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result(s) for
"Invasive plants Seychelles."
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tolerance of island plant-pollinator networks to alien plants
1. Invasive alien plant species pose a severe threat to native plant communities world‐wide, especially on islands. While many studies focus on the direct impact of alien plants on native systems, indirect effects of plant invaders on co‐flowering natives, for example through competition for pollination services, are less well studied and the results are variable. 2. We used six temporally and taxonomically highly resolved plant-pollinator networks from the island of Mahé, Seychelles, to investigate the indirect impact of invasive alien plant species on remnant native plant communities mediated by shared pollinators. We employed fully quantitative network parameters and information on plant reproductive success, and pollinator diversity and behaviour, to detect changes in plant-pollinator networks along an invasion gradient. 3. The number of visits to and fruit set of native plants did not change with invasion intensity. Weighted plant linkage and interaction evenness, however, was lower at invaded sites than at less invaded sites. These patterns were primarily driven by shifts in interactions of the most common pollinator, the introduced honey bee Apis mellifera, while weak interactions and strong native interactions remained unchanged. 4. Synthesis. The implications of these findings are twofold: first, quantitative network parameters are important tools for detecting underlying biological patterns. Secondly, alien plants and pollinators may play a greater role in shaping network structure at high than low levels of invasion. We could not show, however, whether the presence of invasive plants result in a simplification of plant-pollinator networks that is detrimental to native plants and pollinators alike.
Journal Article
Marked differences in foraging area use and susceptibility to predation between two closely-related tropical seabirds
Ecological theory predicts that closely-related species must occupy different niches to coexist. How marine top predators achieve this during breeding, when they often gather in large multi-species colonies and are constrained to central-place foraging, has been mostly studied in productive temperate and polar oceans with abundant resources, but less so in poorer, tropical waters. Here, we track the foraging movements of two closely-related sympatric seabirds—the white-tailed and red-tailed tropicbirds Phaethon lepturus and P. rubricauda—breeding on Aldabra Atoll, Seychelles, to investigate potential mechanisms of niche segregation and shed light on their contrasting population trends. Combining data from GPS, immersion, depth and accelerometry loggers, we show that the two species have similar behaviour at sea, but are completely segregated spatially, with red-tailed tropicbirds flying further to feed and using different feeding areas than white-tailed tropicbirds. Using nest-based camera traps, we show that low breeding success of both species—which likely drives observed population declines—is caused by high nest predation. However, the two species are targeted by different predators, with native avian predators mainly targeting red-tailed tropicbird nests, and invasive rats raiding white-tailed tropicbird nests when they leave their eggs unattended. Our findings provide new insight into the foraging ecology of tropicbirds and have important conservation implications. The extensive range and spatial segregation highlight the importance of considering large-scale protection of waters around tropical seabird colonies, while the high level of nest predation provides evidence in support of rat eradication and investigating potential nest protection from native avian predators.
Journal Article
Wider spectrum of fruit traits in invasive than native floras may increase the vulnerability of oceanic islands to plant invasions
Plant–animal mutualisms such as seed dispersal can play an important role in enabling some species to become invasive. For example, an introduced species could become invasive because birds prefer its fruits to those of native plants. To investigate this possibility, we compared various measures of fruit quality of 22 of the most common native and invasive woody species on the oceanic island Mahé (Seychelles, Indian Ocean). Individual measures of food quality tended to vary much more amongst invasive species than amongst native species; thus, whereas for particular traits the fruits of some invasive species had higher values than any native species, others had relatively low values. However, invasive species consistently produced fruits with a lower water content, resulting in a higher relative yield (i.e. dry pulp weight to total wet fruit weight ratio), and a higher energy content. The fruits of the most abundant invasive tree Cinnamomum verum (Lauraceae) were of particularly high nutritional quality, with individual berries containing 3.5 times more protein and 55 times more lipid than the median values of the native species. We suggest that our results may reflect a general tendency for island plants to produce fruits of low energy content, perhaps reflecting reduced competition for dispersal agents on isolated islands. In addition, we argue that inconsistent results on the relevance of fruit quality for plant invasions reported in the literature may be resolved by comparing the widths of trait spectra for native and alien floras rather than average values. Gaps in the native fruit trait spectrum may provide opportunities for particular invasive species, and weaken the resistance of regional floras to invasions. Such empty niche opportunities may occur for several reasons, including generally broader trait spectra in globally assembled alien than regional native floras (especially on oceanic islands), or the loss of native species due to human activities. More generally, a focus on trait variation rather than average trends may help to advance generalisation in invasion biology.
