Spatial variation in coevolutionary dynamics

Summary Computer simulations of coevolutionary dynamics between two hosts and two parasites show that extensive spatial variation in polymorphism can be maintained among environmentally identical patches. Spatial variation can be maintained under frequent migration when the dynamics within patches are locally unstable, and the cycles in host and parasite abundances remain out of phase among patches. Additionally, spatial variation can be maintained when host-parasite interactions cause local extinctions, and migration subsequently allows for recolonization. The temporal dynamics that cause spatial variation are difficult to study directly because of the long time scale over which they occur. The simulations suggest that sampling over space at one or a few points in time may provide much information about temporal dynamics.     

Plankton time — space dynamics in German Bight — a systems approach

Summary Long term continuous plankton measurements at Helgoland (North Sea) have provided a set of data which could be used for ecological functional analysis with respect to prey predator theory. Local dynamics display trophic feedback between selected populations. Phytoplankton, small copepods, Noctiluca miliaris, Pleurobrachia pileus and Beroe gracilis conform with theoretical assumptions. The functional relationships are less significant in averaged population dynamics compared with single ear processes. The local prey-predator cycles are to be understood as population waves travelling through German Bight. Such population waves over an area of 18 000 km² have been investigated and are displayed for the above given zooplankton populations as computer graphics and analysed in their progression for the population of P. pileus.     

Area-dependent migration by ringlet butterflies generates a mixture of patchy population and metapopulation attributes

Interpretation of spatially structured population systems is critically dependent on levels of migration between habitat patches. If there is considerable movement, with each individual visiting several patches, there is one ”patchy population”; if there is intermediate movement, with most individuals staying within their natal patch, there is a metapopulation; and if (virtually) no movement occurs, then the populations are separate (Harrison 1991, 1994). These population types actually represent points along a continuum of much to no mobility in relation to patch structure. Therefore, interpretation of the effects of spatial structure on the dynamics of a population system must be accompanied by information on mobility. We use empirical data on movements by ringlet butterflies, Aphantopus hyperantus, to investigate two key issues that need to be resolved in spatially-structured population systems. First, do local habitat patches contain largely independent local populations (the unit of a metapopulation), or merely aggregations of adult butterflies (as in patchy populations)? Second, what are the effects of patch area on migration in and out of the patches, since patch area varies considerably within most real population systems, and because human landscape modification usually results in changes in habitat patch sizes? Mark-release-recapture (MRR) data from two spatially structured study systems showed that 63% and 79% of recaptures remained in the same patch, and thus it seems reasonable to call both systems metapopulations, with some capacity for separate local dynamics to take place in different local patches. Per capita immigration and emigration rates declined with increasing patch area, while the resident fraction increased. Actual numbers of emigrants either stayed the same or increased with area. The effect of patch area on movement of individuals in the system are exactly what we would have expected if A. hyperantus were responding to habitat geometry. Large patches acted as local populations (metapopulation units) and small patches simply as locations with aggregations (units of patchy populations), all within 0.5 km². Perhaps not unusually, our study system appears to contain a mixture of metapopulation and patchy-population attributes.     

Incidence and population dynamics of the leaf beetle Gonioctena olivacea in dynamic habitats

In natural as well as in cultural landscapes, disturbance and succession are responsible for the emergence and subsequent disappearance of suitable habitat patches. The dynamics of habitat patches has important consequences for the spatial structure and dynamics of regional populations. However, there are only few studies quantifying both patch dynamics and incidence of insect species in a dynamic landscape over several years. I studied the incidence and population dynamics of the leaf beetle Gonioctena olivacea in a system of dynamic patches of the host plant Scotch broom Cytisus scoparius . The incidence of the beetle was most strongly affected by patch area, whereas connectivity, patch quality, patch age, and landscape context had no or only a minor effect when analysed with logistic regression. The size of local beetle populations was highly fluctuating between the years; however, the population dynamics of the local populations was not synchronous. Adjacent patches did not show higher degrees of synchrony than patches separated by large distances. In the three years of study, local populations became extinct through demographic or environmental stochasticity and patch destruction. Each year >10% of the patches disappeared. The extinction rate of beetles in persistent patches was decreasing with increasing patch area. On the other hand, patches newly emerged and were rapidly colonized by the beetle. The colonization rate depended on patch connectivity. Obviously, Gonioctena olivacea was capable of persisting in this system with high turnover of patches owing to its high dispersal power.     

