Deforestation and Avian Extinction on Tropical Landbridge Islands

Abstract: There are few empirical data, particularly collected simultaneously from multiple sites, on extinctions resulting from human‐driven land‐use change. Southeast Asia has the highest deforestation rate in the world, but the resulting losses of biological diversity remain poorly documented. Between November 2006 and March 2008, we conducted bird surveys on six landbridge islands in Malaysia and Indonesia. These islands were surveyed previously for birds in the early 1900s, when they were extensively forested. Our bird inventories of the islands were nearly complete, as indicated by sampling saturation curves and nonparametric true richness estimators. From zero (Pulau Malawali and Pulau Mantanani) to 15 (Pulau Bintan) diurnal resident landbird species were apparently extirpated since the early 1900s. Adding comparable but published extinction data from Singapore to our regression analyses, we found there were proportionally fewer forest bird extinctions in areas with greater remaining forest cover. Nevertheless, the statistical evidence to support this relationship was weak, owing to our unavoidably small sample size. Bird species that are restricted to the Indomalayan region, lay few eggs, are heavier, and occupy a narrower habitat breadth, were most vulnerable to extinction on Pulau Bintan. This was the only island where sufficient data existed to analyze the correlates of extinction. Forest preservation and restoration are needed on these islands to conserve the remaining forest avifauna. Our study of landbridge islands indicates that deforestation may increasingly threaten Southeast Asian biodiversity.     

Range Size and Extinction Risk in Forest Birds

Abstract:  Small geographical range size is the single best predictor of threat of extinction in terrestrial species. Knowing how small a species' range has to be before authorities consider it threatened with extinction would allow prediction of a species' risk from continued deforestation and warming climates and provide a baseline for conservation and management strategies aspiring to mitigate these threats. To determine the threshold at which forest-dependent bird species become threatened with extinction, we compared the range sizes of threatened and nonthreatened species. In doing so, we present a simple, repeatable, and practical protocol to quantify range size. We started with species' ranges published in field guides or comparable sources. We then trimmed these ranges, that is, we included only those parts of the ranges that met the species' requirements of elevation and types of forest preferred. Finally, we further trimmed the ranges to the amount of forest cover that remains. This protocol generated an estimate of the remaining suitable range for each species. We compared these range estimates with those from the World Conservation Union Red List. We used the smaller of the two estimates to determine the threshold, 11,000 km², below which birds should be considered threatened. Species considered threatened that have larger ranges than this qualified under other (nonspatial) red list criteria. We identified a suite of species (18) that have not yet qualified as threatened but that have perilously small ranges—about 11% of the nonthreatened birds we analyzed. These birds are likely at risk of extinction and reevaluation of their status is urgently needed.     

Application of Extinction and Conservation Theories for Forest Birds in Nicaragua

Abstract: An increasing number of empirical studies have been done on the effects of tropical forest fragmentation on avian communities, but few researchers have applied these theories to assess the vulnerability of birds in poorly researched countries such as Nicaragua. I used a logistic regression to determine which natural-history characteristics were most important in predicting a list of threatened birds known to occur in Nicaragua. The best model included five macroecological variables ( body weight, habitat specificity, trophic group, forest preference, and biogeography within Nicaragua). I used this model to generate predicted probabilities of extinction for all forest birds in Nicaragua. The predicted probability of extinction from the best model ranked 63% of the extinction-prone birds from La Selva, Costa Rica, and 59% of the extinction-prone birds from Barro Colorado, Panama, in the first quartile of all forest birds recorded in Nicaragua. This method provides a first-order approximation of which species deserve global and national priorities for conservation. The central and Atlantic regions of Nicaragua deserve high priority for conservation at a global scale, whereas the Atlantic region deserves the highest priority for conservation at a national scale. The Nicaraguan Ministry of Natural Resources and the Environment has done an adequate job of identifying areas for conservation based on the proportion of decreed nature reserves in each biogeographic region and the distribution of forest birds with a high predicted probability of extinction. Forest birds in central Nicaragua, however, may currently be the most vulnerable to local extinction because of low forest cover within decreed reserves.     

