Abstract
Anthropogenic climate change and habitat alterations increase the importance of understanding the causes and consequences of variation in phenological traits. Although the timing of phenological events may vary in response to both direct and mediated effects, methods to measure and distinguish direct and mediated effects have seldom been used. We used a Bayesian structural equation model (SEM) to evaluate potential direct and mediated effects of intrinsic individual and environmental factors on the timing and progression of spring molt in bighorn sheep. The SEM showed that molt phenology varied across years, was earlier in prime-aged and in heavier individuals, slower in males, and later in lactating ewes, especially if they were light. These results highlight how individual variation in intrinsic traits and life-history leads to substantial variation in a phenological trait. Indirect effects in the SEM predicted a delay in sheep molt phenology at high population density mediated through negative density effects on body mass and lactation probability. Cooler temperatures in late spring were also predicted to delay molt phenology via a negative effect on body mass. Finally, lactation reduced ewe mass which was predicted to delay molt phenology. This mediated effect thus increased the total delay (sum of direct and mediated effects) in molt phenology experience by lactating ewes. Our results underline the importance of estimating direct and indirect effects when modeling phenological traits. Because indirect effects could substantially affect estimates of total plasticity, they should be critically important to accurately predict phenological mismatches and demographic consequences of environmental change.
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Acknowledgements
We thank all the wildlife biologists, field assistants and graduate students who helped collect data at Ram Mountain throughout the years. We thank J. Jorgenson, C. Feder and A. Hubbs for their contributions to field logistics. The Ram Mountain bighorn sheep research project is financially supported by the Natural Sciences and Engineering Research Council of Canada (Discovery Grants to MFB and FP). Additional support was provided by the Canada Research Chair in Evolutionary Demography and Conservation (grant to FP 2009 and 2014), the Alberta Conservation Association, Alberta Environment and Parks, the Fonds de Recherche Nature et Technologies – Québec, the Ministère de l'Éducation et de l'Enseignement supérieur du Québec and the Université de Sherbrooke.
Funding
This research was funded by the Natural Sciences and Engineering Research Council of Canada (Discovery Grants to MFB and FP), the Canada Research Chair in Evolutionary Demography and Conservation (grant to FP 2009 and 2014), the Alberta Conservation Association, Alberta Environment and Parks, the Fonds de Recherche Nature et Technologies—Québec, the Ministère de l'Éducation et de l'Enseignement supérieur du Québec and the Université de Sherbrooke.
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BL, MFB and FP collected data. BL conceived the project, analyzed the data and wrote the manuscript. MFB and FP contributed to writing and revising the manuscript.
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Larue, B., Pelletier, F. & Festa-Bianchet, M. Determinants of spring molt in bighorn sheep: life-history, plasticity and phenology. Oecologia 199, 809–817 (2022). https://doi.org/10.1007/s00442-022-05231-7
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DOI: https://doi.org/10.1007/s00442-022-05231-7