Trends in Ecology & Evolution 16(2)
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Copyright Notice This electronic reprint is provided by the author(s) to be consulted by fellow scientists. It is not to be used for any purpose other than private study, scholarship, or research. Further reproduction or distribution of this reprint is restricted by copyright laws. If in doubt about fair use of reprints for research purposes, the user should review the copyright notice contained in the original journal from which this electronic reprint was made. Review TRENDS in Ecology & Evolution Vol.16 No.2 February 2001 89 El Niño effects on the dynamics of terrestrial ecosystems Milena Holmgren, Marten Scheffer, Exequiel Ezcurra, Julio R. Gutiérrez and Godefridus M.J. Mohren New studies are showing that the El Niño Southern Oscillation (ENSO) has Effects on plant communities major implications for the functioning of different ecosystems, ranging from Short-term responses in arid ecosystems deserts to tropical rain forests. ENSO-induced pulses of enhanced plant The short-term response of vegetation to wet ENSO productivity can cascade upward through the food web invoking unforeseen events is often spectacular. In the barren, arid islands feedbacks, and can cause open dryland ecosystems to shift to permanent of the Gulf of California, plant cover ranges between 0 woodlands. These insights suggest that the predicted change in extreme and 4% during ‘normal’ years, but during rainy El Niño climatic events resulting from global warming could profoundly alter periods, it rises to 54–89% of the surface area available biodiversity and ecosystem functioning in many regions of the world. Our for growth. This is largely caused by a profusion of increasing ability to predict El Niño effects can be used to enhance annuals and some drought-susceptible perennial management strategies for the restoration of degraded ecosystems. species8 (Fig. 2). The fast and dramatic response of annuals has been described for several ecosystems9. Extensive regions around the world are influenced by Landscape topographical features usually affect water the El Niño Southern Oscillation (ENSO). El Niño availability for plants and, therefore, induce large events occur irregularly but typically once every three spatial variation in the effects of rainy events. For to six years1. Although the effects of global warming instance, in semiarid north-central Chile, ephemeral on ENSO oscillations are difficult to predict, recent plant cover during an El Niño year increased by over high resolution climatic models suggest that the five times on a north-facing dry slope, by three times on frequency of El Niño-like conditions are expected to a south-facing mesic slope, and only moderately in the increase over the coming decades2. During an El Niño relatively moist ravine10. episode, rainfall dramatically increases in certain Although massive germination of annuals is the areas of the world, whereas severe droughts occur in most spectacular effect of wet episodes in dry regions, other regions (Fig. 1). The rainfall during El Niño perennial herbs, shrubs and trees also tend to show years can be four to ten times higher than average, significant increases in growth, flowering and fruit and the phenomenon lasts approximately one year. production8,9. However, this usually only results in The next phase, known as La Niña, produces roughly small changes in cover10. Importantly, top-down the opposite climate patterns to those found during regulation by relatively inconspicuous animals such as an El Niño episode. small rodents might interact strongly with rainfall Milena Holmgren* El Niño events have strong ecological3 and effects in regulating plant biomass in these dry areas. Godefridus M.J. Mohren economic consequences4. The best-studied effects on A large-scale field experiment conducted in northern Silviculture and Forest Ecology Group ecosystems are those in marine environments, where Chile since 1989 has shown that shrub and perennial *e-mail: this climatic phenomenon is correlated with dramatic cover can increase significantly when either rodent Milena.Holmgren@btbo. changes in the abundance and distribution of many herbivores are directly excluded, or when predators bosb.wau.nl organisms, and the collapse of fisheries5. By contrast, (birds of prey and foxes) are allowed to prey on Marten Scheffer the effects on terrestrial ecosystems have been poorly herbivores9. Interestingly, the opposite is true for Aquatic Ecology and explored. Only the spectacular greening and ephemeral herbaceous species, especially exotic Water Quality 6 Management Group, Dept flowering of deserts , and the crash of agricultural annuals, which actually decrease in cover when of Environmental crops7 in the core region of El Niño had been rodents are excluded. Apparently, small rodents Sciences, Wageningen commonly noted. In recent years, however, results reduce shrub cover by browsing, and increase soil University, PO Box 342, from several systematic long-term studies have disturbance, which facilitates the colonization of 6700 AH