Pekka Helle

LANDSCAPE FRAGMENTATION AND FOREST COMPOSITION EFFECTS ON GROUSE BREEDING SUCCESS IN BOREAL FORESTS

We examined the breeding success of forest grouse in relation to anthropogenic forest fragmentation in Finland. Employing Geographic Information Systems (GIS) and grouse data derived from Finnish wildlife triangle censuses conducted during 1989–1994, we combined the locations of 2267 Black Grouse (Tetrao tetrix) and 1060 Capercaillie (T. urogallus) females after the breeding season in mid-August with landscape data. The indicators of breeding success were the proportion of grouse hens with a brood and brood size. Two study areas (each 45 000 km2) in the boreal zone were selected for investigation. The breeding success of grouse was negatively correlated with both fragmentation of forest area per se by farmland and the decreasing proportion of older forest as a result of clear-cutting. The extent of landscape accounting best for variation in nesting success was an order of magnitude larger (∼100 km2) than the area most probably used by a grouse female and her brood during the summer, which suggests that la...

Abundances of red fox and pine marten in relation to the composition of boreal forest landscapes.

The effects of human-caused fragmentation of boreal forest on the abundance of red fox Vulpes vulpes L. and pine marten Martes martes L. were studied by combining the Finnish wildlife-triangle snow-track data (1990-94) with land-use and forest resources data employing the GIS. Two study areas (each 45 000 km(2) ) located in northern and southern Finland were selected for the investigation. The extent of landscape that best explained predator abundance (tracks per 10 km 24 h(-1) ) was the same (about 100 km(2) ) in both species and study areas. The decreasing proportion of older forest and the increasing proportions of young forest and agricultural land in the landscape positively affected track density of red fox. The relationship between agricultural land and fox abundance, however, was characterized by a convex curve peaking at 20-30% of agricultural land. With the habitat classification used, landscape composition explained 26% and 11% of the spatial variation in fox abundance in the northern and southern study area, respectively. The relationship between landscape composition and pine marten abundance was not as clear as in that of red fox. Landscape composition explained 10% and 6% of spatial variation in pine marten abundance in the northern and southern study area, respectively. In both areas a positive impact occurred with the increasing proposition of young forest in the landscape, but in the northern area the negative effect of increasing proportion of agricultural land was dominant. The abundances of red fox and pine marten were not negatively correlated, indicating that competition or intraguild predation by red fox do not determine abundance of pine marten on a landscape scale. A general increase in predation pressure by generalist predators in fragmented forest landscapes has been an intensively discussed conservation problem during recent years. We conclude that the red fox is a species potentially able to cause elevated predation pressure in boreal landscapes fragmented by human activities, but that the evidence against the pine marten is weaker.

Short- and long-term population dynamical consequences of asymmetric climate change in black grouse

Temporal asymmetry in patterns of regional climate change may jeopardize the match between the proximate and ultimate cues of the timing of breeding. The consequences on short- and long-term population dynamics and trends as well as the underlying mechanisms are, however, often unknown. Using long-term data from Finland, we demonstrate that black grouse (Tetrao tetrix) have responded to spring warming by advancing both egg-laying and hatching. However, early summer (the time of hatching) has not advanced, and chicks have to face colder post-hatching conditions. Demonstrating that these conditions are critical to post-hatching survival, we show that chicks are increasingly suffering higher mortality because they hatch too early. Consequently, breeding success and population size has severely declined over the past four decades. Finally, we modelled the impact of this particular climate change scenario on population dynamics and show that the mismatch can further explain the observed collapse of cyclic fluctuations. Because the evolutionary response of grouse is lagging behind the novel selective pressures, seasonally asymmetric climate change is likely to constitute an important determinant of future short- and long-term changes in the dynamics of black grouse populations.

Population trends of North Finnish land birds in relation to their habitat selection and changes in forest structure

Long-term trends in the 22 most abundant land bird species breeding in northern Finland were related to their habitat selection patterns. Independent data sets were used to describe long-term population trends (POP), edge preferences (EDGE) and the effects of the changing age structure of the forests on bird density (AGE). AGE combined the present age preferences of birds with data on the changing age structure of North Finnish forests from the 1950s to the 1970s. EDGE and AGE correlated positively though not significantly with POP. When EDGE and AGE were taken into account simultaneously, a significant correlation with POP emerged (67% of the variance accounted). Southern species were an exception; their population trends seem to depend on changes in southern Finland. The sedentary species of old forests have plummeted during the recent decades in northern Finland. The results for this group (Parus montanus, P. cristatus, P. cinctus, Certhia familiaris, Perisoreus infaustus) agreed well with the finding that fragmentation and changes in the age structure of the forests are mainly responsible for the recent trends among the abundant forest birds in the north. Our results seemingly indicate a close tracking of the environmental resources by the bird community. However, as the geographical scale of the study is broad (northern Finland as opposed to one population site) and the temporal scale decades rather than years, a simple saturation hypothesis is not an inevitable inference from the data; and if there is close tracking of the environmental resources, it is rather in terms of populations and not of the whole community. An alternative hypothesis is provided by changing numbers of high-quality and low-quality population sites; in this view, the regional pattern is a result of complicated dynamics in a mosaic of local populations.

