Phylogenetic and functional clustering illustrate the roles of adaptive radiation and dispersal filtering in jointly shaping late-Quaternary mammal assemblages on oceanic islands

Weaker Plant-Frugivore Trait Matching Towards the Tropics and on Islands

Biotic interactions play an important role in species diversification and maintenance and, thus, are regarded as the architecture of biodiversity. Since Darwin and Wallace, biologists have debated whether biotic interactions are stronger towards the tropics and on continents, when compared to temperate regions and islands. Here, based on 354 avian frugivory networks accounting for 22,199 interactions between 1247 bird species and 2126 plant species, we quantified trait matching strength, which reflects interaction strength and specificity, across gradients of latitude and insularity globally. We found that matching between beak size and fruit size was significantly stronger towards the poles and on continents, when compared with the tropics and on islands. As underlining ecological factors, trait matching was stronger with a larger proportion of frugivory (measured as the mean proportion of fruits in bird diets) and network-level mean beak size, and with a smaller proportion of fleshy-fruited species (measured as the proportion of fleshy-fruited plant species in the botanical country where the network was located). These findings suggest that the latitudinal and insular patterns in trait matching are driven by biotic factors that may relate to trait co-evolution between interacting species and optimal foraging for bird species.

Taxonomic, phylogenetic, and functional nestedness of mammal assemblages in the Zhoushan Archipelago, China

Nestedness is an important part of the theoretical framework of island biogeography and community ecology. However, most previous studies focused on taxonomic dimension and overlooked functional and phylogenetic nestedness. Here, we simultaneously investigated taxonomic, functional, and phylogenetic nestedness of terrestrial mammals on 39 land-bridge islands in the Zhoushan Archipelago, China. As mammals' response to the environment may depend on their body size, we performed analyses for three mammal assemblages separately: overall species, large and medium-sized species, and small species. The taxonomic nestedness was quantified by organizing the species incidence matrix, while the functional and phylogenetic nestedness were estimated by combining the similarity of their ecological traits and phylogeny. Island characteristics (island area, three isolation indices, land use intensity, and habitat diversity) and species traits (body size, litter size, habitat specificity, geographic range size, and minimum area requirement) were used as predictors of nestedness. Overall and small species were significantly nested in 3 facets of nestedness, and results supported the selective extinction, selective colonization, and habitat nestedness hypotheses. Large and medium-sized species were functionally and phylogenetically nested when matrices were ordered by increasing distance to mainland, supporting the selective colonization hypothesis. Overall, differences in nestedness and its underlying mechanisms were detected not only in 3 facets of nestedness but also in the 3 mammal assemblages. Therefore, frameworks that incorporate taxonomic, phylogenetic, and functional nestedness can contribute to a more comprehensive understanding of nestedness processes. Additionally, it also improves our ability to understand the divergent responses of mammal assemblages to the insular environment.

Phylogeographic Structure and Population Dynamics of Baoxing Osmanthus (Osmanthus serrulatus), an Endemic Species from the Southwest Sichuan Basin, China

The mountainous regions of southwest China are recognized as pivotal centers for the origin and evolution of Osmanthus species. Baoxing Osmanthus (Osmanthus serrulatus Rehder), a rare and endemic species known for its spring blooms, is sparsely distributed within the high altitude evergreen broad-leaved forests surrounding the southwestern Sichuan Basin. However, persistent anthropogenic disturbances and habitat fragmentation have precipitated a significant decline in its natural population size, leading to the erosion of genetic resources. To assess the genetic status of O. serrulatus and formulate effective conservation strategies, we conducted sampling across ten wild populations, totaling 148 individuals in their natural habitats. We employed two cpDNA fragments (matK and trnS-trnG) to elucidate the phylogeographic structure and historical population dynamics. The results revealed low species-level genetic diversity, alongside pronounced regional differentiation among populations (FST = 0.812, p < 0.05) and a notable phylogeographic structure (NST = 0.698 > GST = 0.396, p < 0.05). Notably, genetic variation was predominantly observed among populations (81.23%), with no evidence of recent demographic expansion across the O. serrulatus distribution range. Furthermore, divergence dating indicated a timeline of approximately 4.85 Mya, corresponding to the late Miocene to early Pleistocene. This temporal correlation coincided with localized uplift events in the southwestern mountains and heightened Asian monsoons, suggesting pivotal roles for these factors in shaping the current phylogeographic pattern of O. serrulatus. These findings support the effective conservation of O. serrulatus germplasm and offer insights into the impact of Quaternary climate oscillations on companion species within evergreen broad-leaved forests. They also enhance our understanding of the origin and evolution of these forests in the southwestern mountains, aiding biodiversity conservation efforts in the region.

