The repeated evolution of large seeds on islands

Symbiotic status alters fungal eco-evolutionary offspring trajectories

Despite host-fungal symbiotic interactions being ubiquitous in all ecosystems, understanding how symbiosis has shaped the ecology and evolution of fungal spores that are involved in dispersal and colonization of their hosts has been ignored in life-history studies. We assembled a spore morphology database covering over 26,000 species of free-living to symbiotic fungi of plants, insects and humans and found more than eight orders of variation in spore size. Evolutionary transitions in symbiotic status correlated with shifts in spore size, but the strength of this effect varied widely among phyla. Symbiotic status explained more variation than climatic variables in the current distribution of spore sizes of plant-associated fungi at a global scale while the dispersal potential of their spores is more restricted compared to free-living fungi. Our work advances life-history theory by highlighting how the interaction between symbiosis and offspring morphology shapes the reproductive and dispersal strategies among living forms.

Fruit Size in Indo-Malayan Island Plants Is More Strongly Influenced by Filtering than by In Situ Evolution

AbstractCommunity trait assembly, the formation of distributions of phenotypic characteristics across coexisting species, can occur via two main processes: filtering of trait distributions from the regional pool and in situ phenotypic evolution in local communities. But the relative importance of these processes remains unclear, largely because of the difficulty in determining the timing of evolutionary trait changes and biogeographic dispersal events in phylogenies. We assessed evolutionary and biogeographic transitions in woody plant species across the Indo-Malay archipelago, a series of island groups where the same plant lineages interact with different seed disperser and seed predator assemblages. Fruit size in 2,650 taxa spanning the angiosperm tree of life tended to be smaller in the Sulawesi and Maluku island groups, where frugivores are less diverse and smaller bodied, than in the regional source pool. While numerous plant lineages (not just small-fruited ones) reached the isolated islands, colonists tended to be the smaller-fruited members of each clade. Nearly all of the evolutionary transitions to smaller fruit size predated, often substantially, organismal dispersal to the islands. Our results suggest that filtering rather than within-island evolution largely determined the distribution of fruit sizes in these regions.

Phenotypic divergence of traits that mediate antagonistic and mutualistic interactions between island and continental populations of the tropical plant, Tribulus cistoides (Zygophyllaceae)

Island systems have long served as a model for evolutionary processes due to their unique species interactions. Many studies of the evolution of species interactions on islands have focused on endemic taxa. Fewer studies have focused on how antagonistic and mutualistic interactions shape the phenotypic divergence of widespread nonendemic species living on islands. We used the widespread plant Tribulus cistoides (Zygophyllaceae) to study phenotypic divergence in traits that mediate antagonistic interactions with vertebrate granivores (birds) and mutualistic interactions with pollinators, including how this is explained by bioclimatic variables. We used both herbarium specimens and field‐collected samples to compare phenotypic divergence between continental and island populations. Fruits from island populations were larger than on continents, but the presence of lower spines on mericarps was less frequent on islands. The presence of spines was largely explained by environmental variation among islands. Petal length was on average 9% smaller on island than continental populations, an effect that was especially accentuated on the Galápagos Islands. Our results show that Tribulus cistoides exhibits phenotypic divergence between island and continental habitats for antagonistic traits (seed defense) and mutualistic traits (floral traits). Furthermore, the evolution of phenotypic traits that mediate antagonistic and mutualistic interactions partially depended on the abiotic characteristics of specific islands. This study shows the potential of using a combination of herbarium and field samples for comparative studies on a globally distributed species to study phenotypic divergence on island habitats.

Dispersal ability and its consequences for population genetic differentiation and diversification

Dispersal ability is known to influence geographical structuring of genetic variation within species, with a direct relationship between low vagility and population genetic structure, which can potentially give rise to allopatric speciation. However, our general understanding of the relationship between dispersal ability, population differentiation and lineage diversification is limited. To address this issue, we sampled mitochondrial DNA variation within lineages of beetles and spiders across the Canary Islands to explore the relationships between dispersal ability, differentiation within lineages and diversification. We found positive relationships between population genetic structure and diversification for both beetles and spiders. Comparisons between dispersive and non-dispersive lineages revealed significant differences for both lineage differentiation and diversification. For both taxa, non-dispersive lineages had stronger population genetic structure. Genus-level endemic species richness and proxies for diversification rate within genera were higher in non-dispersive taxa for both beetles and spiders. Comparisons of average and maximum node divergences within genera suggest that species turnover may be higher in non-dispersive genera. Our results reveal a model where dispersal limitation may shape the diversity of lineages across evolutionary timescales by positively influencing intraspecific and species diversity, moderated by higher extinction rates compared to more dispersive lineages.

Philomatry in plants: why do so many species have limited seed dispersal?

Many have noted limited seed dispersal of plants in diverse environments and attempted evolutionary explanations for it. Although philopatric ("love of fatherland") is used by zoologists to describe organisms that remain near their place of origin, philomatric ("love of motherland") is proposed as more appropriate for plants because seeds develop on the maternal parent, fecundity and dispersal are maternally influenced characteristics, and the term dovetails with the mother-site hypothesis (MSH) for the evolution of restricted dispersal. Proximate reasons for philomatry include intrinsic drivers such as morphological features of diaspores and where on the maternal parent they are produced. Extrinsic drivers include local environmental conditions, surrounding vegetation, and ineffective dispersal agents. The MSH proposes that selection should favor philomatry in a population adapted to a particular habitat because offspring will likewise be adapted to that same habitat. Several studies show philomatry can mitigate distance-dependent costs of dispersing into surrounding inhospitable areas. Undispersed diaspores can eliminate energetic costs of accessory structures or biochemicals needed by dispersible diaspores, but it is unclear whether these costs are significant to the evolution of philomatry. Disadvantages of limited dispersal are inability to escape deteriorating habitat conditions, inability to colonize new habitats, and inbreeding among offspring. Heterocarpic species offset these disadvantages by producing dispersed plus undispersed diaspores. A conceptual framework is presented relating dispersal distance to the probability of seedling establishment. Future research should recognize dispersal as a covarying syndrome of multiple life history traits and focus on ecological selection agents that favor philomatry.

Seed morphological traits as a tool to quantify variation maintained in ex situ collections: a case study in Pinus torreyana

Understanding the within- and among-population distribution of trait variation within seed collections may provide a means to approximate standing genetic variation and inform plant conservation. This study aimed to estimate population- and family-level seed trait variability for existing seed collections of Torrey pine (Pinus torreyana), and to use these data to guide sampling of future collections. We quantified variation in 14 seed morphological traits and seedling emergence within and among Torrey pine populations. Using a simulation-based approach, we used estimates of within-population variance to assess the number of maternal families required to capture 95 % of trait variation within each existing seed collection. Substantial structure was observed both within and among Torrey pine populations, with island and mainland seeds varying in seed size and seed coat thickness. Despite morphological differences, seedling emergence was similar across populations. Simulations revealed that 83 % and 71 % of all maternal families within island and mainland seed collections respectively needed to be resampled to capture 95 % of seed trait variation within existing collections. From a conservation perspective, our results indicate that to optimize genetic diversity captured in Torrey pine seed collections, maximizing the number of maternal families sampled within each population will be necessary.

A roadmap to plant functional island biogeography

Island biogeography is the study of the spatio-temporal distribution of species, communities, assemblages or ecosystems on islands and other isolated habitats. Island diversity is structured by five classes of process: dispersal, establishment, biotic interactions, extinction and evolution. Classical approaches in island biogeography focused on species richness as the deterministic outcome of these processes. This has proved fruitful, but species traits can potentially offer new biological insights into the processes by which island life assembles and why some species perform better at colonising and persisting on islands. Functional traits refer to morphological and phenological characteristics of an organism or species that can be linked to its ecological strategy and that scale up from individual plants to properties of communities and ecosystems. A baseline hypothesis is for traits and ecological strategies of island species to show similar patterns as a matched mainland environment. However, strong dispersal, environmental and biotic-interaction filters as well as stochasticity associated with insularity modify this baseline. Clades that do colonise often embark on distinct ecological and evolutionary pathways, some because of distinctive evolutionary forces on islands, and some because of the opportunities offered by freedom from competitors or herbivores or the absence of mutualists. Functional traits are expected to be shaped by these processes. Here, we review and discuss the potential for integrating functional traits into island biogeography. While we focus on plants, the general considerations and concepts may be extended to other groups of organisms. We evaluate how functional traits on islands relate to core principles of species dispersal, establishment, extinction, reproduction, biotic interactions, evolution and conservation. We formulate existing knowledge as 33 working hypotheses. Some of these are grounded on firm empirical evidence, others provide opportunities for future research. We organise our hypotheses under five overarching sections. Section A focuses on plant functional traits enabling species dispersal to islands. Section B discusses how traits help to predict species establishment, successional trajectories and natural extinctions on islands. Section C reviews how traits indicate species biotic interactions and reproduction strategies and which traits promote intra-island dispersal. Section D discusses how evolution on islands leads to predictable changes in trait values and which traits are most susceptible to change. Section E debates how functional ecology can be used to study multiple drivers of global change on islands and to formulate effective conservation measures. Islands have a justified reputation as research models. They illuminate the forces operating within mainland communities by showing what happens when those forces are released or changed. We believe that the lens of functional ecology can shed more light on these forces than research approaches that do not consider functional differences among species.