Cyclic dynamics in simulated plant populations

Despite the general interest in nonlinear dynamics in animal populations, plant populations are supposed to show a stable equilibrium that is attributed to fundamental differences compared with animals. Some studies find more complex dynamics, but empirical studies usually are too short and most modelling studies ignore important spatial aspects of local competition and establishment. Therefore, we used a spatially explicit individual-based model of a hypothetical, non-clonal perennial to explore which mechanisms might generate complex dynamics, i.e. cycles. The model is based on the field-of-neighbourhood approach that describes local competition and establishment in a phenomenological manner. We found cyclic population dynamics for a wide spectrum of model variants, provided that mortality is determined by local competition and recruitment is virtually completely suppressed within the zone of influence of established plants. This destabilizing effect of local processes within plant populations might have wide-ranging implications for the understanding of plant community dynamics and coexistence.     

The dynamics of two diffusively coupled predator-prey populations

I analyze the dynamics of predator and prey populations living in two patches. Within a patch the prey grow logistically and the predators have a Holling type II functional response. The two patches are coupled through predator migration. The system can be interpreted as a simple predator-prey metapopulation or as a spatially explicit predator-prey system. Asynchronous local dynamics are presumed by metapopulation theory. The main question I address is when synchronous and when asynchronous dynamics arise. Contrary to biological intuition, for very small migration rates the oscillations always synchronize. For intermediate migration rates the synchronous oscillations are unstable and I found periodic, quasi-periodic, and intermittently chaotic attractors with asynchronous dynamics. For large predator migration rates, attractors in the form of equilibria or limit cycles exist in which one of the patches contains no prey. The dynamical behavior of the system is described using bifurcation diagrams. The model shows that spatial predator-prey populations can be regulated through the interplay of local dynamics and migration.     

Post-settlement emigration affects mortality estimates for two Bahamian wrasses

Understanding the dynamics of open marine populations is inherently complex, and this complexity has led to decades of debate regarding the relative importance of pre- versus post-settlement processes in structuring these populations. Movement between patches may be an important modifier of patterns established at settlement, yet local immigration and emigration have received less attention than other demographic rates. I examined loss rates from tagged populations of juvenile wrasses (yellowhead wrasse Halichoeres garnoti and bluehead wrasse Thalassoma bifasciatum) at two sites in the Bahamas. Assuming that all losses were due solely to mortality would have significantly underestimated survivorship of yellowhead wrasse by 29% and bluehead wrasse by 14%. On average, per capita mortality and emigration rates were higher for yellowhead than bluehead wrasse, but neither demographic rate differed between sites for either species. With respect to within-species density, bluehead wrasse mortality was density-dependent at the patch reef site, but mortality rates of yellowhead wrasse were consistently density-independent. Evaluating the effects of between-species density, yellowhead wrasse mortality increased with a decrease in bluehead wrasse density, but this effect was limited to the patch reef site. Emigration rates were not a function of either within-species or between-species density, but instead varied inversely with isolation distance. Numerous previous studies of coral-reef fish, conducted on patch reefs separated by only a few meters of sand and often using untagged fish, may have confounded losses due to emigration with those due to mortality. A better understanding of the factors affecting emigration in marine fishes is important to their effective management using spatial tools such as marine protected areas.     

