Long-lived species like Agassiz’s desert tortoise (Gopherus agassizii) hedge against bad years by keeping reproductive output modest and fairly stable — but climate change could push that output below sustainable levels across an entire range. This study pooled 12 datasets spanning two decades and over 500 individual-year observations to build a range-wide Bayesian model linking annual egg production to climate variables and individual attributes (body size and whether a female reproduced the previous year).
Climate turned out to matter more than individual traits: reproductive output rose with cumulative precipitation in the 13 months before nesting and with an earlier start to the spring activity season, while a wider temperature range in early spring pushed output down. Projecting these relationships across the tortoise’s range from 1990-2018 revealed a gradual but significant decline in expected egg production over time, with the exception of a single unusually favorable year (2004) — a warning sign that further reductions in precipitation under climate change could meaningfully depress reproduction across the species’ range.
Citation: Mitchell, C.I., Friend, D.A., Phillips, L.T., Hunter, E.A., Lovich, J.E., Agha, M., Puffer, S.R., Cummings, K.L., Medica, P.A., Esque, T.C., Nussear, K.E., Shoemaker, K.T. 2021. “Unscrambling” the drivers of egg production in the Mojave desert tortoise: Climate and individual attributes predict reproductive output. Endangered Species Research 44:217-230. https://doi.org/10.3354/esr01103