Ile Aux Aigrettes Research Articles

Understanding demographic limiting factors to species recovery: Nest-site suitability and breeding ecology of Phelsuma guentheri on Round Island, Mauritius

The island of Mauritius in the Mascarenes has already suffered high levels of extinction, and habitat destruction and invasive alien species have pushed many of the remaining endemic species to the brink of extinction. Round Island, a small island northeast of Mauritius, is home to remnant populations of several reptile species that were once distributed across Mauritius. One of the most threatened of these is Günther’s gecko Phelsuma guentheri, where despite extensive management efforts, population size remains small and below expectation. We identify determinants of nest-site suitability across Round Island, and examine whether suitable nest sites are a limiting factor for the abundance of P. guentheri. Over one breeding season, we recorded and monitored a high number (269) of nest sites harbouring a total of 1475 eggs. We used the number of eggs at each nest, and the hatching success as proxies for nest-site suitability. To study the reproductive behaviour of P. guentheri, we installed trail cameras at three nest sites. We did not find the expected high frequency of use of hardwood trees as nest sites, as found on Ile aux Aigrettes to the southeast of Mauritius, probably because hardwood trees are still relatively sparse and small on Round Island. Instead, the endemic palm Latania loddigesii was the most used nest sites, harbouring 78.8% of all nests with 56.3% of all eggs. Hatching success was high for palm nest sites (90.6%), rock surfaces (91.8%), and on the endemic screw pine Pandanus vandermeeschii (96%), rates that are similar to that reported from hardwood trees on Ile aux Aigrettes (~90%). Communal nesting occurred on all substrates, with up to five individuals using the same nest site. Egg-laying took place at dusk or at night, lasting around 30 min, whereas hatching occurred mostly during the day, lasting 2–107 min. Females laid eggs every 34 days on average, and produced up to four clutches of 1–2 eggs each within the season. We conclude that P. guentheri does not suffer from nest site limitation on Round Island, and that reproductive success is high in terms of both numbers of eggs laid and hatching success, likely due to ongoing habitat restoration. Further research is needed to investigate the factors inhibiting abundance and distribution of P. guentheri on Round Island.

Entangling (non)human isolation and connectivity: island nature conservation on Ile aux Aigrettes, Mauritius

Entangling (non)human isolation and connectivity: island nature conservation on Ile aux Aigrettes, Mauritius

The effects of parameter choice on defining molecular operational taxonomic units and resulting ecological analyses of metabarcoding data

The combination of DNA barcoding and high-throughput (next-generation) sequencing (metabarcoding) provides many promises but also serious challenges. Generating a reliable comparable estimate of biodiversity remains a central challenge to the application of the technology. Many approaches have been used to turn millions of sequences into distinct taxonomic units. However, the extent to which these methods impact the outcome of simple ecological analyses is not well understood. Here we performed a simple analysis of dietary overlap by skinks and shrews on Ile Aux Aigrettes, Mauritius. We used a combination of filtering thresholds and clustering algorithms on a COI metabarcoding dataset and demonstrate that all bioinformatics parameters will have interacting effects on molecular operational taxonomic unit (MOTU) recovery rates. These effects generated estimates covering two orders of magnitude. However, the effect on a simple ecological analysis was not large and, despite the wide variation in estimates of niche overlap, the same ecological conclusion was drawn in most cases. We advise that a conservative clustering programme coupled with larger sequence divergences to define a cluster, the removal of singletons, rigorous length filtering, and stringent match criteria for Molecular Identifier tags are preferable to avoid MOTU inflation and that the same parameters be used in all comparative analyses.

