Island Species Richness Research Articles

Forest and Isolation Determine Bat Species Richness on Mediterranean Islands: Conservation Implications

ABSTRACTBackgroundThe study of island species richness has gone far beyond traditional theories, as more and more studies attempt to include new predictor variables besides island area. Despite the rich bat diversity of Mediterranean islands, no studies have examined the factors influencing chiropteran species richness in such a geographic context.AimsWe used Greek islands as a model to detect drivers of bat species richness based on a broad species occurrence dataset from islands of the Aegean and Ionian Seas, which was explored using 10 potential predictor variables.Materials and MethodsInitially, we comprehensively evaluated all available data and sources for 53 Greek islands, deemed essential. This resulted in a reduced set of 28 islands. Predictors were grouped into three categories: island area, landscape diversity and island isolation. Generalised linear models were employed to identify the most significant factors explaining bat species richness in the 28 islands.ResultsTree cover and the distance between the islands and their nearest coast (mainland or island) best performed to predict bat species richness on Greek islands.DiscussionWe show the important role of landscape composition and degree of isolation in influencing the distribution of bat species on islands.ConclusionWe highlight the importance of forest habitats and the vital role of their management and protection, especially in the insular systems of the Mediterranean basin, where the risk of forest degradation or loss due to wildfires and land use change is especially high.

The herpetofauna of the Sporades Islands (Aegean Sea, Greece): New discoveries and a review of a century of research

The Sporades are one of the most biologically important archipelagos in the Aegean Sea (Greece) and have received priority conservation over the last 50 years. However, despite numerous early efforts, its herpetofauna is only partially described, resulting in many distributional gaps that have prevented adequate understanding and management of the resident species communities. We review one century of bibliography from the Northern Sporades and combine this information with a review of museum specimens and insights from numerous extensive field surveys over the last near-decade to provide for the first time a comprehensive picture of the reptiles and amphibians of the archipelago. We report here on 26 new island records and find that the herptile communities of the region are largely derived from the herpetofauna of the nearby Thessaly mainland, with only a few introduced taxa. There is also a small but significant set of endemic taxa in the archipelago. Island species richness declines with decreasing island size and increasing duration of island isolation. Herptile communities on smaller islands are progressively nested subsets of the communities on larger islands. The presence of reptile species depends sensitively on the condition and management of native ecosystems. While non-aquatic species maintain largely healthy populations, most populations are under pressure from the combined effects of rampant tourist development, the destruction and degradation of rare wetland habitats, and the abandonment of traditional agricultural landscapes. We provide recommendations regarding sustainable management of the local reptile and amphibian populations.

Island area and diet predict diversity and distribution of bats in a Pacific Northwest archipelago

Abstract The island biogeography theory predicts that species richness in islands and island-like systems is the ultimate result of island isolation and area. Species with high dispersal capabilities are predicted to be less affected by these factors because of their capacity to move more efficiently between islands or habitats, and here we test this idea in bats, the only mammals capable of flight. We conducted mist net and acoustic surveys across 21 islands in the San Juan Archipelago (Washington State, United States) and adjacent northwest mainland to: (i) investigate the effects of island area, distance from mainland, and habitat on bat diversity; and (ii) evaluate whether differences in morphological (body mass, forearm length, wing loading) and ecological (dietary niche breadth, foraging guild) traits among species influence their prevalence across islands. We found that island size strongly influenced patterns of species richness, with larger islands having a greater number of bat species. However, neither island distance from mainland nor any measure of habitat availability was a significant predictor of species richness at the scale of this study. Additionally, we found that dietary niche breadth, as opposed to any morphological trait, best predicted the prevalence of species across the islands. Our results suggest that species with more specialized diets may be more vulnerable to habitat fragmentation, and provide insight into how geographic and ecological factors affect the diversity of insular bat communities, adding to growing knowledge about the role of species traits as mediators of their responses to large-scale landscape structure.

Island biodiversity in peril: Anticipating a loss of mammals' functional diversity with future species extinctions.

