Academic literature on the topic 'Trophic Mutualism'

The collapse of mutualisms owing to anthropogenic changes is contributing to losses of biodiversity. Top predators can regulate biotic interactions between species at lower trophic levels and may contribute to the stability of such mutualisms, but they are particularly likely to be lost after disturbance of communities. We focused on the mutualism between the fig tree Ficus microcarpa and its host-specific pollinator fig wasp and compared the benefits accrued by the mutualists in natural and translocated areas of distribution. Parasitoids of the pollinator were rare or absent outside the natural range of the mutualists, where the relative benefits the mutualists gained from their interaction were changed significantly away from the plant's natural range owing to reduced seed production rather than increased numbers of pollinator offspring. Furthermore, in the absence of the negative effects of its parasitoids, we detected an oviposition range expansion by the pollinator, with the use of a wider range of ovules that could otherwise have generated seeds. Loss of top-down control has therefore resulted in a change in the balance of reciprocal benefits that underpins this obligate mutualism, emphasizing the value of maintaining food web complexity in the Anthropocene.

This article compares 13 network properties of food webs of 31 Ukrainian grasslands. The properties are: network size N, trophic link number L, trophic classes Cl, system connectance C, link density LD, total system throughflow TST, network cycling FCI, ascendency AS, developmental capacity DC, indirect effects dominance IE, system aggradation AI, system synergism SI, and mutualism MI. Our results show these properties are highly correlated and can be aggregated into the three latent factors. The first factor includes N, TST, AS, DC, SI, L, and LD, where network size N appears to be a central defining variable. The second latent factor includes FCI, AI, and Cl, and is driven by indirect effects dominance IE. The third factor includes mutualism MI and connectance C, the last being the driving variable. Network Synergism SI is negatively correlated with the other network properties, while all others are positively intercorrelated. Network connectance appears to be a scale invariant property, while link density is highly sensitive to network size. Our data also show that network mutualism MI is more tied to the network complexity than simply to system scale or number of feeding links.

This paper presents the first five variable model of mutualism motivated by the interaction between ants and homopterans. In this mutualism, homopterans benefit both directly through increased feeding rates and indirectly through predator protection. Results of our analyses show oscillatory, complex, and chaotic dynamic behavior. In addition, we show that intraspecies interactions are crucial for closing trophic levels and stabilizing the dynamic system from potential “chaotic” behavior.

Figs ( Ficus ) have a reciprocally obligate mutualism with tiny, short-lived (1–2 days) fig wasps (Agaonidae). The small size and short life of these pollinators is expected to make them more vulnerable to climate change than their larger and longer-lived hosts. We experimentally tested the thermal tolerances of four species of adult female fig wasp from equatorial Singapore. The results suggest that an increase of 3°C or more above the current temperatures experienced across much of the equatorial tropics would markedly decrease the active adult lifespan of all four species. Fig plants are the centre of an intricate web of specialist and generalist animals. Unless fig wasps can acclimate or adapt to warmer temperatures in time, these responses may disrupt the mutualism, potentially affecting multiple trophic levels.

Interactions across trophic levels influence plant diversity effects on ecosystem functions, but the complexity of these interactions remains poorly explored. For example, the interplay between different interactions (e.g. mutualism, predation) might be an important moderator of biodiversity–ecosystem function relationships. We tested for relationships between trophobioses (facultative ant–hemipteran mutualism) and leaf chewer herbivory in a subtropical forest biodiversity experiment. We analysed trophobiosis and herbivory data of more than 10 000 trees along a tree species richness gradient. Against expectations, chewing damage was higher on trees with trophobioses. However, the net positive relationship between trophobioses and overall herbivory depended on tree species richness, being most pronounced at low richness. Our results point to indirect, positive effects of ant-tended sap suckers on leaf chewers, potentially by altering plant defences. Direct antagonistic relationships of trophobiotic ants and leaf-chewing herbivores—frequently reported to drive community-wide effects of trophobioses in other ecosystems—seemed less relevant. However, antagonistic interactions likely contributed to the attenuating effect of tree species richness, because trophobiotic ant and herbivore communities changed from monocultures to species-rich mixtures. Our findings, therefore, suggest that biodiversity loss might lead to complex changes in higher trophic level effects on ecosystem functions, mediated by both trophic and non-trophic interactions.

Multi-trophic interactions maintain critical ecosystem functions. Biodiversity is declining globally, while responses of trophic interactions to biodiversity change are largely unclear. Thus, studying responses of multi-trophic interaction robustness to biodiversity change is crucial for understanding ecosystem functioning and persistence. We investigate plant–Hemiptera (antagonism) and Hemiptera–ant (mutualism) interaction networks in response to experimental manipulation of tree diversity. We show increased diversity at both higher trophic levels (Hemiptera and ants) and increased robustness through redundancy of lower level species of multi-trophic interactions when tree diversity increased. Hemiptera and ant diversity increased with tree diversity through non-additive diversity effects. Network analyses identified that tree diversity also increased the number of tree and Hemiptera species used by Hemiptera and ant species, and decreased the specialization on lower trophic level species in both mutualistic and antagonist interactions. Our results demonstrate that bottom-up effects of tree diversity ascend through trophic levels regardless of interaction type. Thus, local tree diversity is a key driver of multi-trophic community diversity and interaction robustness in forests.

The diversity and structure of ecosystems has been found to depend both on trophic interactions in food webs and on other species interactions such as habitat modification and mutualism that form non-trophic interaction networks. However, quantification of the dependencies between these two main interaction networks has remained elusive. In this study, we assessed how habitat-modifying organisms affect basic food web properties by conducting in-depth empirical investigations of two ecosystems: North American temperate fringing marshes and West African tropical seagrass meadows. Results reveal that habitat-modifying species, through non-trophic facilitation rather than their trophic role, enhance species richness across multiple trophic levels, increase the number of interactions per species (link density), but decrease the realized fraction of all possible links within the food web (connectance). Compared to the trophic role of the most highly connected species, we found this non-trophic effects to be more important for species richness and of more or similar importance for link density and connectance. Our findings demonstrate that food webs can be fundamentally shaped by interactions outside the trophic network, yet intrinsic to the species participating in it. Better integration of non-trophic interactions in food web analyses may therefore strongly contribute to their explanatory and predictive capacity.

Photosymbioses, intimate interactions between photosynthetic algal symbionts and heterotrophic hosts, are well known in invertebrate and protist systems. Vertebrate animals are an exception where photosynthetic microorganisms are not often considered part of the normal vertebrate microbiome, with a few exceptions in amphibian eggs. Here, we review the breadth of vertebrate diversity and explore where algae have taken hold in vertebrate fur, on vertebrate surfaces, in vertebrate tissues, and within vertebrate cells. We find that algae have myriad partnerships with vertebrate animals, from fishes to mammals, and that those symbioses range from apparent mutualisms to commensalisms to parasitisms. The exception in vertebrates, compared with other groups of eukaryotes, is that intracellular mutualisms and commensalisms with algae or other microbes are notably rare. We currently have no clear cell-in-cell (endosymbiotic) examples of a trophic mutualism in any vertebrate, while there is a broad diversity of such interactions in invertebrate animals and protists. This functional divergence in vertebrate symbioses may be related to vertebrate physiology or a byproduct of our adaptive immune system. Overall, we see that diverse algae are part of the vertebrate microbiome, broadly, with numerous symbiotic interactions occurring across all vertebrate and many algal clades. These interactions are being studied for their ecological, organismal, and cellular implications. This synthesis of vertebrate–algal associations may prove useful for the development of novel therapeutics: pairing algae with medical devices, tissue cultures, and artificial ecto- and endosymbioses.

