyllaceae) in arid coastal Chile

Soil nutrients and density and biomass of annual plants underneath and outside the canopy of Porlieria chilensis shrubs were measured at the end of the growing season in a protected arid coastal site in Chile. Levels of soil nitrogen, phosphorus and organic matter were significantly higher underneath than outside the canopies of shrubs. Almost 4 times as many plants occurred outside than underneath shrubs, but no significant differences in total aboveground biomass were found. Several species had higher densities and/or biomass outside rather than underneath shrubs, whereas others showed the oppsite trend. Species richness was lower underneath P. chilensis canopy. The spatial microdistribution of ephemeral species may be explained by differential water and nutrient requirements. Comparison of the patterns observed in our protected site versus surrounding unprotected areas supports the generalization that man, by removing shrubs and trees, has changed a previous heterogeneous spatial distribution of nutrients to a more homogenous one.

Population dynamics of two sympatric rodents in a variable environment: rainfall, resource availability, and predation

Precipitation plays an important role in the dynamics of species found in arid and semiarid environments. However, population fluctuations generally are driven by a combination of multiple factors whose relative contribution may vary through time and among species. We monitored fluctuations of species in three trophic levels for >17 years at a semiarid community in north-central Chile. The region is strongly affected by the El Niño Southern Oscillation, resulting in high variation in rainfall that triggers dramatic changes in food resource availability, with strong effects on upper trophic levels. We focused our analyses on the role played by endogenous and exogenous (climatic) factors on the dynamics of two important rodent species in the community, Octodon degus and Phyllotis darwini. We documented population fluctuations of several orders of magnitude in response to wet and dry episodes of different strength and duration. P. darwini reached similar maximum densities, regardless of the duration of high-rainfall events, whereas O. degus showed additive effects of multiple wet years. Time series diagnostic tools revealed oscillations with a 5-year periodicity in rainfall, which may be the cause of the same periodicity and a weak second-order signal observed in the rodent dynamics. However, the dynamics of both rodent species were dominated by strong first-order processes, suggesting an important role of direct density dependence. Intraspecific competition, expressed as the ratio of rodent density/rainfall (or food resources) explained more than two-thirds of the variation in the population rate of change, whereas less than one-third was explained by lagged rainfall (or food resources). We detected no significant effects of predation. Our results contribute to a growing number of examples of dynamics governed by the combined effect of density dependence and climatic forcing. They also reveal strong bottom-up regulation that may be common in other arid environments.

Effects of small mammals and vertebrate predators on vegetation in the Chilean semiarid zone

Abstract We monitored the cover and seed bank response of shrubs, perennial herbs, and ephemeral plants to experimental exclusion of both the principal rodent herbivore, Octodon degus, and its vertebrate predators from 1989 to 1994 in a semiarid Chilean mediterranean site. Although both richness and species composition of the plant community at the study site were largely determined by abiotic factors (mainly rainfall and soil nutrients), predator and herbivore exclusion had significant effects on the relative abundance of several plant species. Experimental exclusion of herbivores was associated with increased cover of some shrubs and a perennial grass, and decreased cover and seed densities of several ephemerals, especially those exotic or restricted to areas underneath shrubs. Herbivores apparently reduced shrubs through browsing and indirectly affected herb cover and seed densities by opening up areas under shrubs and/or modifying physical and chemical conditions of the soil. Plant responses to predator exclusion were less clear. Nevertheless, higher cover of some shrubs and ephemerals in the presence of predators suggests tritrophic effects through changes in small mammal densities and/or foraging behavior.