Review of small mammal trophic structure in drylands: resource availability, use, and disturbance

Early studies of trophic structure from North American deserts set up a paradigm with granivory as a general and integral phenomenon driving the assembly and structure of small mammal communities. This paradigm encouraged many comparative studies seeking support for convergent evolution. A comprehensive review of studies of dietary habits of aridland mammals provided little support for the paradigm. Subsequent research making use of ternary phase diagrams eventually showed that in small mammal communities trophic structure is strongly related to resource use. A synthesis of results across 5 continents shows a gradation in the distribution of trophic groups across regions. The percentage of species in each dietary category from regional pools shows that no one trophic group is dominant in all desert regions studied: South America has 58% herbivores; South Africa, 52% omnivores; North America, 50% granivores; Australia, 49% insectivores; and Eurasia, roughly equal proportions of granivores, herbivores, and omnivores (similar to 30% each). Seed availability shows substantial overlap across regions but with greater maxima in North America. Granivory appears to be a less important factor in aridlands subject to disturbance regimes, and this could be a result of reduced reliability of seed resources. The impact of fire upon arid environments is one factor common across the Southern Hemisphere but with a lesser effect in the Northern Hemisphere. Fire, rainfall, and particularly extreme climatic events such as El Nino can, at times, outweigh the importance of biotic factors such as competition or predation, emphasizing the importance of resource pulses associated with disturbances. The concepts of history of place and history of lineage can be important factors determining community structure, as has been well demonstrated with the impact of the extensive evolution of the granivorous Heteromyidae in North America and the radiations of marsupials in aridlands of Australia, 2 extremes of the trophic structure shown by desert small mammal communities.