A Report from Michael J. Lannoo, U.S. Coordinator for DAPTF
In the United States, we currently recognize 288 extant amphibian species: 103 species of frogs and 185 species of salamanders, although the identity and relationships of species in several genera remain unresolved. Through DAPTF, Partners in Amphibian and Reptile Conservation (PARC), the USGS's Amphibian Research and Monitoring Initiative (ARMI), and several other partners we have recently completed our 2-volume Status and Conservation of U.S. Amphibians project. The first volume, consisting of 52 conservation essays, gives us our context—how we think about amphibian declines and malformations. The second volume, consisting of species accounts covering all species and authored by experts, and newly developed, digitally based, distribution maps gives us the range of facts we need to support and expand both our theoretical and practical perspectives.
In the United States, but not just in the United States, a problem with documenting amphibian declines is that in most regions, and for most species, we have few historical data to compare to the data from our current studies. A second problem is that amphibian populations fluctuate with environmental conditions—wet years favor reproduction, drought years do not. Therefore, to properly document amphibian declines, we have tended to focus our studies on areas where historical data are available and sample across many years under a variety of conditions. Despite these limitations, several studies have now been done that document declines. For example, Drost and Fellers have documented the decline of several frog species in the Yosemite region of the California Sierra Nevada. Also in the Sierra Nevada, Bradford and colleagues note the role that introduced fishes play in reducing anuran populations, a theme that Knapp and Matthews have expanded upon. Introduced American bullfrogs (Rana catesbeiana) are also playing a role in reducing native amphibian numbers in many parts of the West (but see the work of M. Hayes and Jennings).
In the midwestern United States, fish and American bullfrog introductions are playing the same role in decimating amphibian populations that they have played in California. Long-term data from the Wisconsin Frog and Toad Survey indicate that numbers of most amphibians are in steady decline, at rates of between 1% and 4% per year (Mossman and colleagues).
In the Southeast, the conversion of native forests to pine plantations appears to be decimating amphibian populations (Means and colleagues).
Contradicting reports of declines, other studies designed to look at amphibian abundance indicate that numbers are stable. Long-term studies at the Savannah River Ecology Laboratory show large population fluctuations but little evidence for declines over the past two decades (Pechmann and colleagues). From these studies and others, a general picture emerges. Within species, populations may be in decline in some regions, yet stable in others.
A second general pattern is also evident. Within regions of the United States, some species are in decline while others are not. For example, in North America, northern populations of Blanchard's cricket frogs (Acris crepitans blanchardi) have declined, while southern populations remain robust. These declines in northern populations of cricket frogs have not been mirrored by concomitant declines in syntopic species such as northern leopard frogs (Rana pipiens), American toads (Bufo americanus), western chorus frogs (Pseudacris triseriata), and tiger salamanders (Ambystoma tigrinum).
The reasons underlying amphibian declines are in some cases thought to be known, while in others they are completely unknown. In the United States, and indeed the world, ultraviolet light, acid rain, commercial collecting, invasive species, pesticide use, and global warming have all been implicated. In fact, under the 2 CO2 model (doubling the ambient levels of atmospheric carbon dioxide) of global warming, the lush Southeastern forests, currently home to the most stable North American amphibian populations and the highest richness of species, will transform into a dry chaparral-like ecosystem (R. Neilson, personal communication). Such dry conditions are inconsistent with the life history requirements of the current native amphibian species assemblage of this region.
Despite the attention given to these causes of amphibian declines, up to this point in time the rather pedestrian causes of habitat loss and alteration undoubtedly have been the largest factors contributing to amphibian declines.
While the trend in experimental studies has been to focus on single potential causes of amphibian declines, animals in nature rarely face only one threat. Recognizing this, recent studies (e.g., by Kiesecker, Blaustein, and colleagues) have focused on synergistic effects of anthropogenic disturbances on amphibians. A second trend in experimental studies has been to assess the level of insult needed to kill. But again, recognizing the situation in nature, more recent studies (e.g., T. Hayes and colleagues) have focused on the role of sublethal effects.
In the United States there are several conclusions that can be drawn from recent work:
1. Despite our knowledge, we still do not have a complete picture of the conservation status of amphibians. In essence, by defining what we know, we have also defined what we do not know—it is this lack of information, and its magnitude, that now draws our attention.
2. Amphibian species are responding in various ways to the environmental pressures presented by current land management practices and by compromises in air and water quality.
3. We now have enough information on many U.S. amphibian species to begin to make informed decisions about their management.
4. In assessing amphibian declines we must distinguish between naturally rare species and declines for unnatural reasons.
5. Current abundance does not equate with future conservation status.
6. Developmental malformations can cause amphibian declines, but declines are occurring in the absence of high rates of malformations.
7. Aquatic species are being affected more than terrestrial species. In aquatic species malformation rates are higher (Hoppe and colleagues) and declines are greater. While roughly 70% of United States species have an aquatic life history stage, 88% (22/25) of the currently listed U.S. Federal Threatened and Endangered species have an aquatic life history stage.
8. While amphibian declines are currently receiving a great degree of publicity, they serve as a proxy for declines that are co-occurring in many other non-game species, including (and especially) aquatic species.
9. The notion of amphibians as bioindicators of conditions for humans is real, but needs some qualification. While it is true that amphibians have a number of characteristics that make them potential bioindicators, including unprotected permeable skin and a lack of long-range dispersal capability, amphibians cannot in truth tell us any more about human health than a careful epidemiologist could tease apart from an examination of either historical or comparative data.
10. As environmental indicators, amphibians show the effects of compromised ecosystems. But in certain cases the situation is reversed—rather than amphibians being affected because ecosystems are sick, ecosystems become sick because amphibians are affected. For example, in chytrid fungal outbreaks, amphibians themselves are the targets and, with their loss, ecosystems are damaged. These cause-effect relationships may not be as simple as we imagine they are. Sublethal exposures of pollutants or UV-B may stress amphibians to the point of making them susceptible to pathogens such as chytrid fungi. The factor (or a coincident one) causing malformations at the Crow Wing site in Minnesota is also causing behavioral modifications in northern leopard frogs that is affecting their ability to reproduce (Hoppe and colleagues). So, amphibians are affected when ecosystems are affected, and amphibians themselves can be targeted even when ecosystems otherwise appear to be healthy.
11. While the focus of the high-profile amphibian malformations investigation has been on determining proximal causes and their relative importance, the solution to this problem is relatively simple and largely independent of proximal cause. From our recent work in Minnesota, we realize that the hottest of the hot spots all share one feature: they are altered wetlands. While the nature of these alterations varies by site (eutrophication, erosion, septic system leakage, partial filling, excavation, cattle usage) these alterations are easily identified and if corrected would undoubtedly reduce, if not eliminate, the malformation problem at these sites.
Contact: Michael J. Lannoo, Muncie Center for Medical Education, Indiana School of Medecine, Ball State University, Muncie, IN 47306-0230, USA. mlannoo@bsu.edu