The principal aim of the study which was conducted between May 1995 and May 1997 was to collect and analyze information that would be considered vital to any future management actions that may be applied to the deer living in the wild in the Bali Barat National Park ecosystem in Indonesia. The systems approach used sought to analyze the complex interactions between the soil, plant, animal and human activity subsystems. In particular, interaction between Rusa deer and Muntjac deer was compared where possible, although the principal focus of the study was on the population of Muntjac deer. The soils in habitats frequented by deer in Bali Barat National Park were found to be of relatively low fertility status. Chemical analysis of the soil revealed that all of the mineral element contents considered in this study were in the lowest range for soils, in general. During this study the population of Muntjac deer in the Bali Barat National Park was submitted to phylogenetic analysis to determine whether the Bali population is distinct. Preliminary results indicate that these deer are apart of a diverse, but monophyletic group of Muntiacus Muntjac. The potential unique status of Muntjac deer in Bali Barat National Park, and the need to preserve them as part of the natural resource base that constitutes the Indonesian archipelago increased the importance of this study of the ecosystem and social system surrounding Bali Barat National Park. / Doctor of Philosophy (PhD)

Most studies of wetlands tend to focus on the biotic and abiotic interactions within the aquatic habitat. Though wetlands and associated biota may appear to be somewhat isolated from the influence of the wider landscape, wetland habitats are critically linked with adjacent terrestrial habitats and other wetlands through the two-way flows of energy and nutrients and provision of structure. While an understanding of these inter-habitat linkages is breaking down the perceived boundaries between &quotaquatic" and &quotterrestrial" ecosystems, there is more limited knowledge on the ecology of wetland animals that must meet critical needs in both aquatic and terrestrial habitats at some time during their life or seasonal cycles. Here, I examine the terrestrial ecology of a freshwater turtle, the eastern long-necked turtle (Chelodina longicollis) in the temporally dynamic and heterogeneous landscape of Booderee national park in south-east Australia by 1) providing a description of terrestrial behaviours, 2) identifying the factors driving terrestrial behaviour and its functional significance, 3) examining factors that may limit or constrain terrestrial behaviour and 4) demonstrating how various terrestrial behaviours can factor prominently in the overall biology of a nominally aquatic animal. Chelodina longicollis used terrestrial habitats for reasons other than nesting, including aestivation and movements between wetlands. Radio-telemetry of 60 turtles revealed that nearly 25 % of all locations were in terrestrial habitats up to 505 m from the wetland, where turtles remained for extended periods (up to 480 consecutive days) buried under sand and leaf litter in the forest. Individuals also maintained an association with a permanent lake and at least one temporary wetland within 1470 m, though some inter-wetland dispersal movements were much longer (5248 m). As a result of their associations with several wetlands and terrestrial aestivation sites, C. longicollis traversed large areas and long distances (13.8 +/- 2.8 ha home range, 2608 +/- 305 m moved), indicating that this species is highly vagile. In fact, a three-year capture-mark-recapture study conducted in 25 wetlands revealed that 33% of the population moved overland between wetlands. After scaling this rate to the number of generations elapsed during the study, C. longicollis moved between discrete water bodies at a rate of 88-132% per generation. This rate is not only high for freshwater turtles, but is among the highest rates of inter-patch movement for any vertebrate or invertebrate. Chelodina longicollis demonstrated an impressive capacity for individual variation in nearly every aspect of its behaviour examined. Most of the variation in space use, movements, terrestrial aestivation and activity could be attributed to extrinsic local and landscape factors, seasonal influences and rainfall, whereas intrinsic attributes of the individual such as sex, body size, body condition and maturity status were less important. Turtles increased movement distance and home range size in regions where inter-wetland distances were farther and with increasing wetland size. Individuals spent more time in terrestrial habitats with decreasing wetland hydroperiod and increasing distance to the nearest permanent lake. Overland movements between wetlands were correlated with rainfall, but the directionality of these movements and the frequency with which they occurred varied according to the prevalent rainfall patterns; movements were to permanent lakes during drought, but turtles returned to temporary wetlands en masse upon the return of heavy rainfall. However, deteriorating conditions in drying wetlands forced turtles to move even in the absence of rainfall. Captures at a terrestrial drift fence revealed that immature turtles as small as 72.3 mm plastron length may move overland between wetlands with similar frequency as larger adults. Taken together, these results suggest that C. longicollis behaviour is in part conditional or state-dependent (i.e., plastic) and shaped by the spatiotemporal variation and heterogeneity of the landscape. Perhaps the