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Pflanzenökologie
& Systematik
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Species interactions in a changing world: Herbivory and ecosystem engineering drive ecosystem functioning in anthropogenic landscapes
Research interests | Curriculum vitae | Publications I am interested in the ways species
interactions are contributing to
ecosystem functioning and how these mechanisms are qualitatively and/or
quantitatively modified by anthropogenic influences. We use
leaf-cutting ants
of the genus Atta as model
organisms
for our investigations that are conduced partly in the Brazilian
Atlantic Forest
(cooperation with Prof. Dr. Marcelo Tabarelli and Prof. Dr. Inara Lea,
Both
Federal University of Pernamuco, Recife, Brazil) and partly in the
laboratory
at the University of Kaiserslautern (lab group of Dr. Rainer Wirth).
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Ecosystem effects of leaf-cutting ants in
fragmented forests Fragmentation of habitats, especially of
tropical rainforests, ranks globally among the most pervasive man-made
disturbances of ecosystems. There is growing evidence for long-term
effects of forest fragmentation and the accompanying creation of
artificial edges on ecosystem functioning and forest structure, which
are altered in a way that generally transforms these forests into early
successional systems. Edge-induced disruption of species interactions
can be among the driving mechanisms governing this transformation.
These species interactions can be direct (trophic interactions,
competition, etc.) or indirect (modification of the resource
availability for other organisms). Such indirect interactions are
called ecosystem engineering. Leaf-cutting ants of the genus Atta are dominant herbivores and
keystone-species in the Neotropics and have been called ecosystem
engineers. In contrast to other prominent ecosystem engineers that have
been substantially decimated by human activities some species of
leaf-cutting ants profit from anthropogenic landscape alterations.
Thus, leaf-cutting ants are a highly suitable model to investigate the
potentially cascading effects caused by herbivores and ecosystem
engineers in modern anthropogenic landscapes following fragmentation.
The present thesis aims to describe this interplay between consequences
of forest fragmentation for leaf-cutting ants and resulting impacts of
leaf-cutting ants in fragmented forests.
This research project started out with a review of 55 published articles
demonstrating that herbivores, especially generalists, profoundly
benefit from forest edges, often due to (1) favourable
microenvironmental conditions, (2) an edge-induced increase in food
quantity/quality, and (3; less well documented) disrupted top-down
regulation of herbivores (Wirth, Meyer et al. 2008; Progress in Botany
69:423-448). Field investigations in the heavily fragmented
Atlantic Forest of Northeast Brazil (Coimbra forest) were subsequently
carried out to evaluate patterns and hypotheses emerging from this
review using leaf-cutting ants of the genus Atta
as a model system. Colony densities of both Atta
species occuring in the area changed similarly with distance to the
edge but the magnitude of the effect was species-specific. Colony
density of A. cephalotes
was low in the forest interior (0.33 ± 1.11 /ha, pooling all
zones >50 m into the forest) and sharply increased by a
factor of about 8.5 towards the first 50 m (2.79 ±
3.3 /ha), while A. sexdens
was more uniformly distributed (Wirth, Meyer et al. 2007; Journal of
Tropical Ecology 23:501-505). The accumulation of Atta
colonies persisted at physically stable forest edges over a four-year
interval with no significant difference in densities between years
despite high rates of colony turnover (little less than 50% in 4
years). Stable hyper-abundant populations of leaf-cutting ants accord
with the constantly high availability of pioneer plants (their
preferred food source) as previously demonstrated at old stabilised
forest edges in the region (Meyer et al. submitted). In light of the hyper-abundance of leaf-cutting
ants within the forest edge zone (first 50 m), their
potentially far-reaching ecological importance in anthropogenic
landscapes is apparent. Based on previous colony-level estimates, we
extrapolated that herbivory by A. cephalotes
removes 36% of the available foliage at forest edges (compared to 6% in
the forest interior). In
addition, A. cephalotes acted as ecosystem engineers constructing large nests (on average 55 m2:
95%-CI: 22-136) that drastically altered forest structure. The ants
opened gaps in the canopy and forest understory at
nest sites, which allowed three times as much light to reach the nest
surface as compared to the forest understory. This was accompanied by an increase in soil temperatures and a
reduction in water availability. Modifications of microclimate and forest
structure greatly surpassed previously published estimates. Since
higher light levels were detectable up to about 4 m away from
the nest edge, an area roughly four times as big as the actual nest
(about 200 and 50 m2, respectively) was impacted
by every colony, amounting to roughly 6% of the total area at the
forest edge (Meyer et al. submitted). The hypothesized impacts of high cutting
pressure and microclimatic alterations at nest sites on forest
regeneration were directly tested using transplanted seedlings of six
species of forest trees. Nests of A. cephalotes
differentially impacted survival and growth of seedlings. Survival
differed highly significantly between habitats and species and was
generally high in the forest, yet low on nests where it correlated
strongly with seed size of the species. These results indicate that the
disturbance regime created by leaf-cutting ants differs from other
disturbances, since nest conditions select for plant species that
profit from additional light, yet are large-seeded and have resprouting
abilities, which are best suited to tolerate repeated defoliation on a
nest (Meyer et al. in preparation).
