The western world has witnessed a rising epidemic of chronic inflammatory disorders, such as allergies and asthma. This epidemic is expected to spread also to the rest of the world, where allergies have to date been practically absent, along with adoption of western lifestyle. In parallel, biological diversity is globally declining. This inspired Ilkka Hanski, together with medical doctors, to formulate the biodiversity hypothesis of allergic disease. This hypothesis proposes that reduced contact with natural environments, including natural microbial diversity, is associated with unhealthy human microbiota, less able to educate the immune system. Contact with beneficial bacteria, particularly early in life, seems to be instrumental to the normal development of immune responses. Changes in lifestyle and diet, destruction of natural environments, and urbanisation threaten our natural exposure to these beneficial bacteria and thus also reduce their impact on our physiology. To ensure a healthy life, we need to preserve biodiversity in the environment and make sure it finds a favourable home in us. In this review, we will focus on the role of commensal microbiota in human health and wellbeing, as well as the interaction between our microbiota and environmental microbiota, highlighting the contribution of Ilkka Hanski.

Ilkka Hanski is most widely known for his seminal contributions to metapopulation ecology, both theoretical and empirical. But he also made many important and wide-ranging contributions to other arenas of ecological inquiry, including in particular predator—prey, host—parasitoid, and host—pathogen interactions. This paper provides an overview of his work in trophic ecology, ranging from individual behavior of foraging predators and fleeing prey, up to effects of spatial patchiness on the persistence of specialist natural enemies, and even the determinants of food chain length. In recent years, his work on food web interactions took on an increasingly genetic and evolutionary slant. In this paper, I also return to two theoretical models that I discussed over the years with Ilkka, where we contemplated carrying out collaborative work, but never managed to do so. The first of these models involves how habitat patchiness might moderate apparent competition between prey species. The second of these has to do with splicing evolutionary dynamics into metapopulation models, so as to craft hypotheses about how food chain length might be influenced by the coevolutionary struggle between predators and their prey. I developed simple models of the sort we should have explored together, in the style we would have used at the time of our conversations on these themes, in homage to my memory of our interactions.

Outcrops of serpentine bedrock bear as close a resemblance to islands as any terrestrial habitat, with floras often highly distinct from those of the surrounding matrix. In 30 years of research in California, I found that the sizes and spatial distribution of serpentine outcrops may influence extinction and colonization, alpha and beta diversity, reproductive success and genetics, and other facets of plant (and animal) ecology in ways consistent with spatial ecological theory. Such ‘spatial effects’ were detected primarily at scales of 10^2–103 meters separating habitat patches, whereas habitat quality and climate effects predominated at smaller and larger extents, respectively. Most regions containing sets of small outcrops (< 10 ha) also contain large ones (> 10² ha), making the detection of spatial effects dependent on careful site selection. This review highlights the interplay between testing theory and understanding the most important processes shaping biodiversity in a particular study system.

Community composition is determined by attributes of the environment, individual species, and interactions among species. We studied the distributions of a seed weevil and its parasitoid and hyperparasitoid wasps in a fragmented landscape. The occurrence of the weevil was independent of the measured attributes of the landscape (patch connectivity and resource availability). However, between habitat-patch networks, weevil density decreased with increasing parasitism, suggesting top-down control, especially in the north. Parasitism was mostly due to a specialist and a generalist that appeared to compete strongly. This competitive interaction was strongest at high patch-connectivity, perhaps due to a trade-off of local competitive ability and dispersal. Finally, the abundance of the generalist hyperparasitoid was unrelated to landscape or host-species abundance. The snapshot presented by these data can best be explained by top-down effects, interactions among species, host ranges, and patch configuration in the landscape, but not by local host-plant abundance.

