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ESR prepress abstract   -  DOI: https://doi.org/10.3354/esr01103

‘Unscrambling’ the drivers of egg production in Agassiz’s desert tortoise: climate and individual attributes predict reproductive output

Corey I. Mitchell*, Derek A. Friend, Lauren T. Phillips, Elizabeth A. Hunter, Jeffrey E. Lovich, Mickey Agha, Shellie R. Puffer, Kristy L. Cummings, Philip A. Medica, Todd C. Esque, Kenneth E. Nussear, Kevin T. Shoemaker

*Corresponding author:

ABSTRACT: The ‘bet hedging’ life history strategy of long-lived iteroparous species reduces short-term reproductive output to minimize the risk of reproductive failure over a lifetime. For desert-dwelling ectotherms living in variable and unpredictable environments, reproductive output is further influenced by precipitation and temperature via effects on food availability and limits on activity. We assembled multiple (n = 12) datasets on egg production for the threatened Agassiz’s desert tortoise Gopherus agassizii across its range and used these data to build a range-wide predictive model of annual reproductive output as a function of annual weather variation and individual-level attributes (body size and prior-year reproductive status). Climate variables were more robust predictors of reproductive output than individual-level attributes, with overall reproductive output positively related to prior-year precipitation and an earlier start to the spring activity season, and negatively related to spring temperature extremes (monthly temperature range in March–April). Reproductive output was highest for individuals with larger body sizes that reproduced in the previous year. Expected annual reproductive output from 1990-2018 varied from 2–5 eggs per female per year to 6-–12 eggs, with a weak decline in expected reproductive output over this time (p = 0.02). Climate-driven environmental variation in expected reproductive output was highly correlated across all 5 Recovery Units for this species (Pearson’s r > 0.9). Overall, our model suggests that climate change could strongly impact Agassiz’s desert tortoise reproductive output, and could have a negative population-level effect if precipitation is significantly reduced across the species’ range as predicted under some climate models.