Solving the enigma of granivory rates in Patagonia and throughout other deserts of the world: is thermal range the explanation?

In order to gain insight about why Patagonia has low levels of granivory activity, percentages of world-wide granivory rates were compared for rodents, ants, and birds in seven desertic areas of the world. Using a series of multivariate analyses, we classified and ordinated these areas according to percentages of granivory rates, environmental features of the sites, and granivore biodiversity. Granivory activity was clearly separated into two groups, one representing the Northern Hemisphere which comprised Sonora, Great Basin, and Israel, and another, representing the Southern Hemisphere with Monte, Patagonia, South Africa, and Australia. The ordination analyses did not discrimínate any clear groups using the combined environmental variables. Separate correlations between the ordination axes of granivory and each environmental variable, and granivore richness, showed that only thermal range (the difference between the extreme annual mean temperatures) successfully correlated with the differences ín overall granivory between deserts and hemispheres. Our results show that all sites from the Northern Hemisphere, which had a very high continentality (high land/ocean ratio), and therefore high thermal range, were the ones with greater levels of granivory. A linear regression analysis showed that 72 % of the variation in overall granivory, mainly driven by rodent activity, was explained by thermal range. We propose that the only strategy that can evolve in environments with high thermal range ís granivory, as seeds are the only high quality food that can be stored. We propose that the combination of strong selective pressures with the chance that a certain taxa has for being at a certain place and time determines the relative importance of different taxa as granivores. Murid rodents had arrived earlier in the Northern Hemisphere than ín the Southern, and therefore, had greater opportunity to develop the granivore syndrome. Ants -which are as old as rodents at some of the studied sites but are poikilotherms- cannot deal as efficiently as rodents with very harsh environments. Birds, finally, avoid bad situations by migrating to more favourable habitats, and therefore circumvent those selective pressures with their great vagility. The more beningn the environment (lower thermal range due to low land/ocean ratios, as in the Southern Hemisphere), the less selective pressure for granivory, a reason that can also account for the high number of omnivores in South America and Australia, and the low granivory rates in Patagonia.