Colonization across oceanic islands, and how to estimate it

Colonization across oceanic islands is a central topic in island biogeography. PAICE, a new methodological tool to estimate colonization events using floristics, genetics, and accounting for sample size. PAICE is designed to perform comparisons among organisms and archipelagos, and can be used to test explicit biogeographic hypotheses such as the difference in colonization success between species with or without long-distance dispersal traits.

Above: Schematic representation of colonization events.

Because oceanic islands emerged lifeless from the bottom of the sea floor with no connection to any continent, they are ideal systems to study complex biogeographic processes. In particular, the colonization of oceanic islands has intrigued scientists for centuries given that all their land life initially arrived in them from another distant territory. Consequently, species with dispersal abilities have traditionally been assumed to be more successful colonizers.

Editors’ choice article: (Free to read online for two years.)
Coello, A. J., Fernández-Mazuecos, M., Heleno, R. H. & Vargas, P. (2022). PAICE: A new R package to estimate the number of inter-island colonizations considering haplotype data and sample size. Journal of Biogeography, 49, xxx– xxx.

However, many species challenge this assumption. For example, Cistus monspeliensis is a plant with capsules and small seeds, that is, without any long-distance dispersal specialization, but it is a good colonizer as inferred by numerous colonization events among islands of the Canarian archipelago. This result goes against the classical dispersal hypothesis and thus encouraged us to compare the number of inter-island colonization events among numerous plant and animal species. As a general assumption, a species is considered a more successful colonizer when displaying a higher number of inter-island colonization events across a given archipelago. To our surprise, in a previous study we found that the number of estimated inter-island colonization events was highly influenced by sample size, and thus it was not possible to compare among species without some degree of bias. In fact, although there are several methods available to reconstruct inter-island colonization events, none of them considers sampling size.

In this study we propose PAICE (Phylogeographic Analysis of Island Colonization Events), a new approach implemented in an R package that not only uses floristics and haplotype sharing among islands (like previous studies) but also sample sizes in the estimation of the number of inter-island colonization events for any species within an archipelago. Based on haplotype diversity of uniparental inherited DNA regions, PAICE calculates the number of inter-island colonization events considering haplotype sharing, haplotype networks and rarefaction curves at both sampling levels (field and genetic). As a result, this approach estimates the number of inter-island colonization events accounting for sample size.

Cistus monspeliensis.

After developing PAICE, we applied it to 10 animal and plant species with data taken from the literature and noticed some problems when trying to compare their numbers of inter-island colonization events. In particular, a considerable number of case studies showed a sample size that was too small to estimate a reliable number of colonization events (birds like Buteo galapagoensis or Setophaga petechia aureola, plants like Canarina canariensis, Croton scouleri or Juniperus cedrus). Despite this challenge, a comparative estimation of colonization events suggests a higher colonization ability for species that were previously considered poor colonizers. Interestingly, animals with high flying capacity such as the carpenter bee (Xylocopa darwini) and the bird S. petechia aureola of the Galápagos Islands were considered poor colonizers in previous studies. However, the estimated number of inter-island colonization events provided by PAICE revealed that many colonization events were hidden in those previous studies because a very frequent haplotype was distributed across many islands. This increase in the estimate of colonization events was possible due to the application of rarefaction curves, which had not been use before for this purpose. In contrast, both Cistus monspeliensis and Olea europaea subsp. guanchica had similar numbers of inter-island colonization events (c. 20 – 45), although O. europaea subsp. guanchica is an endozoochorous plant while C. monspeliensis does not have dispersal specializations. Two additional plants were suggested to have a high number of inter-island colonization events, specifically Juniperus brevifolia (> 100 colonizations) and Picconia azorica (> 75 colonizations), but more studies are needed to refine these estimates.

We believe that PAICE paves the road for future studies aiming to compare colonization success among species in insular systems. In fact, this approach can also be applied to study movements among territories in other island-like systems such as lakes and mountain tops.

We hope that future island biogeographic studies will benefit from PAICE to evaluate species colonization success, as well as the relative importance of dispersal and establishment in the colonization process. In particular, classical hypotheses in island biogeography, such as the higher colonization success of species with long-distance dispersal abilities, can be addressed using PAICE. Although PAICE provides user-friendly R functions, the corresponding authors offer to guide any phylogeographical studies aiming to estimate numbers of colonization events and thus colonization success across islands and island-like system.

The carpenter bee (Xylocopa darwini)

Written by:
Alberto J. Coello (1), Mario Fernández-Mazuecos (2), Ruben H. Heleno (3) & Pablo Vargas (4)
(1) PhD candidate, Department of Biodiversity and Conservation, Real Jardín Botánico (RJB-CSIC)
(2) Lecturer, Department of Biology (Botany), Facultad de Ciencias, Universidad Autónoma de Madrid
(3) Assistant Professor, Centre for Functional Ecology, Associate Laboratory TERRA, Department of Life Sciences, University of Coimbra
(4) Professor, Department of Biodiversity and Conservation, Real Jardín Botánico (RJB-CSIC)

Additional information:
Alberto J. Coello: @albertojcoello (Twitter)
Mario Fernández-Mazuecos: @mfmazuecos (Twitter), (website)
Ruben H. Heleno: @ruben_heleno (Twitter),
Pablo Vargas: @pablovargas1111 (Twitter)

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