Loss of top-down biotic interactions changes the relative benefits for obligate mutualists

Abstract

The collapse of mutualisms owing to anthropogenic changes is contributing to losses of biodiversity. Top predators can regulate biotic interactions between species at lower trophic levels and may contribute to the stability of such mutualisms, but they are particularly likely to be lost after disturbance of communities. We focused on the mutualism between the fig tree Ficus microcarpa and its host-specific pollinator fig wasp and compared the benefits accrued by the mutualists in natural and translocated areas of distribution. Parasitoids of the pollinator were rare or absent outside the natural range of the mutualists, where the relative benefits the mutualists gained from their interaction were changed significantly away from the plant's natural range owing to reduced seed production rather than increased numbers of pollinator offspring. Furthermore, in the absence of the negative effects of its parasitoids, we detected an oviposition range expansion by the pollinator, with the use of a wider range of ovules that could otherwise have generated seeds. Loss of top-down control has therefore resulted in a change in the balance of reciprocal benefits that underpins this obligate mutualism, emphasizing the value of maintaining food web complexity in the Anthropocene.

Are nematodes costly to fig tree–fig wasp mutualists?

“…The biology and impact of nematodes on the fig-fig wasp mutualism remains under studied, with most studies focusing on the taxonomy, diversity, and phylogeny of nematodes (Zeng et al, 2011;Davies et al, 2015;Susoy et al, 2016;Sun et al, 2017). Figs, fig wasps, and their parasitic nematodes provide a comparative system for assessing the influence of third parties on mutualism persistence and the cost of parasitism Dunn et al, 2008a;Van Goor et al, 2018;Wang et al, 2019b). However, the effects of nematodes on the fig-fig wasp mutualism are not always clear (but see Van Goor et al, 2018).…”

“…Figs, fig wasps, and their parasitic nematodes provide a comparative system for assessing the influence of third parties on mutualism persistence and the cost of parasitism (Herre et al, ; Dunn et al, ; Van Goor et al, ; Wang et al, ). However, the effects of nematodes on the fig–fig wasp mutualism are not always clear (but see Van Goor et al, ).…”

Molecular mechanisms of mutualistic and antagonistic interactions in a plant–pollinator association

“…The plants reward the larvae of pollinating wasps with nutrition and protection, and each mutualist wasp species is both pollinator and herbivore 12 . Each individual wasp spends the majority of its lifespan at the larval stage (from three weeks up to nine months) and develops inside a single galled ovule of a female floret located inside the enclosed inflorescences characteristic of the genus (figs or 'syconia') [13][14][15] (Fig. 1a).…”

Habitat fragmentation changes top‐down and bottom‐up controls of food webs

“…The ongoing worldwide change in land use has fundamentally altered the otherwise intact landscapes and threatened the survivorship of many plants and animals, disintegrating the intricacies of long‐evolved biotic interactions in nature (Wang et al 2019 a ). The scale dependence of bottom‐up and top‐down effects found in this study shows that habitat loss and fragmentation could entirely change the process of species regeneration in an ecosystem.…”