Journal Article
Late stage dynamics of a successful feral goat eradication from the UNESCO World Heritage site of Aldabra Atoll, Seychelles
Feral goats Capra hircus, considered among the world’s most destructive invasive mammals, were introduced to Aldabra Atoll, a UNESCO World Heritage site in the Seychelles, before 1878. An eradication programme to remove goats from Aldabra was initiated in 1987, after severe ecological impacts were recorded. Eradication and control efforts continued intermittently for the next 20 years, and a final campaign was launched in 2007 using the Judas goat method. We present the methods, eradication dynamics, outcomes and financial costs of the final eradication campaign between 2007 and 2012. This effort was divided into three phases; (1) establishment of Judas goats and intensive hunting (4 months); (2) monitoring of Judas goats (4 years); and (3) Judas goat elimination and verification of success (8 months). In the focal 5-year period, 227 goats were culled (of 2297 across the entire 25-year period); 202 in phase 1, 21 in phase 2, and four remaining Judas goats in phase 3. The eradication was completed and confirmed successful in August 2012, following the use of multiple measures to confirm the absence of goats. The total cost of the eradication was US$ 185,105, an average of US$ 815/goat, or US$ 31/ha. The eradication, although ultimately successful, posed a unique combination of challenges. We discuss key lessons learned and put the project in context of other major island goat eradications. The financial details, context and lessons are expected to be of value to future practitioners.
Journal Article
Herbicide application as a habitat restoration tool: impact on native island plant communities
QUESTIONS: Which physical or chemical removal method is most effective in controlling woody invasive alien plant (IAP) species in sensitive conservation areas, while minimizing the impact on different life stages of native plant communities? Is there a distinct influence of removal method on successional trajectories of post‐treatment seedling communities? LOCATION: Mid‐altitude inselbergs, Mahé, Seychelles, Indian Ocean. METHODS: We conducted two field experiments. First, to test the effects of four IAP control methods (cut, 5%, 10%, 15% Tordon 101 herbicide concentration) on ten woody IAP species, we treated a total of 320 adults and monitored mortality for 18 wk. Second, we set up five blocks each with four plots (4 m × 4 m) at three inselberg sites (N = 60 plots) and recorded the response of seedlings, saplings and adults of all native and IAP species to four Tordon 101 (a.i. picloram and 2,4‐D) herbicide and manual control treatments (cut, cut‐stump, foliar spray, control) over 22 mo. Treatment effects were analysed with GLMM models, and community responses were investigated with NMDS and multivariate ANOVA. RESULTS: The first experiment showed that IAP mortality (98.3%) was similar across all three herbicide concentrations, but markedly higher than mortality of cut plants (42.5%). In the second experiment, abundance, but not diversity, of native seedlings declined with herbicide application in the first 6 mo after treatment. Ordination analysis showed that seedling communities experienced high turnover driven by early successional IAP species and abundance, and that between‐plot variation was determined by site‐specific differences, with only a weak treatment effect. Foliar spray resulted in high mortality of native saplings (52.7%) and adults (18.3%), and both herbicide applications reduced adult growth over a post‐treatment period of 12 mo. CONCLUSION: IAP removal controlled plant invasion processes and altered community composition in the short term, regardless of removal method. Post‐treatment trajectories of seedling communities are determined by site‐specific differences. Chemicals can strongly increase the efficacy of IAP control, but short‐term detrimental effects on native saplings and adults may be expected. As re‐invasion by IAPs is likely, a long‐term restoration plan is required. Management strategies that aim to conserve β‐diversity should employ a landscape approach to habitat restoration to buffer against site idiosyncrasies.
Journal Article
Influence of light and nutrient conditions on seedling growth of native and invasive trees in the Seychelles
Several recent studies have shown that plant invasions can occur in resource-poor and relatively undisturbed habitats. It is, therefore, important to investigate whether and how life-history traits of species invasive in such habitats differ from those of species that are only invasive in disturbed and resource rich habitats. We compared the growth of seedlings of native and invasive tree species from nutrient-poor secondary forests in the tropical Seychelles. We hypothesised that the relative performance of the two groups would change predictably along resource gradients, with native species performing better at low levels of resource availability and invasive species performing better at higher levels. To test this hypothesis, we performed a common garden experiment using seedlings of six invasive and seven native tree species grown under three levels of light (65, 11 and 3.5% of ambient light) and two of nutrients (low and high). Due to large variation among species, differences in growth rates (RGR) were not significant among seedlings of the native and the invasive species. However, seedlings of the invasive species showed higher specific leaf areas (SLA) and higher leaf nutrient contents than seedlings of the native species. They also exhibited greater plasticity in biomass and nutrient allocation (i.e., greater plasticity in LAR, RSR and leaf nutrient contents) in response to varying resource availability. However, differences between the mean values of these parameters were generally small compared with variation within groups. We conclude that successful invaders on nutrient-poor soils in the Seychelles are either stress-tolerant, possessing growth traits similar to those of the native species, or fast-growing but adapted to nutrient-poor soils. In contrast, the more typical, fast-growing alien species with no particular adaptations to nutrient-poor soils seem to be restricted to relative nutrient-rich sites in the lowlands. The finding—that some introduced species thrive in resource-poor habitats—suggests that undisturbed habitats with low resource availability may be less resistant to plant invasions than was previously supposed.
