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The rate of biodiversity loss is not slowing despite global commitments, and the depletion of animal species can reduce the stability of ecological communities. Despite this continued loss, some substantial progress in reversing defaunation is being achieved through the intentional movement of animals to restore populations. We review the full spectrum of conservation translocations, from reinforcement and reintroduction to controversial conservation introductions that seek to restore populations outside their indigenous range or to introduce ecological replacements for extinct forms. We place the popular, but misunderstood, concept of rewilding within this framework and consider the future role of new technical developments such as de-extinction.

Ecological replacement involves the introduction of non-native species to habitats beyond their historical range, a factor identified as increasing the risk of failure for translocations. Yet the effectiveness and success of ecological replacement rely in part on the ability of translocatees to adapt, survive and potentially reproduce in a novel environment. We discuss the welfare aspects of translocating captive-reared non-native tortoises, Aldabrachelys gigantea and Astrochelys radiata, to two offshore Mauritian islands, and the costs and success of the projects to date. Because tortoises are long-lived, late-maturing reptiles, we assessed the progress of the translocation by monitoring the survival, health, growth, and breeding by the founders. Between 2000 and 2011, a total of 26 A. gigantea were introduced to Ile aux Aigrettes, and in 2007 twelve sexually immature A. gigantea and twelve male A. radiata were introduced to Round Island, Mauritius. Annual mortality rates were low, with most animals either maintaining or gaining weight. A minimum of 529 hatchlings were produced on Ile aux Aigrettes in 11 years; there was no potential for breeding on Round Island. Project costs were low. We attribute the success of these introductions to the tortoises' generalist diet, habitat requirements, and innate behaviour. Feasibility analyses for ecological replacement and assisted colonisation projects should consider the candidate species' welfare during translocation and in its recipient environment. Our study provides a useful model for how this should be done. In addition to serving as ecological replacements for extinct Mauritian tortoises, we found that releasing small numbers of captive-reared A. gigantea and A. radiata is cost-effective and successful in the short term. The ability to release small numbers of animals is a particularly important attribute for ecological replacement projects since it reduces the potential risk and controversy associated with introducing non-native species.

Starting in the late 1970s, ecologists began unraveling the role of recently extinct large vettebrates in evolutionary ecology and ecosystem dynamics. Three decades later, practitioners are now considering the role of ecological history in conservation practice, and some have called for restoring missing ecological functions and evolutionary potential using taxon substitutes — extant, functionally similar taxa — to replace extinct species. This pro-active approach to biodiversity conservation has proved controversial. Yet, rewilding with taxon substitutes, or ecological analogues, is now being integrated into conservation and restoration programmes around the world. Empirical evidence is emerging that illustrates how taxon substitutions can restore missing ecological functions and evolutionary potential. However, a major roadblock to a broader evaluation and application of taxon substitution is the lack of practical guidelines within which they should be conducted. While the International Union for Conservation of Nature's reintroduction guidelines are an obvious choice, they are unsuitable in their current form. We recommend necessary amendments to these guidelines to explictly address taxon substitutions. A second impediment to empirical evaluations of rewilding with taxon substitutions is the sheer scale of some proposed projects; the majority involves large mammals over large area. We present and discuss evidence that large and giant tortoises (family Testudinidae) are a useful model to rapidly provide empirical assessments of the use of taxon substitutes on a comparatively smaller scale. Worldwide, at least 36 species of large and giant tortoises went extinct since the late Pleistocene, leaving 32 extant species. We examine the latent conservation potential, benefits, and risks of using tortoise taxon substitutes as a strategy for restoring dyfunctional ecosystems. We highlight how, especially on island, conservation practitioners are starting to employ extant large tortoises in ecosystems to replace extinct tortoises that once played keystone roles.

