A Debreceni Déri Múzeum Évkönyve 1986 (Debrecen, 1987)
Természettudomány - Juhász, Lajos–Bozsko, Szvetlana: The Ornithofauna of Debrecen II. Synecological Analysis
b) Species diversity conditions of the ornithofauna in the urban biotopes The biological value of the bird communities of the particular biotopes is well demonstrated by the species diversity (H s ) and evenness (J) data of the given community. The species diversity conditions are changing in parallel with the aspectual changes in species composition and individual bird count (Table 10). Referring the species diversity values to the whole town —as compared to the spring period —they decrease in the nesting period, increase in autumn and fall again in winter. This regularity is a consequence of the annual mobility of the species and individual count. The maximum diversity in the spring period is shown by the outskirts (3.3540), the bird community of which reaches the highest individual count in this migration period (See Table 10). This can be accounted for by the poor selective power of this biotope. The preponderance due to adaptation to the urban environment cannot yet be manifest among the species appearing in the biotope. In addition to this, we can emphasize the high species diversity of the ornithofauna of the Great Forest, the Public Cemetery, the small urban parks and of the garden city. The evenness values, in general, differ only slightly from the possible values, which, at the same time, denotes the nearly maximum „exploitation" of some biotopes. The diversity values of the centrally located biotopes show a difference of an order of magnitude. In the mirror of a lower species count the high abundance values of the mass species come better to the foreground, which is indicated by the negative changes in the species diversity values. The lowest H s value, together with a J=90%, can be encountered in the housing estate, which proves an extremely unifacial bird community (See Table 10). The ornithofauna of most urban biotopes has already come to a state of stabilization by the time of the nesting period. The richness of species in the large green areas, and the dispersion of the species is pronounced in this period, too, which is the result of a process involving the high exploitation of a highly differentiated niche. In this period the maximum diversity can be found in the Cemetery (3.2744). A similarly rich and well-ordered fauna is shown by the diversity values of the Great Forest and the outskirts (See Table 10). The small parks and the garden city still show ecologically significant composition in spite of the preponderous dominance of the individual count of some species. The other urban biotopes require high specialization from the birds, which leads to uneven species diversity distribution (Table 10). The faunistic changes beginning in autumn, the migration, result in greater absolute individual diversity values, whereas the richness in species, characteristic of spring, is not manifested. After the onset of winter a levelling of species and individual counts is observed, which is verified by the calculated diversity values, too. The homogeneity of the urban ornithofauna is pronounced in this period, mainly due to the winterig guests and wandering birds of identical species. Out of the particular biotopes the strikingly low diversity value (1.6545) of the old city centre is worth mentioning, which is associated with a decreased evenness value (0.5893). This is due to the appearance of Turdus pilaris and Bombycilla garrulus in large masses. The summa D values for the two species reach nearly 70%, with very high abundance values (See Table 10), thus, the state of equilibrium is overturned. c) Analysis of the evenness value (J) of the bird communities of the urban biotopes The changes in the bird communities in the particular biotopes induce changes in the individual count, dominance and evenness values too. The intensive spring movement of the birds within the town results in relatively high 50