Marine ecologists have been slow at adopting the metapopulation paradigm in their research. As explained in the landmark book Marine Metapopulations, marine ecology was focused on local processes and used mainly experimental approaches that neglected the potential role of demographic connections among local populations, but this is no longer the case. Metapopulation theory is now integrated into marine ecology research and is being used as a helpful framework for describing spatial population processes, for asking new research questions, and for planning the management and conservation of marine species. Nevertheless, the use of the metapopulation paradigm in marine sciences is challenging because of the complex life histories of marine species and the peculiarities of the marine environment. Here, I highlight the many challenges faced by ecologists who study marine metapopulations and mention some of the approaches that can be used to overcome them. In particular, I focus on the problem of estimating connectivity, an important attribute of metapopulations that represents a fundamental knowledge gap in marine ecology. Following the repeated calls for integrative approaches that combine all sources of information I propose the use of the hierarchical Bayesian framework and provide an example that considers the joint analysis of the three leading approaches in connectivity research, genetics, microchemistry and biophysical modelling.

Stochastic models of closed populations predict eventual extinction with certainty. Consequently, their behavior is often characterized by the quasi-stationary state, i.e. the long-term distribution of population sizes conditional on non-extinction. In contrast, models which allow for immigration exhibit a regular stationary state. At the limit of a low immigration rate, a population is expected to alternate between three states: the quasi-stationary state of a closed population, the extinction state, and the transient phase during which a newly arrived immigrant either establishes a new population or fails to do so. We develop this argument into a simple and intuitive framework that can be used to assess the effect of immigration in a general class of population models. We exemplify the framework for models in which immigrants arrive either singly or in groups, for models with an Allee effect, for models with environmental stochasticity, and for models leading to metapopulation dynamics.

The dispersal ability of a species will be critical for how population dynamics are realized in spatially structured systems. To date, the effect of group dispersal on metapopulation dynamics is poorly understood. Here, we investigate how group dispersal and Allee effects shape metapopulation dynamics identifying conditions in which group dispersal can be an advantage over independent dispersal. We approach this question by building and analysing a Markovian random walk for metapopulation dynamics including group dispersal and Allee effect. This Markovian random walk is analogous to the discrete-time Stochastic Patch Occupancy Model (SPOM). We find that intermediate group sizes may lead to larger and more sustainable metapopulations in the presence of an Allee effect. Hence, understanding how group size variation and realized (meta)population dynamics are linked offers an exciting future venue for research that is expected to yield key insights into the ecology and evolution of populations occupying spatially structured environments.

Extinction—recolonization dynamics is known to promote “fugitive coexistence” in patchy environments: a species that is an inferior competitor but a better colonizer may persist by exploiting the period between its colonization of a patch and the arrival of a superior competitor. Here I use a simple model to demonstrate the plausibility of a different type of fugitive coexistence. I show that, under some circumstances, a “jack of all trades” plastic species can persist despite competition from an adaptable species that has the genetic potential to adapt locally and outcompete the plastic species in every patch. This persistence can be mediated by two forces that impede local adaptation: gene flow and extinction—recolonization dynamics. In the latter case, the persistence of the plastic species is of a fugitive nature; however, it is not mediated by an earlier colonization. Rather, it relies on the fact that, following recolonization, the adaptable species is often locally maladapted. This opens a time window for the plastic species to multiple and produce propagules before the other species becomes locally adapted, and thus competitively superior.

Ilkka Hanski may be best known for his work on insect and metapopulation dynamics, but he also contributed significantly to small mammal research. In the early 1980s he became interested in shrew dynamics, energetics, and of course, shrew metapopulations. He aimed at understanding the population biological consequences of body size in different shrew species. Feeding habits and environmental stochasticity affect shrew species in profoundly different ways: due to their short survival time small species have high extinction rates but their dispersal and colonization capacity is high which enables them to survive as metapopulations. After Hansson and Henttonen reported the Fennoscandian gradients in vole dynamics in the mid-1980s, Hanski became interested in vole and lemming cycles. The first models on this were published with Henttonen and Hansson in 1991 where the roles of specialist and generalist predators were assessed. Later, the models were further developed with Korpimäki and Turchin, with model parametrization from Microtus biology and including both specialist mammalian predators as well as avian predators. A special case was the model with Henttonen on competing vole species with a shared predator (apparent competition), which was related to the long-term fading out of vole cycles in Finnish Lapland in the mid-1980s (which though returned in the early 2010s). Later Hanski became interested in the work of Sittler and Gilg in Greenland. Together they modelled the very simple vertebrate community and showed how stoats played a pivotal role in generating a population cycle in the collared lemming. In addition to these specific works, Hanski was leading collaborator in several reviews on small rodent cycles and predation. He intended to return to shrew biology, but that never realized. Hanski was a fearless field biologist, but he always aimed at understanding natural phenomena at more general, theoretical level.