A simple null model predicts the island rule

The island rule is a putative pattern in island evolution, where small species become larger on islands and large species become smaller. Despite decades of study, a mechanistic explanation for why some taxonomic groups obey the island rule, while others do not, has yet to be identified. Here, we explore whether the island rule might result from evolutionary drift. We derived a simulation model that predicts evolutionary size changes on islands based on random evolutionary trajectories along bounded trait domains. The model consistently predicted the island rule and could account for its occurrence in plants inhabiting islands in the Southwest Pacific. When support for the island rule was not detected, insular gigantism was often observed, suggesting that natural selection was at work. Overall results indicate that evolutionary drift can provide a parsimonious explanation for the island rule, suggesting future work should focus on circumstances where it does not occur.

Seed dispersal as a search strategy: dynamic and fragmented landscapes select for multi-scale movement strategies in plants

BACKGROUND

Plant dispersal is a critical factor driving ecological responses to global changes. Knowledge on the mechanisms of dispersal is rapidly advancing, but selective pressures responsible for the evolution of dispersal strategies remain elusive. Recent advances in animal movement ecology identified general strategies that may optimize efficiency in animal searches for food or habitat. Here we explore the potential for evolution of similar general movement strategies for plants.

METHODS

We propose that seed dispersal in plants can be viewed as a strategic search for suitable habitat, where the probability of finding such locations has been optimized through evolution of appropriate dispersal kernels. Using model simulations, we demonstrate how dispersal strategies can optimize key dispersal trade-offs between finding habitat, avoiding kin competition, and colonizing new patches. These trade-offs depend strongly on the landscape, resulting in a tight link between optimal dispersal strategy and spatiotemporal habitat distribution.

RESULTS

Our findings reveal that multi-scale seed dispersal strategies that combine a broad range of dispersal scales, including Lévy-like dispersal, are optimal across a wide range of dynamic and patchy landscapes. At the extremes, static and patchy landscapes select for dispersal strategies dominated by short distances, while uniform and highly unpredictable landscapes both select for dispersal strategies dominated by long distances.

CONCLUSIONS

By viewing plant seed dispersal as a strategic search for suitable habitat, we provide a reference framework for the analysis of plant dispersal data. Consideration of the entire dispersal kernel, including distances across the full range of scales, is key. This reference framework helps identify plant species' dispersal strategies, the evolutionary forces determining these strategies and their ecological consequences, such as a potential mismatch between plant dispersal strategy and altered spatiotemporal habitat dynamics due to land use change. Our perspective opens up directions for future studies, including exploration of composite search behaviour and 'informed searches' in plant species with directed dispersal.

Higher evolutionary rates in life-history traits in insular than in mainland palms

Isolated islands, due to the reduced interspecific competition compared to mainland habitats, present ecological opportunities for colonizing lineages. As a consequence, island lineages may be expected to experience higher rates of trait evolution than mainland lineages. However, island effects on key life-history traits of vascular plants remain underexplored at broad spatiotemporal scales, even for emblematic island clades such as palms. Here, we used phylogenetic comparative methods to evaluate potential differences in size and macroevolutionary patterns of height and fruit diameter among mainland, continental, and volcanic island palms. Further, phylogenetic beta-diversity was used to determine if lineage turnover supported an adaptive radiation scenario on volcanic islands. Volcanic island palms were taller than their continental island and mainland counterparts, whereas continental island palms exhibited smaller fruit size. Height and fruit size of palms evolved under evolutionary constraints towards an optimal value. However, scenarios of adaptive radiation and niche conservatism were not supported for the height and fruit size of volcanic and mainland palm clades, respectively, as expected. Instead, continental island palms exhibited higher evolutionary rates for height and fruit size. Insular palm assemblages (continental and volcanic) are composed of unique lineages. Beyond representing evolutionary sources of new palm lineages, our results demonstrate that insular habitats are important in shaping palm trait diversity. Also, the higher phenotypic evolutionary rates of continental island palms suggest disparate selection pressures on this habitat type, which can be an important driver of trait diversification over time. Taken together, these results stress the importance of insular habitats for conservation of functional, phylogenetic, and taxonomic diversity of palms.

