Efeito do pulso de inundação na assembleia de formigas em ilhas fluviais na Amazônia

Abstract

Disturbances are one the main factors affecting species assemblages' dynamics and structure. The flood pulse, observed in rivers in the Amazon, is a natural and seasonal disturbance that floods large tracts of land annually. The water level of most rivers varies from 10-15 meters between dry and flooded peaks and modifies the functioning of these ecosystems. The natural fluvial islands of the Amazon are also heavily affected, being wholly flooded during the flooding phase, temporarily transforming terrestrial environments into aquatic ones. The disturbance effect is expected to be stronger for sessile and terrestrial organisms, such as ants, which have few adaptations to the flood period. In this way, we evaluated how flooding and isolation of river islands act as an environmental filter, structuring taxonomic and guild diversity of ant assemblage. For this, we sampled 36 transects, 12 in each type of environment: terra-firme forest (non-flooded forest), igapó (flooded forest connected with terra-firme), and fluvial island (flooded forest isolated from terra-firme). We collected terrestrial ants, using soil pitfall, and arboreal ants combining arboreal pitfall and beating, at these sites during the dry season in Anavilhanas National Park, Amazonas. We recorded 210 species and morphospecies, representing 52 genera and seven subfamilies. The ant richness of the arboreal stratum was similar between the environments, but in the terrestrial stratum, greater richness was sampled in the terra-firme. Ant composition differed between environments in arboreal and terrestrial strata. The ant assemblage in the terrestrial stratum had a nested pattern according to the distance from the terra firme. Nested pattern was not evident in the arboreal stratum. We also observed that the proportion of species’ guild most associated with the soil (leaf cutters, cryptobiotic atines, raid hunter predators, and ground-dwelling omnivores) had a very low or non-existent proportion on the islands. Our results suggest that the flood pulse modifies the structure of ant assemblages that live in seasonally flooded forests, reducing the number and changing the identity of species in these environments. The environmental filter acted more strongly on terrestrial ants because, with the suppression of the soil during the flooded phase, there is a reduction in nesting sites and the availability of food resources. Thus, the ant species and guilds that live on the ground are possibly only able to survive in places where the influence of floods is less and end up disappearing on the river islands. On the other hand, arboreal ants-maintained richness between environments, probably due to the high food similarity of the species and the increase in dominance, regardless of the type of environment. Disturbance and isolation seem to favor the presence of dominant and opportunistic species as there is an increase in these species’ guilds on river islands. Fluvial islands subject to flooding and isolated from the terra-firme forest generally harbor an impoverished diversity, which can alter the functions performed by ants in these environments. Disturbance and isolation may, therefore, represent important factors for ant assemblages and may be operating as filters for dispersal and competition.