Islands are biodiversity hotspots that host unique assemblages. However, a substantial proportion of island species are threatened and their long-term survival is uncertain. Identifying and preserving vulnerable species has become a priority, but it is also essential to combine this information with other facets of biodiversity like functional diversity, to understand how future extinctions might affect ecosystem stability and functioning. Focusing on mammals, we (i) assessed how much functional space would be lost if threatened species go extinct, (ii) determined the minimum number of extinctions that would cause a significant functional loss, (iii) identified the characteristics (e.g., biotic, climatic, geographic, or orographic) of the islands most vulnerable to future changes in the functional space, and (iv) quantified how much of that potential functional loss would be offset by introduced species. Using trait information for 1474 mammal species occurring in 318 islands worldwide, we built trait probability density functions to quantify changes in functional richness and functional redundancy in each island if the mammals categorized by IUCN as threatened disappeared. We found that the extinction of threatened mammals would reduce the functional space in 63% of the assessed islands, although these extinctions in general would cause a reduction of less than 15% of their overall functional space. Also, on most islands, the extinction of just a few species would be sufficient to cause a significant loss of functional diversity. The potential functional loss would be higher on small, isolated, and/or species-rich islands, and, in general, the functional space lost would not be offset by introduced species. Our results show that the preservation of native species and their ecological roles remains crucial for maintaining the current functioning of island ecosystems. Therefore, conservation measures considering functional diversity are imperative to safeguard the unique functional roles of threatened mammal species on islands.

Digest: A model for speciation in island systems.

Ecological pulses, marked by rapid environmental shifts, shape species dynamics such as speciation. Princepe et al. 2024 explore how intermittent migration, seabed depth, and sea-level changes influence species richness in islands. Their model, revealing nuanced richness patterns and migration impacts, contributes to understanding island evolution and offers insights into real-world cases. Examining the Philippines Archipelago, known for rich biodiversity, parallels emerge, showcasing the complex interplay of isolation, land bridges, and sea-level shifts in shaping genetic diversity. Overall, the findings enhance our understanding of population evolution in dynamic island environments.

Big eyes on the Island: First record of Chiroderma villosum of Cozumel Island, México and bat species richness in the Caribbean islands

El registro más cercano de Chiroderma villosum a Isla Cozumel, está documentado en la ciudad de Playa del Carmen, Quintana Roo. En este trabajo documentamos el primer registro del murciélago C. villosum para Isla Cozumel y compilamos la información sobre la riqueza de especies de murciélagos en las Islas del Caribe. El 5 de mayo de 2021 se realizó una salida de campo, se instalaron redes de niebla a nivel del suelo. Posteriormente se tomaron los siguientes datos de cada individuo capturado: medidas del antebrazo con un vernier con una precisión de 1 mm, se tomó el peso con una báscula digital con precisión de 1 g, y se identificó la especie siguiendo la clave de murciélagos de México. Durante la noche de trabajo se capturaron 8 individuos de Artibeus jamaicensis y un ejemplar macho adulto no reproductivo de C. villosum a las 21:45 hr en una red de niebla colocada cruzando una brecha dentro de la selva mediana en el noroeste de la zona arqueológica de San Gervasio. En Isla Cozumel se han documentado 19 especies de murciélagos correspondientes a 5 familias y 15 géneros. Con el registro de este espécimen, se incrementa a 20 el número de especies y a 16 géneros de quirópteros presentes en la isla. Aunque todavía faltan más estudios, es muy probable que esta especie tenga una distribución amplia en todas las áreas de bosque en Cozumel.

Influence of geographic predictors on beta diversity of insular snakes communities

Islands biogeography theory establishes that species richness and composition in islands result from ecological and evolutionary processes, such as immigration/emigration rate and the balance between speciation and extinction. Islands' geographic distribution, area, and distance from the mainland are important features in structuring communities of different taxonomic groups. Here, we aimed to evaluate the influence of geographic predictors on the variation of snakes' community composition. The study area encompassed 17 oceanic islands in the state of São Paulo, southeast Brazil. We recorded 40 species of snakes, with an average of 8.53 ± 8.11 species per island. We observed that islands closer to the mainland are more similar in snake composition, and more distant islands have a greater variation in composition among themselves. We verified a positive correlation between beta diversity and distance from the mainland, turnover component and distance from the mainland, nested component and distance among islands, and nestedness and island area. So, we conclude that the spatial pattern of island snake composition seems to be related to species dispersal. However, it can also be a relictual fauna pattern due to the isolation of populations due to rising sea level and the subsequent formation of the studied islands. Our results reinforces the pivotal role of geographic factors in shaping spatial patterns of insular communities.

Island biogeography, competition, and abiotic filtering together control species richness in habitat islands formed by nurse tree canopies in an arid environment

ABSTRACT The theory of island biogeography predicts that island size is a key predictor of community species richness. Islands can include any habitat surrounded environments that are inhospitable to the resident species. In arid environments, nurse trees act as islands in an environment uninhabitable to many plant species, and the size of the canopy controls the size of the understory plant community. We predicted that plant species richness will be affected by the area of the habitat and decrease with habitat isolation. We sampled the adult and seedling plant communities at canopy center, canopy edge, and outside canopy microhabitats. We found that species richness in both adult and seedling communities increases with increasing island area. However, richness in seedling communities was greater than in adult communities, and this effect was greatest at the canopy center microhabitat. Competition has been demonstrated to be more important in controlling species distributions near the canopy center, and stress is more important near the canopy edge. Thus, our results suggest that neutral forces, biotic interactions, and abiotic filtering act together to control species richness in these island communities.

Distribution pattern of plant species richness of uninhabited islands in the Bohai Sea area

Distribution pattern of plant species richness of uninhabited islands in the Bohai Sea area

Mixed evidence for the small‐island effect in a replicated colonisation experiment

AbstractAimPlant species richness increases with island/patch area, but the effect of spatial scale on the shape of the species‒area relationship remains debatable. We asked whether the small‐island effect (SIE; i.e. different relationship between island area and species richness on smaller compared to larger plots) occurs in plants naturally colonising extremely small de novo plots.LocationNorthern Europe.MethodsWe established 16 experimental plots with bare ground (two plots of each of eight sizes, from 2 to 0.01 m2) in each of four localities, and monitored colonisation of these plots by vascular plants from 2008 to 2010. We modelled species‒area relationships in these plots by linear and breakpoint regressions, and we analysed plant functional traits with respect to plot size.ResultsSmall‐seeded plants were over‐represented among the primary colonisers, and plant functional traits changed with plot size. On average, the number of plant species was six times greater in our largest (2 m2) than in our smallest (0.01 m2) plots. The SIE was observed in half of our analyses; the semi‐log models identified SIE more often than the log‐log models did (71.1% and 28.9% of all analyses respectively).ConclusionsThe evidence for the small‐island effect in boreal forest plants colonising patches of bare ground is mixed. Despite extremely small patch size and habitat uniformity, half of our data showed a “classic” species‒area relationship. Our findings hint that the probability of SIE occurrence increases with decreases in the species pool of primary colonisers and with decreases in the time from the beginning of colonisation. We suggest that the analysis of plant functional traits in study systems where SIE does occur be conducted to uncover the drivers of this interesting biogeographical phenomenon.

Evaluating the correlation between area, environmental heterogeneity, and species richness using terrestrial isopods (Oniscidea) from the Pontine Islands (West Mediterranean)

Area and environmental heterogeneity influence species richness in islands. Whether area or environmental heterogeneity is more relevant in determining species richness is a central issue in island biogeography. Several models have been proposed, addressing the issue, and they can be reconducted to three main hypotheses developed to explain the species-area relationship: (1) the area-per se hypothesis (known also as the extinction-colonisation equilibrium), (2) the random placement (passive sampling), and the (3) environmental heterogeneity (habitat diversity). In this paper, considering also the possible influence of geographic distance on island species richness, we explore the correlation between area, environmental heterogeneity, and species richness by using faunistic data of Oniscidea inhabiting the Pontine Islands, a group of five small volcanic islands and several islets in the Tyrrhenian Sea, located about 60 km from the Italian mainland. We found that the colonisation of large Pontine Islands may occur via processes independent of geographic distance which could instead be an important factor at a much smaller scale. Such processes may be driven by a combination of anthropogenic influences and natural events. Even in very small-size island systems, environmental heterogeneity mostly contributes to species richness. Environmental heterogeneity could influence the taxocenosis structure and, ultimately, the number of species of Oniscidea via direct and indirect effects, these last mediated by area which may or may not have a direct effect on species richness.

What defines insularity for plants in edaphic islands?

The theory of island biogeography postulates that size and isolation are key drivers of biodiversity on islands. This theory has been applied not only to true (e.g. oceanic) islands but also to terrestrial island‐like systems (e.g. edaphic islands). Recently, a debate has opened as to whether terrestrial island‐like systems function like true islands. However, identifying the effect of insularity in terrestrial systems is conceptually and methodologically challenging because recognizing species source(s) and measuring isolation is not as straightforward as for true islands. We contribute to the debate by proposing an approach to contextualize the definition of insularity and to identify the role of isolation in terrestrial island‐like systems. To test this approach, we explored the relationship between insularity predictors and specialist species richness of edaphic islands in three systems in Europe (spring fens, mountaintops, and outcrops). We detected that insularity affected specialist richness of edaphic islands through island size and target effect (i.e. an emergent property of islands depending on their isolation and size). As predicted by the Theory of Island Biogeography, species richness decreased with increasing isularity. Given the comprehensiveness and ease of implementation of our approach, we encourage its extension to other island‐like systems.

Nonlinear thresholds in the effects of island area on functional diversity in woody plant communities

Abstract Threshold nonlinearities in the relationship between island area and species richness can result in dramatic declines in richness with a seemingly small decline in area near the threshold. What is not known, is whether threshold declines in richness are also accompanied by nonlinear changes in functional trait space and non‐random shifts of trait group composition in response to declining area. Plant species richness was recorded, and four functional diversity (FD) indices calculated based on 12 traits for 76 tree species in 5,082 plots (5 × 5 m) on 29 islands in the Thousand Island Lake region, Zhejiang, China. We tested for threshold nonlinearities in richness and FD relationships using segmented regression, with randomized resampling using a null model to account for potential sampling artefacts. We clustered tree species into trait groupings based on trait dissimilarity, and used multinomial segmented models to test whether threshold area effects on FD were associated with shifts in trait group composition. Species richness and three of the four FD indices showed significant threshold nonlinearities with declining island area below a threshold of ca 1.16–6.88 ha. K‐means clustering identified two trait groups, reflecting species at the ‘fast’ versus ’slow’ ends of the trait spectrum. Significant area thresholds in trait group composition were driven by increasing relative richness, but decreasing relative abundance, of species with resource‐conservation traits on islands below the threshold. Synthesis. We found a threshold collapse in richness on islands below c. 1 ha, resulting in a significant decrease in functional trait space (functional richness) and an increase in the degree of niche differentiation among species in the community (functional divergence) on small islands. Threshold effects were associated with non‐random re‐assembly of trait group composition, favouring the relative richness of species at the ‘slow’ resource‐conservation end of the trait spectrum, but the relative abundance of a few species with ‘fast’ resource‐acquisition traits. These results suggest that disturbance‐driven dynamics potentially drive a functional shift in the ecosystem state at a critical threshold. We show that a minimum critical habitat area is required to maintain functional diversity in woody plant communities.

Unravelling the small‐island effect through phylogenetic community ecology

AbstractAimThe small‐island effect (SIE) describes a different relationship between island area and species richness on smaller compared to larger islands. The pattern has recently gained widespread support. However, few studies have attempted to identify the actual mechanisms driving the SIE. Here, we use a phylogenetic community framework to study the SIE, based on the assumption that if the dominant assembly processes differ between small and large islands, patterns of phylogenetic community structure should shift across the area and habitat diversity gradient.LocationThe Aegean Archipelago, Greece.TaxonPlants.MethodsWe used a large dataset of 3,262 vascular plant species distributed across 173 islands, in combination with a species‐level phylogeny. The phylogenetic community structure of each island was calculated using a null modelling framework and was quantified using effect sizes (ES); negative values indicating phylogenetic clustering and positive values overdispersion. Habitat diversity, species richness, phylogenetic diversity (PD) and ES values were regressed against log10‐transformed area and we tested for a SIE using piecewise regression models. We also assessed differences in taxonomic and phylogenetic composition between small and large islands using a beta diversity framework.ResultsWe found evidence of a SIE using species richness, PD and phylogenetic community structure (ES values). Small islands displayed low variation in habitat diversity and tended to be more phylogenetically clustered, while large islands shifted from phylogenetic clustering towards phylogenetic overdispersion with increasing area and habitat diversity. In addition, we showed that phylogenetic composition tended to be more similar between small islands than expected.Main conclusionOverall, our results provide an example of a SIE in the analysis of island phylogenetic community structure, and point to a role of habitat diversity in driving the SIE more generally.

