Models of oceanic island biogeography:
changing perspectives on biodiversity
dynamics in archipelagoes
Lawrence R. Heaney 1*, Danilo S. Balete 1 , and Eric A. Rickart 2
1 Field Museum of Natural History, 1400 S Lake Shore Drive, Chicago, IL 60605, USA;
2 Natural History Museum of Utah, 301 Wakara Way, Salt Lake City, UT 84108, USA;
frontiers of biogeography, Volume 5, Issue 4, 2013,pp 249 - 257
Models of biogeographic processes can both enhance and inhibit our ability to ask questions that guide our understanding of patterns and processes. The two ‘traditional’ models of island biogeog‐ raphy, the Equilibrium Model and the Vicariance Model, raise important and insightful questions about relevant processes, but both fail to raise many crucial questions. An example involving the non‐volant mammals of the Philippine archipelago shows that both models highlight some, but not all, relevant pat‐ terns and processes. The more recently proposed General Dynamic Model successfully combines many of the positive aspects of the two traditional models, but leaves some important questions unasked. We pose a number of questions here that may help guide further development of models of island biogeog‐ raphy.
Keywords. Colonization, Equilibrium Model, extinction, General Dynamic Model, geomorphology, hot spots, Philippines, speciation, subduction zones, timescale, Vicariance Model
The progress of science is guided by the models that we use. Much useful science can be con‐ ducted that is ‘model‐free’, but by definition it is purely descriptive. When we choose to ask ques‐ tions and to make comparisons, and especially in deciding how to interpret our results, we rely on models, explicitly or implicitly. New models often emerge when the existing ones have increasing difficulty with allowing us to ask or answer rele‐ vant questions, or when patterns become evident that are not addressed by the existing models. Models and paradigms (the larger conceptual framework for a model) are useful to the extent that they help us ask good questions; they are ob‐ structive to the extent they prevent us from ask‐ ing good questions
A change in models may represent a simple shift from one model to another while remaining in the same paradigm, but sometimes a change in models is associated with a fundamentally differ‐ ent way of thinking—a paradigm shift. We have become convinced that such a paradigm shift is taking place in island biogeography, with some rather profound effects on how we can and do conceptualize the dynamics of the origin and maintenance of biological diversity on islands (Heaney 2007, 2011a). This paradigm shift is asso‐ ciated with what we view as a movement away from the two ‘classical’ models of island biogeog‐ raphy, both of which came to prominence nearly simultaneously in the 1970s: the Equilibrium Model of MacArthur and Wilson (1963, 1967), and the Vicariance Model developed by Brundin, Croi‐ zat, Nelson, Platnick, Rosen, and others (e.g., Rosen 1978, Nelson and Platnick 1981).
This shift in paradigms is not taking place because either of these two models is ‘wrong’ in the sense that they focus on processes that either do not exist or do not describe important phe‐ nomena; rather, it is apparent that the processes are quite real, and that the issues are quite impor‐ tant. Instead, it has become apparent that each model is limited in scope and fails to ask essential questions about processes of great importance, and each may lead to some profound misunder‐ standings about the nature of island life. Most strikingly, though intending to provide an under‐ standing of biodiversity dynamics of the same or‐ ganisms on the same islands, the two models are nearly mutually exclusive, with virtually no over‐ lap in the processes and perspectives that they consider (Whittaker and Fernández‐Palacios 2007). Although they existed side‐by‐side for dec‐ ades without attempts at integration, new per‐ spectives and syntheses have emerged recently, pointing the way to a deeper understanding of island biogeography
This brief paper is offered not as a compre‐ hensive review of the subject; instead, it is greatly streamlined, and deliberately somewhat provoca‐ tive. It is intended primarily as an invitation to is‐ land biogeographers to further consider these is‐ sues, and to engage in a discussion about the chal‐ lenges that are implied. Island biogeography has been one of the most influential fields of biodiver‐ sity science since the study of biological diversity began; it has great potential to remain one of the most dynamic and forward‐looking.
Our descriptions of the two foundational models that follow here are brief to the point of being caricatures. We do this to highlight the con‐ trasts between the two as they have usually been treated in the published literature. More nuanced versions of both exist, and many exceptions to each model have been noted. However, in their essential features, they each have core perspec‐ tives that are simple and unambiguous – and are mutually incompatible in crucial respects. Our de‐ scription of the most prominent of the new mod‐ els is equally brief, and also intended to highlight what we view as limitations and areas for further development.
This view of the dynamics of biodiversity in oce‐ anic island archipelagoes is far more complex than what is considered under either the Equilibrium or Vicariance Model. We view the General Dynamic Model as representing a substantial step forward in developing a comprehensive model that will lead to an integrated, realistic understanding of these very complex geological and biological inter‐ actions. However, we also believe that many questions have not been adequately defined or investigated, and still others have barely been considered. The following is a brief, undoubtedly incomplete list of issues that we consider to be of broad importance in moving forward in develop‐ ing a comprehensive model.
