Predicting the consequences of disperser extinction: richness matters the most when abundance is low

Beatriz Rumeu, Mariano Devoto, Anna Traveset, Jens M. Olesen, Pablo Vargas, Manuel Nogales and Ruben Heleno

The ongoing rapid loss of biodiversity often entails replacement of disturbance-sensitive species by disturbance-tolerant ones and thus disruption of the ecosystem functions they provide. In this line, the loss of fruit-eating (frugivorous) animals may negatively affect the long-term persistence of the plant communities whose seeds they disperse. However, little is known on how the loss of frugivore richness and abundance affects the seed dispersal services provided to plants. Here we assessed the functional consequences of defaunation on a seed dispersal network from the Galápagos Islands under five simulated extinction scenarios based on current threats to the archipelago biota.

We independently evaluated the consequences of declines in abundance and species richness of frugivores, assuming that when a particular species becomes extinct, the remaining species will compensate for such loss by increasing their own abundance. We found that both abundance and species richness of frugivores significantly affect the seed dispersal function, which we measured as the number of plant species dispersed after frugivore extinctions. Additionally, species richness becomes increasingly important as frugivore abundance declines. The sequence of animal extinctions also has profound implications for the service available to the plant community. In the Galapagos, abundant species with generalist diets like lava lizards can to a large extent mitigate the loss of more specialized dispersers. Yet, the early extinction of these key generalist frugivores can lead to the rapid collapse of seed dispersal services. Therefore, at the same time that generalist frugivores provide functional redundancy and secure high levels of seed dispersal, the identity of the species lost is also critical to understand the consequences for the number of plant species dispersed.

This study highlights that not only richness but also abundance of frugivores is an important, yet often neglected, driver of seed dispersal services. Hence, the coupled loss of animal abundance and richness can accelerate the loss of essential functions such as seed dispersal in degraded ecosystems.

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Image caption: Microlophus indefatigabilis on Santa Cruz Island, Galapagos. Photo credit: Ruben Heleno

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