Measuring specialization in species interaction networks

BACKGROUND: Network analyses of plant-animal interactions hold valuable biological information. They are often used to quantify the degree of specialization between partners, but usually based on qualitative indices such as 'connectance' or number of links. These measures ignore interaction frequencies or sampling intensity, and strongly depend on network size. RESULTS: Here we introduce two quantitative indices using interaction frequencies to describe the degree of specialization, based on information theory. The first measure (d') describes the degree of interaction specialization at the species level, while the second measure (H(2)') characterizes the degree of specialization or partitioning among two parties in the entire network. Both indices are mathematically related and derived from Shannon entropy. The species-level index d' can be used to analyze variation within networks, while H(2)' as a network-level index is useful for comparisons across different interaction webs. Analyses of two published pollinator networks identified differences and features that have not been detected with previous approaches. For instance, plants and pollinators within a network differed in their average degree of specialization (weighted mean d'), and the correlation between specialization of pollinators and their relative abundance also differed between the webs. Rarefied sampling effort in both networks and null model simulations suggest that H(2)' is not affected by network size or sampling intensity. CONCLUSION: Quantitative analyses reflect properties of interaction networks more appropriately than previous qualitative attempts, and are robust against variation in sampling intensity, network size and symmetry. These measures will improve our understanding of patterns of specialization within and across networks from a broad spectrum of biological interactions

Tropical parabiotic ants: Highly unusual cuticular substances and low interspecific discrimination

<p>Abstract</p> <p>Background</p> <p>Associations between animal species require that at least one of the species recognizes its partner. Parabioses are associations of two ant species which co-inhabit the same nest. Ants usually possess an elaborate nestmate recognition system, which is based on cuticular hydrocarbons and allows them to distinguish nestmates from non-nestmates through quantitative or qualitative differences in the hydrocarbon composition. Hence, living in a parabiotic association probably necessitates changes of the nestmate recognition system in both species, since heterospecific ants have to be accepted as nestmates.</p> <p>Results</p> <p>In the present study we report highly unusual cuticular profiles in the parabiotic species <it>Crematogaster modiglianii </it>and <it>Camponotus rufifemur </it>from the tropical rainforest of Borneo. The cuticle of both species is covered by a set of steroids, which are highly unusual surface compounds. They also occur in the Dufour gland of <it>Crematogaster modiglianii </it>in high quantities. The composition of these steroids differed between colonies but was highly similar among the two species of a parabiotic nest. In contrast, hydrocarbon composition of <it>Cr. modiglianii </it>and <it>Ca. rufifemur </it>differed strongly and only overlapped in three regularly occurring and three trace compounds. The hydrocarbon profile of <it>Camponotus rufifemur </it>consisted almost exclusively of methyl-branched alkenes of unusually high chain lengths (up to C₄₉). This species occurred in two sympatric, chemically distinct varieties with almost no hydrocarbons in common. <it>Cr. modiglianii </it>discriminated between these two varieties. It only tolerated workers of the <it>Ca. rufifemur </it>variety it was associated with, but attacked the respective others. However, <it>Cr. modiglianii </it>did not distinguish its own <it>Ca. rufifemur </it>partner from allocolonial <it>Ca. rufifemur </it>workers of the same variety.</p> <p>Conclusion</p> <p>We conclude that there is a mutual substance transfer between <it>Cr. modiglianii </it>and <it>Ca. rufifemur</it>. <it>Ca. rufifemur </it>actively or passively acquires cuticular steroids from its <it>Cr. modiglianii </it>partner, while the latter acquires at least two cuticular hydrocarbons from <it>Ca. rufifemur</it>. The cuticular substances of both species are highly unusual regarding both substance classes and chain lengths, which may cause the apparent inability of <it>Cr. modiglianii </it>to discriminate <it>Ca. rufifemur </it>nestmates from allocolonial <it>Ca. rufifemur </it>workers of the same chemical variety.</p

In search of cues: dung beetle attraction and the significance of volatile composition of dung

Volatile organic compounds (VOCs) provide animals with multiple cues about location, type, and condition of valuable resources such as food. In particular, odour cues are often essential for the localization and discrimination of resources with patchy distribution. Dung beetles (Scarabaeoidea) rely on such scented resources to locate food for their own diet and to provision their progeny. Despite the beetles’ mostly generalist choice across dung types, several studies showed that the beetles prefer some dung types over others. Yet, the importance of VOCs for dung localization and differentiation remains unclear. In this study, we used six single chemical components (indole, skatole, phenol, butyric acid, 2-butanone, and p-cresol), two different blends of these components, and six different dung types for a detailed behavioural analysis of dung beetles. We found very little specialization of beetle species towards specific VOCs. We found that dung baits and baits with synthetic compounds attracted similar communities of dung beetles, but the visitors of synthetic baits exhibited much lower diversity and abundance. The analysis of dung scent profiles of six types of dung revealed both, unique patterns in composition and ubiquitous components such as p-cresol. However, when we used a six-component blend of synthetic compounds, it turned out to be as attractive as three of the most attractive dung types in the field. Our findings highlight the significance of key VOCs, but, moreover, that dung beetles use a blend of specific components for resource localization

An automated device for the digitization and 3D modelling of insects, combining extended-depth-of-field and all-side multi-view imaging

