Avian translocations restore dual interaction networks in an island ecosystem

Many island bird species have been driven to extinction by introduced predators. Although poorly understood, these extinctions could have a 2-fold impact on bird–plant mutualisms, because island bird species can serve as both pollinators and seed dispersers. We investigated how avian translocations into a mammal-free reserve in New Zealand affected the structure of bird–flower and bird–fruit interactions. We observed bird–fruit and bird–flower interactions over a 9-year period to establish (1) the extent to which native birds are both nectivorous and frugivorous (i.e. “dual mutualists”) and (2) how avian translocations and conservation reestablished nectivory and frugivory networks. Results showed that all but one native bird species were dual mutualists. Pairwise species interaction frequencies were positively correlated between networks. However, overall levels of nectivory by each bird species were unrelated to levels of frugivory. Interaction specialization and species strength also did not differ between networks. The reintroduction of threatened and endangered bird species appeared to have restored both interaction networks, and the sequence of species recovery accelerated restorative changes. Overall results indicate that not only does the extinction of dual mutualists have a 2-fold, negative effect on mutualistic interactions with plants, they can also accelerate the recovery of ecosystem services following restoration efforts.     

Differences in behavioural traits among native and introduced colonies of an invasive ant

Identifying the factors that promote the success of biological invasions is a key pursuit in ecology. To date, the link between animal personality and invasiveness has rarely been studied. Here, we examined in the laboratory how Argentine ant populations from the species’ native and introduced ranges differed in a suite of behaviours related to species interactions and the use of space. We found correlations among specific behavioural traits that defined an explorative-aggressive syndrome. The Main “European” supercolony (introduced range) more readily explored novel environments, displayed more aggression, detected food resources more quickly, and occupied more space than the Catalonian supercolony (introduced range) and two other Argentine supercolonies (native range). The two native supercolonies also differed in their personalities; one harbouring the less invasive personality, while the other is intermediate between the two introduced supercolonies. Therefore, instead of a binary pattern, Argentine ant supercolonies display a behavioural continuum that is independent on their geographic origin (native/introduced ranges). Our results also suggest that variability in personality traits is correlated to differences in the ecological success of Argentine ant colonies. Differences in group personalities may facilitate the persistence and invasion of animals under novel selective pressures by promoting adaptive behaviours. We stress that the concept of animal personality should be taken into account when elucidating the mechanisms of invasiveness.     

Spatial dynamics of supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes, on Christmas Island, Indian Ocean

Key to the management of invasive species is an understanding of the scope of an invasion, the rate of proliferation and the rate at which invaded habitats become degraded. This study examines the spatial dynamics of high-density supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes , on Christmas Island, Indian Ocean, and the associated impacts at their boundaries. Since the early 1990s, A. gracilipes supercolonies have occupied over 30% of the 10,000 ha of rainforest on Christmas Island. Thirty-four discrete high-density supercolonies formed between 1989 and 2003, ranging in size across nearly three orders of magnitude from 0.9 to 787 ha. Supercolonies boundaries are diffuse, and ants were observed in low densities in some cases up to 200 m from the main high-density supercolony. The 13 boundaries examined were all dynamic over a 10–20 observation month period: nine boundaries expanded, and the maximum rate of spread was 0.5 m day⁻¹. Across boundary transition zones, between high-density supercolonies and intact rainforest, yellow crazy ants reduced other ant species richness, occupied red crab burrows and killed resident red crabs, which was the trigger for 'invasional meltdown' on Christmas Island. The highly variable and unpredictable nature of A. gracilipes boundaries poses a challenge for incorporation into a predictive framework, as well as for their management.     

Supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes, on an oceanic island: Forager activity patterns, density and biomass

Summary. Ants have the capacity to reach unusually high densities, mostly in their introduced ranges. Numerical dominance is often cited as key to the ability of exotic ants to displace native ant species, reduce the abundance of invertebrates and negatively impact upon bird, land crab and other vertebrate populations. On Christmas Island, Indian Ocean, the yellow crazy ant, Anoplolepis gracilipes (Jerdon), forms supercolonies, where extremely high densities of foraging ants have contributed to ‘invasional meltdown’ in rainforest areas. Densities of up to 2254 foraging ants per m² and a biomass of 1.85 g per m² were recorded, and nest densities reached 10.5 nest entrances per m². Populations of A. gracilipes can overcome and kill red endemic land crabs (Gecarcoidea natalis) over 100 times their own biomass. This is the highest recorded density of foraging ants, and adds another element to the definition of ‘supercolony’ of unicolonial ants. This paper documents one extreme in a continuum of densities of unicolonial, invasive ant species and highlights the need to incorporate forager densities into invasive ant research.Received 17 November 2004; revised 14 February 2005, accepted 21 February 2005.     

Implications of newly-formed seed-dispersal mutualisms between birds and introduced plants in northern California,USA

I examined the role of bird dispersal in invasiveness of three non-native plant species in California, USA: Triadica sebifera, Ligustrum lucidum, and Olea europaea. I selected these species because their invasiveness in California is uncertain, but a survey of ornithologists highlighted them as likely bird-dispersed. I quantified bird frugivory of these plants, compared them with a native species (Heteromeles arbutifolia), and explored the management implications of dispersal mutualisms for these and other incipient invasive plants. Fruit removal by birds was sufficient to permit spread for all study species. Seed dispersers (rather than seed predators) and pulse feeders (flocking species with potential for long distance dispersal) performed most fruit removal for the non-native species, a pattern indicative of an effective dispersal regime. The number of fruiting trees per stand was a significant predictor of bird visitation. Founding population size may thus be important in management of invasive, bird-dispersed plants. Disperser-defined niches were relatively narrow because a few disperser species performed the majority of fruit removal from study trees, but each fruit species was consumed by a variety of potential dispersers. This results in strong pairwise niche overlap between some plant species. Ordinated by bird use, study site-species combinations clustered more by geographic location than by plant species, emphasizing the opportunistic nature of bird foraging. None of the non-native focal plant species appears dispersal limited, and all have formed novel mutualisms in California. It is possible that these plants are now in lag phases preceding bird-mediated invasion. Consideration of bird dispersal when evaluating invasiveness is therefore an imperative.     

Seed dispersal of fleshy-fruited invasive plants by birds: contributing factors and management options

The ecology of seed dispersal by vertebrates has been investigated extensively over recent decades, yet only limited research has been conducted on how suites of invasive plants and frugivorous birds interact. In this review, we examine how plant fruit traits (morphology, colour and display, nutritional quality, accessibility and phenology), avian traits (fruit handling techniques, gut passage time and effect, bird movements and social behaviour and dietary composition) and landscape structure (fruit neighbourhood, habitat loss and fragmentation and perch tree effects) affect frugivory and seed dispersal in invasive plants. This functional approach could be used to develop generic models of seed dispersal distributions for suites of invasive plant species and improve management efficiencies. Four broad research approaches are described that could direct management of bird‐dispersed invasive plants at the landscape scale, by manipulating dispersal. First, research is needed to quantify the effect of biological control agents on dispersal, particularly how changes in fruit production and/or quality affect fruit choice by frugivores, dispersal distributions of seed and post‐dispersal processes. Second, we explore how seed dispersal could be directed, such as by manipulating perch structures and/or vegetation density to attract frugivorous birds after they have been foraging on invasive plant fruits. Third, the major sources of seed spread could be identified and removed (i.e. targeting core or satellite infestations, particular habitats and creating barrier zones). Fourth, alternative food resources could be provided for frugivores, to replace fruits of invasive plants, and their use quantified.     

Diets of fruit-eating birds: what are the causes of interspecific differences?