Island‐biogeographic patterns of spider communities on epiphytes depend on differential space use among functional groups

AbstractAimPositive relationships between island size and species richness have been found for oceanic and non‐oceanic islands. However, the effect of isolation, also predicted by island‐biogeographical theory, seems challenging to test. Species‐specific mechanisms drive the perception of distance and space for animals, and therefore isolation patterns are masked when described at the community level. We used epiphytic plants (canopy islands) and their spider communities to study how functional group and species‐specific use of space modifies the effect of island‐level predictors on species richness.LocationA coffee plantation near Xalapa, Mexico.TaxonSpiders (Araneae), vascular epiphytes (Bromeliaceae, Piperaceae, Orchidaceae, Araceae, Pteridophyta).MethodsWe collected canopy islands from three trees and recorded their three‐dimensional position. Afterwards, we collected all epiphyte‐dwelling spiders. We used Generalized Linear Models (GLMs) to analyse the effect of island size and isolation on the species richness of spider communities and community subgroups (guilds, mobility groups and families). Isolation was calculated using two metrics (aerial and walking distances) associated with different ways of mobility in the canopy (bridging and wandering respectively). We evaluated species‐level associations with island features and tested for sample‐size effects on GLMs using null‐model analysis of ecological networks.ResultsAlthough unimportant at the community level, isolation was more important than island size for some spider subgroups, while having negligible effects for others. Species‐specific responses to isolation and island size were responsible for the variability in species richness patterns observed for broader subgroups (families, guilds and mobility groups). Specific responses of mobility groups to isolation metrics were in line with two ways of moving between islands; one using tree branches as pathways (walking isolation) and the other using silk threads to connect points directly across the air (aerial isolation).Main conclusionsSpatial patterns of epiphyte‐dwelling spiders were explained better when functional traits were considered, and the effect of isolation became apparent only when analysed per functional group, whereas it was masked at the community level. This supports the hypothesis that hunting behaviour and mobility affect the specific perception of space and distance even at the small spatial scale of canopy islands.

Remoteness promotes biological invasions on islands worldwide

One of the best-known general patterns in island biogeography is the species-isolation relationship (SIR), a decrease in the number of native species with increasing island isolation that is linked to lower rates of natural dispersal and colonization on remote oceanic islands. However, during recent centuries, the anthropogenic introduction of alien species has increasingly gained importance and altered the composition and richness of island species pools. We analyzed a large dataset for alien and native plants, ants, reptiles, mammals, and birds on 257 (sub) tropical islands, and showed that, except for birds, the number of naturalized alien species increases with isolation for all taxa, a pattern that is opposite to the negative SIR of native species. We argue that the reversal of the SIR for alien species is driven by an increase in island invasibility due to reduced diversity and increased ecological naiveté of native biota on the more remote islands.

The true tempo of evolutionary radiation and decline revealed on the Hawaiian archipelago.

Establishing the relationship between rates of change in species richness and biotic and abiotic environmental change is a major goal of evolutionary biology. Although exquisite fossil and geological records provide insight in rare cases, most groups lack high-quality fossil records. Consequently, biologists typically rely on molecular phylogenies to study the diversity dynamics of clades, usually by correlating changes in diversification rate with environmental or trait shifts. However, inferences drawn from molecular phylogenies can be limited owing to the challenge of accounting for extinct species, making it difficult to accurately determine the underlying diversity dynamics that produce them. Here, using a geologically informed model of the relationship between changing island area and species richness for the Hawaiian archipelago, we infer the rates of species richness change for 14 endemic groups over their entire evolutionary histories without the need for fossil data, or molecular phylogenies. We find that these endemic clades underwent evolutionary radiations characterized by initially increasing rates of species accumulation, followed by slow-downs. In fact, for most groups on most islands, their time of evolutionary expansion has long past, and they are now undergoing previously unrecognized long-term evolutionary decline. Our results show how landscape dynamism can drive evolutionary dynamics over broad timescales, including driving species loss that is not readily detected using molecular phylogenies, or without a rich fossil record. We anticipate that examination of other clades where the relationship between environmental change and species richness change can be quantified will reveal that many other living groups have also experienced similarly complex evolutionary trajectories, including long-term and ongoing evolutionary decline.

