Rarity patterns of woody plant species are associated with life form and diversification rates in Pacific islands forests.

PREMISE: Rarity is a complex and central concept in ecology and conservation biology. Yet, it is still poorly understood why some species are rare and others common. Here, we aimed to understand the drivers of species rarity patterns in woody plant communities. METHODS: We analyzed the local abundance and landscape frequency of 121 woody plant species across 238 plots on American Samoa and Hawaiian islands. We first assessed whether taxonomy, life form (shrub, small tree, large tree), and dispersal syndrome (dispersed by animals or by other means) are associated with the rarity of species. We then analyzed phylogenetic patterns in plant rarity and tested whether rarity patterns are associated with species evolutionary distinctiveness and the number of species within genera and families. RESULTS: Large trees were less abundant but more frequent than shrub species. Animal-dispersed species tended to be less abundant than species dispersed by other means, while species frequency was not associated with dispersal syndromes. Relative frequency in Hawai'i exhibited a more robust phylogenetic signal than did abundance. Both evolutionary distinctiveness and taxa species richness were significantly associated with the frequency of shrub species in Hawai'i. CONCLUSIONS: Life form appears consistently associated with the rarity of species. High diversification rate is probably a key factor explaining landscape-scale rarity of native species on isolated archipelagos like Hawai'i. At the landscape scale, rarity appears to be inversely associated with evolutionary distinctiveness, but at the local scale, species abundance may be not associated with evolutionary distinctiveness.

Postzygotic barriers isolate sympatric species of Cyrtandra (Gesneriaceae) in Hawaiian montane forest understories.

PREMISE OF THE STUDY: Recent reviews of reproductive isolation (RI) in plants propose that boundaries between closely related species are maintained predominantly through prezygotic mechanisms. However, few experimental studies have explored how boundaries are maintained in long-lived species. Hawaiian Cyrtandra presents an intriguing challenge to our understanding of RI, as it comprises 60 shrub or small tree species that are almost exclusively restricted to wet forests, where sympatry of multiple species is common. METHODS: We assessed the relative strengths of pre- and postzygotic barriers among four species of Cyrtandra occurring at the extremes of the main Hawaiian Island's natural island-age gradient, Kaua'i (4.7 Myr) and Hawai'i Island (0.6 Myr), to contrast the strengths and stages of reproductive isolation among species at different stages of divergence. KEY RESULTS: A combination of F1 seed germination, F1 seedling survival, and F1 seedling growth isolated (61-91%) three of the species from sympatric relatives. In contrast, the fourth species was isolated (59%) from its sympatric relative through phenological differences alone. Significant postzygotic barriers in between-island crosses were also observed in one species. CONCLUSIONS: Results suggest that boundaries between sympatric Cyrtandra species in Hawaii are maintained predominantly through postzygotic barriers. Observations from between-island crosses indicate that postzygotic barriers can arise in allopatry, which may be important in the initial divergence of populations. Future studies of RI in Cyrtandra should include a broader range of species to determine if postzygotic isolating barriers are foremost in the maintenance of species boundaries in this large genus.

Patterns and processes in complex landscapes: testing alternative biogeographical hypotheses through integrated analysis of phylogeography and community ecology in Hawai'i.

The Island of Hawai'i is a dynamic assemblage of five volcanoes with wet forest habitat currently existing in four distinct natural regions that vary in area, age and geographical isolation. In this complex landscape, alternative assumptions of the relative importance of specific habitat characteristics on evolutionary and ecological processes predict strikingly different general patterns of local diversity and regional similarity. In this study, we compare alternative a priori hypotheses against observed patterns within two distinct biological systems and scales: community composition of wet forest vascular plant species and mitochondrial and nuclear genes of Drosophila sproati, a wet-forest-restricted endemic. All observed patterns display strong and similar regional structuring, with the greatest local diversity found in Kohala and the windward side of Mauna Loa, the least in Ka'u and Kona, and a distinctive pattern of regional similarity that probably reflects the historical development of this habitat on the island. These observations largely corroborate a biogeographical model that integrates multiple lines of evidence, including climatic reconstruction, over those relying on single measures, such as current habitat configuration or substrate age. This method of testing alternative hypotheses across biological systems and scales is an innovative approach for understanding complex landscapes and should prove valuable in diverse biogeographical systems.

Diurnal temperature range as a key predictor of plants' elevation ranges globally.

