Upper Midwest Environmental Sciences Center

ARMI - Amphibian declines
folder.gif Amphibian Research and Monitoring Initiative, Midwest Region

Amphibian Declines

  • Declines of populations of amphibians around the world have been a prominent issue in conservation biology for more than a decade (Hayes and Jennings 1986; Barinaga 1990; Bradford 1991; Carey 1993; Fellers and Drost 1993; Kagarise Sherman and Morton 1993; Blaustein 1994a, b; Drost and Fellers 1996; Lannoo 1998; Lips 1998; Knapp and Matthews 2000; Reaser 2000; Alexander and Eischeid 2001; Carey et al. 2001; Young et al. 2001).

  • Quantifying declines and understanding their ecological relevance is challenging because there have been few long-term studies of populations.
  • We need to obtain new baseline data for populations and subsequently monitor them over time.
  • Data from such studies help us establish statuses and trends and understand the context of fluctuations in populations over time.
  • Environmental stressors that can cause declines include loss of habitat, disease, pollutants, climate change, ultrDecember 29, 2010ation by introduced species, among others (Bradford 1991; Griffiths and Beebee 1992; Blaustein et al. 1994; Bradford et al. 1994; Pounds and Crump 1994; Carey and Bryant 1995; Kupferberg 1997; Adams 2000; Carey 2000; Knapp and Matthews 2000; Alford et al. 2001; Davidson et al. 2001; Gillespie 2001; Kiesecker et al. 2001; Pounds 2001; Blaustein and Kiesecker 2002; Hayes et al. 2003).

  • The relevance and intensity of environmental stressors vary in space and time and tolerances to those stressors vary with individuals, populations, and species (Griffiths and Beebee 1992; Alford et al. 2001; Blaustein and Kiesecker 2002).
  • Evaluating the relevance and intensity of stressors requires measuring and monitoring appropriate environmental variables; understanding tolerances requires manipulative experiments to determine dose-response relationships and thresholds.
  • Given inherent fluctuations in populations, quantifying declines can take years (Griffiths and Beebee 1992; Pechmann and Wilbur 1994; Blaustein and Kiesecker 2002).
  • Even when a decline has been described, demonstrating its relation to specific stressors at requisite multiple scales can be daunting (Corn and Fogleman 1984; Griffiths and Beebee 1992; Davidson et al. 2001; Young et al. 2001; Blaustein and Kiesecker 2002).
  • Scientific approaches to these challenges require accumulating complementary evidence in support or rejection of specific hypotheses pertaining to the status of a population and detrimental effects of specific stressors on that population.
  • Given the complexity of these issues, the cumulative lines of evidence necessary to unequivocally link a decline to specific causes can prove elusive, especially for declines that occurred in the past.

Declines in the Midwest Region of ARMI

  • Some populations of amphibians in the Midwest have declined and face the same threats as populations in other locations (Lannoo et al. 1994; Lannoo 1998).

  • Dramatic declines have not been reported from this region on a scale similar to those reported from the western United States (e.g., Corn and Fogleman 1984; Bradford 1991; Carey 1993; Kagarise Sherman and Morton 1993; Bradford et al. 1994; Drost and Fellers 1996; Lannoo 1998; Knapp and Matthews 2000; Reaser 2000; Carey et al. 2001).

  • The lack of reports of dramatic declines in this region could reflect a combination of less actual recent declines, relative stability among populations currently, and a lack of appropriate data to describe declines adequately.

  • Many of the declines in the western states have been observed in mountainous regions where relatively long-term, large-scale studies have been conducted by visual encounter surveys (Carey 1993; Kagarise Sherman and Morton 1993; Blaustein et al. 1994; Bradford et al. 1994; Kiesecker and Blaustein 1995; Drost and Fellers 1996; Knapp and Matthews 2000).
  • Few comparable studies have been reported from the UMR of ARMI.
  • We do not understand historical variation or current status well enough (Pechmann and Wilbur 1994) to describe current population levels or their trajectories for most populations in the UMR of ARMI (Lannoo 1998).
  • Populations of northern leopard frogs (Rana pipiens) and Blanchard’s cricket frogs (Acris crepitans blanchardi) declined over the past several decades (e.g., Corn and Fogleman, 1984; Lannoo et al. 1994; Hay 1998; Lannoo 1998; Moriarity 1998).
  • We do not know the locations of or trends in the distributions and abundances of most remaining populations well enough to evaluate their relative stabilities.