Effects of habitat fragmentation on species diversity of ground dwelling beetles in the Thousand-Island Lake region, Zhejiang, China.

The ground dwelling beetles on 20 land-bridge islands and 8 mainland sites in the Thousand-Island Lake region were collected by pitfall trap methods for six times from July 2012 to April 2014 (in three seasons, spring, summer and autumn). The species abundance, composition, diversity and seasonal dynamics of ground dwelling beetles were analyzed. Besides, the relationship between island spatial attributes (e.g., island area and isolation) and the diversity of ground dwe-lling beetles were studied. There were 3370 litter-dwelling beetle individuals belonging to 26 families and 101 species.For species richness of ground dwelling beetles, there were significant diffe-rences between the large islands and mainland sites, the mean density on small islands was significantly higher than on mainland sites. The species turnover was higher among mainland sites than among islands. The species richness on islands was significantly associated with island area, and the density was significantly associated with the distance to mainland. The species richness of islands in summer was higher than that in spring and autumn. Shannon, Simpson and Pielou indices of islands and mainlands were all highest in summer and lowest in autumn.

Extinction debt on reservoir land-bridge islands

Large dams cause extensive inundation of habitats, with remaining terrestrial habitat confined to highly fragmented archipelagos of land-bridge islands comprised of former hilltops. Isolation of biological communities on reservoir islands induces local extinctions and degradation of remnant communities. “Good practice” dam development guidelines propose using reservoir islands for species conservation, mitigating some of the detrimental impacts associated with flooding terrestrial habitats. The degree of species retention on islands in the long-term, and hence, whether they are effective for conservation is currently unknown. Here, we quantitatively review species' responses to isolation on reservoir islands. We specifically investigate island species richness in comparison with neighbouring continuous habitat, and relationships between island species richness and island area, isolation time, and distance to mainland and to other islands. Species' responses to isolation on reservoir islands have been investigated in only 15 of the >58,000 large-dam reservoirs (dam height >15m) operating globally. Research predominantly originates from wet tropical forest habitats and focuses on mammals, with species richness being the most widely-reported ecological metric. Terrestrial taxa are, overall, negatively impacted by isolation on reservoir islands. Reservoir island species richness declines with isolation time, and although the rate of loss is slower on larger islands, all islands exhibit depauperate species richness <100years after isolation, compared to continuous mainland habitats. Such a pattern of sustained and delayed species loss following large-scale habitat disturbance is indicative of an extinction debt existing for reservoir island species: this pattern is evident across all taxonomic groups and dams studied. Thus, reservoir islands cannot reliably be used for species conservation as part of impact mitigation measures, and should instead be included in area calculations for land impacted by dam creation. Environmental licensing assessments as a precondition for future dam development should explicitly consider the long-term fate of island communities when assessing biodiversity loss vs energy output.

Maintenance of biodiversity on islands.

MacArthur and Wilson's theory of island biogeography predicts that island species richness should increase with island area. This prediction generally holds among large islands, but among small islands species richness often varies independently of island area, producing the so-called 'small-island effect' and an overall biphasic species-area relationship (SAR). Here, we develop a unified theory that explains the biphasic island SAR. Our theory's key postulate is that as island area increases, the total number of immigrants increases faster than niche diversity. A parsimonious mechanistic model approximating these processes reproduces a biphasic SAR and provides excellent fits to 100 archipelago datasets. In the light of our theory, the biphasic island SAR can be interpreted as arising from a transition from a niche-structured regime on small islands to a colonization-extinction balance regime on large islands. The first regime is characteristic of classic deterministic niche theories; the second regime is characteristic of stochastic theories including the theory of island biogeography and neutral theory. The data furthermore confirm our theory's key prediction that the transition between the two SAR regimes should occur at smaller areas, where immigration is stronger (i.e. for taxa that are better dispersers and for archipelagos that are less isolated).