Assessing the potential role of ground beetles (Coleoptera, Carabidae) as bioindicators in poplar stands, with a newly proposed ecological index (FAI)

Poplar plantations host a number of organisms which can live and reproduce in this habitat. Among these, ground beetles (Coleoptera, Carabidae) show strict relations with the environmental conditions and are therefore potential bioindicators of the ecosystem stability or stress. For this reason ground beetle assemblages have been studied for about 10 years (1989–1999) in poplar stands of different ages in the Po Valley (Italy) as well as in reference habitats, i.e. natural woods and agricultural crops, in order to assess their potential as indicators of anthropogenic impact. Sampling was carried out by homogeneous pitfall trapping. The number of species in each assemblage varied between 10 and 46, without any recognisable correlation with factors such as habitat type or age of the poplar stand; even the Shannon–Wiener index was extremely variable and in no way correlated with the factors above. Evenness values were generally quite low in all poplar plantations, due to the simplified dominance structures of these carabid assemblages, which are characterised by only a few dominant and sub-dominant species; on the contrary, in natural woods the quantitative distribution of species within the community was fairly well balanced. The forest affinity index (FAI) is a newly proposed ecological index which expresses an evaluation of the relative quality of a habitat compared to another reference habitat; in the case of poplar plantations, which are basically a woody agricultural crop, the FAI index is devised to measure their similarity with (or distance from) natural woodlands. This index displayed a high variability; nevertheless in a linear regression model it appeared to be significantly dependent on the age of the poplar stands. FAI values ranged between −0.35 and −0.45 in agricultural crops and in young poplar stands, and was positive only in a few 7-year-old (and older) poplar plantations (top value=0.25); on the contrary the FAI index was highly positive in natural woods (top value=0.92 in a beech wood). Increasing FAI values in poplar stands throughout the cultural cycle are due to the immigration of more specialised forest carabid species, even if they never reach the values typical of forest habitats. In conclusion, the FAI index proved to be the only parameter potentially useful to assess the ecological complexity of poplar stands.