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I Hanski

Publications and source records attributed to I Hanski.

10 recordsLinked to original sources
PubMed2001

Patterns of 2-year population cycles in spatially extended host-parasitoid systems.

Coexisting but temporally separated cohorts of insects with a multiannual life cycle may have dissimilar average abundance, resulting in periodically fluctuating population density. In the case of the boreal moth genus Xestia with a 2-year life cycle and a distinct abundance difference between the two coexisting cohorts, empirical results and a simple model suggest that the oscillatory dynamics are maintained by interaction with a parasitoid wasp. Here we report theoretical results on a spatially extended version of the basic model and relate the modeling results to empirical observations. A spatially extended model may have domains oscillating in different phases as is the case between western and eastern Finnish Lapland. Spatial heterogeneity tends to fix the location of phase boundaries. In contrast, spatially homogeneous temporal fluctuations tend to synchronize populations in large regions.

Animals↗
PubMed2001

Spatially structured metapopulation models: global and local assessment of metapopulation capacity.

We model metapopulation dynamics in finite networks of discrete habitat patches with given areas and spatial locations. We define and analyze two simple and ecologically intuitive measures of the capacity of the habitat patch network to support a viable metapopulation. Metapopulation persistence capacity lambda(M) defines the threshold condition for long-term metapopulation persistence as lambda(M)>delta, where delta is defined by the extinction and colonization rate parameters of the focal species. Metapopulation invasion capacity lambda(I) sets the condition for successful invasion of an empty network from one small local population as lambda(I)>delta. The metapopulation capacities lambda(M) and lambda(I) are defined as the leading eigenvalue or a comparable quantity of an appropriate "landscape" matrix. Based on these definitions, we present a classification of a very general class of deterministic, continuous-time and discrete-time metapopulation models. Two specific models are analyzed in greater detail: a spatially realistic version of the continuous-time Levins model and the discrete-time incidence function model with propagule size-dependent colonization rate and a rescue effect. In both models we assume that the extinction rate increases with decreasing patch area and that the colonization rate increases with patch connectivity. In the spatially realistic Levins model, the two types of metapopulation capacities coincide, whereas the incidence function model possesses a strong Allee effect characterized by lambda(I)=0. For these two models, we show that the metapopulation capacities can be considered as simple sums of contributions from individual habitat patches, given by the elements of the leading eigenvector or comparable quantities. We may therefore assess the significance of particular habitat patches, including new patches that might be added to the network, for the metapopulation capacities of the network as a whole. We derive useful approximations for both the threshold conditions and the equilibrium states in the two models. The metapopulation capacities and the measures of the dynamic significance of particular patches can be calculated for real patch networks for applications in metapopulation ecology, landscape ecology, and conservation biology.

Animals↗
PubMed2001

Spatially realistic theory of metapopulation ecology.

Much of spatial ecology since the late 1960s has been dominated by two theories, the dynamic theory of island biogeography and the classical metapopulation theory. The latter theory largely replaced the former one in the 1980s, especially in conservation applications. It is only recently that ecologists have fully realized that a relatively simple general theory can be readily constructed that makes some of the simplifying assumptions of the two earlier theories unnecessary. The spatially realistic metapopulation theory thereby provides a more unified framework for spatial ecology than the island theory or the classical metapopulation theory. This article describes the application of the spatially realistic metapopulation theory to real metapopulations living in highly fragmented landscapes. I discuss the principal messages for population ecology and conservation biology, and I also place this theory into a broader context of other approaches to spatial ecology.

Animals↗
PubMed2000

The metapopulation capacity of a fragmented landscape.

Ecologists and conservation biologists have used many measures of landscape structure to predict the population dynamic consequences of habitat loss and fragmentation, but these measures are not well justified by population dynamic theory. Here we introduce a new measure for highly fragmented landscapes, termed the metapopulation capacity, which is rigorously derived from metapopulation theory and can easily be applied to real networks of habitat fragments with known areas and connectivities. Technically, metapopulation capacity is the leading eigenvalue of an appropriate 'landscape' matrix. A species is predicted to persist in a landscape if the metapopulation capacity of that landscape is greater than a threshold value determined by the properties of the species. Therefore, metapopulation capacity can conveniently be used to rank different landscapes in terms of their capacity to support viable metapopulations. We present an empirical example on multiple networks occupied by an endangered species of butterfly. Using this theory, we may also calculate how the metapopulation capacity is changed by removing habitat fragments from or adding new ones into specific spatial locations, or by changing their areas. The metapopulation capacity should find many applications in metapopulation ecology, landscape ecology and conservation biology.

