Your boss, Councilman Pierce, needs to know what is causing the frogs of Slow Water Park to disappear. What do you think the cause is?

The ANSWER is: cane toads, a non-native species, have invaded the park and are eating the young native frogs.

To solve the mystery, you need to talk to one of the two people who are closest to "the scene of the crime" - SANDY DENIM or DEEPTI GUPTA.

If you talked with Sandy the park ranger, you know that there is something new in the park: long strings of eggs like frog eggs, floating in the water. These are different from the small egg clusters of the Rana viridiana frog.

If you talked with Deepti, you know that something strange began to happen in the neighborhood near Slow Water Park, at just the same time as the Rana viridiana frogs began to disappear. Pets began to get mysteriously sick, and some died.

Both of these clues point to the same culprit: Bufo marinus, the cane toad. The cane toad lays many eggs in big long strings. And the cane toad has poison glands on its shoulders, which release a toxin if any predator tries to swallow the cane toad.

The Queensland province of Australia has been invaded by Bufo marinus, and people there have experienced on a grand scale what you are seeing in miniature: the cane toads grow in numbers very quickly, they can displace (or eat) native frogs, and both pets and natural predators are often found dead of cane toad toxin.


OTHER POSSIBLE ANSWERS? Although a cane toad invasion explains the facts very neatly, there are other hypotheses which also explain many of the facts, or at least are not disproved by the evidence we have so far. A good scientist keeps many hypotheses in mind when investigating a question, and an open mind to new hypotheses as new evidence is found.

WATER POLLUTION. What if pollution is not killing the frogs, but just making them sick or slow, so that their natural predators find it easy to eat more of them? This could also explain why the pets are getting sick - they are able to catch and eat more of the frogs than they used to, and the frogs are now polluted.

HUMAN POACHERS. What if the poachers want young frogs, instead of adult frogs, for some reason? That could explain why young frogs are disappearing.


UNLIKELY ANSWERS - AND HERE'S WHY. Some of the hypotheses we presented are unlikely to be true. Here are some of the reasons why you should have discounted them as the answer:

EGGS AND TADPOLES BEING EATEN BY RAINBOW TROUT? Abby Nguyen reported that the city had recently done a fish survey. In such a survey, people are expressly looking for any new fish type to be present. Trout did not show up on the survey, so they are probably not present in the water.

INSECTS, THE FROGS' FOOD SOURCE, ARE BEING EATEN BY BATS? Not likely. Bats eat flying insects that are out at night. The frogs eat crawling and flying insects that are out during the day.

YOUNG FROGS BEING EATEN BY MEXICAN FREE-TAILED BATS? Not likely. These bats are small and eat insects. Frog-eating bats are much larger, and live only in South America.

TOO MANY PEOPLE ARE USING THE PARK, WHICH IS DESTROYING THE FROG HABITAT? Not likely. There's no evidence that more people are using the park than before, and people familiar with the park haven't noticed any change.


"POINTERS." When puzzling out a question, a good scientist looks for some "symptoms" of the problem that might indicate a solution. In this mystery, you might have noticed:

VERY SUDDEN DECLINE. After years of stable and predictable growth patterns, the frog population declined very rapidly in just a few months. This points at a significant habitat change, usually sparked by an event such as a pollution spill - or the rise of an aggressive invader species.

YOUNG FROGS MISSING. If you look carefully at the data Marjorie Mol collected, you'll see that the total number of frogs is declining because young frogs are not growing up. This points to a cause which affects young frogs much more than older ones - such as a predator.

NO FROG BODIES! If frogs were being killed in great numbers by pollution, you would expect that people familiar with the park would find some frog bodies (and an examination of the bodies would lend more clues about why they were dying). But no one mentions finding any bodies - which points toward predators or poaching.


Since CROAK was written, scientists have uncovered more evidence about possible causes of the worldwide amphibian decline. Search the web to find the latest science about the crisis.

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We hope you enjoyed CROAK. Save the frogs!

CROAK - Fact/Fiction - About this Story. . . . . .An Access Excellence Science Mystery