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.     

Drivers of extinction: the case of Azorean beetles

Oceanic islands host a disproportionately high fraction of endangered or recently extinct endemic species. We report on species extinctions among endemic Azorean beetles following 97% habitat loss since AD 1440. We infer extinctions from historical and contemporary records and examine the influence of three predictors: geographical range, habitat specialization and body size. Of 55 endemic beetle species investigated (out of 63), seven can be considered extinct. Single-island endemics (SIEs) were more prone to extinction than multi-island endemics. Within SIEs restricted to native habitat, larger species were more extinction-prone. We thus show a hierarchical path to extinction in Azorean beetles: species with small geographical range face extinction first, with the larger bodied ones being the most threatened. Our study provides a clear warning of the impact of habitat loss on island endemic biotas.     

Testing causal structure in the biogeography of avian extinctions on oceanic islands

In their recent comment in this journal, T. M. Blackburn and colleagues called into question the use of standardized partial regression modelling (also called path analysis and structural equation modelling) when null expectations for regression coefficients are not zero. Here, we answer their critique by showing how randomization can be used to illuminate and interpret causal modelling in analyses that have non-zero expectations. Causal modelling is a powerful tool that can yield novel insights in biogeography when properly interpreted.     

Pleistocene extinctions as drivers of biogeographical patterns on the easternmost Canary Islands

Subtropical islands are often viewed as refuges where Quaternary climatic shifts driving global episodes of extinction were buffered. Island biodiversity, however, may have been impacted by climatic fluctuations at local scales, particularly in spatially heterogeneous island systems. In this study, we generated a conceptual framework for predicting the potential impact of Pleistocene extinctions on the biogeographical pattern of the Canarian spermatophyte flora, with a focus on the easternmost Canarian islands (ECI). Then, we performed an exhaustive bibliographic revision (270 studies) to examine whether taxonomic, phylogenetic and phylogeographical data support our predictions. Although molecular information is limited for many lineages, the available data suggest that the majority of extant ECI plant taxa may be the result of relatively recent (<1 Ma) dispersal from surrounding insular and mainland areas. Different lines of evidence are compatible with the idea of a Pleistocene period of frequent lineage extirpation on ECI. Extinction may thus have provided new ecological opportunities for recent (re)colonization, with some cases of recent establishment mediated by facilitation. Considering background extinction on ECI, we describe five general patterns of colonization for Canarian plant lineages. In addition to factors related to island ontogeny and long‐distance dispersal, we suggest that Pleistocene extinctions may have significantly contributed to extant biogeographical patterns in the Canarian archipelago, such as the biased distribution ranges of island plants and the low endemic richness on ECI. This new scenario provides testable hypotheses for future studies dealing with the phylogeography, taxonomy and conservation of terrestrial biodiversity on the Canarian islands, and possibly, on other near‐shore islands.     

Inbreeding and extinction: Effects of rate of inbreeding

Deleterious alleles may be removed (purged) bynatural selection in populations undergoinginbreeding. However, there is controversyregarding the effectiveness of selection inreducing the risk of extinction due toinbreeding, especially in relation to the rateof inbreeding. We evaluated the effect of therate of inbreeding on reducing extinction risk,in populations of Drosophila melanogastermaintained using full-sib mating (160replicates), or at effective population sizes(N _e) of 10 (80) or 20 (80).Extinction rates in the populations maintainedusing full-sib mating occurred at lower levelsof inbreeding than in the larger populations,whereas the two larger populations did notdiffer significantly from each other.Inbreeding coefficients at 50% extinction were0.62, 0.79 and 0.77 for the full-sib (N _e = 2.6), N _e = 10 and N _e = 20 treatments, respectively. Populations of N _e = 20 that remained extant after 60 generations, showed inbreeding depression, with the mean fitness of these populations being only 45% of the outbredcontrols. There was considerable variationamong the 31 inbred populations in fitness, butnone of the N _e = 20 populations hadfitness that was higher than the outbredcontrol. We conclude that purging may slow therate of extinction slightly, but it cannot berelied on to eliminate the deleterious effectsof inbreeding.     

