Rodent Scatterhoarders as Conditional

Seeds dispersed by vertebrates can face potentially very different seed fate pathways. One of these is via vertebrates that pose little threat to the seed itself, either dropping the seed in the course of fruit handling or passing the seed relatively unharmed through the gut (‘benign dispersers’ or ‘legitimate’ dispersers of Schupp (1993)). Alternatively, seeds could be dispersed by vertebrates that potentially act as either seed predators or seed dispersers by consuming and damaging a significant proportion of seeds they handle. One set of animals that act in this way are those that ingest seeds and pass them through the gut (Janzen, 1981; see Beck, Chapter 6, this volume). Another set includes animals that temporarily cache seeds for later consumption, and occasionally fail to recover some of these seeds, thereby acting inadvertently as seed dispersers (Vander Wall, 1990). Because of the large cost in seeds destroyed by the latter two groups, and the difficulty in estimating the relative benefits of dispersal by these animals to plant recruitment, whether they play the role of antagonistic seed predator or mutualistic seed disperser is often equivocal (see Hulme, 2002; Beck, Chapter 6, this volume). Variation in the outcome of species interactions like that exhibited by animals that can potentially destroy or disperse seeds has been documented in several systems that traditionally have been viewed as mutualisms (e.g. ant-membracid mutualisms; Cushman and Whitham, 1989; Billick and Tonkel, 2003). In these systems, termed ‘conditional mutualisms’, variation in ecological and life-history factors in either space or time can shift the outcome of a species interaction to multiple points along the continuum from antagonistic to mutualistic (Bronstein, 1994). In this chapter, I argue that the interaction between plants and the rodents that scatterhoard their seeds (i.e. cache seeds individually or in small groups in numerous cache locations) can be viewed as a conditional mutualism that depends in part on two factors that can vary in time and space: