The significance of reserve networks, ecological corridors, stepping stones, and source-sink dynamics in maintaining biodiversity in boreal forests

Protected fragments of old-growth forest comprise reserve networks, which are essential for the long-term survival of specialist old-growth species (Hanski & Gilpin 1997). Small forest fragments may function as stepping stones, increasing connectance in the network, and long and narrow stretches of habitat suitable for movement are meant to facilitate dispersal. Complex forest landscapes include great variation in habitat quality, from sources to sinks (Pulliam 1988), which may have unexpected ecological consequences. All these elements are critical to landscape ecological planning and to planning of reserve networks. However, our empirical understanding of the functioning of networks of forest reserves, stepping stones, corridors, and source-sink dynamics is still very limited. In this task we conduct empirical research in real forest landscapes to fill gaps in our knowledge.

The aims of this research task are

  • To assess the significance of small fragments of old-growth forest as stepping stones, parts of more extensive reserve networks, and as locations where species may have populations temporarily
  • To assess the significance of source-sink dynamics in boreal forest-dwelling species
  • To empirically test the significance of ecological corridors and stepping stones in facilitating the movements of forest taxa.

These studies will mainly be carried out in two areas, in Evo in southern Finland and in Kainuu in northeastern Finland.

The research group is described in the following table (PI=Principal Investigator, FEI=Finnish Environment Institute, UH=University of Helsinki)

Name (organization)
Raimo Virkkala (FEI) PI

Paula Siitonen (FEI)
Ilkka Hanski (UH)
Timo Pakkala (UH)
Ilpo K Hanski (UH)
Reijo Penttilä (UH)
post graduate (UH)
undergraduates (UH)

reserve networks and source-sink dynamics
connectivity studies,
metapopulation modelling
landscape ecology
biology of the flying squirrel
wood-rotting fungi
small forest fragments
source-sink dynamics, corridors and stepping stones

1 Background and present knowledge

Individual protected areas comprise a reserve network at a regional scale. The aim of a reserve network is to maintain ecosystems and species. A reserve network should be both representative and complementary (Pressey et al. 1993), which means that all habitat types of natural or semi-natural ecosystems should be represented. A network should be comprehensive enough to accommodate several local populations of focal species; exactly how many are needed is a question for modelling (Section 4.4). In a landscape of managed boreal forests in Finland, viable metapopulations of species with specific habitat requirements, such as species associated with old-growth forest, can be preserved only by an adequate reserve network. Fundamental characteristics of old-growth forests include long continuity, old individual trees, and large volume of decaying wood (Esseen et al. 1992), which are essential for the persistence of a range of organisms, for example many polyporous fungi, saproxylic beetles and hole-nesting bird species.

Habitat quality typically differs between habitat patches in a forest landscape. According to the source-sink model, sources are demographically viable local populations, whereas sinks are demographically inviable local populations (Pulliam 1988). Sink populations will ultimately become extinct unless they receive immigrants from source populations. Preserving source populations is therefore of utmost importance.

The purpose of movement corridors and stepping stones, or small patches of suitable habitat, is to facilitate dispersal of species between larger areas of suitable habitat, e.g. old-growth reserves. The Finnish Forest and Park Service, which manages the state-owned land, has recently initiated landscape ecological forest planning (Hallman et al. 1996). The planning involves among other things ecological corridors and stepping stones and the maintenance of structural characteristics of mature forest to mimic natural forest dynamics. Landscape ecological planning should ensure the maintenance of biodiversity in a landscape consisting of managed and protected forests. However, at present scientific knowledge of the effects of different silvicultural practices on biota is not sufficient. There is practically no critical data to say anything about the value of corridors and stepping stones in facilitating movement.

2 Aims of the research

The primary aim of this research is to find out how species specialising in old-growth forests

  • survive in small patches of old-growth forest
  • are affected by source-sink dynamics
  • are able to disperse and move between old-growth forest areas

The context in which small patches of old-growth forest occur in southern and northern Finland is very different. In southern Finland the great majority of protected old-growth forest stands are small, less than 1 km2 in area. In northern Finland there exists a larger number of protected (or planned protected) old-growth forest areas including also large forest areas, 10 km2 or more in size (e.g. Virkkala 1996). In southern Finland the proportion of protected forest land, the majority of which is old-growth forest, is less than 1%, whereas in northeastern Finland (Kainuu) this figure is about 5%, assuming that the planned conservation programmes will be fully implemented. Finally, fragmentation has occured a long time ago in the south whereas only during the past few decades in the north.

