Ameisen als Schlüsseltiergruppe in einem Grasland
Studien zu ihrer Bedeutung für die Tiergemeinschaft, das Nahrungsnetz und das Ökosystem
Ants as keystone species in a dry grassland
Studies of their importance for animal community, food web and ecosystem function
by Christian Karl-Johannes Platner
Date of Examination:2004-06-30
Date of issue:2004-10-28
Advisor:Prof. Dr. Matthias Schaefer
Referee:Prof. Dr. Matthias Schaefer
Referee:Prof. Dr. Ulrich Ehlers
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Description:Dissertation
Abstract
English
Ants as eusocial predators living in the soil and ground litter are able to directly or indirectly affect soil organisms, herbivores and plants via trophic interactions ("top-down") and altering the physical and chemical environment ("bottom up"). Mound building ants may emancipate themselves from some environmental influences by creating a modified surrounding. However, even these ecosystem engineers need to select a suitable microclimate out of the range locally available.The aim of this study was the analysis of the structure of these interactions driven by ants. Three components of this engineer-web were investigated descriptively and experimentally:1. Environmental impact and community structureThe effect of spatial structure of environmental factors on ant diversity and abundance, dominance and trophic structure of the soil macrofauna was studied in a grassland-wood-gradient of a xerothermous limestone slope in the "Werratal" near Witzenhausen (Hesse, Germany). Animals were extracted from soil cores taken in a hexagonal 5 m raster including a near-natural mixed stand of beech and pine, different forest edges, some bushes, a dry grassland, a set-aside meadow, a former orchard and a shrub belt (chapter 4).Animal taxa and trophic groups responded differentially to mesoscalic habitat differences: in both types of grasslands densities of Formicidae, Araneida, Auchenorrhyncha, Heteroptera, Aphidoidea and Ortheziidae exceeded those of the wood or shrub overgrown patches. Lithobiidae, Julidae, most detritivore beetles, diptera larvae and Pseudoscorpionida were most abundant in forest-like habitats. Furthermore, direct gradient analysis (RDA) indicated that soil pH, which is positively correlated with the maximum soil temperature, and the overshadow represent major structuring forces for the soil animal community.In a nearby dry grassland on limestone with high density of Lasius flavus mounds the effects of experimentally reduced solar radiation and reduced/increased precipitation on biotic and abiotic soil conditions and on the abundance of soil animal groups were investigated. Radiation was attenuated by dark roofs, which were permeable for rain. Precipitation was reduced by perspex roofs and increased by irrigation (chapter 5). The modification of solar radiation and precipitation strongly affected most groups of soil animals and modified soil microbiological characteristics in the short and long term. The density of detritivores increased with increased water input. Ants and spiders responded negatively to shading, while harvestmen thrived in the shadow but in the long run effects of the structure of the vegetation dominated. Shading resulted in a strong decline in aboveground and belowground plant biomass, diversity of ants, and soil carbon content. Microbial carbon correlated positively with precipitation, soil C and water content. Microclimatic conditions are important structuring forces for ant communities and, since ants are keystone species in many grassland habitats, for the whole animal food web.2. Trophic level of ants and their role in the food webTo investigate the structure of the food web, ratios of stable isotopes of ants, spiders and other invertebrates were analysed for a ranking into trophic levels. Multitrophic level interactions were studied in a laboratory experiment using a plant ? plant lice - ant community (Phaseolus vulgaris, Aphis fabae, Lasius niger) in mesocosms (chapter 6).Species of spiders and ants showed a high overlap and variance of their 15N/14N ratio. Lasius flavus and L. alienus had a lower 15N level than Formica cunicularia because of higher rates of trophobiosis. Mainly predatory ants from the genera Myrmica and Myrmecina were more enriched in 15N. Adults of wandering spiders had a higher 15N/14N ratio compared to juveniles, indicative of a higher trophic level. The ants in the laboratory mesocosms showed a strong dependence from the availability of sugar as a carbon resource.To study the role of predation by ants and spiders in a dry grassland, a manipulative field experiment was conducted using fenced plots. The densities of ants and spiders were manipulated (natural and low density) in a two-factorial design and the resulting