most surprising aspect of individual variation was the alternate responses to wetland drying. Turtles either aestivated in terrestrial habitats (for variable lengths of time), or moved to other wetlands. Movement to other wetlands was the near universal strategy when only a short distance from permanent lakes, but the proportion of individuals that aestivated terrestrially increased with distance to the nearest permanent lake. When long distances must be travelled, both behaviours were employed by turtles in the same wetland, suggesting that individuals differentially weigh the costs and benefits of residing terrestrially versus those of long-distance movement. I propose that diversity in response to wetland drying in the population is maintained by stochastic fluctuations in resource quality. The quality of temporary wetlands relative to permanent wetlands at our study site varies considerably and unpredictably with annual rainfall and with it the cost-benefit ratio of each strategy or tactic. Residency in or near temporary wetlands is more successful during wet periods due to production benefits (high growth, reproduction and increased body condition), but movement to permanent wetlands is more successful, or least costly, during dry periods due to the fitness benefits of increased survival and body condition. I used the doubly-labelled water (DLW) method to provide the first estimates of water and energy costs of aestivation and overland movement for any freshwater turtle behaving naturally in the field. Chelodina longicollis remained hydrated while terrestrial with water flux rates (14.3-19.3 ml kg-1 d-1) on par with those of strictly terrestrial turtles, but field metabolic rate during aestivation (20.0-24.6 kJ kg-1 d-1) did not indicate substantial physiological specializations in metabolism during aestivation. Energy reserves, but not water, are predicted to limit survival in aestivation to an estimated 49-261 days, which is in close agreement with the durations of natural aestivation. The energy costs of overland movement were 46-99 kJ (kg d)-1, or 1.6-1.7 times more expensive than aestivation. When a wetland dries, a turtle that foregoes movement to other wetlands can free sufficient energy to fuel up to 134 days in aestivation. The increasing value of this energy &quottrade-off" with travel distance fits our behavioural observations of variance in response to wetland drying. Taken together, this evidence indicates that terrestrial habitats provide more than just organic and structural inputs and filtering services and that nearby wetlands are important for reasons other than potential sources of occasional colonists to a population. Terrestrial habitats are used for aestivation in response to wetland drying and different wetlands are diverse in their functions of meeting the annual or life-cycle requirements of C. longicollis in temporally dynamic wetland systems. As overland movements between these various habitat types are in response to spatiotemporal variation in habitat quality and associated shifts in the fitness gradient between them, I suggest that terrestrial and different aquatic habitats in Booderee offer complementary resources contributing to regional carrying capacity and population persistence of the turtle population. Thus, important ecological processes regulating C. longicollis in a focal wetland should not be viewed as operating independently of other nearby wetlands and their adjacent terrestrial habitats. Collectively, these findings highlight the complex and dynamic associations between a population of freshwater turtles and the wider terrestrial and aquatic landscape, demonstrating that turtle populations and the factors that impact them can extend well beyond the boundaries of a focal wetland.

Thesis (M.S.) -- Mississippi State University. Department of Biological Sciences. / Title from title screen. Includes bibliographical references.

<p>The work in this thesis deals with effects of changed species richness on process rates among stream-living macroinvertebrates. Global biodiversity is decreasing rapidly and it is poorly known what the consequences of this loss may be for ecosystems and the services they provide. Hence, it is important to investigate the potential effects of losing species. In streams, deforestation, introduction of non-native species, pollution and channelization are examples of events that may affect species richness negatively. In this thesis emphasis is on changes in species richness within functional feeding groups (FFGs) of stream-living macroinvertebrates. The FFGs used were shredding detritivores, grazers, filter feeders and predators - all of which uphold important ecological processes in streams. Along with an observational field study, species richness was manipulated in laboratory and field experiments to investigate the effects of changed species richness on process rates and thus ecosystem functioning.</p><p>The results show that effects of changed species richness on process rates may be dramatic. Among the shredding detritivores there were negative effects on leaf mass loss, regardless whether fixed, random or predicted sequences of species loss was investigated. These effects could be attributed to either species richness per se or species composition. However, among the other FFGs the relationship between species richness and process rates was less consistent. In filter feeders, there was no or a negative effect of decreasing species richness while both grazers and predators showed positive effects of species loss.