Comparing disturbance-sensitivity between plants and birds: a fine-grained analysis in a suburban remnant wetland

Abstract

Hemeroby is a concept widely employed in assessment of the effect of human activities on vegetation. In this study, we apply the concept to a set of bird species occurring in a Mediterranean remnant wetland. The aim was to obtain an average hemeroby index for two seasonally related bird assemblages (i.e. breeding and wintering) based on the information related to two levels of plant hemeroby. In a grid of 47 cells 100×100 m-wide, we sampled the fine-grained distribution of plant communities (Braun-Blanquet method/cell) in parallel with birds (point count method; one point count/cell), assigning an independent score of hemeroby to plants and birds on a scale from I to V, from pristine habitats with a lack of natural and/or anthropogenic disturbance (score = I) to completely artificial habitats (score = V). Whereas bird species ranged from categories II to V, vegetation types spanned only the categories III and IV. Therefore, bird species showed a higher variability in hemeroby. By comparing hemeroby scores, we can deduce the effect that the vegetation disturbance may have on bird species. The mean hemeroby for breeding birds, calculated on all the species occurring in a determined plant hemeroby category, is not significantly different between sites with higher (= IV) and lower (= III) plant hemeroby (i.e. higher and lower level of disturbance). The mean hemeroby of the wintering birds was significantly different in the two levels of plant hemeroby (i.e. higher vs. lower hemeroby). Our data suggest that only the wintering birds had a hemeroby distribution pattern related to that of the plants, while the distribution of breeding birds showed no association, i.e. they appear in similar distribution in both plant hemeroby classes. This pattern may reflect the characteristics of the habitat types in relation to bird seasonality: a large section of wintering bird species are strictly water-related, linked to habitats with low plant hemeroby, so appearing more sensitive to change in plant hemeroby when compared to breeding species. Although explorative, our data may be useful in wildlife management, implying that in wetland–grassland mosaics the more sensitive wintering bird species are suitable as indicators aimed to test the effect of natural and anthropogenic disturbances.