We are currently faced with the global challenge of conserving biological diversity while also increasing food production to meet the demands of a growing human population. Land-use change, primarily resulting from conversion to production land, is currently the leading cause of biodiversity loss. This occurs through habitat loss, fragmentation of remaining natural habitats, and resulting edge effects. Land-sparing and land-sharing approaches have been discussed as alternative ways to engineer landscapes to mitigate biodiversity loss while meeting production objectives. However, these represent extremes on a continuum of real-world landscapes, and it will be important to understand the mechanisms by which adjacent land use affects natural remnant ecosystems in order to make local land-management decisions that achieve conservation, as well as production, objectives. This thesis investigates the impact of juxtaposing production and natural forest on the community-wide interactions between lepidopteran herbivores and their parasitoids, as mediated by parasitoid spillover between habitats. The first and overarching objective was to determine whether herbivore productivity drives asymmetrical spillover of predators and parasitoids, primarily from managed to natural habitats, and whether this spillover alters trophic interactions in the recipient habitat. The study of trophic interactions at a community level requires understanding of both direct and indirect interactions. However, community-level indirect interactions are generally difficult to predict and measure, and these have therefore remained understudied. Apparent competition is an indirect interaction mechanism thought to be very important in structuring host-parasitoid assemblages. However, this is known primarily from studies of single species pairs, and its community-wide impacts are less clear. Therefore, my second objective was to determine whether apparent competition could be predicted for all species pairs within an herbivore assemblage, based on a measure of parasitoid overlap. My third objective was to determine whether certain host or parasitoid species traits can predict the involvement of those species in apparent competition. My key findings were that there is a net spillover of generalist predators and parasitoids from plantation to native forest, and that for generalists, this depends on herbivore abundance in the plantation forest. Herbivore populations across the edge were linked by shared parasitoids in apparent competition. Consequently, an experimental reduction of herbivore density in the plantation forest changed parasitism rates in the natural forest, as predicted based on parasitoid overlap. Finally, several host and parasitoid traits were identified that can predict the degree to which host or parasitoid species will be involved in apparent competition, a finding which may have extensive application in biological control, as well as in predicting spillover edge effects. Overall, this work suggests that asymmetrical spillover between production and natural habitats occurs in relation to productivity differences, with greater movement of predators and parasitoids in the managed-to-natural forest direction. The degree to which this affected species interactions has implications for landscape design to achieve conservation objectives in production landscapes.