Dietary competition between the alien Asian Musk Shrew (Suncus murinus) and a re‐introduced population of Telfair's Skink (Leiolopisma telfairii)

Re-introduction of rare species to parts of their historical range is becoming increasingly important as a conservation strategy. Telfair's Skinks (Leiolopisma telfairii), once widespread on Mauritius, were until recently found only on Round Island. There it is vulnerable to stochastic events, including the introduction of alien predators that may either prey upon it or compete for food resources. Consequently, skinks have been introduced to Ile aux Aigrettes, another small Mauritian island that has been cleared of rats. However, the island has been invaded by Asian Musk Shrews (Suncus murinus), a commensal species spread by man well beyond its natural Asian range. Our aim was to use next-generation sequencing to analyse the diets of the shrews and skinks to look for niche competition. DNA was extracted from skink faeces and from the stomach contents of shrews. Application of shrew- and skink-specific primers revealed no mutual predation. The DNA was then amplified using general invertebrate primers with tags to identify individual predators, and then sequenced by 454 pyrosequencing. 119 prey MOTUs (molecular taxonomic units) were isolated, although none could be identified to species. Seeding of cladograms with known sequences allowed higher taxonomic assignments in some cases. Although most MOTUs were not shared by shrews and skinks, Pianka's niche overlap test showed significant prey overlap, suggesting potentially strong competition where food resources are limited. These results suggest that removal of the shrews from the island should remain a priority.

The Welfare Implications of Using Exotic Tortoises as Ecological Replacements

BackgroundEcological 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.Methodology/Principal FindingsBecause 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.Conclusions/SignificanceFeasibility 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.

Does a giant tortoise taxon substitute enhance seed germination of exotic fleshy-fruited plants?

Aims The use of exotic species as taxon substitutes to restore lost ecological interactions is currently hotly debated. Aldabrachelys gigantea giant tortoises have recently been introduced to three islands in the Mascarene archipelago (Ile aux Aigrettes, Round Island and Rodrigues) to resurrect herbivory and seed dispersal functions once performed by extinct giant tortoises. However, potential unintended impacts by frugivore substitutes on native ecosystems, e.g. whether they will facilitate the germination of exotic plant species, are largely unknown. We investigated whether A. gigantea introduced to Rodrigues in 2006 could enhance the germination percentage of four widespread fleshy-fruited exotic species on the island. Using germination trials to forecast unintended impacts that could arise from the introduction of a frugivorous taxon substitute enables conservation managers to limit potential adverse negative interactions before they occur. Methods In germination trials that ran over 4 months, we investigated the effects of ingestion (gut passage and deposition in faeces) by subadult and adult A. gigantea on the germination percentage of four exotic fleshy-fruited plant species introduced to Rodrigues. We fed fruits of these plant species to sub-adult and adult A. gigantea to test how variation in age and size of the frugivore would affect seed germination. Feeding of distinctly coloured plastic pellets together with the fruits allowed us to test for individual tortoise effects on seed germination. Important Findings Ingestion by A. gigantea increased the percentage of seeds germinating of Mimusops coriacea and Lantana camara ,b ut not percentage of germination of Veitchia merrillii or Wikstroemia indica. Seeds were more likely to germinate following ingestion by sub-adult rather than adult tortoises, which may be a consequence of the shorter gut passage time observed for sub-adults. Our results demonstrate that introduced frugivorous taxon substitutes could facilitate germination of exotic and invasive plants and highlight the need for conservation managers to weigh the risk of taxon substitutes potentially facilitating the germination and recruitment of exotic fleshy-fruited plants against the benefit of restoring lost seed dispersal functions of threatened indigenous plants. Our findings also highlight the importance of considering age and size variation in frugivores, in particular in long-lived taxa such as giant tortoises, when studying ingestion effects on the germination performance of plants.

Resurrecting Extinct Interactions with Extant Substitutes

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.

