Island rewilding with giant tortoises in an era of climate change

Ecological restoration and rewilding: two approaches with complementary goals?

As we enter the UN Decade on Ecosystem Restoration (2021-2030) and address the urgent need to protect and restore ecosystems and their ecological functions at large scales, rewilding has been brought into the limelight. Interest in this discipline is thus increasing, with a large number of conceptual scientific papers published in recent years. Increasing enthusiasm has led to discussions and debates in the scientific community about the differences between ecological restoration and rewilding. The main goal of this review is to compare and clarify the position of each field. Our results show that despite some differences (e.g. top-down versus bottom-up and functional versus taxonomic approaches) and notably with distinct goals - recovery of a defined historically determined target ecosystem versus recovery of natural processes with often no target endpoint - ecological restoration and rewilding have a common scope: the recovery of ecosystems following anthropogenic degradation. The goals of ecological restoration and rewilding have expanded with the progress of each field. However, it is unclear whether there is a paradigm shift with ecological restoration moving towards rewilding or vice versa. We underline the complementarity in time and in space of ecological restoration and rewilding. To conclude, we argue that reconciliation of these two fields of nature conservation to ensure complementarity could create a synergy to achieve their common scope.

Fine-scale foraging ecology and habitat use of sympatric green and hawksbill turtles in the Western Indian ocean

Using stable isotope analysis of carbon and nitrogen of turtle tissues and putative prey items, we investigated the diet of immature green turtles and hawksbill turtles foraging in the lagoon of Aldabra Atoll, a relatively undisturbed atoll in the southern Seychelles. Aldabra offers a unique environment for understanding sea turtle ecology. Green turtles mostly consumed seagrass and brown algae while hawksbill turtles mainly consumed mangroves and invertebrates. Green turtles showed a dietary shift with size (a proxy for age). There was minimal niche overlap between species and evidence of small-scale foraging site fidelity with turtle tissue reflecting site-specific prey. This highlights the ecological importance of seagrass and mangrove habitats and suggests that turtles play a role in controlling algal biomass at Aldabra. This study is the first to closely examine the foraging ecology of these sympatric turtle species in the Western Indian Ocean, a globally important region for both species.

Defaunation and species introductions alter long-term functional trait diversity in insular reptiles

Biodiversity loss poses a major threat to ecosystem function, which has already been severely impacted by global late-Quaternary defaunation. The loss of mammalian megafauna from many insular systems has rendered reptiles into key modulators of many ecosystem services, such as seed dispersal and pollination. How late-Quaternary extinction events impacted reptile functional diversity remains unclear but can provide critical guidance on traits that render reptiles vulnerable to extinction, as well as anthropogenic, environmental, and evolutionary histories that may promote stability and resilience. This study reconstructs the trajectory of functional diversity change in the Caribbean reptile fauna, a speciose biota distributed over a diverse set of islands with heterogeneous histories of human habitation and exploitation. Human-induced Quaternary extinctions have completely removed key functional entities (FEs)-groupings of species with similar traits that are expected to provide similar ecosystem services-from the region, but functional redundancy on large islands served as a buffer to major functional diversity loss. Small islands, on the other hand, lose up to 67% of their native FEs with only a few exceptions, underscoring the importance of a place's anthropogenic history in shaping present-day biodiversity. While functional redundancy has shielded ecosystems from significant functional diversity loss in the past, it is being eroded and not replenished by species introductions, leaving many native FEs and the communities that they support vulnerable to extinction and functional collapse. This research provides critical data on long-term functional diversity loss for a taxonomic group whose contributions to ecosystem function are understudied and undervalued.

Behavioural Impact of Captive Management Changes in Three Species of Testudinidae

Reptile behaviour and welfare are understudied in comparison with mammals. In this study, behavioural data on three species (Astrochelys radiata, Stigmochelys pardalis, Aldabrachelys gigantea) of tortoises were recorded before and after an environmental change which was anticipated to be positive in nature. The environmental changes differed for each population, but included a substantial increase in enclosure size, the addition of substrate material, and a change in handling procedure. A tortoise-specific ethogram was created to standardise data collection. Focal behaviour sampling was used to collect behavioural data. Changes in the duration of performance of co-occupant interaction and object interaction in the leopard (Stigmochelys pardalis) and Aldabra (Aldabrachelys gigantea) tortoises were observed following the environmental changes. The Shannon–Weiner diversity index did not yield a significant increase after the changes but had a numerical increase which was relatively greater for the leopard tortoise group, which had experienced the greatest environmental change. The leopard tortoises also demonstrated changes in a greater number of behaviours compared to the other species, and this was sustained over the study period. However, this included a behaviour indicative of negative affect: aggression. Whilst we are unable to conclude that welfare was improved by the management changes, there are suggestions that behavioural diversity increased, and some promotion of positive social behaviours occurred.