</p><p>The results also show that the most important interactions between species in an experiment, thus potentially in a natural community, are likely to determine what the effect of species loss on process rates will be. Facilitation and niche differentiation lead to reduced process rates if species are lost, while mechanisms, such as interspecific resource or interference competition, produce the opposite effect. Furthermore, in systems with a diminishing resource, the first two mechanisms may become more important over time enhancing the effect of species loss in the long term.</p><p>In conclusion, effects of species loss may be dramatically negative or positive even if lost species are classified as redundant. The effect in the short term most likely depends on which species are lost, on the original species composition and on the underlying mechanisms. Questions remaining to be answered are how important the observed effects are in more complex systems and if they are persistent over time? Future studies will tell. </p>

Résumé Ce travail est un premier pas dans l’étude de la qualité des eaux du fleuve Niger à Niamey. Il est basé sur la physico-chimie et les macroinvertébrés de cet écosystème aquatique. L’approche physico-chimique met en évidence les changements de la qualité des eaux, le long des rives, en relation avec les rejets de la ville et en fonction du régime du fleuve. L’impact de ces rejets est très négligeable à l’échelle de l’écosystème à cause de la forte dilution. La demande chimique en oxygène, l’oxygène dissous, l’ammonium, les orthophosphates et le phosphore total sont les principaux facteurs environnementaux qui déterminent la pollution des eaux liée aux matières organiques et oxydables, aux matières azotées et phosphorées dans ce fleuve. L’étude des macroinvertébrés fait ressortir 83 taxons répartis le long du fleuve Niger à Niamey. La description des peuplements de macroinvertébrés montre une variation de structure des communautés, en passant de l’amont à l’aval des points de rejets, caractérisée par une baisse de la richesse et de la diversité taxonomique en aval immédiat des rejets. La présence, dans certaines stations en aval immédiat des rejets, des taxons tels que Chironomus gr. plumosus, Melania sp., les familles des Syrphidae et des Culicidae, est une indication de la dégradation de la qualité des eaux liée à la matière organique. Par contre, les taxons comme Thraulus sp., Elassoneuria sp., Afronurus sp., Centroptiloides sp., Adenophlebia sp., Dipseudopsis sp. et Neoperla sp., appartenant aux Ephéméroptères, Trichoptères et Plécoptères, reflètent une meilleure qualité de l’eau dans les stations de référence situées en amont de tous les points de rejets. L’importante diversité de la famille des Chironomidae (20 taxons) dans ce cours d’eau offre de bonnes perspectives pour son utilisation dans l’étude de la qualité des eaux du fleuve Niger à Niamey. Les descripteurs qui expliquent significativement la distribution des communautés de macroinvertébrés le long du fleuve Niger à Niamey sont la matière organique (demande chimique en oxygène) et les nutriments (ammonium, orthophosphates et phosphore total). La technique d’échantillonnage au filet troubleau explique beaucoup plus d’inertie que les substrats artificiels. Quant à la jacinthe d’eau, elle présente peu d’intérêt dans l’étude de la qualité des eaux du fleuve Niger à Niamey à cause de sa présence uniquement en période de crue. Summary This survey is a first step in the study of water quality of the River Niger in Niamey. It is based on physicochemical factors and the macroinvertebrates inhabiting this aquatic ecosystem. The physicochemical approach underlines the changes in water quality, along the banks, in relation with wastewater from the city of Niamey, and as a response to seasonal variations of water level. The impact of wastewater on the scale of the entire ecosystem appears very limited, because of the strong dilution. Chemical oxygen demand, dissolved oxygen, ammonium, phosphates and total phosphorus levels are the main environmental factors that determine both the pollution due to organic (and oxidizable) matter, and the pollution resulting from nitrogen and phosphorus-based chemicals in this river. In this survey 83 taxa of macroinvertebrates were found along the River Niger in Niamey. The description of macroinvertebrates shows a variation of community structure, from upstream to downstream, that is characterized by a decrease of taxonomic richness and diversity downstream wastewater discharges. The presence of Chironomus gr. plumosus, Melania sp., families of Syrphidae and Culicidae indicates a deterioration of water quality due to organic matter in some stations near wastewater discharges. However, taxa like Thraulus sp., Elassoneuria sp., Afronurus sp., Centroptiloides sp., Adenophlebia sp., Dipseudopsis sp. and Neoperla sp. reflect a better quality of water in the reference stations located upstream of all wastewater discharges. The high diversity of Chironomidae (20 taxa) in this river makes them a good candidate taxonomic group to be used for the assessment of water quality in the River Niger near Niamey. The factors which significantly explain the distribution of macroinvertebrates communities along the River Niger in Niamey are organic matter (chemical oxygen demand) and nutrients (ammonium, phosphates, total phosphorus). Results from net hand samples explain a higher fraction of total inertia than the ones obtained using artificial substrata. Water hyacinths play a limited role in the survey of water quality in the Niger near Niamey, because they are only present during floodperiods.