Possible control of introduced giant African land snails (Achatina spp.) by the reintroduced endemic skink Leiolopisma telfairii, Ile aux Aigrettes, Mauritius

The giant African land snail (Achatinafulica) is one of the world’s worst invasive species, out‐competing endemic snails, consuming native vegetation and potentially altering nutrient cycles. Attempts to eradicate the snail from islands have only been successful with incipient populations. We present correlative evidence that native island predators may act as an effective control agent for the snail. In 2000 a population of between 37,300 and 45,100 African land snails was estimated on the 26ha nature reserve island of Ile aux Aigrette, Mauritius. Between 2006 and 2007, 260 endemic Telfair’s skink Leiolopisma telfairii were reintroduced to the reserve. Snail population surveys in 2008 and 2009 showed that the introduced snail population had declined to 5,569 (± 3,630) and 6,871 (±5,379), respectively. Previous studies showed that the introduced snails were selective over other invertebrate prey items. We suggest that predation by the endemic skink has been an important causal factor behind the snail population decline.

Rodrigues Island: Hope thrives at the François Leguat Giant Tortoise and Cave Reserve

Rodrigues Island: Hope thrives at the François Leguat Giant Tortoise and Cave Reserve

The untamed shrew: on the termination of an eradication programme for an introduced species

Summary One option in managing introductions of non‐native pest species is to eradicate the introduced species by trapping or otherwise removing individuals. A central issue in undertaking such a programme is deciding when it is reasonable to terminate the programme because all individuals have been removed. This decision requires knowledge of the probability that complete eradication has been achieved. An objective Bayesian method is presented to determine the probability that all individuals of an introduced species have been removed. The method is based on the excess trapping times of individuals beyond a specified threshold, assuming a generalized Pareto distribution for excess trapping time. The method is applied to data from an unsuccessful trapping programme of the Asian musk shrew Suncus murinus on the Mauritian island of Ile aux Aigrettes. The results suggest that, had the method been used, managers would have realized that the probability of complete eradication was not high and premature termination could have been avoided. Synthesis and applications. A central issue facing the manager of an eradication programme for an introduced or pest species is deciding when it is reasonable to terminate the programme because all individuals have been removed. If termination is premature, the programme will fail; if termination is delayed beyond the point of eradication, unnecessary costs will be incurred. The statistical method described here for calculating the probability of complete eradication based on the record of removals provides information that is useful either in an informal assessment of progress or in more formal decision making regarding termination.

The incidence of the parasitic disease trichomoniasis and its treatment in reintroduced and wild Pink PigeonsColumba mayeri

The recovery programme for the endangered Pink PigeonColumba mayeriPrévost, 1843 of Mauritius has involved intensive management to promote rapid population growth. As the Pink Pigeon population has increased in size and distribution, disease problems have emerged that have affected wild and reintroduced birds. Trichomoniasis first appeared in the remnant wild population in 1992, and became established in a reintroduced subpopulation on the offshore island of Ile aux Aigrettes. We aimed to control the disease by treating individuals and, on Ile aux Aigrettes, by routinely treating the population with medicated drinking water. On Ile aux Aigrettes, 49% of squabs were clinically diseased and only 27% of all squabs survived without treatment. Treatment of squabs that were not clinically diseased improved survival from 36 to 82%, suggesting that subclinical infections also affected survival. Subclinical infections were demonstrated by microscopy in 22% of Pink Pigeons and 17% of exotic doves on Ile aux Aigrettes. Clinical disease was observed in 4.5% of adults and 3.1% of juveniles, but more juveniles than adults died from the disease. It is likely that Trichomoniasis and other diseases will continue to affect the Pink Pigeon's recovery and the species may require long‐term management to counteract the effects of disease and other limiting factors.

Mechanisms underlying the failure of an attempt to eradicate the invasive Asian musk shrew Suncus murinus from an island nature reserve