Islands rarely exist as single, isolated enti‐ ties; rather, they most often occur in groups, due to the geological processes that produce them, and organisms (and the dynamic processes that influence them) thus exist within these complex geomorphological groupings. Oceanic archipela‐ goes (island groups that have had no dry‐land con‐ nections to more species‐rich continental areas) are of two broad types: hot‐spot archipelagoes, which form over a single plume of magma (Hawaii is a premiere example), and plate‐margin archi‐ pelagoes, which form beside subduction zones (the Philippine archipelago is such a group). The former typically occur in a nearly linear string of islands that form quickly then erode away; the latter form through a series of eruptions over the period of existence of the subduction zone, and often undergo progressive (though irregular) in‐ crease in area over time, persist much longer than hot‐spot islands, and often undergo mergers be‐ tween formerly isolated islands as volcanic materi‐ als progressively fill intervening basins. These dif‐ fering ‘life histories’ for the two types of archi‐ pelagoes lead to the question, do hot‐spot vs. plate‐margin archipelagoes differ consistently in their species‐richness dynamics? For example, does indigenous phylogenetic diversification re‐ sult in more species richness in the latter than in the former, given their typically greater persis‐ tence and greater area? When islands merge in plate‐margin archipelagoes, to what extent do species spread out and increase total species rich‐ ness, and to what extent do these ‘invasions’ pre‐ cipitate a wave of extinctions? What geological processes promote speciation within old, geologi‐ cally complex islands individually and collectively as archipelagoes, as dictated by the processes of volcanic dynamics and geomorphological develop‐ ment? Within an archipelago, how much biodiver‐ sity results from inter‐island vs. intra‐island speci‐ ation, for organisms of varying vagility?
The General Dynamic Model explicitly mod‐ els a general interaction between colonization and phylogenetic diversification as they together fill ‘ecological space’, raising some additional ques‐ tions. To what extent do ‘adaptive radiations’ fill ‘niche space’ (and to what extent may a radiation generate new ‘niches’), and do they do so to the extent that later colonizers are inhibited or pre‐ vented from invading successfully? Does each suc‐ cessful colonization by a given taxon (e.g., by a previously absent family of beetles, flowering plants, or rodents) into an archipelago result in an increase in standing diversity, or does the new colonizer cause the extinction of an ’old en‐ demic’? Conversely, does the presence of ‘old en‐ demic’ species cause the failure of colonization by the newly arrived species? In general, are species/ lineages prone to extinction or to persistence? To what extent do ecological and geological proc‐ esses influence taxa and the communities in which they live, and how do these two types of proc‐ esses interact?
On a still broader level, we might ask, how much do rates of diversification differ based on dispersal ability, ‘newness’ of habitat, body size, trophic level, etc., of the arriving taxa? Do island biotas exist in a state of equilibrium, or disequilib‐ rium? Indeed, is there such a thing as an equilib‐ rium value of species richness, and if so, is it ever actually achieved in a geomorphologically active archipelago? To what extent are the processes that produce species richness patterns determinis‐ tic, probabilistic, or random? Finally, how do these processes influence the response of island com‐ munities to habitat disturbance or invasion by in‐ troduced exotics?
We believe that these questions, and un‐ doubtedly many more not posed here, must be asked, and empirical data developed so that they can be answered, if we are to develop an accurate and fully‐formed understanding of the dynamics of species diversity in oceanic island archipelagoes. We believe that few of these questions could have been posed as long as ‘ecological’ and ‘evolutionary’ island biogeography were treated as distinct topics. The General Dynamic Model prompts us to integrate ecological and evolution‐ ary perspectives in more complex and therefore more realistic ways than prior models, but remains limited by focusing on one island at a time, and by application to islands with only the ‘hot‐spot is‐ land life history’. By broadening our models to in‐ corporate the dynamics of archipelagoes and more complex island life histories, we will allow even broader and more integrative conceptualizations of island biogeography dynamics to be developed.
We find it noteworthy that in Island Life, A. R. Wallace’s (1880) grand synthesis that estab‐ lished island biogeography as a distinct field of scientific study, Wallace strongly emphasized both the reality of long‐term geological change and its impact on organismal distribution and diversity, and the reality of both long‐timescale and short‐ timescale ecological factors (such as climate change) and dispersal (Heaney 2013). His explicit call for integrative approaches that recognize mul‐ tiple processes, patterns, and timescales within any given archipelago serves us well as a frame‐ work in our current reconsideration of the dynam‐ ics of biodiversity on islands.
This commentary grew out of a presentation by Heaney at the Early Career Conference of the In‐ ternational Biogeography Society, held at Oxford University in September 2011. We thank Rob Whittaker for the invitation to present the paper, and Mike Dawson for his encouragement to pre‐ pare this version for publication. We thank Mike Dawson, Jake Esselstyn, Joaquin Hortal, Kostas Triantis, and an anonymous reviewer for com‐ ments that improved earlier drafts of the manu‐ script.
The research on Philippine mammals men‐ tioned here, and the opportunity to develop the perspectives expressed here, has been possible only because of the support and collaboration of colleagues at many institutions, including the Phil‐ ippine Department of Environment and Natural Resources, National Museum of the Philippines, Silliman University, Haribon Foundation, Conser‐ vation International – Philippines, University of the Philippines, Florida State University, University of Minnesota, University of Kansas, and Smith‐ sonian Institution. Funding has been provided by the US National Science Foundation, John D. and Catherine T. MacArthur Foundation, the Field Mu‐ seum (Barbara Brown, Ellen Thorne Smith, and Marshall Field Funds), Grainger Foundation, and especially the Negaunee Foundation.