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Current decline of the ‘‘Dodo Tree’’: a case of broken-down interactions with extinct species or the result of new interactions with alien invaders

Authors:
In W. F. Laurance and C.A. Peres. (eds). 2006.
Emerging Threats to TropicalForests. The University
of Chicago University Press, Chicago.
... A decline among smaller diameter woody native plants (>2.5 to <10 cm DBH) is also apparent when comparing with studies done in the 1980s [87,88]. A closer tree by tree monitoring through time between adjacent weeded and nonweeded native forests showed native tree mortality to be increased by the presence of invasive alien plants at both community [89] and population levels [90]. Even native trees that largely overtop the alien plants are dying faster when growing within stands of alien plants [90], indicating that root interactions alone suffice in increasing mortality of native trees. ...
... A closer tree by tree monitoring through time between adjacent weeded and nonweeded native forests showed native tree mortality to be increased by the presence of invasive alien plants at both community [89] and population levels [90]. Even native trees that largely overtop the alien plants are dying faster when growing within stands of alien plants [90], indicating that root interactions alone suffice in increasing mortality of native trees. This elevated native tree mortality is generating an inexorable loss of suitable host plants for epiphytic orchids. ...
... The invasion by alien plants has also been shown to be reducing the extent of flowering of native woody plants [90,94] as well as the production of fruits [90,94,95] hence of seeds too, resulting in a much lowered native plant regeneration in forests invaded by alien plants [93]. Invasive plants also reduce growth rate of native trees [90], which in turn is expected to result in smaller trees with fewer opportunities for epiphytic orchids to grow on, as well as feeding back into reduced seed production and regeneration because smaller trees typically carry fewer fruits. ...
Chapter
Mauritius was one of the last places on Earth to be colonized by humans offering one of the most complete history of what native species occurred originally and what was lost, when, and why. This situation can therefore serve as a laboratory to study human impacts in the current age of human-driven species extinction. Mauritius is also one of the most human-impacted places, thereby reflecting what awaits much of the tropics as human impacts intensify. We used available literature, herbarium samples, and personal observations and studies on the Orchidaceae to characterize its diversity, distribution and ecology, and the human-induced threats they face, to better inform their conservation in Mauritius. There are 91 native orchid species from 30 genera recorded on the island. Twenty species (22%) appear extinct, although some may survive undetected. New species and records continue to be added. Only 10% of the species are endemic to Mauritius, and 80% are unique to the south-west Indian Ocean islands. Most species are epiphytic, and the highest diversity occurs in native forests of the wet uplands. Mauritian orchids, particularly the larger ones, face many threats, some inexorably worsening. There exists much room to improve knowledge about Mauritian orchids that would better inform their conservation which is today still very neglected. This includes taxonomic research, detection of ecological patterns and trends, ecology of the species, as well as quantification and hierarchization of threats to prioritize conservation management. Studying Mauritius native orchids helps understand how devastating, sustained, and accelerating the many threats that human activities pose to orchid biodiversity can be and which await other countries currently less human-impacted than Mauritius.
... A decline among smaller diameter woody native plants (>2.5 to <10 cm DBH) is also apparent when comparing with studies done in the 1980s [87,88]. A closer tree by tree monitoring through time between adjacent weeded and nonweeded native forests showed native tree mortality to be increased by the presence of invasive alien plants at both community [89] and population levels [90]. Even native trees that largely overtop the alien plants are dying faster when growing within stands of alien plants [90], indicating that root interactions alone suffice in increasing mortality of native trees. ...
... A closer tree by tree monitoring through time between adjacent weeded and nonweeded native forests showed native tree mortality to be increased by the presence of invasive alien plants at both community [89] and population levels [90]. Even native trees that largely overtop the alien plants are dying faster when growing within stands of alien plants [90], indicating that root interactions alone suffice in increasing mortality of native trees. This elevated native tree mortality is generating an inexorable loss of suitable host plants for epiphytic orchids. ...