Differences in fruit choice among the bird species of a Spanish shrubland were related to the size of fruits and to the lipid content of pulp. Lipid-rich fruits were selected by the bird species with slower food passage rates through the digestive tract. These bird species also fed frequently on insects and seeds. Bird species with faster food passage rates fed less on insects and seeds, and ate mainly fruits with pulp poor in lipids (rich in sugars and water). Studies of digestion in birds indicate that lipids require slower food passage rates for efficient digestion and intestinal transport than simple sugars. The available evidence indicates that the European bird species that show stronger preferences for lipid-rich fruits are no better as seed dispersers, from the point of view of the plants, than species choosing lipidpoor fruits. Thus, the degree of frugivory of birds, their fruit choice patterns and their effects on seed dispersal do not seem to be related to each other in the ways expected by the early models of the evolution of fleshy fruits.     

Can Functional Traits Predict Ecological Interactions? A Case Study Using Rain forest Frugivores and Plants in Australia

In rain forest, the large numbers of species of fleshy-fruited plants and frugivorous animals result in a large number of potential fruit–frugivore interactions, which are challenging to survey in the field. Yet, knowledge of these relationships is needed to predict consequences of changes in the frugivore assemblage for seed dispersal. In the absence of comprehensive dietary information, it may be possible to delineate between frugivores that disperse different plants using ‘functional traits,’ or morphological and behavioral attributes of frugivores that interact with differences in salient characteristics of plant species. Here we use data on the consumption of 244 Australian rain forest plant species by 38 bird species to test for associations between patterns of frugivory and birds': (1) degree of frugivory, (2) gape width, and (3) seed treatment (seed crushing or seed dispersing). Degree of frugivory and gape width explain 74 percent of the variation in the sizes of fruits consumed by frugivorous birds. Among birds that consume a substantial dietary proportion of fruit, birds with wider gapes consume larger fruits. In contrast, this relationship was not shown by birds for which fruit is only a minor dietary component. Degree of frugivory and gape width, together with seed treatment, also strongly predict the overall taxonomic composition and diversity of plants consumed by bird species. Functional classifications of frugivore species may prove useful in developing a predictive understanding of fruit–frugivore interactions in other rain forest regions where detailed dietary information is not available for most frugivores.     

Introduction of mammalian seed predators and the loss of an endemic flightless bird impair seed dispersal of the New Zealand tree Elaeocarpus dentatus

Understanding the mutualistic services provided by species is critical when considering both the consequences of their loss or the benefits of their reintroduction. Like many other Pacific islands, New Zealand seed dispersal networks have been changed by both significant losses of large frugivorous birds and the introduction of invasive mammals. These changes are particularly concerning when important dispersers remain unidentified. We tested the impact of frugivore declines and invasive seed predators on seed dispersal for an endemic tree, hinau Elaeocarpus dentatus, by comparing seed dispersal and predation rates on the mainland of New Zealand with offshore sanctuary islands with higher bird and lower mammal numbers. We used cameras and seed traps to measure predation and dispersal from the ground and canopy, respectively. We found that canopy fruit handling rates (an index of dispersal quantity) were poor even on island sanctuaries (only 14% of seeds captured below parent trees on islands had passed through a bird), which suggests that hinau may be adapted for ground‐based dispersal by flightless birds. Ground‐based dispersal of hinau was low on the New Zealand mainland compared to sanctuary islands (4% of seeds dispersed on the mainland vs. 76% dispersed on islands), due to low frugivore numbers. A flightless endemic rail (Gallirallus australis) conducted the majority of ground‐based fruit removal on islands. Despite being threatened, this rail is controversial in restoration projects because of its predatory impacts on native fauna. Our study demonstrates the importance of testing which species perform important mutualistic services, rather than simply relying on logical assumptions.     

The worldwide expansion of the Argentine ant

Aim The aim of this study was to determine the number of successful establishments of the invasive Argentine ant outside native range and to see whether introduced supercolonies have resulted from single or multiple introductions. We also compared the genetic diversity of native versus introduced supercolonies to assess the size of the propagules (i.e. the number of founding individuals) at the origin of the introduced supercolonies. Location Global. Methods We used mitochondrial DNA (mtDNA) markers and microsatellite loci to study 39 supercolonies of the Argentine ant Linepithema humile covering both the native (n = 25) and introduced range (n = 14). Results Data from three mitochondrial genes and 13 nuclear microsatellites suggest that the introduced supercolonies studied originated from at least seven founding events out of the native area in Argentina (primary introductions). The distribution of mtDNA haplotypes also suggests that supercolonies in the introduced range each derive from a single source supercolony and that one of these source supercolonies has been particularly successful, being the basis of many introduced populations spread across the world. Comparison of the genetic diversity of supercolonies based on the five most diverse loci also revealed that native and introduced supercolonies have greatly overlapping ranges of diversity, although the genetic diversity is on average less in introduced than in native supercolonies. Main conclusions Both primary introductions (from the native range) and secondary introductions (from sites with established invasive supercolonies) were important in the global expansion of the Argentine ant. In combination with the similar social organization of colonies in the native and introduced range, this indicates that invasiveness did not evolve recently as a unique and historically contingent event (e.g. reduction of genetic diversity) in this species. Rather, native L. humile supercolonies have characteristics that make them pre‐adapted to invade new – and in particular disturbed – habitats when given the opportunity. These results have important implications with regard to possible strategies to be used to control invasive ants.     

Consistency and reciprocity of indirect interactions between tree species mediated by frugivorous birds

Despite the recognized importance of indirect plant–plant interactions for community structure, we still need to improve our current knowledge on how their outcomes are consistent in space and time, as well as reciprocal between participating species. These caveats are especially relevant in the case of indirect interactions mediated by animals, whose behavior may show high variability. We studied consistency and reciprocity of frugivore‐mediated interactions between fleshy‐fruited trees. For three years we examined the influence of crop size and neighborhood characteristics (con‐ and heterospecific fruit abundance and forest cover) on frugivory rates on Crataegus monogyna and Ilex aquifolium, two coexisting species in the secondary forests of the Cantabrian range that share a guild of frugivorous birds. Crop size and neighborhood characteristics influenced frugivory on C. monogyna and I. aquifolium. Both con‐ and heterospecific fruit abundance affected frugivory, evidencing the occurrence of indirect interactions between trees, although the strength and sign of these effects varied between tree species as well as across years within species. By showing complex temporal patterns in the consistency and reciprocity of indirect interactions, this study emphasizes the need for multispecific, long‐term studies to assess the actual contribution of animal‐mediated plant–plant indirect interactions to community dynamics.     

Frugivory and seed dispersal by vertebrates in the Oriental (Indomalayan) Region

Current knowledge of frugivory and seed dispersal by vertebrates in the Oriental Region is summarized. Some degree of frugivory has been reported for many fish and reptile species, almost half the genera of non-marine mammals and more than 40% of bird genera in the region. Highly frugivorous species, for which fruit dominates the diet for at least part of the year, occur in at least two families of reptiles, 12 families of mammals and 17 families of birds. Predation on seeds in fleshy fruits is much less widespread taxonomically: the major seed predators are colobine monkeys and rodents among the mammals, and parrots, some pigeons, and finches among the birds. Most seeds in the Oriental Region, except near its northern margins, are dispersed by vertebrate families which are endemic to the region or to the Old World. Small fruits and large, soft fruits with many small seeds are consumed by a wide range of potential seed dispersal agents, including species which thrive in small forest fragments and degraded landscapes. Larger, bigger-seeded fruits are consumed by progressively fewer dispersers, and the largest depend on a few species of mammals and birds which are highly vulnerable to hunting, fragmentation and habitat loss.     

Across islands and continents,mammals are more successful invaders than birds

Many invasive species cause ecological or economic damage, and the fraction of introduced species that become invasive is an important determinant of the overall costs caused by invaders. According to the widely quoted tens rule, about 10% of all introduced species establish themselves and about 10% of these established species become invasive. Global taxonomic differences in the fraction of species becoming invasive have not been described. In a global analysis of mammal and bird introductions, I show that both mammals and birds have a much higher invasion success than predicted by the tens rule, and that mammals have a significantly higher success than birds. Averaged across islands and continents, 79% of mammals and 50% of birds introduced have established themselves and 63% of mammals and 34% of birds established have become invasive. My analysis also does not support the hypothesis that islands are more susceptible to invaders than continents, as I did not find a significant relationship between invasion success and the size of the island or continent to which the species were introduced. The data set used in this study has a number of limitations, e.g. information on propagule pressure was not available at this global scale, so understanding the mechanisms behind the observed patterns has to be postponed to future studies.     

