I showed in this thesis the potential effects of trophic cascades, grazing disturbance and nutrient loading on the structuring processes (i.e. plant interactions) of submersed plant communities. Hence, it opens a large area of research in order to clarify the potential key-effects of plant interactions for driving plant communities in freshwater ecosystems and how they may be key drivers mediating ecosystem function and services (Smith 2003, Carpenter 2005), particularly under increased eutrophication which is considered as one of the most visible examples of changes to the biosphere (Smith 2003) and one of the most important threats to biodiversity (Thomas et al. 2004, Suding et al. 2005, Hillebrand et al. 2007).
The use of a short eutrophication gradient did not allow detecting any collapse in facilitation under eu-hypertrophic conditions (Chapter IV). Using gradients with higher resolution, i.e. several levels within a stress factor may confirm the occurrence of such a collapse as experimentally demonstrated along both abiotic and biotic gradients in terrestrial
Concluding remarks and future researches
(Kawai and Tokeshi 2007, Levenbach 2009) and freshwater ecosystems (Chapter III). I also illustrated in Chapter IV the opposite effect of indirect facilitation to direct competition among macrophyte to maintain diverse macrophyte communities with increased nutrient loading as reported by Hillebrand et al. (2007). Such a result illustrates the potential role of indirect facilitation for governing the thresholds of regime shifts. Disentangling these two opposite forces of indirect facilitation and competition may clarify how they interact to control submersed plant diversity an eutrophication gradient and how they drive the thresholds of alternative stable states.
The plant community literature mainly focuses on responses of plant interactions to environmental constraints by manipulating the beneficiary species. However, Brooker et al.
(2008) also stressed the importance to focus on neighbouring effects. Particularly, an interesting experiment would consist in the use of several macrophyte species with contrasting effects on nutrient uptakes as neighbour plants and growing alone or in mixture (increased diversity of neighbours). The increase in diversity may efficiently out-compete the phytoplankton and stabilize the system in a clear water state (Engelhardt and Richie 2001, 2002). As a result, indirect facilitation among macrophyte plants may increase with increased diversity of neighbouring plants.
Ultimately, I advocate for a deeper focus on the potential links between indirect facilitation, nutrient enrichments, grazing pressure and the thresholds of regime shifts. An experimental test of the effects of differences in the intensity of indirect facilitation on shifts in alternative states (see Fig 4, Chapter IV) should be an important step for future theoretical developments. Manipulating of intensity of both, indirect facilitation (see above) and fertilisation (in order to mimic eutrophication) may show when and where ecological thresholds of regime shifts will be surpassed. Finally, such an experiment may integrate other factors such as grazing disturbance in order to focus on how their interacting effects with
Zusammenfassung
Zusammenfassung
Untersuchungen zu Interaktionen zwischen Pflanzen sind fundamental wichtig für das Verständnis der Prozesse, die die Struktur und Funktion von Pflanzengemeinschaften und Ökosystemen steuern. Das Auftreten von positiven Interaktionen („Facilitation“) sowie der Einfluss, den die Antwort der Pflanzengemeinschaft auf Veränderungen in der Umwelt haben ist aus terrestrischen und marinen Ökosystemen bekannt, allerdings ist nur wenig darüber bekannt, wie sie submerse Pflanzengemeinschaften (Makrophyten) steuern. Aufgrund ihrer strukturierenden Funktion in Süßwasser-Ökosystemen (z.B. bei der Sedimentation, der Nährstoffdynamik oder der Stabilität von eutrophierten Ökosystemen) ist das Verständnis, wie positive Interaktionen diese Gemeinschaften beeinflussen von grundlegender Bedeutung.
In meiner Dissertation untersuchte ich als erstes im Rahmen eines Review-Artikels das Zusammenspiel von direkten und indirekten positiven Interaktionen entlang von Umwelt- und Grazing-Gradienten bzw. auf die Struktur und Stabilität von Pflanzengemeinschaften in vielen verschiedenen Ökosystemen. Im Folgenden untersuchte ich experimentell die Funktion von indirekten positiven Interaktionen als ein potentieller Mechanismus zur Regulierung von Makrophyten-Gemeinschaften in drei verschiedenen Zusammenhängen: Ich konzentrierte mich auf 1) den potentiellen indirekten positiven Effekt von Fischen auf Makrophyten durch Verminderung der Herbivoren (indirekte positive Interaktion über zwei trophische Ebenen), 2) die mögliche indirekte positive Interaktion zwischen Makrophyten bei steigendem Fraßdruck und 3) mit steigender Eutrophierung. Als Fischart, Herbivore und Makrophyten wählte ich für diese Untersuchungen dominante und weit verbreitete Arten.