A prominent hypothesis in ecology is that larger species ranges are found in more variable climates because species develop broader environmental tolerances, predicting a positive range size-temperature variability relationship. However, this overlooks the extreme temperatures that variable climates impose on species, with upper or lower thermal limits more likely to be exceeded. Accordingly, we propose the 'temperature range squeeze' hypothesis, predicting a negative range size-temperature variability relationship. We test these contrasting predictions by relating 88,000 elevation range sizes of vascular plants in 44 mountains to short- and long-term temperature variation. Consistent with our hypothesis, we find that species' range size is negatively correlated with diurnal temperature range. Accurate predictions of short-term temperature variation will become increasingly important for extinction risk assessment in the future.

Evolutionary genomics of oceanic island radiations.

A recurring feature of oceanic archipelagos is the presence of adaptive radiations that generate endemic, species-rich clades that can offer outstanding insight into the links between ecology and evolution. Recent developments in evolutionary genomics have contributed towards solving long-standing questions at this interface. Using a comprehensive literature search, we identify studies spanning 19 oceanic archipelagos and 110 putative adaptive radiations, but find that most of these radiations have not yet been investigated from an evolutionary genomics perspective. Our review reveals different gaps in knowledge related to the lack of implementation of genomic approaches, as well as undersampled taxonomic and geographic areas. Filling those gaps with the required data will help to deepen our understanding of adaptation, speciation, and other evolutionary processes.

"The upper limits of vegetation on Mauna Loa, Hawaii": a 50th-anniversary reassessment.

In January 1958, a survey of alpine flora was conducted along a recently constructed access road across the upper volcanic slopes of Mauna Loa, Hawaii (2525-3397 m). Only five native Hawaiian species were encountered on sparsely vegetated historic and prehistoric lava flows adjacent to the roadway. A resurvey of roadside flora in 2008 yielded a more than fourfold increase to 22 species, including nine native species not previously recorded. Eight new alien species have now invaded this alpine environment, although exclusively limited to a few individuals in ruderal habitat along the roadway. Alternative explanations for species invasion and altitudinal change over the past 50 years are evaluated: (1) changes related to continuing primary succession on ameliorating (weathering) young lava substrates; (2) local climate change; and (3) road improvements and increased vehicular access which promote enhanced car-borne dispersal of alien species derived from the expanding pool of potential colonizers naturalized on the island in recent decades. Unlike alpine environments in temperate latitudes, the energy component (warming) in climate change on Mauna Loa does not appear to be the unequivocal driver of plant invasion and range extension. Warming may be offset by other climate change factors including rainfall and evapotranspiration.

A phylogenetic basis for species-area relationships among three Pacific Island floras.

PREMISE OF THE STUDY: The angiosperm floras of the Hawaiian, Society, and Marquesas archipelagoes are remarkably comparable ecologically and evolutionarily, a result of similar geologic history, climate, and isolation. METHODS: We characterized variation in species richness among islands and whole archipelagoes by analyzing species-area relationships (SARs). By partitioning each flora into putative phylogenetic lineages each derived from a given colonization event, we explored several ways in which speciation contributes to SARs. KEY RESULTS: Specifically, these groups exhibit expected island SARs and a whole archipelago SAR characterized by a steep slope. The number of species added by net cladogenesis increases with area much more quickly than the number contributed by net colonization from outside. In each of the three archipelagoes, most colonists do not speciate, while many species occur in a few diverse colonist lineages. Colonization events that are unique to a given archipelago are in more prone to speciation than lineages with close relatives in the other archipelagoes. Most lineages with relatives in all three archipelagoes have one species in each, suggesting a similar tendency not to diversify. On the other hand, a correlation between lineage size in one archipelago and that of related lineages in other archipelagoes suggests a consistent tendency among diverse groups to speciate extensively. Lineages with multiple species in each archipelago also tend to have far more species in the largest archipelago, the Hawaiian Islands. CONCLUSIONS: The most diverse lineages exhibit a strong response to archipelago area. These diverse, area-sensitive lineages contribute substantially to the slope of the inter-archipelago SAR. Regional species pools elsewhere may exhibit similar steep-sloped SARs; thus, these findings may inform how the behavior of lineages with different responses to increasing shapes these patterns.

Speciation in Hawaiian angiosperm lineages: cause, consequence, and mode.

