Jiro Kikkawa

Genetic consequences of sequential founder events by an island-colonizing bird

The importance of founder events in promoting evolutionary changes on islands has been a subject of long-running controversy. Resolution of this debate has been hindered by a lack of empirical evidence from naturally founded island populations. Here we undertake a genetic analysis of a series of historically documented, natural colonization events by the silvereye species-complex (Zosterops lateralis), a group used to illustrate the process of island colonization in the original founder effect model. Our results indicate that single founder events do not affect levels of heterozygosity or allelic diversity, nor do they result in immediate genetic differentiation between populations. Instead, four to five successive founder events are required before indices of diversity and divergence approach that seen in evolutionarily old forms. A Bayesian analysis based on computer simulation allows inferences to be made on the number of effective founders and indicates that founder effects are weak because island populations are established from relatively large flocks. Indeed, statistical support for a founder event model was not significantly higher than for a gradual-drift model for all recently colonized islands. Taken together, these results suggest that single colonization events in this species complex are rarely accompanied by severe founder effects, and multiple founder events and/or long-term genetic drift have been of greater consequence for neutral genetic diversity.

MICROEVOLUTION IN ISLAND FORMS: THE ROLES OF DRIFT AND DIRECTIONAL SELECTION IN MORPHOLOGICAL DIVERGENCE OF A PASSERINE BIRD

Abstract.— Theory predicts that in small isolated populations random genetic drift can lead to phenotypic divergence; however this prediction has rarely been tested quantitatively in natural populations. Here we utilize natural repeated island colonization events by members of the avian species complex, Zosterops lateralis, to assess whether or not genetic drift alone is an adequate explanation for the observed patterns of microevolutionary divergence in morphology. Morphological and molecular genetic characteristics of island and mainland populations are compared to test three predictions of drift theory: (1) that the pattern of morphological change is idiosyncratic to each island; (2) that there is concordance between morphological and neutral genetic shifts across island populations; and (3) for populations whose time of colonization is known, that the rate of morphological change is sufficiently slow to be accounted for solely by genetic drift. Our results are not consistent with these predictions. First, the direction of size shifts was consistently towards larger size, suggesting the action of a nonrandom process. Second, patterns of morphological divergence among recently colonized populations showed little concordance with divergence in neutral genetic characters. Third, rate tests of morphological change showed that effective population sizes were not small enough for random processes alone to account for the magnitude of microevolutionary change. Altogether, these three lines of evidence suggest that drift alone is not an adequate explanation of morphological differentiation in recently colonized island Zosterops and therefore we suggest that the observed microevolutionary changes are largely a result of directional natural selection.

Morphological shifts in island-dwelling birds: the roles of generalist foraging and niche expansion.

Abstract Passerine birds living on islands are usually larger than their mainland counterparts, in terms of both body size and bill size. One explanation for this island rule is that shifts in morphology are an adaptation to facilitate ecological niche expansion. In insular passerines, for instance, increased bill size may facilitate generalist foraging because it allows access to a broader range of feeding niches. Here we use morphologically and ecologically divergent races of white‐eyes (Zosteropidae) to test three predictions of this explanation: (1) island populations show a wider feeding niche than mainland populations; (2) island‐dwelling populations are made up of individual generalists; and (3) within insular populations there is a positive association between size and degree of foraging generalism. Our results provide only partial support for the traditional explanation. In agreement with the core prediction, island populations of white‐eye do consistently display a wider feeding niche than comparative mainland populations. However, observations of individually marked birds reveal that island‐dwelling individuals are actually more specialized than expected by chance. Additionally, neither large body size nor large bill size are associated with generalist foraging behavior per se. These latter results remained consistent whether we base our tests on natural foraging behavior or on observations at an experimental tree, and whether we use data from single or multiple cohorts. Taken together, our results suggest that generalist foraging and niche expansion are not the full explanation for morphological shifts in island‐dwelling white‐eyes. Hence, we review briefly five alternative explanations for morphological divergence in insular populations: environmental determination of morphology, reduced predation pressure, physiological optimization, limited dispersal, and intraspecific dominance.

Inter-related age-dependent patterns of ecology and behaviour in a population of silvereyes (Aves: Zosteropidae)

(1) The winter pattern of ecological differences between adults and juveniles was examined in the population of silvereyes (Zosterops lateralis chlorocephala) on Heron Island, Australian Great Barrier Reef. (2) Adults were more likely than juveniles to survive over winter. (3) Adults and juveniles tended to segregate into different habitats. The adults were more likely than juveniles to use the preferred breeding habitat (Pisonia forest) in winter. (4) Adults ranged over smaller areas than did juveniles, and were less likely to join foraging flocks. (5) Adults were socially dominant to juveniles, and were more discriminating in their defence of fig trees and figs, which were common in the clearings and edges of the Pisonia forest.

4000 YEARS OF PHENOTYPIC CHANGE IN AN ISLAND BIRD : HETEROGENEITY OF SELECTION OVER THREE MICROEVOLUTIONARY TIMESCALES

Abstract Pronounced phenotypic shifts in island populations are typically attributed to natural selection, but reconstructing heterogeneity in long-term selective regimes remains a challenge. We examined a scenario of divergence proposed for species colonizing a new environment, involving directional selection with a rapid shift to a new optimum and subsequent stabilization. We provide some of the first empirical evidence for this model of evolution using morphological data from three timescales in an island bird, Zosterops lateralis chlorocephalus. In less than four millennia since separation from its mainland counterpart, a substantial increase in body size has occurred and was probably achieved in fewer than 500 generations after colonization. Over four recent decades, morphological traits have fluctuated in size but showed no significant directional trends, suggesting maintenance of a relatively stable phenotype. Finally, estimates of contemporary selection gradients indicated generally weak directional selection. These results provide a rare description of heterogeneity in long-term natural regimes, and caution that observations of current selection may be of limited value in inferring mechanisms of past adaptation due to a lack of constancy even over short time-frames.

