Mikko Kuussaari

Inbreeding and extinction in a butterfly metapopulation

It has been proposed that inbreeding contributes to the decline and eventual extinction of small and isolated populations,. There is ample evidence of fitness reduction due to inbreeding (inbreeding depression) in captivity and from a few experimental, and observational field studies,, but no field studies on natural populations have been conducted to test the proposed effect on extinction. It has been argued that in natural populations the impact of inbreeding depression on population survival will be insignificant in comparison to that of demographic and environmental stochasticity,. We have now studied the effect of inbreeding on local extinction in a large metapopulation of the Glanville fritillary butterfly (Melitaea cinxia). We found that extinction risk increased significantly with decreasing heterozygosity, an indication of inbreeding, even after accounting for the effects of the relevant ecological factors. Larval survival, adult longevity and egg-hatching rate were found to be adversely affected by inbreeding and appear to be the fitness components underlying the relationship between inbreeding and extinction. To our knowledge, this is the first demonstration of an effect of inbreeding on the extinction of natural populations. Our results are particularly relevant to the increasing number of species with small local populations due to habitat loss and fragmentation.

Extinction debt: a challenge for biodiversity conservation

Local extinction of species can occur with a substantial delay following habitat loss or degradation. Accumulating evidence suggests that such extinction debts pose a significant but often unrecognized challenge for biodiversity conservation across a wide range of taxa and ecosystems. Species with long generation times and populations near their extinction threshold are most likely to have an extinction debt. However, as long as a species that is predicted to become extinct still persists, there is time for conservation measures such as habitat restoration and landscape management. Standardized long-term monitoring, more high-quality empirical studies on different taxa and ecosystems and further development of analytical methods will help to better quantify extinction debt and protect biodiversity.

Habitat fragmentation causes immediate and time‐delayed biodiversity loss at different trophic levels

Intensification or abandonment of agricultural land use has led to a severe decline of semi-natural habitats across Europe. This can cause immediate loss of species but also time-delayed extinctions, known as the extinction debt. In a pan-European study of 147 fragmented grassland remnants, we found differences in the extinction debt of species from different trophic levels. Present-day species richness of long-lived vascular plant specialists was better explained by past than current landscape patterns, indicating an extinction debt. In contrast, short-lived butterfly specialists showed no evidence for an extinction debt at a time scale of c. 40 years. Our results indicate that management strategies maintaining the status quo of fragmented habitats are insufficient, as time-delayed extinctions and associated co-extinctions will lead to further biodiversity loss in the future.

Metapopulation persistence of an endangered butterfly in a fragmented landscape

We describe an extensive metapopulation study on the Glanville fritillary Melitaea cinxia, in a network of 1502 discrete habitat patches, comprising the entire distribution of this butterfly species in Finland. A thorough survey of the easily detected larval groups revealed a local population in 536 patches (dry meadows). We demonstrate that this system satisfies the four necessary conditions for a species to persist in a balance between stochastic local extinctions and recolonizations. Patterns of patch occupancy support several qualitative and quantitative model predictions. With decreasing regional density and average area of habitat patches, the butterfly occurs in a diminishing fraction of suitable habitat. To our knowledge, this is the first conclusive demonstration, based on a comparison of many conspecific metapopulations, of declining habitat occupancy and hence of increasing threat to survival caused by increasing habitat fragmentation

An experimental study of migration in the Glanville fritillary butterfly Melitaea cinxia

1. We studied factors affecting emigration and immigration behaviour in the butterfly Melitaea cinxia by releasing 882 newly emerged marked butterflies into 16 habitat patches in a network of 64 empty patches on an isolated island (area 1.6 km 2 ). 2. Of the 363 butterflies that were recaptured at least once, 40% were recorded in a new patch during their lifetime. Females emigrated earlier and moved further away than males. One-third of males appeared to remain permanently in, whereas females gradually drifted away from, the release patch. 3. High density of butterflies, great abundance of flowers, and large patch area decreased emigration, whereas open landscape around the patch increased emigration. Females that emigrated were on average larger than females that stayed in the patch of release. 4. In total, 152 immigrants were recorded in 32 patches. Numbers of immigrants increased with patch area and abundance of flowering plants. 5. Results on emigration suggest that conserving an isolated butterfly population is more successful in an area with physical barriers to migration than in an open landscape. The possible tendency of butterflies to leave a patch with low density should be taken into consideration in introductions of butterflies to empty habitat patches. 6. Results on immigration indicate the significance of nectar sources and patch size in successful colonization of empty habitat patches.

