Temporary Water Holes May Benefit the Breeding of the Common Skipper Frog Euphlyctis Cyanophlyctis
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ecologies Article Temporary Water Holes May Benefit the Breeding of the Common Skipper Frog Euphlyctis cyanophlyctis (Anura: Dicroglossidae) Mohammad Abdul Wahed Chowdhury * , Shimu Rani Shil and Md. Mizanur Rahman Department of Zoology, University of Chittagong, Chattogram 4331, Bangladesh; [email protected] (S.R.S.); [email protected] (M.M.R.) * Correspondence: [email protected] Abstract: Seasonal fluctuations in adult Euphlyctis cyanophlyctis density were recorded in two perma- nent ponds in Southeastern Bangladesh. From a two-year dataset of seven habitat parameters, the seasonal suitability of two different pond habitats was examined to understand these fluctuations of frog density. The combined influence of the parameters on the density was significant in all three seasons. One combination was found to be optimal: water temperature of 24 ◦C, a pH of approximately 6, and a water depth of 2.29 m. These conditions supported more than 24 frogs per acre in the round pond. In the forestry pond, the optimal parameters were a water temperature of 27 ◦C and a water depth of less than 1 m, which accommodated 27 frogs per acre. The result indicates that a slight increase in any of these parameters was found to sharply lower the accommodation capacity of the studied ponds to less than half of the aforesaid densities, especially in the rainy season. During this season, the adults were found to breed in the small waterholes adjacent to the ponds. This may be the response of anuran to temporal unsuitability, for example, the depth of water and the pH of the permanent ponds. Citation: Chowdhury, M.A.W.; Shil, Keywords: habitat; parameter; population; null abundance; pond; temporary water hole S.R.; Rahman, M.M. Temporary Water Holes May Benefit the Breeding of the Common Skipper Frog Euphlyctis cyanophlyctis (Anura: Dicroglossidae). 1. Introduction Ecologies 2021, 2, 138–149. https:// Studies on amphibian populations and habitats have become a major research inter- doi.org/10.3390/ecologies2010007 est of ecologists in order to uncover the reasons for the rapid extinction of amphibians globally [1,2]. How amphibian populations behave over a short period, viz., seasons, in Received: 11 November 2020 a variety of habitats has also been studied [3,4]. The seasonal conditions of habitats can Accepted: 29 January 2021 explain anuran population fluctuations [5]. Moreover, unfavorable habitat conditions may Published: 5 February 2021 affect the metamorphosis of tadpoles [6], which may cause the low abundance of a popula- tion [7]. This can lead to the sudden extinction of amphibians in a habitat [8]. Synergistic Publisher’s Note: MDPI stays neutral interaction of parameters, pathogens, and warm–dry environments was associated with with regard to jurisdictional claims in the sudden decline of Atelopus ignescens in inter-Andean valleys [9]. For amphibians, basic published maps and institutional affil- iations. conservation activities should focus on preventing changes in suitable habitat conditions that result in poor accommodation capacity of a habitat. Being amphibious, aquatic parameters play an important role in every part of the anuran life cycle. Water temperature has a profound influence on development and growth in the embryonic and larval stages of anurans [10,11]. Embryos at constant temperature Copyright: © 2021 by the authors. (15 ◦C) hatched earlier than those in fluctuating temperatures (4 ◦C to 15 ◦C) [10]. For Licensee MDPI, Basel, Switzerland. breeding, the adult anurans usually select habitats with suitable parameters. For example, This article is an open access article Bufo calamita choose breeding sites with a pH above 3.8 (3.8–6.9) [12]. Dissolved oxygen also distributed under the terms and conditions of the Creative Commons influences the choice of breeding pools [13]. Pearman [14] studied the multilevel influence Attribution (CC BY) license (https:// of the size of the water body—calculated from the pond surface area and depth—on tadpole creativecommons.org/licenses/by/ populations of Rana clamitans. Increased area and depth of the water bodies led to lower 4.0/). survival in tadpoles of this species [14]. Ecologies 2021, 2, 138–149. https://doi.org/10.3390/ecologies2010007 https://www.mdpi.com/journal/ecologies Ecologies 2021, 2 139 In Bangladesh, about 20% of amphibian species are categorized as threatened, and the biggest threat is habitat alteration [15]. However, studies on amphibian habitats and populations in Bangladesh are limited [16]. The largest portion of the recently published research has focused on genetic analyses [17], description of new species [18], or work on amphibian communities in a specific locality [19]. Those studies have enriched the list of amphibians of Bangladesh by adding more than 23 new records in the last 15 years [15]. Of the anurans, the common skipper frog, Euphlyctis cyanophlyctis (Schneider, 1799), is a species complex [17,20]. Seven new