treatment effects on arthropods were assessed. The epigeic and hypergeic fauna was sampled on three occasions between June and September 2002 using a D-vac suction sampler or by heat extraction from soil cores (chapter 7). The experiment revealed strong intra-guild interactions between ants and spiders. The abundance of the web-building spiders, especially of Linyphiidae, was influenced negatively by higher densities of ants.However, the density of wandering spiders was nearly unaffected by the presence of ants. The negative effect of ants on the abundance of web-building spiders was strong in June and September resulting in maximum activity of ants on these occasions. In June, the density of web-building spiders was 2,5x higher and in September 3x higher in the plots without ant colonies. Higher densities of wandering spiders negatively affected the population size of Collembola and Auchenorrhyncha. In contrast to this finding, populations of other herbivores seemed to be not affected by spiders and ants. Strong intra-guild interactions of spiders and ants may relax the impact on herbivore populations. The abundance of Ortheziidae and Thysanoptera was strongly positively correlated with the densities of ants showing a strong mutualistic interaction.3. Influence on the soil and effects for the ecosystemTo evaluate the role of ants as ecosystem engineers the effect of yellow ants (Lasius flavus) on nutrient content, microbial activity, soil fauna and soil fertility was studied in a dry grassland on limestone. Nest mound substrate was sampled during spring and summer and was compared to substrate away from mounds in a factorial design (chapter 8).The modification of the microhabitats in the mounds of the subterranean ants strongly affected soil conditions and also soil animals and microorganisms. Nest mound soil had significantly higher available phosphorus and nitrogen levels than the surrounding soil. In the mounds total soil N was significantly increased whereas total soil C was significantly decreased. Substrate moisture and pH did not differ between mounds and controls, but in the mounds the temporal fluctuations in moisture content were considerably more pronounced than in the surrounding soil. Microbial biomass, microbial respiration and nematode density fluctuated, but overall were significantly increased in ant mounds.A laboratory bioassay experiment using different plant species (Arrhenatherum elatius, Galium album, Trifolium repens) in different mixtures growing in microcosms indicated that the distinctive differences in biotic and abiotic soil conditions caused by ants strongly effect plant growth and nutrient leaching.The direct effects of Lasius flavus on soil fauna and plants were investigated by manipulating ant density in a one-year mesocosm experiment. The outdoor mesocosms, which were permeable to ants, were placed in and beside ant hills in a dry grassland on limestone. They contained defaunated sieved soil and plant species from different functional groups including grasses, forbs and legumes (chapter 9).Both, structure of the soil invertebrate community and root growth were modified strongly by a 100-fold difference in ant density in the mesocosms. The densities of spiders and oribatid mites were reduced in the mesocosms incubated inside ant hills, whereas the densities of plant lice and elaterid larvae, total mites, the microbial biomass and biomass of plant roots increased in the high ant density treatments.It is concluded that subterranean ants play an important role as soil engineers and therefore modulate bottom-up forces for the whole food web by altering the resource supply to other species. But they also directly affect above- and belowground populations of different trophic levels including plants, by top-down control via predation, interference and disturbance, and tending species of aphids and coccids.However, comparing to the strong belowground bottom-up impacts the top-down forces driven by ants are relatively weak. Beside trophic interactions the structuring forces are an important point of influence in near-natural terrestrial ecosystems, especially in the soil.
Keywords: ants; spiders; Araneida; planthopper; leafhopper; earthworms; ecology; forest; grassland; microclimate; stable isotops 15N 13C; mites; collembola; Hymenoptera; nest mound soil; intraguild predation; biodiversity; food web; top-down bottom-up; ecosystem engineering
Other Languages
Ameisen können als boden- und streubewohnende
eusoziale Prädatoren durch ihre zentrale Stellung im
Nahrungsnetz das Gefüge der Interaktionen sowohl von
der Basis her durch Modulation der
Ressourcenverfügbarkeit ("bottom-up"-Wirkungen) als
auch von der Spitze her durch Fraßdruck auf ihre
Beutetiere ("top-down"-Kontrolle) beeinflussen.