On an ecosystem scale leaf-cutting ants might
amplify edge-driven microclimatic alterations by very high rates of
herbivory and the maintenance of canopy gaps above frequent nests. By
allowing for an increased light penetration Atta
may, ultimately, contribute to a dominating, self-replacing pioneer
communities at forest edges, possibly creating a positive feed-back
loop. Based on the persisting hyper-abundance of
leaf-cutting ants at old edges of Coimbra forest and the multifarious
impacts documented, we conclude that the ecological importance of leaf-cutting
ants in pristine forests, where they are commonly believed to be
keystone species despite very low colony densities, is greatly
surpassed in anthropogenic landscapes
In fragmented forests, Atta
has been identified as an
essential component of a disturbance regime that causes a
post-fragmentation retrogressive succession. Apparently, these
forests have reached a new self-replacing secondary state. I suggest
additional human interference in form of thoughtful management in order
to break this cycle of self-enhancing disturbance and to enable forest
regeneration along the edges of threatened forest remnants. Thereby the
situation of the forest as a whole can be ameliorated and the chances
for a long-term retention of biodiversity in these landscapes increased. |
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Feeding preference of leaf-cutting ants for drought stressed food plants Despite centuries of research on food plant
selection of leaf-cutting ants it is rather unknown how drought stress,
acting on the ant's food plants, affects their host plant selection.
Published data on interactions of leaf water content and host plant
selection are inconsistent. Yet a discrimination between differentially
stressed plants can potentially explain some observed herbivory
patterns and a preference for stressed plants is of special interest in
the context of forest fragmentation (edge creation) and changing
climatic conditions with more frequent and often more severe dry spells. By employing two types of bioassays we showed that drought stressed plants are attractive to leaf-cutting ants of the species Atta colombica. In experiments with whole plants of the species Piper marginatum the amount of leaf material harvested of stressed plants was doubled compared to vigorous control plants. In pick up assay were 80% of the collected leaf disks those from stressed plants. There is evidence that the preference for stressed plants is caused by an increase in osmolyte concentrations. In leaves of stressed plants proline and non structural carbohydrates have been shown to increase by an factor of 3,5 and 2 respectively. The results also provide some explanation for intra-plant patchiness of leaf-cutting ant herbivory, because the ants showed a preferences for stressed leaves within an individual plant additionally to the intraspecific one. Samples taken from favoured leaves contained more non structural carbohydrates and more proline than those representing the whole plant. The attractiveness of a leaf caused by its stress level was influenced by the other leaves within a plant. This was shown by an analysis of correlations between individual leaf attractiveness and leaf water content (as an indicator for the stress level). Our results support the Plant Stress Hypothesis by White (1984), which predicts that stressed plants are more attractive to herbivores. The host plant selection of the ants has potentially a huge impact on their food plant populations, because it is selecting against drought-sensitive individuals.
