Isolation‐driven functional assembly of plant communities on islands

The physical and biotic environment is often considered the primary driver of functional variation in plant communities. Here, we examine the hypothesis that spatial isolation may also be an important driver of functional variation in plant communities where disturbance and dispersal limitation may prevent species from occupying all suitable habitats. To test this hypothesis, we surveyed the vascular plant composition of 30 islands in the Gulf of Maine, USA, and used available functional trait and growth form data to quantify the functional composition of these islands. We categorized species based on dispersal mode and used a landscape metric of isolation to assess the potential role of dispersal limitation as a mechanism of isolation‐driven assembly. We tested for island and species level effects on functional composition using a hierarchical Bayesian framework to better assess the causal link between isolation and functional variation. Growth form composition and the community mean value of functional traits related to growth rate, stress tolerance, and nutrient use varied significantly with island isolation. Functional traits and growth forms were significantly associated with dispersal mode, and spatial isolation was the strongest driver of primary trait variation, while island properties associated with environmental drivers in our system were not strong predictors of trait variation. Despite the species‐level association of dispersal mode and functional traits, dispersal mode only accounted for a small proportion of the overall isolation effect on community‐level trait variation. Our study suggests that spatial isolation can be a key driver of functional assembly in plant communities on islands, though the role of particular dispersal processes remains unclear.     

Plant‐driven changes in soil microbial communities influence seed germination through negative feedbacks

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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.     

Long‐term stasis and short‐term divergence in the phenotypes of microsnails on oceanic islands

Phenotypic divergence is often unrelated to genotypic divergence. An extreme example is rapid phenotypic differentiation despite genetic similarity. Another extreme is morphological stasis despite substantial genetic divergence. These opposite patterns have been viewed as reflecting opposite properties of the lineages. In this study, phenotypic radiation accompanied by both rapid divergence and long‐term conservatism is documented in the inferred molecular phylogeny of the micro land snails Cavernacmella (Assimineidae) on the Ogasawara Islands. The populations of Cavernacmella on the Sekimon limestone outcrop of Hahajima Island showed marked divergence in shell morphology. Within this area, one lineage diversified into types with elongated turret shells, conical shells and flat disc‐like shells without substantial genetic differentiation. Additionally, a co‐occurring species with these types developed a much larger shell size. Moreover, a lineage adapted to live inside caves in this area. In contrast, populations in the other areas exhibited no morphological differences despite high genetic divergence among populations. Accordingly, the phenotypic evolution of Cavernacmella in Ogasawara is characterized by a pattern of long‐term stasis and periodic bursts of change. This pattern suggests that even lineages with phenotypic conservatism could shift to an alternative state allowing rapid phenotypic divergence.     

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.     

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.     

Combining plant–frugivore networks for describing the structure of neotropical communities

Frugivory and seed dispersal are key processes for the maintenance of biodiversity. This is particularly true in the Neotropics, where most plant species depend on animals to disperse their seeds and most birds and mammals include fruits in their diets. We performed a continental‐scale literature review to build a database of interactions between neotropical fruits and fruit‐eating birds and mammals. Our objective was to evaluate the viability of combining literature data from different studies to describe the structure of highly diverse fruit–frugivore neotropical communities. We investigated sites that had been the focus of studies of at least four different avian and/or mammalian taxonomic orders and we included in our database only those conducted for at least a 6‐month period in order to account for the seasonality in fruit availability. In spite of a large number of study sites investigated for frugivory (n = 156), we found a huge gap in the knowledge of community‐wide fruit–frugivore interactions in the Neotropics, since most studies focused on single or a few species. Nevertheless, we were able to construct diverse plant–frugivore qualitative networks for 17 areas unevenly spread throughout the neotropical region. Using complex network analyses, we found that these networks were highly informative and non‐randomly organized. Most networks were both significantly nested and modular, characteristics related to stability and resilience in biological systems. We concluded that it is possible to use merged data to build networks for sites of conservation interest. The main advantage of using this approach is to optimize resources, avoiding exhaustive, costly and time‐consuming fieldwork when data is already available. Whilst bearing in mind the shortcomings of this methodology, these results can be used in studies aiming to understand the ecological processes structuring different communities in the neotropical region and to support conservation and restoration actions.     

Agriculture erases climate‐driven β‐diversity in Neotropical bird communities

Earth is experiencing multiple global changes that will, together, determine the fate of many species. Yet, how biological communities respond to concurrent stressors at local‐to‐regional scales remains largely unknown. In particular, understanding how local habitat conversion interacts with regional climate change to shape patterns in β‐diversity—differences among sites in their species compositions—is critical to forecast communities in the Anthropocene. Here, we study patterns in bird β‐diversity across land‐use and precipitation gradients in Costa Rica. We mapped forest cover, modeled regional precipitation, and collected data on bird community composition, vegetation structure, and tree diversity across 120 sites on 20 farms to answer three questions. First, do bird communities respond more strongly to changes in land use or climate in northwest Costa Rica? Second, does habitat conversion eliminate β‐diversity across climate gradients? Third, does regional climate control how communities respond to habitat conversion and, if so, how? After correcting for imperfect detection, we found that local land‐use determined community shifts along the climate gradient. In forests, bird communities were distinct between sites that differed in vegetation structure or precipitation. In agriculture, however, vegetation structure was more uniform, contributing to 7%–11% less bird turnover than in forests. In addition, bird responses to agriculture and climate were linked: agricultural communities across the precipitation gradient shared more species with dry than wet forest communities. These findings suggest that habitat conversion and anticipated climate drying will act together to exacerbate biotic homogenization.     

Environment‐driven changes in diversity of riparian plant communities along a mountain river

The study of changes in species richness and composition along rivers has focused on large spatial scales. It has been ignored that in different sections of the river (high mountain area, middle zone, and mouth of the river) the specific environmental conditions can generate different longitudinal patterns of the species richness and composition. In this study, we determine whether species richness and composition of the riparian plant communities change along a mountain river and whether these changes are related to environmental variables. We expect an increase in species richness and turnover along the river, that the upstream communities would be a subset of the downstream communities, and that such would be related to edaphic and hydrologic conditions. To test this, we sampled three strata of the riparian vegetation (upper: individuals with <1 cm of ND, middle: individuals with >1 cm of ND, low: individuals with >1 m tall) in a set of 15 sites that we place along a mountain river. Additionally, we recorded topographic, hydrological, morphological, and soil variables. We performed correlation analyzes to determine whether changes in species richness and turnover were related to increased distance to the origin of the river. Also, we obtained the nestedness and evaluated the importance of environmental variables with GLM, LASSO regression, and CCA. With the increase in distance, the species richness decreases in the upper stratum, but not in the middle and the low stratum (although the highest values were observed near the origin of the river), the turnover increase in all strata and the upstream communities were not a subset of the downstream communities. The changes in species richness and composition were related to topographic (altitude), hydrological (flow), and edaphic (conductivity and pH) variables. Our results indicate that at small spatial scales the patterns of richness and composition differ from what has been found at larger spatial scales and that these patterns are associated with environmental changes in the strong altitude gradients of mountain rivers.     

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.     

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.