Kostas A. Triantis

Island Species Richness Increases with Habitat Diversity

Species richness is commonly thought to increase with habitat diversity. However, a recent theoretical model aiming to unify niche and island biogeography theories predicted a hump‐shaped relationship between richness and habitat diversity. Given the contradiction between model results and previous knowledge, we examine whether the relationship between species richness and habitat diversity is consistently monotonically increasing and under which circumstances, if at all, such relationships could be hump shaped. We review the empirical evidence about the shape of such relationships and show that species richness on islands usually increases with habitat diversity and that it never decreases. We also critically examine the assumptions of the theoretical model and modify them to incorporate a less restrictive definition of niche width. The modified assumptions lead to simulations that better capture real patterns, using either simple parameters or observed distributions of niche breadth. Further work is needed to incorporate ecological interactions and metacommunity dynamics if the aim is to merge niche and island biogeography theories in a realistic modeling framework.

Functional biogeography of oceanic islands and the scaling of functional diversity in the Azores

Significance Biogeographic theory builds upon a long history of analyzing species-diversity patterns of remote islands, but no previous studies have attempted to investigate corresponding patterns in functional traits on islands. Our analyses of functional diversity (FD) for spiders and beetles in the Azorean archipelago reveal that FD increases with species richness, which, in turn scales with island area regardless of the taxa and distributional group considered (endemics, natives, and exotics). Our results also support the hypothesis that each group contributes to FD in proportion to their species richness and that, being dominant, exotic species have significantly extended the realized trait space of the Azorean islands. Further analyses in other archipelagos are needed to establish whether our findings are representative of oceanic islands. Analyses of species-diversity patterns of remote islands have been crucial to the development of biogeographic theory, yet little is known about corresponding patterns in functional traits on islands and how, for example, they may be affected by the introduction of exotic species. We collated trait data for spiders and beetles and used a functional diversity index (FRic) to test for nonrandomness in the contribution of endemic, other native (also combined as indigenous), and exotic species to functional-trait space across the nine islands of the Azores. In general, for both taxa and for each distributional category, functional diversity increases with species richness, which, in turn scales with island area. Null simulations support the hypothesis that each distributional group contributes to functional diversity in proportion to their species richness. Exotic spiders have added novel trait space to a greater degree than have exotic beetles, likely indicating greater impact of the reduction of immigration filters and/or differential historical losses of indigenous species. Analyses of species occurring in native-forest remnants provide limited indications of the operation of habitat filtering of exotics for three islands, but only for beetles. Although the general linear (not saturating) pattern of trait-space increase with richness of exotics suggests an ongoing process of functional enrichment and accommodation, further work is urgently needed to determine how estimates of extinction debt of indigenous species should be adjusted in the light of these findings.

Species richness can decrease with altitude but not with habitat diversity.

In the paper by Allouche et al. (1), the authors suggested that species richness decreases at high levels of habitat diversity because the area available per habitat decreases [area–heterogeneity tradeoff hypothesis (AHTO)]. They showed a hump-shaped relationship between Catalonian bird richness and altitudinal range in grid cells, the authors’ surrogate for environmental heterogeneity. However, birds select habitats mainly based on vegetation structure and floristic composition (2). Catalonian high altitudes are dominated by uniform coniferous forests or simple habitats with low vegetation cover (outcrops, grasslands, and scrublands) that are known to be poor in bird richness. Furthermore, high-altitude grid cells have the largest altitudinal ranges but much fewer habitats than lower altitudes (using 48 habitat categories obtained from Inventario Nacional Forestal III, 2007–2008, Spanish Ministerio de Medio Ambiente). Elevation range in Catalonia is tightly correlated with maximum altitude (r = 0.951; P < 0.0001) and mean elevation (r = 0.858; P < 0.0001), but poorly correlated with environmental heterogeneity (number of habitats per cell: r = 0.049, P = 0.349; Shannon index across all habitat categories: r = 0.119, P = 0.024). When analyzed together (Generalized Additive Model; Fig. 1), bird-species richness shows a hump-shaped relationship with mean elevation [nonlinear P (n-Lp) = 0.003], a negative linear relationship with altitudinal range (P = 0.001; n-Lp = 0.426), and a positive, monotonic relationship with habitat diversity (P < 0.001; n-Lp = 0.336), as predicted from ecological theory (3). Thus, the unimodal relationship between altitudinal range and richness (1) merely reflects the well-known hump-shaped relationship between species richness and altitude (4), not a tradeoff between richness and environmental heterogeneity.