species have already been described; they were previously included under the single taxon E. cyanophlyctis [18,21,22]. In our study, we considered the lentic-occurring E. cyanophlyctis. This aquatic frog is active both day and night all around the year, and they usually inhabit any water body. Adults are voracious feeders of surface insects, and tadpoles are organic component feeders from the bottom of the water body. Adult breeds mostly in the rainy season (July through September) [15]. The population dynamics of this freshwater skipper frog is unknown [15]. A wide fluctuation in the population of the common skipper frog with a lack of sightings was recorded in seven different water habitats in southeastern Bangladesh in 2013 [23] and three in 2014 [24]. Another study in 2015 recorded similar absences and wide population fluctuations of this species (0–51 individuals/acre) in three ponds in the same area [25]. To understand this phenomenon, we studied seven habitat variables of those ponds in 2013 and in 2015. In the present study, we analyzed the influence of habitat variables on the accommoda- tion capacity of two permanent pond- habitats. We will test the hypothesis that variations of the variables shape the seasonal suitability of the habitat and the seasonal density of adult frogs as the response variable. Moreover, we compared the habitat suitability based on the cumulative influence of the variables between breeding seasons and non-breeding seasons. 2. Materials and Methods The study area, Chittagong University Campus, includes over seven square kilometers of land between 22◦27025.6900 and 22◦27026.3400 N and between 91◦48010.1200 and 91◦48010.9600 E. This area—situated in Hathazari Thana of Chattogram division in Bangladesh—includes the hilly terrain of the Sitakunda hill range and plains [26] (Figure1). Two permanent water bodies were selected to represent the perennial lentic water bodies of this area. Structurally, both ponds have a steep bank, though some eroded parts of the bank formed slopes. The depressions surrounding these water bodies and the poorly maintained drainage frequently turn into temporary waterholes [26] in which E. cyanophlyctis can be observed during the monsoon season. The littoral sediments of the ponds are predominantly sandy with clay [27]. The land adjacent to the ponds is mainly used for plantation. 2.1. Site-1 (Round Pond) This study site is a round perennial pond (22◦27058.100 N 91◦47031.400 E) with a surface area of 0.59 acres and 55 m diameter. The mean depth is 2.55 m, and the edges are vertical. An exit pipe expels excessive water. The bank is heavily vegetated by native shrubs along with planted coconut (Cocos nucifera) and mahogany trees (Swietenia mahogany). Rainfall and seepage are the main sources of the refill. 2.2. Site-2 (Forestry Pond) This is a triangular pond (22◦27044.300 N 91◦47046.600 E) of 1.5 acres in size with a mean depth of 0.79 m. The bank of this pond is characterized by a hill slope with planted teak (Tectona grandis) and dense undergrowth of climbers and shrubs on one side. The ornamental plant false ashoka (Polyalthia longifolia) dominates another side. This pond also has a pipe to expel excessive water. The area is subject to a tropical maritime climate with three main seasons: a humid summer from March to May, a tropical Monsoon from June to October, and dry winter from November to February [28]. The semi-evergreen forest of the campus consists of 665 plant species from 126 families and 404 genera [29]. Ecologies 2021, 2, FOR PEER REVIEW 3 (Tectona grandis) and dense undergrowth of climbers and shrubs on one side. The orna- mental plant false ashoka (Polyalthia longifolia) dominates another side. This pond also has a pipe to expel excessive water. The area is subject to a tropical maritime climate with three main seasons: a humid Ecologies 2021, 2 summer from March to May, a tropical Monsoon from June to October, and dry winter140 from November to February [28]. The semi-evergreen forest of the campus consists of 665 plant species from 126 families and 404 genera [29]. Figure 1. Map of Bangladesh andand thethe locationlocation ofof thethe studystudy areaarea (asterisk) (asterisk) in in the the Southeastern Southeastern Division Division of of Bangladesh Bangladesh with with a closea close view view of of the the ponds. ponds. Inset: Inset: the the common common skipper skipper frog frog (Euphlyctis (Euphlyctis cyanophlyctis cyanophlyctis). ). 2.3. Data Collection We spent two to four days of every month collectingcollecting fieldfield data in 2013 and 2015. We maintained a minimum seven-day gap between two observation days, except September 2013, in in which which only only one one observation observation was was made. made. Both Both ponds ponds were were sampled sampled on oneach each obser- ob- vationservation day. day. A total A total of 62 of observations 62 observations were were considered, considered, which which covered covered the three the three basic basic sea- sonsseasons in Bangladesh in Bangladesh (Supplementary (Supplementary Table Table S1).