Abstract
The periodic flood pulse of the Amazon River has been the main controlling factor in the local riverine ecosystem for at least two million years. The disturbance caused by the annual inundation is reflected in poorer species diversity in affected lowlands compared to upland areas. Seasonality of flooding, however, has enabled long-term acclimatisation in resident flora and fauna. Numerous adaptations, in some cases along with speciation, have evolved in local terrestrial invertebrates, such as Poratia obliterata (Kraus, 1960). This small millipede, which probably originates from the Andes, is currently known from a remarkably broad range of Central Amazonian biotopes, i.e. various seasonal inundation forests, upland forest and plantations. Like most native millipedes, P. obliterata appears to escape flooding by tree ascents. Such developed survival strategies adaptive to annual inundation can either reflect ecological plasticity or implicate ecological speciation, .i.e. ‘biotope-specific races’ or ecotypes. To assess the causal mode of adaptation, I combined ecological studies with genetic analyses.
To compare life history strategies of hypothesised ecotypes in non-flooded versus seasonally flooded biotopes, I surveyed the behaviour and phenology of P. obliterata populations on a local upland plantation and in a white-, black- and mixedwater inundation forest. In addition, I conducted comparative experiments with regard to migration potential and possible factors regulating reproduction. I found that millipedes in the inundation forests migrate in direct response to the rising and receding waters and take refuge on tree trunks during the aquatic phase. Experiments suggest that disposition for such vertical migrations is higher in animals native to these forests than in those originating from upland areas. In the former situation, animals then take shelter under bark, aggregate in groups close to the water line, probably avoiding dehydration, and repeatedly socialise with co-occurring millipede species of similar size. In contrast, specimens on the upland plantation dwell on the ground in moist decaying plant debris without aggregation or species interactions. This population shows a plurivoltine life cycle, i.e. continuous reproduction, whereas a univoltine life cycle, i.e. lack of offspring during flooding, seems characteristic for inundation forest populations. Here, oviposition is apparently restricted to the terrestrial phase after females returned to the ground. Experiments indicate an optional adult dormancy on tree trunks, but failed to show regulating effects of abiotic factors, such as habitat substrate, temperature, humidity and resources. Both development and maturation in offspring were faster for populations from inundation forests compared to upland ones. This appears to be field-derived and may allow for the floodplain populations to cope with discontinuous reproduction due to inundation by a faster establishment of succeeding generations during the terrestrial phase.
Seasonal vertical migration, aggregation and a univoltine life cycle appear to be adaptive traits of individuals living in inundation forests. Specimens inhabiting flooded and non-flooded sites thus may be considered as different ecotypes. Therefore, I examined whether these ecological adaptations involve ecologically driven speciation processes, i.e. genetically differentiated populations. Eleven subpopulations of P. obliterata from ten different sites in Central Amazonia, including a non-flooded plantation and various inundation forests, and one upland locality in Amapá State were compared on the basis of six polymorphic enzyme loci. Even though the behaviour and environment of the ecotypic forms are in marked contrast, the allozyme data did not support biotope-specific genetic adaptations of P. obliterata in Central Amazonia. Genetic diversity in P. obliterata (A = 3.5; H = 0.393) is higher than the average value in millipedes, providing a potential for future evolution, while outcrossing rates appear to be high, since most of the genetic variation (ca. 90 %) resides within rather than among subpopulations (FST = 0.079). Hence, the alternative life strategies of populations in floodplain areas might be a phenotypic response to environmental constrains, suggesting ecological plasticity in this species. Estimates of gene flow indicate long-distance dispersal, probably downstream drift along the rivers, and/or recent expansion to the study area, on plantations likely due to introductions with plant material.
Comparing the distribution, biotope range, population subdivision and genetic diversity of different millipedes, the species P. obliterata appears to feature a generalist strategy. My results show low divergence between Amazonian populations of this diverse and widespread species, which seems to cope well with abiotically varying biotopes and thus successfully invaded seasonal inundation forests.