... The invasion by alien plants has also been shown to be reducing the extent of flowering of native woody plants [90,94] as well as the production of fruits [90,94,95] hence of seeds too, resulting in a much lowered native plant regeneration in forests invaded by alien plants [93]. Invasive plants also reduce growth rate of native trees [90], which in turn is expected to result in smaller trees with fewer opportunities for epiphytic orchids to grow on, as well as feeding back into reduced seed production and regeneration because smaller trees typically carry fewer fruits. ...
Chapter
Mauritius was one of the last places on Earth to be colonized by humans offering one of the most complete history of what native species occurred originally and what was lost, when, and why. This situation can therefore serve as a laboratory to study human impacts in the current age of human driven species extinction. Mauritius is also one of the most human-impacted places, thereby reflecting what awaits much of the tropics as human impacts intensify. We used available literature, herbarium samples, and personal observations and studies on the Orchidaceae to characterize its diversity, distribution and ecology, and the human-induced threats they face, to better inform their conservation in Mauritius. There are 91 native orchid species from 30 genera recorded on the island. Twenty species (22%) appear extinct, although some may survive undetected. New species and records continue to be added. Only 10% of the species are endemic to Mauritius, and 80% are unique to the south-west Indian Ocean islands. Most species are epiphytic, and the highest diversity occurs in native forests of the wet uplands. Mauritian orchids, particularly the larger ones, face many threats, some inexorably worsening. There exists much room to improve knowledge about Mauritian orchids that would better inform their conservation which is today still very neglected. This includes taxonomic research, detection of ecological patterns and trends, ecology of the species, as well as quantification and hierarchization of threats to prioritize conservation management. Studying Mauritius native orchids helps understand how devastating, sustained, and accelerating the many threats that human activities pose to orchid biodiversity can be and which await other countries currently less human-impacted than Mauritius.
... One such strategy could be the control of invasive alien species, particularly those that may reduce natural food resources for flying foxes directly (e.g. long-tailed macaques on Mauritius (Baider & Florens, 2006)) or indirectly (for example through foraging habitat degradation). Invasive alien species are major drivers of habitat degradation, particularly on islands (Caujapé-Castells et al., 2010). ...
... Additionally, invasive alien plants can compete with native plants reducing their fitness (Brown & Mitchell, 2001;McKinney & Goodell, 2010;Stinson et al., 2006;Vilà et al., 2011) and may contribute to a major decrease in large tree density (Florens, Baider, Marday et al., 2017). Conversely, habitat restoration by controlling alien plants can increase growth rate, flowering and fruiting of native trees (Baider & Florens, 2006;Monty, Florens & Baider, 2013), leading to increased native tree regeneration and species richness (Baider & Florens, 2011). Notably, this includes species whose fruits are eaten by Pteropus (Florens, Baider, Marday et al., 2017). ...
... G. Krivek, et al. Journal for Nature Conservation 54 (2020) 125805 previously noted in several other species bearing fleshy fruits (Auchoybur & Florens, 2005;Baider & Florens, 2006;Monty, Florens, & Baider, 2013), suggesting that this trend is general. However, other processes may affect this outcome. ...
Article
Full-text available
Flying foxes play keystone ecological roles in plant reproduction. Yet, they face numerous threats, including persecution for eating commercial fruits. This human-wildlife conflict has recently escalated to culling campaigns of a threatened flying fox on Mauritius. Finding non-lethal solutions to this human-wildlife conflict on the island is therefore extremely important. We hypothesized that invasive alien plants may reduce native fruit availability through competition and that weeding alien plants could improve the native foraging habitat quality of flying foxes – in turn, reducing their consumption of commercially important fruits. We compared native fruit production and foraging intensity of the Mauritian flying fox (Pteropus niger) in forests weeded of alien plants a decade previously and adjacent non-weeded forests. Fruits and ejecta were collected weekly during five months under 144 randomly chosen native trees of two canopy species whose fruits are eaten by flying foxes. Intraspecific variations in tree and fruit traits were used to examine flying fox foraging preference. Native fruit production was significantly higher in weeded forests for both tree species, and this was matched by higher flying fox foraging intensity. Flying foxes preferred large trees and fed predominantly on large and ripe fruits. The predominant consumption of ripe fruits emphasizes the importance of flying foxes as seed dispersers. Our results indicate that alien plant invasion substantially reduces native fruit production and that weeded forests provide a much better habitat for flying foxes. Our findings lend support to invasive alien plant control as a management strategy in mitigating such human-wildlife conflicts.