Im Rahmen der Untersuchung zur Entwicklung der pelagischen Gemeinschaft des ehemals sauren Tagebauseenkomplexes Goitsche (pH~3) während dessen Flutung und Neutralisierung wurden Wechselwirkungen zwischen der Zusammensetzung von Organismengemeinschaften und der Variabilität des abiotischen Umfeldes untersucht.Im Mittelpunkt standen zwei von ihrer Kausalität her unterschiedliche Aspekte: • Der erste Aspekt betraf die Reifung von Ökosystemen: War der Reifungsprozess von pelagischen Gemeinschaften anhand der von Odum (1969) formulierten Kriterien zur Energetik der Gemeinschaft, zu den Nährstoffkreisläufen sowie zu strukturellen Merkmalen auf Ökosystem- und Individuenebene zu erfassen? Führten der physiologische Stress durch den niedrigen pH und physikalische Störungen der Schichtung durch das einströmende Flutungswasser zu einer Umkehr des Reifungsprozesses? Auf welchen Organisationsebenen der Lebensgemeinschaften waren die Auswirkungen dieser Stressoren erkennbar? • Der zweite Aspekt behandelte die Entwicklung der Artenzahl, die Gleichverteilung der Dominanz von Arten (Eveness) und die Diversität von Planktongemeinschaften entlang des Produktivitätsgradienten. Speziell wurde untersucht, ob die Artenzahl und die Diversität eine monoton positive oder eine unimodale Funktion der Produktivität waren und ob die Eveness eine monoton abnehmende Funktion der Produktivität war. Zur besseren Vorhersagbarkeit der Entwicklung dieser Indizes wurden in einem nächsten Schritt zusätzliche biotische und abiotische Faktoren (z.B. Konsumenteneffekte, physikalische Störung, Immigration) berücksichtigt. Zuletzt wurde die Hypothese getestet, dass unter dem Einfluss von extremem physiologischem Stress keine Abhängigkeit zwischen den betrachteten Indizes und der Produktivität von Ökosystemen besteht. Die Untersuchungen der vorliegenden Arbeit führten zu folgenden Ergebnissen und Schlussfolgerungen: 1) Der Reifungsprozess der Planktongemeinschaft war unter neutralen Bedingungen nicht eindeutig an einzelnen Kriterien festzumachen. Vielmehr schienen idiosynkratische Effekte einzelner Arten auf die Zusammensetzung und Funktion der Organismengemeinschaft von Bedeutung. Coenobiumbildende Kieselalgen sowie größere Cladoceren und Copepoden dominierten sehr rasch die Planktongemeinschaft und vermittelten den Eindruck eines reifen Ökosystems fast unmittelbar nach der Neutralisierung des Tagebausees. 2) Der Einfluss von physiologischem Säurestress und physikalischer Störung der Schichtung durch das eintretende Flutungswasser war gegenüber einem neutralen, ungestörten Teilbecken des Tagebausees (Referenzzustand) eindeutig zu erkennen. Die isolierte Betrachtung der Wirkung der Stressoren lieferte hinsichtlich fast aller Kriterien Anzeichen einer Verjüngung des Systems sensu Odum (1969, 1985). 3) Im betrachteten Ökosystem existierte eine Hierarchie innerhalb der Stressoren. Der Einfluss des Säurestresses dominierte gegenüber dem Einfluss der physikalischen Störung, wahrscheinlich indem er die Reaktionsmöglichkeiten der Planktongemeinschaft einschränkte. 4) Für Primärproduzenten war die Artenzahl eine monoton positive Funktion der realisierten Biomasse (einem Surrogatparameter für die Produktivität des Systems). Die Eveness war eine monoton negative Funktion der Produktivität. Die beobachtete unimodale Beziehung zwischen der Diversität und der Produktivität der Primärproduzenten muss als eine Folge der mathematischen Formulierung dieser Indizes betrachtet werden. 5) Die Ergebnisse multivariater Modelle zur Vorhersage der Artenzahl und der Eveness der Primärproduzenten in Abhängigkeit zusätzlicher erfassbarer biotischer und abiotischer Faktoren ermöglichten eine differenziertere Betrachtung der Ergebnisse: • Im Tagebausee Goitsche war die Eveness hauptsächlich von dichteabhängigen Prozessen gesteuert (negative Abhängigkeit zum Quadrat der Biomassen und zu den Lichtverhältnissen). • Die Entwicklung der Artenzahl war neben dem primären Einfluss der zunehmenden Biomasse auch durch qualitative und quantitative Aspekte der Konsumentengemeinschaft (Diversität und Biomasse des Zooplanktons) beeinflusst. Der Einfluss einer erhöhten Immigration auf die Artenzahl wurde nur zu Beginn der Flutung des Tagebausees beobachtet. 6) Auf Ebene der Konsumenten war die einzige eindeutig feststellbare Abhängigkeit ein Anstieg der Artenzahl mit steigender Biomasse. Das Fehlen von weiteren Beziehungen zwischen Diversitätsindizes und dem Produktivitätsgradienten wird darauf zurückgeführt, dass auf den unteren trophischen Ebenen der Primärproduzenten Ressourceneffekte (Bottom-Up) stärker ausgeprägt sind, wohingegen auf höheren trophischen Ebenen Konsumenteneffekte (Top-Down) dominieren. 7) In durch physiologischen Stress beeinflussten Systemen bestand keine Abhängigkeit zwischen den Diversitätsindizes (Artenzahl, Eveness und Diversität) und der Produktivität. / The objective of this study was to investigate the development of the pelagic plankton community of the formerly acidic open mining lake Goitsche (pH~3) during its flooding and neutralization. The emphasis was on the interaction between the community composition of an ecosystem and the variability of the abiotic environment. The focus was on two aspects, differing in their causality: • The first aspect concerned the maturation process of the studied ecosystem: Was it possible to picture the development of the pelagic community, based on the criteria formulated by Odum (1969) about the energetics of communities, the nutrient cycling and the structural characteristics at the level of the whole system and of individuals? Do the physiological stress and the physical disturbances, caused by the acidic environment and the inflowing water, induce a reversal of the maturation process? And on which levels of organization can the effect of these stressors be detected? • The second aspect concerned the relationship between selected diversity indices of the community (Species richness, Eveness and Diversity) and the whole-system productivity gradient. I looked whether the Species richness and the Diversity were a monotonous increasing or a unimodale function of the productivity, and if the Eveness was a monotonous decreasing function of the productivity. In a next step, to assure a better predictability of these indices, I took additional biotic and abiotic variables (e.g. consumer effect, physical disturbance, and immigration) into consideration. Lastly, I tested the hypothesis that under the influence of extreme physiological stress there would be no relationship between the diversity indices and the productivity. Main results and conclusions are as follows: 1) The maturation process of the plankton community under neutral conditions could not clearly be depicted by single criteria alone. The structure and function of the community seemed much more driven by idiosyncratic effects of individual species. Coenobial diatoms as well as larger Cladocera and Copepoda, which rapidly dominated the plankton community, made the ecosystem look mature almost immediately after the neutralization of the mining lake. 2) The influence of the physiological stress and the physical disturbances on the maturation process was observed, when compared to an unimpaired reference basin of the open mining lake. When the effects of the two stressors were considered alone, nearly all criteria confirmed a reversal of the maturation process sensu Odum (1969, 1985). 3) In the ecosystem there existed a hierarchy within stressors. The influence of the acid stress dominated over the influence of the physical disturbance, probably by restraining the reaction potential of the plankton community. 4) For primary producers, the Species richness was a monotonous positive function of the realized biomass (a surrogate for the productivity of the ecosystem), and the Eveness a monotonous negative function of the productivity. The observed unimodale relationship between the Diversity and the productivity of primary producers must be seen as a consequence of the mathematical formulation of these indices. 5) The results of multivariate models regarding the forecast of both Species richness and Eveness of the primary producers in relation to the additionally considered biotic and abiotic factors revealed following dependences: • In the mining lake Goitsche the realized Eveness was mainly explained by density dependent processes (negative dependence to the square of the producer biomass and to the light level). • Besides the main influence of increasing biomasses, the Species richness was a function of qualitative and quantitative aspects of the consumer community (Diversity and biomass of the zooplankton). A significant impact of species immigration from the regional pool on the realized Species richness was observed only at the beginning of the flooding of the mining lake. 6) At the consumer level, the only significant relationship was an increase of the Species richness with increasing biomass. The absence of further dependencies between diversity indices and the productivity gradients is attributed to the fact that on lower trophic levels Bottom-Up effects play a major role in the regulation of the community structure, whereas on higher trophic levels Top-Down effects dominate. 7) In ecosystems affected by physiological stress no relationship existed between the diversity indices (Species richness, Eveness and Diversity) of the plankton community and the productivity.

