The Effects of Road De-Icing Salts on Two Amphibian Species, Rana Sylvatica and Rana Pipiens, and Their Trematode Parasites

Dino Milotic

doi:10.32920/ryerson.14660994

Abstract

With ongoing amphibian declines, it is essential to determine possible contributors such as diseases and environmental contaminants that may increase susceptibility. A potential contaminant is road salt (mainly NaCl), which leaches into aquatic environments. I examined whether road salts make larval amphibians (tadpoles) more susceptible to trematode parasite infection, and also how these affect free-living trematode infectious stages (cercariae). I exposed Rana sylvatica (wood frogs) and R. pipiens (northern leopard frogs) to control, medium (400 mg/L), and high salt (800 mg/L) treatments, and then to trematodes. High salt tended to reduce wood frog anti-parasite behaviour and resistance to infection but the opposite was seen for R. pipiens, although these tadpoles had elevated lymphocyte counts in high salinity. Trematodes were differentially affected by increased salinities. The results suggest that host-parasite-environment interactions are complex, with species differentially affected by contaminants, which may lead to community shifts in predominant hosts and parasite species.

Highlights

There was no overall effect of road salts on amphibian growth but when considering pairwise comparisons, it was seen that wood frog tadpoles reared in the medium salt treatment had a significant reduction in growth compared to the control treatment
Larval amphibian growth reduction in saline conditions has been reported in literature (Findlay and Kelly, 2011; Wood and Welch, 2015), possibly because increased saline concentrations mimic a drying pool of water (Denver et al, 1998), whereby larval amphibian development is increased at the expense of mass gain to escape drying conditions in ponds (Denver, et al, 1998; Kiesecker and Skelly, 2001; Koprivnikar, et al, 2014)
Reduced growth results in smaller frogs at metamorphosis that have a lower fecundity, are less competitive, and are more vulnerable to predation (Sanzo and Hecnar, 2006). This has implications for the population at large, especially if it allows other amphibian species that are not impacted by road salt to outcompete wood frogs

Summary

Introduction

Amphibian habitats are undergoing anthropogenic disturbances that are related to degradation and the introduction of contaminants (Semlitsch, 2000; Koprivnikar et al, 2007). The herbicide atrazine has been shown to increase susceptibility of larval amphibians to trematode infection (Koprivnikar et al, 2007). This is important because amphibians are undergoing global declines, and infectious diseases have been implicated (Daszak et al, 1999, Daszak et al, 2003). One indirect mechanism by which human activities can harm organisms is by the introduction of contaminants into the ecosystem, such as pesticides, heavy metals, and road salts (Stuart et al, 2004; Kaushal et al, 2005; Koprivnikar et al, 2007) Some of these chemicals have been shown to increase susceptibility of larval amphibians to trematode (flatworm) parasite infection (Koprivnikar et al, 2007). The miracidia and cercariae of trematodes are fragile and have limited energy reserves (Pietrock and Marcogliese, 2003)

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