The Asteraceae, commonly known as the daisy or sunflower family, stands out as one of the most diverse plant families globally, boasting between 32,000 and 34,000 species. A comprehensive study examining island ecosystems reveals that this Plant Family is also exceptionally diverse in insular environments. Researchers have identified 6,135 Asteraceae species native to islands, with a remarkable 3,535 (58%) being exclusively endemic to these isolated lands. Focusing specifically on oceanic islands, the study found 1,833 native Asteraceae species, of which 955 (52%) are island endemics.
Fig. 1: Angiosperm Plant Family Diversity on Islands. A comparative analysis of the ten most diverse angiosperm plant families on islands, showing native and endemic species richness across all islands (A) and specifically oceanic islands (B). Observed species numbers are contrasted with null model expectations. Plant families below expectation are highlighted in red.
Asteraceae species are often considered effective long-distance dispersers and successful island colonizers. This might lead to the expectation that the proportion of native and endemic island species within the Asteraceae plant family would be higher than in other large plant families, and greater than what would occur by chance. However, surprisingly, a comparison of angiosperm family diversity on islands indicates that Asteraceae is not the most species-rich family across all islands (Fig. 1). In fact, when considering its overall global diversity, Asteraceae are underrepresented in terms of island species. This finding aligns with previous research on island plant disharmony, suggesting that while the Asteraceae plant family is widespread on islands, it is less dominant than its mainland diversity might suggest. Notably, Orchidaceae and Rubiaceae exhibit even higher numbers of native island species, with 11,188 and 6,188 species respectively. The substantial island diversity of these families is particularly concentrated on large, tropical continental islands and archipelagos, regions that are not particularly rich in Asteraceae species. Conversely, on oceanic islands, the Asteraceae plant family emerges as the most diverse, both in terms of native and endemic species counts. Despite this high species richness relative to other plant families on oceanic islands, Asteraceae diversity remains lower than predicted based on their global diversity (Fig. 1B).
Island species constitute 18% of the total species diversity within the Asteraceae plant family. Statistical analysis confirms that the observed number of native island Asteraceae species is significantly different from, and lower than, what would be expected based on the global proportion of Asteraceae among angiosperms (10%). Furthermore, comparisons with island community simulations reinforce that the observed number of island species is lower than the null expectation for both all islands and oceanic islands (Fig. 1).
Family age could be a factor influencing the variation in island diversity among plant families. The mainland species pool diversity of each plant family is shaped by its age and global diversification rates. Intriguingly, among the ten most diverse plant families on islands, Asteraceae are the youngest. This younger age may contribute to their underrepresentation on islands, as younger families might have had smaller mainland species pools when islands emerged, thus initially hindering their colonization potential. However, most islands in the study are considerably younger than the plant families themselves, suggesting that lower colonization and/or diversification rates since island formation are more likely the primary biogeographic reasons for this underrepresentation. As more phylogenetic data on island species becomes available, island biogeography models can be employed to estimate colonization, speciation, and extinction rates for entire island communities of each plant family across evolutionary timescales.
Global Distribution of Asteraceae Plant Family Across Islands
Native Asteraceae species exhibit a truly global distribution across the world’s islands (Fig. 2). Global checklists reveal that the Asteraceae plant family occurs natively on 791 islands, including 308 oceanic islands, and across 146 archipelagos. Their distribution extends from the Svalbard Islands and Greenland in the north (80°N and 75°N) to Macquarie Island and Heard Island in the south (55°S and 53°S). Island diversity within the family ranges from a minimum of one native species to as many as 550, with 29 islands (4%) harboring over 100 native species and 155 islands (20%) having only a single native species (the dataset only includes islands with at least one native Asteraceae species). Across all island types, 128 islands (16%) are home to at least one endemic species of Asteraceae.
Fig. 2: Worldwide Distribution of Asteraceae Plant Family on Islands. (A) Illustrates the global distribution of Asteraceae across all island types, with marker shape denoting island type, size representing species number, and color indicating endemic species count. (B) Focuses on the global distribution of Asteraceae on archipelagos.