The Asian musk shrew Suncus murinus is an invasive predator that has had a considerable impact on biodiversity. An eradication was attempted from Ile aux Aigrettes, a 25-ha island reserve off the coast of Mauritius as part of a wider restoration program. A total of 759 shrews were removed using up to 1112 live-traps, in six trapping sweeps of the island. After the first four sweeps, in which 705 shrews were caught, complete eradication appeared to have been achieved, with only four shrews caught in the next three months. However, thereafter shrew captures increased and after six months shrews were again being captured in large numbers. We examine three hypotheses for the failure of the eradication program: (i) low efficiency of all traps (‘low efficiency’ hypothesis); (ii) low trap efficiency in certain locations (‘refuge’ hypothesis); and (iii) presence of trap-shy individuals (‘trap-shy’ hypothesis). We constructed an individual-based simulation of the first four trapping sweeps to predict expected patterns of shrew capture for different shrew densities and capture probabilities and in the presence of trapping ‘refugia’. The observed patterns of capture were not consistent with the expected patterns of capture under the ‘low efficiency’ and ‘refuge’ hypotheses, but were intuitively consistent with the ‘trap-shy’ hypothesis. We use the results from this study to help plan future eradication programs for S. murinus.

A test of methods for estimating population size of the invasive land snail Achatina fulica in dense vegetation

Summary Physical inaccessibility often complicates censuses of poorly mobile organisms. We therefore assessed the effectiveness of using a sample of quadrat counts to generate a population estimate corrected for inaccessible areas. The result is directly applicable to management of the introduced snail Achatina fulica on Ile aux Aigrettes, a small island off Mauritius, but also has implications for counting this and similar species elsewhere. Accurate counting of A. fulica is important given that this species is such a widespread and serious pest. Counts were made in 17 quadrats taken from a grid covering the island. These were used to produce one population estimate by interpolating for the rest of the grid using GIS software (method 1). A second estimate assumed equal density of snails in accessible and inaccessible parts of the 17 quadrats, again with the population estimate interpolated (method 2). Four further quadrats were cleared of vegetation and, by comparison of counts before and after clearance, the relationship between initial count and true snail number was estimated. This resulted in two further population estimates, with the relationship used to adjust counts in the 17 experimental quadrats before interpolation (methods 3 and 4). All four estimates were tested using 35 additional quadrats of two types. Type 1 quadrats were physically cleared of vegetation; type 2 quadrats were fully accessible without clearance. Predicted counts in these quadrats were assessed for accuracy by comparison with actual counts. The method 1 estimate was clearly inadequate; method 4 gave a consistent overestimate; method 2 gave the smallest error in both quadrat types. In type 1 quadrats, method 2 and 3 estimates were not significantly different and method 2 had a slight tendency to underestimate. Overall, for studies of A. fulica, method 2 is recommended. However, it should be noted that the study took place towards the end of the wet season. In the dry season, damp refuges under inaccessible vegetation may be more important and methods 3 and 4 may then give a better estimate. The population of A. fulica with shell length > 10 mm on Ile aux Aigrettes near the end of the wet season in 2000 was between 37 300 and 45 100, with 39 700 being the best estimate. The results underline the importance of considering inaccessible areas when accurate counts of species are needed, and a method is suggested by which a simple census technique can be adjusted. In the case of A. fulica, more accurate estimates of population size and distribution are invaluable in the management, monitoring and eradication of this invasive species.

