the green frog

The green frog (Lithobates [Rana] clamitans) is the classic frog of my East Coast childhood: often the size of your hand, mostly green, and visible sitting placidly on lily pads in ponds throughout eastern North America. Thus, seeing it in Kingston, Wash. in 2019 initially brought a smile to my face. The conservation biologist in me followed up just as quickly with “Uh oh, they’re not supposed to be here.” Volunteers at Stillwaters Environmental Center in Kingston had taken photos of the species in a private pond along Carpenter Creek two years previously, but it wasn’t until the landowner brought us a bucket of 19 green frogs some young visitors caught in his yard in May 2019 that we were able to attract wildlife agency attention to the discovery. Later that month, Mark Hayes, an amphibian biologist with the Washington Department of Fish and Wildlife (WDFW), visited to collect our frogs, tour the sites in the lower Carpenter Creek watershed where we’d been hearing them, and confirmed that they were indeed introduced green frogs.

The green frog has a long history in parts of the Pacific Northwest. It was probably introduced to western North America for food, fish bait, and education tools at about the same time as its larger cousin, the American bullfrog (Lithobates [Rana] catesbeiana), and as pets starting in the early 1900s. While the bullfrog has greatly expanded its range and is considered invasive and widely viewed as a predator and competitor of native frog species throughout the west, green frogs have made much less of a splash. Outside its native range, established green frog populations have only been documented in southeastern British Columbia and southern Vancouver Island, in Stevens and Whatcom Counties in Washington, and near Ogden, Utah. Surprisingly, these populations do not appear to have expanded their ranges significantly in the last 50 years, although numbers have increased locally, particularly on Vancouver Island.

Male green frog at county stormwater pond #147. photo by Elizabeth Springborn

WDFW advised Stillwaters on various ways to try eradicating the green frogs in Carpenter Lake and beyond, all of which are both labor intensive and likely problematic for native species, including trapping and dredging ponds. Carpenter Lake is a natural reserve and wildlife sanctuary located in the middle of a sphagnum bog and is largely inaccessible by boats that would be necessary to conduct control efforts there. After consulting with amphibian biologist Dr. Tom Doty, who had retired to North Kitsap from teaching at Roger Williams University in Rhode Island, Stillwaters decided to adopt the WCAMP approach of monitoring green frog distribution in hopes that an agency or academic researcher will be interested in conducting a study on their impact here someday.

The green frog situation led Tom and I to some interesting discussions because of differences in our initial levels of concern about green frogs being potentially invasive in North Kitsap. I was initially quite concerned that they could become a species “whose introduction does or is likely to cause economic or environmental harm or harm to human health,” as invasives are defined by the National Invasive Species Council. Tom felt that native amphibian declines due to pollution, disease, and habitat loss far outweighed potential declines due to a non-native amphibian outcompeting them and that the former were problems more deserving of our limited time, money, and attention. The more I read about introduced green frog populations in the Pacific Northwest and observed native amphibians here, the more I came to agree that green frogs, in North Kitsap at least, did not appear to be a threat commensurate with time and effort it would take to eradicate them.

Freshly laid green frog egg mass floating on surface of Kingston Hills stormwater pond #148, May 14 2021.

Many local amphibians start breeding as early as December and January, taking advantage of innumerable seasonal wetlands that appear during our winter rains, hatching as soon as water temperatures allow, and metamorphosing relatively quickly before these water bodies dry up as they typically do in late summer.

Green frogs are adapted to living in waterbodies with predatory fish (like trout in Carpenter Lake), which native amphibians breeding in more seasonal waterbodies don’t have to deal with. Like bullfrogs, they start breeding when the water is warmer, have eggs that develop relatively quickly, and have a longer tadpole or larval stage, often more than a year, before metamorphosing. 

Green frogs may do well in permanent water bodies like Carpenter Lake (where no bullfrogs have been reported), but many of the stormwater and small backyard ponds they’ve also been reported in may not be able to support successful reproduction in most years. Thus, green frogs’ impacts on native species may be largely restricted to larger ponds and lakes.

Ecologists and others have observed the growing impact of non-native species on biological diversity around the world with alarm, but biologists who study invasive and potentially invasive species have come to appreciate the importance of taking an objective look at the role of non-native species in their new environments. Are negative outcomes the mostly likely result when a non-native species gains a foothold in a new location? Not necessarily. There are many examples of non-native species which have major disruptive impacts on ecosystems and decimate native species. There are also examples of non-native species that have become important in conservation efforts because they are filling a niche emptied due to local extirpation or extinction — like non-native birds in Hawaii that pollinate and disperse seeds of native plants whose survival depends on them now that native nectivores and frugivores have gone extinct. In Puerto Rico, non-native plantation trees, which are better at colonizing disturbed areas, create conditions that can facilitate the re-establishment of native trees. 

Non-native species are also spreading, not just by human-mediated introductions (intentional or accidental), but via range shifts tracking anthropogenic changes to the environment like deforestation, reforestation, and global climate change. Decisions about how to manage non-native species are best made on a case-by-case basis, particularly in human-disrupted environments, and the case of range-shifting species requires even more careful consideration, especially in the face of climate change.

Male green frog in Kingston Hills stormwater pond #148.

Green phase Pacific tree frog (sex unknown but possibly a female swollen with eggs) near Kingston Hills stormwater pond #147.

Green frogs also have a distinctive call, most often described as a “banjo twang,” quite unlike that of Pacific tree frogs and bullfrogs. The red-legged frog’s call is a low “stutter” that is rarely heard unless one is practically standing on top of them; they often call underwater making it even harder for us to hear them.

Please report green frog calls or sightings (with photos if possible) to:

melissa@stillwatersenvironmentalcenter.org

Accepting responsibility for our role in the movement of species and the problems that result also means accepting greater stewardship and management roles in the new ecological communities we’ve created. As our responsibilities to the environment expand, managers and scientists need more help than ever from local communities to gather data needed to make decisions about how best to spend our limited time and resources managing new and established species in changing ecosystems. Citizen or community science has become an invaluable tool for collecting large amounts of quality data quickly and inexpensively. There are numerous opportunities to participate in outside projects that match your interests and passions in the Pacific Northwest.

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Thanks so much for your interest and your support.