Assessing the Impacts of Urbanization on Sex Ratios of Painted Turtles (Chrysemys picta)

Abstract

Turtles are particularly susceptible to the negative impacts of urbanization due to low mobility and a life history strategy emphasizing long generation times and high adult survival. In addition to declines directly through habitat loss, urbanization has been hypothesized to limit populations of aquatic turtles through changes in population structure, as adult females are disproportionally killed on and near roads, leading to male-biased populations, which can lead to population declines or local extirpations. The purpose of this study was to better understand how urbanization impacts the sex ratios of painted turtles (Chrysemys picta) in an urban ecosystem, as empirical results linking male-biased turtle populations to roads and urbanization are mixed. Using eight years of trapping data from a long-term monitoring program in a suburb of Chicago, IL, USA, we report one of the most male-biased populations (x = 75% male) of turtles in the USA, consistent with prevailing road mortality hypotheses. However, we found no evidence that male-biased populations were related to road density or the amount of protected area around a sampling location and found that impervious surface (a metric of urbanization) was weakly related to less male-biased populations. Our results highlight the importance of replicating ecological studies across space and time and the difficulty in assessing population structure in aquatic turtles. We suggest that active conservation measures may be warranted for the continued persistence of urban turtle populations.Diversity 2019, 11, 72 2 of 13 and degradation [10,13], road mortality [11,[14][15][16], and increased depredation rates by subsidized predators [17][18][19][20]. Understanding how turtles respond to urbanization is imperative in a world with a rapidly increasing urban footprint [4,21] and rapidly shrinking turtle populations [22,23].Urbanization has been hypothesized to further limit populations of aquatic turtles through changes in population structure, as adult females (relative to males and juveniles) are disproportionally killed on and near roads [24][25][26][27][28], and Gibbs and Steen [27] show that turtle populations in the US have become more male-biased over time, coinciding with the proliferation of roads. This problem is particularly insidious as turtle populations can become functionally extinct well before the last turtle in a population actually dies [22,29]. However, empirical results of urbanization impacting the sex ratios of turtle populations are mixed and limited to two species with a limited geographic scope [25,[30][31][32], and a series of recent papers has failed to find an association between roads and male-biased populations [33][34][35][36]. Notably, Bowne et al. [36] found the opposite relationship, and hypothesized that that urbanization could be resulting in warmer nesting locations, leading to more females being produced via environmental sex determination [37] (but see Lambert and Steen [38]).In this study, we attempt to better understand how urbanizat...

A broader approach to understanding urbanization effects on freshwater turtles: reply to Lambert and Steen 2019

“…We recognize our findings are not applicable to every study system but argue that a blanket generalization about roads and urbanization causing a decrease in the proportion of adult females is not supported by the published literature (e.g., Dorland et al. ; Buchanan ; Carstairs et al ; Vanek & Glowacki ). It is also important to acknowledge that road mortality can increase extinction probability of turtle populations even if mortality is equal between the sexes (Howell & Seigel ).…”

“…Paper submitted April 9, 2019;revised manuscript accepted July 15, 2019. mechanism contributing to the relationship (e.g., excess deaths of males and altered thermal nesting environment) leads to population declines. We recognize our findings are not applicable to every study system but argue that a blanket generalization about roads and urbanization causing a decrease in the proportion of adult females is not supported by the published literature (e.g., Dorland et al 2014;Buchanan 2017;Carstairs et al 2018;Vanek & Glowacki 2019). It is also important to acknowledge that road mortality can increase extinction probability of turtle populations even if mortality is equal between the sexes (Howell & Seigel 2019).…”

Peer Review #2 of "Historical museum collections and contemporary population studies implicate roads and introduced predatory bullfrogs in the decline of western pond turtles (v0.1)"

“…Most turtle species are long-lived; our study taxa, the two species of WPT, live to 45 years (Holland, 1994), and female-biased adult mortality, with or without reduced recruitment, can have devastating consequences for long-lived species. Multiple studies have observed or proposed that male-biased populations result in population declines or extirpations of freshwater turtles (Ceballos et al, 2016;Vanek & Glowacki, 2019), and a long-term study of two wood turtle (Glyptemys insculpta) populations in Connecticut, USA documented sharp declines coinciding with human-mediated reductions in adult females (Garber & Burger, 1995). Reduced juvenile production, the ultimate result of increased female mortality, has also been found to result in freshwater turtle population declines or extirpations (Burgin and Ryan, 2008;Howey & Dinkelacker, 2013).…”

“…Roads and their associated vehicle traffic can have profound impacts on wildlife (Spellerberg, 1998), and studies of other species have suggested that vehicular mortality results in male biased turtle populations near roads (Marchand & Litvaitis, 2004;Steen & Gibbs, 2004;Aresco, 2005). Countering this, a study of painted turtles (Chrysemys picta) near Chicago, IL found a large male bias (75% male), but no evidence of road density as a causal factor (Vanek & Glowacki, 2019), and a population model predicted that populations of small-bodied pond turtles, like the WPT, should not be threatened by road mortality anywhere in the United States (Gibbs & Shriver, 2002). In Ontario, Canada, one study of painted turtles found no higher frequency of males at sites closer to major roads than more remote sites further from roads (Dorland et al, 2014).…”

Historical museum collections and contemporary population studies implicate roads and introduced predatory bullfrogs in the decline of western pond turtles

“…In the United States, a relatively recent review of the literature on turtle population sex ratios, published between 1928 and 2003 covering 36 different species, reported an average 22% increase in the intrapopulation proportion of males, and that this bias was most pronounced in aquatic species (Gibbs & Steen, 2005). Other studies have found similar increases in male bias in turtle populations (Aresco, 2005;Vanek & Glowacki, 2019). Although this body of work clearly indicates an increase in male bias across many aquatic turtle species, there is less certainty about the cause of these shifts.…”