Effects of Vegetation Traits on Habitat Preferences of Frugivorous Birds in Atlantic Rain Forest

Structural and floristic components of vegetation and fruit production were examined to evaluate the relative importance of each vegetation aspect on patterns of habitat use and to determine whether differences in degree of frugivory would affect sensitivity of birds to vegetation components. Abundances of 12 bird species were quantified in four different habitat types in the Southeastern Atlantic rain forest of Brazil using captures with mist nets. Structural and floristic traits and fruit production were sampled within plots along the mist net lines. Best explanations for the variation in species abundance were obtained by different combinations of the vegetation components (generalized linear models, Akaike information criterion; R ² mean values = 0.48, Δ AIC c = 0). Although each species had a particular preference for some vegetation variables, floristic components stood out in those relationships, generating models with a stronger explanatory capacity ( R ² > 0.40) and higher levels of empirical support (Δ AIC c = minimum values). The variation in bird sensitivities to the floristic component was associated with bird dependence on a fruit diet: there was a correlation between the explanatory capacity of models built with only floristic composition and a species' degree of frugivory (Spearman rank correlation, r = 0.66, P = 0.04), indicating that floristics had a higher effect in more frugivorous birds. For the other vegetation components, there was no clear pattern with statistical support. These results indicate that sensitivities of bird species to vegetation aspects are associated with their dependence on a fruit diet.     

Interspecific Aggression and Resource Monopolization of the Invasive Ant Anoplolepis gracilipes in Malaysian Borneo

Invasions by introduced ant species can be ecologically destructive and affect a wide range of taxa, particularly native ants. Invasive ant species often numerically dominate ant communities and outperform native ant species in effective resource acquisition. Here, we describe interactions between the invasive ant Anoplolepis gracilipes (Smith) and resident ant species in disturbed habitats in NE Borneo. We measured interference competition abilities of A. gracilipes by performing arena bioassays between two A. gracilipes colonies and seven local ant species, and measured its effective resource competition at baits within supercolonies and at supercolony boundaries. Furthermore, we compared ant species diversity and composition at baits among (A) core areas of A. gracilipes supercolonies, (B) supercolony boundaries and (C) outside supercolonies. Anoplolepis gracilipes was behaviorally dominant over most ant species except Oecophylla smaragdina. Within supercolonies, A. gracilipes discovered all food baits first, and monopolized the vast majority throughout the course of the experiment. At supercolony boundaries, A. gracilipes discovered baits later than resident ant species, but subsequently monopolized half of the baits. Furthermore, the activity and diversity of the ant community within A. gracilipes supercolonies was lower than at its boundaries and outside supercolonies, and the ant communities differed significantly between infested and noninfested areas. Our study supports the hypothesis that successful establishment of A. gracilipes in anthropogenically disturbed habitats may negatively affect resident ant communities through high levels of direct interspecific aggression and almost complete monopolization of resources within high‐density supercolonies.     

Invasive Argentine ants reduce fitness of red maple via a mutualism with an endemic coccid

Many invasive ant species form mutualisms with honeydew-producing Hemiptera and their aggressive presence deters the natural enemies of the Hemiptera. Invasive ant species like the Argentine ant have often been associated with hemipteran outbreaks in urban, agricultural and natural ecosystems. We investigated the effects of a mutualism between the invasive Argentine ant and the endemic terrapin scale on coccid density and the fitness of the host of this mutualism, the endemic red maple, situated in a commercial park. The terrapin scale has numerous natural enemies and we predicted that the high terrapin scale numbers associated with tending Argentine ants would collapse once Argentine ants were excluded from the host tree canopy. We predicted that excluding the Argentine ant from the tree canopy would result in an indirect net fitness benefit to the host. Terrapin scale numbers collapsed when Argentine ants were excluded from the host tree canopy. Red maples with Argentine ants excluded from their canopy had higher seed mass and larger early leaves indicating that this invasive ant-endemic scale mutualism imposed a net fitness cost to the host tree. The Argentine ant has yet to invade closed-canopy forest within its introduced range. The red maple is common in adjacent closed-canopy forest fragments and recent work has shown that invasion of these forest fragments by the Argentine ant is limited by a steady carbohydrate resource. We discuss the implications to forest invasion posed by a mutualism involving the Argentine ant and an endemic coccid.     