Animals↗
PubMed1998

Sexual Reproduction and Stable Coexistence of Identical Competitors.

Current competition theory predicts that species must be ecologically distinct in order to stably coexist in the same community. This prediction is based on the implicit assumption that the consequences of sexual reproduction can be ignored. If anything, it is generally assumed that sex will only add problems, such as failure of insemination and inbreeding depression to species that have become rare, thus hastening competitive exclusion. Here we suggest that sexual reproduction may also bring rare species advantages in terms of the rate of per capita population growth. The key to our argument is that species-specific density dependence in population growth can occur when sexual reproduction is explicitly considered. We show that density-dependent sex ratio, sexual conflict, and sexually transmitted diseases can all promote species coexistence without separate niches.Copyright 1998 Academic Press

Journal Article↗
PubMed1997

Uniting Two General Patterns in the Distribution of Species

Two patterns in the distribution of species have become firmly but independently established in ecology: the species-area curve, which describes how rapidly the number of species increases with area, and the positive relation between species' geographical distribution and average local abundance. There is no generally agreed explanation of either pattern, but for both the two main hypotheses are essentially the same: divergence of species along the ecological specialist-generalist continuum and colonization- extinction dynamics. A model is described that merges the two mechanisms, predicts both patterns, and thereby shows how the two general, but formerly disconnected, patterns are interrelated.

Journal Article↗
PubMed1997

Habitat Deterioration, Habitat Destruction, and Metapopulation Persistence in a Heterogenous Landscape

Levins's unstructured metapopulation model predicts that the equilibrium fraction of empty habitat patches is a constant function of the fraction h of suitable patches in the landscape and that this constant equals the threshold value for metapopulation persistence. Levins's model thus suggests that the minimum amount of suitable habitat necessary for metapopulation persistence can be estimated from the fraction of empty patches at steady state. In this paper we construct several more realistic structured metapopulation models that include variation in patch quality and the rescue effect. These models predict both positive and negative correlations between the fractions of suitable patches and empty patches. The type of correlation depends in an intricate manner on the strength of the rescue effect and on the quality distribution of the patches to be destroyed. Empty patches can be considered as the resource limiting metapopulation growth. Our results demonstrate that the correlation between the fractions of suitable patches and empty patches is positive if and only if the average value of the resource decreases as the number of patches increases. Copyright 1997 Academic Press. Copyright 1997 Academic Press

Journal Article↗
PubMed1995

Developing microsatellite markers for insect population structure: complex variation in a checkerspot butterfly.

We isolated and characterized two microsatellite markers from the genome of the endangered checkerspot butterfly Melitaea cinxia L. In Finland, this species only survives on the Aland islands, where it exhibits a highly fragmented metapopulation structure on small meadows. Four alleles were observed at the locus CINX1 and nine at CINX4; the total gene diversities at the two loci were HT = 0.34 and 0.80, respectively. A pilot survey showed moderate gene frequency differentiation among meadows (local populations; FLM = 0.1) and among metapopulations c. 30 km apart (FMT = 0.2). Contrary to prior expectation, distinct feeding larval groups collected in the spring did not represent offspring of single females. There was a conspicuous excess of homozygotes within local populations (FIL = 0.35), which can hardly be attributed to population structure alone; this urges caution in straightforward interpretation of microsatellite phenotype data.

Animals↗
PubMed1993

Population oscillations of boreal rodents: regulation by mustelid predators leads to chaos.

The four-year cycle of microtine rodents in boreal and arctic regions was first described in 1924 (ref. 1). Competing hypotheses on the mechanisms underlying the small mammal cycle have been extensively tested, but so far the sustained rodent oscillations are unexplained. Here we use two mutually supportive approaches to investigate this question. First, building on studies of the interaction between rodents and their mustelid predators, we construct a predator-prey model with seasonality. Second, we use a new technique of nonlinear analysis to examine empirical time-series data, and compare them with the model dynamics. The model parameterized with field data predicts dynamics that closely resemble the observed dynamics of boreal rodent populations. Both the predicted and observed dynamics are chaotic, albeit with a statistically significant periodic component. Our results suggest that the multiannual oscillations of rodent populations in Fennoscandia are due to delayed density dependence imposed by mustelid predators, and are chaotic.

Animals↗