On the origin of giant seeds: the macroevolution of the double coconut (Lodoicea maldivica) and its relatives (Borasseae, Arecaceae)

Seed size shapes plant evolution and ecosystems, and may be driven by plant size and architecture, dispersers, habitat and insularity. How these factors influence the evolution of giant seeds is unclear, as are the rate of evolution and the biogeographical consequences of giant seeds. We generated DNA and seed size data for the palm tribe Borasseae (Arecaceae) and its relatives, which show a wide diversity in seed size and include the double coconut (Lodoicea maldivica), the largest seed in the world. We inferred their phylogeny, dispersal history and rates of change in seed size, and evaluated the possible influence of plant size, inflorescence branching, habitat and insularity on these changes. Large seeds were involved in 10 oceanic dispersals. Following theoretical predictions, we found that: taller plants with fewer-branched inflorescences produced larger seeds; seed size tended to evolve faster on islands (except Madagascar); and seeds of shade-loving Borasseae tended to be larger. Plant size and inflorescence branching may constrain seed size in Borasseae and their relatives. The possible roles of insularity, habitat and dispersers are difficult to disentangle. Evolutionary contingencies better explain the gigantism of the double coconut than unusually high rates of seed size increase.

Linking Plant Functional Ecology to Island Biogeography

The study of insular systems has a long history in ecology and biogeography. Island plants often differ remarkably from their noninsular counterparts, constituting excellent models for exploring eco-evolutionary processes. Trait-based approaches can help to answer important questions in island biogeography, yet plant trait patterns on islands remain understudied. We discuss three key hypotheses linking functional ecology to island biogeography: (i) plants in insular systems are characterized by distinct functional trait syndromes (compared with noninsular environments); (ii) these syndromes differ between true islands and terrestrial habitat islands; and (iii) island characteristics influence trait syndromes in a predictable manner. We are convinced that implementing trait-based comparative approaches would considerably further our understanding of plant ecology and evolution in insular systems.

Plants obey (and disobey) the island rule

The island rule predicts that small animals evolve to become larger on islands, while large animals evolve to become smaller. It has been studied for over half a century, and its validity is fiercely debated. Here, we provide a perspective on the debate by conducting a test of the island rule in plants. Results from an extensive dataset on islands in the southwest Pacific illustrate that plant stature and leaf area obey the island rule, but seed size does not. Our results indicate that the island rule may be more pervasive than previously thought and that support for its predictions varies among functional traits.

Island disharmony revisited using orchids as a model group

One central concept in island biology is that island assemblages form subsets of the mainland species pool, being disproportionately rich or poor in certain taxonomic groups. This unbalanced composition, termed 'disharmony', is generally explained using a taxon-centred approach, linking the over- or under-representation of taxa to their colonisation abilities. However, islands may also harbour 'functionally' disharmonic flora, being disproportionately rich or poor in species with certain traits, which may offer greater insights into the processes driving island colonisation. Here, we use orchids as a model to illustrate key processes involved in the formation of functionally disharmonic island floras, including filtering effects (for example biotic interactions), and speciation. Our synthesis is based on a comprehensive orchid dataset of 27 637 species and combines both a literature review and simple exploratory analyses to show that orchids are significantly under-represented on islands relative to mainland regions and that insular orchids display shifts in functional traits, from the shortening of nectar spurs to facilitate ornithophily to changes in colour associated with generalist insect pollinators. We highlight that taxa are simply coarse proxies and that we need to consider species traits and interactions to gain a full understanding of the processes constraining plant assembly on islands.

How repeatable is microevolution on islands? Patterns of dispersal and colonization-related plant traits in a phylogeographical context

BACKGROUND AND AIMS

Archipelagos provide a valuable framework for investigating phenotypic evolution under different levels of geographical isolation. Here, we analysed two co-distributed, widespread plant lineages to examine if incipient island differentiation follows parallel patterns of variation in traits related to dispersal and colonization.

METHODS

Twenty-one populations of two anemochorous Canarian endemics, Kleinia neriifolia and Periploca laevigata, were sampled to represent mainland congeners and two contrasting exposures across all the main islands. Leaf size, seed size and dispersability (estimated as diaspore terminal velocity) were characterized in each population. For comparison, dispersability was also measured in four additional anemochorous island species. Plastid DNA data were used to infer genetic structure and to reconstruct the phylogeographical pattern of our focal species.