Mutualistic interactions between species are balanced on a delicate scale of net benefits to both interacting partners. The dynamics of such interactions could change depending on the context in which these interactions occur. One of the most well-studied models for interspecies mutualisms are myrmecophytic systems, also known as ant-plant systems, where the host plant (myrmecophyte) provides shelter (domatia), solely or along with food resources, for ant partners, while the domatia-resident ants intensively patrol and protect the host plant from herbivory. In some cases, nutrient flux has also been reported from the ant-derived debris in the domatia to the host plant. Such mutualisms are often vulnerable to exploitation by non-mutualist organisms or interlopers such as non-protective ants and other invertebrates that use the plant rewards without any returns. Since provision of domatia and food imposes costs on the host plants, the trajectory of evolution in such cases where protection is partial or absent needs investigation to understand the evolution of myrmecophytism. In this thesis, we investigate the possibility of evolution of myrmecophytism in the absence of universal protection by partner ants, using the unique semi-myrmecophyte (domatia are not expressed in all individuals of the species) Humboldtia brunonis as the study model. H. brunonis is endemic to the tropical wet evergreen forests of the Western Ghats of India. Being locally abundant in its distribution range, this plant species has also been used in characterising forest types in the Western Ghats. H. brunonis provides domatia (modified stem internodes) and food for ants in the form of extrafloral nectar (EFN) on leaves and bracts of floral buds in all individuals. Each domatium has a self-opening slit, which could have led to the domatia being accessed and inhabited by numerous ants and other non-ant invertebrates throughout its distribution range. Of these, only one ant species, Technomyrmex albipes, has been reported to be significantly protective against herbivores, and the protection received by the plant is reported to be restricted only to one site where T. albipes is most abundant. In the light of the above, the possible explanation for continued expression of rewards (domatia as well as EFN) in the absence of universal protection was investigated. Chapter 1: Introduction This chapter starts with a brief history of the concept of mutualism, evolution and maintenance of mutualism, and trophic mutualism amongst organisms in general. This is followed by a description of ant–plant mutualisms, and the various interactions that drive the interaction in such systems, with a more detailed emphasis on trophic mutualism in ant-plants, and stable isotope analysis as a technique that is used to study trophic mutualism in ant-plants. The study system, Humboldtia brunonis is introduced, and all the studies on this system preceding this current thesis are discussed in the light of findings in other ant-plant systems. Lastly, the objectives of the thesis are briefly introduced as separate chapters. Chapter 2: Context dependency of rewards and services in an Indian ant–plant interaction: southern sites favour the mutualism between plants and ants (published in Journal of Tropical Ecology) Earlier studies on the H. brunonis system have shown that there is geographic variation in the occupancy of the domatia, with domatia in the northern part of the H. brunonis range being dominantly occupied by an arboreal earthworm species, while domatia in the south are mostly occupied by ants, especially the sole protective ant T. albipes. Further, it has been reported that herbivory is significantly reduced in the presence of ants in the south. In the present study, conducted at 5 sites spanning the distribution range of H. brunonis, we observed that there is a geographic variation in various ant-related plant traits such as abundance of domatia-bearing individuals, number of nectaries per leaf, size of nectaries, and volume and composition of the EFN, with a clear north–south increasing gradient. However, strong protection mutualism was observed only at one site in the south where herbivory pressure was highest. By comparing our results with earlier findings, we show that in addition to geographic variation, there is also temporal variation in the strength of protection mutualism, and that protection mutualism in this system is context-dependent. These results provide new perspectives on the evolution of myrmecophytism. Chapter 3: Leaf expansion and foliar extrafloral nectar as defence strategies in a paleotropical ant-plant Humboldtia brunonis (Fabaceae) (a section of this chapter is submitted to Biotropica) Despite the absence of universal protection against herbivory, H. brunonis plants constitutively secrete EFN and domatia. We therefore explored other non-chemical defences in this system, and investigated possible explanations for the continued reward production. We observed rapid rate of leaf expansion during the early and most vulnerable phase of leaf phenology, and propose this as a strategy to escape herbivory. The young leaves are also subject to being infested by phloem-feeding Hemiptera, but there was seldom any case of ants tending Hemiptera for honeydew (sugary material excreted by the Hemiptera) on the plant. We analysed the sugar and amino acids compositions of EFN, honeydew and phloem sap, and found that EFN composition was much richer and more attractive (to ants) than honeydew, thereby suggesting that EFN could possibly function to distract ants from tending Hemiptera on the plant, thereby avoiding further damage to the plant. We also observed that EFN composition was much richer than phloem sap, and thereby confirmed that EFN is not mere phloem exudate; rather, our results suggests that EFN could possibly be synthesized actively in the secretory cells of the extrafloral nectary. Anatomical observations of the foliar nectaries further support the synthesis of EFN in the secretory cells of the nectary. Chapter 4: Nutritional benefits from domatia-inhabitants in an ant–plant interaction: interlopers do pay the rent (published in Functional Ecology) In this chapter, we explore how a myrmecophytic system could evolve in the absence of protection benefits from the partner ants. We investigate non-protective benefits, specifically trophic (nutrient) benefits, from the protective and non-protective ants and other invertebrates to the host plant, using stable isotope techniques. We selected three representative inhabitant species for our analysis, viz., the protective ant T. albipes, a non-protective ant Crematogaster dorhni, and the arboreal earthworm Perionyx pullus. We observed that earthworms contributed approximately 9% while protective or non-protective ants contributed approximately 17% of the nitrogen to the plant tissues nearest to the domatium. We also observed from 15N labelling experiments that that nutrients from the domatia are not restricted solely to the domatia-bearing branch but could travel to distant non-domatia bearing branches as well. This study demonstrated for the first time that non-protective ants and non-ant invertebrates that inhabit the domatia, and hitherto referred to as interlopers, could be in a trophic mutualism with the host plant, thereby proposing the possibility of trophic mutualism as a factor for the evolution and maintenance of the domatia trait in addition to or in the absence of protection mutualism. It is also possible that fitness benefits of bearing domatia, acquired via trophic mutualism, could later facilitate the establishment of a specialised ant–plant protection mutualism. Chapter 5: Structure and development of the caulinary domatia of Humboldtia brunonis In this chapter, we investigate the morphology of domatia at different ontogenetic stages in order to understand the mode of development of the domatia. Our observations show that the domatium of H. brunonis is formed spontaneously near the terminal end of a growing branch, next to the young apical shoot. It appears as a young swollen internode which is soft and fleshy with the pith tissue still present. As the domatia grows and expands, the collective effect of both schizogeny as well as lysogeny, act on on the pith region. We also observed acropetal lignification of the pith cells around the hollow chamber. We investigate micro-scale anatomy of the inner wall of the domatia using scanning electron microscopy, and observed that the inner lining of the domatia cavity have canaliculated, lignified sclerenchyma with numerous plasmodesmata (intercellular pits) that could facilitate the flow of occupant-derived nutrients supporting trophic interaction between the plant and its domatia inhabitants. We also observed fungal mycelia-like structures in ant-occupied domatia that suggests the possibility of a fungus as a third party in the ant–plant trophic mutualism, as is observed in some other myrmecophytic systems. This aspect however needs further investigation. Chapter 6: Conclusion In this chapter, the main findings of the preceding chapters are summarised. A general conclusion of the thesis is provided, and future directions leading from the present thesis are also listed. The present thesis has explored the dynamics of interactions between a unique semi-myrmecophyte and its domatia-inhabitants; while the unprotected host plant resorts to an escape strategy to evade foliar herbivory, the ants (and other invertebrates) seem to have bid “a farewell to arms” and yet maintain a mutualism with its host via nutrient exchange. The results of this thesis contribute to furthering our current understanding of the evolution and stability of inter-species mutualisms.

