Yrjö Haila

Small-scale heterogeneity in the spatial distribution of carabid beetles in the southern Finnish taiga

Small-scale distribution of ground beetles (Cole- optera, Carabidae) was examined, based on catches among 300 pitfall traps, in a coniferous forest in southern Finland. The sample from the whole snow-free season comprised 2405 individuals of twenty-two species. Each of the most numerous species (Pterostichus oblongopunctatus (F.), Calathus micropterus (Dft.), Leistus terminatus (Hellw. in Pz.), Notiophilus biguttatus (F.) and Cychrus caraboides (L.)) was non-randomly distributed and formed aggrega- tions within the site of c. 1.3 ha in area. Although the five species occurred in every microhabitat defined in the plot, four of them (N. biguttatus was an exception) were more abundantly found in certain microhabitat types than in the others. In addition to vegetation around the traps, the numbers of Formica ants in the traps correlated with the numbers of carabids caught, mostly negatively. The consid- erable variation in catches and species richness among single traps and among blocks of 16 traps tended to even out, when larger subsamples were taken from the total pool (blocks of 48 traps). The distribution patterns observed within the plot were compared to a reference data set from similar habitat in the same region. Species distributions among microhabitats were slightly different in the reference set and the predic- tive success was relatively poor, probably due to different scales of study in the two data sets. Variation in species distribution in the two spatial scales studied (within a habitat patch and among them) is suppos- edly due to different factors. Although no direct evidence is available from our study area, we suggest that active micro- habitat selection explains the small-scale distribution within the study plot, whereas dynamics of local populations, influenced by regional-scale differences in habitat composi- tion, are the most likely explanation for the distribution pat- terns among habitat patches.

Colonisation of clearcut forests by ants in the southern Finnish taiga: a quantitative survey

We assessed the colonisation of clearcut forests by ants by using pitfall-trap data collected from clearcut areas of different age in southern Finland. We compared mature forests (age over 120 yr) with areas cleared 0, 2 and 10 yr before trapping. We concluded that the old-forest wood-ant systems are destroyed by clear-cutting, and that this is probably based on the loss of food resources. In the colonisation stage the role of stochasticity is counter-balanced by a number of deterministic species-specific factors in the succession of ant assemblages

Calculating and miscalculating density: the role of habitat geometry

The density of a population is a quantitative variable with spatial attributes: density is calculated on the basis of the geometric configuration of a selected area on a two-dimensional plane. This means that a density estimate implies a decision as to what is an ecologically realistic configuration for a study area, but errors in determining the appropriate value for A in the D = N/A equation may have serious consequences for the value of D. I illustrate problems created by such decisions with examples taken from breeding bird distributions in fragmented habitats.

Colonization of islands by land birds: prevalence functions in a Finnish archipelago

Land birds breeding on forty-four islands (size range 0.5-582 ha) in the Vargskar archipelago of the kland Islands (SW Finland) were censused in three (or two) breeding seasons in 1976-80 by one-visit methods. The islands were divided into five size classes and expected population sizes of land birds were estimated on the basis of (1) habitat composition of the islands and (2) population densities of each species in equivalent habitats in the source area, Main Aland. Prevalence is defined as the ratio between observed and expected population sizes. Observed densities of most of the fifty-six species studied did not deviate from expected values. Eleven species were more abundant on the islands than expected, including dominant generalists and species favoured by edges and bushy, heterogeneous habitat structure. Fourteen species had smaller populations on the islands than expected, but only seven of them consistently so in all census years and all island size classes. Habitat micro-structure seems important for three low-prevalence species, nest predation is invoked in two cases, in one case poor wintering capability seems a plausible factor, while one case remains obscure. Rarities were observed in the island censuses about as frequently as on Main Aland. Interspecific competition seems a plausible explanation for the absence of one species from the archipelago. One species, Turdus pilaris, changed its prevalence pattern dramatically in the study area by disappearing from the archipelago after the exceptionally cold winter of 1978/ 79; recolonization has taken place very slowly. The only species possibly showing density compensation is the dominant generalist Fringilla coelebs. Its populations on small islands are about doubled compared with the expecta- tions. Total densities are also higher than expected on small islands, but in most cases F. coelebs contributes less than half to the increase. In addition, there are no puzzling absences from small islands, but the poverty of their bird com- munities seems to result from reduced habitat diversity. Therefore autecological population responses rather than density compensation seem a plausible explana- tion of excessive population sizes of single species such as F. coelebs. We con- clude that, in these northern conditions, it seems appropriate to think of island communities as assemblages of species colonizing the islands according to their autecological (especially habitat) requirements. Ubiquitous habitat require- ments and flexibility in habitat utilization seem important correlates of coloniza-

The Role of Theoretical Concepts in Understanding the Ecological Theatre: A Case Study on Island Biogeography

To do science means to construct theories and to adopt theoretical concepts in order to explain facts of nature (or man); if the world were what it looks like, science would be unnecessary. But how are the prevailing concepts in science in general, or in ecology in particular, vindicated?

On the semiotic dimension of ecological theory: The case of island biogeography

The Macarthur-Wilson equilibrium theory of island biogeography has had a contradictory role in ecology. As a lasting contribution, the theory has created a new way of viewing insular environments as dynamical systems. On the other hand, many of the applications of the theory have reduced to mere unimaginative curve-fitting. I analyze this paradox in semiotic terms: the theory was mainly equated with the simple species-area relationship which became a ‘signifier’ of interesting island ecology. The theory is, however, better viewed as a theoretical framework that suggests specific hypotheses on the ecology of colonization of insular environments. This paradox is inherent in the use of simplifying analytic models. Analytic models are necessary and fruitful in the work of ecologists, but they ought to be supplemented with a broader, pluralistic appreciation of the role of theories in general.