Lost, gained, and regained functional and phylogenetic diversity of European mammals since 8000 years ago

Mammals have experienced high levels of human-mediated extirpations but have also been widely introduced to new locations, and some have recovered from historic persecution. Both of these processes-losses and gains-have resulted in concern about functional losses and changes in ecological communities as new ecological states develop. The question of whether species turnover inevitably leads to declines in functional and phylogenetic diversity depends, however, on the traits and phylogenetic distinctiveness of the species that are lost, gained, or regained. Comparing ~8000 years ago with the last century, we show that extirpations and range retractions have indeed reduced the functional and phylogenetic diversity of mammals in most European regions (countries and island groups), but species recoveries and the introduction of non-native species have increased functional and phylogenetic diversity by equivalent or greater amounts in many regions. Overall, across Europe, species richness increased in 41 regions over the last 8000 years and declined in 1; phylogenetic diversity increased in 33 and declined in 12, while functional diversity results showed 20 increases and 25 decreases. The balance of losses (extirpations) and gains (introductions, range expansions) has, however, led to net increases in functional diversity on many islands, where the original diversity was low, and across most of western Europe. Historically extirpated mega- and mesofaunal species have recolonized or been reintroduced to many European regions, contributing to recent functional and phylogenetic diversity recovery. If conservation rewilding projects continue to reintroduce regionally extirpated species and domestic descendants of "extinct" species to provide replacement grazing, browsing, and predation, there is potential to generate net functional and phylogenetic diversity gains (relative to 8000 years ago) in most European regions.

The EICAT+ framework enables classification of positive impacts of alien taxa on native biodiversity

Species introduced through human-related activities beyond their native range, termed alien species, have various impacts worldwide. The IUCN Environmental Impact Classification for Alien Taxa (EICAT) is a global standard to assess negative impacts of alien species on native biodiversity. Alien species can also positively affect biodiversity (for instance, through food and habitat provisioning or dispersal facilitation) but there is currently no standardized and evidence-based system to classify positive impacts. We fill this gap by proposing EICAT+, which uses 5 semiquantitative scenarios to categorize the magnitude of positive impacts, and describes underlying mechanisms. EICAT+ can be applied to all alien taxa at different spatial and organizational scales. The application of EICAT+ expands our understanding of the consequences of biological invasions and can inform conservation decisions.

Evaluating unintended consequences of intentional species introductions and eradications for improved conservation management

Increasingly intensive strategies to maintain biodiversity and ecosystem function are being deployed in response to global anthropogenic threats, including intentionally introducing and eradicating species via assisted migration, rewilding, biological control, invasive species eradications, and gene drives. These actions are highly contentious because of their potential for unintended consequences. We conducted a global literature review of these conservation actions to quantify how often unintended outcomes occur and to elucidate their underlying causes. To evaluate conservation outcomes, we developed a community assessment framework for systematically mapping the range of possible interaction types for 111 case studies. Applying this tool, we quantified the number of interaction types considered in each study and documented the nature and strength of intended and unintended outcomes. Intended outcomes were reported in 51% of cases, a combination of intended outcomes and unintended outcomes in 26%, and strictly unintended outcomes in 10%. Hence, unintended outcomes were reported in 36% of all cases evaluated. In evaluating overall conservations outcomes (weighing intended vs. unintended effects), some unintended effects were fairly innocuous relative to the conservation objective, whereas others resulted in serious unintended consequences in recipient communities. Studies that assessed a greater number of community interactions with the target species reported unintended outcomes more often, suggesting that unintended consequences may be underreported due to insufficient vetting. Most reported unintended outcomes arose from direct effects (68%) or simple density-mediated or indirect effects (25%) linked to the target species. Only a few documented cases arose from more complex interaction pathways (7%). Therefore, most unintended outcomes involved simple interactions that could be predicted and mitigated through more formal vetting. Our community assessment framework provides a tool for screening future conservation actions by mapping the recipient community interaction web to identify and mitigate unintended outcomes from intentional species introductions and eradications for conservation.

Galapagos land iguanas as ecosystem engineers

BACKGROUND

Declines of large-bodied herbivorous reptiles are well documented, but the consequences for ecosystem function are not. Understanding how large-bodied herbivorous reptiles engineer ecosystems is relevant given the current interest in restoration of tropical islands where extinction rates are disproportionately high and reptiles are prominent as herbivores.

METHODS

In this study, we measured the ecosystem-level outcomes of long-term quasi-experiment represented by two adjacent islands within the Galapagos Archipelago, one with and the other without Galapagos land iguanas (Conolophus subcristatus), large-bodied herbivores known to feed on many plant species. We characterized plant communities on each island by developing high-resolution (<1 cm²) aerial imagery and delineating extent of plant associations and counting individual plants on each.

