Mechanisms That Generate Resource Pulses in a Fluctuating Wetland

RESEARCH ARTICLE Mechanisms That Generate Resource Pulses in a Fluctuating Wetland Bryan A. Botson1*, Dale E. Gawlik1, Joel C. Trexler2 1 Department of Biological Sciences, Florida Atlantic University, Boca Raton, Florida, United States of America, 2 Department of Biological Sciences, Florida International University, Miami, Florida, United States of America * a11111 Abstract OPEN ACCESS Citation: Botson BA, Gawlik DE, Trexler JC (2016) Mechanisms That Generate Resource Pulses in a Fluctuating Wetland. PLoS ONE 11(7): e0158864. doi:10.1371/journal.pone.0158864 Editor: Judi Hewitt, University of Waikato (National Institute of Water and Atmospheric Research), NEW ZEALAND Received: September 28, 2015 Accepted: June 23, 2016 Published: July 22, 2016 Copyright: © 2016 Botson et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This work was funded by Florida Atlantic University and Florida International University. DEG was funded by South Florida Water Management District contracts CP040319 and 4500042572, and contracts CP040130 and 4600001083 to JCT (http:// www.sfwmd.gov/portal/page/portal/sfwmdmain/home %20page). JCT was also funded by Florida Coastal Everglades Long-Term Ecological Research program under National Science Foundation Grant No. DBI0620409, and Grant No. DEB-1237517. The funders had no role in study design, data collection and Animals living in patchy environments may depend on resource pulses to meet the high energetic demands of breeding. We developed two primary a priori hypotheses to examine relationships between three categories of wading bird prey biomass and covariates hypothesized to affect the concentration of aquatic fauna, a pulsed resource for breeding wading bird populations during the dry season. The fish concentration hypothesis proposed that local-scale processes concentrate wet-season fish biomass into patches in the dry season, whereas the fish production hypothesis states that the amount of dry-season fish biomass reflects fish biomass production during the preceding wet season. We sampled prey in drying pools at 405 sites throughout the Florida Everglades between December and May from 2006–2010 to test these hypotheses. The models that explained variation in dry-season fish biomass included water-level recession rate, wet-season biomass, microtopography, submerged vegetation, and the interaction between wet-season biomass and recession rate. Crayfish (Procambarus spp.) biomass was positively associated with wet-season crayfish biomass, moderate water depth, dense submerged aquatic vegetation, thin flocculent layer and a short interval of time since the last dry-down. Grass shrimp (Palaemonetes paludosus) biomass increased with increasing rates of water level recession, supporting our impression that shrimp, like fish, form seasonal concentrations. Strong support for wet-season fish and crayfish biomass in the top models confirmed the importance of wet-season standing stock to concentrations of fish and crayfish the following dry season. Additionally, the importance of recession rate and microtopography showed that local scale abiotic factors transformed fish production into the high quality foraging patches on which apex predators depended. Introduction When food is spatially and temporally variable, animals must track resources efficiently to match the costs of their feeding efforts to the energetic demands of their life history [1,2]. Reproduction is energetically costly, greatly elevating these demands, compelling foragers to PLOS ONE | DOI:10.1371/journal.pone.0158864 July 22, 2016 1 / 20 Mechanisms That Generate Resource Pulses in a Fluctuating Wetland analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. target highly rewarding prey patches to sustain breeding [3–5]. A strategy employed by many organisms is to time breeding with resource pulses—infrequent, large magnitude, and short duration events of dramatically increased resource availability [6,7]. Resource pulses can occur intermittently, such as insect outbreaks [8,9], mast fruiting by trees [10–12], and irruptions of small mammal populations [13], or they can be seasonally recurrent events such as annual salmon spawning [14,15], seasonal inundation of river floodplains [16], and spawning of Pacific Herring (Clupea pallasii) [17]. Species living in environments where the spatial and temporal variability in food is integrally tied to recurrent pulses may evolve to completely rely on them [18]. Nesting wading birds (Pelecaniformes, Ciconiiformes), top predators in wetland ecosystems, are often limited by food [19–23] and may depend on ephemeral pulses of concentrated prey to sustain themselves during their breeding season [24–26]. In one large wetland, the Florida Everglades, wading birds are largely absent during the wet season, when water levels are deep, and prey are dispersed. During the dry season, large numbers of breeding wading birds come to exploit the resource pulses generated by receding water concentrating prey in shallow depressions [26,27]. Much is known about how birds respond to water level fluctuations [23,28] and which factors produce prey populations during times of high water [29]; however, little is known about factors that control resource pulses just as the marsh is drying and wading birds are using the resource. Much of the evidence that wading birds are food-limited is based on the observed sensitivity of wading birds to hydrologic conditions, assumed to be reflective of food availability. This stems from evidence that populations of fish, the primary prey for wading birds, respond positively to increases in water levels [29–31] and negatively to drought [32–34]. There is not a clear relationship between crayfish and increases in water levels, but crayfish have been shown to respond positively following droughts due to a reduction in fish, which may release crayfish from predation [35]. However, droughts also can cause direct mortality to crayfish in shorthydroperiod wetlands [36]. Grass shrimp numbers are often low and slow to recover following drought [37], but their density and trophic position increases with time since dry-down [38,39]. These patterns led to the generalization that hydrologic conditions drive the production of aquatic prey organisms in wetlands [30,40,41]. However, the relative effect of particular hydrologic parameters on prey is not clear. Production of prey is not the same as availability to wading birds because availability includes factors that affect the vulnerability of prey animals to being captured [26]. Moreover, the t (...truncated)