The downsizing of insular frugivore communities

This Aldabra giant tortoise Aldabrachelys gigantea is eating the fruit of an endemic Pandanus plant on Rodrigues. There were no frugivores left that were large enough to be able to swallow and disperse their seeds until these tortoises were introduced to replace the extinct Cylindraspis giant tortoises. (Photograph by Dennis Hansen)

By Julia Heinen and W. Daniel Kissling

Biodiversity is decreasing worldwide, but local extirpations and species extinctions are not taking place at random. For instance, extinctions can be non-random in geographic space because of extrinsic variation in climatic conditions or availability of habitat space (e.g. on island), or taxonomically non-random because of intrinsic characteristics of species such as body mass or inability to fly. Although such extrinsic and intrinsic factors of extinction risk have been previously studied, their generality across broad spatial scales and different taxonomic groups remains little explored.

In our study, we provide a comprehensive overview of insular frugivore extinctions worldwide. We focused on frugivorous vertebrates such as birds, mammals and reptiles because they provide an important ecological function for plants and ecosystems due to their seed dispersal services. Especially in the tropics and subtropics, many plants depend on vertebrates for seed dispersal as it improves recruitment success, gene flow and the colonization ability of plants.

We quantified insular extinctions of frugivores across 74 tropical and subtropical oceanic islands within 20 archipelagos worldwide. We focused on islands because they tend to have more extinctions than mainland areas, and because island conservation increasingly focuses on restoring species interactions that have been lost in the past. To provide a biogeographic overview we compiled a new database from published sources, sub-fossil records, expert estimates and morphometric calculations. We made the database open access in the Dryad Digital Repository.

For the analysis of the dataset we developed two statistical models to test whether and to what extent extinction probability is related to island characteristics and species traits. Our key findings were:

I. Global distribution of extinctions

Extinctions of frugivorous vertebrates are recorded on 33 of the 74 islands. Those islands have on average lost a third of their initial frugivore community. Extinctions are particularly pronounced on Hawaii, Cook Islands, Tonga Islands, Mascarenes and the Seychelles.

II. Island characteristics

Islands that have lost the highest proportion of their original frugivore communities are small, isolated and topographically diverse. Extinction was not related to temperature or precipitation (all analyzed islands are located in the tropics and sub-tropics).

III. Intrinsic traits

Large and flightless species showed a higher extinction probability than small and volant species. Examples of extinct taxa include flightless birds such as the dodo and mammals such as native rodents. The amount of fruit in the diet did not show an influence on extinction probability.

IV. Community-level body mass

On most islands, especially large species were lost from the original frugivore communities. These extinctions have resulted in a strong reduction of the mean and maximum body mass of the frugivore community. This phenomenon is often referred to as downsizing or trophic downgrading of ecological communities.

At least two key conclusions can be drawn from our study. First, predictions of extinction risk need to consider both extrinsic and intrinsic factors. For instance, island biogeography (e.g. island size, isolation and topography) as well as species characteristics (e.g. body size and inability to fly) play a key role here. Second, our results suggest that targeted conservation and rewilding efforts on islands are urgently needed, especially to restore the defaunation of megafauna. However, rigorous scientific studies to support this restoration strategy are currently often lacking.

Video abstract: https://youtu.be/MtRjveZFfG8

The article

Heinen, J. H., van Loon, E. E., Hansen, D. M., & Kissling, W. D. (2017) Extinction‐driven changes in frugivore communities on oceanic islands. Ecography. doi: 10.1111/ecog.03462

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