The role of refuges in the persistence of Australian dryland mammals

Irruptive population dynamics are characteristic of a wide range of fauna in the world's arid (dryland) regions. Recent evidence indicates that regional persistence of irruptive species, particularly small mammals, during the extensive dry periods of unpredictable length that occur between resource pulses in drylands occurs as a result of the presence of refuge habitats or refuge patches into which populations contract during dry (bust) periods. These small dry‐period populations act as a source of animals when recolonisation of the surrounding habitat occurs during and after subsequent resource pulses (booms). The refuges used by irruptive dryland fauna differ in temporal and spatial scale from the refugia to which species contract in response to changing climate. Refuges of dryland fauna operate over timescales of months and years, whereas refugia operate on timescales of millennia over which evolutionary divergence may occur. Protection and management of refuge patches and refuge habitats should be a priority for the conservation of dryland‐dwelling fauna. This urgency is driven by recognition that disturbance to refuges can lead to the extinction of local populations and, if disturbance is widespread, entire species. Despite the apparent significance of dryland refuges for conservation management, these sites remain poorly understood ecologically. Here, we synthesise available information on the refuges of dryland‐dwelling fauna, using Australian mammals as a case study to provide focus, and document a research agenda for increasing this knowledge base. We develop a typology of refuges that recognises two main types of refuge: fixed and shifting. We outline a suite of models of fixed refuges on the basis of stability in occupancy between and within successive bust phases of population cycles. To illustrate the breadth of refuge types we provide case studies of refuge use in three species of dryland mammal: plains mouse (Pseudomys australis), central rock‐rat (Zyzomys pedunculatus), and spinifex hopping‐mouse (Notomys alexis). We suggest that future research should focus on understanding the species‐specific nature of refuge use and the spatial ecology of refuges with a focus on connectivity and potential metapopulation dynamics. Assessing refuge quality and understanding the threats to high‐quality refuge patches and habitat should also be a priority. To facilitate this understanding we develop a three‐step methodology for determining species‐specific refuge location and habitat attributes. This review is necessarily focussed on dryland mammals in continental Australia where most refuge‐based research has been undertaken. The applicability of the refuge concept and the importance of refuges for dryland fauna conservation elsewhere in the world should be investigated. We predict that refuge‐using mammals will be widespread particularly among dryland areas with unpredictable rainfall patterns.     

Models for spatially distributed populations: The effect of within-patch variability

This paper studies population models which have the following three ingredients: populations are divided into local subpopulations, local population dynamics are nonlinear and random events occur locally in space. In this setting local stochastic phenomena have a systematic effect on average population density and this effect does not disappear in large populations. This result is an outcome of the interaction of the three ingredients in the models and it says that stochastic models of systems of patches can be expected to give results for average population density that differ systematically from those of deterministic models. The magnitude of these differences is related to the degree of nonlinearity of local dynamics and the magnitude of local variability. These results explain those obtained from a number of previously published models which give conclusions that differ from those of deterministic models. Results are also obtained that show how stochastic models of systems of patches may be simplified to facilitate their study.     

Application of network methods for understanding evolutionary dynamics in discrete habitats

In populations occupying discrete habitat patches, gene flow between habitat patches may form an intricate population structure. In such structures, the evolutionary dynamics resulting from interaction of gene‐flow patterns with other evolutionary forces may be exceedingly complex. Several models describing gene flow between discrete habitat patches have been presented in the population‐genetics literature; however, these models have usually addressed relatively simple settings of habitable patches and have stopped short of providing general methodologies for addressing nontrivial gene‐flow patterns. In the last decades, network theory – a branch of discrete mathematics concerned with complex interactions between discrete elements – has been applied to address several problems in population genetics by modelling gene flow between habitat patches using networks. Here, we present the idea and concepts of modelling complex gene flows in discrete habitats using networks. Our goal is to raise awareness to existing network theory applications in molecular ecology studies, as well as to outline the current and potential contribution of network methods to the understanding of evolutionary dynamics in discrete habitats. We review the main branches of network theory that have been, or that we believe potentially could be, applied to population genetics and molecular ecology research. We address applications to theoretical modelling and to empirical population‐genetic studies, and we highlight future directions for extending the integration of network science with molecular ecology.     

Long‐term metapopulation study of the Glanville fritillary butterfly (Melitaea cinxia): survey methods,data management,and long‐term population trends

Long‐term observational studies conducted at large (regional) spatial scales contribute to better understanding of landscape effects on population and evolutionary dynamics, including the conditions that affect long‐term viability of species, but large‐scale studies are expensive and logistically challenging to keep running for a long time. Here, we describe the long‐term metapopulation study of the Glanville fritillary butterfly (Melitaea cinxia) that has been conducted since 1991 in a large network of 4000 habitat patches (dry meadows) within a study area of 50 by 70 km in the Åland Islands in Finland. We explain how the landscape structure has been described, including definition, delimitation, and mapping of the habitat patches; methods of field survey, including the logistics, cost, and reliability of the survey; and data management using the EarthCape biodiversity platform. We describe the long‐term metapopulation dynamics of the Glanville fritillary based on the survey. There has been no long‐term change in the overall size of the metapopulation, but the level of spatial synchrony and hence the amplitude of fluctuations in year‐to‐year metapopulation dynamics have increased over the years, possibly due to increasing frequency of exceptional weather conditions. We discuss the added value of large‐scale and long‐term population studies, but also emphasize the need to integrate more targeted experimental studies in the context of long‐term observational studies. For instance, in the case of the Glanville fritillary project, the long‐term study has produced an opportunity to sample individuals for experiments from local populations with a known demographic history. These studies have demonstrated striking differences in dispersal rate and other life‐history traits of individuals from newly established local populations (the offspring of colonizers) versus individuals from old, established local populations. The long‐term observational study has stimulated the development of metapopulation models and provided an opportunity to test model predictions. This combination of empirical studies and modeling has facilitated the study of key phenomena in spatial dynamics, such as extinction threshold and extinction debt.     

