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Positive and negative species interactions in fragmented landscapes

Final Activity Report Summary - SPECIES INTERACTIONS (Positive and negative species interactions in fragmented landscapes)

Mechanisms and consequences of coexistence of species have intrigued scientists since the seminal works by Darwin, Lotka and Volterra. The major mechanism among species occurring at the same trophic level and sharing similar resource needs has been assumed to be negative, i.e. interspecific competition. Recent studies, however, have shown that in many plant and sessile animal communities competing species may actually facilitate each other in certain conditions. Such positive interactions are therefore possible and can have major impacts on local community structure (biodiversity) but very little is known whether such mechanism exists in mobile animal communities.

The aims of this project were to examine potential variation in species interactions from positive to negative and its mechanisms and consequences. Bird communities, and especially resident (titmice, Parus spp.) and migratory (collared flycatcher, Ficedula albicollis) birds, were used as a model system. Titmice and flycatchers use similar breeding sites (cavities) and food during breeding time and they have been shown to compete with each other. The assumed mechanism that was expected to create variation in interactions is information on environmental conditions. Because competing species have overlap in resource needs, presence, density, behaviour and performance of competitors can convey information to other species about local environmental conditions and thereby facilitate their decision-making. In my study system, resident birds were assumed to be information sources because of their resident status and knowledge about local conditions, and migratory birds were assumed to be information users.

More specifically, I examined whether,
(i) migrant birds use density of resident birds as information about habitat quality in breeding habitat selection. Low resident density may indicate poor local resources and/or high predator abundance that reduce resident densities.
(ii) At the nest site scale, I examined whether migrant birds use the location of resident nest as a cue for high quality site and whether migrants also may gain benefits via social interactions with residents. This study also examined the effects of migrants on the breeding success of residents.
(iii) I examined whether migrants use residents' investment in offspring as information about habitat quality and in their own investment decisions.

The results of most studies were in line with my predictions and suggest that migratory flycatchers indeed use resident titmice as a source of information, which in turn affects interactions between those species. At the landscape scale, flycatchers use the local density of tits in assessing the quality of potential breeding habitats, which reflects flycatchers' habitat preference, onset of egg-laying and offspring investment (clutch size) decisions. Flycatchers prefer and make highest investment in offspring at intermediate tit density habitats compared with low and high tit densities. For flycatchers, low tit density apparently indicates either poor quality resources or high predation pressure or both, whereas high tit density habitats indicate high costs of interspecific competition. At the nest site scale, flycatchers use tit nest locations as a cue for good quality breeding sites and are attracted to the vicinity of breeding tits. Surprisingly, resident titmice suffer from the presence of flycatchers in terms of lowered offspring number and condition. This suggests that flycatchers are actually exploiting titmice. Investment of titmice in offspring (clutch size) may also somewhat affect flycatchers investment decisions but its effect is not so clear.

The results of these studies challenge current theories of species coexistence showing that the effect of competitors is not always negative but may also be positive via information utilisation, and may even evoke interactions resembling parasitism. Interspecific information utilisation affects the distribution of individuals and species in the landscape via breeding habitat selection and therefore contributes to the local diversity of organisms.