Scientific study
Morey, S. R. and Reznick, D. N. 2004. The relationship between habitat permanence and larval development in California spadefoot toads: field and laboratory comparisons of development plasticity. Oikos 104: 172-190.
Most amphibian populations persist in metapopulations, a network of sub-populations connected by migration. American spadefoot toads are good examples of the metapopulations. Adult spadefoot toads migrate to breeding grounds each year as a means of linking sub-populations together through breeding season migration. This study provides a comparative analysis of three spadefoot toad species located in California: Spea intermontana (Great Basin spadefoot), Spea hammondii (western spadefoot), and Scaphiopus couchii (desert spadefoot). There is no species overlap between the three spadefoot toad species. The study takes place in a multitude of spadefoot toad natural breeding habitats, as well as in "common garden" or a laboratory settings. The researchers determine if, species from stable environments show more levels of responsiveness to a changing environment that are distinguishable from the pattern of those that regularly encounter variability, among three closely related species. North American spadefoot toads provide a good example of a species with a large variation in larval environments and development. Some spadefoot toads breed in temporary environments, so breeding and development must be rapid to escape desiccation. Other species prefer more permanent breeding habitat, which allows for a longer breeding and development period. The study focuses on an in depth look at the larval ecology and development amongst the three spadefoot toad species.
The field portion of the study evaluates: larval growth and development, pool duration, age, and size at metamorphosis and risk of catastrophic mortality due to drying, and the changing abiotic environment. The laboratory portion of the study evaluates: reaction norm, comparisons of growth in nature and lab, plastic response of developmental rate, and the age and size at metamorphosis. The known breeding site tendencies for each of the spadefoot toad species is as follows: S. couchii populations prefer to breed in summer monsoon puddles and therefore have a short breeding/ larval development phase. S. hammondii populations prefer to breed in pools created by winter snow melt off and have an intermediate time period to complete breeding/ larval development. S. intermontana prefer permanent water supplies fed by snowmelt and have the longest time period to complete breeding/ larval development.
Field Observations/ Results
Larval growth and development- The growth patterns for all three spadefoot toad species is found to be somewhat universal. S. intermontan and S. hammondii however, are larger than S. couchii. S. intermontana breeds asynchronously, meaning the breeding season is somewhat sporadic throughout the spring/ summer months and does not always follow a rainfall. S. hammondii is a synchronous breeder; breeding only occurs during March and is restricted to breeding immediately after a heavy rainfall. S. couchii is a synchronous breeder, breeding only occurs during heavy rainfalls. Larval development for S. intermontana took 48 days with a range of 36 to 60 days. Larval development for S. hammondii took 58 days with a range of 30 to 79 days. Larval development for S. couchii took 7.4 days with a range of 7 to 8 days. S. couchii are on average the smallest spadefoot toads upon metamorphosis, which means there is a correlation between larval development period, location, and size. S. couchii are the smallest spadefoots because they prefer to breed in habitats that are not permanent (desiccation is an issue much faster in less permanent breeding habitats), which limits the amount of time to metamorphosis.
Pool duration, age and size at metamorphosis and risk of catastrophic mortality due to drying- As previously mentioned, S. intermontana choose breeding habitat that are permanent, such as quiet pools, while the other two species choose temporary habitats for breeding habitat. The correlation between mortality and breeding habitat supports permanent breeding habitats and S. intermontana. A permanent breeding habitat allows for more time to develop because the risk of drying in a permanent water body is minute in comparison to choosing a temporary water source for breeding. The S. couchii population prefer the least stable of breeding habitats and as a result, S. couchii, have the highest mortality of the three species.
The changing abiotic environment- The temperature of a breeding habitat, depending on the classification for the specific breeding habitat, tends to fluctuate. Temporary breeding habitats, preferred by S. couchii, tend to stay at the same temperature for the duration of the breeding bout. Permanent breeding habitats, such as those chosen by S. intermontana, tend to fluctuate in temperature. The daytime temperature for permanent breeding habitats increases steadily throughout the breeding/ larval development phase. The water chemistry for permanent breeding habitat remains stable for the breeding/ larval development time period. The water chemistry of the temporary habitats fluctuates as the water supply dries (increases in the amount of dissolved substances, such as ammonium nitrate). Water temperature and chemistry both correlate to survivor ability of spadefoot larva.
Laboratory Observations/ Results
Reaction norm- All three species were subject to controlled food availability for an unspecified amount of time. With minimum food availability, all three spadefoot species tend to take a longer period of time to reach the metamorphosis stage of the larval development. In contrast, with maximum food availability all three species reached a certain uniform species specific size and then went through metamorphosis. The findings suggest that there is a maximum body size at metamorphosis.
Comparisons of growth in the lab and nature- The approximate milligram amount of food consumed by each species of spadefoot was estimated in the wild and replicated for lab use to compare consumption and growth rates in the lab versus nature. Minimum food availability results in high mortality, both in a lab setting and in a natural setting. Maximum food availability supports low mortality both in a lab setting and in a natural setting. Given the opportunity to eat more than an average amount of food, larva in a lab setting typically do not eat more than they would in a natural setting.
Plastic responses of development/ Age and size at metamorphosis- S. couchii develop the fastest from the time of hatching to metamorphosis, usually taking about 9.02 days. S. hammondii develop the second fastest, taking 17.73 days to develop from the time of hatching to metamorphosis. S. intermontana develop the slowest, taking about 20.13 days to develop from the time of hatching to metamorphosis. With maximum food availability, the size for each species dramatically increased at the time of metamorphosis. With maximum food availability the given species takes a longer amount of time to develop, and upon the time of metamorphosis the given species is at maximum body sizes.
References:
www.californiaherps.com/frogs/pages/s.intermontana.sounds.html
http://www.npwrc.usgs.gov/resource/herps/amphibid/species/spinterm.htm
http://www.dnr.wa.gov/nhp/refdesk/herp/html/4spin.html
http://wlapwww.gov.bc.ca/sir/fwh/wld/atlas/species/spadefoot.html
http://www.natureserve.org
http://ndis.nrel.colostate.edu/herpatlas/
http://animaldiversity.ummz.umich.edu/site/accounts/classification/Spea.html#Spea
Morey, S. R. and Reznick, D. N. 2004. The relationship between habitat permanence and larval development in California spadefoot toads: field and laboratory comparisons of development plasticity. Oikos 104: 172-190.
Created By: Casey Clark
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