While the Asteraceae plant family’s distribution is global, its diversity is not uniform geographically, with certain island regions standing out as diversity hotspots. Madagascar is the most diverse island overall for both native (550) and endemic (487) Asteraceae species. The Caribbean, particularly the Greater Antilles, is another significant center of island Asteraceae diversity, with 671 native and 430 endemic species. At the individual island level, Cuba, Hispaniola, and Jamaica are among the top ten globally for endemic species richness within this plant family. The Caribbean’s importance for Asteraceae endemism is further supported by prior research highlighting this region’s highest number of endemic genera within the Asteraceae plant family globally. Among oceanic islands, Macaronesia, the Hawaiian Islands, and the Mascarenes are prominent diversity hotspots. The Canary Islands are the most diverse oceanic archipelago with 299 native species, and seven of the ten most diverse oceanic islands for native species are within the Canaries, with Tenerife leading with 159 species. The Hawaiian Islands are the second most diverse oceanic archipelago, with 102 native and 95 endemic species, exhibiting a remarkably high endemism proportion (93%), followed by the Mascarenes with 79 native and 64 endemic species.
In contrast to these hotspots, the British Isles and Iceland appear as diversity anomalies. While seemingly hotspots of island diversity for the Asteraceae plant family, the majority of species in these regions are apomictic. Apomixis, an asexual reproduction method where seeds are produced without fertilization, is a complex trait in Asteraceae and poses challenges to traditional taxonomic species concepts. To assess the influence of apomictic species, a sensitivity analysis excluding apomictic genera was performed, revealing minor shifts in the ranking of top island hotspots but no significant impact on the broader findings.
Diversity within the Asteraceae plant family on islands is also unevenly distributed across major clades and taxonomic tribes (Fig. 3). The tribe Cichorieae has the highest number of native island species (1660 spp.). While Cichorieae is a significant component of island floras, particularly in regions like Macaronesia and the Juan Fernández Islands, its overall diversity is partly inflated by the high prevalence of apomictic species within this tribe. Excluding Cichorieae, the three most diverse tribes for both native and endemic island species are Astereae, Senecioneae, and Gnaphalieae. These four widespread tribes collectively account for nearly 60% of all native insular Asteraceae species. While these tribes are also among the largest within the plant family, comparisons of observed island diversity to expected diversity based on tribe size indicate that island species are overrepresented in Cichorieae, Astereae, and Gnaphalieae, and within the expected range for Senecioneae. Notably, Feddeeae and Distephaneae exhibit the highest proportions of native island species relative to their total diversity. Feddeeae is a monotypic tribe with a single species endemic to Cuba. The Distephanus clade, distributed across Africa, Madagascar, and the Mascarenes, also shows a significant overrepresentation of island species.
Fig. 3: Tribal Island Diversity within the Asteraceae Plant Family. (A) Time-calibrated phylogeny of Asteraceae tribes and clades. (B) Comparison of native island species number versus global species number per tribe, with island species percentage indicated. Illustrations highlight clades with high island diversity.
Intrafamily diversity patterns are influenced by both a tribe’s global distribution and the availability of islands within its range. Asteraceae clades with an overrepresentation of island species, despite limited island availability in their overall range, likely possess intrinsic traits that are particularly well-suited to island environments. Furthermore, several tribes with high island species richness, such as Gnaphalieae and Senecioneae, are known for containing widespread “weedy” species. These species often possess a combination of traits that facilitate colonization, rapid growth, and dominance, traits advantageous in novel island habitats. Future research focusing on these weedy species within the Asteraceae plant family on islands could provide valuable insights into the traits promoting successful colonization and establishment in new island ecosystems.
Factors Driving Island Species Richness and Endemism in the Asteraceae Plant Family
Generalized linear mixed models (GLMMs) were used to investigate island features and environmental variables that might drive native species richness (NSR) and the proportion of single-island endemics (pSIE) within the Asteraceae plant family across islands. Diversity patterns in Asteraceae align with classical island biogeography theory, with island area and isolation being the strongest predictors of both NSR and pSIE (Fig. 4). Larger island area is positively associated with both NSR and pSIE. This area-species relationship is consistent with established ecological theory and observed patterns in various island systems across different taxonomic groups. While isolation is a strong predictor for both NSR and pSIE, it has opposing effects on these diversity measures. Isolation negatively impacts NSR, meaning more isolated islands tend to have fewer native species, but it positively affects pSIE, indicating that more isolated islands have a higher proportion of endemism. This increase in endemism with isolation is also a key prediction of island biogeography, reflecting a shift from dispersal-driven species accumulation to evolution-driven diversification on more remote islands.
Fig. 4: Predictors of Asteraceae Plant Family Diversity on Islands. (A) Coefficient estimates and confidence intervals for global models predicting native species richness (NSR) and proportion of single-island endemics (pSIE). (B) Marginal effects plots illustrating the predicted response of NSR and pSIE to island and environmental variables.
