Relative Importance Of Biotic Interactions Research Articles

Limits to species distributions on tropical mountains shift from high temperature to competition as elevation increases

AbstractSpecies turnover with elevation is a widespread phenomenon and provides valuable information on why and how ecological communities might reorganize as the climate warms. It is commonly assumed that species interactions are more likely to set warm range limits, while physiological tolerances determine cold range limits. However, most studies are from temperate systems and rely on correlations between thermal physiological traits and range limits; little is known about how physiological traits and biotic interactions change simultaneously along continuous thermal gradients. We used a combination of correlational and experimental approaches to investigate communities of Drosophila flies in rainforests of the Australian Wet Tropics, where there is substantial species turnover with elevation. Our experiments quantified individual‐level and population‐level responses to temperature, as well as the impact of interspecific competition under different temperature regimes. Species' distributions were better explained by their performance at extreme temperatures than by their thermal optima. Upper thermal limits varied less among species than lower thermal limits. Nonetheless, these small differences were associated with differences in the centered elevation of distribution. Low‐elevation species were not those with the lowest tolerance to cold, suggesting that cold temperatures were not limiting their abundance at high elevations. Instead, under upland temperature regimes, abundances of these low‐elevation species were reduced by competition with a high‐elevation species, in both short‐ and long‐term competition experiments. Our results demonstrate that high‐elevation species are confined to their current ranges by high temperatures at lower elevations, indicating that their ranges will be highly sensitive to future warming. Counter to expectation, species interactions strongly influenced community composition at cooler, high‐elevation sites. Together, these results raise the possibility that tropical communities differ from better‐studied temperate communities in terms of the relative importance of biotic interactions and abiotic factors in shaping community composition and how the impact of these factors will change as temperatures increase.

Interspecific Host Variation and Biotic Interactions Drive Pathogen Community Assembly in Chinese Bumblebees.

Bumblebees have been considered one of the most important pollinators on the planet. However, recent reports of bumblebee decline have raised concern about a significant threat to ecosystem stability. Infectious diseases caused by multiple pathogen infections have been increasingly recognized as an important mechanism behind this decline worldwide. Understanding the determining factors that influence the assembly and composition of pathogen communities among bumblebees can provide important implications for predicting infectious disease dynamics and making effective conservation policies. Here, we study the relative importance of biotic interactions versus interspecific host resistance in shaping the pathogen community composition of bumblebees in China. We first conducted a comprehensive survey of 13 pathogens from 22 bumblebee species across China. We then applied joint species distribution modeling to assess the determinants of pathogen community composition and examine the presence and strength of pathogen-pathogen associations. We found that host species explained most of the variations in pathogen occurrences and composition, suggesting that host specificity was the most important variable in predicting pathogen occurrences and community composition in bumblebees. Moreover, we detected both positive and negative associations among pathogens, indicating the role of competition and facilitation among pathogens in determining pathogen community assembly. Our research demonstrates the power of a pluralistic framework integrating field survey of bumblebee pathogens with community ecology frameworks to understand the underlying mechanisms of pathogen community assembly.

Nurse shrubs influence plant biodiversity across environmental gradients in rocky outcrops and surrounding rangelands

AbstractQuestionRocky outcrops generally restrict recruitment and survival of plant species due to their environmental conditions including low soil moisture, nutrient shortages and microclimate harshness. Under such severe conditions, nurse plants may play critical roles in preserving plant biodiversity. Although the nurses' effects on plant biodiversity are fully understood in different major biomes, the relative effects of abiotic factors and nurse plants as important factors have rarely been studied in rocky outcrops, and compared to those in the surrounding rangelands. Therefore, we sought to assess the effects of two dominant shrub species (Artemisia aucheri and Prunus pseudoprostrata) on all biodiversity components including taxonomic, functional and phylogenetic diversity across some environmental gradients such as precipitation and soil gradients in rocky outcrops and their surrounding rangelands.LocationThe study was conducted on six sites located in rocky outcrops and their surrounding rangelands across a precipitation gradient (160–910 mm) and under different soil fertility levels in northern Iran.MethodsWe established 80 1‐m2 (1 m × 1 m) plots in each site with respect to the presence of nurse shrubs, and measured biodiversity components within the plots using Rao Q entropies (q0, q1 and q2) and mean pairwise distance (MPD) indices. The relative interaction index (RII) was also calculated to quantify the interaction type and intensity. Finally, variation partitioning analysis was performed to determine the relative importance of biotic interactions and abiotic factors such as precipitation, electrical conductivity (EC) and phosphorus on structuring biodiversity components.ResultsPlant–plant interactions and abiotic factors consistently influenced different components of biodiversity in rocky outcrops and their surrounding rangelands. However, nurse shrubs showed stronger effects on plant biodiversity than abiotic factors by providing suitable microenvironments to harbor less‐tolerant plant species under the severe conditions of outcrops. In addition, microhabitats beneath nurse shrubs significantly responded to soil EC and precipitation gradients, with a decrease in competition intensity under intermediate levels of the gradients. In contrast, nurse shrubs negatively affected different components of biodiversity, with a decrease in biodiversity under intermediate levels of environmental gradients in surrounding rangelands.ConclusionsOur findings show critical roles of nurse shrubs for maintaining plant biodiversity in rocky outcrops depending on their growth form and the biodiversity component considered. In this case, nurse shrubs could reduce double environmental filtering, consistent with establishing a functional paradox trade‐off to support a wide range of plant species with functionally divergent relatedness.

Competitive release during fire succession influences ecological turnover in a small mammal community.

Ecologists have long debated the relative importance of biotic interactions versus species-specific habitat preferences in shaping patterns of ecological dominance. In western North America, cycles of fire disturbance are marked by transitions between North American deermice (Peromyscus maniculatus), which predominate after wildfires, and southern red-backed voles (Myodes gapperi), which gradually replace deermice 3-4 years postfire and maintain dominance as forests mature. While this shift has been frequently documented, the processes that mediate this turnover are debated. One possibility is competitive release, which predicts a reduction in vole competition may contribute to niche expansion and population growth in deermice. Alternatively, turnover in both species may be shaped by differences in their preferred habitat and resource base, as predicted by optimum foraging theory. We evaluate these hypotheses using stable isotopes and spatial mark-recapture of deermouse and vole populations sampled prior to and following a fire as part of a longitudinal study in the Greater Yellowstone Ecosystem. Fire disturbance was associated with a 94% decrease in vole abundance but a 102% increase in deermice. Even after accounting for microhabitat, vole and deermouse populations were negatively correlated spatially and temporally (R=-0.45), and competitor abundance was more important prefire than postfire. When vole abundance was high (prefire), vole dietary niche space was seven times broader than that of deermice. Postfire, deermouse dietary niche nearly tripled and was enriched in 13 C (i.e., more C4 plants), while voles occupied a slightly reduced dietary niche (79% of prefire breadth). Our results suggest deermice are experiencing ecological release due to a reduction in vole competition but vole shifts are largely driven by habitat preferences.

Annual grass invasion in sagebrush steppe: the relative importance of climate, soil properties and biotic interactions.

The invasion by winter-annual grasses (AGs) such as Bromus tectorum into sagebrush steppe throughout the western USA is a classic example of a biological invasion with multiple, interacting climate, soil and biotic factors driving the invasion, although few studies have examined all components together. Across a 6000-km(2) area of the northern Great Basin, we conducted a field assessment of 100 climate, soil, and biotic (functional group abundances, diversity) factors at each of 90 sites that spanned an invasion gradient ranging from 0 to 100% AG cover. We first determined which biotic and abiotic factors had the strongest correlative relationships with AGs and each resident functional group. We then used regression and structural equation modeling to explore how multiple ecological factors interact to influence AG abundance. Among biotic interactions, we observed negative relationships between AGs and biodiversity, perennial grass cover, resident species richness, biological soil crust cover and shrub density, whereas perennial and annual forb cover, tree cover and soil microbial biomass had no direct linkage to AG. Among abiotic factors, AG cover was strongly related to climate (increasing cover with increasing temperatureand aridity), but had weak relationships with soil factors. Our structural equation model showed negative effects of perennial grasses and biodiversity on AG cover while integrating the negative effects of warmer climate and positive influence of belowground processes on resident functional groups. Our findings illustrate the relative importance of biotic interactions and climate on invasive abundance, while soil properties appear to have stronger relationships with resident biota than with invasives.

Biotic interactions and habitat drive positive co‐occurrence between facilitating and beneficiary stream fishes

AbstractAimThe role of biotic interactions in determining species distributions is difficult to decouple from abiotic factors. Most research to date has focused on negative biotic interactions, but the importance of positive interactions such as mutualism and facilitation at large scales is less understood. We used a two‐species occupancy modelling approach to decouple the relative effects of abiotic factors and biotic interactions between a habitat‐modifying North American stream fish (Nocomis leptocephalus) and two of its nearly‐obligate beneficiary species (Chrosomus oreas and Clinostomus funduloides).LocationSixty‐one sites on tributaries to the New River basin in the Appalachian mountains of the eastern United States.MethodsWe sampled fishes using backpack electrofishing in the summers of 2012–2014. We gathered 10 habitat covariates from the National Hydrography Dataset and from instream measurements. We reduced dimensionality in habitat variables, corrected them for spatial autocorrelation and selected two eigenvectors as habitat covariates. We then used an information‐theoretic approach to compare two‐species occupancy models representing hypotheses specifying the importance of only habitat, only biotic interactions, or combinations of both.ResultsThe best model for each species combination specified the importance of biotic interactions. For both associate species, probabilities that associates would occur in the absence of their host were considerably lower than probabilities of co‐occurrence. Species interaction factors indicated positive patterns of co‐occurrence between hosts and associates. Models suggested that habitat variables mediated host‐associate interactions for C. oreas but not for C. funduloides.Main conclusionsThis study provides some of the first large‐scale quantitative evidence of positive co‐occurrence among vertebrates, and demonstrates the importance of abiotic context for mediating interspecific interactions. Two‐species occupancy modelling may be superior to traditional co‐occurrence analyses for parsing out the relative importance of biotic interactions and habitat variables for determining species distributions. However, experiments and small‐scale behavioural observations will also be necessary to confirm mechanisms.

Empirical evidence for the scale dependence of biotic interactions

AbstractAimAlthough it is recognized that ecological patterns are scale dependent, the exact scales over which specific ecological processes operate are still a matter of controversy. In particular, understanding the scales over which biotic interactions operate is critical for predicting changes in species distributions in the face of the ongoing biodiversity crisis. It has been hypothesized that biotic interactions operate predominately at fine grains, yet this conjecture has received relatively little empirical scrutiny. We use US woodpeckers as a model system to assess the relative importance of biotic interactions, environmental suitability and geographic proximity to other intraspecific occurrence sites, across scales.LocationConterminous United States.MethodsWe combined species occurrence data from the North American Breeding Bird Survey (BBS) with a large pair‐wise interaction matrix describing known interactions among woodpeckers and other bird species. Using a logistic mixed modelling framework we then established the relative importance of biotic interactions as predictors of woodpecker occurrences in relation to environment and geographic proximity to intraspecific occurrence sites.ResultsWe found that geographic proximity becomes a stronger predictor of woodpecker occurrence as grain becomes coarser, while environment is grain‐invariant. As opposed to environment and geographic proximity, we found that when the focal species experienced positive biotic interactions, the importance of interactions decreased with increased grain. However, positive interactions remained important up to a grain size of entire BBS routes (c. 40 km), which is much coarser than the grain size used by most species distribution models. In contrast, when the focal species experienced negative interactions we did not find clear grain dependence.Main conclusionsBiotic interactions (both positive and negative) are important predictors of species occurrences. While these interactions are strongest at fine grains, they can remain important even at coarse grains, and are thus critical for predicting distributional changes in the face of the ongoing biodiversity crisis.

The relative importance of biotic and abiotic processes for structuring plant communities through time

Summary The question of the relative importance of biotic interactions versus abiotic drivers for structuring plant communities is highly debated but largely unresolved. Here, we investigate the relative importance of mean July air temperature, nitrogen (N) availability and direct plant interactions in determining the millennial‐scale population dynamics through the Holocene (10 700–5200 cal. years bp) for four temperate tree taxa in the Scottish Highlands. A variety of dynamic population models were fitted to our palaeoecological time‐series data to determine the mechanism(s) by which each driver affected the population biomass dynamics of Betula (birch), Pinus (pine), Alnus (alder) and Quercus (oak). Akaike information criterion weights identified the best model(s) for describing the relationship between each population and driver. The relative importance of these drivers was then assessed by the ability of each model to predict the observed population biomass dynamics. We also measured the change in goodness‐of‐fit of each model over time. We found that models of intra‐ and interspecific plant interactions described the plant population dynamics better than temperature‐ or N‐dependent population growth models over the 5000‐year study period. The best‐fitting models were constant over time for pine, alder and oak. However, the plant–N availability and plant–temperature models provided a progressively better fit to the birch data when temperatures rose and N availability declined, suggesting increasing importance of these abiotic factors coincident with changing conditions. Synthesis. Multiple mechanistic models were applied to palaeoecological data to infer the most likely processes driving millennial‐scale plant biomass dynamics in a woodland ecosystem. Direct plant interactions provided a better explanation for population biomass dynamics than growing season temperature or N availability over the full study period. The relative importance of all drivers we assessed here varied by species and – in the case of birch – over time in response to warming and reduced N availability.

Biotic interactions overrule plant responses to climate, depending on the species' biogeography.

This study presents an experimental approach to assess the relative importance of climatic and biotic factors as determinants of species' geographical distributions. We asked to what extent responses of grassland plant species to biotic interactions vary with climate, and to what degree this variation depends on the species' biogeography. Using a gradient from oceanic to continental climate represented by nine common garden transplant sites in Germany, we experimentally tested whether congeneric grassland species of different geographic distribution (oceanic vs. continental plant range type) responded differently to combinations of climate, competition and mollusc herbivory. We found the relative importance of biotic interactions and climate to vary between the different components of plant performance. While survival and plant height increased with precipitation, temperature had no effect on plant performance. Additionally, species with continental plant range type increased their growth in more benign climatic conditions, while those with oceanic range type were largely unable to take a similar advantage of better climatic conditions. Competition generally caused strong reductions of aboveground biomass and growth. In contrast, herbivory had minor effects on survival and growth. Against expectation, these negative effects of competition and herbivory were not mitigated under more stressful continental climate conditions. In conclusion we suggest variation in relative importance of climate and biotic interactions on broader scales, mediated via species-specific sensitivities and factor-specific response patterns. Our results have important implications for species distribution models, as they emphasize the large-scale impact of biotic interactions on plant distribution patterns and the necessity to take plant range types into account.

Facilitative plant interactions and climate simultaneously drive alpine plant diversity

Interactions among species determine local-scale diversity, but local interactions are thought to have minor effects at larger scales. However, quantitative comparisons of the importance of biotic interactions relative to other drivers are rarely made at larger scales. Using a data set spanning 78 sites and five continents, we assessed the relative importance of biotic interactions and climate in determining plant diversity in alpine ecosystems dominated by nurse-plant cushion species. Climate variables related with water balance showed the highest correlation with richness at the global scale. Strikingly, although the effect of cushion species on diversity was lower than that of climate, its contribution was still substantial. In particular, cushion species enhanced species richness more in systems with inherently impoverished local diversity. Nurse species appear to act as a 'safety net' sustaining diversity under harsh conditions, demonstrating that climate and species interactions should be integrated when predicting future biodiversity effects of climate change.

Biotic interactions limit species richness in an alpine plant community, especially under experimental warming

The determinants of local species richness in plant communities have been the subject of much debate. Is species richness the result of stochastic events such as dispersal processes, or do local environmental filters sort species into communities according to their ecological niches? Recent studies suggest that these two processes simultaneously limit species richness, although their relative importance may vary in space and time. Understanding the limiting factors for species richness is especially important in light of the ongoing global warming, as new species establish in resident plant communities as a result of climate‐driven migration. We examined the relative importance of dispersal and environmental filtering during seedling recruitment and plant establishment in an alpine plant community subjected to seed addition and long‐term experimental warming. Seed addition increased species richness during the seedling recruitment stage, but this initial increase was cancelled out by a corresponding decrease in species richness during plant establishment, suggesting that environmental filters limit local species richness in the long term. While initial recruitment success of the sown species was related to both abiotic and biotic factors, long‐term establishment was controlled mainly by biotic factors, indicating an increase in the relative importance of biotic interactions once plants have germinated in a microhabitat with favourable abiotic conditions. The relative importance of biotic interactions also seemed to increase with experimental warming, suggesting that increased competition within the resident vegetation may decrease community invasibility as the climate warms.

Microhabitat amelioration and reduced competition among understorey plants as drivers of facilitation across environmental gradients: towards a unifying framework.

Studies of facilitative interactions as drivers of plant richness along environmental gradients often assume the existence of an overarching stress gradient equally affecting the performance of all the species in a given community. However, co-existing species differ in their ecophysiological adaptations, and do not experience the same stress level under particular environmental conditions. Moreover, these studies assume a unimodal richness-biomass curve, which is not as general as previously thought. We ignored these assumptions to assess changes in plant-plant interactions, and their effect on local species richness, across environmental gradients in semi-arid areas of Spain and Australia. We aimed to understand the relative importance of direct (microhabitat amelioration) and indirect (changes in the competitive relationships among the understorey species: niche segregation, competitive exclusion or intransitivity) mechanisms that might underlie the effects of nurse plants on local species richness. By jointly studying these direct and indirect mechanisms using a unifying framework, we were able to see how our nurse plants (trees, shrubs and tussock grasses) not only increased local richness by expanding the niche of neighbouring species, but also by increasing niche segregation among them, though the latter was not important in all cases. The outcome of the competition-facilitation continuum changed depending on the study area, likely because the different types of stress gradient considered. When driven by both rainfall and temperature, or rainfall alone, the community-wide importance of nurse plants remained constant (Spanish sites), or showed a unimodal relationship along the gradient (Australian sites). This study expands our understanding of the relative roles of plant-plant interactions and environmental conditions as drivers of local species richness in semi-arid environments. These results can also be used to refine predictions about the response of plant communities to environmental change, and to clarify the relative importance of biotic interactions as a driver of such responses.

From the laboratory to the field: contrasting effects of multi‐trophic interactions and agroforestry management on coffee pest densities

AbstractOnly few factors influencing pest populations can be studied in the laboratory, but many population‐driving factors interact in the field. Therefore, complementary laboratory and field approaches are required for reliable predictions of real‐world patterns and processes. Laboratory and field experiments with the red spider mite, Oligonychus ilicis McGregor (Acari: Tetranychidae), and the coffee leaf miner, Leucoptera coffeella Guérin‐Méneville (Lepidoptera: Lyonetiidae), on coffee plants, Coffea arabica L. (Rubiaceae), were combined to study the relative importance of biotic interactions, including resource preferences and natural‐enemy impact, and habitat factors, such as agroforestry type and management intensity, on coffee pest densities. In the laboratory, leaf discs cut from undamaged coffee plants were significantly preferred by red spider mites over those from plants infested with conspecific mites, leaf rust pathogens [Hemileia vastatrix Berkeley & Broome (Uredinales)], or coffee leaf miners, resulting in higher reproductive success. Similarly, undamaged plants were preferred by coffee leaf miners over red spider mite‐infested plants. However, in the field, red spider mite densities were positively correlated with coffee leaf miner and leaf rust densities, thereby contrasting with laboratory predictions. Hence, our study suggests that the importance of resource preferences and fitness expected based on laboratory experiments was suppressed by environmental conditions in the field, though other unassessed biotic interactions could also have played a role. Furthermore, intensified agroforestry was characterized by higher red spider mite densities, whereas densities of its major natural enemy, the predatory mite Amblyseius herbicolus Chant (Acari: Phytoseiidae), were not related to agroforestry management. Densities of coffee leaf miner and its main natural enemy, a eulophid parasitoid (Hymenoptera), were not affected by management practices. In conclusion, patterns found in the laboratory did not hold for the field, emphasizing the difficulties of extrapolating small‐scale experiments to larger spatial scales and the need to combine both approaches.

RECRUITMENT LIMITATION IN DUNGENESS CRAB POPULATIONS IS DRIVEN BY VARIATION IN ATMOSPHERIC FORCING

Ecologists have long debated the relative importance of biotic interactions vs. abiotic forces on the population dynamics of both marine and terrestrial organisms. Investigation of stock size in Dungeness crab (Cancer magister) is a classic example of this debate. We first tested the hypothesis that adult population size was set by larval success. We found that during a five-year sampling period, adult crab population size from Oregon through central California, USA, as measured by the commercial catch, varied directly with the number of terminal-stage larvae (megalopae) returning to Coos Bay, Oregon, four years earlier; adult population size was largely determined (> 90% of the variation) by success during the larval stage. We then tested whether biotic interactions or abiotic forces caused the variation in larval success. Most of the variation (> 90%) in the number of returning megalopae is explained by the timing of the spring transition, a seasonal shift in atmospheric forcing that drives ocean currents along the west coast of the United States. Early spring transitions lead to larger numbers of returning Dungeness megalopae, while in four other crab taxa, species with very different life history characteristics, early-spring transitions lead to lower numbers of returning megalopae. During the past roughly 30 years, the size of the commercial catch of Dungeness crab is significantly and negatively correlated with the date of the spring transition throughout the California Current system. Long-term variation in the date of the spring transition may explain a major crash in the Dungeness crab fishery in central California, which began in the late 1950s. The data suggest that Dungeness crab population size is determined by variation in larval success and that a significant portion of this variation is due to the timing of the spring transition, a large-scale climatic forcer.

The macrobenthos of Lake Ontario during 1964 to 1966, and subsequent changes in the deepwater community

From January 1964 through January 1966, Dr. F.P. Ide and colleagues at the University of Toronto Great Lakes Institute (GLI) collected benthic macroinvertebrates from up to 63 stations during each of 16 cruises on Lake Ontario. This intensive survey was conducted at a time of rapidly increasing anthropogenic impact. Total invertebrate abundance decreased with increasing depth; this distribution was bimodal on the northern side of the lake, with peaks at ca. 35 and 80 m. Oligochaeta were more abundant along the southern than on the northern side of the lake, except in Humber Bay. The amphipodDiporeia was rarely collected at most stations near the mouth of the Niagara River. Chironomidae were most abundant in the Kingston Basin but were common throughout the lake at depths less than 50 m. Sphaeriidae were confined to water <100 m deep and were most abundant along the southern shore. When compared with the results of subsequent surveys of the deepwater zone, the GLI data provide evidence of two major shifts in the abundance of benthic animals. Numbers of Oligochaeta decreased by about 40% during 1965 and remained at the lower levels through 1990. The abundance ofDiporeia changed little from 1964 through 1977, but increased sharply between 1977 and 1981. These shifts do not appear to reflect implementation of controls on the discharge of phosphorus, or direct predation by fish. Changes in fish and zooplankton communities and inputs of toxic contaminants seem equally likely to be responsible for the shifts in the deepwater benthic community. We have proposed several predictions which could test the relative importance of biotic interactions or chemical stressors.

