Lower genetic diversity and hatchability in amphibian populations isolated by urbanization

Abstract

Many amphibian populations worldwide have declined rapidly and been threatened with extinction in the past few decades because of human impacts on the environment. It is well known that urbanization reduces the genetic diversity of isolated populations. The concept that a reduction in genetic diversity leads to lower reproductive fitness has been predominantly supported by studies involving laboratory organisms, but has rarely been tested in wild populations. Here we examined whether genetic diversity affected hatchability, a population parameter related to reproductive success, in the populations of two pond‐breeding amphibian species (Hynobius tokyoensis and Rana ornativentris) affected by urbanization. We surveyed 81 populations in the southwestern Kanto region of Japan. Mean hatchability of the populations was determined and genetic diversities were estimated via mitochondrial or microsatellite DNA analyses. Firstly, Random Forests (an ensemble machine learning method) models were applied to clarify the effects of environmental factors on both hatchability and genetic diversity. Subsequently, the relationships among environmental factors, genetic diversity, and mean hatchability were evaluated using path analysis. Mean hatchability was significantly affected by both urbanization and genetic diversity. This result shows that loss of genetic diversity may decrease a population's reproductive success in the field.

Effects of landscape features on gene flow among urban frog populations

“…As a result, small isolated populations will be more subject to deleterious inbreeding effects. It has been reported that R. ornativentris hatchability tends to be low in isolated urban populations (Okamiya & Kusano, 2018;Shiga, 2014). A similar phenomenon of fitness reduction due to the erosion of gene flow and genetic diversity in fragmented habitats has been corroborated for several taxa, ranging from invertebrates to mammals (i.e., Bani et al, 2018;Takahashi et al, 2016;Unfried et al, 2013).…”

The importance of long-term genetic monitoring of reintroduced populations: inbreeding in the natterjack toad (Epidalea calamita)

“…Our sites had lower heterozygosity than the least genetically diverse population in their study and thus are likely to show severe fitness consequences as a result. Low genetic diversity has also been linked to reduced hatching success in two other pond breeding amphibians (Hynobius tokyoensis and Rana ornativentris) when sites became isolated (Okamiya & Kusano, 2018). However, low genetic diversity at neutral loci is not an indicator of low diversity at loci under selection, which are key to functionality in relation fitness (Beebee, 2018;Mable, 2019).…”

Importance of Genetic–Fitness Correlations for the Conservation of Amphibians

“…[61] Hynobius tokyoensis Rana ornativentris Genetic diversity and egg survival were positively affected by the forested area. [33] Bombina variegata Bd prevalence and Bd intensity were highest in two inbred, isolated populations.…”

“…From all studies included in this review (Table S1), thirteen found negative effects of isolation on different demographic fitness proxies (Table 3). Egg and tadpole survival was reduced in small or isolated populations in four European (B. calamita, Rowe and Beebee 2003; H. arborea [54]; R. latastei [55]; and R. temporaria [37]) one North American (Lithobates sevosus [61] and two Japanese species (Hynobius tokyoensis and Rana ornatriventris [33]). With respect to Bd infection the results are more divergent: One study on yellow-bellied toads (B. variegata) found that Bd prevalence and intensity was highest in two isolated and inbred populations [65], while in Litoria verrauxii from Australia Bd infection was absent in an isolated population [46].…”

“…In total we found 35 studies published in a diversity of international journals that contained information about GD and at least one fitness-related trait for one up to 38 study sites (SS) in different amphibian species (Table S1). Most studies only included one species, two studies included two species [32,33] and one study included six species [34]. The later study, however, tested for the association of GD and fitness traits between, but not within, species and hence is not directly comparable to the rest of studies.…”