Effects of Culling on Mesopredator Population Dynamics

Anthropogenic changes in land use and the extirpation of apex predators have facilitated explosive growth of mesopredator populations. Consequently, many species have been subjected to extensive control throughout portions of their range due to their integral role as generalist predators and reservoirs of zoonotic disease. Yet, few studies have monitored the effects of landscape composition or configuration on the demographic or behavioral response of mesopredators to population manipulation. During 2007 we removed 382 raccoons (Procyon lotor) from 30 forest patches throughout a fragmented agricultural ecosystem to test hypotheses regarding the effects of habitat isolation on population recovery and role of range expansion and dispersal in patch colonization of mesopredators in heterogeneous landscapes. Patches were allowed to recolonize naturally and demographic restructuring of patches was monitored from 2008–2010 using mark-recapture. An additional 25 control patches were monitored as a baseline measure of demography. After 3 years only 40% of experimental patches had returned to pre-removal densities. This stagnant recovery was driven by low colonization rates of females, resulting in little to no within-patch recruitment. Colonizing raccoons were predominantly young males, suggesting that dispersal, rather than range expansion, was the primary mechanism driving population recovery. Contrary to our prediction, neither landscape connectivity nor measured local habitat attributes influenced colonization rates, likely due to the high dispersal capability of raccoons and limited role of range expansion in patch colonization. Although culling is commonly used to control local populations of many mesopredators, we demonstrate that such practices create severe disruptions in population demography that may be counterproductive to disease management in fragmented landscapes due to an influx of dispersing males into depopulated areas. However, given the slow repopulation rates observed in our study, localized depopulation may be effective at reducing negative ecological impacts of mesopredators in fragmented landscapes at limited spatial and temporal scales.     

Phenotypic plasticity and differentiation in an invasive freshwater microalga

Recent studies show that both marine and limnic microalgal species often consist of several genetically distinct populations. This is also valid for the nuisance freshwater algae Gonyostomum semen, which originates from acidic, brown water swamp lakes, but can nowadays also be found in clearer lakes with close to neutral pH. We hypothesized that the observed genetic differentiation among G. semen lake populations, reported in earlier studies, is connected to adaptation to local environmental conditions. In the present study we performed controlled laboratory experiments to test whether 12 strains originating from five lakes varied in their response to five to six different pHs, light intensities and DOC concentrations. Overall, growth (0.01–0.37 day⁻¹) was observed over a wide range of light intensities and pHs, demonstrating high potential for photoacclimation and extensive plasticity of individual strains. Moreover, we found similar growth rates and consistent growth optima for specific pHs by strains from the same lake, suggesting genetic differentiation of populations into distinct phenotypes. However, observed strain specific preferences did not always reflect environmental conditions in the lakes of origin and provided limited evidence for the hypothesized local adaptation. Instead, the observed phenotypic differentiation may indicate resilient effects of founder events. We suggest that the wide phenotypic plasticity in this species enables it to thrive in fluctuating and variable environments, and may play a role in its ability to colonize new habitats.     

Habitat selection reduces extinction of populations subject to Allee effects

Theoretical studies indicate that a single population under an Allee effect will decline to extinction if reduced below a particular threshold, but the existence of multiple local populations connected by random dispersal improves persistence of the global population. An additional process that can facilitate persistence is the existence of habitat selection by dispersers. Using analytic and simulation models of population change, I found that when habitat patches exhibiting Allee effects are connected by dispersing individuals, habitat selection by these dispersers increases the likelihood that patches persist at high densities, relative to results expected by random settlement. Populations exhibiting habitat selection also attain equilibrium more quickly than randomly dispersing populations. These effects are particularly important when Allee effects are large and more than two patches exist. Integrating habitat selection into population dynamics may help address why some studies have failed to find extinction thresholds in populations, despite well-known Allee effects in many species.     