Island type (oceanic vs. continental) also influences both NSR and pSIE. Oceanic islands, representing different geological origins and connectivity histories, tend to have fewer native Asteraceae species but a higher proportion of single-island endemics compared to continental islands. Maximum elevation positively correlates with NSR, with higher islands supporting more native species. Temperature seasonality is the strongest climatic predictor for NSR (positive effect), while mean annual temperature is the best climatic predictor for pSIE (positive effect). Island age, when considered specifically for oceanic islands, did not show an additional significant effect on NSR or pSIE in this study.
The models developed for NSR and pSIE demonstrate substantial predictive power in explaining island Asteraceae diversity. The archipelago, included as a random factor in the models, significantly contributes to the explained variance, likely due to the shared biogeographic history of islands within an archipelago. Importantly, the main model patterns and relationships with predictor variables remain consistent even when apomictic species are excluded, indicating the robustness of the findings.
Island Radiations of the Asteraceae Plant Family: A Global Phenomenon
Beyond well-known examples of island radiations in animals, the Asteraceae plant family is increasingly recognized for its extensive evolutionary diversification on islands, exemplified by iconic radiations like the Hawaiian silverswords. A comprehensive review of island radiations within Asteraceae identified 39 phylogenetically confirmed insular radiations and 69 putative taxonomy-based radiations across both continental and oceanic islands, totaling 108 island radiations globally within this plant family. The confirmed radiations range in size from 3 to 160 species, averaging 18 species per radiation. New Zealand and its surrounding islands are home to the largest radiations, including the Celmisia group and the Raoulia alliance. Among oceanic islands, the Polynesian Bidens, Hawaiian Silversword alliance, and woody Sonchus alliance on Macaronesia are notable examples of significant radiations within the Asteraceae plant family. The majority of these radiations are relatively recent diversification events, with most having a crown age younger than 5 million years.
Fig. 5: Island Radiations within the Asteraceae Plant Family. (A) Overview of total, confirmed, and putative insular radiations. (B) Geographic distribution of radiations by region. (C) Traits associated with confirmed radiations.
Asteraceae radiations have occurred across a broad geographic range of islands, from large continental islands like Madagascar to oceanic archipelagos like the Galápagos, and across diverse island types and ecosystems. While most confirmed radiations are found on oceanic islands, this might reflect research bias towards oceanic systems. Regions particularly rich in confirmed radiations include New Zealand, Macaronesia, and the Hawaiian Islands. Considering putative radiations further emphasizes the remarkable extent of speciation within this plant family across islands. Regions with a high number of putative radiations include the Caribbean and Madagascar, highlighting areas for future phylogenetic research.
While taxonomy-based assessments of putative radiations can have limitations, they provide valuable insights into the potential magnitude of radiations and guide future phylogenetic investigations. The combined count of confirmed and putative radiations underscores the extraordinary capacity of the Asteraceae plant family to radiate across a wide spectrum of islands. Compared to other groups, the Asteraceae plant family appears to be exceptionally rich in island radiations. While direct comparisons with other flowering plant families require further research, evidence suggests that Asteraceae may have a higher propensity for island radiation than even some broader taxonomic groups.
Future research should move beyond identifying radiations to understanding the underlying evolutionary processes. Analysis of traits associated with confirmed radiations reveals a strong link with woodiness, supporting the idea that secondary woodiness is a key innovation in insular plant radiations. A variety of dispersal mechanisms are observed in Asteraceae island radiations, with wind dispersal common on less isolated archipelagos and bird dispersal more prevalent on isolated ones. Hybridization and polyploidy are also associated with island radiations in Asteraceae to some extent. Breeding system data, particularly self-compatibility, is currently limited, indicating an area for further investigation.
This global analysis of Asteraceae diversity and distribution on islands provides a crucial foundation for future research. Moving beyond well-studied oceanic islands to less explored regions like the Caribbean and New Guinea is essential. The uneven distribution of Asteraceae diversity, both geographically and within the plant family, raises questions about the intrinsic traits and extrinsic factors driving these patterns. The alignment of Asteraceae diversity patterns with island biogeography theory highlights their value as a model group in biogeography. The large number of species and radiations within the Asteraceae plant family offers unique opportunities for circumventing the limitations of species-poor insular systems in evolutionary studies. The vast number of potential undiscovered radiations within Asteraceae promises exciting discoveries in the years to come, further illuminating the evolution of this remarkable plant family.
