Habitat use by man is one of the most important factors affecting terrestrial ecosystems in global scale and leads very often to strong biodiversity impoverishment. Negative influence of human activity on biodiversity is noticeable especially in farmlands. For example, birds disappearing from an agricultural landscape is common in Europe, lately also in Poland. Among habitats present in typical lowland farmland, midfield afforestations play a particularly important role for birds. They constitute breeding habitat for up to 80 species. In spite of many studies and their syntheses, mechanisms which shape avifauna in afforestations, its dynamics and long-term changes have not been fully understood. Especially importance of afforestation surrounding, called “landscape context” remains controversial.
The aim of own study carried out in 1999-2002 was hierarchization of factors affecting breeding bird communities occurring in small, isolated afforestations, and in result – valorization of afforestation structure, their adjacent areas and landscape around afforestations with respect to their importance for bird diversity protection in the agricultural landscape.
The study was carried out in the Kościańska Plain, within the Dezydery Chłapowski Landscape Park and neighbouring areas. It covered 66 afforestations – wood “islands” distributed in the area of 200 km2. The structure of bird life habitat was quantified with respect to its three different elements: afforestation structure, habitat structure in nearest neighbourhood of afforestations and landscape structure around afforestations. Bird population densities were estimated with the aid of cartographic method. The avifauna was characterized in relation to three “levels” of its organization: whole community, individual species and their groups (habitat groups): crop field species,
wood-crop ecotone species, wood edge species, wood “generalists”, wood interior species and others. In result of the performed analysis of habitat features variability and their interrelations, a group of variables has been selected, which describe the above mentioned
aspects of bird life habitat comprehensively and in detail. These variables were used for analysis of relationship between habitat and landscape structure and bird communities of afforestations.
Among 61 observed breeding species the most common were: chaffinch, yellow hammer, blackbird, black cap, great tit, blue tit, gold finch, and spotted flycatcher. The results show that species richness in small afforestations covering usually 1-3% of farmland area is high. The data confirm that the species number of farmland breeding avifauna may reach the level of 1/3-1/4 of breeding avifauna of Poland. According to species number, most abundant were wood species – wood interior, wood edge and wood “generalists” species. They constituted together almost 60% of all the species. Woodcrop ecotone species represented less than 1/4 of all the species number. Mean bird density amounted to 15 pairs/ha, and most abundant species were: chaffinch, black cap, yellow hammer, icterine warbler and blackbird. Share of particular habitat groups was more differentiated. Wood “generalists” consituted more than 40% of community while ecotone species – more than 1/3 of community. Although the group of wood interior birds was rich in species, its share in total density was very low i.e. 2%. Bird communities in particular afforestations were strongly differentiated. Mean density in particular
afforestations was equal to 4.4-47.9 pair/ha, mean pair number – 0.7-42.3, mean number of species per year – 1-18, total number of species per afforestation – 3-27. Species turn-over rate was similar in consecutive years and was equal to about 35%. It should be
underlined that although bird communities in particular afforestations were very changeable, the total bird species number and density with respect to total group of the studied afforestations were highly stable in 1999-2002. It indicates that bird diversity protection in a farmland can be based on managing of the system of small but numerous
midfield afforestations. Stability of bird communities and their species turn-over rate in individual afforestations were dependent mainly on the afforestation area but some features of landscape structure like heterogeneity (high fragmentation degree) and presence of grasslands have also positive although slight influence. The results of the study show also that communities have individualistic characters
– the importance of habitat as well as landscape structure was stronger when lower level of community organization was considered. The relationships between habitat and landscape structure and individual species were strongest, the relationships between habitat
and landscape structure and habitat groups were weaker, and relationships between whole bird community and structure of habitat and landscape were weakest. Most important for shaping breeding bird communities in afforestations was their area, consistently with well-known, common rule of increasing number of species with the area studied. However, it should be stressed that adaptive ability of many bird species, even those typical for wood interior, is significant. These species colonized even very small (< 1 ha) afforestations. Apart from the afforestation area, other aspects of their structure
played an important role. For groups preferring wood edge afforestation structure was less important but neighbourhood habitat structure was more significant. For group of wood interior species most important were features dealing with tree stand while shrub
and herb layer did not have any effect. Habitat structure in nearest neighbourhood of afforestations an played important role for bird communities in afforestations, especially heterogeneity of this habitat and – in case of a few species – share of crop fields. The impact of these factors was strongest for ecotone species but invisible for wood interior species. Also landscape structure had a significant influence on bird communities in afforestations. This influence was strongly differentiated in relation to the level of community organization considered. It is only slightly noticeable in relation to the whole
community but plays an important role for individual species or their habitat groups, especially for wood species. Summing up, all the three studied aspects of bird life habitat have a significant impact on bird communities in afforestations – structure of these afforestations, structure of habitat in nearest neighbourhood of afforestations and structure of landscape. Only considering the influence of all the three groups of habitat features on birds offers a possibility for complete understanding the reasons for spatio-temporal variability of
farmland bird community and for possibility for effective avifauna protection by conscious managing of habitats in an agricultural landscape.