... The few remnants of native Mauritian forests are highly invaded ) and the progressing invasion poses a threat to native plant communities and also to their associated biodiversity Florens et al. 2010). Invasive alien species on Mauritius also cause high mortality among canopy trees used by bats (Baider & Florens 2006), but removal of invasive species reverses this trend (Baider & Florens 2011;Vleut et al. 2013). Weeding out invasive plants improves the fitness of individual trees and of native plant communities by increasing density and species richness of seedlings, individual growth and survival rates, and flower and fruit production of native canopy and understory species (Baider & Florens 2006;Baider & Florens 2011;Florens 2008;Monty et al. 2013). ...
... Invasive alien species on Mauritius also cause high mortality among canopy trees used by bats (Baider & Florens 2006), but removal of invasive species reverses this trend (Baider & Florens 2011;Vleut et al. 2013). Weeding out invasive plants improves the fitness of individual trees and of native plant communities by increasing density and species richness of seedlings, individual growth and survival rates, and flower and fruit production of native canopy and understory species (Baider & Florens 2006;Baider & Florens 2011;Florens 2008;Monty et al. 2013). Increased flower and fruit availability, accessibility and variety of native food plants are positively correlated with fruit bat abundance (Vleut et al. 2013). ...
... Since the studied forests are similar in all aspects except for the management of invasive alien plants, the recorded increased fruit yield of native species is likely because of enhanced plant fitness after weed removal. Indeed, invasive alien plants are known to have a negative impact on the resource acquisition capacity of native trees, ultimately reducing their flower and fruit production (Baider & Florens 2006;Baider & Florens 2011;Monty et al. 2013). As fruit abundance is likely to determine P. niger foraging habitat selection, weeded areas provide a better foraging habitat for fruit bats due to increased native fruit production. ...
Thesis
Full-text available
The last surviving Mauritian fruit bat species, Pteropus niger, although endangered, provides essential seed dispersal and pollination services for the threatened native flora. However, their foraging habitats in native forest remnants are dominated by alien invasive plants and are inhabited by invasive alien animals like long-tailed macaques. Competition for food resources and a decrease of native fruiting trees appear to force fruit bats to top up their diet with cultivated fruits. Consequently, a rapidly escalating conflict with local farmers resulted in a mass cull initiated by the Mauritian government that decimated around 50% of the fruit bat population. Here, I investigate the impact of invasive alien plant control on the foraging habitat quality of fruit bats, by quantifying fruit production and fruit bat foraging intensity in weeded and nonweeded forest areas using ground quadrats. Moreover, I examine how native tree traits and fruit characteristics affect tree and fruit selection of flying foxes and long-tailed macaques. Weeding significantly increased both native fruit production and fruit bat foraging intensity. Fruit bats preferred to forage in weeded areas on tall trees with a large crown. In addition, fruit bats preferred large and ripe fruits, while long-tailed macaques more often used small unripe fruits. Peak of native fruit use by macaques coincided with lower use by fruit bats, suggesting resource competition. The results suggest that weeded forests provide a better foraging habitat for P. niger than non-weeded areas, and emphasize the importance of weeding for the recovery of native plant communities and their associated biodiversity. I conclude that weeding and controlling of the introduced long-tailed macaque population may be solutions to mitigate the current humanwildlife conflict. In the long term, failure to improve native foraging habitats of fruit bats is likely to increase their extinction risk on Mauritius.
... In this case, i.e. "hand-cleaning of seeds", germination can be seen, but in the case of "flesh persistence" modality, only the emergence of seedlings from the fruit can be properly assessed. Photos: A. Gorissen Seed and fruit predation by Rattus rattus have already been reported for Sapotaceae seeds in the Mascarenes (Baider and Florens, 2006;Eric Rivière, pers. comm.), that is why we took into account the potential impact of this introduced seed predator. ...
