Demographic factors affecting population growth in giant gartersnakes

Abstract

Demographic models provide insight into which vital rates and life stages contribute most to population growth. Integral projection models (IPMs) offer flexibility in matching model structure to a species’ demography. For many rare species, data are lacking for key vital rates, and uncertainty might dissuade researchers from attempting to build a demographic model. We present work that highlights how the implications of uncertainties and unknowns can be explored by building and analyzing alternative models. We constructed IPMs for the threatened giant gartersnake (Thamnophis gigas) based on published studies to determine where management efforts could be targeted to have the greatest effect on population persistence and what unknowns remain for future research. Given uncertainty in the survival of snakes during their first year, and in the form of the size‐survival relationship, we modeled a range of scenarios and evaluated where models agree about factors influencing population growth and where discrepancies exist. For most scenarios, the survival of large adult females had the greatest influence on population growth, but the relative importance of juvenile versus adult somatic growth for population growth was dependent on the recruitment probability and the shape of the size‐survival function. More data on temporal variation and covariance among vital rates would improve stochastic models for the giant gartersnake. This paper demonstrates the effectiveness of IPMs for studying the demography of reptiles and the value of the model‐building process for formalizing what is known and unknown about the demography of rare species. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.

Heavy and persistent rainfall leads to brood reduction and nest failure in a passerine bird

“…Life history of an animal is explained by traits that on one hand affect reproduction, like clutch size, number of broods per season, age at first reproduction and lifespan, and on the other hand affect survival. Both reproduction and survival highly affect population growth, but survival seems to be the strongest to model demographic processes (Schmidt et al , Arlt et al , Rose et al ), as animals can only contribute to the local population if they survive until they start to reproduce (Stearns ). In birds, a physiologically demanding and critical timeframe of survival until first reproduction is the period between hatching and fledging (Ricklefs ).…”

Targeting eradication of introduced watersnakes using integral projection models

“…snakes – Weatherhead et al ., 1999; Rose et al ., 2018a, 2018b, lizards – Zúñiga‐Vega, Méndez‐de la Cruz & Cuellar, 2008, turtles – Doak, Kareiva & Klepetka, 1994, crocodilians – Dunham, Dinkelacker & Miller, 2014). IPMs also have the flexibility to model the relationship between vital rates and the state variable using complex, non‐linear functions (Dahlgren, García & Ehrlén, 2011; Rose et al ., 2019). Furthermore, it is possible to incorporate both age‐ and size dependence into an IPM for cases in which some vital rates vary as a function of one characteristic or the other, or both (Ellner & Rees, 2006).…”

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“…Female fecundity also increases with size (Rose et al 2018a), and the greater reproductive value of large adult females means that the survival of this life stage has the greatest influence on population growth rates . The growth and survival rates of juvenile (1 year old) giant gartersnakes also have an important influence on population growth, especially when the probability of recruitment from neonate to 1 year old is higher (Rose et al 2019). The value of large females and growth and survival of juvenile snakes suggests the importance of prey availability to the life history of giant gartersnakes.…”

“…Fish and Wildlife Service (USFWS), CDFW, or both must also include long-term management and not just preservation of habitats (Bunn et al 2013). Although typically not accessible to the general public, these lands offer research opportunities similar to public conservation lands, and conservation banks such as Gilsizer Slough Giant Garter Snake Conservation Complex and Natomas Basin Conservancy reserves have provided long-term access to research and monitoring (e.g., Halstead et al 2012Halstead et al , 2015Halstead et al , 2016Rose et al 2018cRose et al , 2018bRose et al , 2019.…”

“…Conducting these activities during the active season or on short stretches of a single side of a canal could minimize negative effects of canal management. Maintaining large, contiguous areas of rice agriculture to the extent possible would increase survival of adult giant gartersnakes and benefit giant gartersnake populations (Halstead et al 2019;Rose et al 2019). Finally, maintaining a mosaic of open water and emergent vegetation, especially tules, in canals can provide cover and foraging opportunities for giant gartersnakes.…”