Die Ökosysteme unserer Erde sind durch das rasante Artensterben infolge von Umweltveränderungen durch den Menschen und des globalen Klimawandels stark betroffen. Mit den Auswirkungen dieses Artenverlustes und der damit einhergehenden Veränderung der Diversität beschäftigt sich die heutige Biodiversitätsforschung. Spezieller wird der Effekt der Diversität auf Ökosystemprozesse wie beispielsweise den Biomasseaufbau von Primärproduzenten oder der Resistenz einer Gemeinschaft gegen die Einwanderung neuer Arten untersucht. Die Quantifizierung des Einflusses der Diversität auf die Primärproduktion und das Verständnis der zugrunde liegenden Mechanismen ist von besonderer Wichtigkeit. In terrestrischen Pflanzengemeinschaften wurde bereits ein positiver Diversitätseffekt auf die Gemeinschaftsbiomasse beobachtet. Dies wird hauptsächlich durch den Komplementaritäts- und/oder den Dominanzeffekt erklärt. Die Komplementarität zwischen Arten ist beispielsweise bei Unterschieden in der Ressourcenausnutzung gegeben (z.B. unterschiedliche Wurzeltiefen). Diese kann zu einer besseren Nährstoffausnutzung in diverseren Gemeinschaften führen, die letztlich deren höhere Biomassen erklärt. Der Dominanzeffekt hingegen beruht auf der in diverseren Gemeinschaften höheren Wahrscheinlichkeit, eine hochproduktive Art anzutreffen, was letztlich die höhere Biomasse der Gemeinschaft verursacht. Diversitätseffekte auf Ökosystemprozesse wurden bisher hauptsächlich auf der Gemeinschaftsebene untersucht. Analysen über die Reaktionen, die alle Arten einer Gemeinschaft einschließen, fehlen bisher. Daher wurde der Einfluss der Diversität auf die individuelle Performance von Pflanzenarten innerhalb des Biodiversitätsprojektes „Das Jena Experiment“ untersucht. Dieses Experiment umfasst 60 Arten, die charakteristisch für Mitteleuropäische Graslandschaften sind. Die Arten wurden in die 4 funktionellen Gruppen Gräser, kleine Kräuter, große Kräuter und Leguminosen eingeteilt. Im Freilandversuch zeigte sich, dass mit steigender Artenzahl die individuelle Pflanzenhöhe zunahm, während die individuelle oberirdische Biomasse sank. Der positive Diversitätseffekt auf die pflanzliche Gemeinschaftsbiomasse kann folglich nicht auf der individuellen oberirdischen Biomassezunahme beruhen. Überdies reagierten die einzelnen funktionellen Gruppen und sogar die einzelnen Arten innerhalb einer funktionellen Gruppe unterschiedlich auf Diversitätsveränderungen. Folglich ist zu vermuten, dass einige Ökosystemprozesse auf Gemeinschaftsebene durch die Reaktionen von bestimmten funktionellen Gruppen bzw. Arten hervorgerufen werden. Diversitätseffekte auf Gemeinschaftsbiomassen wurden bislang hauptsächlich mit terrestrischen Pflanzen und weniger mit frei-schwebenden Algenarten (Phytoplankton) erforscht. Demzufolge wurde der Einfluss der Diversität auf die Biomasse von Phytoplankton-Gemeinschaften experimentell untersucht, wobei es sowohl zu negativen als auch positiven Diversitätseffekten kam. Eine negative Beziehung zwischen Diversität und Gemeinschaftsbiomasse zeigte sich, wenn schnell-wüchsige Algenarten nur geringe Biomassen in Mono- und Mischkultur aufbauten. Die vorhandenen Nährstoffe in der Mischkultur wurden von den schnell-wüchsigen Arten monopolisiert und folglich standen sie den langsam-wüchsigen Algenarten, welche viel Biomasse in Monokultur aufbauten, nicht mehr zur Verfügung. Zu einem positiven Diversitätseffekt auf die Gemeinschaftsbiomasse kam es, wenn die Artengemeinschaft eine positive Beziehung zwischen Wachstumsrate und Biomasse in Monokultur zeigte, sodass die schnell-wüchsige Algenarten viel Biomasse aufbauten. Da diese schnell-wüchsigen Algen in der Mischkultur dominant wurden, bestand die Gemeinschaft letztlich aus hoch-produktiven Algenarten, was zu einer erhöhten Gesamtbiomasse führte. Diese beiden Versuchsansätze verdeutlichen Mechanismen für die unterschiedlichen Reaktionen der Gemeinschaften auf Diversitätsveränderungen, welche auch für terrestrische Pflanzengemeinschaften gefunden wurden. Ein anderer wichtiger Ökosystemprozess, der von der Diversität beeinflusst wird, ist die Anfälligkeit von Gemeinschaften gegenüber invasiven Arten (Invasibilität). Die Invasibilität wird von einer Vielzahl von Faktoren beeinflusst und demzufolge wurde der Effekt der Diversität und der Produktivität (Nährstoffgehalt) auf die Invasibilität von Phytoplankton-Gemeinschaften in An- und Abwesenheit eines Herbivoren untersucht. Die zwei funktionell unterschiedlichen invasiven Arten waren die Blaualge Cylindrospermopsis raciborskii (schlecht fressbar) und der Phytoflagellat Cryptomonas sp. (gut fressbar). Es zeigte sich, dass der Fraßdruck, welcher selber durch die Produktivität beeinflusst wurde, einen bedeutenden Effekt auf die Invasibilität von Phytoplankton-Gemeinschaften hat. Die funktionellen Eigenschaften der invasiven und residenten Arten waren zudem bedeutender als die Artenzahl. / To date, positive relationships between