... Thus, the reward that has been selected to effectively disperse large seeds in a highly competitive environment via frugivory (Eriksson et al., 2000), may become alone a major handicap for large-seeded plants when mutualist partners disappear from the ecosystem. However, our results are not in line with those of Baider and Florens (2006) that claim seed cleaning to have no effect on the germination rate of Sideroxylon grandiflorum (Sapotaceae) on Mauritius. Even if their weak sampling (only 32 fruits) may considerably limit the scope of their results, the negative impact of flesh persistence might be all the more important as the ratio of pulp to seed is high. ...
Thesis
Full-text available
Les forêts tropicales sont largement dominées par les plantes à fruits charnus dont la dispersion est assurée par les vertébrés frugivores. L'effondrement global des grands vertébrés interroge donc quant à la résilience de ces écosystèmes, en particulier dans les îles qui concentrent l’essentiel des extinctions documentées. Les Mascareignes sont un remarquable système d'étude des ruptures d'interactions de frugivorie car la faune d'origine, pléthorique jusqu'à la colonisation humaine au 17ème siècle et aujourd'hui largement éteinte, est bien connue tout comme sa flore diversifiée qui compte parmi les plus menacées. La Réunion abrite encore des forêts indigènes le long de puissants gradients environnementaux et un volcanisme actif offrant l'opportunité d'explorer sur le long terme les conséquences de la défaunation. De plus, les niveaux variables d'extinction de vertébrés forestiers entre La Réunion (principal frugivore relictuel, masse=55 g) et Maurice (450 g) permet d'utiliser ces îles comme pseudo-réplicats pour tester diverses hypothèses. Cette thèse s'organise en trois parties qui visent à (1) décrire les patrons de distribution spatiale des traits de dispersion à La Réunion et Maurice, et comprendre les implications pour l'extinction de la faune qui a été fulgurante à La Réunion ; (2) évaluer les conséquences de la rupture des interactions de frugivorie sur la reconstruction des écosystèmes forestiers sur les coulées de lave du Piton de la Fournaise ; (3) évaluer les conséquences de la rupture des interactions de frugivorie sur le maintien de la diversité dans les forêts de l'archipel établies avant la colonisation humaine. (1) Les proportions de plantes à fruits charnus dans les communautés de plantes ligneuses chutent avec l'altitude et cette diminution est d'autant plus forte que les fruits sont gros. En comparant les principaux facteurs d'extinction de vertébrés entre La Réunion et Maurice, nous montrons que la destruction précoce des habitats favorables de basse altitude à La Réunion a probablement joué un rôle central dans la fulgurance des extinctions. (2) Après avoir étoffé la chronoséquence des coulées du Piton de la Fournaise, nous montrons que la disparition des populations de frugivores a profondément altéré la capacité des forêts de basse altitude à se rebâtir dès le 18ème siècle et que la refaunation des écosystèmes avec des frugivores introduits profite essentiellement aux plantes exotiques à fruits charnus. Néanmoins, en restaurant la dispersion, les plantes à grosses graines sont capables de s'installer sur les coulées historiques où recrutent très majoritairement des plantes envahissantes. (3) En comparant les forêts de référence de La Réunion et Maurice, nous montrons que la roussette noire permet un bien meilleur recrutement de nombreuses espèces ligneuses à Brise-Fer que le bulbul de La Réunion à Mare-Longue, excepté pour les plantes à grosses graines qui se régénèrent assez mal dans les deux îles. Une expérimentation à Mare-Longue montre enfin comment la persistance de la pulpe seule peut limiter fortement le recrutement, mais que ce dernier peut être notablement influencé par la faune introduite. Nos résultats inquiétants montrent l'urgence de protéger les grands frugivores indigènes où ils existent encore et de favoriser leur retour quand ils ont disparu. Parallèlement, des semis à large échelle devraient être envisagés dans les aires protégées où le maintien, voire le retour de la dynamique forestière indigène sont impératifs.
... However, hundreds of hectares of native forests were cleared of invasive alien plants since early 1990s, much of which coincides with the bird's distribution. Such a conservation measure should have elicited population recovery for a bird like H. olivaceus given its diet of fruits, nectar and insects (Safford, 1996) as alien plant control leads to greater abundances of those resources (Baider & Florens, 2006;Florens, Mauremootoo, Fowler, Winder, & Baider, 2010;Monty, Florens, & Baider, 2013). Therefore, some other compensatory or limiting factor appears to be preventing recovery of the bird population. ...
