Seasonal fecundity is not related to geographic position across a species’ global range despite a central peak in abundance
The range of a species is determined by the balance of its demographic rates across space. Population growth rates are widely hypothesized to be greatest at the geographic center of the species range, but indirect empirical support for this pattern using abundance as a proxy has been mixed, and demographic rates are rarely quantified on a large spatial scale. Therefore, the texture of how demographic rates of a species vary over its range remains an open question. We quantified seasonal fecundity of populations spanning the majority of the global range of a single species, the saltmarsh sparrow (Ammodramus caudacutus), which demonstrates a peak of abundance at the geographic center of its range. We used a novel, population projection method to estimate seasonal fecundity inclusive of seasonal and spatial variation in life history traits that contribute to seasonal fecundity. We replicated our study over 3 years, and compared seasonal fecundity to latitude and distance among plots. We observed large-scale patterns in some life history traits that contribute to seasonal fecundity, such as an increase in clutch size with latitude. However, we observed no relationship between latitude and seasonal fecundity. Instead, fecundity varied greatly among plots separated by as little as 1 km. Our results do not support the hypothesis that demographic rates are highest at the geographic and abundance center of a species range, but rather they suggest that local drivers strongly influence saltmarsh sparrow fecundity across their global range.
KeywordsLatitudinal gradients Fecundity Species range Biogeography Ammodramus caudacutus
This work was primarily funded by a Competitive State Wildlife Grant (U2-5-R-1) via the United States Fish and Wildlife Service, Federal Aid in Sportfish and Wildlife Restoration to the states of Delaware, Maryland, Connecticut, and Maine. We received additional funding from the United States Fish and Wildlife Service (Region 5, Division of Natural Resources, National Wildlife Refuge System), the United States Department of Agriculture (National Institute of Food and Agriculture NH McIntire-Stennis Project 225,575), the New York Department of Environmental Conservation (AM08634), and the National Science Foundation (DEB-1340008). This project was supported by the USDA National Institute of Food and Agriculture, project number #ME0-H-6-00492-12. This is Maine Agricultural and Forest Experiment Station Publication Number 3501. Graduate students were also funded in part by the National Science Foundation, the National Park Service Gateway Learning Center Fellowship, the University of Maine, the University of New Hampshire, and the University of Connecticut. Thank you to our many collaborators, land owners who allowed access to the plots, and to the dozens of field technicians who helped to collect these data. We also thank BJ McGill for his help at various stages of writing this manuscript and MD Correll for the use of her map. Appropriate animal care was ensured by the Institutional Animal Care and Use Committee of the University of Maine under approval A2011-04-02, University of New Hampshire under approvals 100605 and 130604, State University of New York College of Environmental Science and Forestry under approval 120101, University of Connecticut under approval A11-013, and the University of Delaware under approval AUP1157-2015-2. The findings and conclusions of this article are those of the authors and do not necessarily represent the views of the USFWS or any other funding agency.
Author contribution statement
KJR wrote the paper with comments from all authors, particularly MAE, TPH, and BJO; MAE developed the models MCestimate and MCnest that were used in this analysis; all authors participated in data collection; BJO, TPH, CSE, WGS, AIK, and JBC obtained funding for this project.
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