Hügelbauende Arten können sich zu einem gewissen Maße
von den Umweltbedingungen emanzipieren, indem sie sich
eine günstigere Umgebung selbst gestalten; doch selbst
solche Ökosystem-Ingenieure brauchen bestimmte
mikroklimatische Bedingungen. Drei Bereiche dieser
Funktion von Ameisen als Schlüsseltierarten im
Grasland-Ökosystem wurden deskriptiv und experimentell
untersucht:Auf einem Halbtrockenrasen wurden die
Verteilungsmuster der Ameisenarten und weiterer
wichtiger Bodentiere auf einem Wald-Wiese-Gradienten in
Abhängigkeit von den jeweiligen Umweltbedingungen
raumbezogen erfasst und mit multivariaten Analyse- und
Ordinationsverfahren ausgewertet. Ameisen, Spinnen und
Schnabelkerfe hatten ihre höchsten Dichten auf der
Glatthaferwiese. Steinläufer, Schnurfüßer, saprophage
Käferlarven und Pseudoskorpione waren hingegen in den
Waldbereichen deutlich häufiger.Für Ameisen waren die maximale Bodentemperatur und
die Beschattung die wichtigsten verteilungsprägenden
Umweltparameter. Der Einfluss des Mikroklimas auf die
Ameisen und die Bodentiergemeinschaft wurde durch
experimentelle Manipulation der Faktoren
Sonneneinstrahlung und Niederschlag untersucht. Ameisen
und Schnabelkerfe hatten in den beschatteten Varianten
deutlich niedrigere Dichten, für Spinnen und
Weberknechte spielte die Habitatstruktur eine große
Rolle, Saprophage waren auf den beregneten Parzellen
signifikant häufiger. Durch Analyse der natürlichen
Gehalte der stabilen Isotope 13C und 15N wurden
verschiedene dominante Ameisenarten in trophische
Ebenen des Nahrungsnetzes eines Halbtrockenrasens
eingeordnet.Im Frühjahr schient die Bodenfauna eine wichtige
Rolle als Nahrungsgrundlage für die Prädatoren zu
spielen, Ameisen wechseln im Sommer verstärkt auf
Honigtau der Blattläuse um; im Laborversuch hatte
Zuckerzufütterung die deutlichsten positiven
Auswirkungen auf die Ameisen. Durch ein faktorielles
Ausschlussexperiment von Ameisen und Spinnen wurde die
Funktion wichtiger Prädatoren im Nahrungsnetz
erforscht. Vagante Spinnen und Ameisen reduzieren v.a.
die Dichte der netzbauenden Spinnen. Neben der starken
Intragildenprädation wurden nur schwache
top-down-Effekte nachgewiesen.Der bottom-up Einfluss von Ameisen durch
Bodenveränderungen in der Rhizosphäre auf die
Diversität der übrigen Bodenfauna und die
Wuchsleistungen verschiedener Pflanzenarten wurde durch
Untersuchung der biotischen und abiotischen Parameter
und mit einem Mikrokosmosexperiment im Labor genauer
analysiert. Die Nährstoffverhältnisse und die
Bedingungen für die Mikroflora waren in den Nesthügeln
signifikant günstiger als auf der umgebenden Wiese, was
auch in defaunierter Erde im Labor zu besserer
Pflanzenentwicklung in Erde aus den Hügelnestern
führte.Die Bedeutung der trophischen top-down Effekte über
Herbivore in Relation zu den bottom-up Wirkungen durch
?ecosystem engineering? wurde durch ein
Mikrokosmosexperiment im Freiland überprüft. Direkte
Top-down-Effekte sind eher selten, allerdings
beeinflusst die Habitatmodifikation durch die Ameisen
signifikant das Bodennahrungsnetz und damit auch
wichtige Ökosystemprozesse wie die
Stickstoffmineralisation.Aus den Untersuchungen wird gefolgert, dass Ameisen
eine wichtige Funktion als Ökosystemingenieure
übernehmen und verglichen mit den deutlichen
bottom-up-Wirkungen nur eine relativ schwache
top-down-Kontrolle auf ihre Beutetiere ausüben. Neben
dem Einfluss auf die Ressourcenverfügbarkeit für das
ganze Nahrungsnetz beeinflussen Ameisen die übrigen
Tiere direkt durch Interferenz und Störung und durch
die Trophobiose mit Pflanzenläusen. Anhand des
Ingenieursnetzes mit den Wechselwirkungen der Ameisen
zu ihrer Umwelt wird deutlich, dass neben den
trophischen Beziehungen Struktur-schaffende Wirkungen,
besonders im Boden, mit ihren Interaktionen eine
wesentliche Funktion in naturnahen terrestrischen
Ökosystemen haben.
Schlagwörter: Ameisen; Formicidae; Ökosystem; Wiese; Wald; Boden; Spinnen; Zikaden; Regenwürmer; Mikroklima; Nahrungsnetzanalyse mit stabilen Isotopen 15N 13C; Multitrophische Interaktionen