Indirect Facilitation of Granivorous Birds by Desert Rodents: Experimental Evidence from Foraging Patterns

Abstract

In order to quantify the response of birds to experimental manipulations of seed availability and densities of granivorous rodents and ants, we counted granivorous birds and measured diurnal and nocturnal seed removal on 24 plots during winter months in the Chihuahuan Desert. Removal of single, widely spaced millet seeds provided a reliable measure of bird and rodent foraging activity. Avian foraging activity increased in response to supplemental seeds, but decreased in response to long—term removal of all rodents and all ants. Although birds potentially compete for seeds with rodents and ants, these results suggest that, in the long term, indirect facilitation dominates the interactions among all three taxa. The positive effect of the other granivores on birds may be mediated through changes in habitat structure (e.g., reduction in the density of annual plants). The increase of avian foraging in response to seed addition and the high spatial and temporal variance in the patterns of seed removal by birds indicate that in desert habitats granivorous birds use their mobility to find and exploit high concentrations of seeds. Thus, birds may reduce the spatial variation in seed abundance and reduce the densities of seed available to other granivores. The foraging behavior of birds and rodents revealed by these experiments clarifies the response of granivores to experimental manipulations reported earlier, such as the lack of biomass compensation by rodents in response to seed addition.

Responses of birds to rainfall and seed abundance in the southern Karoo, South Africa

“…Clear evidence of close tracking of resources has been found in many species of birds from different dietary guilds (Wiens, 1989). Changes in species richness and numbers of granivorous birds with changes in seed abundance in semi-arid habitats have been shown in Chaco, Argentina (Capurro & Bucher, 1982), in the Chihuahuan and Sonoran Deserts, south-western U.S.A. (Brown et al, 1986;Thompson, et al, 1991) and in Kenya, the U.S.A., Brazil and Argentina (Schluter & Repasky, 1991). It follows that nomadic granivorous birds should track resources closely, and that there is likely to be a correlation between nomadic bird density and local seed abundance.…”

Long-term dynamics of rodent populations in arid Australia: the influence of rainfall

“…In semi-arid Chile, predation has been demonstrated to depress rodent numbers, especially species with predictable population cycles (Meserve et al 1993(Meserve et al , 1996. Indirect and facilitatory interactions may also occur among rodents and other granivores (Thompson et al 1991), but population-level effects have been little studied.…”

Toward reliable estimates of seed removal by small mammals and birds in the Neotropics

“…rodents) animals (e.g. Thompson et al 1991;Pizo and Vieira, 2004). However the application of temporal exclusions are subject to several constraints: 1) the study plot has to be relatively small, allowing the monitoring of seed stations at short intervals of time; 2) long-term studies are difficult, because seed removal must be recorded twice a day; and 3) some animals like agoutis and squirrels have activity schedules during the day, violating the assumption of diurnal seed removal only by birds.…”

“…obs.). Commercial seeds have also been employed in other studies of seed removal worldwide (Thompson et al, 1991;Kelt et al, 2004).…”

“…The role of birds as seed predators is being increasingly recognized, especially in arid and semiarid ecosystems (Thompson et al, 1991;Marone et al, 2000;Kelt et al, 2004). In more mesic habitats however, and in the Neotropics in particular, birds are often considered seed predators of less importance when compared to rodents or granivorous ants (Hulme, 1998).…”