Abstract
AbstractMost plants flower and fruit at times of year when probabilities of pollination and seedling establishment are high. Fig trees (Ficus spp.) are often considered as keystone resources for vertebrate frugivores, in part because of year‐round fig production. This unusual fruiting phenology results in the maintenance of fig wasp populations, but in seasonal environments this means fruiting occurs during periods when the chances of seedling establishment are low. Under these circumstances, selection is expected to favour any individuals that reduce or eliminate fruiting at these times. Here, we describe a large‐scale survey of the extent of dry season fruiting by three riparian Ficus species in Gonarezhou National Park, Zimbabwe. Few trees of two monoecious species, F. sycomorus and F. abutilifolia, had figs, and most crops of F. sycomorus were far smaller than the trees were capable of producing. Large stands of the dioecious F. capreifolia were present, but fig densities were low and no mature female (seed containing) figs were recorded. Even though fig trees may have been the only species bearing fruit, the consequences of the low investment in reproduction by the three Ficus species were clear—there were too few figs for a landscape‐scale keystone role.
Highlights
Fig trees (Ficus spp., Moraceae) are a group of mainly tropical and subtropical plants that are often regarded as keystone mutualists (Dev, Kjellberg, Hossaert-McKey, & Borges, 2011; Lambert & Marshall, 1991; Terborgh, 1986)
The genus is characterised by its unusual enclosed inflorescences which after pollination develop into compound accessory fruits. Their keystone status reflects the importance of figs in the diets of many tropical mammals and birds, with more vertebrates recorded as eating figs than any other fleshy fruits (Shanahan, So, Compton, & Corlett, 2001) and an increasing realisation that the insects associated with the figs are themselves important in the diet of birds (Mackay, Gross, & Rossetto, 2018; Matthews, CotteeJones, Bregman, & Whittaker, 2017)
The importance of figs for vertebrates is a result of several biological features: fig trees can be abundant, they can produce large crops, figs are easy to eat and have a high calcium content, different species of fig trees produce figs that vary in size and location, thereby favouring different groups of vertebrates, and figs are often produced at times of the year when few other fruits are available (Foster, 2014; Lambert & Marshall, 1991; O'Brien et al, 1998; Shanahan & Compton, 2001; Shanahan et al, 2001)
Summary
Fig trees (Ficus spp., Moraceae) are a group of mainly tropical and subtropical plants that are often regarded as keystone mutualists (Dev, Kjellberg, Hossaert-McKey, & Borges, 2011; Lambert & Marshall, 1991; Terborgh, 1986). It is only the male trees that support pollinator populations and so are required to have at least some individuals fruiting throughout the year This difference between sexes frees female trees to concentrate their fruiting efforts during those parts of the year when seeds are more likely to establish successfully (Kjellberg & Maurice, 1989; Patel, 1996). Fig production during the dry season can contribute to water stress on the trees (Patiño, Herre, & Tyree, 1994) These factors suggest that natural selection should favour individuals that reduce their reproductive investment at such times (Kjellberg & Maurice, 1989), because individuals which only fruit during optimum periods (or release fig wasps which pollinate such individuals) should be at an advantage. To assess the extent of reproductive investment and the likely value of the trees to frugivores, we recorded the densities of trees, the numbers of figs present, if any, and their developmental stages
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