A tale of two ecosystems: Long term rodent biomass in a southern hot desert (Mapimi, Mexico) and northern temperate Forests (Oswego, New York)

Background/Question/Methods Differences in net primary production among ecosystems related to dissimilarities in precipitation, solar radiation and temperature have been well documented.  Less is known, however, about the relative differences in total biomass of primary consumers among ecosystems. If primary consumer biomass is directly related to primary production, we would predict that the more highly productive ecosystems support a higher diversity and a higher biomass of primary consumers. To test this hypothesis we compared rodent assemblages from a northern temperate mixed deciduous forest, where net primary production has been measured at 513 g/m²/yr to 1156 g/m²/yr  (Newman et al. 2006) and the Chihuahuan Desert, where net primary production varies from  51.1 g/m²/yr  to 59.2 g/m²/yr.
We monitored the assemblages of rodents from 1997 to 2007 both at Rice Creek Field Station (RCFS), Oswego, NY, EUA (43^o 25’ N, 76^o 33’ W’) and at the Mapimi Biosphere Reserve (MBR), Durango, Mexico (26^o 40’ N, 103^o 40’ W).  Trapping was done annually using traditional trapping grids (RCFS) or trapping webs (MBR).  

Results/Conclusions Seven rodent species were caught at RCFS:  Glaucomys volans, Microtus pennslyvanicus, Peromyscus leucopus, Sciurus carolinensis, Tamiasciurus hudsonicus, Tamias striatus and Zapus hudsonius while  fourteen species were trapped at MBR: Chaetodipus nelsonii, Chaetodipus penicillatus, Dipodomys merriami, Dipodomys nelsonii, Neotoma albigula, Onychomys torridus, Perognathus flavus, Peromyscus eremicus, Peromyscus maniculatus, Reithrodontomys fulvescens, Reithrodontomys megalotis, Sigmodon hispidus, Spermophilus mexicana and Spermophilus spilosoma.
Weighs of RCFS species ranged from 20.8 g ± 2.1 g (P. leucopus) to 260.4 ± 3.9 g (S. carolinensis) whereas weights of species at MBR ranged from 6.3 + 0.1 g (P. flavus) to 165.3 + 1.4 (N. albigula). The average total biomass of rodents in RCFS was 13.4 ± 0.4 g/ha/yr and the corresponding biomass at MBR was 1,400 ± 0.3 g/ha/yr. Biomass of rodent primary consumers was two orders of magnitude greater in the desert ecosystem than in the forest ecosystem despite a biomass of primary production less than one tenth as great. Temperate forest ecosystems have a much higher primary productivity than arid systems, but much of it is in the form of woody tree stems and leaf production, not readily accessible to most rodent species.  Thus, net primary productivity of an ecosystem ultimately determines levels of biomass production, but it is the accessibility of that productivity to the different assemblages of primary consumers that determines the level of biomass of primary consumers.