Stormwater wetlands can function as ecological traps for urban frogs

Abstract

Around cities, natural wetlands are rapidly being destroyed and replaced with wetlands constructed to treat stormwater. Although the intended purpose of these wetlands is to manage urban stormwater, they are inhabited by wildlife that might be exposed to contaminants. These effects will be exacerbated if animals are unable to differentiate between stormwater treatment wetlands of varying quality and some function as "ecological traps" (i.e., habitats that animals prefer despite fitness being lower than in other habitats). To examine if urban stormwater wetlands can be ecological traps for frogs, we tested if survival, metamorphosis-related measures, and predator avoidance behaviors of frogs differed within mesocosms that simulated stormwater wetlands with different contaminant levels, and paired this with a natural oviposition experiment to assess breeding-site preferences. We provide the first empirical evidence that these wetlands can function as ecological traps for frogs. Tadpoles had lower survival and were less responsive to predator olfactory cues when raised in more polluted stormwater wetlands, but also reached metamorphosis earlier and at a larger size. A greater size at metamorphosis was likely a result of increased per capita food availability due to higher mortality combined with eutrophication, although other compensatory effects such as selective-mortality removing smaller individuals from low-quality mesocosms may also explain these results. Breeding adults laid comparable numbers of eggs across wetlands with high and low contaminant levels, indicating no avoidance of the former. Since stormwater treatment wetlands are often the only available aquatic habitat in urban landscapes we need to better understand how they perform as habitats to guide management decisions that mitigate their potential ecological costs. This may include improving wetland quality so that fitness is no longer compromised, preventing colonization by animals, altering the cues animals use when selecting habitats, pretreating contaminated water prior to release, providing off-line wetlands nearby, or simply not constructing stormwater treatment wetlands in sensitive areas. Our study confirms the potential for urban stormwater treatment wetlands to function as ecological traps and highlights the need for greater awareness of their prevalence and impact at landscape scales.

Using conservation behavior to manage ecological traps for a threatened freshwater fish

“…To our knowledge, this is the first demonstration of an ecological trap for a freshwater fish, although they have been hypothesized to form previously (Pelicice andAgostinho 2008, Jeffres andMoyle 2012). For example, traps for birds are often caused by predation (Ekroos et al 2012), whereas aquatic species might be more susceptible to traps that arise in other ways, such as the effects of stormwater pollution on frogs (Sievers et al 2018b). For example, traps for birds are often caused by predation (Ekroos et al 2012), whereas aquatic species might be more susceptible to traps that arise in other ways, such as the effects of stormwater pollution on frogs (Sievers et al 2018b).…”

The influence of potential stressors on oviposition site selection and subsequent growth, survival and emergence of the non‐biting midge (Chironomus tepperi)

“…In (b), Cha = Chandler, Che = Cheltenham, Lyn = Lynbrook, Woo = Woodlands. Chandler and Cheltenham are two more polluted sites that have been shown to be ecological traps for spotted marsh frog Lynodynastes tasmaniensis when compared to Lynbrook and Woodlands, with the number in parenthesis showing the heavy metal quotient at each site (Sievers, Parris, Swearer, et al, ), which is an integrative measure of likely pollutant levels. In (c), black and gray circles indicate data from 2016 and 2017, respectively.…”

“…Recent results have also shown that larvae reared in sediments from these sites had high (85%–100%) and comparable survival (K. Jeppe unpublished data). The two more polluted sites here are ecological traps for native frogs (Sievers, Parris, Hale, & Swearer, ). The traits and behaviors of animals likely to increase susceptibility to ecological traps have been described, for example, slow rate of evolution, slow generation time, and low capacity for learning (Battin, ; Hale, Treml, & Swearer, ), and recent work has begun to explore taxonomic variability in the likelihood that closely related animals (e.g., different insect families) are susceptible to ecological traps (Robertson et al, ,).…”

“…Theory predicts that ecological traps are most likely in scenarios when the environment is changing rapidly and animals are exposed to evolutionary novel situations (Sih, ). With this in mind and to address Aim 2, we exposed C. tepperi to water from four urban stormwater treatment wetlands, two of which are ecological traps for frogs (Sievers, Parris, Swearer, & Hale, ), and tested whether fitness and preference vary among habitats. Stormwater wetlands are being constructed around many cities to treat polluted stormwater runoff (Malaviya & Singh, ), and these wetlands may have high loads of novel pollutants (i.e., synthetic biocides) but also characteristics of “high quality” habitats (e.g., native vegetation), creating a mismatch between habitat suitability and preference (e.g., Tilton, ; Hale, Coleman, Pettigrove, & Swearer, ).…”

“…Two of these sites, Chandler Road (38°00’00.56”S, 145°10’46.19”E) and Cheltenham Road (37°59’27.68”S, 145°09’12.58”E), had high concentrations of bioavailable metals in sediments (e.g., zinc 2,390–3,790 mg/kg), and the other two Lynbrook Estate (38°03’20.84”S, 145°15’03.89”E) and Woodlands Lake (38°00’11.49”S, 145°07’18.82”E) wetlands had relatively lower concentrations (e.g., zinc 209–606 mg/kg). For detailed summaries of metal and pesticide concentrations in sediment at these sites, and other environmental variables, see Sievers, Parris, Swearer, et al (). We collected water in 20 L carboys, and in each trial gave females a choice between four petri dishes each filled with water from one of the study sites.…”