Risk of Local Extinction of Odonata Freshwater Habitat Generalists and Specialists

Understanding the risk of a local extinction in a single population relative to the habitat requirements of a species is important in both theoretical and applied ecology. Local extinction risk depends on several factors, such as habitat requirements, range size of species, and habitat quality. We studied the local extinctions among 31 dragonfly and damselfly species from 1930 to 1975 and from 1995 to 2003 in Central Finland. We tested whether habitat specialists had a higher local extinction rate than generalist species. Approximately 30% of the local dragonfly and damselfly populations were extirpated during the 2 study periods. The size of the geographical range of the species was negatively related to extinction rate of the local populations. In contrast to our prediction, the specialist species had lower local extinction rates than the generalist species, probably because generalist species occurred in both low‐ and high‐quality habitat. Our results are consistent with source–sink theory. Riesgo de Extinción Local de Odonatos de Agua Dulce Generalistas y Especialistas de Hábitat     

Effects of Habitat Area on the Distribution of Grassland Birds in Maine

We used multiple and logistic regression analysis to study the breeding-area requirements of 10 species of grassland and early-successional birds at 90 grassland-barren sites in Maine. The incidence of six of the species was clearly sensitive to the area of grassland. Upland Sandpipers, the species with the largest area requirements, were infrequent at sites of less than 50 ha and reached 50% incidence at those of about 200 ha. Grasshopper Sparrows reached 50% incidence at about 100 ha, Vesper Sparrows at about 20 ha, and Savannah Sparrows at about 10 ha. Incidence for three edge species, Brown Thrasher, Common Yellowthroat, and Song Sparrow, was negatively correlated with open area, and incidence for Field Sparrows was not strongly influenced by grassland size. These results indicate that grasslands need to be approximately 200 ha in area if they are to be likely to support a diverse grass land bird fauna. However, large grasslands or grassland-barrens are rare; random samples indicated that in Maine only 1% of hayfields and only 8% of grassland-barrens were more than 64 ha in area. Conservation efforts seeking to protect habitat for rare grassland birds need to consider sites of at least 100 ha—and preferably 200 ha—in size, and these are notably rare in Maine and probably throughout New England and eastern North America. Airports provide some of the last extensive patches of grassland habitat in the northeast. To maintain viable populations of area-sensitive grassland birds, management of these sites for nesting birds will become increasingly important.     

Extinction, Colonization, and Metapopulations: Environmental Tracking by Rare Species

Extinction and metapopulation theories emphasize that stochastic fluctuations in local populations cause extinction and that local extinctions generate empty habitat patches that are then available for recolonization. Metapopulation persistence depends on the balance of extinction and colonization in a static environment. For many rare and declining species, I argue (1) that extinction is usually the deterministic consequence of the local environment becoming unsuitable (through habitat loss or modification, introduction of a predator, etc.); (2) that the local environment usually remains unsuitable following local extinction, so extinctions only rarely generate empty patches of suitable habitat; and (3) that colonization usually follows improvement of the local environment for a particular species (or long-distance transfer by humans). Thus, persistence depends predominantly on whether organisms are able to track the shifting spatial mosaic of suitable environmental conditions or on maintainance of good conditions locally.     

Fragmentation of Phragmites Habitats, Minimum Viable Population Size, Habitat Suitability, and Local Extinction of Moths, Midges, Flies, Aphids, and Birds

Results from populations of insects and birds inhabiting Phragmites habitats were used to analyze effects of fragmentation. Flush-crash cycles of the stem-boring moth Archanara geminipuncta (Lepidoptera, Noctuidae) showed regionally concurrent, local extinctions despite an originally enormous population size (more than 180,000 adults), emphasizing the importance of metapopulation dynamics. Further, A. geminipuncta could be considered a keystone species, since shoot damage facilitated more than twenty species of herbivores, saprovores (of the caterpillars' feces), and their parasitoids. The gall midge Lasioptera arundinis could survive only in side shoots induced by shoot damage of A. geminipuncta .
Small Phragmites stands had thinner shoots (due to a water or nutrient deficiency) and shoots with more leaves (due to a better light supply) than large stands, thereby influencing species-specific demands for habitat suitability and nutritiousness of reed tissue. In other words significance of habitat fragmentation could not be assessed by area alone. For example, two chloropid flies depending on thin, yellowish shoots survived only in small habitats or in the unmown edges of large habitats.
Local persistence of Phragmites herbivores depended on much larger population sizes than could be expected from a population size sufficient to maintain genetic variation. At least 11,000 adults of the gall midge Giraudiella inclusa (or more than 84,000 galls) were necessary to avoid local extinction.
With regard to conservation management of reed habitats, nature reserves should consist of old and unmown reeds, have fewer disturbed (particularly, fewer mown) habitat edges, measure more than two hectares (priority should go to the largest remaining fragments), and be surrounded by nearby reed habitats providing reservoir populations and diverse shoot types.     