Journal Article
Strong below-ground competition shapes tree regeneration in invasive Cinnamomum verum forests
1 Plant species invading nutrient-poor ecosystems are likely to have their greatest impact on the native plant community by competing for resources below-ground. We investigated how root competition by an invasive tree, Cinnamomum verum, affects regeneration in nutrient-poor tropical secondary forests, in the Seychelles. 2 We performed three trenching experiments to investigate the effects of severing the root systems of mature trees on the growth of juveniles. These experiments had the following objectives: (i) to compare the responses of native and invasive saplings to release from root competition; (ii) to compare how seedlings (< 50 cm tall) and saplings (> 50 cm tall) of C. verum respond to trenching; and (iii) to compare the response of C. verum seedlings to trenching in forest stands with and without C. verum as the dominant species. 3 The results indicate that the dense topsoil root mat produced by mature C. verum trees suppresses the growth of young trees, mainly by increasing competition for scarce nutrients. Growth responses to trenching were stronger for seedlings than saplings, and stronger for juveniles of invasive than of native species. We conclude that stands of C. verum exert a strong below-ground filtering effect on seedling regeneration. This effect is likely to influence secondary forest succession by selectively reducing the establishment of invasive and small-seeded species. 4 Because of the bias in invasion biology towards relatively nutrient-rich, productive ecosystems, few studies have investigated the role of below-ground resource competition in plant invasions. Our results for an infertile, phosphorus-poor ecosystem show that root competition by an alien species can exert a strong influence on forest regeneration. We suggest that this mechanism may be of general importance in nutrient-poor tropical forests invaded by alien tree species.
Journal Article
Impact of a mutualism between an invasive ant and honeydew-producing insects on a functionally important tree on a tropical island
Mutualisms between invasive ants and honeydew-producing Hemiptera have the potential to result in unusually high population levels of both partners, with subsequent major changes to ecosystem composition and dynamics. We assessed the relationship between the invasive ant,
Pheidole megacephala
, and its hemipteran mutualists,
Dysmicoccus
sp. and
Pulvinaria urbicola
, on Cousine Island, Seychelles. We also assessed the impacts of the mutualism on the condition of the hemipteran host plant,
Pisonia grandis
, a native and functionally important tree species. There was a strong positive relationship between
Ph. megacephala
activity and hemipteran abundance, and the exclusion of ants from
Pi. grandis
resulted in a significant decline in
Pu. urbicola
abundance. High abundance of the mutualists was strongly associated with damage to the
Pi. grandis
forest. This indicates that the mutualism is contributing to the massive increase in the population levels of the mutualist species, and is intensifying their impacts on the island. The widespread trophobiosis and its associated high densities of mutualists pose serious threats to the ecosystem, highlighting the need to control the ant and associated hemipteran populations.
Journal Article
Restoration of a tropical island: Cousine Island, Seychelles
Tropical island species and ecosystems are threatened worldwide as a result of increasing human pressure. Yet some of these islands also lend themselves to restoration, as they are physically defined units that can be given focused attention, as long as resources are available and clear conservation targets are set. Cousine Island, Seychelles, is a tropical island that has received such intensive restoration. From a highly degraded island in the 1960s, the island has now been restored to what is believed to be a semblance of the natural state. All alien vertebrates have been eradicated, as have 25 invasive alien plants. Cultivated plants are now confined to one small section of the island. Poaching of nesting marine turtles has been stopped, leading to an increase in turtle breeding numbers. The shearwater population has increased in size with poaching activities under control. The Sooty tern has also returned to the island to breed. The coastal plain has been restored with over 2,500 indigenous shrubs and trees, which have now grown into a forest carpet. There are strict quarantine procedures on the island, keeping it free of rats, mice, various alien invertebrates and potentially invasive alien plants. Three threatened Seychelles endemic land birds (Seychelles warbler, Seychelles magpie robin and Seychelles white-eye) have been introduced and are thriving, with these introductions contributing to both the magpie robin and the white-eye being downgraded from CR to EN (the warbler remains at VU). Ecotourism, and nature conservation for the local inhabitants, have been introduced in a way that does not reduce the improved compositional, structural and functional biodiversity of the island. The result of the restoration effort appears to be sustainable in the long term, although challenges still remain, especially with regards to adequate clean water and a non-polluting power supply on the island. Cousine is thus paving the way in the art and science of tropical island restoration as a legacy for future generations.
Journal Article