Numerical and behavioural responses of migrant passerines to experimental manipulation of resident tits (Parus spp.): heterospecific attraction in northern breeding bird communites?

SummaryWe studied experimentally interspecific competition among foliage-gleaning passerine birds by manipulating the density of resident tits. In 1988 tit density was experimentally increased on three small islands in a central Finnish lake, and decreased on three other islands by tit removal. In order to avoid the effects of between-island differences in habitat quality, the role of the islands was reversed when the experiment was repeated in the following year. Censuses and observations on foraging and feeding behaviour were conducted to assess the numerical and behavioural responses of migrant conguilders (mainly chaffinches and willow warblers) with respect to the manipulated abundance of the tits. We also measured whether variation in food consumption of tits affected the frequency with which the migrants found food by calculating average intervals between successful prey captures, time lags to prey-capture and giving-up times. Our results indicate that interspecific competition is of minor importance in structuring breeding bird assemblages and species feeding ecologies on the study islands. No consistent difference in foraging or feeding niches of chaffinches and willow warblers was found between low and high tit density conditions. Niche overlap analysis showed no avoidance by chaffinches and willow warblers of the microhabitats which tits used. Tit abundance had no significant effect on feeding success or behaviour. Experimentally increased abundance of resident birds was associated with increased abundance of breeding migrants, however. This pattern was found not only in the foliage gleaning guild but also with all passerine birds, indicating that food was not an important contributor to this pattern. We elaborate a hypothesis suggesting heterospecific attraction in northern breeding bird assemblages. Habitat generalist migrants may use the presence of residents as an indicator of safe and/or productive breeding sites in northern unpredictable circumstances.

Population genetic structure of male black grouse (Tetrao tetrix L.) in fragmented vs. continuous landscapes

We investigated the association of habitat fragmentation with genetic structure of male black grouse Tetrao tetrix. Using 14 microsatellites, we compared the genetic differentiation of males among nine localities in continuous lowland habitats in Finland to the genetic differentiation among 14 localities in fragmented habitats in the Alps (France, Switzerland and Italy). In both areas, we found significant genetic differentiation. However, the average differentiation, measured as θ, was more than three times higher in the Alps than in Finland. The greater differentiation found in the Alps is probably due to the presence of mountain ridges rising above natural habitats of the species, which form barriers to gene flow, and to a higher influence of genetic drift resulting from lower effective sizes in highly fragmented habitats. The detection of isolation by distance in the Alps suggests that gene flow among populations does occur. The genetic variability measured as gene diversity HE and allelic richness A was lower in the Alps than in Finland. This could result from the higher fragmentation and/or from the fact that populations in the Alps are isolated from the main species range and have a lower effective size than in Finland. This study suggests that habitat fragmentation can affect genetic structure of avian species with relatively high dispersal propensities.

Ecological Sustainability of Birds in Boreal Forests

We review characteristics of birds in boreal forests in the context of their ecological sustainability under both natural and anthropogenic disturbances. We identify the underlying ecological factors associated with boreal bird populations and their variability, review the interactions between boreal bird populations and disturbance, and describe some tools on how boreal bird populations may be conserved in the future. The boreal system has historically been an area with extensive disturbance such as fire, insect outbreaks, and wind. In addition, the boreal system is vulnerable to global climate change as well as increasing pressure on forest and water resources. Current knowledge indicates that birds play an important role in boreal forests, and sustaining these populations affords many benefits to the health of boreal forests. Many issues must be approached with caution, including the lack of knowledge on our ability to mimic natural disturbance regimes with management, our lack of understanding on fragmentation due to logging activity, which is different from permanent conversion to other land uses such as agriculture or residential area, and our lack of knowledge on what controls variability in boreal bird populations or the linkage between bird population fluctuations and productivity. The essential role that birds can provide is to clarify important ecological concerns and variables that not only will help to sustain bird populations, but also will contribute to the long−term health of the boreal forest for all species, including humans.