diversity and community biomass have been mainly found, especially in terrestrial ecosystems due to the complementarity and/or dominance effect. In this thesis, the effect of diversity on the performance of terrestrial plant and phytoplankton communities was investigated to get a better understanding of the underlying mechanisms in the biodiversity-ecosystem functioning context. In a large grassland biodiversity experiment, the Jena Experiment, the effect of community diversity on the individual plant performance was investigated for all species. The species pool consisted of 60 plant species belonging to 4 functional groups (grasses, small herbs, tall herbs, legumes). The experiment included 82 large plots which differed in species richness (1-60), functional richness (1-4), and community composition. Individual plant height increased with increasing species richness suggesting stronger competition for light in more diverse communities. The aboveground biomass of the individual plants decreased with increasing species richness indicating stronger competition in more species-rich communities. Moreover, in more species-rich communities plant individuals were less likely to flower out and had fewer inflorescences which may be resulting from a trade-off between resource allocation to vegetative height growth and to reproduction. Responses to changing species richness differed strongly between functional groups and between species of similar functional groups. To conclude, individual plant performance can largely depend on the diversity of the surrounding community. Positive diversity effects on biomass have been mainly found for substrate-bound plant communities. Therefore, the effect of diversity on the community biomass of phytoplankton was studied using microcosms. The communities consisted of 8 algal species belonging to 4 functional groups (green algae, diatoms, cyanobacteria, phytoflagellates) and were grown at different functional richness levels (1-4). Functional richness and community biomass were negatively correlated and all community biomasses were lower than their average monoculture biomasses of the component species, revealing community underyielding. This was mainly caused by the dominance of a fast-growing species which built up low biomasses in monoculture and mixture. A trade-off between biomass and growth rate in monoculture was found for all species, and thus fast-growing species built up low biomasses and slow-growing species reached high biomasses in monoculture. As the fast-growing, low-productive species monopolised nutrients in the mixtures, they became the dominant species resulting in the observed community underyielding. These findings suggest community overyielding when biomasses of the component species are positively correlated with their growth rates in monocultures. Aquatic microcosm experiments with an extensive design were performed to get a broad range of community responses. The phytoplankton communities differed in species diversity (1, 2, 4, 8, and 12), functional diversity (1, 2, 3, and 4) and community composition. The species/functional diversity positively affected community biomass, revealing overyielding in most of the communities. This was mainly caused by a positive complementarity effect which can be attributed to resource use complementarity and/or facilitative interaction among the species. Overyielding of more diverse communities occurred when the biomass of the component species was correlated positively with their growth rates in monoculture and thus, fast-growing and high-productive species were dominant in mixtures. This and the study mentioned above generated an emergent pattern for community overyielding and underyielding from the relationship between biomass and growth rate in monoculture as long as the initial community structure prevailed. Invasive species can largely affect ecosystem processes, whereas invasion is also influenced by diversity. To date, studies revealed negative and positive diversity effects on the invasibility (susceptibility of a community to the invasion by new species). The effect of productivity (nutrient concentration ranging from 10 to 640 µg P L-1), herbivory (presence/absence of the generalist feeder) and diversity (3, 4, 6 species were randomly chosen from the resident species pool) on the invasibility of phytoplankton communities consisting of 10 resident species was investigated using semi-continuous microcosms. Two functionally diverse invaders were chosen: the filamentous and less-edible cynaobacterium C. raciborskii and the unicellular and well-edible phytoflagellate Cryptomonas sp. The phytoflagellate indirectly benefited from grazing pressure of herbivores whereas C. raciborskii suffered more from it. Diversity did not affect the invasibility of the phytoplankton communities. Rather, it was strongly influenced by the functional traits of the resident and invasive species.

