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Seasonal fecundity is not related to geographic position across a species' global range despite a central peak in abundance
Citation:
Ruskin, K., M. Etterson, T. Hodgman, A. Borowske, J. Cohen, C. Elphick, C. Field, R. Kern, E. King, A. Kocek, A. Kovach, K. O’Brien, N. Pau, W. Shriver, J. Walsh, AND B. Olsen. Seasonal fecundity is not related to geographic position across a species' global range despite a central peak in abundance. OECOLOGIA. Springer, New York, NY, 183(1):291-301, (2017).
Impact/Purpose:
The US EPA's Markov Chain Nest Productivity Model Estimating Tool (MCestimate) which estimates probabilities of nest failure in the presence of competing risks was applied in an academic setting to test Dobzhanksy’s hypothesis that individuals face greater stress from abiotic stresses at high latitudes compared with biotic stress at low lattitudes. The MCestimate model was applied in this work to examine the importance of biotic influences on nest survival compared with abiotic causes of failure (adverse weather, tidal flooding). Results from the performance of a study comparison like this one help to explain or eliminate causal effects of stressors on individual bird species. While these results did not fully support the hypothesis, they did indicate that there are many factors that must be considered when examining bird populations and their success rate. Use of the model in support of assisting academic researchers attempting to apply the model to diverse study systems and questions, can help to improve and validate the MCestimate model, and broaden the scenarios for which the model can be applied by the Agency.
Description:
AimSixty-five years ago, Theodosius Dobzhansky suggested that individuals of a species face greater challenges from abiotic stressors at high latitudes and from biotic stressors at their low-latitude range edges. This idea has been expanded to the hypothesis that species’ ranges are limited by abiotic and biotic stressors at high and low latitudes, respectively. Support has been found in many systems, but this hypothesis has almost never been tested with demographic data. We present an analysis of fecundity across the breeding range of a species as a test of this hypothesis.Location575 km of tidal marshes in the northeastern United States.MethodsWe monitored saltmarsh sparrow (Ammodramus caudacutus) nests at twenty-three sites from Maine to New Jersey, USA. With data from 840 nests, we calculated daily nest failure probabilities due to competing abiotic (flooding) and biotic (depredation) stressors.ResultsWe observed that abiotic stress (nest flooding probability) was greater than biotic stress (nest depredation probability) at the high-latitude range edge of saltmarsh sparrows, consistent with Dobzhansky’s hypothesis. Similarly, biotic stress decreased with increasing latitude throughout the range, whereas abiotic stress was not predicted by latitude alone. Instead, nest flooding probability was best predicted by date, maximum high tide, and extremity of rare flooding events.Main conclusionsOur results provide support for Dobzhansky’s hypothesis across the global range of a species. We observed predictable variation in competing biotic and abiotic stressors to saltmarshsparrow nest survival across the range. However, our results do not indicate a direct tradeoff between abiotic and biotic stressors along a single gradient. Rather, we found that abiotic and biotic stressors were geographically independent.
