Effects of experimental small-scale grassland fragmentation on the population dynamics of invertebrates

摘要

One of the great questions in ecology is what determines and maintains biodiversity. Thisudquestion is receiving increased attention as biodiversity is at risk. Species go extinct at such audhigh rate that some scientists speak of a man-made mass extinction. As biodiversity is relatedudto ecosystem functioning also wide ranging consequences of the current species loss onudecosystem services can be expected.udIn addition to habitat loss, habitat fragmentation threatens biodiversity. Small andudisolated fragments are expected to host less species than larger and better connected habitatudpatches. Fragmentation also reduces genetic diversity and disrupts interactions betweenudspecies. Invertebrates, in particular insects, contribute considerably to species richness of audhabitat.udIn the present thesis I focus on the effects of an experimental fragmentation onudinvertebrate diversity in calcareous grasslands. These species-rich, extensively used grasslandsudhave been created by man and are today threatened by changes in agricultural practices and byudfragmentation. In a long-term experiment at the University of Basel, fragments of different sizeud(0.5 m2, 2.25 m2 and 20.25 m2) have been isolated and maintained by regularly moving theudsurrounding vegetation. Corresponding control plots were situated in adjacent continuousudgrassland. The experimental set-up consisted of 48 fragments and 48 control plots, distributedudover three study sites in the northern Swiss Jura mountains. I examined effects of theudexperimental fragmentation on invertebrate population dynamics 3 to 6 years after theudinitiation of the fragmentation.udSpecies richness of grasses increased in fragments while species richness of forbs, ants,udaphids, gastropods and grasshoppers were not affected by the fragmentation. Only inudbutterflies, the most mobile animal group examined, a negative response to the fragmentationudwas found. The time frame used in the present experiment may have been to short to monitorudextinction processes. However, the experimental fragmentation altered the abundance of singleudspecies and altered interactions between species. As predicted by theory, many commonudspecies were even more abundant in fragments than in control plots. Furthermore, aphids, audgroup of herbivorous insects, benefited from the fragmentation. However, the increase in aphidududdensity was not a result from reduced parasitation pressure, but rather a result of a higheruddegree of ant-tending and an increased plant productivity. The fragmentation also alteredudcompetitive interactions among ant species. With increasing density of the dominant antudspecies Lasius paralienus species richness and forager density of the other ant speciesuddecreased in fragments but not in control plots. The densities of foragers of the other species atudnatural and artificial sugar resources were not affected by L. paralienus forager density. Thisudcould be a result of an increased density of natural sugar resources in fragments and thusuddecreased competition for them. The fragmentation also affected the spatial distribution andudpersistence of ant nests. These findings were a result of altered abiotic conditions at the edge ofudfragments and were more pronounced for the dominant species than for all ant speciesudtogether.udThe experimental fragmentation increased plant productivity. Changes at the base of theudfood chain could impact higher trophic levels. Therefore, I examined the relationships betweenudproductivity (above-ground phytomass) and plant species richness and between productivityudand species richness and biomass of consumers (gastropods and grasshoppers) at three spatialudscales in two successive years. Only the control plots of the fragmentation experiment wereudused. The shape of the relationship between productivity and species richness varied betweenudgroups and depended on the spatial scale of the investigation.