Impact of Invasive Cane Toads on the Invertebrate Community and Endangered Waterbirds at James Campbell National Wildlife Refuge
Taylor Smith¹ ², Jeff Burgett¹, Randi Riggs¹, and Bethany Chagnon¹ (¹Fish & Wildlife Service, ²Kupu Americorps)
Cane toads (Rhinella marina) were introduced to Oahu in 1932 to control sugar cane beetles (Easteal 1981), and are now abundant. Unlike other invaded locations, there is no evidence that these nocturnal toads prey on Hawaii’s endemic waterbirds, the Hawaiian Stilt (Himantopus mexicanus knudseni), Hawaiian Moorhen (Gallinula galeata sandvicensis), and Hawaiian Coot (Fulica alai). However, cane toads could potentially diminish the biomass of invertebrates eaten by these endangered waterbirds (Greenlees et al. 2006).
On James Campbell National Wildlife Refuge, cane toads are not targeted, but are captured during management actions for bullfrogs, a known predator on waterbird chicks. Biannual surveyed population size of Cane toads at JCNWR averaged 539 in 2017. The Refuge is considering the option of managing the cane toad population.
This project aims to describe the toad’s diet so that future work can examine the degree of overlap with the diets of the endangered waterbirds, especially chicks. Chicks may be more susceptible to competition with cane toads because of their similar size and potential greater need than adults for invertebrates high in protein. This study could also estimate toads’ impact on the available prey base.
Introduction
Cane toads (Rhinella marina) were introduced to Oahu in 1932 to control sugar cane beetles (Easteal 1981), and are now abundant. Unlike other invaded locations, there is no evidence that these nocturnal toads prey on Hawaii’s endemic waterbirds, the Hawaiian Stilt (Himantopus mexicanus knudseni), Hawaiian Moorhen (Gallinula galeata sandvicensis), and Hawaiian Coot (Fulica alai). However, cane toads could potentially diminish the biomass of invertebrates eaten by these endangered waterbirds (Greenlees et al. 2006).
On James Campbell National Wildlife Refuge, cane toads are not targeted, but are captured during management actions for bullfrogs, a known predator on waterbird chicks. Biannual surveyed population size of Cane toads at JCNWR averaged 539 in 2017. The Refuge is considering the option of managing the cane toad population.
This project aims to describe the toad’s diet so that future work can examine the degree of overlap with the diets of the endangered waterbirds, especially chicks. Chicks may be more susceptible to competition with cane toads because of their similar size and potential greater need than adults for invertebrates high in protein. This study could also estimate toads’ impact on the available prey base.
Easteal, Simon. 1981. The history of introductions of Bufo marinus (Amphibia: Anura); a natural experiment in evolution. Biological Journal of the Linnaean Society 16:93-113
Greenlees, M.J., G.P. Brown, J.K. Webb, B.L. Phillips, and R. Shine. 2006. Effects of an invasive anuran [the cane toad (Bufo marinus)] on the invertebrate fauna of a tropical Australian floodplain. Animal Conservation 9:431-438
U.S. Fish and Wildlife Service. 2011. Recovery Plan for Hawaiian Waterbirds, Second Revision. U.S. Fish and Wildlife Service, Portland Oregon. 30-57 pp.
Toads were collected at night (approx. 7-10 p.m.) on January 25 (352 toads) and February 15, 2018 (432 toads). Using lights, gloves and hand nets, Refuge staff and volunteers captured toads on the edges of Refuge ponds and on the roads between ponds. A total of 784 toads were collected over two nights from an area of approximately 55,000 m². Captured toads were euthanized by chilling at the end of the night and stored frozen until dissection.
Weights and snout-vent lengths were recorded together with the wet weight of total stomach contents from a random subset of the captured toads (n=252). Presence of distinct types of prey items were recorded for each stomach. Prey items and other stomach contents were identified by visual examination and identified to at least taxonomic order.
Methods & Materials
Toads were collected at night (approx. 7-10 p.m.) on January 25 (352 toads) and February 15, 2018 (432 toads). Using lights, gloves and hand nets, Refuge staff and volunteers captured toads on the edges of Refuge ponds and on the roads between ponds. A total of 784 toads were collected over two nights from an area of approximately 55,000 m². Captured toads were euthanized by chilling at the end of the night and stored frozen until dissection.
Weights and snout-vent lengths were recorded together with the wet weight of total stomach contents from a random subset of the captured toads (n=252). Presence of distinct types of prey items were recorded for each stomach. Prey items and other stomach contents were identified by visual examination and identified to at least taxonomic order.
