Life history and spatial distribution of the enchytraeid worm Cognettia sphagnetorum (Oligochaeta) in metal-polluted soil: Below-ground sink-source population dynamics?

We studied the life history, metal-avoidance behavior, spatial distribution , and population growth of enchytraeid worms (Cognettia sphagnetorum [Oligo chaeta)) originating from two sites: one uncontaminated, and another patchi ly polluted by heavy metals. Effects of patchy soil contamination on popula tions were studied in microcosms. In uncontaminated soil, worms from the po lluted site had lower viability and reduced growth rate as juveniles but hi gher growth rate as adults compared to worms from the unpolluted site. They were also smaller in size at fragmentation (reproduction). Worms from the polluted site reached a larger population size than worms from the unpollut ed site. Hence, worms from the polluted site seemed to allocate more energy to reproduction, because such a strategy can be highly adaptive when livin g in a less-polluted patch (the source) in, on average, a highly polluted e nvironment. The C. sphagnetorum actively avoided Cu-contaminated soil, with the response being stronger in individuals from the contaminated site. Whe n an unpolluted patch (the source) was surrounded by a toxic environment (t he sink), worms were found in the sink, either because intraspecific compet ition overrode the avoidance behavior or because of random walk migration. As a result, total population size in these microcosms were as high as thos e in totally uncontaminated microcosms. Our results indicate that avoidance behavior and increased allocation to reproduction, together with the obser ved dispersal pattern, can be an adaptive strategy for C. sphagnetorum in t he presence of small, less-polluted patches in, on average, a hostile envir onment. Hence, sink-source population regulation can explain spatial distri bution and survival of C. sphagnetorum in patchily polluted field soils. Wh en such population dynamics occur, this pattern should be taken into accoun t when population densities in the field are monitored for ecological risk- assessment procedures concerning toxicants.