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Hierarchy of responses to resource pulses in arid and semi-arid ecosystems


In arid/semi-arid ecosystems, biological resources, such as water, soil nutrients, and plant biomass, typically go through periods of high and low abundance. Short periods of high resource abundance are usually triggered by rainfall events, which, despite of the overall scarcity of rain, can saturate the resource demand of some biological processes for a time. This review develops the idea that there exists a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses, such that small pulses only trigger a small number of relatively minor ecological events, and larger pulses trigger a more inclusive set and some larger ecological events. This framework hinges on the observation that many biological state changes, where organisms transition from a state of lower to higher physiological activity, require a minimal triggering event size. Response thresholds are often determined by the ability of organisms to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses can only affect surface dwelling organisms with fast response times and high tolerance for low resource levels, such as some species of the soil micro-fauna and -flora, while it takes more water and deeper infiltration to affect the physiology, growth or reproduction of higher plants. This review first discusses how precipitation, climate and site factors translate into soil moisture pulses of varying magnitude and duration. Next, the idea of the response hierarchy for ecosystem processes is developed, followed by an exploration of the possible evolutionary background for the existence of response thresholds to resource pulses. The review concludes with an outlook on global change: does the hierarchical view of precipitation effects in ecosystems provide new perspectives on the future of arid/semiarid lands?

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This research was supported by the US National Science Foundation grant DEB #0222313, the InterAmerican Institute for Global Change Research, the Agencia Nacional de Promoción Científica y Técnica, and the University of Buenos Aires. We thank all the participants of the workshop “Resource pulse utilization in arid and semiarid ecosystems” for stimulating discussion, and especially Jim Ehleringer for his leadership role in developing the idea for this workshop. We also wish to thank Jayne Belnap and two anonymous reviewers for their suggestions on the manuscript.

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Correspondence to Susanne Schwinning.

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Schwinning, S., Sala, O.E. Hierarchy of responses to resource pulses in arid and semi-arid ecosystems. Oecologia 141, 211–220 (2004).

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  • Climate change
  • Ecosystem structure
  • Precipitation thresholds
  • Precipitation variability
  • Rainfall size