  • Various state and conservation organizations have designated other species of conservation concern due to declines or threats (Table 1; Figure 6), but similar uncertainties exist regarding the statuses of most of those populations.
Distribution of species of concern in the Upper Mississippi Region of ARMI

Potential Causes of Declines in the Midwest Region of ARMI


  • Relatively high frequencies of deformed frogs at some locations have gained the most attention regarding amphibians in the UMR of ARMI (Reaser and Johnson 1997; Lannoo 1998; Gardiner and Hoppe 1999; Burkhart et al. 2000; Helgen et al. 2000; Hopkins et al. 2000; Meteyer et al. 2000; Johnson et al. 2001, 2002).
  • We know that deformities are widespread (North American Reporting Center for Amphibian Malformations [NARCAM] 2003; Blaustein and Johnson 2003) perhaps to some extent because surveys have been conducted more extensively and intensively in recent years.
  • Whereas high frequencies of deformities have occurred at specific locations, frequencies often are inconsistent at and across sites, and high frequencies do not appear to be widespread (NARCAM 2003; D. Green pers. com.), including in the UMR (Converse et al. 2000; NARCAM 2003; R. Cole pers. com.; D. Green pers. com.; M. Knutson pers. com.; D. Sutherland pers. com.).
frog deformity
Deformed northern leopard frog (Rana pipiens)
  • Deformities often are associated with infections by the parasitic trematode Ribeiroia (Johnson et al. 2001, 2002; Blaustein and Johnson 2003).  Johnson and Chase (2004) reported recently that the snails in which early life stages of Ribeiroia live before they invade tadpoles were more abundant in wetlands with high levels of nitrogen and phosphorous.  However, lines of evidence from the laboratory and field have not been conclusive regarding the combinations of factors that can act on populations to cause deformities and to what extent, if any, deformities are linked to declines (Reaser and Johnson 1997; Ankley et al. 1998; Burkhart et al. 1998; Gardiner and Hoppe 1999; Dournon et al. 1998; Helgen et al. 2000; Hopkins et al. 2000; Sower et al. 2000; Gillilland et al. 2001; Johnson et al. 2001, 2002; Blaustein and Johnson 2003).

  • Deformities can result from biotic and abiotic factors and get considerable attention in the scientific and popular presses, but little evidence so far suggests that they are sufficiently frequent or consistent to cause declines.
  • The potential for high frequencies of deformities to indicate some form of ecological instability because of environmental stress (Blaustein and Johnson 2003) could portend future declines of populations of amphibians and warrants monitoring frequencies and types of deformities.
  • We continue to survey and monitor animals for deformities and send appropriate specimens to Dr. David Green at the USGS National Wildlife Health Center for analyses, but we do not plan to devote our efforts to studying deformities more intensively at this time.

  • Our efforts could change in future years with improved understanding of any roles deformities play in declines of populations.

Habitat loss, agricultural practices, and climate change

Habitat loss, including fragmentation

  • Enormous quantities of wetlands and upland habitat have been lost to urban development and alteration for agriculture across the Midwest (Lannoo 1998)
    (Figure 7).
agricultural development
Fragmented agricultural landscape