Global patterns in the establishment and distribution of exotic birds

I use three separate data bases to examine recipient community and site factors that might be influencing the establishment, persistence, and distribution of avian exotics. All in all, about half the variance between islands/regions in their numbers of successfully and unsuccessfully introduced species can be accounted for by recipient site-specific variables; the most important correlate of success is the number of native species extinctions over about the last 3000 years, which reflects the degree of human activity and habitat destruction and deterioration through intrusions of exotic predators, herbivores, and parasites. Consequently, the number of exotic species gained is close to the number of species lost through extinction. Even after controlling for avian extinctions, island area correlates positively with introduced species number. Invasion success does not decline significantly with the richness of the native avifauna (after controlling for the effects of extinctions and island area) nor the variety of potential mammalian predators. The relative proportion of extinct native species across islands/regions is negatively correlated with area and positively correlated with introduced species number and the number of endemic species. A strong correlation exists between the number of successes and the number of failures, attesting to the role of persistent acclimatization societies in increasing species numbers despite high failure rates. The relative success to failure rate increases with the number of extinct native species. The correlation between introductions and native extinctions seems to arise because native birds are usually more common, if not restricted, to native habitats while introduced birds are primary occupants of disturbed and open habitats. As more of an island's area is converted to urban, agricultural and disturbed habitats or altered through the introduction of herbivores and exotic predators, most natives lose good living space while most introduced birds, that frequent open and disturbed areas and have evolved in predator-rich areas, gain habitat. I find little support for the notion that rich avifaunas in themselves repel the establishment of avian invaders at the level of whole islands or archipelagoes. However, interactions between established exotics and natives may be influencing habitat distributions of species in both sets within islands. In both man-made habitats and native forest habitats, exotic species number and the relative abundance of exotic birds is negatively related to the number of native species. After accounting for this local variation, exotic species number is positively related to exotic species number for the entire island/region. In local surveys the relative abundance of exotic birds compared to native birds is affected by habitat (non-native habitats have more exotics) and also by the numbers of species of exotics and natives on the island. The relative importance of biotic interactions like competition, apparent competition through differential disease transmission or susceptibility, and predation in shaping the abundance and habitat affinities of exotics and native species can be difficult to unravel when regional affects are so important.

On the instincts of birds

I use three separate data bases to examine recipient community and site factors that might be influencing the establishment, persistence, and distribution of avian exotics. All in all, about half the variance between islands/regions in their numbers of successfully and unsuccessfully introduced species can be accounted for by recipient site-specific variables; the most important correlate of success is the number of native species extinctions over about the last 3000 years, which reflects the degree of human activity and habitat destruction and deterioration through intrusions of exotic predators, herbivores, and parasites. Consequently, the number of exotic species gained is close to the number of species lost through extinction. Even after controlling for avian extinctions, island area correlates positively with introduced species number. Invasion success does not decline significantly with the richness of the native avifauna (after controlling for the effects of extinctions and island area) nor the variety of potential mammalian predators. The relative proportion of extinct native species across islands/regions is negatively correlated with area and positively correlated with introduced species number and the number of endemic species. A strong correlation exists between the number of successes and the number of failures, attesting to the role of persistent acclimatization societies in increasing species numbers despite high failure rates. The relative success to failure rate increases with the number of extinct native species. The correlation between introductions and native extinctions seems to arise because native birds are usually more common, if not restricted, to native habitats while introduced birds are primary occupants of disturbed and open habitats. As more of an island's area is converted to urban, agricultural and disturbed habitats or altered through the introduction of herbivores and exotic predators, most natives lose good living space while most introduced birds, that frequent open and disturbed areas and have evolved in predator-rich areas, gain habitat.I find little support for the notion that rich avifaunas in themselves repel the establishment of avian invaders at the level of whole islands or archipelagoes. However, interactions between established exotics and natives may be influencing habitat distributions of species in both sets within islands. In both man-made habitats and native forest habitats, exotic species number and the relative abundance of exotic birds is negatively related to the number of native species. After accounting for this local variation, exotic species number is positively related to exotic species number for the entire island/region. In local surveys the relative abundance of exotic birds compared to native birds is affected by habitat (non-native habitats have more exotics) and also by the numbers of species of exotics and natives on the island. The relative importance of biotic interactions like competition, apparent competition through differential disease transmission or susceptibility, and predation in shaping the abundance and habitat affinities of exotics and native species can be difficult to unravel when regional affects are so important.