This study is concerned both with the patterns and the processes affecting the occurrence and dynamics of species specialising in old-growth forest in landscapes fragmented by human activity, mainly forestry. Delimitation of old-growth fragments is occasionally trivial but often involves drawing arbitrary boundaries. The important factors likeky to affect the distributional patterns of species are forest patch size, patch isolation, patch quality and fragmentation history.

3 Study areas

The study will be carried out mainly in two areas: in Evo (Lammi, see Task 3, Section 4.3) in southern Finland and in Kainuu in northeastern Finland. Evo belongs to the southern boreal zone. This study will include about 30 forest areas within 100 km of Evo, with sizes ranging from 10 to 300 ha. These forest patches are among the most valuable old-growth forests that are left in southwestern Finland. The other study area in eastern Kainuu is situated between the middle and northern boreal zones. The study area is 500 km2 in size and it consists of old-growth forests of different sizes, with the largest continuous areas being some tens of km2 and the smallest ones few hectares in size. In addition, fragmentation history is studied also in Koitajoki area in Ilomantsi, eastern Finland, where the study area is 200 km2 in size. Eastern Kainuu and Koitajoki are connected to Russian Karelia where a landscape of more continuous old-growth forests still prevails.

4 Research task

A. The significance of small fragments of old-growth forests

Local populations inhabiting individual fragments of old-growth forest have different extinction probabilities depending on the size, quality, and isolation of the fragment, and also of course on the ecological properties of the species. Small fragments of old-growth forest may act as stepping stones between larger forest areas, even though the small fragments would not have permanent populations of their own. Small old-growth fragments may also serve as temporal refuges, possibly allowing some species to persist until the quality of the surrounding landscape has improved (Schelhas & Greenberg 1996, DeGraaf & Miller 1996).

The empirical studies take advantage of the contrast between an area where relatively large old-growth forest areas still remain and where fragmentation is recent (Kainuu and Ilomantsi), and an area where only very small fragments exist and where fragmentation has occurred mostly long time ago (Evo region). Additionally, we have the opportunity to compare these areas with extensive virgin forest areas in Russian Karelia. It is likely that little or no effects of fragmentation can be detected in an area which has become recently fragmented (see e.g. Ås 1993), hence in an empirical study it is important to study fragments of different ages (see section Fragmentation history and temporal connectivity below). Different taxa are also likely to show differences, and species with great individual longevity are especially likely to show a long delay in their response to fragmentation.

The effects of fragmentation on dynamics of species

This work consists of assessing the presence and population sizes of taxa in forest fragments, and relating these results to the size and age of the fragments. In practice, we will select a range of different-sized fragments from the study areas, keeping isolation and habitat quality (such as volume of decaying wood) as constant as possible. We will select a range of taxa with different population turnover rates. We have much expertise on saproxylic beetles, polyporous fungi and epiphytic lichens which all show relatively slow dynamics. The two former groups are particularly dependent on decaying wood, whereas the diversity of the tree stand is most essential to the epiphytes. We will attempt to identify species with relatively fast dynamics during the first year of the project. We will attempt to model the extinction process of populations in isolated fragments using models of population extinction (Section 4.4)

We anticipate a number of practical problems with this project. Some taxa can be surveyed efficiently, e.g. species in Task 1 and the darkling beetle Bolitophagus reticulatus ( Rukke & Midtgaard 1996), but many others can only be sampled with unknown efficiency. However, in the latter case a comparative study is possible. Selection of study areas and taxa is a major undertaking, which we do during the first year of the project.

Fragmentation history and temporal connectivity

Fragmentation history is an important determinant of species occurrence. Populations that are too small to be viable may persist in a small remnant for long periods simply because of longevity of individuals. Distribution of many long-lived organisms, like some polyporous fungi and epiphytic lichens, are affected by time lags. Presence of a species in a forest remnant does not yet guarantee that its population is viable. Fragmentation history should be considered when assessing the value of small remnants of old-growth forests or other habitat patches. The aim of this study is to find out the effects of fragmentation history on the occurrence and abundance of polyporous fungi and epiphytic lichens. The main purpose is to find out how the presence and abundance of these species is dependent on time since isolation and is affected by quality and size of the forest patch both at this moment and before isolation.

Fragmentation history is studied in Kainuu and Ilomantsi analysing historical forestry inventory data and aerial photos by GIS and FRAGSTAT (for fragmentation history in forest-agricultural landscape, see Task 3, Section 4.3). The forestry inventory data and aerial photos are available from 1936 to 1995.