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Set up of the bioassays with whole plants. The stressed plant is offered on the left hand
side, the vigorous one on the right. Ants coming from the colony in the
background enter the arena in the centre. |
Set up of the pick up assays. Photos were taken in regular time intervals to
assess the leaf disk removal by the ants. |
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Recently these experiments have been repeated
with free living colonies of the leaf-cutting ant species Atta cephalotes and Atta
sexdens in the Brazilian Atlantic forest using a set of
different plant species. This research conducted as part of the Diploma
thesis of Meike Passlack
(University of Kaiserslautern) and the Master thesis of José
Domingos Ribeiro Neto (Federal University of Pernambuco, Recife,
Brazil) confirms the strong preference of leaf-cutting ants for drought
stressed food plants and allows to generalize the documented mechanisms
to a whole range of leaf-cutting ant and food plant species. |
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Research
Awards
Reviewer for the Journal of Applied Entomology (1) and Journal of Applied Ecology (1) |
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Büdel B, Meyer ST, Reichenberger R (in
preparation) Photosynthetic plasticity in water relations of free
living and lichenized cyanobacteria (Croococcidiopsis sp.) depends on thallus morphology. Meyer ST, Leal IR, Tabarelli M & Wirth
R (in preparation) Plant recruitment on and around nests of Atta cephalotes: Ecological filters in a
fragmented forest. Meyer ST, Leal IR, Tabarelli M & Wirth
R (submitted) Ecosystem engineering in a fragmented forest: Altered
forest structure and microclimate at nest sites of Atta
cephalotes. Meyer ST, Leal IR & Wirth R (submitted)
Persisting hyper-abundance of keystone herbivores (Atta
spp.) at the edge of an old Brazilian Atlantic Forest fragment. Wirth R, Meyer ST, Leal IR & Tabarelli
M (2008) Plant-herbivore interactions at the forest edge. Progress in
Botany 69, 423-448. Wirth R, Meyer ST, Almeida WR,
Araújo Jr. MV, Barbosa VS & Leal IR (2007)
Increasing densities of leaf-cutting ants (Atta spp.) with proximity to
the edge in a Brazilian Atlantic forest. Journal of Tropical Ecology 23, 501-505, pdf. Meyer ST, Roces F & Wirth R (2006)
Selecting the drought stressed: Effects of plant stress on
intraspecific and within-plant herbivory patterns of the leaf-cutting
ant Atta colombica. Functional Ecology 20,
973-981 pdf. Talks at scientific meetings Meyer ST, Leal IR, Tabarelli M & Wirth
R (2008) Hyper-abundant ecosystem engineer (Atta cephalotes) impacts
microclimate and plant communities at the forest edge. Annual meeting
of the Association for Tropical Biology (ATBC). Paramaribo, Suriname. Leal IR, Wirth R, Meyer ST & Tabarelli
M (2008) Edge creation promotes proliferation of herbivores. Annual
meeting of the Association for Tropical Biology (ATBC). Paramaribo,
Suriname. Meyer ST, Leal IR, Tabarelli M & Wirth
R (2007) Leaf-cutting ants as ecosystem engineers: Microclimatic
gradients in the vicinity of Atta cephalotes nests. Annual meeting of
the Association for Tropical Biology (ATBC). Morelia, Mexico. Leal IR, Wirth R, Meyer ST, Tabarelli M (2007)
Proliferação de herbívoros em bordas
de florestas. VIII Congresso de Ecologia do Brasil, Caxambu,
Brazil Meyer ST, Roces F & Wirth R (2005)
Selecting the drought stressed: Effects of plant stress on
intraspecific and within-plant herbivory patterns of leaf-cutting ants.
Annual meeting of the Association for Tropical Biology (ATBC). Meyer ST, Roces F & Wirth R (2005)
Selecting the drought stressed: Effects of plant stress on
intraspecific and intraindividual herbivory patterns of leaf-cutting
ants. 18th annual meeting of Society for Tropical Ecology
(GTÖ). Posters at scientific meetings Jürgens S, Meyer ST, Leal IR, Wirth R
(2007) Uncluttered leafcutters: Atta cephalotes promotes canopy and
understory gaps. 20th annual GTÖ meeting, Bonn, Germany Passlack M, Meyer ST, Leal IR, Wirth R (2007)
Cutting the drought stressed: Food plant preference of Atta cephalotes.
20th annual GTÖ meeting, Bonn, Germany Meyer ST, Dohm C, Almeida WR ,
Araújo Jr. MV, Wirth R & Leal IR (2006)
Nidificando na matriz, mas forrageando na floresta:
Distribuição de colônias de formigas
cortadeiras em floresta Atlântica nordestina. XXI Congresso
Brasiliero de Entomologia, Recife, Brazil Schwarzkopf M, Hartard B, Meyer ST, Lakatos M
(2006) Tropical lichens as microclimatic indicator? 19th annual GTÖ meeting, Dohm C, Meyer ST, Leal IR, Wirth R (2006)
Living outside but cutting within: high colony densities of
leaf-cutting ants in front of the forest edge. 19th annual GTÖ
meeting, Meyer ST & Engelbrecht BMJ (2003) The
combined impacts of clipping an drought on seedling mortality for two
tropical tree species. Special Symposium of the BES and Annual Meeting
of the ATBC, Meyer ST, Hartard B, Herz H & Berger J
(2003) Effect of tannin and total phenols on the feeding preference of
a spezialized herbivore. Jahrestagung der Gesellschaft für
Tropenökologie, Rostock, Germany. |
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