Seed dispersal is a key process in plant population dynamics (Harper 1977). Although it is implicitly assumed that animal dispersers can influence plant population dynamics, their role has seldom been evaluated (Estrada & Fleming 1986; Jordano 1992; Willson 1992; Fleming & Estrada 1993; Herrera 1995). Over the last two decades, most studies of seed dispersal have focused on just one of the several stages leading to recruitment (but see Herrera et al. 1994; Houle 1998; Clark et al. 1999). Conclusions about the evolutionary and ecological importance of dispersers may therefore be limited by an absence of information about the remaining stages (Schupp 1995; Schupp & Fuentes 1995). If however, we view dispersal as one event within a sequence, it is possible to determine the relative importance of dispersers in driving recruitment dynamics by contrasting their effect with subsequent interactions.
The result of recruitment in heterogeneous habitats can be expressed in terms of the number and spatial distribution of new individuals incorporated into the population, both of which can be influenced by seed dispersers. Dispersers can limit recruitment if the probability of a seed being dispersed is lower than the probability of a dispersed seed becoming an established plant. However, fleshy-fruited trees usually produce large crops that are dispersed by a coterie of animal species (Jordano 1992; Herrera 1995), and we therefore hypothesize that dispersal is unlikely to be the limiting process in the recruitment dynamics of such species.
Nevertheless, dispersers may disseminate different numbers of seeds in a variety of microhabitats, and thus either shape the spatial distribution of the seedlings or limit recruitment by depositing large numbers of seeds in places that are unsuitable for recruitment. Processes acting at different recruitment stages are likely to be independent and evidence of such ‘uncoupling’ (Jordano & Herrera 1995) would indicate the complexity of selective forces in play during the life cycle of the plant, including seed–seedling conflicts (Schupp 1995). Different outcomes for each stage in different microhabitats (spatial discordance, Jordano & Herrera 1995) are partly a consequence of uncoupling because the recruitment-driving processes, which may have a strong effect on spatial pattern, can vary between microhabitats (Shibata & Nakashizuka 1995; Houle 1998). We hypothesize that the spatial pattern of recruitment will initially be established by seed dispersers but may then be altered by uncoupling and spatial discordance (Schupp 1995; Schupp & Fuentes 1995).
We use a demographic approach to explore our hypotheses linking seed dispersal to recruitment in Olea europaea var. sylvestris. Brot. (wild olive, hereafter Olea), a Mediterranean bird-dispersed tree. We use a simple model of the plant life cycle, composed of life stages connected by transition probabilities (Harper 1977; Jordano & Herrera 1995) that are determined by a number of processes. A process is any event that affects the probability of a propagule moving from one stage to the next: pre-dispersal seed loss (whereby the seed fails to reach the dispersed seed stage) is a process and may be caused by biotic factors, like granivorous insect larvae, or by abiotic factors, such as frost. Our approach is exploratory rather than predictive. We analyse the influence of biotic and abiotic factors on each recruitment stage (pre-dispersed seed, dispersed seed, seedling and sapling), and then estimate the transition probabilities (hereafter TP) between stages and the overall probability of a seed in a ripe fruit becoming an established juvenile plant (i.e. overall probability of recruitment, hereafter OPR). Comparison of TPs and OPRs between microhabitats allows us to explore the extent and influence of spatial discordance, and, by analysing each process, to determine which one influences the final spatial pattern and dynamics of recruitment.
Our design allows us to address some of the most relevant and current questions concerning the link between seed dispersal and recruitment (see, e.g. Jordano & Herrera 1995; Schupp & Fuentes 1995; Schupp 1995; Houle 1998; Clark et al. 1999); specifically: (i) which are the critical life stages and processes in recruitment, (ii) how do processes operating at different stages interact to affect final recruitment patterns, (iii) are the best sites for seeds also the best for seedlings, i.e. is seed–seedling conflict in operation, and (iv) does the activity of seed dispersers determine the spatial distribution of recruitment, or is it counteracted by the existence of strong spatial discordance?