Land-use change interacts with island biogeography to alter bird community assembly

Anthropogenic activities have reshaped biodiversity on islands worldwide. However, it remains unclear how island attributes and land-use change interactively shape multiple facets of island biodiversity through community assembly processes. To answer this, we conducted bird surveys in various land-use types (mainly forest and farmland) using transects on 34 oceanic land-bridge islands in the largest archipelago of China. We found that bird species richness increased with island area and decreased with isolation, regardless of the intensity of land-use change. However, forest-dominated habitats exhibited lower richness than farmland-dominated habitats. Island bird assemblages generally comprised species that share more similar traits or evolutionary histories (i.e. functional and/or phylogenetic clustering) than expected if assemblages were randomly assembled. Contrary to our expectations, we observed that bird assemblages in forest-dominated habitats were more clustered on large and close islands, whereas assemblages in farmland-dominated habitats were more clustered on small islands. These contrasting results indicate that land-use change interacts with island biogeography to alter the community assembly of birds on inhabited islands. Our findings emphasize the importance of incorporating human-modified habitats when examining the community assembly of island biota, and further suggest that agricultural landscapes on large islands may play essential roles in protecting countryside island biodiversity.

The environmental conditions of endemism hotspots shape the functional traits of mammalian assemblages

Endemic (small-ranged) species are distributed non-randomly across the globe. Regions of high topography and stable climates have higher endemism than flat, climatically unstable regions. However, it is unclear how these environmental conditions interact with and filter mammalian traits. Here, we characterize the functional traits of highly endemic mammalian assemblages in multiple ways, testing the hypothesis that these assemblages are trait-filtered (less functionally diverse) and dominated by species with traits associated with small range sizes. Compiling trait data for more than 5000 mammal species, we calculated assemblage means and multidimensional functional metrics to evaluate the distribution of traits across each assemblage. We then related these metrics to the endemism of global World Wildlife Fund ecoregions using linear models and phylogenetic fourth-corner regression. Highly endemic mammalian assemblages had small average body masses, low fecundity, short lifespans and specialized habitats. These traits relate to the stable climate and rough topography of endemism hotspots and to mammals' ability to expand their ranges, suggesting that the environmental conditions of endemism hotspots allowed their survival. Furthermore, species living in endemism hotspots clustered near the edges of their communities' functional spaces, indicating that abiotic trait filtering and biotic interactions act in tandem to shape these communities.

Interspecific sociality alters the colonization and extinction rates of birds on subtropical reservoir islands

Island biogeography theory has proved a robust approach to predicting island biodiversity on the assumption of species equivalency. However, species differ in their grouping behaviour and are entangled by complex interactions in island communities, such as competition and mutualism. We here investigated whether intra- and/or interspecific sociality may influence biogeographic patterns, by affecting movement between islands or persistence on them. We classified bird species in a subtropical reservoir island system into subcommunities based on their propensity to join monospecific and mixed-species flocks. We found that subcommunities which had high propensity to flock interspecifically had higher colonization rates and lower extinction rates over a 10-year period. Intraspecific sociality increased colonization in the same analysis. A phylogenetically corrected analysis confirmed the importance of interspecific sociality, but not intraspecific sociality. Group-living could enable higher risk crossings, with greater vigilance also linked to higher foraging efficiency, enabling colonization or long-term persistence on islands. Further, if group members are other species, competition can be minimized. Future studies should investigate different kinds of island systems, considering positive species interactions driven by social behaviour as potential drivers of community assembly on islands. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Nearby large islands diminish biodiversity of the focal island by a negative target effect