The biota of Hawaiian Islands is derived entirely from long distance dispersal, often followed by in situ speciation. Species descended from each colonist constitute monophyletic lineages that have diverged to varying degrees under similar spatial and temporal constraints. We partitioned the Hawaiian angiosperm flora into lineages and assessed morphological, ecological, and biogeographic characteristics to examine their relationships to variation in species number (S). Lineages with external bird dispersal (through adhesion) were significantly more species-rich than those with abiotic dispersal, but only weakly more species-rich than lineages with internal bird dispersal (involving fleshy fruits). Pollination mode and growth form (woody vs. herbaceous) had no significant effect on S, in contrast to studies of angiosperm families. S relates positively to the geographic and ecological range size of whole lineages, but negatively to local abundance and mean range sizes of constituent species. Species-rich lineages represent a large proportion of major adaptive shifts, although this appears to be an artifact of having more species. Examination of 52 sister species pairs in numerous lineages provides evidence for allopatric (including peripheral isolates) and parapatric (ecological) modes, with 15 cases of each. Although postspeciational dispersal may obscure these modes in many of the remaining cases, instances of sympatric and hybrid speciation are also discussed. Because speciation is both a consequence and a cause of ecological and biogeographic traits, speciation mode may be integral to relationships between traits. We discuss the role of speciation in shaping the regional species pool.

How old is the Hawaiian biota? Geology and phylogeny suggest recent divergence.

This study quantifies long-term landscape changes in the Hawaiian archipelago relating to dispersal, speciation and extinction. Accounting for volcano growth, subsidence and erosion, we modelled the elevations of islands at time intervals of 0.5 Myr for the last 32 Myr; we also assessed the variation in the spacing of volcanoes during this period. The size, spacing and total number of volcanic islands have varied greatly over time, with the current landscape of large, closely spaced islands preceded by a period with smaller, more distantly spaced islands. Considering associated changes in rates of dispersal and speciation, much of the present species pool is probably the result of recent colonization from outside the archipelago and divergence within contemporary islands, with limited dispersal from older islands. This view is in accordance with abundant phylogenetic studies of Hawaiian organisms that estimate the timing of colonization and divergence within the archipelago. Twelve out of 15 multi-species lineages have diverged within the lifetime of the current high islands (last 5 Myr). Three of these, and an additional seven (mostly single-species) lineages, have colonized the archipelago within this period. The timing of colonization of other lineages remains uncertain.

Modeling Hawaiian ecosystem degradation due to invasive plants under current and future climates.

Occupation of native ecosystems by invasive plant species alters their structure and/or function. In Hawaii, a subset of introduced plants is regarded as extremely harmful due to competitive ability, ecosystem modification, and biogeochemical habitat degradation. By controlling this subset of highly invasive ecosystem modifiers, conservation managers could significantly reduce native ecosystem degradation. To assess the invasibility of vulnerable native ecosystems, we selected a proxy subset of these invasive plants and developed robust ensemble species distribution models to define their respective potential distributions. The combinations of all species models using both binary and continuous habitat suitability projections resulted in estimates of species richness and diversity that were subsequently used to define an invasibility metric. The invasibility metric was defined from species distribution models with <0.7 niche overlap (Warrens I) and relatively discriminative distributions (Area Under the Curve >0.8; True Skill Statistic >0.75) as evaluated per species. Invasibility was further projected onto a 2100 Hawaii regional climate change scenario to assess the change in potential habitat degradation. The distribution defined by the invasibility metric delineates areas of known and potential invasibility under current climate conditions and, when projected into the future, estimates potential reductions in native ecosystem extent due to climate-driven invasive incursion. We have provided the code used to develop these metrics to facilitate their wider use (Code S1). This work will help determine the vulnerability of native-dominated ecosystems to the combined threats of climate change and invasive species, and thus help prioritize ecosystem and species management actions.

Modeling Hawaiian ecosystem degradation due to invasive plants under current and future climates.

Occupation of native ecosystems by invasive plant species alters their structure and/or function. In Hawaii, a subset of introduced plants is regarded as extremely harmful due to competitive ability, ecosystem modification, and biogeochemical habitat degradation. By controlling this subset of highly invasive ecosystem modifiers, conservation managers could significantly reduce native ecosystem degradation. To assess the invasibility of vulnerable native ecosystems, we selected a proxy subset of these invasive plants and developed robust ensemble species distribution models to define their respective potential distributions. The combinations of all species models using both binary and continuous habitat suitability projections resulted in estimates of species richness and diversity that were subsequently used to define an invasibility metric. The invasibility metric was defined from species distribution models with <0.7 niche overlap (Warrens I) and relatively discriminative distributions (Area Under the Curve >0.8; True Skill Statistic >0.75) as evaluated per species. Invasibility was further projected onto a 2100 Hawaii regional climate change scenario to assess the change in potential habitat degradation. The distribution defined by the invasibility metric delineates areas of known and potential invasibility under current climate conditions and, when projected into the future, estimates potential reductions in native ecosystem extent due to climate-driven invasive incursion. We have provided the code used to develop these metrics to facilitate their wider use (Code S1). This work will help determine the vulnerability of native-dominated ecosystems to the combined threats of climate change and invasive species, and thus help prioritize ecosystem and species management actions.