The work in this thesis deals with effects of changed species richness on process rates among stream-living macroinvertebrates. Global biodiversity is decreasing rapidly and it is poorly known what the consequences of this loss may be for ecosystems and the services they provide. Hence, it is important to investigate the potential effects of losing species. In streams, deforestation, introduction of non-native species, pollution and channelization are examples of events that may affect species richness negatively. In this thesis emphasis is on changes in species richness within functional feeding groups (FFGs) of stream-living macroinvertebrates. The FFGs used were shredding detritivores, grazers, filter feeders and predators - all of which uphold important ecological processes in streams. Along with an observational field study, species richness was manipulated in laboratory and field experiments to investigate the effects of changed species richness on process rates and thus ecosystem functioning. The results show that effects of changed species richness on process rates may be dramatic. Among the shredding detritivores there were negative effects on leaf mass loss, regardless whether fixed, random or predicted sequences of species loss was investigated. These effects could be attributed to either species richness per se or species composition. However, among the other FFGs the relationship between species richness and process rates was less consistent. In filter feeders, there was no or a negative effect of decreasing species richness while both grazers and predators showed positive effects of species loss. The results also show that the most important interactions between species in an experiment, thus potentially in a natural community, are likely to determine what the effect of species loss on process rates will be. Facilitation and niche differentiation lead to reduced process rates if species are lost, while mechanisms, such as interspecific resource or interference competition, produce the opposite effect. Furthermore, in systems with a diminishing resource, the first two mechanisms may become more important over time enhancing the effect of species loss in the long term. In conclusion, effects of species loss may be dramatically negative or positive even if lost species are classified as redundant. The effect in the short term most likely depends on which species are lost, on the original species composition and on the underlying mechanisms. Questions remaining to be answered are how important the observed effects are in more complex systems and if they are persistent over time? Future studies will tell.

During the last few decades, coral reefs have become a disappearing feature of tropical marine environments, and those reefs that do remain are severely threatened. It is understood that humans have greately altered the environment under which these ecosystems previously have thrived and evoloved. Overharvesting of fish stocks, global warming and pollution are some of the most prominent threats, acting on coral reefs at several spatial and temporal scales. Presently, it is common that coral reefs have been degraded into alternative ecosystem regimes, such as macroalgae-dominated or sea urchin-barren. Although these ecosystems could potentially return to coral dominance in a long-term perspective, when considdering current conditions, it seems likely that they will persist in their degraded states. Thus, recovery of coral reefs cannot be taken for granted on a human timescale. Multiple stressors and disturbances, which are increasingly characteristic of coral reef environments today, are believed to act synergistically and produce ecological surprises. However, current knowledge of effects of compounded disturbances and stress is limited. Based on five papers, this thesis investigates the sublethal response of multiple stressors on coral physiology, as well as the effects of compounded stress and disturbance on coral reef structure and function. Adaptive responses to stress and disturbance in relation to prior experience are highlighted. The thesis further explores how inherent characteristics (traits) of corals and macroalgae may influence regime expression when faced with altered disturbance regimes, in particular overfishing, eutrophication, elevated temperature, and enhanced substrate availability. Finally, possibilities of affecting the resilience of macroalgae-dominaed reefs and shifting the community composition towards a coral-dominated regime are explored.

Coral reefs are suffering unprecedented declines in coral cover and species diversity. These changes are often associated with  substantial shifts in community structure to new dominant organisms. Ultimately, these “phase shifts” can be persistent and very difficult to return from. Building insurance against degradation and decreasing the likelihood of reefs undergoing shifts to undesirable states will require sustainable management practices that uphold coral reef resilience. This thesis consists of five papers that contribute new knowledge useful for managing the resilience of coral reefs, and other marine ecosystems. Paper I shows how the morphology of natural substrate (dead coral colonies) can significantly influence coral recruitment patterns. Paper II focuses on larval lipid levels, a key determinant of coral dispersal potential, in a common Caribbean coral (Favia fragum). It shows that i) lipid levels exhibit a significant, non-linear reduction throughout the larval release period of F. fragum and ii) exposure to a common pollutant (copper) could potentially lead to a more rapid lipid consumption in the larvae. Paper III presents a broader analysis of the different undesirable states a coral reef can shift to as a consequence of reef degradation. It concludes that different states are caused by different driving factors and that management must explicitly acknowledge this. Paper IV proposes a suite of resilience indicators that can help managers assess when a coral-dominated reef might be moving towards a shift to an undesirable state. These indicators capture key-processes occuring on different temporal and spatial scales and signal resilience loss early enough for managers to take appropriate measures. Finally, Paper V reviews the feedback loops that reinforce the undesirable states of five important marine ecosystems and suggests certain strategies that can ease the restoration back to healthier conditions. Managing these critical feedbacks will recquire monitoring the processes underpinning these feedbacks, breaking already established feedbacks loops through large-scale management trials and acknowledging transdisciplinary solutions that move management beyond the discipline of ecology / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 5: In progress.