Rodent biomass changes in the Chihuahuan Desert, Mexico

Background/Question/Methods Energy flow is one of the most fundamental aspects of ecosystem dynamics. Energy captured in a trophic level, represented by net biomass, is the key to energy availability for the next trophic level. Identifying the factors that affect biomass capture within a trophic level then becomes important in understanding community structure and energy flow.  However, all net biomass is not equal relative to use by higher tropic levels, e.g. increases in seed biomass will favor granivore more than folivore species.  Thus changes in the species or functional contribution of net biomass at one trophic level affects energy availability to the components in subsequent levels.  Small mammals, especially in desert environments, are considered an important trophic link for energy transfer energy to subsequent secondary consumers. Because of predator specialization, the distribution of net biomass among the different species of small mammals can directly impact the carnivores within a system.  For example, increases in biomass of nocturnal small mammals benefits nocturnal owls but is completely unavailable to diurnal hawks.  Thus, it is important to also understand the factors that affect net biomass distribution within the different small mammal species of a community. We used an 11 year data set on small mammal abundance in the Chihuahuan Desert in Mexico to test the prediction that net biomass production of small mammals was directly related to annual precipitation levels.  We also analyzed the change in distribution of the net biomass among the 13 small mammal species found there. 

Results/Conclusions

Rodent biomass of one year was significantly related to annual precipitation of the previous year (r² = 0.54, P = 0.006).  At lower biomass levels, biomass distribution was approximately equally dominated by a granivore species (Dipdomys merriami, 34.9 ± 2.7%) and a folivore species (Neotoma albigula 28.6 ± 3.9 %). At higher biomass levels, the contribution of D. merriami declined to 15.9 ± 2.7%  and was dominated by  N. albigula 65.3 ± 5.8 % (range = 54.6-74.7%). Our study showed that biomass did increase with higher precipitation and that the distribution of this biomass shifted from a predominately graniovore species to folivore species. We discuss the possible reasons for this shift and the implications relative to energy flow to the subsequent carnivore level.