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Current
research topics
Linking
phytophagous insects with ecosystem processes
Studies on the biogeochemistry of
ecosystems usually do not consider herbivores as
an important component. This is partly due to the
difficulty to experimentally manipulate their
distribution or abundance and evaluate their role
for the behaviour of ecosystems or maintenance of
ecosystem services. Some species, however, hold
key positions because their activities and
abundance affects the distribution of many other
species. For example, aphids feed on the phloem
sap of their host plant and produce a sugary
excreta (honeydew),
which is used as energy resource by more than 250
insect species and micro-organisms.
They can also have economic
importance because many aphids significantly
reduce the growth performance of their host trees.
Butterfly larvae feeding on leaves often produce
large quantities of faeces, rich in carbon and
nitrogen. We investigate infestation patterns on
spruce and beech/oak forests and the effects on
local ecosystem processes, e.g., on the carbon and
nitrogen cycles from the phyllosphere down to the
soil. The connection of the ecology of key
functional organisms with ecosystem processes is
an important step to further our understanding of
the temporal variability and spatial heterogeneity
in the flows of elements and nutrients in forested
ecosystems.
Exotic pest species
The spread of introduced organisms in new
environements has repeatedly caused severe damage
to other organisms and ecosystems. There are
numerous examples of intentionally and
accidentally introduced species, which developed
into major forest pests. In the eastern
United States, e.g., the Hemlock
woolly adelgid, (Adelges tsugae, HWA)
was introduced from Asia in the 1920's where it is
now mainly found on hemlock (Tsuga canadensis).
This insect has now grown to a major problem
because it kills most trees within 5-15 years. As
a consequence, other species like black birch
invade formerly pure hemlock stands and change
ecosystem functions and community composition. All
tree age classes are affected and needles of
infested trees have a much higher nitrogen content
than uninfested needles. With progressing needle
loss more light is reaching the forest floor.
Higher temperatures are likely to accelerate
litter decomposition. In addition,
leaching from the canopy increases the nitrogen
and carbon input to the forest floor providing better growth conditions for birch saplings.
Currently HWA moves about 30 km to the north in
the eastern North America. The expectation is a strong
change in the composition of the forest community
and likely a change in vital ecosystem functions. So far
there is no means to stop them from moving northward.
Biodiversity
and ecosystem functioning
The local species composition and
dynamics of communities are most likely very
important for a number of ecosystem functions and
services. However, the consequences of an altered
biological diversity for the behaviour of
ecosystems is difficult to evaluate because
informative long term experiments are very
difficult to perform. Different species have
different importance at different periods of time
for the structure of communities and for ecosystem
functions. To identify these key organisms/key
communities and the associated dynamics under
changing environmental conditions is likely to
increase in importance in the future, especially
in the growing field of ecosystem management.
Evolution of myrmecophile-ant
interactions
Antagonistic and mutualistic
interactions between different organisms determine
the structure of communities. Aphids and ants
developed an array of interactions from
non-attendance to obligate myrmecophily. The
selection pressure that shaped this continuum of
associations are largely unknown. Ants often
consume honeydew of aphids, coccids and some
lycaenid larvae and provide service functions,
such as protection from natural enemies, in return.
There are, however, a number of constraints
associated with these interactions for both
partners, and as a consequence, net benefits might
be severely constrained. The strength of these
interactions depends on several factors (i.e.,
host type, predator pressure, habitat type) and is
temporarily and spatially highly dynamic. The costs
and benefits of these interactions are
investigated at different levels of organisation (individual,
colony, community) to better understand the contribution of
each of these scales of observation for the
development of mutualistic relationships.
Life
history strategies of clonal insects
Adaptation to environmental changes is a
necessary prerequisite if organisms are to remain
successful on an evolutionary time scale. The
optimisation approach to life history strategies
is a central theme in the fields of theoretical
ecology and evolutionary biology. In combining
experiments on the reproductive physiology and
behaviour with models we seek to understand the
phenotypic plasticity and trade-offs in key
life-history characters on the fitness
consequences for the respective organisms. Clonal
organisms are especially useful to investigate
such problems, because cause and effects can be
followed across a number of genetically identical
generations.
Aphids are characterised by a "telescoping"
of generations, a mechanism by which
parthenogenetic females not only produce daughters
but also granddaughters at the same time in their
bodies. Therefore, environmental effects might not
only be seen in the current generation but feed
forward to future generations more quickly than in
bisexually reproducing organisms. This mode of
reproduction is very effective to adapt to
changing environmental conditions.
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