Digitization of natural history collections is a major challenge in archiving biodiversity. In recent years, several approaches have emerged, allowing either automated digitization, extended depth of field (EDOF) or multi-view imaging of insects. Here, we present DISC3D: a new digitization device for pinned insects and other small objects that combines all these aspects. A PC and a microcontroller board control the device. It features a sample holder on a motorized two-axis gimbal, allowing the specimens to be imaged from virtually any view. Ambient, mostly reflection-free illumination is ascertained by two LED-stripes circularly installed in two hemispherical white-coated domes (front-light and back-light). The device is equipped with an industrial camera and a compact macro lens, mounted on a motorized macro rail. EDOF images are calculated from an image stack using a novel calibrated scaling algorithm that meets the requirements of the pinhole camera model (a unique central perspective). The images can be used to generate a calibrated and real color texturized 3Dmodel by ‘structure from motion’ with a visibility consistent mesh generation. Such models are ideal for obtaining morphometric measurement data in 1D, 2D and 3D, thereby opening new opportunities for trait-based research in taxonomy, phylogeny, eco-physiology, and functional ecology

Indices, Graphs and Null Models: Analyzing Bipartite Ecological Networks

Many analyses of ecological networks in recent years have introduced new indices to describe network properties. As a consequence, tens of indices are available to address similar questions, differing in specific detail, sensitivity in detecting the property in question, and robustness with respect to network size and sampling intensity. Furthermore, some indices merely reflect the number of species participating in a network, but not their interrelationship, requiring a null model approach. Here we introduce a new, free software calculating a large spectrum of network indices, visualizing bipartite networks and generating null models. We use this tool to explore the sensitivity of 26 network indices to network dimensions, sampling intensity and singleton observations. Based on observed data, we investigate the interrelationship of these indices, and show that they are highly correlated, and heavily influenced by network dimensions and connectance. Finally, we re-evaluate five common hypotheses about network properties, comparing 19 pollination networks with three differently complex null models: 1. The number of links per species (“degree”) follow (truncated) power law distributions. 2. Generalist pollinators interact with specialist plants, and vice versa (dependence asymmetry). 3. Ecological networks are nested. 4. Pollinators display complementarity, owing to specialization within the network. 5. Plant-pollinator networks are more robust to extinction than random networks. Our results indicate that while some hypotheses hold up against our null models, others are to a large extent understandable on the basis of network size, rather than ecological interrelationships. In particular, null model pattern of dependence asymmetry and robustness to extinctio

In search of cues: dung beetle attraction and the significance of volatile composition of dung

Volatile organic compounds (VOCs) provide animals with multiple cues about location, type, and condition of valuable resources such as food. In particular, odour cues are often essential for the localization and discrimination of resources with patchy distribution. Dung beetles (Scarabaeoidea) rely on such scented resources to locate food for their own diet and to provision their progeny. Despite the beetles’ mostly generalist choice across dung types, several studies showed that the beetles prefer some dung types over others. Yet, the importance of VOCs for dung localization and differentiation remains unclear. In this study, we used six single chemical components (indole, skatole, phenol, butyric acid, 2-butanone, and p-cresol), two different blends of these components, and six different dung types for a detailed behavioural analysis of dung beetles. We found very little specialization of beetle species towards specific VOCs. We found that dung baits and baits with synthetic compounds attracted similar communities of dung beetles, but the visitors of synthetic baits exhibited much lower diversity and abundance. The analysis of dung scent profiles of six types of dung revealed both, unique patterns in composition and ubiquitous components such as p-cresol. However, when we used a six-component blend of synthetic compounds, it turned out to be as attractive as three of the most attractive dung types in the field. Our findings highlight the significance of key VOCs, but, moreover, that dung beetles use a blend of specific components for resource localization

Societies Drifting Apart? Behavioural, Genetic and Chemical Differentiation between Supercolonies in the Yellow Crazy Ant Anoplolepis gracilipes

Background: In populations of most social insects, gene flow is maintained through mating between reproductive individuals from different colonies in periodic nuptial flights followed by dispersal of the fertilized foundresses. Some ant species, however, form large polygynous supercolonies, in which mating takes place within the maternal nest (intranidal mating) and fertilized queens disperse within or along the boundary of the supercolony, leading to supercolony growth (colony budding). As a consequence, gene flow is largely confined within supercolonies. Over time, such supercolonies may diverge genetically and, thus, also in recognition cues (cuticular hydrocarbons, CHC’s) by a combination of genetic drift and accumulation of colony-specific, neutral mutations. Methodology/Principal Findings: We tested this hypothesis for six supercolonies of the invasive ant Anoplolepis gracilipes in north-east Borneo. Within supercolonies, workers from different nests tolerated each other, were closely related and showed highly similar CHC profiles. Between supercolonies, aggression ranged from tolerance to mortal encounters and was negatively correlated with relatedness and CHC profile similarity. Supercolonies were genetically and chemically distinct, with mutually aggressive supercolony pairs sharing only 33.1%617.5% (mean 6 SD) of their alleles across six microsatellite loci and 73.8%611.6% of the compounds in their CHC profile. Moreover, the proportion of alleles that differed between supercolony pairs was positively correlated to the proportion of qualitatively different CHC compounds. These qualitatively differing CHC compounds were found across various substance classes including alkanes, alkenes and mono-, di- and trimethyl-branched alkanes. Conclusions: We conclude that positive feedback between genetic, chemical and behavioural traits may further enhance supercolony differentiation through genetic drift and neutral evolution, and may drive colonies towards different evolutionary pathways, possibly including speciation