The extinction of large herbivores, often keystone species, can dramatically modify plant communities and impose key biotic thresholds that may prevent an ecosystem returning to its previous state and threaten native biodiversity. A potentially innovative, yet controversial, landscape-based long-term restoration approach is to replace missing plant-herbivore interactions with non-native herbivores. Aldabran giant (Aldabrachelys gigantea) and Madagascan radiated (Astrochelys radiata) tortoises, taxonomically and functionally similar to the extinct Mauritian giant tortoises (Cylindraspis spp.), were introduced to Round Island, Mauritius, in 2007 to control the non-native plants that were threatening persistence of native species. We monitored the response of the plant community to tortoise grazing for 11 months in enclosures before the tortoises were released and, compared the cost of using tortoises as weeders with the cost of using manual labor. At the end of this period, plant biomass; vegetation height and cover; and adult, seedling, flower, and seed abundance were 3-136 times greater in adjacent control plots than in the tortoise enclosures. After their release, the free-roaming tortoises grazed on most non-native plants and significantly reduced vegetation cover, height, and seed production, reflecting findings from the enclosure study. The tortoises generally did not eat native species, although they consumed those native species that increased in abundance following the eradication of mammalian herbivores. Our results suggest that introduced non-native tortoises are a more cost-effective approach to control non-native vegetation than manual weeding. Numerous long-term outcomes (e.g., change in species composition and soil seed bank) are possible following tortoise releases. Monitoring and adaptive management are needed to ensure that the replacement herbivores promote the recovery of native plants. La extinción de grandes herbívoros, casi siempre especies clave, puede modificar dramáticamente a las comunidades de plantas e imponer umbrales bióticos claves que pueden impedir que un ecosistema regrese a su estado previo y que amenacen a la biodiversidad nativa. Una aproximación potencialmente innovadora, a largo plazo basada en el paisaje, pero controversial, consiste en reemplazar las interacciones herbívoro-planta faltantes con herbívoros no-nativos. Aldabrachelys gigantea y Astrochelys radiata son tortugas terrestres similares funcional y taxonómicamente a la especie extinta Cylindraspis spp., que fueron introducidas a Round Island, Mauricio, en 2007 para controlar a las plantas no-nativas que estaban amenazando la persistencia de especies nativas. Monitoreamos la respuesta de la comunidad vegetal al forrajeo de las tortugas durante 11 meses en encierros antes de que las tortugas fueran liberadas y comparamos el costo de usar a las tortugas como eliminadores de hierbas con el costo de usar labor manual. Al final de este periodo la biomasa vegetal, la altura y cobertura de la vegetación, y la abundancia de adultos, plántulas, flores y semillas eran entre 3-316 veces más grandes en terrenos control adyacentes que en los encierros con tortugas. Después de su liberación, las tortugas forrajearon a casi todas las plantas no-nativas y redujeron significativamente la cobertura de la vegetación, altura y producción de semilla, igualando los resultados del estudio de los encierros. Las tortugas en general no comieron especies nativas aunque sí consumieron aquellas especies nativas cuya abundancia incrementó después de la erradicación de mamíferos herbívoros. Nuestros resultados sugieren que las tortugas no-nativas introducidas son una aproximación más económica que el deshierbe manual para controlar a la vegetación no-nativa. Numerosos resultados a largo plazo (e.g., cambios en la composición de especies y el banco de semillas en el suelo) son posibles una vez liberadas las tortugas. Se requiere de monitoreo y manejo adaptativo para asegurar que los herbívoros de reemplazo promuevan la recuperación de las plantas nativas.

Aim: To test whether ingestion by endemic frugivores differentially affects the seed germination time, germination percentage and seedling survival of endemic, native and exotic fleshy fruited plant species, and to identify the principal processes and attributes driving such effects. Location: Round Island, Mauritius. Methods: We conducted a germination and seedling survival experiment for 3 months to test whether ingestion (gut passage and deposition in faeces) by the endemic Telfair's skink (Leiolopisma telfairii) had a differential effect on the germination time, germination percentage and seedling survival of two endemic, four native and two exotic fleshy fruited plant species. To assess the importance of factors involved in the ingestion process, we used a factorial design with gut passage (gut-passed vs. not gut-passed), depulping (whole fruit vs. manually depulped seed) and the presence of faecal material (faeces vs. without faeces). In addition, the roles of species-specific traits, seed size and deposition density (average number of seeds per faeces) were examined. Results: Exotic species had a higher germination percentage than indigenous (native and endemic) species when not ingested. Following skink ingestion, there was no longer a difference, as ingestion enhanced germination percentage most in endemic species. The exotic species still germinated faster overall than the indigenous species, despite ingestion accelerating the germination time of endemics. However, ingestion strongly reduced seedling survival of the exotic species, while having no negative effect on the survival of indigenous seedlings. Overall, ingested indigenous seeds were more likely to germinate and the seedlings more likely to survive than ingested exotic seeds and seedlings. Seed size, deposition density and the removal of fruit pulp by either manual depulping or gut passage were important predictors of germination time, germination percentage and seedling survival. Main conclusions: These endemic frugivores can enhance the competitiveness of endemic compared with exotic fleshy fruited plants at the critical germination and seedling establishment stage. Consequently, conservation and restoration of mutualistic endemic plant-animal interactions may be vital to mitigating the degradation of habitats invaded by exotic plants, which is of particular relevance for island ecosystems in which large numbers of endemics are threatened by exotic invaders.

Could committing half of Earth's surface to conservation stave off the next mass extinction? Imagine a chain of uninterrupted wild landscapes spanning the savannas and forests of southern and central Africa, the woodlands of Poland and Belarus, and the tropical forests of Central and South America. By committing half of the planet's terrestrial and marine surface to nature, Edward O. Wilson believes that we can save millions of species, as well as humanity. This is Wilson's audacious proposal to stave off the coming biological apocalypse, the sixth mass extinction event, which he outlines in Half-Earth: Our Planet's Fight for Life.

Imagine a chain of uninterrupted wild landscapes spanning the savannas and forests of southern and central Africa, the woodlands of Poland and Belarus, and the tropical forests of Central and South America. By committing half of the planet's terrestrial and marine surface to nature, Edward O. Wilson believes that we can save millions of species, as well as humanity. This is Wilson's audacious proposal to stave off the coming biological apocalypse, the sixth mass extinction event, which he outlines in Half-Earth: Our Planet's Fight for Life. Half-Earth Our Planet's Fight for LifeEdward O. Wilson Liveright, 2016. 261 pp.