Wageningen, 9 The Netherlands. become available, revealing how ENSO events can disturbed patches by annuals . A less well-studied but have pronounced effects on plant and animal obviously important consequence of the pulse in Exequiel Ezcurra San Diego Natural History communities in a wide range of terrestrial primary productivity is that the seed bank in the soil Museum, PO Box 121390, ecosystems. We discuss the main results, showing can increase sharply following rainy ENSO episodes. San Diego, CA 92112- first the effects on plant dynamics in dry and moist In northern Chile, for example, the seed bank can 1390, USA. habitats, then the cascading effects on animal increase almost sixfold during rainy El Niño years9,10. Julio R. Gutiérrez communities and evidence for effects on the evolution Departamento de of species. Finally, we discuss ways of using these Long-term effects in arid ecosystems Biología, Universidad de La Serena, Casilla 599, insights in the management of degraded arid Tree ring analysis provides impressive records of La Serena, Chile. ecosystems. ENSO effects on the growth of trees and shrubs, even http://tree.trends.com 0169–5347/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0169-5347(00)02052-8 90 Review TRENDS in Ecology & Evolution Vol.16 No.2 February 2001 (a) (b) TRENDS in Ecology & Evolution Fig. 1. Regions showing in regions such as Israel that are usually thought to Consequently, although initial settlement might increased precipitation be little affected by this climatic oscillation11. Perhaps require wet climatic conditions, woodland can be (blue) and drier more importantly, extremely wet ENSO events might resilient and, once established, can persist for a conditions (orange) 14 during El Niño (a) and La cause long-lasting effects on semiarid vegetation, as long time . For instance, in the Niña (b) phases of the they can open rare windows of opportunity for the shrubland–grassland transition zone of the ENSO phenomenon. recruitment of trees and shrubs. For instance, the Chihuahuan desert, large increases in shrub cover Modified, with permission, from Ref. 1. recruitment of shrubs and cacti in the Sonoran desert over the past decades have been related to episodes appears to be related to El Niño events: stands of of increased winter precipitation that seem to Larrea shrubs usually consist of many size classes in broadly coincide with El Niño events15. Similarly, in relatively mesic places, whereas in drier areas they semiarid Australia, successful seedling often contain only one cohort, suggesting that establishment of mulga16, as well as Eucalyptus and recruitment in drier areas occurs only in conifer woodlands17, has been linked with wetter exceptionally favorable years12. Cactus recruitment is periods during La Niña episodes. However, severe also restricted to rainy El Niño years in the Sonoran ENSO-related droughts can also induce episodes of desert, but has an additional indirect relation to tree mortality, as demonstrated in the open donkey grazing; nurse shrubs, under which cactus Austrocedrus woodlands near the forest-steppe seedlings survive the drier periods, have been ecotone of northern Patagonia18,19. extirpated by these animals, resulting in a complete lack of cactus recruitment13. Effects on humid plant communities Adult trees and shrubs tend to be less vulnerable Several recent studies show that El Niño effects are to desiccation and herbivory than are seedlings, by no means restricted to drylands20–31. For example, which, once established, provide a relatively benign El Niño events have been linked to the almost microclimate for subsequent seedlings. complete defoliation of mangroves20, as well as http://tree.trends.com Review TRENDS in Ecology & Evolution Vol.16 No.2 February 2001 91 Fig. 2. Winter precipitation large source of carbon to the atmosphere, although in (circles) and mean other years it acts as a carbon sink28. percent plant cover (triangles) on islands of ENSO droughts can also induce widespread forest 0.4 the Gulf of California 150 fires, such as those observed in the Amazonian showing the effect of the tropical forests29,30, or in the pine forests of North 1992–1993 El Niño. 120 0.3 31 Modified, with America . These catastrophic fires have profound permission, from Ref. 8. 90 effects on the structure and dynamics of these 0.2 ecosystems29–31. 60 0.1 30 Effects on animal communities One would expect that an increase in primary Winter precipitation (mm) 1990 1991 1992 1993 1994 productivity causes a subsequent increase in Mean percentage of plant cover herbivores, followed by an increase in carnivores. TRENDS in Ecology & Evolution Indeed, several long-term monitoring studies have revealed that such bottom-up effects can be seen changes in growth and phenology of alpine forbs21, several years after the occurrence of the El Niño event and boreal forests22. In most of the wet tropics, El itself 8,26,32–34. However, these studies also show that Niño events are known to bring drier, warmer and top-down effects can complicate the story sunnier conditions. In contrast to what happens considerably.