Extinction debt on oceanic islands

Habitat destruction is the leading cause of species extinctions. However, there is typically a time‐lag between the reduction in habitat area and the eventual disappearance of the remnant populations. These “surviving but ultimately doomed” species represent an extinction debt. Calculating the magnitude of such future extinction events has been hampered by potentially inaccurate assumptions about the slope of species–area relationships, which are habitat‐ and taxon‐specific. We overcome this challenge by applying a method that uses the historical sequence of deforestation in the Azorean Islands, to calculate realistic and ecologically‐adjusted species–area relationships. The results reveal dramatic and hitherto unrecognized levels of extinction debt, as a result of the extensive destruction of the native forest:>95%, in<600 yr. Our estimations suggest that more than half of the extant forest arthropod species, which have evolved in and are dependent on the native forest, might eventually be driven to extinction. Data on species abundances from Graciosa Island, where only a very small patch of secondary native vegetation still exists, as well as the number of species that have not been found in the last 45 yr, despite the extensive sampling effort, offer support to the predictions made. We argue that immediate action to restore and expand native forest habitat is required to avert the loss of numerous endemic species in the near future.     

Influence of invasion history on rapid morphological divergence across island populations of an exotic bird

There is increasing evidence that exotic populations may rapidly differentiate from those in their native range and that differences also arise among populations within the exotic range. Using morphological and DNA‐based analyses, we document the extent of trait divergence among native North American and exotic Hawaiian populations of northern cardinal (Cardinalis cardinalis). Furthermore, using a combination of historical records and DNA‐based analyses, we evaluate the role of founder effects in producing observed trait differences. We measured and compared key morphological traits across northern cardinal populations in the native and exotic ranges to assess whether trait divergence across the Hawaiian Islands, where this species was introduced between 1929 and 1931, reflected observed variation across native phylogeographic clades in its native North America. We used and added to prior phylogenetic analyses based on a mitochondrial locus to identify the most likely native source clade(s) for the Hawaiian cardinal populations. We then used Approximate Bayesian Computation (ABC) to evaluate the role of founder effects in producing the observed differences in body size and bill morphology across native and exotic populations. We found cardinal populations on the Hawaiian Islands had morphological traits that diverged substantially across islands and overlapped the trait space of all measured native North American clades. The phylogeographic analysis identified the eastern North American clade (C. cardinalis cardinalis) as the most likely and sole native source for all the Hawaiian cardinal populations. The ABC analyses supported written accounts of the cardinal's introduction that indicate the original 300 cardinals shipped to Hawaii were simultaneously and evenly released across Hawaii, Kauai, and Oahu. Populations on each island likely experienced bottlenecks followed by expansion, with cardinals from the island of Hawaii eventually colonizing Maui unaided. Overall, our results suggest that founder effects had limited impact on morphological trait divergence of exotic cardinal populations in the Hawaiian archipelago, which instead reflect postintroduction events.     

The island biogeography of exotic bird species

Aim   A recent upsurge of interest in the island biogeography of exotic species has followed from the argument that they may provide valuable information on the natural processes structuring island biotas. Here, we use data on the occurrence of exotic bird species across oceanic islands worldwide to demonstrate an alternative and previously untested hypothesis that these distributional patterns are a simple consequence of where humans have released such species, and hence of the number of species released.
Location   Islands around the world.
Methods   Statistical analysis of published information on the numbers of exotic bird species introduced to, and established on, islands around the world.
Results   Established exotic birds showed very similar species–area relationships to native species, but different species–isolation relationships. However, in both cases the relationship for established exotics simply mimicked that for the number of exotic bird species introduced. Exotic bird introductions scaled positively with human population size and island isolation, and islands that had seen more native species extinctions had had more exotic species released.
Main conclusion   The island biogeography of exotic birds is primarily a consequence of human, rather than natural, processes.     

Worldwide patterns of bird colouration on islands

Island environments share distinctive characteristics that offer unique opportunities to investigate parallel evolution. Previous research has produced evidence of an island syndrome for morphological traits, life‐history strategies and ecological niches, but little is known about the response to insularity of other important traits such as animal signals. Here, we tested whether birds' plumage colouration is part of the island syndrome. We analysed with spectrophotometry the colouration of 116 species endemic to islands and their 116 closest mainland relatives. We found a pattern of reduced brightness and colour intensity for both sexes on islands. In addition, we found a decrease in the number of colour patches on islands that, in males, was associated with a decrease in the number of same‐family sympatric species. These results demonstrate a worldwide pattern of parallel colour changes on islands and suggest that a relaxation of selection on species recognition may be one of the mechanisms involved.     