Four mutually incompatible Argentine ant supercolonies in Japan: inferring invasion history of introduced Argentine ants from their social structure

In recent years, highly invasive ant species successively invaded warm regions of Asia. In Japan, the Argentine ant, Linepithema humile, has become established in several coastal regions. This species forms unusual social organizations called supercolonies consisting of numerous mutually non-aggressive nests. We studied the behavioral relationships, similarity of cuticular hydrocarbon profiles (nestmate recognition cue), and genetic relationships among the introduced Argentine ant populations of Japan. The Japanese populations were divided into four behaviorally, chemically, and genetically distinct supercolonies, which may have derived from independent source populations. The result represents the recent trend of increasing invasions of invasive ants to Asia. The discontinuous distribution of one supercolony throughout most of the Japanese range suggests rapid expansion of the supercolony via human-mediated jump dispersal. Meanwhile, localization of the other three supercolonies in Kobe Port provides the first evidence for multiple invasions of distinct supercolonies into a base for international trade.     

Impact of avian frugivores on dispersal and recruitment of the invasive <Emphasis Type="Italic">Prunus serotina</Emphasis> in an agricultural landscape

Although seed dispersal is considered to be a key process determining the spatial structure and spread of non-native plant populations, few studies have explicitly addressed the link between dispersal vector behaviour, seed distribution and seedling recruitment to gain insight into the process of exotic species invasion within a fragmented landscape context. The present study analyses the relationship between avian frugivory and spatial patterns of seed deposition and seedling recruitment for an expanding population of the invasive Prunus serotina in a hedgerow network landscape in Flanders, Belgium. We quantified fruit production, observed frugivores, and determined the spatial distribution of bird droppings and P. serotina seedlings. A relatively diverse assemblage of frugivores visited P. serotina seed trees, with Columba palumbus and Turdus merula being by far the most important dispersers. Landscape structure strongly affected dispersal vector behaviour and the spatial distribution of perching birds, droppings and seedlings. Frugivorous birds non-randomly dispersed seeds to perching sites and an association between perching birds, seed deposition and seedling recruitment was found. Results indicate that landscape structure contributes to non-random seed deposition of P. serotina by common local frugivores. Cutting the larger seed trees is proposed as the most feasible measure to slow down the invasion rate.     

生物入侵对鸟类的生态影响

生物入侵是全球生物多样性面临的最主要威胁之一, 入侵种在改变入侵地环境的同时也使当地的生物受到极大影响。鸟类在生态系统中处于较高的营养级, 生态系统中任何一个环节的变化都可能对鸟类造成一定的影响。本文回顾了哺乳动物、鸟类、无脊椎动物和植物等不同生物类群的入侵对本地鸟类生态影响方面的研究进展。外来生物对鸟类的影响主要表现在以下几方面: (1)外来哺乳动物对成鸟、幼鸟或鸟卵的捕食作用; (2)外来鸟类与本地鸟类竞争栖息地和食物资源, 与当地的近缘种杂交而造成基因流失; (3)外来无脊椎动物改变本地鸟类的栖息环境和食物状况, 甚至直接捕食本地鸟类; (4)外来植物入侵改变入侵地的植物群落组成和结构, 造成本地鸟类的栖息地丧失或破碎化, 并通过改变入侵地生态系统的食物链结构而对高营养级的鸟类产生影响。最后, 作者还提出了该领域有待解决的问题和今后可能的研究方向。