Modeling the Effects of Habitat Fragmentation on Source and Sink Demography of Neotropical Migrant Birds

Many songbird populations in the midwestem United States are structured as a network of sources and sinks that are linked by dispersal. We used a modeling approach to examine explicitly how populations respond to incremental fragmentation of source habitat and how this response may vary depending upon two life-history attributes: fidelity to natal habitat type and reproductive strength of the source. Fragmentation of source habitat led to a predictable decline in population for both attributes examined, but the manner in which populations declined varied depending upon the reproductive strength of the source and the level of fidelity. When the source was weak and produced few excess individuals, fragmentation of source habitats resulted in a predictable and parallel population decline of adults in both the source and the sink. In this situation high fidelity to natal habitats was important for maintenance of population size and structure. Low fidelity to weak sources resulted in population extinction; populations experienced a demographic cost by dispersing from high quality source habitat to low quality sink habitat. In contrast, when the source was strong and produced many excess individuals, fragmentation of the source led to population declines in both the source and the sink, but this decline was more abrupt in sink habitats. When the source was strong and produced a large excess of individuals, nonfidelity to natal habitats had little effect on metapopulation size and structure.     

Habitat and reef-fish assemblages of banks in the Northwestern Hawaiian Islands

The moderately deep terraces and banks of the Northwestern Hawaiian Islands (NWHI) were surveyed to describe their habitat and reef-fish assemblages. These tracts of bottom at 30–40 m comprise more than 4,500 km² of the regions reef area. The habitat was found to be dominated by algal meadows (65% cover of exposed bottom), with infrequent relief features. Annual monitoring of select stations for 4 years at Necker Bank indicated that the relative difference in algal abundance between stations persisted from year to year (at least in summer). Temperature records from year-long deployments of archival thermistors in high-cover (>70%) and low-cover (<30%) algal biotopes were indistinguishable, providing no explanation of the algal differences between stations. At all banks, Microdictyon was the primary alga, averaging 1.22 kg/m². In spite of the extensive standing primary production, and a historical lack of fishing, bank reef-fish populations were impoverished. Mean densities, sizes, and biomass of trophic groups were considerably less than values reported for NWHI reef shallows. An overall mean biomass was estimated at 22.5 g/m², which is a fifth of that reported for shallow reefs of the region. Fish biomass of all trophic groups was associated with the few sources of relief available on the banks. Apex predators (sharks, jacks, and snappers), common on all surveys (with a mean of five per station), were proposed to constrain fish populations to sparse sources of relief resulting in a skewed size structure of the two primary fish trophic components. Sizes of lower-level carnivores were tightly correlated with sources of relief whereas the size of herbivores were not, indicating that herbivores more often venture out and risk the exposed algal meadows. These bank summits are a rare example of a near pristine reef system with high benthic primary productivity and low fish biomass, and are a stark contrast to shallower coral-reef ecosystems of the NWHI.Communicated by P.W. Sammarco, Chauvin     

Interspecific Patterns of Genetic Diversity in Birds: Correlations with Extinction Risk

Abstract:  Birds are frequently used as indicators of ecosystem health and are the most comprehensively studied class in the animal kingdom. Nevertheless, a comprehensive, interspecific assessment of the correlates of avian genetic diversity is lacking, even though indices of genetic diversity are of considerable interest in the conservation of threatened species. We used published data on variation at microsatellite loci from 194 bird species to examine correlates of diversity, particularly with respect to conservation status and population size. We found a significant decline in mean heterozygosity with increasing extinction risk, and showed, by excluding species whose heterozygosity values were calculated with heterospecific primers, that this relationship was not dependent on ascertainment bias. Results of subsequent regression analyses suggested that smaller population sizes of threatened species were largely responsible for this relationship. Thus, bird species at risk of extinction are relatively depauperate in terms of neutral genetic diversity, which is expected to make population recovery more difficult if it reflects adaptive genetic variation. Conservation policy will need to minimize further loss of diversity if the chances of saving threatened species are to be maximized.     