Polyporous fungi and habitat characteristics are investigated in forest patches with different fragmentation history. Special attention is paid to the long-lived old-growth specialists with perennial fruiting bodies that are easy to find. Preliminary data on the occurrence of polyporous fungi have already been gathered in old-growth forest patches in Kainuu (Penttilä 1994). Abundance of polyporous fungi can be assessed by calculating the amount of occupied trunks (Renvall 1995). About 30 epiphytic lichen species will be used to study the effect of fragmentation on epiphytic communities. The selected taxa include both some fairly common species (e.g. Alectoria sarmentosa) and rare old-growth specialists (Cybebe gracilenta, Calicium adspersum). We have already collected preliminary data about the occurrence and abundance of these species in almost 100 old-growth forest patches in Kainuu in 1994-1996 (Kuusinen et al. 1995, Kuusinen & Kokko, unpubl.). Effects of fragmentation history and species dynamics will be modelled in Task 4 (Section 4.4).

Network of forest fragments

In Kainuu, the landscape now consists of a complex mosaic of forest fragments. We will survey a large network of fragments for the presence and population sizes of selected taxa, including those described in Task 1 (Section 4.1). The distributional patterns are modelled (Section 4.4) to test hypotheses about colonisation-extinction processes. This study will elucidate the potential role of small fragments between larger forest areas as stepping stones. Because fragmentation is relatively recent in Kainuu, we attempt to identify species that have fast turnover rates and hence respond quickly to fragmentation.

B. Source-sink dynamics

In preserving threatened species in boreal forests, source habitats with demographically viable populations are critical. Very little is known about source-sink dynamics in boreal forests. In this project, we will focus on two bird species which are likely to exhibit a source-sink metapopulation structure. The empirical work involves the determination of their abundances in different types of habitat, study of their movement behaviour, and the measurement of habitat specific demographic parameters. These studies involve colour-ringing of adults and nestlings.

The three-toed woodpecker

The three-toed woodpecker (Picoides tridactylus) is a species that is dependent on decaying wood, as it feeds mainly on wood-boring beetle larvae. The woodpecker prefers old-growth forests (Virkkala et al. 1994a) and it has declined in Finland in the past decades (Järvinen & Väisänen 1979). The three-toed woodpecker may also occur in managed forests but the survival and permanence of these populations is not known. They may be sink populations dependent on recruitment from source populations in the remaining old-growth forests in protected areas. The population dynamics of this species will be studied in the Lammi and Padasjoki region.

The crested tit

The crested tit (Parus cristatus) is a hole-nesting bird whose abundance has decreased markedly in Finland (Järvinen & Väisänen 1979). The species prefers old-growth forest (Virkkala et al. 1994a), though it also occurs in different kinds of managed forests provided that there are enough decaying snags for nest building. The crested tit forages on spiders and other invertebrates in forest canopy. The density of invertebrates has been found to be manyfold on the branches of trees in virgin old-growth forests (200 years) compared to those in managed mature forests (100 years, Pettersson et al. 1995). Therefore, it may be expected that offspring production and survival of the crested tit are highest in extensive areas of old-growth forest. This study will be conducted in Kainuu using nest-box populations.

C. Connectivity and ecological corridors

Effects of corridor width on microclimate and occurrence of epiphytic lichens

Ecological corridors may combine isolated forest areas and facilitate the movement of individuals from one patch to another in a landscape matrix of otherwise unfavourable habitat (Hudson 1991). As corridors are rather narrow habitat patches they may be microclimatically adverse for species of old-growth forests. Thus, an important question to study is the depth of climatological change in a corridor caused by the nearby open habitat. We will use epiphytic lichens and bryophytes to address this question, as these taxa are known to be sensitive indicators of the forest microclimate (Barkman 1958). Change in the epiphytic lichen and bryophyte communities on basal trunks of spruce and pine from the edge into the forest interior is studied in relation to microclimatic parameters (wind velocity, moisture, etc.) in study plots with known history of forest edge. Our preliminary data from Evo indicates that there are significant changes in the relative abundances of species and usually a slight decrease of total species richness in epiphytic communities at clear-cut edges (Kuusinen, unpubl.).

Movements along corridors

We plan to conduct four specific projects on dispersal and the use of corridors by fungi, beetles, birds, and mammals.