Drivers of extinction: the case of Azorean beetles

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

Biogeography, land snails and incomplete data sets: the case of three island groups in the Aegean Sea

This research aimed to study the land snails of three island groups (Skyros, Astypalaia, Kalymnos) in the Aegean Sea and the biogeographical patterns arising, with reference to the other Aegean islands. A total of 91 species was found on the 37 islands surveyed, with 89 being extant. For the majority of the islands the malacofauna is reported for the first time. Similarity indices, chorotypes and nestedness were studied for each of the island groups. The islands belonging to the same island group were clustered together, exhibiting a quite delimited malacofauna. The Mediterranean and Palaearctic elements dominate, with the Aegean element increasing in the Astypalaia group. The three island groups, despite their different geological histories and the different level of the present‐day geographical isolation, have been formed quite recently in geological time. This “recent” formation of the groups results in islands, especially the small ones, which still behave as parts of a continuous land mass. In the context of the new results from the present work, the completeness of the knowledge of the terrestrial malacofauna of the Aegean islands is discussed, and the caution needed for drawing general biogeographical conclusions based on the available data is emphasized.

Biogeographic patterns of tenebrionid beetles (Coleoptera, Tenebrionidae) on four island groups in the south Aegean Sea

New records on Tenebrionidae from four island groups of the southern Aegean (Santorini, Astypalaia, Nisyros and Kalymnos, with their satellite islets – 26 islands in total) allowed investigation of the biogeography of the taxon in the area. All tenebrionid records from small islets are new, with the exception of those from Pserimos and the satellite islets of Santorini. Chorological and nestedness analyses, species‐area relationships and various measures of faunal similarity based on binary and quantitative data matrices were applied in order to explore biogeographic patterns. The Anatolian and east Mediterranean faunal elements dominate on the island groups of Kalymnos and Nisyros, with a considerable decrease on Astypalaia and Santorini, where circum‐Mediterranean elements prevail. Analyses of faunal similarities clustered together the well defined island groups of Santorini and Astypalaia, while Kalymnos and Nisyros groups were clustered together rather vaguely. Astypalaia and Santorini were also the most nested island groups in the studied area. The biogeographic position of Astypalaia within the central Aegean islands is well supported by the data.

Biodiversity patterns of terrestrial isopods from two island groups in the Aegean Sea (Greece): Species–area relationship, small island effect, and nestedness

ABSTRACT Although almost one third of Aegean islands have an area smaller than 1 km2, very few studies have concentrated on this spatial scale. We investigate biodiversity patterns of terrestrial isopods in 2 island groups (Kalymnos and Astypalaia) of the Aegean Sea, consisting mainly of small islands and characterized by different geological histories, isolation, and species source pools. We focus on the species–area relationship (SPAR), community nestedness and the investigation of the small island effect (SIE) in these island groups. The slopes of the SPARs for terrestrial isopods of the 2 groups are almost identical and place the 2 island groups within the intraprovincial category. The SIE investigated was detected in both island groups, with the Kalymnos group exhibiting a higher upper limit (the island size threshold under which an increase of species number with increase of area in small islands is not observed). Both island groups are highly nested and exhibit similar levels of nestedness. Despite the different geological histories, isolation, and source pools, the 2 island groups exhibit great similarity in all the patterns investigated. The islands studied still “behave” as parts of a continuous land mass, with high numbers of species even on the smallest islands, limited net effects of island size, and a significant contribution of the interrelationship between area and environmental heterogeneity to the total species richness. Nomenclature: Schmalfuss, 2003.