Because there was no significant sexual difference in lengths (mean: m=102.11 mm, f=99.06 mm) or weights (mean: m=105.75 g, f=105.92 g), males and females were considered together in subsequent analyses.
Fig. 1
Thirty distinct prey items were found in the cane toad stomachs, of which seven were found in more than 10% of the stomachs observed (Fig. 1). Items found at lower frequencies included crayfish (Procambarus clarkii), slugs (Veronicella cubensis ), earwigs (dermaptera), snails (Achatinoidea), cockroaches (Blattodea), aphids (Homoptera), insect eggs, crickets (Orthoptera), insect wings, spiders (Araneae), bees (Hymenoptera), blind snakes (Indotyphlops braminus), mosquitos (Diptera), bird feathers, water striders (Hemiptera), and moths (Lepidoptera).
Fig.2
Stomach contents could be up to twenty-one percent of total body weight, but there was a widely varying mass of stomach contents for any given toad weight (Fig. 2), with no significant correlation (R=0.140, n=252).
Results
Because there was no significant sexual difference in lengths (mean: m=102.11 mm, f=99.06 mm) or weights (mean: m=105.75 g, f=105.92 g), males and females were considered together in subsequent analyses.
Fig. 1
Thirty distinct prey items were found in the cane toad stomachs, of which seven were found in more than 10% of the stomachs observed (Fig. 1). Items found at lower frequencies included crayfish (Procambarus clarkii), slugs (Veronicella cubensis ), earwigs (dermaptera), snails (Achatinoidea), cockroaches (Blattodea), aphids (Homoptera), insect eggs, crickets (Orthoptera), insect wings, spiders (Araneae), bees (Hymenoptera), blind snakes (Indotyphlops braminus), mosquitos (Diptera), bird feathers, water striders (Hemiptera), and moths (Lepidoptera).
Fig.2
Stomach contents could be up to twenty-one percent of total body weight, but there was a widely varying mass of stomach contents for any given toad weight (Fig. 2), with no significant correlation (R=0.140, n=252).
Little is known about the diet of Hawaii’s endemic waterbirds. They are opportunistic generalist feeders, and food items may include seeds and leaves of aquatic plants, invertebrates including snails, crustaceans, and aquatic or terrestrial insects, tadpoles, and small fish (U.S. Fish and Wildlife Service 2011). Cane toads eat a wide variety of prey items, and could compete with waterbirds and their chicks at James Campbell NWR. This project provides targets for DNA screening of waterbird excrement to determine potential prey overlap with cane toads.
While cane toads consume a variety of prey types, our sampling methodology would need to improve to have a greater confidence in estimating their impact on prey biomass. Two sources of error make it difficult to clearly relate toad size to mass of stomach contents. First, toads were not chilled immediately upon capture, allowing digestion to continue for a variable amount of time. Second, collection over a period of hours gave toads collected later a longer time to forage.
In conclusion, we determined that cane toads consume a variety of food sources at JCNWR with the most common being vegetation, coleoptera, and crustacean larvae. Further research on both the diet of Hawaii's endangered waterbirds and overall prey availability at JCNWR is needed to further clarify the level of potential impact invasive cane toads could have as competitors with waterbirds.
Discussion
Little is known about the diet of Hawaii’s endemic waterbirds. They are opportunistic generalist feeders, and food items may include seeds and leaves of aquatic plants, invertebrates including snails, crustaceans, and aquatic or terrestrial insects, tadpoles, and small fish (U.S. Fish and Wildlife Service 2011). Cane toads eat a wide variety of prey items, and could compete with waterbirds and their chicks at James Campbell NWR. This project provides targets for DNA screening of waterbird excrement to determine potential prey overlap with cane toads.
While cane toads consume a variety of prey types, our sampling methodology would need to improve to have a greater confidence in estimating their impact on prey biomass. Two sources of error make it difficult to clearly relate toad size to mass of stomach contents. First, toads were not chilled immediately upon capture, allowing digestion to continue for a variable amount of time. Second, collection over a period of hours gave toads collected later a longer time to forage.
In conclusion, we determined that cane toads consume a variety of food sources at JCNWR with the most common being vegetation, coleoptera, and crustacean larvae. Further research on both the diet of Hawaii's endangered waterbirds and overall prey availability at JCNWR is needed to further clarify the level of potential impact invasive cane toads could have as competitors with waterbirds.
Thanks to Randi Riggs, Bethany Chagnon, Jeff Burgett, Ty Spangler, Kristy Okimoto, Enoch Candari, and Boy Scout troop 360 for all their help with this project and poster.