The transfer of materials such as nutrients from one ecosystem to another can establish gradients that affect the distribution and performance of species. We examined how a natural resource gradient influences the density and flowering of butterwort (Pinguicula vulgaris), a carnivorous plant. Mývatn, a lake in Iceland, produces large midge populations, generating a gradient in midge abundance on shore. The midge gradient was associated with butterwort flowering probability, which decreased by roughly 50% moving away from the shore. In a midge addition experiment, plants receiving midges had a four-fold increase in their subsequent likelihood of flowering, implicating carnivory in the flowering gradient. Nonetheless, the density of butterworts increased with distance from shore, in negative association with soil nutrient content and the abundance of grass species. This suggests that the positive effect of the midge gradient on butterwort flowering is overwhelmed by the negative effect of competition with grasses.

Cavity-nesting birds can save time and energy by reusing old cavities. We studied cavity reuse and its connections to nesting success and timing in the three-toed woodpecker Picoides tridactylus in a 170-km² area in southern Finland during 1987–2015. The data include 520 nest trees, 645 nest cavities and 833 nestings in 86 territory sites, including 211 cases of cavity reuse. Twenty-five percent of nestings was in previously used cavities. Twenty-eight percent of cavities and 25% of nest trees were used more than once. Reuse improved nesting success and facilitated early nesting in the year following first nesting. Reuse of nest trees with several cavities was observed in 15% of nest trees, and 62% of reused cavities were in those multi-cavity trees. Cavity reuse and multi-cavity trees were most abundant in long-term territories with stable habitats. In boreal forests, cavity and tree reuse can be an important adaptation allowing efficient nesting during a short breeding season.

Within the framework of adaptive dynamics we consider the evolution by natural selection of reproductive strategies in which individuals may adjust their reproductive behaviour in response to changing environmental conditions. As a specific example we considered a discrete-time model in which possible fluctuations in the environmental conditions are caused by predator—prey interaction. Our main findings include: (1) Coexistence between two fixed strategies (i.e., strategies that do not adjust to changing environmental conditions) is impossible; there exists a best fixed strategy, which invades and ousts all other fixed strategies. (2) A necessary condition for conditional (adjustable) strategies to evolve is that there are fluctuations in the environmental conditions. Predator—prey interactions may cause such fluctuations and under natural assumptions there exists an optimal conditional strategy which is uninvadable and invades and ousts all other strategies.

Clumped distributions of herbivorous insect eggs often result from independent assessments of individual plants by different ovipositing females. Here we ask whether, in addition, plants might be rendered more or less attractive to ovipositing Melitaea cinxia butterflies by presence of conspecific eggs and/or by prior larval attack. Both eggs and larval damage rendered Veronica spicata plants significantly more acceptable; the effect of eggs was particularly strong. Larval damage caused a marginally significant increase in acceptability of Plantago lanceolata, but there was no trend for an effect of eggs on this host. Variable oviposition preferences of Melitaeine butterflies are known to drive their metapopulation dynamics by affecting rates of emigration and patch colonization. Therefore variable host acceptability, as documented here, should do likewise, reducing emigration rates at high population densities where V. spicata is present in the landscape and complementing Allee effects that are already known in this system.

