Local abundance of species, which can be represented by either the distance between neighbouring individuals (density) or the number of individuals (population size) (Kunin 1997) tends to decrease when human activities lead to habitat fragmentation and destruction. Changes in both size and spatial structure may have profound effects on ecological interactions and population dynamics.
Stochastic processes will, for example, have more influence on the dynamics and genetic composition of small populations (e.g. Lande 1988; Barrett & Kohn 1991; Ellstrand & Elam 1993; Schemske et al. 1994), while important ecological interactions, in particular the mutualistic relationship between plants and pollinators, are likely to be disrupted in small and isolated populations (McKey 1989; Rathcke & Jules 1993; Aizen & Feinsinger 1994a).
Larger populations of plants are likely to be more attractive to pollinators resulting in higher visitation rates and therefore pollination success (Sih & Baltus 1987; Ågren 1996), whereas small populations may suffer from insufficient pollen transfer and consequently lower seed set (e.g. Jennersten 1988a; Lamont et al. 1993; Ågren 1996; Fischer & Matthies 1998). In addition, the level of inbreeding may be higher in small, isolated populations (e.g. Barrett & Kohn 1991; Falconer & Mackay 1996) because of the higher rate of selfing and more frequent matings between close relatives. The resulting inbreeding depression can reduce the fitness of these plants compared with those in larger populations (Menges 1991; Aizen & Feinsinger 1994a; Ouborg & Van Treuren 1994; Heschel & Paige 1995).
The spacing of individuals within a population may also affect visitation rates and the foraging behaviour of pollinators and consequently, the level of out-crossing and inbreeding (reviewed in Handel 1983). Previous studies indicate that higher plant density is associated with higher visitation rates of both hummingbirds (Feinsinger et al. 1991) and insect pollinators (Kunin 1993), and this is likely to have a greater impact for self-incompatible species which cannot compensate for lower pollinator abundance by selfing. Moreover, plant density probably affects the behaviour of pollinators which are more likely to move between individuals in dense populations, where flight distances are shorter, than in sparse populations where increased visits within a plant may favour within-plant pollen transfer (geitonogamy, reviewed in de Jong et al. 1993).
Both the size and the density of a population are known to affect pollination and subsequent reproductive performance (e.g. Sih & Baltus 1987; Feinsinger et al. 1991; Kunin 1993, 1997; Bosch & Waser 1999), but strong correlations between these two factors (Ågren 1996) mean that experimental manipulations are needed to separate their effects. Unlike earlier studies of population density (e.g. Kunin 1997; Bosch & Waser 1999), we allowed population area to vary as well as size and density of populations, because the area occupied by a plant population of a particular size will increase when the distance between individuals increases. Specifically, we address the following questions:
(1) Does the pollinator community and the rate at which it visits a plant population dependent on size and density?
(2) Do the size and density of a plant population affect the behaviour of its pollinators? and
(3) Do the size (number of individuals) and density (distance between individuals) of a population affect the reproductive success of its component plants?