RESULTS

In the presence of iguanas there was dramatically less woody plant cover, more area with seasonal grasses, and many fewer cacti. Cacti had a more clumped distribution where iguanas were absent than where iguanas were present.

DISCUSSION

This study provided strong evidence that Galapagos land iguanas can substantially engineer the structure of terrestrial plant communities; therefore, restoration of large-bodied reptilian herbivores, such as land iguanas and giant tortoises, should be regarded as an important component of overall ecosystem restoration, especially for tropical islands from which they have been extirpated.

Cascading Impacts of Seed Disperser Loss on Plant Communities and Ecosystems

Seed dispersal is key to the persistence and spread of plant populations. Because the majority of plant species rely on animals to disperse their seeds, global change drivers that directly affect animals can cause cascading impacts on plant communities. In this review, we synthesize studies assessing how disperser loss alters plant populations, community patterns, multitrophic interactions, and ecosystem functioning. We argue that the magnitude of risk to plants from disperser loss is shaped by the combination of a plant species’ inherent dependence on seed dispersal and the severity of the hazards faced by their dispersers. Because the factors determining a plant species’ risk of decline due to disperser loss can be related to traits of the plants and dispersers, our framework enables a trait-based understanding of change in plant community composition and ecosystem functioning. We discuss how interactions among plants, among dispersers, and across other trophic levels also mediate plant community responses, and we identify areas for future research to understand and mitigate the consequences of disperser loss on plants globally.

Fecal Microbiota Characterization of Seychelles Giant Tortoises (Aldabrachelys gigantea) Living in Both Wild and Controlled Environments

A microbiome is defined as a complex collection of microorganisms and their genetic material. Studies regarding gut microbiomes of different animals have provided ecological and evolutionary information showing a strong link between health and disease. Very few studies have compared the gut microbiota of animals housed under controlled conditions and those in wild habitats. Little research has been performed on the reptile gut microbiota, and what studies do exist are mainly focused on carnivorous reptiles. The aim of this study was first to describe the overall microbiota structure of Aldabra giant tortoises (Aldabrachelys gigantea) and, second, to compare the microbiota of tortoises living under natural conditions and tortoises living in controlled environments, such as zoological and botanical parks, in Italy and in the Seychelles. Seventeen fecal samples were collected from giant tortoises located on Curieuse Island (CI, n = 8), at the Botanical Garden (BG, n = 3) in Mahé (Seychelles Islands) and at Parco Natura Viva-Garda Zoological Park (PNV, n = 6) in Verona (Italy). The V3-V4 region of the 16S rRNA gene was amplified in order to characterize the gut microbiota profile. Overall, the major phyla identified were Bacteroidetes 42%, Firmicutes 32%, and Spirochaetes 9%. A higher microbial diversity (alpha indices) was observed for the BG samples as compared to the PNV samples (Shannon: 5.39 vs. 4.43; InvSimpson: 80.7 vs. 25; Chao1: 584 vs. 377 p < 0.05). The results in the present study showed a significant difference in beta diversity between the samples from CI, BG, and PNV (p = 0.001), suggesting a different bacterial fecal profile of giant tortoises at the different habitats. This study provided novel insights into the effects of different environmental conditions on the gut microbial communities of giant tortoises. In particular, differences were reported regarding the bacterial gut community structure between tortoises in natural and in controlled environments. These results could help to improve the management of giant tortoises under human care, thus enhancing ex-situ conservation efforts far from the species geographic range.

Where to rewild? A conceptual framework to spatially optimize ecological function

Rewilding is an approach aiming at restoring ecosystems to a self-sustaining state by restoring ecological function through active reintroductions or passive management. Locations for most rewilding-through-reintroduction efforts today are selected based on the suitability of the habitat for the reintroduced species, often with little consideration of where the ecological function is most needed. We developed the Spatial Planning of Rewilding Effort (Spore) framework to identify priority locations for rewilding projects through simultaneous consideration of habitat suitability and provisioning of ecological function. We use the island of Guam as a case study for a potential rewilding project, as the island has functionally lost all native seed dispersers as a result of the invasive brown treesnake (Boiga irregularis). The Såli (Micronesian starling, Aplonis opaca) is a good candidate for rewilding to restore ecological function, because it is an effective seed disperser with a localized remnant population. Using Spore, we identify three priority areas for the restoration of seed dispersal, each subdivided into management units. By recognizing the influence of landscape structure and the behaviour of the reintroduced species on the spatial pattern of the function provided by that species, this approach should lead to improved ecological outcomes.