American Toads: How Sustainable Are Amphibian Populations?

This Fall, I took a course in Conservation Biology, which provided me with the opportunity to ask my professor, David Green, what species of toad I happened to stumble upon in my yard last summer. According to him, an expert in the field of amphibian research, this is a female American Toad, scientifically known as the Bufo americanus. They can be found across the southern region of Canada from Manitoba to the Maritimes, as well as throughout all of the eastern United States, except Florida. Within this range, the species prefers habitat that contains ‘accumulated leaf litter, sandy or loamy soil for burrowing, moist hiding places, and an abundance of food’ (6). Shallow bodies of water without any fish are used for breeding activity, such as bogs, ponds, and roadside ditches. My yard consists of sandy, forested land, which is thus perfect American toad territory. However, the time at which I had found the toad – mid-June – is rather unusual, as they tend to be active at night to avoid the dryness of the summer heat (6).

In terms of general characteristics about the American toad, their colouring can be brown, grey, tan, olive, or reddish. What makes them distinguishable is their black, spotted pattern and a lighter line marking down their middorsal. They range from 50 to 90 centimeters in length, with females being larger than males (6). As a result, females take longer to mature, which creates uneven sex ratios amongst the population. It doesn’t help that the breeding season is also a short one (5). Unlike most toads, which respond to the scent of algal bloom formation to decipher their breeding time, American toads respond to changes in ground-level air temperature (4). The males then gather at a usual breeding spot, typically within 80 yards, and begin calling the females in a series of chorus vocalizations, each lasting six to 30 seconds (5 & 6). The females will lay 2000 to 20,000 ‘eggs in double, gelatinous stands’, which hatch three to 12 days later (6). The young then spend two months as tadpoles developing into toadlets, when they then transfer from an aquatic environment to a terrestrial environment. They then spend a lifespan of two to three years maturing into juveniles, and then into adults (3 & 6). The American toad diet differs depending on the stage of growth. Tadpoles tend to eat detritus, algae, dead fish, and even other weaker tadpoles; whereas, insects such as ants, grubs, beetles, and moths serve as prey for adult toads (6).

Toad Pic3

Although the American toad is of least concern from a conservation standpoint, there are several serious threats to the entire Amphibia class compared to all other vertebrates. This graph was shown in one of my lectures, demonstrating the much lower mean population trend of amphibians as opposed to other animal classes (2). Industrial waste and farm runoff can infect water sources with contaminants, which can ‘affect feeding, locomotion, development, and survival of amphibians’ (1). For example, nitrate and malathion can strongly influence the growth development, survivorship, and disease exposure of toad tadpoles (1). Climate change is another factor affecting toad populations, such as causing temperature fluctuations that can confuse toads as to when to breed and increases in UV-B radiation that can kill embryos (3). Perhaps the biggest threat is habitat destruction by human urbanization. I happened to find this American toad while my mother was mowing the lawn. I took the toad and deposited her into the forest where she would be safe. However, humans are not the only danger, as adult American toads are the prey of snakes, birds, and mammals. They face predators as tadpoles as well, such as ‘diving beetles, giant water bugs, dragonfly naiads, crayfish, and birds’ (6). Perhaps it would have been safer for the toad to have remained in my mother’s garden, but it goes to show that we, as humans, must be much more vigilant if we are to help amphibian populations from declining in any way at all.


References

1.     Krishnamurthy, Sannanegunda V, and Geoffrey R. Smith. “Growth, abnormalities, and mortality of tadpoles of American toad exposed to combinations of malathion and nitrate.” Environmental Toxicology and Chemistry, vol. 29, no. 12, (2010), pp. 2777-2782.

2.     Leung, B, Greenberg, D. A, and D. M. Green. “Trends in mean growth and stability in temperate vertebrate populations.” Diversity and Distributions, vol. 23, no. 12, (2017), pp. 1372-1380.

3.     McGill University Lecture: Declining Amphibian Populations. By Professor David Green. BIOL-465-001 Fall 2017.

4.     Oldham, R.S. “Initiation of breeding behavior in the American toad, Bufo americanus.” Canadian Journal of Zoology, vol. 47, no. 5, (1969), pp. 1083-1086.

5.     Oldham, R.S. “Spring movements in the American toad, Bufo americanus.” Canadian Journal of Zoology, vol. 44, no. 1, (1966), pp. 63-100.

6.     Partymiller, Lindsay. “American Toad (Bufo [Anaxyrus] americanus).” Savannah River Ecology Laboratory UNIVERSITY OF GEORGIA. URL: https://srelherp.uga.edu/anurans/bufame.htm

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