  • Such loss of habitat undoubtedly has eliminated or reduced the distribution and abundance of many populations of amphibians and continues to threaten many others (Hager 1998; Lannoo 1998; Knutson et al. 1999; Kolozsvary and Swihart 1999; Lehtinen et al. 1999; Semlitsch 2000; 2002; Gaggiotti 2003; Hazell 2003; Houlahan and Findlay 2003; Weyrauch and Grubb 2004).
  • Meaningful analyses of the statuses of populations of amphibians in the UMR of ARMI cannot be conducted without understanding the threats posed by the extensive habitat loss and fragmentation in this region.
  • For example, a population and its habitat might be or become fragmented because of habitat destruction or alteration.  Such changes also could cause the population to become isolated from that of other populations because of the loss of habitat in between.  Thus, the population might exist today, but it could go extinct due to loss of too much local habitat or to being cutoff from immigrants from nearby populations (Bradford et al. 1993; Lehtinen et al. 1999; Semlitsch 2000; Ovaskainen and Hanski 2003).
  • We are working with researchers from Iowa State University and the USGS EROS Data Center to develop methods to assess the connectedness of populations across the landscape and to study relationships between habitat loss, connectedness, and vulnerability of populations to extinction.

Agricultural practices

  • In addition to destruction and fragmentation of habitats, agricultural use of pesticides and fertilizers poses potential threats to populations by lethal and sublethal mechanisms (Lannoo 1998; Bishop et al. 1999; Marco and Blaustein 1999; Bridges et al. 2000; Bridges and Semlitsch 2000; Semlitsch et al. 2000; Davidson et al. 2001; Marco et al. 2001; Sparling et al. 2001; Kiesecker 2002; Relyea 2003, 2004).
  • Hayes et al. (2002, 2003) and Carr et al. (2003) published evidence recently that atrazine can induce abnormal gonadal development in frogs, as was suggested earlier by Reeder et al. (1998).

  • Atrazine is the most heavily used herbicide in the Midwest and the United States (Figure 8 ).  Thus, these reports are cause for concern with respect to potential past and present effects on populations of amphibians (Hayes et al. 2002; 2003).
  • We are measuring levels of atrazine and similar compounds in the water at breeding sites we are monitoring.  We also are collecting frogs from some of these sites to analyze them for gonadal abnormalities.

Climate change

  • Concern among scientists over human-induced changes in global climate continues to increase (e.g., Hughes 2000; McCarty 2001; Pounds 2001; Walther et al. 2002; Flanagan et al. 2003; O’Reilly 2003; Trenberth 2003; Thomas et al. 2004).
  • Evidence suggests that global warming is associated with intensification of the hydrologic cycle, leading to greater extremes of floods and droughts in many regions, including much of the Mississippi River Basin (Houghton et al. 1996; Karl and Knight 1998; Knox 2000; Groisman et al. 2001; Milly et al. 2002).
  • Rainfall increased in the Upper Mississippi River Basin over the last 30 years, driving increased runoff and transport of nitrate (and presumably pesticides) into tributaries from agricultural lands (Randall and Mulla 2001; Donner et al. 2002; Donner and Kucharik 2003) .
weather pattern radar world weather
Three different views of weather patterns on November 17, 2003.

  • In addition, higher spring temperatures in the Upper Mississippi River Basin may have directly influenced land use, resulting in earlier planting of crops (Kucharik in prep.) and applications of fertilizer (Donner et al. in review) and pesticides.
  • Amphibians are sensitive to climate-driven variation in hydroperiod, humidity, and temperature (Pounds and Crump 1994; Blaustein et al. 2001).
  • Droughts have been associated with declines of populations of amphibians in the Sierra Nevada Mountains (Kagarise Sherman and Morton 1993).
  • Changes in climate have been associated with declines in Costa Rica (Pounds and Crump 1994), breeding phenology in Great Britain (Beebee 1995), and the incidence of UVB-induced mortality in the Cascade Mountains of Oregon (Kiesecker et al. 2001).
  • Cold temperatures and insufficient precipitation can result in frozen embryos and breeding sites can dry before metamorphosis can occur (Sadinski pers. obs.).
  • Insufficient snowfall can result in soils and water freezing to depths that could kill or harm overwintering amphibians (Irwin et al. 1999).
  • Higher frequencies of such conditions could cause substantially reduced recruitment in populations of amphibians than has occurred in the recent past.
  • Climate change, among all of the potential universal stressors, has a uniquely powerful potential to affect populations of amphibians, either by directional changes in temperature and precipitation or higher frequencies of extreme weather.
  • We are working to establish sites where we can monitor relationships between the dynamics of populations and climate over time.

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