There is concern that climate change may cause mismatches between timing of flowering and activity of pollinators (phenology). However, concluding that mismatches will occur, and have serious consequences for pollination services, requires assumptions that have not yet been tested. I begin by discussing a set of these assumptions, bringing past research into the context of mismatch. Briefly, the assumptions are that 1) dates of first-flowering or emergence (DFFE) correctly describe phenology (and therefore mismatch); 2) differences in DFFE represent the magnitude of mismatch; 3) advancement of DFFE will be the primary phenological change; 4) shifts will be random and independent for each species; 5) populations of plants and pollinators are “bottom-up” regulated by their mutualistic interactions; 6) all interactions are of similar strength and importance; 7) dispersal, and the spatial context of phenological mismatches can be ignored; and ecological processes including 8) phenotypic plasticity and adaptive evolution of phenology, 9) competition and facilitation, and 10) emergence of novel interactions, will not affect mismatches. I then describe novel experiments, which could help to account for some of these assumptions, clarifying the existence and impacts of mismatches. Next, I present an original field experiment on factors affecting seed set in an alpine meadow in the Coast Range of British Columbia, Canada. I found evidence contradicting the assumption that seed set is primarily limited by pollination. My data highlight the roles of phenology, temperature (degree-days above 15°C, and frost hours), and interactions with pollinators (mutualists) and seed-predators (floral antagonists) in driving patterns of seed set. Seed set of early and late-flowering species responded differently to a 400m elevation gradient, which might be explained by phenology of bumble bees. My data suggest that the consequences of mismatch may be smallest for plants that are fly-pollinated and self-fertile. Non-selfing, bee-pollinated species might be more prone to reproductive limitation through mismatch (affected by snowmelt and cumulative degree-days). Plants that are limited by seed-predators might be negatively affected by warming temperatures with fewer frost hours, and extreme events such as late-season frosts and hail storms can prevent plants from setting seed entirely. Overall, my work emphasizes the importance of complementing theory, data-driven simulations, and meta-analyses with experiments carried out in the field.
Graduate