Prehistoric species richness of birds on oceanic islands

Using data on prehistoric and modern birds from seven islands in the Kingdom of Tonga, we demonstrate that there is no positive relationship between species richness (S) and island area (A) over the observed range of A (1.8–259 km²). The uniform S‐values occur across more than three orders of magnitude of A when prehistoric data are included, and the strongest predictor of S on any island is the level of fossil sampling (number of identified bones). Below a minimum value for A (in Tonga < 1.8 km²), S declines to zero as A does the same. Within the ranges of island elevation (E) and inter‐island isolation (I) among the seven islands, neither E (11–312 m) nor I (0.6–38 km) has much if any effect on S. Under natural (pre‐human) conditions, a positive species‐area relationship may not be a valid generalization for birds on oceanic islands.     

The biogeography of avian extinctions on oceanic islands

Aim To test the influences of island area, island isolation, and human‐introduced mammalian predators on avian extinctions that have occurred on oceanic islands worldwide. Location The oceanic islands of the world. Methods We augmented and re‐examined an existing data set for 218 oceanic islands by means of causal modelling using path analysis (a form of structural equation modelling) and a null model. Results The number of extinctions was not a simple function of the number of bird species on the various islands. Whereas introduced mammalian predators had an influence on the number of extinctions, island area (via indirect influences) and isolation (via direct influences) were equally or more important. Main conclusions The multiple influences of physical and biotic factors on past extinctions can be revealed through modelling the causal influences of physical attributes of islands on biological characteristics, and the causal influences of both physical and biological characteristics on extinctions.     

Biodiversity on oceanic islands:a palaeoecological assessment

Islands are bounded ecosystems and serve as excellent laboratories for assessing changes in Biodiversity. Some oceanic islands, such as Madagascar, Bermuda and notably the islands in the Pacific (e.g. Hawaii), are home to unique forms of endemic plants and animals that have evolved in isolation over millions of years. The palaeoecological record indicates that such islands are characterized by waves of extinctions concomitant with colonization by humans. By way of contrast, the biota of the islands of the north Atlantic (Greenland, Iceland and Faroe) do not follow the expected pattern and the few extinctions recorded are very localized. This is not a result of the scale of human impact, which is as great as on other islands, but relates to the virtual lack of endemics. The dearth of endemic forms and the disharmonic nature of these island communities indicates a youthful biota and the operation of severe filters and sweepstakes during colonisation over the last 10,000 years. This paper draws upon an extensive invertebrate fossil record to contrast and examine these spatial and temporal patterns in island Biodiversity.     

Tracking macroalgae introductions in North Atlantic oceanic islands

The Azores archipelago was selected as a case study since there are few studies on macroalgae introduction in oceanic islands. While at a global scale, around 3 % of macroalgae are considered non-indigenous; in the remote oceanic islands of the Azores, over 6 % of the marine algal flora is non-indigenous. The taxa distribution pattern of non-indigenous species in the Azores is significantly different from the distribution pattern in the globe. The most representative group was Rhodophyta species, being 84 % of the total non-indigenous macroalgae, mainly introduced via maritime traffic. This study highlights the vulnerability of remote islands to the introduction of macroalgae and the need to develop further studies on other archipelagos to understand whether the observed vulnerability is generally characteristic of oceanic islands. The development of local monitoring and mitigation programs and the necessity of regulatory and preventive measures for the maritime traffic vector are strongly suggested.     

The biogeography of avian extinctions on oceanic islands revisited

A recent paper by Karels et al ., 'The biogeography of avian extinctions on oceanic islands' ( Journal of Biogeography , 2008, 35 , 1106–1111), uses structural equation modelling to assess the causes of the number of island bird species driven extinct in the historical period. Here, we critically assess the conclusions of the paper and argue that it does not provide the new insights into the causes of extinction in island birds that its authors claim.     