The Potential for Conservation of Polynesian Birds through Habitat Mapping and Species Translocation

Abstract: The dependency of highly endemic island floras on few potential pollinators in depauperate island faunas suggests that pollinators and seed dispersers may be crucial in the preservation of biodiversity in isolated oceanic islands. We discuss the hypothesis that flying foxes are "strong interactors" in South Pacific islands where they setwe as the principal pollinators and seed dispersers, This suggests that the ongoing decline and ultimate extinction of flying fox species on Pacific islands may lead to a cascade of linked plant extinctions. We propose an empirical test of this hypothesis: comparisons of plant reproductive success in Guam, which has virtually lost its flying fox populations, and Samoa, where signifcant populations remain.     

Habitat Change and Plant Demography: Assessing the Extinction Risk of a Formerly Common Grassland Perennial

Abstract:  An important aim of conservation biology is to understand how habitat change affects the dynamics and extinction risk of populations. We used matrix models to analyze the effect of habitat degradation on the demography of the declining perennial plant Trifolium montanum in 9 calcareous grasslands in Germany over 4 years and experimentally tested the effect of grassland management. Finite population growth rates (λ) decreased with light competition, measured as leaf-area index above T. montanum plants. At unmanaged sites λ was <1 due to lower recruitment and lower survival and flowering probability of large plants. Nevertheless, in stochastic simulations, extinction of unmanaged populations of 100 flowering plants was delayed for several decades. Clipping as a management technique rapidly increased population growth because of higher survival and flowering probability of large plants in managed than in unmanaged plots. Transition-matrix simulations from these plots indicated grazing or mowing every second year would be sufficient to ensure a growth rate ≥1 if conditions stayed the same. At frequently grazed sites, the finite growth rate was approximately 1 in most populations of T. montanum . In stochastic simulations, the extinction risk of even relatively small grazed populations was low, but about half the extant populations of T. montanum in central Germany are smaller than would be sufficient for a probability of survival of >95% over 100 years. We conclude that habitat change after cessation of management strongly reduces recruitment and survival of established individuals of this perennial plant. Nevertheless, our results suggest extinction processes may take a long time in perennial plants, resulting in an extinction debt. Even if management is frequent, many remnant populations of T. montanum may be at risk because of their small size, but even a slight increase in size could considerably reduce their extinction risk.     

Fragments as Islands: a Synthesis of Faunal Responses to Habitat Patchiness

Abstract:  Scientific interest in the impact of habitat fragmentation on biodiversity is increasing, but our understanding of fragmentation is clouded by a lack of appreciation for fundamental similarities and differences across studies representing a wide range of taxa and landscape types. In an effort to synthesize data describing ecological responses of animals to fragmentation across two classes of independent variables (taxonomic group and landscape), we sampled 148 studies of five major faunal groups from the primary literature and analyzed data on 13 variables extracted from those studies. We focused our analyses on three classes of dependent variables (effects of area and isolation on species richness, z values, and nestedness and species composition). Area ranged over more orders of magnitude than isolation and tended to explain more variation in species richness than isolation. There were few matrix or taxon effects on the patterns we investigated, although we did find that sky islands tended to manifest isolation effects on both species richness and nestedness more frequently than other patch types. Sky islands may offer insight into the future of habitat patches fragmented by contemporary habitat loss, and because they show a stronger effect of isolation than other patch types, we suggest that isolation will play an increasing role in the biology of habitat fragments. We use multiple lines of evidence to suggest that our understanding of the role of isolation on community assembly in fragmented landscapes is inadequate. Finally, our observation that consistent taxonomic differences in community patterns were minimal suggests that conservation actions intended to mitigate the negative effects of extinction may have far-reaching effects across taxonomic groups.