Population ecology of wild sunflowers: effects of seed density and post-dispersal vertebrate seed predators

Assessing the effects of seed density on the population dynamics of wild plant species with crop relatives will be vital in determining the potential effects of introducing traits into wild populations as a result of crop-to-wild gene flow. We examined experimental sunflower (Helianthus annuus) patches in eastern Kansas to determine the effects of seed density and predation on seedling recruitment and seed production in the next generation. High seed density treatment plots had significantly more seedlings and adult plants than did low seed density treatment plots. Overwinter vertebrate seed predator exclusion treatments resulted in increases in plant density compared to plots in which vertebrates were not excluded. Control patches (no seeds added) contained virtually no plants. Head production and estimated total seed production for a patch were not statistically different among treatments (excluding control plots). Although initial seed density and vertebrate post-dispersal seed predation do appear to have effects on seedling recruitment, neither appear to be limiting seed production of competing adult plants. Therefore, variation in seed densities (over the range examined) may have limited effects on local population dynamics. It is important to note that the choice of seed densities may affect the results obtained: the seed densities used in this study may, in retrospect, be higher than in the small roadside populations typical in eastern Kansas, yet other natural sites have much larger densities. Further, the effects of increased seed density at a local site may have other important effects such as altering metapopulation dynamics through increased long-distance dispersal or increased local seed bank size.     

Patterns of differentiation among wild rabbit populations Oryctolagus cuniculus L. in arid and semiarid ecosystems of north-eastern Australia

Feral rabbit populations in Australia have generally been managed using localized control procedures. While these procedures may result in local extinctions, persistence of populations will depend on the probability of recolonization. Genetic markers developed using temperature gradient gel electrophoresis (TGGE) combined with heteroduplex analysis (HA) of mitochondrial DNA (mtDNA) were used to characterize the degree of subdivision and extent of gene flow within and among rabbit populations distributed over large distances (up to 1000 km) in southern Queensland (QLD) and north-west New South Wales (NSW), Australia. TGGE analyses revealed significant heterogeneity in mtDNA control region haplotype frequencies. From heterogeneity χ² tests, it was evident that the differentiation observed was largely attributable to five sites which were located in the semiarid eastern region, whereas haplotype frequencies were homogeneous throughout the arid western region. These results suggest that there are independent population systems within the study area. The extent of gene flow among local populations within each system is related to the spatial configuration of acceptable habitat patches and the persistence of the populations is determined by the probability of recolonization following local extinction. These data suggest that to provide better overall control of rabbit populations, different management strategies may be necessary in arid and semiarid ecosystems. In arid south-west QLD and north-west NSW, where extensive gene flow occurs over large distances, rabbit populations should be managed at a regional level. In semiarid eastern QLD, where gene flow is restricted and populations are more isolated, localized control procedures may provide effective short-term relief. These results indicate that in nonequilibrium systems with patchy distribution of individuals, the interpretation of migration rate from estimates of gene flow obtained using existing genetic models must include an understanding of the spatial and temporal scales over which population processes operate.     

Metapopulation dynamics of the bog fritillary butterfly: movements between habitat patches

Recent studies on metapopulation dynamics have emphasized the need for improved methods for quantifying individual movements between local populations and habitat patches. In this paper, we report on a 6-yr study in which a network of 12 habitat patches occupied by the bog fritillary, Proclossiana eunomia , was surveyed, with special focus on quantifying movements between the habitat patches. We applied the Virtual Migration model which has been designed to estimate survival and migration parameters in a metapopulation of several connected local populations. The model was parameterized using mark-release-recapture data collected during 6 yr. Generally, the estimated parameter values indicated a high level of movements, with roughly half of butterfly-days spent outside the natal patch. Mortality within patches was higher in males than in females. Females tended to be more mobile and spent more time outside their natal patch than males. Further analysis of the MRR data shows that in this protandrous species males tend to move very little between habitat patches before substantial numbers of females have emerged.     