Das Wissen über Mechanismen, die einen Einfluss auf Muster der Artenvielfalt und biotische Interaktionen haben, ist grundlegend für den Schutz von Biodiversität. Darüber hinaus kann es von direktem ökonomischem Nutzen sein, zum Beispiel im biologischen Pflanzenschutz oder bei Bestäubungsdienstleistungen. Die Größe eines Organismus kann ein solcher Faktor sein, der die Artenzahl und Interaktionen der assoziierten Organismen beeinflusst, denn große Organismen sind auffälliger als kleine und ihr Angebot an Ressourcen und Nischen für mit ihnen assoziierte Organismen ist oft reicher. Bezogen auf Pflanzen könnte daher die Größe einer Pflanze einen erheblichen Einfluss auf die Artenzahl der mit ihr assoziierten Arthropoden und ihre biotischen Interaktionen wie Herbivorie oder Bestäubung haben. Trotzdem ist der Einfluss der Pflanzengröße auf mutualistische und antagonistische Interaktionspartner der Pflanze und der sich daraus ergebende Einfluss auf die reproduktive Fitness der Pflanze bisher nicht umfassend und unter standardisierten Bedingungen untersucht worden. In der vorliegenden Studie wurden die Auswirkungen der Pflanzengröße auf die Artenzahl von Herbivoren, deren Gegenspielern und Bestäubern untersucht, sowie die Auswirkungen dieser Interaktionspartner auf die Pflanzenfitness. Dabei wurde zusätzlich zwischen endophagen und ektophagen Herbivoren und deren Gegenspielern unterschieden. Außerdem wurden die Herbivoren einzelner Pflanzenkompartimente und deren Gegenspieler separat analysiert. Des Weiteren wurde der Einfluss der Pflanzengröße auf den Herbivorieschaden an den verschiedenen Pflanzenkompartimenten und deren Einfluss auf die reproduktive Fitness der Pflanze, d.h. auf ihre Samenzahl, Tausendkorngewicht und Samengesamtgewicht, untersucht. Zuletzt wurde besonderes Augenmerk auf den Einfluss der Pflanzengröße auf mutualistische und antagonistische Blütenbesucher und deren Einfluss auf die reproduktive Fitness gelegt und untersucht, ob und inwiefern die reproduktive Fitness letztendlich von der Pflanzengröße abhängig ist. Zur Untersuchung dieser Fragen wurde ein „Common Garden“-Experiment angelegt. Um einen interspezifischen Pflanzengrößengradienten zu erzeugen, wurden 21 annuelle Pflanzenarten aus der Familie der Kreuzblütler (Brassicaceae) ausgewählt, deren Größe von 10 bis 130 cm reichte (gemessen als Pflanzenhöhe vom Boden bis zur Spitze). So konnten die Einflüsse des Habitats und der umgebenden Landschaft für alle Pflanzenarten standardisiert und trotzdem ein breiter Gradient realisiert werden. Dadurch hebt sich diese Studie von den bisherigen ab, die den Effekt von meist intraspezifischer Pflanzengröße auf die assoziierten Tiere anhand wild wachsender Pflanzen untersucht haben. Pflanzengröße sowie Zahl, Biomasse und Größe der unterschiedlichen überirdischen Pflanzenkompartimente (Blüten, Schoten, Blätter, Stängel) sowie Blütendeckung und -farbe wurden aufgenommen. Der Herbivorieschaden an diesen Pflanzenkompartimenten und die reproduktive Fitness (Samenzahl, Tausendkorngewicht und Gesamtsamengewicht) wurden gemessen. An und in Blüten, Schoten, Blättern und Stängeln wurden herbivore, räuberische, parasitäre und bestäubende Arthropoden gezählt. Die Pflanzengröße hatte einen positiven Einfluss auf die Artenzahl von Herbivoren, deren Gegenspielern und Bestäubern. Das traf ebenso auf endophage und ektophage sowie auf mit Blättern und Schoten assoziierte Herbivore und deren Gegenspieler zu. Des Weiteren konnte ein Anstieg des Herbivorieschadens an Blüten und Schoten mit zunehmender Pflanzengröße festgestellt werden, wohingegen der Schaden an Blättern und Stängeln von der Biomasse des entsprechenden Kompartiments positiv beeinflusst wurde. Der Schaden an Blüten hatte den stärksten Einfluss auf die reproduktive Fitness und reduzierte neben der Samenzahl auch das Tausendkorngewicht und das Gesamtsamengewicht der Pflanze. Die genaue Analyse der blütenbesuchenden Insekten ergab einen positiven Einfluss der Pflanzengröße auf die Abundanz und Artenzahl von Bestäubern (allerdings nicht bei extrem großem Blütenangebot), wie auch auf die Abundanz der adulten und juvenilen Rapsglanzkäfer und deren Parasitierungsrate. Steigende Rapsglanzkäferzahlen verringerten die Samenzahl sowie das Tausendkorngewicht, während die Bestäuber sich lediglich auf die Samenzahl positiv auswirkten. Insgesamt führte ein Anstieg der Pflanzenhöhe zu einer Abnahme des Tausendkorngewichts, aber nicht zu einer Veränderung der Samenzahl oder des Gesamtsamengewichts, was auf einen Ausgleich der Effekte von zunehmender Antagonistenzahl und zunehmender Mutualistenzahl hindeutet. Großen Pflanzen entstehen also durch ihre Auffälligkeit und Attraktivität für Herbivore hohe Fitnesskosten, wobei insbesondere der Blütenschaden durch Rapsglanzkäfer einen starken negativen Einfluss auf Samenzahl, Tausendkorngewicht und Gesamtsamengewicht hat. Diesen Fitnesskosten großer Pflanzen wirkt der Nutzen durch ihre Auffälligkeit und Attraktivität für Bestäuber entgegen, die die Samenzahl positiv beeinflussen. Hinsichtlich der Samenzahl sollten also große Pflanzen gegenüber kleineren im Vorteil sein, wenn die Insektengemeinschaft des Habitats von Bestäubern dominiert wird. Wird sie aber von herbivoren Blütenbesuchern dominiert, sollten kleine Pflanzen gegenüber großen einen Vorteil haben. Im Gegensatz dazu sollten große Pflanzen immer einen Nachteil bezüglich des Tausendkorngewichts haben, das von Antagonisten, nicht aber von Mutualisten beeinflusst wurde. Der Einfluss der Pflanzengröße auf biotische Interaktionen wurde bisher oft unterschätzt, obwohl er sich auf komplexe Weise über die mutualistischen und antagonistischen Insekten auf die reproduktive Fitness der Pflanze auswirkt.

