The influence of behaviour and physiology on abundance and distribution relationships

Abstract

Abiotic factors, biotic factors and dispersal ability affect distribution and abundance. Due to their intimate host association, pollinating fig wasps are expected to occupy the entire distribution of their host fig. However, Ceratosolen galili (non-pollinator) has been suggested to be rarer than the pollinating Ceratosolen arabicus in drier areas due to different desiccation tolerance. Both species are closely related and utilise the same oviposition sites on Ficus sycomorus. We question what form of rarity (distribution, abundance or both) is present and the potential mechanism(s) for this. We used ecological niche modelling to determine whether C. galili covers the range of F. sycomorus and whether it is consistently rarer than C. arabicus. We examined emergence times, critical thermal limits, desiccation and starvation tolerances of the wasps to determine the potential behavioural and physiological underpinning of rarity. The suggested rarity of C. galili relates to lower abundance in certain months and at certain localities but a similar number of occurrence records within the distributional range (except in arid regions, e.g. Namibia). Competitive interactions between the species means that the wasp species with highest abundance at a specific time will ‘close’ the figs to entry by the other species, lowering the abundance of the second species. In contrast to C. galili, C. arabicus has a wider thermal range, lives longer under hydrating and dehydrating conditions and is a nocturnal flier. The synergistic effect of physiological tolerances and flight time differences exacerbate the less favourable conditions available to C. galili during flight and location of receptive figs. These factors enable C. arabicus to survive for longer than C. galili, meaning that they are more likely to disperse to trees that are flowering further away in space and time. Combining correlative and mechanistic approaches has aided us in understanding the ecological niches of these species.