Macrophyte Associated Invertebrates and the Effect of Habitat Permanence

Publisher Website
Google Scholar
Abstract
Invertebrate abundance differed in stands of the two characeans Chara tomentosa L. and Nitellopsis obtusa (Desv.) J. Groves in a shallow eutrophic lake. The plants are similar in morphology but differ in growth cycle. In stands of the winter-green Chara tomentosa, Asellus aquaticus L. (Isopoda) dominated in abundance over Chironomidae (Diptera) and Gammarus lacustris G. O. Sars (Amphipoda) occurred frequently. In stands of Nitellopsis obtusa, which dies off during winter, Chironomidae was the dominating taxon, while the abundance of Asellus was one to orders of magnitude lower than in Chara, and Gammarus was not found. Field and laboratory preference tests showed that Asellus and Gammarus did not colonize or prefer Chara over Nitellopsis, suggesting that active avoidance was not the cause of their low abundance in Nitellopsis. Exclusion of fish for 90 d in a Nitellopsis stand did not increase the density of Asellus, but introduction of Asellus to exclosures showed that the species was able to grow rapidly and reproduce in the habitat. In a recently established Chara stand the species structure of associated invertebrates shifted from a dominance of Chironomidae in September of the first year to a dominance of Asellus the following year. These results all support the hypothesis that the die-off of Nitellopsis in autumn limits slow colonizers like Asellus and Gammarus from establishing dense populations in this habitat. The invertebrate community in this seasonally changing habitat is thus suggested to be in an early phase of colonization, where animal interactions are less important and the species composition is mainly determined by colonization ability. In the more permanent Chara habitat, density-dependent animal interactions are suggested to determine the abundance and species composition.