Differences in the climatic debts of birds and butterflies at a continental scale

Climate changes have profound effects on the distribution of numerous plant and animal species(1-3). However, whether and how different taxonomic groups are able to track climate changes at large spatial scales is still unclear. Here, we measure and compare the climatic debt accumulated by bird and butterfly communities at a European scale over two decades (1990-2008). We quantified the yearly change in community composition in response to climate change for 9,490 bird and 2,130 butterfly communities distributed across Europe(4). We show that changes in community composition are rapid but different between birds and butterflies and equivalent to a 37 and 114 km northward shift in bird and butterfly communities, respectively. We further found that, during the same period, the northward shift in temperature in Europe was even faster, so that the climatic debts of birds and butterflies correspond to a 212 and 135 km lag behind climate. Our results indicate both that birds and butterflies do not keep up with temperature increase and the accumulation of different climatic debts for these groups at national and continental scales.

Life-history traits predict species responses to habitat area and isolation: a cross-continental synthesis

There is a lack of quantitative syntheses of fragmentation effects across species and biogeographic regions, especially with respect to species life-history traits. We used data from 24 independent studies of butterflies and moths from a wide range of habitats and landscapes in Europe and North America to test whether traits associated with dispersal capacity, niche breadth and reproductive rate modify the effect of habitat fragmentation on species richness. Overall, species richness increased with habitat patch area and connectivity. Life-history traits improved the explanatory power of the statistical models considerably and modified the butterfly species-area relationship. Species with low mobility, a narrow feeding niche and low reproduction were most strongly affected by habitat loss. This demonstrates the importance of considering life-history traits in fragmentation studies and implies that both species richness and composition change in a predictable manner with habitat loss and fragmentation.

Allee effect and population dynamics in the Glanville fritillary butterfly

The Allee effect a decrease in population growth rate at low density - may be produced by several mechanisms including difficulty in finding mates at low densities. We present evidence for the Allee effect in natural populations of an endangered butterfly. the Glanville fritillary (Melitaea cinxia) and for two mechanisms that cause it. Specifically. we show that emigration rate increases and the fraction of mated females decreases with decreasing local density. We then demonstrate a connection between mating success and population growth rate and more generally, slightly reduced reproductive success in the smallest butterfly populations in an extensive data set gathered over four years. The Allee effect increases the significance of the rescue effect in metapopulations, and thereby the potential for alternative stable states in metapopulation as well as in local dynamics.

Environmental factors driving the effectiveness of European agri‐environmental measures in mitigating pollinator loss – a meta‐analysis

In Europe, agri-environmental schemes (AES) have been introduced in response to concerns about farmland biodiversity declines. Yet, as AES have delivered variable results, a better understanding of what determines their success or failure is urgently needed. Focusing on pollinating insects, we quantitatively reviewed how environmental factors affect the effectiveness of AES. Our results suggest that the ecological contrast in floral resources created by schemes drives the response of pollinators to AES but that this response is moderated by landscape context and farmland type, with more positive responses in croplands (vs. grasslands) located in simple (vs. cleared or complex) landscapes. These findings inform us how to promote pollinators and associated pollination services in species-poor landscapes. They do not, however, present viable strategies to mitigate loss of threatened or endangered species. This indicates that the objectives and design of AES should distinguish more clearly between biodiversity conservation and delivery of ecosystem services.

New insights into butterfly–environment relationships using partitioning methods

Variation partitioning and hierarchical partitioning are novel statistical approaches that provide deeper understanding of the importance of different explanatory variables for biodiversity patterns than traditional regression methods. Using these methods, the variation in occupancy and abundance of the clouded apollo butterfly (Parnassius mnemosyne L.) was decomposed into independent and joint effects of larval and adult food resources, microclimate and habitat quantity. The independent effect of habitat quantity variables (habitat area and connectivity) captured the largest fraction of the variation in the clouded apollo patterns, but habitat connectivity had a major contribution only for occupancy data. The independent effects of resources and microclimate were higher on butterfly abundance than on occupancy. However, a considerable amount of variation in the butterfly patterns was accounted for by the joint effects of predictors and may thus be causally related to two or all three groups of variables. Abundance of the butterfly in the surroundings of the focal grid cell had a significant effect in all analyses, independently of the effects of other predictors. Our results encourage wider applications of partitioning methods in biodiversity studies.