Gregarious, social caterpillars have stimulated research because group size may affect survival, growth rate, thermoregulation, and interactions with other species, yet group size is often variable both within and among populations. We used a combination of observations and experiments to study the importance of group size for egg and larval survival in the Glanville fritillary butterfly, Melitaea cinxia, which lives in groups from egg hatching until the last larval instar. Both experimental manipulation of egg clutches placed in the field and observations of naturally occurring groups showed that survival increased with increasing group size. This pattern was present independently during all four developmental stages studied: eggs, prediapause larvae, diapausing larvae and post-diapause larvae. However, it was significant only during two stages: pre-diapause (in one year only) and diapausing larvae (in all years). Large group size increased survival of entire larval groups as well as that of individual larvae within surviving groups. These results may explain why cluster size is large and why adults oviposit infrequently. Large cluster size, coupled with correlated survival of group members, in turn helps to explain the unstable local dynamics and short average persistence time of local M. cinxia populations.

The evolution of adaptive phenotypic plasticity, whereby organisms respond to cues to produce phenotypes appropriate to their future environment, is not well understood. Male orange-tip Anthocharis cardamines butterflies utilizing the host-plant lady's smock Cardamine pratensis (on which food is limiting) are smaller and emerge earlier than those utilizing garlic mustard Alliaria petiolata. Since small size reduces dispersal rate, this coupling produces an adaptive ‘emerge early and wait' mate location strategy. Here we show that early emergence is abolished when males are subjected to a severely restricted diet, which we take to be the ancestral response. We propose that the derived (early emergence) response on C. pratensis has evolved through sex-specific changes in the timing and efficiency of developmental and metabolic processes in late larval life. Our results offer broad support to the idea that adaptive phenotypic plasticity can evolve through the selection of cue-sensitive modifier genes controlling initially passive plastic responses.

Phylogeny can provide information about the processes that have shaped extant diversity. Here, we complement existing comparative phylogenetic methods by developing a model that couples diversity-dependent diversification rate and range dynamics. Unlike many models, we used Approximate Bayesian Computation to fit the model to the data. We validated the inference by estimating known parameter values from simulated data, and found that within-region speciation and extinction rates cannot be simultaneously estimated most likely due to correlations among parameter values. Since the model can estimate a diversification rate, we applied the model to a monophyletic lineage of 74 species of dung beetles (Canthonini: Nanos and Apotolamprus) endemic to Madagascar. The estimated diversification rate is clearly higher in northern than in eastern or western Madagascar. The current species richness is highest in North where complex topography and a mixture of biomes likely favour ecological diversification. The approach we have developed here is a step towards examining weaknesses and strengths of phylogenetic comparative methods in an explicit spatial context. Further development and testing of the model is needed before its routine application to empirical data.

The Glanville fritillary butterfly inhabits fragmented landscapes in the Åland Islands, Finland, where the long-term persistence of this classic metapopulation depends on frequent re-colonisation events. A number of studies have focused on understanding how dispersal propensity varies within this system, what the underlying mechanisms are, and how variation in dispersal influences metapopulation dynamics. In the mid-2000s the focus turned to a potential candidate gene, phosphoglucose isomerase (Pgi) that was soon found to have effects on phenotypic performance of individuals in regard to their dispersal potential but also other key life-history traits. Variation in Pgi has further been shown to affect population dynamics. Here we review 30 research papers in which associations of Pgi polymorphism with metabolic rate, mobility, fecundity, lifespan, and population growth rate in the Glanville fritillary are discussed. The studies highlight the role of environmental variation and landscape structure in maintaining genetic polymorphism in this study system.

The first lepidopteran genome (Bombyx mori) was published in 2004. Ten years later the genome of Melitaea cinxia came out as the third butterfly genome published, and the first eukaryotic genome sequenced in Finland. Owing to Ilkka Hanski, the M. cinxia system in the Åland Islands has become a famous model for metapopulation biology. More than 20 years of research on this system provides a strong ecological basis upon which a genetic framework could be built. Genetic knowledge is an essential addition for understanding eco-evolutionary dynamics and the genetic basis of variability in life history traits. Here we review the process of the M. cinxia genome project, its implications for lepidopteran genome evolution, and describe how the genome has been used for gene expression studies to identify genetic consequences of habitat fragmentation. Finally, we introduce some future possibilities and challenges for genomic research in M. cinxia and other Lepidoptera.