These calls are done in a rapid series of low-pitched throaty notes (Great1 pp). A study titled, "A Comparative Analysis of Plasticity in Larval Development in Three Species of Spadefoot Toads," reported by David Reznick in the June 01, 2000 issue of Ecology, evaluated four salient features of the Wilbur and Collins (1973) model for amphibian metamorphosis (Reznick pp) H.M. Wilbur and J.P. Collins offered an evolutionary explanation for the labile nature of amphibian metamorphosis (Reznick pp). Their model has provided the most important framework for interpreting phenotypic plasticity in age and size at metamorphosis (Reznick pp). This model is attractive due to its simplicity, and the fact that it focuses on selection at the larval life stage, is time invariant, and ignores complex relationships between larvae and their predators (Reznick pp).

Reznick study performed an experiment on three species of spadefoot toads derived from environments that differ in their degree of ephemerality, in order to evaluate the existence of a threshold, or minimum size, for attaining successful metamorphosis and to evaluate the influence of growth rate, mass, and stage of development on the definition of this threshold (Reznick pp). They further characterized the rate of development after the threshold and the nature of differences in the threshold and post-threshold development among species (Reznick pp).

According to the Reznick study, the threshold in larval development exists in all three species that separates an early larval period during which the larvae are not able to metamorphose due to a decline in food availability and a later period in development when they can metamorphose (Reznick pp). After attaining the threshold, larvae respond to a cessation of feeding by speeding up the rate of development and metamorphosing at an earlier age, and relative to fed controls, are smaller in size (Reznick pp). This response is consistent with the idea that such plasticity is adaptive since it will result in an earlier escape from a deteriorating environment (Reznick pp). After the threshold is exceeded, the degree of delay is relative to the increases with food availability and growth rate, thus rapidly growing individuals remain in the favorable environment longer and grow to a larger size (Reznick pp). Larvae that were exposed to a cessation in food availability after exceeding the threshold continued to respond by accelerating the rate of development throughout the remainder of the larval period (Reznick pp).

In all areas, the responses of Scaphiopus larvae to a cessation of feeding are consistent with the predictions of the Wilbur-Collins model for adaptive plasticity in amphibian metamorphosis (Reznick pp). The net result being that the larvae will metamorphose at an earlier age if they encounter a decline in growth opportunity, if they have exceeded a critical threshold (Reznick pp). One consequence is that they metamorphose at a smaller body size, and if the growth environment is favorable, then they will extend their period of development for a longer period, take advantage of the greater growth opportunities, and metamorphose at a larger body size (Reznick pp).

Reznick's results further demonstrated that the properties of the threshold differed among species in a way that is consistent with its being an adaptation to ephemeral environments (Reznick pp).

Reznick assumed that the growth rate was a good index of environmental quality and that a reduction in growth rate should serve as a good general cue of the degradation of the larval environment (Reznick pp).

The issue was whether simply removing all food was a reasonable surrogate for the signal that a tadpole would receive in nature (Reznick pp). A recent study on Scaphiopus hammondi suggests that this is true (Reznick pp). Researchers simulated a drying environment by either reducing water depth or by inserting netting and thus restricting the larvae to being close to the surface (Reznick pp). All of the experiments were conducted in a constant temperature room, so that water depth was not confounded with temperature (Reznick pp). Either treatment resulted in a rapid cessation of...

Researchers further observed a threshold with identical properties to Reznick's findings, yet with a slight shift in the Gosner stage associated with the threshold (Reznick pp).
Six earlier studies investigated the existence of thresholds for metamorphosis with a species, and in all six studies, growth rate was manipulated by switching larvae from a high food supply to a low food supply and vice versa at different ages, and found that there is a threshold in the ability of larvae to respond with changes in age and size at metamorphosis (Reznick pp). However, if larvae were switched to low food after the threshold, there was no change in the age at metamorphosis, yet there was a decline in the size (Reznick pp). But if larvae were switched to low food before the threshold, then they metamorphosed at a later age and smaller size (Reznick pp). Moreover, a switch from low to high food caused earlier metamorphosis at a larger size if the switch was made before the threshold, yet only larger size if it was made after the threshold (Reznick pp).

Reznick found that the majority of the information for predicting whether or not an individual larva will respond to a loss of growth opportunity by successfully completing larval development is contained in the two correlated variables, body mass and developmental stage (Reznick pp). The relative contributions of these two variables varies across a range of growth rates, with body mass contributing more at high growth rates and developmental stage contributing more at low growth rates (Reznick pp). Although the two variables are often highly correlated, and it would seem that either one should be equally suitable as a measure of the threshold, this is, however not the case for two reasons (Reznick pp). First, the threshold stage of development is progressively earlier in faster growing tadpoles, indicating that size and/or condition play an important role, and second, body mass becomes a poorer predictor of the threshold in extremely low growth environments, therefore making developmental stage more important in those conditions (Reznick pp). Thus, the threshold is defined by both the body mass and the developmental stage (Reznick pp).

Analyses of patterns of age and size at metamorphosis would be greatly facilitated if an external morphological marker for the threshold condition could be found (Reznick pp). Some researchers have suggested that the threshold is marked by the asymptote of the growth curve, and this is probably a fairly accurate marker when growth rate is very low, but is clearly a very poor marker of the threshold when growth is high (Reznick pp).

Within the mountains that divide Saugus from Canyon Country are the Cruzan Mesa vernal pools where the endangered Riverside fairy shrimp flourish (Aidem pp). According to a study by the Los Angeles County Department of Regional Planning, the surface water in these pools is home to two plants on the federal endangered species list, as well as the spadefoot toad (Aidem pp). And in the Great Basin-Mojave Desert region, amphibians are one of the rarest animal groups because of their high water requirements, however four native species of frogs and toads are widely distributed throughout, including the Great Basin spadefoot, western toad, Pacific chorus frog, and northern leopard frog (Great4 pp). Of the twenty-two amphibian species, ten, or forty-five percent, are species of concern or candidates for federal listing, and several others seem to be declining (Great4 pp). According to recent surveys in northern Nevada, leopard frogs, spotted frogs, western toads, chorus frogs, and spadefoots are now difficult or impossible to find, yet they had been abundant earlier in the century (Great4 pp).

Less than a century ago, the wilderness in and around Yosemite National Park flourished with the trilling, croaking songs of frogs and toads, however according to a 1996 study, there has been a large-scale collapse of an entire community of frog species (Kaesuk pp). Biologists found that all seven native species of frogs and toads in the Yosemite region have declined, and three of the species have disappeared completely from the area (Kaesuk pp). By comparing a recent survey of frog and toad species with a 1915 survey, researchers were able to provide the type of long-term data needed by scientists to study amphibian die-offs (Kaesuk pp). In a 1992 study, researchers focused entirely on frogs and toads, and not only scanned for adults, as the 1915 team had done, but also listened for songs, waded into pools and used nets to gather tadpoles for identification (Kaesuk pp). Although the team even searched new sites, the 1992 study consistently turned up far fewer frogs and toads than did the 1915 team (Kaesuk pp). Three species that were not found at any of the original or new sites searched included the red-legged frog, the foothill yellow-legged frog, and the Great Basin Spadefoot (Kaesuk pp). The declines occurred across a wide range of species and in a large area over a wide…