Primate extinction risk and historical patterns of speciation and extinction in relation to body mass

Body mass is thought to influence diversification rates, but previous studies have produced ambiguous results. We investigated patterns of diversification across 100 trees obtained from a new Bayesian inference of primate phylogeny that sampled trees in proportion to their posterior probabilities. First, we used simulations to assess the validity of previous studies that used linear models to investigate the links between IUCN Red List status and body mass. These analyses support the use of linear models for ordinal ranked data on threat status, and phylogenetic generalized linear models revealed a significant positive correlation between current extinction risk and body mass across our tree block. We then investigated historical patterns of speciation and extinction rates using a recently developed maximum-likelihood method. Specifically, we predicted that body mass correlates positively with extinction rate because larger bodied organisms reproduce more slowly, and body mass correlates negatively with speciation rate because smaller bodied organisms are better able to partition niche space. We failed to find evidence that extinction rates covary with body mass across primate phylogeny. Similarly, the speciation rate was generally unrelated to body mass, except in some tests that indicated an increase in the speciation rate with increasing body mass. Importantly, we discovered that our data violated a key assumption of sample randomness with respect to body mass. After correcting for this bias, we found no association between diversification rates and mass.     

Correlates of rediscovery and the detectability of extinction in mammals

Extinction is difficult to detect, even in well-known taxa such as mammals. Species with long gaps in their sighting records, which might be considered possibly extinct, are often rediscovered. We used data on rediscovery rates of missing mammals to test whether extinction from different causes is equally detectable and to find which traits affect the probability of rediscovery. We find that species affected by habitat loss were much more likely to be misclassified as extinct or to remain missing than those affected by introduced predators and diseases, or overkill, unless they had very restricted distributions. We conclude that extinctions owing to habitat loss are most difficult to detect; hence, impacts of habitat loss on extinction have probably been overestimated, especially relative to introduced species. It is most likely that the highest rates of rediscovery will come from searching for species that have gone missing during the 20th century and have relatively large ranges threatened by habitat loss, rather than from additional effort focused on charismatic missing species.     

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.     

Correlates of extinction risk of birds from two Indonesian islands

Size of distributional range, position in the range, body size and diet are some of the ecological traits that may correlate with local abundance. Evolutionary phenomena such as taxon cycles, acting over much greater time periods, may also influence abundance and promote species extinction. This paper assesses which of a wide range of ecological and historic traits best predict the variation in abundance of tropical forest birds on Sumba and Buru islands in Wallacea (Indonesia). In addition we seek to determine which traits predict species' ability to adapt to secondary or logged forest. The most important correlates of both abundance and ability to transfer were those related to the evolutionary history of the species within the Wallacean Archipelago and not the traits that were more directly related to species ecology. These relationships are maintained when allowance is made for phylogenetic relationships. Our interpretation of the results is that recent colonists to an island are initially rare in the indigenous forest habitat but concomitant with an adaptation to local conditions they gradually become more abundant and taxonomically distinct from other populations of the same species. These results apparently contradict the taxon cycle hypothesis but this may be a result of our focus on indigenous forest habitats rather than on a wider range dominated by anthropogenic ones.     

Paleoecological patterns in molluscan extinctions and recoveries: comparison of the Cretaceous-Paleogene and Eocene-Oligocene extinctions in North America