Excess density compensation of island herpetofaunal assemblages

AimSome species reach extraordinary densities on islands. Island assemblages have fewer species, however, and it is possible that island species differ from their mainland counterparts in average mass. Island assemblages could be partitioned differently (fewer species or smaller individuals) from mainland sites without differing in aggregate biomass (density compensation). Our objective was to determine the generality of excess density compensation in island herpetofaunal assemblages.LocationOur bounded removal plot data were obtained from Pacific Island sites (Guam, Saipan and Rota), the West Indies (British Virgin Islands), and the Indian Ocean (Ile aux Aigrettes off Mauritius). The literature values were taken from several locales. Other island locations included Barro Colorado Island, Bonaire, Borneo, Philippine Islands, Seychelle Islands, Barrow Island (Australia), North Brother Island (New Zealand), Dominica and Puerto Rico. Mainland sites included Costa Rica, Ivory Coast, Cameroon, Australia, Thailand, Peru, Brazil, Panama and the USA.MethodWe added our thirty‐nine bounded total removal plots from sixteen island habitats to fifteen literature records to obtain seventy‐five venues with estimable density and biomass of arboreal or terrestrial herpetofaunal assemblages. These biomass estimates were evaluated geographically and in relation to sampling method, insularity, latitude, disturbance regime, seasonality, community richness, vegetative structure and climate. Direct data on trophic interactions (food availability, parasitism and predation pressure) were generally unavailable. Sampling problems were frequent for arboreal, cryptic and evasive species.Results and main conclusionsWe found strong evidence that herpetofaunal assemblages on small islands (mostly lizards) exhibit a much greater aggregate density of biomass (kg ha−1) than those of larger islands or mainland assemblages (small islands show excess density compensation). High aggregate biomass density was more strongly associated with the degree of species impoverishment on islands than it was on island area or insularity per se. High aggregate biomass density was not strongly associated with latitude, precipitation, canopy height or a variety of other physical characteristics of the study sites. The association between high aggregate biomass density and species‐poor islands is consistent with the effects of a reduced suite of predators on depauperate islands, but other features may also contribute to excess density compensation.

Invasion of pollination networks on oceanic islands: importance of invader complexes and endemic super generalists

Abstract. The structure of pollination networks is described for two oceanic islands, the Azorean Flores and the Mauritian Ile aux Aigrettes. At each island site, all interactions between endemic, non‐endemic native and introduced plants and pollinators were mapped. Linkage level, i.e. number of species interactions per species, was significantly higher for endemic species than for non‐endemic native and introduced species. Linkage levels of the two latter categories were similar. Nine types of interaction may be recognized among endemic, non‐endemic native and introduced plants and pollinators. Similar types had similar frequencies in the two networks. Specifically, we looked for the presence of ‘invader complexes’ of mutualists, defined as groups of introduced species interacting more with each other than expected by chance and thus facilitating each other’s establishment. On both islands, observed frequencies of interactions between native (endemic and non‐endemic) and introduced pollinators and plants differed from random. Introduced pollinators and plants interacted less than expected by chance. Thus, the data did not support the existence of invader complexes. Instead, our study suggested that endemic super‐generalist species, i.e. pollinators or plant species with a very wide pollination niche, include new invaders in their set of food plants or pollinators and thereby improve establishment success of the invaders. Reviewing other studies, super generalists seem to be a widespread island phenomenon, i.e. island pollination networks include one or a few species with a very high generalization level compared to co‐occurring species. Low density of island species may lead to low interspecific competition, high abundance and ultimately wide niches and super generalization.

<strong>Bryophytes collected in Mauritius by Gunnar Erdtman</strong>

This small collection contained 8 species of mosses and 3 species of hepatics. Leucobryum boryanum is new to Mauritius but previously known from Réunion. A sample from the seldom visited small island of Ile aux Aigrettes, close to Mauritius, is of particular interest.

A study of the ecological history, vegetation and conservation management of Ile aux Aigrettes, Mauritius

ABSTRACTMuch of the unique native vegetation of Mauritius has been destroyed. Coastal ebony (Diospyros egrettarum I.B.K. Richardson) forest forms an extreme type of Mauritian lowland forest which no longer exists on mainland Mauritius and only survives on one offshore islet, Ile aux Aigrettes. Undisturbed D. egrettarum forest is resistant to invasion by exotic plants, which have now invaded most relict patches of native lowland vegetation in Mauritius. Human disturbance however, has allowed many exotics (particularly Flacourtia indica (Burm. fil.) Merrill) to invade and form new vegetation types. Much of the disturbance was caused by illegal woodcutting up to 1985, prompted by an acute fuelwood shortage in Mauritius. On the basis of 132 4 X 4 m quadrats, we recognize 10 types of natural, semi-natural and exotic vegeta-tion. The conservation of the remaining natural ebony woodland vegetation requires the total cessation of woodcutting and the eradication of Tabebuia pallida (Lindl.) Miers (potentially the most damaging exotic species).