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

One of the great questions in ecology is what determines and maintains biodiversity. This
question is receiving increased attention as biodiversity is at risk. Species go extinct at such a
high rate that some scientists speak of a man-made mass extinction. As biodiversity is related
to ecosystem functioning also wide ranging consequences of the current species loss on
ecosystem services can be expected.
In addition to habitat loss, habitat fragmentation threatens biodiversity. Small and
isolated fragments are expected to host less species than larger and better connected habitat
patches. Fragmentation also reduces genetic diversity and disrupts interactions between
species. Invertebrates, in particular insects, contribute considerably to species richness of a
habitat.
In the present thesis I focus on the effects of an experimental fragmentation on
invertebrate diversity in calcareous grasslands. These species-rich, extensively used grasslands
have been created by man and are today threatened by changes in agricultural practices and by
fragmentation. In a long-term experiment at the University of Basel, fragments of different size
(0.5 m2, 2.25 m2 and 20.25 m2) have been isolated and maintained by regularly moving the
surrounding vegetation. Corresponding control plots were situated in adjacent continuous
grassland. The experimental set-up consisted of 48 fragments and 48 control plots, distributed
over three study sites in the northern Swiss Jura mountains. I examined effects of the
experimental fragmentation on invertebrate population dynamics 3 to 6 years after the
initiation of the fragmentation.
Species richness of grasses increased in fragments while species richness of forbs, ants,
aphids, gastropods and grasshoppers were not affected by the fragmentation. Only in
butterflies, the most mobile animal group examined, a negative response to the fragmentation
was found. The time frame used in the present experiment may have been to short to monitor
extinction processes. However, the experimental fragmentation altered the abundance of single
species and altered interactions between species. As predicted by theory, many common
species were even more abundant in fragments than in control plots. Furthermore, aphids, a
group of herbivorous insects, benefited from the fragmentation. However, the increase in aphid
density was not a result from reduced parasitation pressure, but rather a result of a higher
degree of ant-tending and an increased plant productivity. The fragmentation also altered
competitive interactions among ant species. With increasing density of the dominant ant
species Lasius paralienus species richness and forager density of the other ant species
decreased in fragments but not in control plots. The densities of foragers of the other species at
natural and artificial sugar resources were not affected by L. paralienus forager density. This
could be a result of an increased density of natural sugar resources in fragments and thus
decreased competition for them. The fragmentation also affected the spatial distribution and
persistence of ant nests. These findings were a result of altered abiotic conditions at the edge of
fragments and were more pronounced for the dominant species than for all ant species
together.
The experimental fragmentation increased plant productivity. Changes at the base of the
food chain could impact higher trophic levels. Therefore, I examined the relationships between
productivity (above-ground phytomass) and plant species richness and between productivity
and species richness and biomass of consumers (gastropods and grasshoppers) at three spatial
scales in two successive years. Only the control plots of the fragmentation experiment were
used. The shape of the relationship between productivity and species richness varied between
groups and depended on the spatial scale of the investigation.