A global comparison of plant invasions on oceanic islands

Oceanic islands have long been considered to be particularly vulnerable to biotic invasions, and much research has focused on invasive plants on oceanic islands. However, findings from individual islands have rarely been compared between islands within or between biogeographic regions. We present in this study the most comprehensive, standardized dataset to date on the global distribution of invasive plant species in natural areas of oceanic islands. We compiled lists of moderate (5–25% cover) and dominant (>25% cover) invasive plant species for 30 island groups from four oceanic regions (Atlantic, Caribbean, Pacific, and Western Indian Ocean). To assess consistency of plant behaviour across island groups, we also recorded present but not invasive species in each island group.We tested the importance of different factors discussed in the literature in predicting the number of invasive plant species per island group, including island area and isolation, habitat diversity, native species diversity, and human development. Further we investigated whether particular invasive species are consistently and predictably invasive across island archipelagos or whether island-specific factors are more important than species traits in explaining the invasion success of particular species.We found in total 383 non-native spermatophyte plants that were invasive in natural areas on at least one of the 30 studied island groups, with between 3 and 74 invaders per island group. Of these invaders about 50% (181 species) were dominants or co-dominants of a habitat in at least one island group. An extrapolation from species accumulation curves across the 30 island groups indicates that the total current flora of invasive plants on oceanic islands at latitudes between c. 35°N and 35°S may eventually consist of 500–800 spermatophyte species, with 250–350 of these being dominant invaders in at least one island group. The number of invaders per island group was well predicted by a combination of human development (measured by the gross domestic product (GDP) per capita), habitat diversity (number of habitat types), island age, and oceanic region (87% of variation explained). Island area, latitude, isolation from continents, number of present, non-native species with a known invasion history, and native species richness were not retained as significant factors in the multivariate models.Among 259 invaders present in at least five island groups, only 9 species were dominant invaders in at least 50% of island groups where they were present. Most species were invasive only in one to a few island groups although they were typically present in many more island groups. Consequently, similarity between island groups was low for invader floras but considerably higher for introduced (but not necessarily invasive) species – especially in pairs of island groups that are spatially close or similar in latitude. Hence, for invasive plants of natural areas, biotic homogenization among oceanic islands may be driven by the recurrent deliberate human introduction of the same species to different islands, while post-introduction processes during establishment and spread in natural areas tend to reduce similarity in invader composition between oceanic islands. We discuss a number of possible mechanisms, including time lags, propagule pressure, local biotic and abiotic factors, invader community assembly history, and genotypic differences that may explain the inconsistent performance of particular invasive species in different island groups.     

Floristic homogenization as a teleconnected trend in oceanic islands

Aim We searched for evidence of floristic homogenization in widespread oceanic archipelagos. Location Twelve oceanic archipelagos in the Atlantic and Pacific oceans: Ascension, Azores, Canary, Cape Verde, Desventuradas, Easter, Galápagos, Hawaii, Juan Fernández, Madeira, Puerto Rico and Savage. Methods By using Jaccard’s index, we established the floristic similarity between pairs of archipelagos at two stages: original (pre‐European; J_o) and current flora (J_c). Then, we calculated ΔJ = J_c–J_o, where positive differences imply that similarity has increased floristic homogenization. Results We found that floristic similarity increased fourfold on average, from 1.6% to 6.3% for original and current floras, respectively. In fact, we recorded 64 cases in which floristic similarity increased and only two in which it decreased. The importance of invasions exceeds that of extirpations as a driver of biotic change by more than an order of magnitude (2679 versus 142 species, respectively). Main conclusions The vascular floras of these 12 insular oceanic systems have increased in compositional similarity, a phenomenon consistent with the trend towards biotic homogenization. It can be characterized as a teleconnected process that operates across vast geographical distances, driven by the unprecedented capacity of humans for translocation. Trends in biotic homogenization differ depending on the geographical location (i.e. Pacific < Atlantic archipelagos) and phylogenetic groups (island vascular plants > island birds reported in a previous study), emphasizing the complexity of biotic change.     

Minimizing energy expenditure facilitates vertebrate persistence on oceanic islands

The characteristics of terrestrial vertebrates on oceanic islands are examined. They often include a reduced body size, a tolerance of conspecifics, flightlessness, a reduced basal rate of metabolism, and a propensity to enter torpor. On oceanic islands ectotherms frequently replace endotherms. These changes reduce the energy expenditure and resource requirements of vertebrates. Such reductions are permitted by the absence of mammalian predators and facilitate the survival of island endemics in the face of a restricted resource base and a variable environment through an increase in population size. Some insular species increase body size, but this occurs only when the resource base is large, due either to a fortuitously abundant resource, or to the absence of other species that exploit normally abundant resources. Some questions are posed to guide future work. They examine of the characteristics that permit species to disperse over water barriers, the conditions that require a reduction in resource use, the rapidity of response by immigrants to island conditions, whether supertramps show physiological differentiation with respect to island distance or size, and whether island size is absolute or relative to the size of the immigrants.