Pima County Regional Flood Control District is removing invasive and predatory bullfrogs this summer from the Kino Environmental Restoration Project (KERP).
KERP is a multi-purpose environmental and storm water harvesting project that was constructed northwest of Ajo Way and Country Club Road in 2001by the Army Corps of Engineers and Pima County Regional Flood Control District (RFCD).
Bullfrogs and other non-native aquatic species have invaded and become abundant at KERP during the past five years. In that time, predatory bullfrogs have repeatedly dispersed downstream during storm flows to the Santa Cruz River, where they threaten biodiversity at the best remaining toad-breeding ephemeral – or temporary – ponds in urban Tucson. Jennifer Becker, the RFCD principal hydrologist overseeing the project, says that without removal of the bullfrogs at KERP, we will lose native frog and toad species in central Tucson.
Bullfrogs (Lithobates – or Rana – catesbeianus) are ranked among the 100 worst invasive species worldwide by the IUCN Species Survival Commission (http://www.issg.org/database/welcome/). Native to North America east of the Rocky Mountains, they have spread around the world, beginning before 1900 when they were introduced to other areas for sport or as a food source (frog legs).
On wildlife refuges in Arizona, bullfrogs have nearly eliminated the Mexican garter snake and the threatened Chiricahua leopard frog.
Bullfrogs may lay 20,000 eggs at one time, while native frogs lay 2,000-3,000. Bullfrogs are voracious predators that will eat almost anything, including each other. But their cannibalism does not reduce their threat. Instead, the younger frogs provide a reliable and plentiful food source that allows the adult population to grow even as it depletes other species.
But bullfrogs require perennial – or year-round – water, so the KERP ponds were temporarily drained to remove them. RFCD also completed some necessary maintenance during the dry-down.
The draining was timed for late May through June to best accommodate migrating birds and to take advantage of the monsoon inflows to replenish the ponds. Pima County Stadium District utilized the drained water to irrigate sports fields at the Kino Sports Complex and surrounding landscaped areas.
County contractor and University of Arizona herpetologist Dr. Phil Rosen reported that nearly all the adult bullfrogs have been removed, and bullfrog inspections are ongoing. Since the bullfrog eradication efforts began, native Great Plains toads (Anaxyrus cognatus) and narrow-mouthed toads (Gastrophryne olivacea) have been observed. Tadpoles for these and other native toads are present and expected to thrive now that the water harvesting ponds have refilled with recent monsoon rains. It may be necessary to repeat the eradication efforts at KERP over the next few years to entirely remove the bullfrogs. Any native species present will be conserved.
Managers at the upstream Davis-Monthan Air Force Base facilitated inspections at a pond on the base, and no bullfrogs were found.
Since bullfrogs were not found in upstream ponds, it is suspected that the KERP population may have resulted from wildlife dumping or intentional release, which is a Class 2 misdemeanor subject to fines and possible imprisonment.
The U.S. Fish and Wildlife Service (FWS) and Bureau of Land Management (BLM), the Arizona Game and Fish Department, and the Tucson Audubon Society all expressed support for RFCD’s effort. BLM provided a large truck-mounted pump and another smaller pump for free to support bullfrog eradication.
“No agency in the region has exceeded or equaled Pima County in leadership for urban and metropolitan area biodiversity conservation,” said Jean Calhoun, assistant FWS field supervisor, in a March letter of support.