Abstract In 1999, Daniel Simberloff and Betsy Von Holle introduced the term 'invasional meltdown'. The term and the concept have been embraced and critiqued but have taken a firm hold within the invasion biology canon. The original formulation of the concept argued two key points: first, biologists rarely study how non-natives interact with one another. Second, nearly all the conceptual models about the success and impact of invasive species are predicated on the importance of competitive interactions and an implicit assumption that non-natives should interfere with establishment, spread and impact of other non-natives. In response, Simberloff and Von Holle called for more research on invader interactions and proposed an alternative consequence of non-native species interactions - invasional meltdown - where facilitative interactions among non-natives could increase the invasion rate or ecological impacts in invaded systems. This chapter outlines the primary pathways in which direct and indirect interactions among non-natives could lead to invasional meltdown. It provides examples of how different types of interactions among non-natives could lead to net positive effects on the invasion success of non-native plants or the impact of non-native plants on invaded ecosystems. Direct effects are by far the most commonly explored form of non-native- non- native interaction, primarily focusing on plant mutualisms with pollinators, seed dispersers or soil microbial mutualists. There are, however, also examples of non-native plants that benefit from commensal and even herbivorous interactions with other non-natives. Indirect interactions among non-natives are very infrequently studied. Although examples are scarce, non-natives may indirectly benefit other non-native plants through trophic cascades, apparent competition and indirect mutualisms. It remains unclear whether indirect effects are important pathways to invasional meltdown. More work is needed on studying ecosystems that are invaded by multiple non-native species and we need to consider the full range of interactions among non-natives that could either stymie or promote their spread, population growth and impact. Only then can we address how common facilitative interactions are relative to competitive interactions among non-natives or provide robust suggestions on how to manage ecosystems.

Abstract Many species interactions occur in lotic systems. Most are negative for one of the interactors, as in predation, herbivory, competition, parasitism and disease. Positive interactions, including symbiosis, mutualism, commensalism and facilitation are also important. How significant are species interactions in physically demanding habitats? Communities encompass a network of interactions (‘food webs’), though not all involve feeding. Food webs can simply be counts of all the trophic interactions that occur (‘connectance webs’). More instructive are ‘flow webs’, where links are weighted based on the energy passing along them. A third kind is based on estimating the dynamic effects of interactions on populations of prey and predators. Body size is clearly important in stream webs (size determines who eats whom), while omnivory (feeding at more than one trophic level) may stabilise food webs where interactions are mainly weak. Food webs stand at the interface between organismal biology and ecosystem processes.

