SHANNON L. DAVIS

Education:

Thesis research:  Environmental variation and the demography of a tropical forest rodent.

Many small mammal populations display dramatic fluctuations in their demography (vital statistics), and one of the major goals in ecology is to explain this variability.  Demographic variables include density, sex ratio, age structure, and population growth rate (and its constituents birth, survival, and dispersal rates).  Demographic fluctuations depend largely on environmental variation, and such fluctuations may be patterned (i.e., exhibiting seasonal or long-term variation) or chaotic.  Resource abundance is particularly important in influencing demography because organisms typically track available resources and adjust their behaviors and reproductive activity accordingly.  Environmental factors, such as precipitation, have an influence on fruiting phenology, and also may be directly correlated with demography.  It is often difficult to estimate the demographic effects of environmental variability because of dispersal and community interactions such as predation and competition.

Data on Proechimys semispinosus (the Central American spiny rat) and fruit abundance were collected on four islands in the Panama Canal from 1991 through 1999.  Individuals in these island populations rarely disperse among the islands and are largely free of competition and predation relative to mainland populations.  Six demographic variables and fruit densities were calculated and used to statistically quantify the amount of synchrony over time and space.  More spatial than temporal synchrony was found, although overall there was a high degree of asynchrony.  The demographic variables then were correlated with fruit densities and the quantity of precipitation to analyze the influence of temporally-variable environmental factors.  Each demographic variable also was autocorrelated to test for density-dependence. Results varied among islands and years, but fruit density appeared to affect population density, density of births, and population growth rate, but not survival rates.  Precipitation was not correlated population density, but density of births, population growth rate, overall survival, and proportion of adults were all significantly correlated with the quantity of precipitation.  Strong positive correlations occurred with a lag between population growth rate and population density, which indicated that density-dependence also played a part in influencing the demographic fluctuations of P. semispinosus.
 

Publications:

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Last updated 12 February 2002.