The Equilibrium Theory of Island Biogeography postulates that larger and closer islands support higher biodiversity through the dynamic balance of colonization and extinction processes. The negative diversity-isolation (i.e. the distance to the mainland) relationship is derived based on the assumption that the mainland is the only source pool for island biotas. However, nearby islands could also act as species sources for focal islands via a source effect. In this study, we move a further step and hypothesize that nearby islands may reduce bird colonizers of the focal island and diminish its biodiversity, resulting in a negative target effect. To test our hypothesis, we assessed the effects of island area and isolation (metrics considering both the mainland and nearby islands) on taxonomic (i.e. species richness), functional and phylogenetic diversity of terrestrial breeding birds on 42 islands in the largest archipelago of China, the Zhoushan Archipelago. Furthermore, we compared the predictive power of the distance to the large island under a set of relative area thresholds and the relative area of nearby islands on species richness under a set of distance thresholds to explore the role of nearby islands as a source and/or target island. We found that island area had a positive effect on species richness, phylogenetic diversity and functional diversity, while the distance to the mainland had a negative effect only on species richness. Species richness on the focal island increased with increasing distance to the nearest larger island, indicating the negative target effect. Furthermore, the negative target effect depended on the area of nearby islands relative to the area of the focal island. Our finding of the negative target effect suggests islands located between the mainland and the focal island can be not only sources or stepping stones, but also colonization targets. This result demonstrates the importance of considering multiple geographical attributes of islands in island biogeographic studies, especially the characteristics related to source and/or target effects.

The interactive effects of environmental gradient and dispersal shape spatial phylogenetic patterns

Introduction

The emergence and maintenance of biodiversity include interacting environmental conditions, organismal adaptation to such conditions, and dispersal. To understand and quantify such ecological, evolutionary, and spatial processes, observation and interpretation of phylogenetic relatedness across space (e.g., phylogenetic beta diversity) is arguably a way forward as such patterns contain signals from all the processes listed above. However, it remains challenging to extract information about complex eco-evolutionary and spatial processes from phylogenetic patterns.

Methods

We link environmental gradients and organismal dispersal with phylogenetic beta diversity using a trait-based and eco-evolutionary model of diversification along environmental gradients. The combined effect of the environment and dispersal leads to distinct phylogenetic patterns between subsets of species and across geographical distances.

Results and discussion

Steep environmental gradients combined with low dispersal lead to asymmetric phylogenies, a high phylogenetic beta diversity, and the phylogenetic diversity between communities increases linearly along the environmental gradient. High dispersal combined with a less steep environmental gradient leads to symmetric phylogenies, low phylogenetic beta diversity, and the phylogenetic diversity between communities along the gradient increases in a sigmoidal form. By disentangling the eco-evolutionary mechanisms that link such interacting environment and dispersal effects and community phylogenetic patterns, our results improve understanding of biodiversity in general and help interpretation of observed phylogenetic beta diversity.

Human Disturbance and Geometric Constraints Drive Small Mammal Diversity and Community Structure along an Elevational Gradient in Eastern China

Understanding the mechanisms influencing patterns and processes of biological diversity is critical to protecting biodiversity, particularly in species-rich ecosystems such as mountains. Even so, there is limited knowledge of biodiversity patterns and processes in the mountains of eastern China, especially about small mammals. In this study, we examined the taxonomic, functional, and phylogenetic diversity of small mammal distribution and community structure along the elevational gradient of Qingliang Mountain, eastern China. We then evaluated how they are influenced by space (area and mid-domain effect (MDE)), environment (temperature, precipitation, and normalized difference vegetation index (NDVI)), and human disturbance. The results showed hump-shaped patterns of taxonomic and phylogenetic diversity along elevation gradients, peaking at 1000 m, unlike functional diversity, which peaked at lower elevations (600 m). The mean pairwise distance and mean nearest taxon distance of functional and phylogenetic variance (MFD and MPD, respectively) were also incongruent. The MFD and MPD showed hump-shaped patterns along elevations; however, unlike MFD, which peaked at lower elevations (600 m), MPD peaked at higher elevations (1200 m). The mean nearest functional taxon distance (MNFD) decreased, while the mean nearest phylogenetic taxon distance (MNTD) increased along the elevation gradient. The higher elevations were functionally more clustered, while the lower elevations were phylogenetically more clustered, suggesting that environmental filtering for traits was stronger at higher elevations. In comparison, phylogenetic conservatism of ecological niches had a stronger influence at lower elevations. The diversity and community structure indices were inconsistently explained, with human disturbance and MDE accounting for the biggest proportions of the model-explained variances. Overall, the results confirm that environmental filtering and human disturbance significantly influence small mammals' diversity and community structure. These findings also emphasize the need for increased conservation efforts in the middle and lower elevation regions of Qingliang Mountain.