... For example, invasive alien plants like the strawberry guava (Psidium cattleyanum Sabine), an invader on many other tropical islands worldwide (Tuler, Proença, Carrijo, & Peixoto, 2018), reaches high densities in most wet native vegetation of Mauritius , including all current localities of R. simplex (apart from Pétrin where it has been weeded). These alien plants are known to reduce native plant flowering and fruiting (Baider & Florens, 2006;Monty et al., 2013), tree density and regeneration (Baider & Florens, 2011) and are thus likely to be negatively impacting R. simplex. ...
Article
Oceanic islands harbour a disproportionately large share of extinct and endangered birds worldwide and up to about 6,800 highly threatened plants, stressing the urgency for conservation efforts there. However, effective conservation action can only be as sound as the ecological understanding on which it is based. Knowledge about the ecology of threatened birds and plants can be relatively sketchy even in well-studied oceanic islands and this can potentially misdirect or erode conservation actions’ effectiveness. We used camera traps to document vertebrate flower visitors of a threatened, mono-specific endemic oceanic island plant (Roussea simplex) that produces much nectar and which was abundant until the 1930s before declining severely despite its presence mostly within protected areas. We determined proportions of native and alien flower visitors in four populations and characterised their ecological role (e.g. florivore, nectar robber, pollinator) through observations and exclusion experiments alongside experiments to determine seed sets by agamospermy, autogamy, geitonogamy and xenogamy. Five native and three alien vertebrate species visited flowers (N = 5,085 camera trap-hours), 96.6% of visits being from birds. Among endemics, 74-96% of visits were by the Mauritius Bulbul (Hypsipetes olivaceus), a threatened bird able to effect pollination contrary to the other endemic birds. Roussea simplex is primarily xenogamous, producing 2657 ± 480 seeds, and seed set dropped markedly when the bird was excluded (861.8 ± 91.0 SE, Kruskal-Wallis χ2 = 14.2, p < 0.001). Natural seed set was very low (410.0 ± 85.3 SE) where the bird was locally extinct or very rare. Invasive alien rats (Rattus rattus) and long-tailed macaques (Macaca fascicularis) were important florivores or nectar robbers. Systematic non-intrusive study using camera traps combined with manipulative experiments revealed a mutualism between two relatively well-studied threatened endemic species as well as new threats from alien vertebrates acting as nectar robbers and florivores. Roussea simplex’s major decline within protected areas and its abundant and year round flowering and nectar production point to a major hitherto unrecorded drop in floral resource previously available to at least five endemic species, and particularly to its commonest flower visitor and principal pollinator, the threatened Mauritius Bulbul. These findings exemplify how systematic non-intrusive study of threatened species may radically change conservation managers’ priorities which in the current case should focus primarily on controlling alien rats and macaques and re-instating or reinforcing Bulbul-Roussea mutualism as each would be more impactful than addressing gecko-Roussea mutualism disruption by alien ants which so far was the only recorded threat thought to drive the rapid decline of Roussea simplex. Our study underscores that current conservation efforts should also pay particular attention to medium to longer-term changes in habitat or community composition which may not be obvious from merely considering extent and composition of current habitats.
... The main threat in Mauritius to these CWR is the deleterious effects of invasive alien species, especially strawberry guava, P. cattleyanum, through competition, and by introduced animals such as rats and macaques that destroy large numbers of flowers, fruits and seeds of native species (e.g. Baider and Florens, 2006;Monty et al., 2013;Florens, 2015). Other additional specific threats for some species as Elaeocarpus bojeri R.E. ...