Heterospecific attraction and food resources in migrants' breeding patch selection in northern boreal forest

Abstract We studied experimentally how heterospecific attraction may affect habitat selection of migrant passerine birds in Finnish Lapland. We manipulated the densities of resident tit species (Parus spp.). In four study plots residents were removed before the arrival of the migrants in the first study year, and in four other plots their densities were increased by releasing caught individuals. In the second year the treatments of the areas were reversed, allowing paired comparisons within each plot. We also investigated the relative abundance of arthropods in the study plots by the sweep-net method. This allowed us to estimate the effect of food resources on the abundance of birds. The heterospecific attraction hypothesis predicts that densities of migrant species (especially habitat generalists) would be higher during increased resident density. Results supported this prediction. Densities and number of the most abundant migrant species were significantly higher when resident density was increased than when they were removed. On the species level the redwing (Turdus iliacus) showed the strongest positive response to the increased abundance of tits. Migrant bird abundances seemed not to vary in parallel with relative arthropod abundance, with the exception of the pied flycatcher (Ficedula hypoleuca) which showed a strongly positive correlation with many arthropod groups. The results of the experiment indicate that migrants can use resident tit species as a cue to a profitable breeding patch. The relationship between the abundance of the birds and arthropods suggests that annual changes in food resources during the breeding season probably do not have a very important effect on bird populations in these areas. The results stress the importance of positive interspecific interactions in structuring northern breeding bird communities.

Mixed-species foraging aggregations and heterospecific attraction in boreal bird communities

We investigated whether different species of birds associated with each other while foraging during the breeding season. On average 50% of foraging observations were in mixed-species foraging aggregations, but with extensive interspecific variation. The occurrence of mixed-species foraging aggregations was relatively constant from the end of May until early July, with no indication that the appearance of fledglings increased the frequency of aggregations. These mixed-species foraging aggregations consisted of a wide variety of species in terms of foraging niche : typical flycatchers, specialized arboreal insectivores, and generalists were observed foraging together. The titmice species (Parus spp.) seemed to be preferred as foraging companions. The sites where foraging aggregations were observed did not differ from an equal number of randomly chosen sites with respect to forest vegetation structure. The dispersion of individuals in study areas was analysed using a 50 m x 50 m quadrat as the sampling unit. Altogether, 96 quadrats were surveyed. The dispersion of individual birds with respect to individuals of other species was non-random, and significantly clumped within quadrats. We used two playback experiments to test whether the aggregations were due to active search for heterospecific companions. In the first experiment, 20 randomly assigned sites received two 10-min treatments : willow tit (Parus montanus) song and silence as control. In the second experiment, we had two control treatments at 30 playback sites for the willow tit song, the song of the olive-sided flycatcher (Contopus borealis) and classical music. More species were observed during willow tit song than during any of the controls. This suggests that aggregated dispersion pattern and the existence of mixed-species foraging aggregations were due to active search for heterospecific companions, and also that recognition of neighbouring species was involved. We suggest that birds actively aggregate in mixed-species foraging groups to gain protection against predators and/or enhance feeding efficiency. The results support the view that positive interactions have an importance in structuring northern breeding bird communities.

Large-scale forest corridors to connect the taiga fauna to Fennoscandia.

Finland and Russian Karelia belong to the same biogeographical entity, lying on the same Precambrian bedrock. During the last half century there has been an enormous ‘natural experiment’, in which forestry in Finland has been very intensive, whereas in Karelia forestry has been negligent leaving large primaeval areas untouched. As a result, Russian forests have a much greater diversity of wildlife. In particular, rare species and species favouring old forests are more abundant in Karelia than in Finland. Typical dominant species in Finland are those characteristic of younger successional stages as well as many vole-dependent small carnivores. Finland is situated on the eastern margin of a vast coniferous taiga. The future of the taiga fauna in Fennoscandia is dependent on the condition of the taiga forests in Russia and on the connectivity of Fennoscandian forest areas to the intact taiga, i.e. connectivity at the border between Russia and Finland. In this paper, we focus our attention on the narrow isthmus between the White Sea and Lake Onega, which is an extremely important connection for the northern element of the taiga fauna. The capercaillie Tetrao urogallus may be a good focal species, with its large spatial requirements for lek areas depicting the need for connectivity to maintain viable populations. We suggest that large-scale connections should be planned, ‘forest bridges’ intruding into Finland and even into Sweden, where the proportion of mature forests would be high enough (as much as ⅓ of the total area) to guarantee the connectivity between subpopulations. We argue that this may not necessarily represent additional costs for forestry, provided that actions are taken for a careful large-scale planning of forest harvesting to satisfy the requirements of these corridors. We believe that large-scale preservation of ecosystems will be a better strategy in the future than species-specific conservation programmes for wildlife species.