This chapter examines ecological opportunities that are available to species in various positions within a biological community, with particular emphasis on identifying the criteria necessary for an ecological opportunity to exist. Before discussing what performance capabilities a species must have to fill different types of ecological opportunities and what is required for invasibility of species into different functional positions in a community, the chapter considers the different frameworks that have been used to model species interactions. It then describes resource and apparent competition to show how resource availability from below and predation pressure from above can affect the types of species that can exploit specifc ecological opportunities. It also analyzes communities with three trophic levels, intraguild predation or omnivory, mutualism, the mechanisms that foster coexistence between one plant species and one pollinator species, and the case of one plant species with multiple pollinators.

Now that we have discussed how assemblages of marine soft sediments are structured, we need to consider functional aspects. There are a few main interrelationships that need to be discussed here— inter- and intraspecific competition, feeding and predator–prey interactions, the production of biomass, and the production and delivery of recruiting stages. Other functional aspects, such as the effects of pathogens and parasites and the benefits of association (mutualism, parasitism, symbiosis, etc.) are of less importance in the present discussion. By function we mean the rate processes (i.e. those involving time) that either affect (extrinsic processes) or are inside (intrinsic processes and responses) the organisms that live in sediments. Hence these include primary and secondary production and processes that are mitigated by the organisms that live in sediments, such as nutrient and contaminant fluxes into and out of the sediment. We begin with the historical development of the field since such aspects are often overlooked in these days of electronic searches for references. Functional studies of ecosystems really began with Lindeman´s classic paper (1942) on trophic dynamics. Rather than regarding food merely as particulate matter, Lindeman expressed it in terms of the energy it contained, thereby enabling comparisons to be made between different systems. For example, 1 g of the bivalve Ensis is not equivalent in food value to 1 g of the planktonic copepod Calanus, so the two animals cannot be compared in terms of weight, but they can be compared in terms of the energy units that each gram dry weight contains. The energy unit originally used was the calorie, but this has now been superseded by the joule (J), 1 calorie being equivalent to 4.2 joules. Ensis contains 14 654 J g-1 dry wt and Calanus 30 982 J g-1 dry wt. The basic trophic system is well understood and can be summarized as we showed earlier in Fig. I.8 which gives the links between various trophic levels and the role of competition, organic matter transport, and resource partitioning. In systems fuelled by photosynthesis (so excluding the chemosynthetic deep-sea vent systems), the primary source of energy for any community is sunlight, which is fixed and stored in plant material, which thus constitutes the first trophic level in the ecosystem.

The unusual behavior of cleaner fish has attracted both popular and scientific curiosity since its discovery early in the 20th century. These fish apparently make their living by removing external parasites from “host” fishes of other species (some also remove bacteria or diseased and injured tissue). When they approach cleaners, hosts assume an unusual motionless posture that allows cleaners to feed from their scales, from their gill cavities, or even inside their mouths. For their trouble, cleaner fish get a meal, and hosts get a good cleaning. The interaction between cleaner fish and their hosts is generally classified as a mutualism, or mutually beneficial interaction between species. Stories about this and other mutualisms have become staples of nature documentaries and the popular literature and have helped lure many students into a lifetime of studying biology. From the perspective of evolutionary ecology, however, the cleaner-host relationship is anything but straightforward (Poulin and Grutter 1996). First, it is not at all clear that this interaction confers reciprocal fitness benefits. Despite several decades of effort, only one study has shown that cleaners significantly reduce hosts’ parasite loads (Grutter 1999), and none has yet demonstrated that reducing parasite loads increases host success. Since cleaners often gouge the host’s flesh, particularly when parasites are few, the interaction is often more costly than beneficial. Second, if cleaning does not confer an advantage, it is not evident why hosts should tolerate and even actively solicit cleaners’ attention. In fact, sometimes hosts lure cleaners only to eat them, but the conditions under which it might be beneficial for a host to doublecross its cleaners like this remain unexplored. Third, we don’t really understand how cleaning behaviors arose in the first place, considering that the first individuals that approached hosts to feed on parasites were very likely eaten. Despite this constraint, cleaning has apparently evolved multiple times; it is found in at least five families, in both marine and freshwater species, and in both the temperate zone and the tropics.