We will study empirically the dispersal of spores of selected old-growth specialist polyporous fungi (e.g. Phlebia centrifuga, Fomitopsis rosea, Haploporus odorus). For this purpose, we select suitable old-growth forest patches, inhabited by the focal species and surrounded by open habitats and by potential corridors. Dispersing spores are captured using agar plates containing homocaryotic mycelia of the focal species. As a result of the fusion of two uninucleate cells of the compatible homocaryotic mycelia, a binucleate cell is established. When the binucleate cell is ready to divide, a short branch - the clamp connection - arises between two nuclei. Clamps can be identified by microscope. Clamp formation indicates that spores of the target species were captured by the trap.


Field experiments are conducted to find out whether ecological corridors can facilitate dispersal of beetles associated with old-growth forests. A general problem in mark-recapture studies is that very large numbers of individuals are necessarily needed. We will use a bark-beetle species (Hylurgops glabratus) living on large fallen spruce trunks in shady places and a darkling beetle (Bolitophagus reticulatus), which is a monophagous species living on dead sporocarps of Fomes fomentarius on snags of deciduous trees (mostly birch in Finland). It is possible to obtain thousands of individuals of these species by mass-rearing. Marked individuals are released in corridors and their consequent movements are monitored at different distances from the release point, both along the corridor and outside of it. Window-flight traps baited with either fresh spruce logs or sporocarps of Fomes fomentarius will be used in trapping the beetles.


The Siberian jay (Perisoreus infaustus) is a resident bird species which prefers large old-growth forest areas (e.g. Virkkala 1996). Ecological corridors may be important for the Siberian jay as its dispersal rate is low. The movements of jays in relation to potential corridors and small patches of old-growth forest can be studied with radiotelemetry, particularly during the non-breeding season. The Siberian jay is a relatively large bird species (weighing about 80 g) and hence long-lasting, efficient radiotransmitters (tailmount tags) can be utilised.


The flying squirrel (Pteromys volans) is a threatened species of boreal forests (Hokkanen et al. 1982). The species probably suffers from fragmentation of continuous forest areas, as it cannot easily cross open areas, like clear-cuts and sapling stands, which do not contain an adequate density of large, high trees (I. K. Hanski, pers. comm.). The flying squirrel utilises high trees in moving, has specific foraging preferences (mainly catkins and buds of deciduous trees) and is a hole-nester. The flying squirrel's habitat use, particularly in relation to corridors and small patches of old-growth forests, is studied by radiotelemetry using neck collars. We have extensive experience about radiotelemetric studies of the flying squirrel (I. K. Hanski)

Significance of ecological corridors for the species of old-growth forest.

The aim is to study the significance of ecological corridors as movement routes for the species of old-growth spruce forest by field experiments. In the experiments marked animals are released within or close to an ecological corridor and their movements are followed by recapturing them. The experiments are designed to answer the following kinds of questions:

1) Do the animals remain in the corridor more often than expected by chance and how long distances do they move along the corridors?
2) How readily do individuals move from a larger patch of old-growth forest into a corridor?
3) Do the animals emigrate more willingly along a corridor than along some other type of habitat?
4) Do the animals emigrate along a clear-cut and along the first successional stages of forest?
5) Is a narrow clear-cut area a barrier for movements between two forests?
6) How often the animals move a 0.5 km distance to another area of spruce forest a) within an extensive spruce forest, b) along an ecological corridor and c) when two old-growth forest patches are separated by clear-cuts or other early successional stages of forest?

During the summer 1997 a pilot experiment was done by releasing marked individuals of two moth species (Xestia rhaetica, a specialist species of old-growth spruce forest, and X. speciosa, a less demanding species in habitat choice; n=106 and 94, respectively) into a corridor system of old-growth spruce forest. The movements of the released moths were monitored by recapturing moths with bait traps (n=52) within and outside the spruce forest system.

The study approach proved to be useful and potentially effective way of studying the significance of ecological corridors, but because the abundances of Xestia moths were unusually low in 1997, the results of the 1997 study remained very tentative. The modest number of moths marked and released led to small numbers of recaptured individuals (7.5 and 0% of X. rhaetica and X. speciosa, respectively). Of the 8 recaptured X. rhaetica 7 were recaptured within the spruce forest complex connected by corridors in distances between 100 and 510 m from the point of release. One individual had moved to another patch of old spruce forest and had crossed at least 40 m of unsuitable habitat for the species. Similar kind of experimental studies on the significance of ecological corridors will be continued during next years.

Studies on ecological corridors and connectivity are carried out mostly in the northernmost study area in Kainuu, where it is possible to select appropriate spatial configurations of habitat patches and corridors for the study. This is one of the areas where the Forest and Park Service has initiated work on a landscape ecological forest plan, and we can hence work on ´real´ corridors.