Seasonality of Nesting by Zebra Finches at Armidale, NSW

Nests of Zebra Finches were surveyed from 1961 to 1965 at Saumarez Pond near Armidale. About 200 birds used 220 trees for nesting within an area of five square kilometres. Two types of nest (breeding and roosting) varied in number according to the season from eighteen (January 1964) to ninety-eight (October 1961). The number of breeding nests built in a season varied between eighty-one (1963 - 64) and 154 (1961-62). Nesting success (young fledged from eggs laid) varied little, averaging about forty-four percent (n=382). No laying occurred in June or July (except once), when temperatures were often below freezing. At least two birds born in spring bred in the autumn of the same breeding season. In contrast to the breeding regime in arid areas, nesting activities of Zebra Finches at Armidale were not directly affected by drought; during the autumn of 1965 the peak of breeding activity was reached in March (19 breeding nests) when no rain had fallen for six weeks and the effect of prolonged drought on the vegetation was apparent. The results may be interpreted in several ways: that low temperatures (but not drought) inhibit breeding or that changes of photoperiods or increased availability of food itself stimulate many individuals to breed in this population. From a review of relevant literature, it is concluded that, if there is a proximate factor for the breeding of Zebra Finches, it must be different in different parts of the range of the species. It is suggested that individual differences exist in response to various proximate factors for reproduction and that the populations in the intermediate climatic zone are more polymorphic with respect to types of response than the populations that occur where the causal relations of environmental factors are predictable, if not periodic, in occurrence.

Individual heterogeneity and senescence in silvereyes on Heron Island.

Individual heterogeneity and correlations between life history traits play a fundamental role in life history evolution and population dynamics. Unobserved individual heterogeneity in survival can be a nuisance for estimation of age effects at the individual level by causing bias due to mortality selection. We jointly analyze survival and breeding output from successful breeding attempts in an island population of Silvereyes (Zosterops lateralis chlorocephalus) by fitting models that incorporate age effects and individual heterogeneity via random effects. The number of offspring produced increased with age of parents in their first years of life but then eventually declined with age. A similar pattern was found for the probability of successful breeding. Annual survival declined with age even when individual heterogeneity was not accounted for. The rate of senescence in survival, however, depends on the variance of individual heterogeneity and vice versa; hence, both cannot be simultaneously estimated with precision. Model selection supported individual heterogeneity in breeding performance, but we found no correlation between individual heterogeneity in survival and breeding performance. We argue that individual random effects, unless unambiguously identified, should be treated as statistical nuisance or taken as a starting point in a search for mechanisms rather than given direct biological interpretation.

Determinants of social dominance and inheritance of agonistic behavior in an island population of silvereyes, Zosterops lateralis

SummaryWe examined components of agonistic behavior and dominance in parents and offspring of silvereyes on Heron Island, Great Barrier Reef, to discover their determinants. The data were collected over four years on 1,235 juveniles and their parents. Of four behavioral characters used to compare juveniles and their parents, bill clattering was seen significantly more among young from female parents that exhibited bill clattering. The total absence of submissive behavior and the total absence of aggressive behavior were shown more often than not by young from dominant male parents and subordinate female parents, respectively. Young fledging early in the breeding season tended to be more dominant than those fledging late in the season. The number of brood-mates or the local nest density did not affect the dominance of young. Dominance status appears to be little affected by the performance of parents.

Ecological research for the restoration and management of rivers and reservoirs in Japan

In Japan the River Law was amended in 1997 to expand the traditional roles of flood control and water supply in river management to include environmental conservation. Two major multidisciplinary research groups were also founded to address the environmental issues arising from the management of rivers and watershed areas in Japan. One called the River Ecology Research Group was formed in 1995 to search for an ideal dynamic state of rivers to be managed. Six case studies commenced involving measurements of natural and human impacts on representative rivers and their biota selected from different regions of the country. Restoration of natural rivers has also been attempted. The other, called the Watershed Ecology Research Group, was formed in 1998 to study the natural environment surrounding dams. It consists of four groups concerned with forest ecology in the headwaters, raptor management research, reservoir ecology, and flow regime research. The topics include modeling of regeneration dynamics of riparian forests, GIS mapping of endangered raptor habitats, developing measures to reduce eutrophication of reservoir water, and the use of biodiversity of benthic faunas as an indicator of environmental change in the downstream. In both groups, ecologists collaborate with engineers who are responsible for the river infrastructure, to predict future impacts and keep ecological perspectives for the maintenance of the healthy environment of rivers and reservoirs.

Post-Fledging Parental Investment in the Capricorn Silvereye

We measured relative parental investments of 23 pairs of Capricorn Silvereyes Zosterops lateralis chlorocephala in the post- fledging period to examine the possibility of brood division and to relate parental investment to the survival of parents and offspring. Both parents invested similarly in nestlings and young fledglings. When feeding old fledglings or small broods, one parent tended to invest considerably more, but brood division did not develop. The degree of investment was not related to parental sex, age or dominance status. The strong pair bond, clumping by the fledglings and the small brood size may preclude development of brood division in Silvereyes. Survival of parents and of their young was not related to the relative investment of the members of the pair.