Variation of a floral polymorphism at different spatial scales in the Mediterranean geophyte Narcissus assoanus

Aims Habitat fragmentation impacts the spatial extent and isolation of local populations and communities. Although the biological consequences of these impacts have been well studied at the site level, effects directly related to changes in the spatial configuration of populations in the landscape remain less clear. The objective of this study is to examine how changes in the spatial-scale configuration of populations are associated with variability in morph rations in the floral polymorphic Mediterranean geophyte Narcissus assoanus .Methods We performed a nested analysis of morph ratio variation at three spatial scales: a 50×50 km regional scale in SE France, in fourteen 1×1 km landscapes located in the same region, and within 12 spatially extensive population patches. We also quantified variation in the behaviour of pollinators in population patches of contrasting spatial configuration.Important findings At a regional scale, morph ratios show a geographic pattern of increasing isoplethy (1:1 morph ratio) away from centres of human population development and in upland pastures. At the landscape scale, isoplethy of local population patches is more correlated with decreased isolation among patches than with patch size. Finally, within local isoplethic populations, small patches can show significantly biased morph ratios. In small isolated patches, pollinators perform shorter flight distances among consecutive flower visits than in spatially extensive patches. This study of variation in a genetic polymorphism at multiple spatial scales provides new insights into the scale-dependent effects of habitat fragmentation and the potential occurrence of metapopulation dynamics in natural plant populations.     

On the evolution of cytoplasmic incompatibility in haplodiploid species

The most enigmatic sexual manipulation by Wolbachia endosymbionts is cytoplasmic incompatibility (CI): infected males are reproductively incompatible with uninfected females. In this paper, we extend the theory on population dynamics and evolution of CI, with emphasis on haplodiploid species. First, we focus on the problem of the threshold to invasion of the Wolbachia infection in a population. Simulations of the dynamics of infection in small populations show that it does not suffice to assume invasion by drift alone (or demographic "accident"). We propose several promising alternatives that may facilitate invasion of Wolbachia in uninfected populations: sex-ratio effects, meta population structure, and other fitness-compensating effects. Including sex-ratio effects of Wolbachia allows invasion whenever infected females produce more infected daughters than uninfected females produce uninfected daughters. Several studies on haplodiploid species suggest the presence of such sex-ratio effects. The simple metapopulation model we analyzed predicts that, given that infecteds are better "invaders," uninfecteds must be better "colonizers" to maintain coexistence of infected and uninfected patches. This condition seems more feasible for species that suffer local extinction due to predation (or parasitization) than for species that suffer local extinction due to overexploiting their resource(s). Finally, we analyze the evolution of CI in haplodiploids once a population has been infected. Evolution does not depend on the type of CI (female mortality or male production), but hinges solely on decreasing the fitness cost and/or increasing the transmission efficiency. Our models offer new perspectives for increasing our understanding of the population and evolutionary dynamics of CI.     

The concept of the taxon cycle in biogeography

Taxon cycles are sequential phases of expansion and contraction of the ranges of species, associated generally with shifts in ecological distribution. The important contribution of the taxon cycle to biogeographical analysis is its emphasis on evolutionary and ecological interactions among colonizing and resident species, which influence their extinction dynamics and establish patterns of geographical distribution. Taxon cycles were inferred originally from the distribution of species across island archipelagos, where a correlation was noted between gaps in island occupancy and the degree of phenotypic differentiation. This pattern implied that phases of colonization were followed by range contraction, while endemic Antillean species that were undifferentiated between islands suggested secondary expansion and the beginning of a new cycle. This interpretation was met with scepticism, but reconstruction of phylogenetic relationships from gene sequences has now permitted us to characterize taxon cycles in Lesser Antillean birds. The relative timing of phases of the cycle can be deduced from genetic divergence between island populations. We have found that taxon cycles have periods in the order of 10⁶ years and that cycles in different lineages occur independently of each other and independently of Pleistocene climate cycles. Individual island populations may persist for several millions of years on the larger islands of the Lesser Antilles; occasional expansion phases lead to the replacement of island populations that have disappeared, thus reducing the archipelago‐wide rate of extinction to nil. What drives taxon cycles is unknown, but we speculate that they may be caused by co‐evolution with enemy populations, and a probable mechanism would involve infrequent mutations influencing parasite virulence and avian host disease resistance. Taxon cycles undoubtedly occur on continents, but the geographical configuration of island archipelagos reveals more clearly their presence and invites their study.