KEY RESULTS

In both lineages, mainland-island phenotypic divergence probably started within a similar time frame (i.e. Plio-Pleistocene). Island colonization implied parallel increases in leaf size and dispersability, but seed size showed opposite patterns of variation between Kleinia and Periploca species pairs. Furthermore, dispersability in our focal species was low when compared with other island plants, mostly due to large diaspore sizes. At the archipelago scale, island exposure explained a significant variation in leaf size across islands, but not in dispersability or seed size. Combined analyses of genetic and phenotypic data revealed two consistent patterns: (1) extensive within-island but very limited among-island dispersal, and (2) recurrent phenotypic differentiation between older (central) and younger (peripheral) island populations.

CONCLUSIONS

Leaf size follows a more predictable pattern than dispersability, which is affected by stochastic shifts in seed size. Increased dispersability is associated with high population connectivity at the island scale, but does not preclude allopatric divergence among islands. In sum, phenotypic convergent patterns between species suggest a major role of selection, but deviating traits also indicate the potential contribution of random processes, particularly on peripheral islands.

Can dispersal investment explain why tall plant species achieve longer dispersal distances than short plant species?

Tall plant species disperse further distances than do short species, within and across dispersal syndromes, yet the driver underpinning this relationship is unclear. The ability of taller plants to invest more in dispersal structures may explain the positive relationship between plant height and dispersal distance. Here, we quantify the cross-species relationships between presence of dispersal structures, dispersal investment plant height and dispersal distance. Plant height, dispersal syndrome and dispersal investment data were collated for 1613 species from the literature, with dispersal distance data collated for 114 species. We find that species with high dispersal investment disperse further than do species with low dispersal investment. Tall species have a greater probability of having dispersal structures on their seeds compared with short species. For species with dispersal structures on their seeds, plant height is very weakly related to dispersal investment. Our results provide the first global confirmation of the dispersal investment-distance hypothesis, and show dispersal investment can be used for predicting species dispersal distances. However, our results and those of previous studies indicate plant height is still the best proxy for estimating species dispersal distances due to it being such a readily available plant trait.

Evolutionary cascades induced by large frugivores

Large, fruit-eating vertebrates have been lost from many of the world's ecosystems. The ecological consequences of this defaunation can be severe, but the evolutionary consequences are nearly unknown because it remains unclear whether frugivores exert strong selection on fruit traits. I assessed the macroevolution of fruit traits in response to variation in the diversity and size of seed-dispersing vertebrates. Across the Indo-Malay Archipelago, many of the same plant lineages have been exposed to very different assemblages of seed-dispersing vertebrates. Phylogenetic analysis of >400 plant species in 41 genera and five families revealed that average fruit size tracks the taxonomic and functional diversity of frugivorous birds and mammals. Fruit size was 40.2-46.5% smaller in the Moluccas and Sulawesi (respectively), with relatively depauperate assemblages of mostly small-bodied animals, than in the Sunda Region (Borneo, Sumatra, and Peninsular Malaysia), with a highly diverse suite of large and small animals. Fruit color, however, was unrelated to vertebrate diversity or to the representation of birds versus mammals in the frugivore assemblage. Overhunting of large animals, nearly ubiquitous in tropical forests, could strongly alter selection pressures on plants, resulting in widespread, although trait-specific, morphologic changes.

Plant composition patterns inside an endemic birds’ nest fern (Asplenium goudeyi) on Lord Howe Island: effects of fern size, fern isolation and plant dispersal abilities

The importance of deterministic and stochastic processes in structuring ecological communities is an enduring debate. Although this debate is nearly a century old, the extent to which communities are structured by species interactions or chance events is a central issue in ecology. We examined the assemblages of plants living inside 119 birds’ nest ferns (Asplenium goudeyi), which are endemic to Lord Howe Island. Specifically, we investigated whether patterns of species richness and community composition were influenced by fern size, fern isolation and plant dispersal abilities. Fern size and fern isolation significantly predicted plant community richness. At the community level, plant composition patterns did not deviate from randomized expectations. Individual species occurrences increased with increasing community richness, and no species exclusions were observed. Wind-dispersed taxa, which accounted for 29% of all species, were well represented in isolated ferns. Comparatively, poorer dispersers were confined to ferns nearest the forest at the base of the cliffs. We suggest that dispersal plays a key role in structuring plant communities living within birds’ nest ferns, and that species interactions are less important. Our study emphasizes the importance of epiphytes with a nest-like growth form as habitat for plants in a harsh environment.