Article
Full-text available
The rising need for crop diversification to mitigate the impacts of climate change on food security urges the exploration of crop wild relatives (CWR) as potential genetic resources for crop improvement. This study aimed at assessing the diversity of CWR of the Indian Ocean islands of Mauritius and Rodrigues and proposing cost-effective conservation measures for their sustainable use. A comprehensive list of the native species was collated from The Mauritius Herbarium and published literature. Each species was assessed for the economic value of its related crop, utilization potential for crop improvement, relative distribution, occurrence status and Red List conservation status, using a standard scoring method for prioritization. The occurrence data of the priority species were collected, verified, geo-referenced and mapped. A total of 43 crop-related species were identified for both islands and 21 species were prioritized for active conservation. The CWR diversity hotspots in Mauritius included Mondrain, followed by Florin and Le Pouce Mountain. Although a wide diversity of CWR has been recorded on both islands, most do not relate to major economic crops in use, therefore only a few species may be gene donors to economic crops at the regional and global level. For example, coffee, a major global beverage crop, has three wild relatives on Mauritius, which could potentially be of interest for future predictive characterization.
... D'autre part, le contrôle des plantes exotiques semble secondairement contribuer à l'augmentation de la population des geckos. Des recherches ont montré que dans les forêts mauriciennes où les plantes exotiques ont été contrôlées (comme ce qui a été entrepris à l'Île aux Aigrettes), les composantes 1 de la forêt liées à l'alimentation des Phelsuma, ont été améliorées (Baider & Florens, 2006 ;Monty et al., 2013). Un véritable réseau trophique peut alors se mettre en place avec des cascades dans les consommateurs dans lequel la prolifération des fleurs engendre une abondance de nectar, qui lui même dynamise les populations d'insectes constituant les proies des geckos (Florens et al., 2010 ;Imlay et al., 2012 ;Hagey et al., 2016). ...
Article
Full-text available
Abstract: With the eradication of introduced predators (Feral cat, Felis catus ; Black rat, Rattus rattus), but also with the control of exotic plants and their replacement by native plants, the population of Ornate Day Gecko, Phelsuma ornata, a species endemic to Mauritius, has increased significantly on Île aux Aigrettes. Although the population now appears safe, a monitoring is advisable because the threat of exotic predators or disease could rapidly change the situation. Résumé : Après l’éradication des prédateurs introduits, (Chat haret : Felis catus ; Rat noir : Rattus rattus), puis le contrôle des plantes exotiques et leur remplacement par des plantes indigènes, la population du Gecko orné, Phelsuma ornata Gray, 1825, une espèce endémique de l’Île Maurice a nettement progressé sur l’Île aux Aigrettes. La survie de ce gecko semble aujourd’hui assurée mais une surveillance est recommandée, car la possibilité d’un retour potentiel des prédateurs ou l’introduction de maladies exotiques reste possible.
Article
Bats provide important pollination and seed-dispersal services to native angiosperms. However, many bat species are increasingly threatened by human disturbance, including the Mauritian flying fox (Pteropus niger), an endemic, keystone seed disperser. Native forests are scarce and P. niger frequently feeds in commercial plantations, where it now is considered a pest and subjected to frequent culling, thereby hindering conservation efforts. The invasive long-tailed macaque (Primates: Cercopithecidae, Macaca fascicularis) potentially competes with P. niger for scarce native fruits. We investigated the extent of dietary overlap between M. fascicularis and P. niger on Mauritius by sampling fruit drop for 17 tree species and identifying additional food species along line transects. Fruits of 13 of 17 species were eaten by animals and fruit production across tree replicates generally was low but highly variable. Although M. fascicularis ate only 4% of fruit overall, they consumed 20–100% of the fruits of seven species. Approximately 39% of dropped fruits were intact; based on field observations, most probably were dropped by M. fascicularis. Unlike P. niger, M. fascicularis ate mostly unripe fruit and depleted all fruit of certain species at an unripe stage. Hence, M. fascicularis may restrict P. niger’s diet and potentially disrupt seed dispersal of some tree species. Furthermore, small trees are more prone to fruit depletion at an unripe stage by macaques. In addition, asynchronous fruiting phenology across forest fragments may modulate the provision of native fruits to P. niger throughout the year. Although competition can be demonstrated only by controlled experimental studies that are logistically impossible in our scenario, our results highlight potential detrimental consequences that introduced frugivores may have on keystone seed dispersers. Finally, our results suggest that a more integrative and island-wide approach to forest restoration may be valuable for the conservation of P. niger.
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