Der Review-Artikel legt die Vermutung nahe, dass indirekte und direkte positive Interaktionen insgesamt ein Maximum in Umgebungen mit mittlerem Stress und Störungen verursachen, was unter diesen Umweltbedingungen zu einer sehr diversen
Pflanzengesellschaft führt. Ein Zusammenbruch der positiven Interaktionen in Gemeinschaften mit hohem Stress und vielen Störungen wiederum kann auf verminderte Schutzwirkung der Nachbarn zurückgeführt werden. Dies kann zu einer lokalen Abnahme der Biodiversität, zum Aussterben von Pflanzengemeinschaften und zu Veränderungen im Zustand des Ökosystems führen. Die experimentelle Herangehensweise wiederum hebt das Auftreten von indirekten positiven Effekten in Süßwasser-Ökosystemen, welche den Zustand von Makrophyten (Überleben und Biomasse) positiv beeinflussten, hervor. Die Anwesenheit von Arten mit positivem Einfluss (benefactors) wie Fischen (höhere trophische Ebene) oder Makrophyten (gleiche trophische Ebene) fängt direkte negative Interaktionen (z.B. Pflanzen-Herbivoren oder Pflanzen-Phytoplankton Interaktionen) auf.
Die Erhaltung des Überlebens- und Wachstums-Erfolgs der Pflanzen durch positive Effekte der Nachbarn zeigt die Bedeutung der indirekten positiven Interaktionen für aquatische Pflanzengemeinschaften und wie sie negative biotische (z.B. Fraßdruck) und abiotische Faktoren (z.B. erhöhte Eutrophierung) auffangen können. Darüber hinaus zeigte ich auch, dass der schützende Effekt durch die Nachbarpflanzen unter starken Fraßdruck abnehmen kann und durch den Zusammenbruch der indirekten positiven Interaktionen einen Erfolg der Pflanzen vermindern kann. Die Folgen eines solchen Kollapses sind von großer Bedeutung für die Reaktion der Pflanzengemeinschaften und Ökosysteme auf Umweltveränderungen.
Eine Abnahme des schützenden Effekts der Nachbarpflanzen könnte zu einem Zusammenbruch der gesamten Süßwasser-Vegetation führen, wie es aus anderen Ökosystemen bekannt ist. Demzufolge könnte dies in Süßwasser-Ökosystemen teilweise den Verlust der Klarwasser-Phase, d.h. den Wechsel zu einem trüben, von Phytoplankton dominierten Stadium, erklären.
Summary
Summary
The study of plant interactions is fundamental for understanding the processes involved in driving the structure and functioning of plant communities and ecosystems.
Although the occurrence of facilitation and how it mediates the response of plant communities to environmental change have been reported in terrestrial and marine ecosystems, few is known about its importance for driving submersed aquatic plant (macrophyte) communities.
However, because macrophytes have an important structuring role for freshwater ecosystems (e.g. on sedimentation, nutrient dynamic or stability of ecosystems under eutrophication), it is of primary importance to understand how facilitation may affect these communities. In my thesis, I first explored in a review the interacting role of direct and indirect facilitation occurring along environmental and grazing constraints respectively, on the structure and stability of plant communities in a wide range of ecosystems. Then, I experimentally addressed the role of indirect facilitation as a potential mechanism driving macrophyte communities within three different contexts: I focussed on 1) the potential indirect positive effects of fish on macrophyte plants by decreasing the abundance of herbivores (indirect facilitation across trophic levels), 2) the potential indirect facilitation among macrophytes with increased grazing pressure, and 3) with increased eutrophication. For these approaches, I chose dominant and widely distributed species of fish, herbivores and macrophyte plants.
The review paper suggests that both direct and indirect facilitation interact and produce a peak of facilitation in intermediately stressed and disturbed environments leading to highly diverse communities in these environments. However, a collapse of facilitation in highly stressed and / or disturbed communities occurs due to declined protecting effects of neighbours. This may lead to a local decrease in biodiversity, extinctions of plant communities and changes in ecosystem states. The experimental approaches underlined the
occurrence of indirect facilitation in freshwater ecosystems which positively affected the performances (i.e. both survival and biomass) of macrophytes. The presence of different kinds of neighbouring species (benefactors) such as fish (higher trophic level) or other macrophytes (the same trophic level) buffered direct negative interactions (i.e. herbivore or plant-phytoplankton interactions). The maintenance of plant performance due to protecting neighbouring effects demonstrates the importance of indirect facilitation for structuring aquatic plant communities and how it can buffer the negative effects of both biotic (i.e.
grazing) and abiotic factors (i.e. increased eutrophication). Furthermore, I also showed that the protecting effect of neighbouring plants can decrease under strong grazing pressure resulting in a collapse of indirect positive interactions and thus, a collapse of plant performance. The implications of such a collapse are highly relevant regarding the response of communities and ecosystem functioning to environmental change. A decline in protecting effects of neighbouring plants might contribute to a collapse of vegetation in freshwater ecosystems as reported in other ecosystems and hence may partially explain the loss of clear water states, i.e. the switch to a turbid phytoplankton-dominated state in freshwater ecosystems.
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