Resurrecting Extinct Interactions with Extant Substitutes

Abstract

There is increasing evidence that restoration ecologists should be most concerned with restoring species interactions rather than species diversity per se [1]. Rewilding with taxon substitutes, the intentional introduction of exotic species to replace the ecosystem functions of recently extinct species, is one way to reverse ecosystem dysfunction following the loss of species interactions [2]. This is highly controversial [3], in part because of a lack of rigorous scientific studies [4]. Here we present the first empirical evidence of an in situ rewilding project undertaken as a hypothesis-driven ecosystem management option. On Ile aux Aigrettes, a 25-hectare island off Mauritius, the critically endangered large-fruited endemic ebony, Diospyros egrettarum (Ebenaceae), was seed-dispersal limited after the extinction of all native large-bodied frugivores, including giant tortoises. We introduced exotic Aldabra giant tortoises, Aldabrachelys gigantea, to disperse the ebony seeds. Not only did the tortoises ingest the large fruits and disperse substantial numbers of ebony seeds, but tortoise gut passage also improved seed germination, leading to the widespread, successful establishment of new ebony seedlings. Our results demonstrate that the introduction of these exotic frugivores is aiding the recovery of ebonies. We argue for more reversible rewilding experiments to investigate whether extinct species interactions can be restored.

The conservation physiology of seed dispersal

“…However, even if germination can occur in the absence of suitable dispersers, these are still required to transport the seed away from the parent plant and can thereby enhance germination probability. This was found in the rewilding study by Griffiths et al [25] (see §4). Consequently, resurrecting the interactions between large-bodied seed dispersers and dispersallimited plants is an increasingly important topic for conservation biologists.…”

“…An interesting case study in this regard is the critically endangered large-fruited ebony Diospyros egrettarum, which is endemic to a 25-ha island off Mauritius (Ile aux Aigrettes). Intensive study of the species from 1986 to 2000 found seedlings to occur only under maternal trees [25]. This situation has been linked to the extinction of endemic Cylindraspis tortoises on Mauritian islands by humans in the mid-nineteenth century (within the lifetime of adult trees present now).…”

“…This situation has been linked to the extinction of endemic Cylindraspis tortoises on Mauritian islands by humans in the mid-nineteenth century (within the lifetime of adult trees present now). Griffiths et al [25] report on a rewilding experiment where exotic Aldabra giant tortoises were introduced onto the island so that they could disperse the ebony's seeds. The introduced tortoises ingest the large fruits of this tree and have been demonstrated to disperse large numbers of seeds, with gut passage improving germination rates and successful seedling establishment away from the vicinity of parent trees.…”

“…Rewilding much more mobile and fecund bat, bird or primate dispersers would be much less easy to both monitor and if necessary reverse. Alien tortoises have previously been introduced to other Indian Ocean islands as surrogates for extinct species, apparently without adverse effects, and in the case of Round Island have been implicated in the dispersal of seeds from previously dispersal-limited endemic trees [25].…”

Extinction‐driven changes in frugivore communities on oceanic islands

“…Conservation practitioners are starting to employ large and giant tortoises (family Testudinidae) in island ecosystems such as the Galapagos, the Mascarenes and the Seychelles to replace extinct tortoises with extant analogues (Hansen et al 2010). This can improve seed germination and establishment of endangered largefruited plant species (Griffiths et al 2011). However, implementation of rewilding projects to restore plant-frugivore interactions on islands is still rare (Kaiser-Bunbury et al 2010), and empirical evidence as well as rigorous scientific studies on trophic rewilding are often lacking (Svenning et al 2016).…”