Efeitos da sazonalidade pluviométrica sobre a diversidade e a composição de uma assembléia de formigas (Hymenoptera: Formicidae) em uma área de caatinga

Abstract

Presence of water is a determinant factor for food sources and/or nest sites availability, and is therefore potentially important for terrestrial invertebrate communities, particularly for those with fixed/perennial nests and that live in environments marked by strong rainfall seasonality. The aim of this work was to obtain informations about the seasonal patterns of a soil ant assembly structure of a Caatinga area, and to investigate how diversity and species composition of that assembly vary with rainfall seasonality. The study was performed in a Caatinga area localized at Pentecoste-CE. A 200 meters transect with 20 sampling points at 10 meters intervals was established. Transect sampling was performed once a month during 12 months, in the period 08/2008 – 08/2009. At each sampling point, a pitfall trap (height: 13 cm; diameter: 12 cm) filled with a ethanol/ethylenoglicol solution was placed at the beginning of each month and remained in the field for seven days. The taxonomic structure of the sampled fauna as well as the species (relative) frequency distribution followed the patterns generally found in neotropical regions. Forty-one species (40 in the dry season; 30 in the rainy season) belonging to six subfamilies and 20 genera were collected, with Myrmicinae (21 spp.), Pheidole (10 ssp) and Camponotus (8 spp) as the most specious taxa. Most species showed low (relative) abundance, being found in less than 10% of all the sampling units used in the whole sampling period. Only five species were collected in more than 50% of the sampling units. The collect of the relatively rare species Thaumatomyrmex mutilatus represents the second record of that species for the semi-arid northeastern region, whereas the collect of a new species, actually being described, of the Platythyrea genus represents the first record for the Ceará state. Of the 41 collected species, 12 and 1 (Strumigenys elongata) were found only during the dry and the rainy season respectively. The species-accumulation curves (observed and estimated) indicated that the sampling sufficiency was attained, and that about 90% of the estimated fauna had been collected. The Shannon and the Simpson diversity indexes for the rainy season were both lower compared to the dry season (H’: 2,91 versus 2,78; 1/D: 13,83 versus 12,8), and there was a negative correlation between the monthly Shannon and Simpson diversity indexes and the pluviometric indexes. A reverse relationship was observed with the equability indexes (Pielou equability index and Simpson equability index) (Pielou equability: 0,79 versus 0,83; Simpson equability: 0,35 versus 0,44), with a significant positive correlation between the monthly Simpson equability indexes and the pluviometric indexes. The turnover indexes were low, with a mean of 0,23 between the consecutive months of the sampling period, and a value of 0,19 when comparing the two periods (dry and rainy). The average fauna similarity (Jaccard´s index) for the monthly samplings (0,61) and the fauna similarity for two periods (dry and rainy) (0,67) were high. The group analysis with the hierarchical clustering method (UPGMA) did not allow the detection of two clear-cut groups corresponding to the two periods (dry and rainy). Compared to other Brazilian biomes, the ant species richness found in the study was low, and the results suggest that the dry season is not a limiting factor for the ant communities of the Caatinga biome. Nevertheless, the diversity reduction observed in the rainy season could result from an important reduction of ant activity in response to an atypical/exceptional rainy period in the 2009 year. The reduced biodiversity, when compared to other Brazilian biomes, could result from the permanent or seasonal lack of leaf-litter, an important factor for the specialized soil ant predators, and from the possible carbohydrates seasonal limitation (nectar, honeydew).