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) data sets on egg production for the threatened Agassiz’s desert tortoiseGopherus agassiziiacross 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 to 6-12 eggs female-1yr-1, 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 the reproductive output of Agassiz’s desert tortoise, and could have a negative population-level effect if precipitation is significantly reduced across the species’ range as predicted under some climate models.
Highlights
Variation in reproductive output is an important determinant of population dynamics, and understanding the drivers of this variation is necessary for Endang Species Res 44: 217–230, 2021 habitat quality (Powell & Steidl 2000, Blanchfield & Ridgway 2005) and levels of disturbance (Anthonysamy et al 2014), external pressures such as predation and available forage (Bjurlin & Bissonette 2004, Zanette et al 2006), individual physiological condition (Henen 2002, Vleck & Vleck 2002), and genetic differences between populations (Hagerty & Tracy 2010), among others
With their ‘bet hedging’ life history strategy, long-lived iteroparous species are generally considered to be less vulnerable to short-term disruptions to reproduction than are semelparous species (Stearns 1976, Lovich et al 2015)
We summarized our data at the level of the US Fish and Wildlife designated Recovery Unit (RU) for the tortoise, as this reflects a tangible metric for management of the species, but the RUs are delineated based on estimations of genetic structure of desert tortoise populations (Hagerty & Tracy 2010, USFWS 2011)
Summary
Variation in reproductive output is an important determinant of population dynamics, and understanding the drivers of this variation is necessary for Endang Species Res 44: 217–230, 2021 habitat quality (Powell & Steidl 2000, Blanchfield & Ridgway 2005) and levels of disturbance (Anthonysamy et al 2014), external pressures such as predation and available forage (Bjurlin & Bissonette 2004, Zanette et al 2006), individual physiological condition (Henen 2002, Vleck & Vleck 2002), and genetic differences between populations (Hagerty & Tracy 2010), among others Many of these factors are driven or influenced by annual weather and changes in climate, which can play a major role in reproductive success. It is critical to understand and predict the long-term effects of climate variation on reproductive success for such species
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