Mount Diablo Western Toad
A Preliminary Survey
by Joyce Gross
By Chris Brown, US Geological Survey, Western Ecological Research Center - San Diego Field Station [Public domain], via Wikimedia Commons
In April 1998 I photographed Bufo boreas (western toads) breeding and laying eggs in Little Pine Creek. Of the approximately 20 toads present, females predominated. My previous experience observing amphibian breeding congregations indicated that this was unusual--normally there are more males than females--but I didn't realize how unusual until Bruce Waldman, a herpetologist at the University of Canterbury, New Zealand, saw my photos. Bruce also pointed out that many of the eggs I had photographed were irregularly shaped and not viable, probably "old" eggs, ovulated a week or more prior to actual oviposition.
Bruce and I monitored Little Pine Creek in the spring of 1999. Toads always return to the same locations to breed. However, we saw no evidence of reproductive behavior. We found no toads, no eggs, and no tadpoles. Amphibian population declines have been occurring with alarming frequency all over the world during the past few years. There are many possible causes, including pathogenic organisms (e.g. chytrid fungi), high UV-B radiation, exotic predators, and habitat destruction. One of the least understood possible causes is pollution by pesticides, herbicides, and residues of commercial plastics. These compounds are similar in chemical structure to estrogen and other reproductive hormones.
Thus it is feared that even at very low concentrations, which may pose no toxicological risk to organisms, these chemicals may mimic female hormones and disrupt normal endocrinological processes. The result might be altered reproductive timing that could contribute to reproductive failure. Of course in this case the reasons for our observations may be completely unrelated to the causes about which we are speculating.
In 2000, we received a permit from the park to study the toads and test a few if we found evidence of reproductive abnormalities. I visited the park twice a week from February through June. I found normal breeding activity, resulting in small toadlets by June, in two locations in the park: two miles away at the pond off the Bruce Lee trail in Mitchell Canyon, and two miles in the other direction in a cattle pond near China Wall. At Little Pine Creek there was once again no evidence of reproductive behavior. I found two juvenile toads, but no adult toads in or near the creek, and no eggs and no tadpoles.
What happened to the adult toads in the vicinity of Little Pine Creek in Mt Diablo State Park remains a mystery.