Island biogeography: Taking the long view of nature’s laboratories

Dynamics of island biodiversity Fifty years ago, MacArthur and Wilson published their influential book, The Theory of Island Biogeography. This work provided a quantitative framework for understanding the ecological processes governing the diversity of species on oceanic islands. Whittaker et al. review the subsequent progress in the field, focusing particularly on the integration of the ecological model with island geophysical dynamics. Recent work is showing how immigration, speciation, and extinction respond to the phases of emergence, development, and submergence in oceanic islands. Science, this issue p. eaam8326 BACKGROUND Ever since Darwin, natural scientists have turned to islands for inspiration and for model systems. For the past half century, they have done so largely within the paradigm established by Robert H. MacArthur and Edward O. Wilson’s Theory of Island Biogeography, which provided a quantitative, dynamic framework, based upon assumptions of geographically predictable patterns of immigration, extinction, and speciation. Although this approach has proven productive, its application to remote archipelagos and evolutionary time scales has been hampered by a rather static view of islands themselves, despite mounting evidence of their dynamism as platforms. We review recent progress in integrating the largely ecological thinking of their theory with insights on the longer-term dynamics of both the islands and their biotas. ADVANCES Classification and analysis of marine islands by their geophysical dynamics, and of their species by how they colonized, provides a step toward a more nuanced biogeography out of which new insights are already emerging. This perspective is exemplified by the general dynamic model of oceanic island biogeography, which predicts how immigration, speciation, and extinction respond to the typical life cycle of hotspot islands, with phases of emergence, development, and submergence. The model successfully predicts such emergent patterns as the occurrence of peak diversification on youthful, expanding islands with maximum vacant niche space. Diversity patterns analyzed for large numbers of data sets have confirmed the importance of in situ evolutionary dynamics on remote archipelagos, which typically possess steep island species–area relationships, especially for endemic taxa. We may infer that variations in propagule flow among islands within archipelagos are important in modulating these emergent diversity patterns. There is, for example, good support for an “island progression rule” in which older land masses donate colonists to younger islands (consistent with the generalization of islands as “sinks”), but there is also increasing evidence of “reverse colonization,” including from islands to continental regions. Advances are also being made in linking such island biogeographical models with the classic traits and syndromes of insular species, although this first demands that previous generalizations are rigorously reexamined using expanded data sets and modern techniques of analysis. A classic insular syndrome is the loss of dispersability of formerly dispersive species following island colonization, for which there is now good evidence for several taxa, including many genera of land birds. Yet, paradoxically, and perhaps controversially, it has also been inferred that many species of plants lacking specialized dispersal adaptations can colonize quite remote islands, often by nonstandard means of transport. Unfortunately, island evolutionary syndromes, such as loss of flight in birds, frequently predispose species to heightened extinction risk when islands are colonized and transformed by humans, as we also document. OUTLOOK Developments in theory and in analytical and modeling capabilities within biological and Earth system science, and the pooling of large numbers of data sets, enhancing statistical power, collectively hold the promise of a new synthesis in island biogeography. This synthesis will need to accommodate evidence of the long-term dynamics of remote island systems, whereby some lineages persist far longer than any particular island platform, while others founder as their sole island home sinks under the waves. The promise is of a biogeography in the tradition of the MacArthur–Wilson theory, generating and testing predictive models, but extended to accommodate a more sophisticated suite of insular geological and environmental dynamics, combined with a fuller understanding of patterns and processes of gene flow within and between archipelagos. Islands provide model systems for the investigation of the fundamental biogeographical processes of migration, diversification, and extinction, as discussed herein with emphasis on long-term dynamics. Islands provide classic model biological systems. We review how growing appreciation of geoenvironmental dynamics of marine islands has led to advances in island biogeographic theory accommodating both evolutionary and ecological phenomena. Recognition of distinct island geodynamics permits general models to be developed and modified to account for patterns of diversity, diversification, lineage development, and trait evolution within and across island archipelagos. Emergent patterns of diversity include predictable variation in island species–area relationships, progression rule colonization from older to younger land masses, and syndromes including loss of dispersability and secondary woodiness in herbaceous plant lineages. Further developments in Earth system science, molecular biology, and trait data for islands hold continued promise for unlocking many of the unresolved questions in evolutionary biology and biogeography.

Biogeographical determinants of pteridophytes and spermatophytes on oceanic archipelagos

Using the data from six oceanic archipelagos, we investigated the species richness patterns on islands for all natives, archipelagic endemics and single-island endemics of pteridophytes and spermatophytes. We tested the hypothesis that the descriptive ability of biogeographical factors for species is reduced as we move from native species, to archipelagic endemics and to single island endemics, because of the increasing importance of island ‘idiosyncrasies’ (i.e. unique features of each island shaping its biota, such as catastrophic volcanic eruptions, random colonization events, mega-landslides) in controlling the species richness of endemic species. This hypothesis was addressed using two approaches: (1) the island species–area relationships (ISARs), and (2) a multiple regression approach with variable selection based on permutation, to test the combined effects of island area with other biogeographical factors. Area was an effective predictor of species richness for all native species (R 2 = 0.568 and R 2 = 0.624 for pteridophytes and spermatophytes respectively), but its predictive capacity decreased for archipelagic endemics (R 2 = 0.261 and R 2 = 0.531) and single-island endemics (R 2 = 0.084 and R 2 = 0.438). The reduction of R 2 from all native species, to archipelagic endemics and to single-island endemics was attributed to the increasing effects of the ‘idiosyncrasies’ of each island. The predictive capacity of multiple regression models increased with respect to ISARs, ranging from 27.3% (for single-island endemic pteridophytes) to 83.3% (all native pteridophytes), and included three to five predictors. Island area remained the most important variable for spermatophytes but was less important for pteridophytes. For pteridophytes, elevation was the most important predictor for native species, while isolation-related variables were the most important predictors for archipelagic endemics and single-island endemics. Our results support the hypothesis that as we move from native, to archipelagic endemic and to single-island endemic species the predictive ability of models is reduced, indicating an increased effect of the ‘idiosyncratic’ character of islands.

The hydrobioid freshwater gastropods (Caenogastropoda, Truncatelloidea) of Greece: new records, taxonomic re- assessments using DNA sequence data and an update of the IUCN Red List Categories

Abstract Hydrobioid freshwater gastropods were collected from mainland and insular Greece. Several threatened taxa, such as Graecoanatolica vegorriticola, Pseudamnicola negropontina, Pseudamnicola pieperi, Pseudobithynia eubooensis and Pseudoislamia balcanica, were recorded from new localities. Trichonia trichonica, which has been considered extinct from its type locality for the last twenty eight years, was re-discovered, whereas the presence of Daphniola exigua, G. vegorriticola, Marstoniopsis graeca, P. pieperi and Pseudobithynia trichonis in their type localities was verified. The taxonomic status of P. negropontina and the newly discovered populations of G. vegorriticola was elucidated using COI sequence data. The new data recorded during this survey indicate that the IUCN status of some Greek endemic hydrobioids needs to be updated.