Exposures across the Cretaceous-Tertiary (K-T) and Eocene-Oligocene (E-O) boundaries, in Texas and Mississippi, respectively, probably represent the most complete and best-preserved fossil molluscan sequences across these boundary intervals in the world. Outcrops from both boundaries contain pristine aragonitic and calcitic molluscan shells, which were deposited in fine-grained sediments from open marine environments. The K-T and the E-O extinctions exhibit very different recovery patterns, probably reflecting very different causes as well as magnitudes of extinction.The K-T sequence contains a molluscan fossil record that is consistent with an abrupt extinction event at the K-T boundary and a prolonged initial recovery in hostile oceanographic conditions. The uppermost 10 m of Upper Cretaceous sediments contain a diverse (approximately 40 species) molluscan fauna dominated by suspension feeders. The earliest Paleocene sediments immediately above the tsunami bed contain an impoverished fauna dominated by deposit feeders. The Paleocene fauna slowly climbs in diversity but remains relatively impoverished and dominated by deposit feeders for several hundred thousand years after the extinction in conjunction with anomalous δ¹³C values that suggest prolonged suppression of marine primary productivity. Diverse suspension-feeder dominated molluscan assemblages reappear with the resumption of normal conditions of primary production. In the long term, early to middle Paleocene gamma diversity includes evolutionary “bloom taxa,” families that exhibit unusual speciation bursts that subside in the Eocene. Total diversity for the Gulf Coast does not approach Cretaceous levels until the Late Eocene representing a total recovery interval of nearly 25 million years.While the E-O event also reflects a molluscan extinction rate of over 90% in the Gulf of Mexico, there are no signs of hostile environmental conditions in the recovery fauna. Early Oligocene molluscan assemblages are diverse and dominated by suspension feeders characteristic of normal marine conditions. The hiatus at the E-O boundary, however, could have obscured a short-term recovery fauna. There is also no sign of long-term perturbation by the E-O extinction. There are no bloom taxa and gamma diversity approaches pre-extinction levels within a few million years. The overall pattern of the E-O extinction is consistent with extinction (and/or migration) associated with long-term cooling.     

Integration of palaeontological, historical, and geographical data on the extinction of koa-finches

Identifying the root causes of extinction or endangerment requires long chronological records that begin before a population started to decline and extend until its extinction or functional extinction. We present a case study of the koa‐finches, genus Rhodacanthis, an extinct group of Hawaiian honeycreepers that was specialized to feed on green pods and seeds of the koa tree or other leguminous plants. Six island populations of koa‐finches are known; four in the Holocene fossil record and two that survived until the 1890s. We document the palaeoecological context of the fossils and identify constraints on the age span of the specimen record for each population using stratigraphic contexts, associated radiometric determinations, and museum specimen data. We estimate the potential geographical range of koa‐finches at the time of human arrival using two methods: assessment of their historical and palaeo‐habitats, and geographical information system mapping of the pre‐human distribution of the koa plant (Acacia koa) and its sister species, the koai‘a plant (Acacia koaia). After integrating the foregoing data with chronological records and distributional maps of the potential forcing agents of extinction, we conclude that at least two extinctions of island populations were due to ecological change in the lowlands in the prehistorical and perhaps the early historical periods. In the same time frame, the koa‐finch populations on Hawai‘i Island became rare and restricted to upland refugia, making them vulnerable to the upland forest harvesting and degradation that was accelerating in the 1890s. Neither climatic variation nor mosquito‐vectored diseases are likely to have caused the observed extinctions. This study illustrates an approach that can be applied to many other extinct and endangered island species to better understand the causes of high extinction rates in the human era.     

Scaling the extinction vortex: Body size as a predictor of population dynamics close to extinction events

1. Mutual reinforcement between abiotic and biotic factors can drive small populations into a catastrophic downward spiral to extinction—a process known as the “extinction vortex.” However, empirical studies investigating extinction dynamics in relation to species'' traits have been lacking.
2. We assembled a database of 35 vertebrate populations monitored to extirpation over a period of at least ten years, represented by 32 different species, including 25 birds, five mammals, and two reptiles. We supplemented these population time series with species‐specific mean adult body size to investigate whether this key intrinsic trait affects the dynamics of populations declining toward extinction.
3. We performed three analyses to quantify the effects of adult body size on three characteristics of population dynamics: time to extinction, population growth rate, and residual variability in population growth rate.
4. Our results provide support for the existence of extinction vortex dynamics in extirpated populations. We show that populations typically decline nonlinearly to extinction, while both the rate of population decline and variability in population growth rate increase as extinction is approached. Our results also suggest that smaller‐bodied species are particularly prone to the extinction vortex, with larger increases in rates of population decline and population growth rate variability when compared to larger‐bodied species.
5. Our results reaffirm and extend our understanding of extinction dynamics in real‐life extirpated populations. In particular, we suggest that smaller‐bodied species may be at greater risk of rapid collapse to extinction than larger‐bodied species, and thus, management of smaller‐bodied species should focus on maintaining higher population abundances as a priority.

     

Holocene mammal extinctions in the Carpathian Basin: a review

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