Adult Age-Structure Variability in an Amphibian in Relation to Population Decline

Abstract

Animal populations that normally fluctuate in size in response to stochastic environmental events might be subject to unsustainably high mortality rates in the face of progressive habitat loss or degradation. This should perturb the population's age-structure in predictable ways if the increase in mortality is age-or stage-specific (i.e., a decline in recruitment should result in a progressively older adult population, whereas a decline in adult survivorship should result in a progressively younger adult population). We used skeletochronology to ascertain the ages of adult individuals in a population of Fowler's Toads, Anaxyrus (5 Bufo) fowleri, over 20 yr, spanning a period of regulated population fluctuation from 1992 to 2002 and a period of sustained population decline thereafter until 2011. Age structure was similar between sexes, but was highly variable during both periods. Although there was no temporal trend detected in average age among 420 toads during the predecline period, 1992-2000, there was an increase in average age among 469 toads during the decline period, 2002-2011, with no significant change in adult survivorship. This evidence of an aging population is consistent with a reduction in recruitment in the population, related to the progressive loss of breeding habitat caused by an invasive plant, demonstrating that amphibian populations may be threatened with decline even when mortal threats to adults have not increased.

Early Development Drives Variation in Amphibian Vulnerability to Global Change

“…Our theoretical framework provides a basis from which to predict the impact of multiple threats acting synergistically in the context of each life-history strategy and prioritize data deficient species about which little is known (Blaustein and Kiesecker, 2002;Silva et al, 2018). Clades in which direct development or viviparity are common are of special concern, but we cannot rule out the possibility that species with aquatic larvae are equally imperiled, and their IUCN Red List status simply reflect biases in our status assessments when evaluating species with high fecundities and large geographic ranges (Bielby et al, 2006;Harper et al, 2008;Middleton and Green, 2015;Lion et al, 2019).…”

Road-killed toads as a non-invasive source to study age structure of spring migrating population

“…Some life history traits such as age of maturity, longevity, or potential reproductive life span are good indicators of habitat quality (Sinsch et al 2007). As demonstrated, continuous road mortality causes the decline in numbers of individuals in amphibian populations (Gibbs and Shriver 2005;Cooke 2011) and, according to Middleton and Green (2015), as the population decreases, the mean age increases as the result of lower recruitment.…”

“…Studies on population age structure are crucial to investigate age of maturity, longevity including reproductive life span, mortality, growth rate, and fecundity (Reading 1991;Smirina 1994;Socha and Ogielska 2010). They are also important in understanding the species population dynamics (Hemelaar 1986;Smirina 1994;Middleton and Green 2015). Amphibian body length and mass are more dependent on environmental factors and individual predispositions, thus these parameters should not be used to estimate precise age (Hemelaar 1986;Reading 1991).…”

“…Furthermore, amphibians are very sensitive to negative impact of the human activities, therefore studies of the age structure dynamics can be used in biomonitoring, including effect of climate change, or in estimating probability of threatened populations extinction (Guarino et al 2008;Middleton and Green 2015;Sinsch 2015). These studies are essential for developing suitable conservation plans for populations, and species management in their natural environments (Driscoll 1999).…”

Age, size and body condition do not equally reflect population response to habitat change in the common spadefoot toadPelobates fuscus

“…Age and size at which transitions occur often change plastically in response to environmental conditions (Roff, 1992), and their study allows us to determine population responses and trends over time. As such, age-related parameters obtained by skeletochronology proved valuable tools when studying population responses to environmental stress like habitat loss and degradation (Middleton & Green, 2015), urbanization (Yetman, Mokonoto & Ferguson, 2012), pollution (Spear et al, 2009;Kaczmarski et al, 2016;Otero et al, 2018), parasitic infestation (Gustafson et al, 2015;Sinsch, Kaschek & Wiebe, 2018), or pathogens (Campbell et al, 2018). They were also successfully used to evaluate performance and well-being of amphibian populations inhabiting differently-managed landscapes (e.g., Orchard, Tessa & Jehle, 2019), or to understand the effects of habitat quality on amphibian life histories (Sinsch et al, 2007).…”