PARENT SESSION
Symposium 21: Resource pulses in space and time: Linking species, communities and ecosystems
Organized by: LH Yang, MJ Vanni, WH Nowlin , and KA Schmidt
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 517 C, Level 5, Palais des congrès de Montréal

Pulsed vs. subsidized communities: Similar, different, or inseparable in a hyperarid system?

Anderson, Wendy^*,1, Wait, Alexander², ¹ Drury University, Springfield, MO, USA² Southwest Missouri State University, Springfield, MO, USA

ABSTRACT- As the theoretical bases for the dynamics of spatially subsidized communities emerge, ecologists question whether spatially subsidized communities behave like communities that receive resource pulses. We explore this question with annual plant data from a hyperarid system that is both pulsed and subsidized. Desert islands in the Gulf of California receive significant rains in infrequent pulses (every 2-7 years) that stimulate annual plant growth. Spatial subsidies arrive regularly from the ocean in the form of N and P rich seabird guano on islands where seabirds nest and roost. In wet years, the interior of large islands without seabirds receive only pulses. In dry years, islands with seabirds receive only subsidies. In wet years, islands with seabird guano receive both pulses and subsidies. Plant communities differ substantially among these three conditions. In unsubsidized areas during pulses, apparent plant richness increases from 1-2 to 30-40 annual species, and productivity increases 5-6x. In subsidized areas during a drought, apparent plant richness is 1-2 annual species, and productivity is similar to the pulsed areas during a pulse (10g/m²). In subsidized areas during a pulse, apparent plant richness increases to 3-4 species, and productivity is 20-60x greater than in areas or times without subsidies or pulses. Thus, desert plant communities in pulsed only vs. subsidized only areas have similar productivity but very different richness. However, when subsidized areas also receive a pulse, plant communities have significantly greater productivity while still maintaining very low richness. These general patterns suggest that the selective forces of pulsed environments may differ from those of subsidized environments. Allochthonous nutrient deposition alleviates the severe nutrient limitation found in most desert regions and extends the functional growing period for annuals, thus modifying the typical pulse-reserve strategy that is favored by most desert annuals. However, as is likely the case in many systems, it is difficult to examine the effects of subsidies in the absence of pulses since many systems receive both pulsed and subsidized resources, or resource subsides arrive in pulses.

Key words: allochthonous inputs, seabird guano, deserts, Gulf of California