Rafael L. G. Raimundo

Gatekeeping Twitter: Message diffusion in political hashtags

This article explores the structure of gatekeeping in Twitter by means of a statistical analysis of the political hashtags #FreeIran, #FreeVenezuela and #Jan25, each of which reached the top position in Twitter Trending Topics. We performed a statistical correlation analysis on nine variables of the dataset to evaluate if message replication in Twitter political hashtags was correlated with network topology. Our results suggest an alternative scenario to the dominant view regarding gatekeeping in Twitter political hashtags. Instead of depending on hubs that act as gatekeepers, we found that the intense activity of individuals with relatively few connections is capable of generating highly replicated messages that contributed to Trending Topics without relying on the activity of user hubs. The results support the thesis of social consensus through the influence of committed minorities, which states that a prevailing majority opinion in a population can be rapidly reversed by a small fraction of randomly distributed committed agents.

Daily activity schedule, gregariousness, and defensive behaviour in the neotropical harvestman Goniosoma longipes (Opiliones: Gonyleptidae).

In this paper we provide a field account of some aspects of the behavioural biology of Goniosoma longipes (Roewer), a harvestman which commonly occurs in caves in South-east Brazil. During daytime, solitary and aggregated individuals can be found resting on the cave walls. Just after sunset, however, many individuals leave the cave to forage for live and dead arthropods. Foraging individuals return to the cave before dawn. Aggregations of G. longipes contain on average 34 individuals (range 7-200), and the groups are usually found close to the water source and away from the cave entrance. The main predators of G. longipes are the spider Ctenus fasciatus Mello-Leitao (Ctenidae) and the opossum Philander opossum (L.) (Didelphidae). Upon disturbance solitary and aggregated individuals may either flee, or drop from the cave wall or vegetation. The harvestmen can also release a repugnatory liquid upon manipulation, and aggregated individuals collectively discharge this secretion toward the aggressor before fleeing. The activity schedule of G. longipes shows that individuals need to leave the cave periodically to forage, and therefore the population can be considered trogloxene. Data on the food items collected by G. longipes indicate that the harvestman is a generalist predator which also feeds on dead animal matter. Gregarious behaviour is considered relatively common among harvestmen and has been interpreted in several ways. We suggest that gregarious behaviour in G. longipes may be related with the choice of more suitable microconditions in the cave habitat and/or with group chemical defence.

Long-term temporal variation in the organization of an ant–plant network

BACKGROUND AND AIMS Functional groups of species interact and coevolve in space and time, forming complex networks of interacting species. A long-term study of temporal variation of an ant-plant network is presented with the aims of: (1) depicting its structural changes over a 20-year period; (2) detailing temporal variation in network topology, as revealed by nestedness and modularity analysis and other parameters (i.e. connectance, niche overlap); and (3) identifying long-term turnover in taxonomic structure (i.e. switches in ant resource use or plant visitor assemblages according to taxa). METHODS Fieldwork was carried out at La Mancha, Mexico, and ant-plant interactions were observed between 1989 and 1991, between 1998 and 2000, and between May 2010 and 2011. Occurrences of ants on extrafloral nectaries (EFNs) were recorded. The resulting ant-plant networks were constructed from qualitative presence-absence data determined by a species-species matrix defined by the frequency of occurrence of each pairwise ant-plant interaction. KEY RESULTS Network variation across time was stable and a persistent nested structure may have contributed to the maintenance of resilient and species-rich communities. Modularity was lower than expected, especially in the most recent networks, indicating that the community exhibited high overlap among interacting species (e.g. few species were hubs in the more recent network, being partly responsible for the nested pattern). Structurally, the connections created among modules by super-generalists gave cohesion to subsets of species that otherwise would remain unconnected. This may have allowed an increasing cascade-effect of evolutionary events among modules. Mutualistic ant-plant interactions were structured 20 years ago mainly by the subdominant nectarivorous ant species Camponotus planatus and Crematogaster brevispinosa, which monopolized the best extrafloral nectar resources and out-competed other species with broader feeding habits. Through time, these ants, which are still present, lost their position as network hubs and diminished in their importance in structuring the network; simultaneously, plants gained in importance. CONCLUSIONS The long-term network analysis reveals a decrease in attended plant species richness, a notable increase in plant species participation from 1990 to 2010 (sustained by less plant taxonomic similarity in the older 1990 network), an increase in the number of ant species and a diminishing dominance of super-generalist ants. The structure of the community has remained highly nested and connected with low modularity, suggesting overall a more participative, homogeneous, cohesive interaction network. Although previous studies have suggested that interactions between ants and EFN-bearing plants are susceptible to seasonality, abiotic factors and perturbation, this cohesive structure appears to be the key for biodiversity and community maintenance.

Diversidade e ocorrência de Asteraceae em cerrados de São Paulo

De abril a maio de 2000 a 2002 oito localidades com remanescentes de cerrados sensu stricto no estado de Sao Paulo foram amostradas para o levantamento das especies de asteraceas, uma das familias mais representativas da flora herbaceo-arbustiva nestas formacoes. Foram feitas 23 amostragens e cada area de estudo foi amostrada em media uma vez por ano durante o pico de floracao das plantas. Ao todo foram obtidas 399 amostras, nas quais foram reconhecidas 89 morfoespecies (74 foram identificadas como especies conhecidas). Quarenta por cento das especies foram registradas uma unica vez (unicatas), indicando um grande numero de especies raras. Apenas 10% das especies que ocorreram em mais de uma amostra foram obtidas de uma mesma area (sobreposicao espacial) ou de um mesmo ano de estudo (sobreposicao temporal). A riqueza de especies em cada area foi estimada por meio de transecoes e depois comparada a riqueza total observada em cada area de estudo, sendo esta na maioria das vezes mais alta que a estimada com base nas transecoes. A lista de especies obtida para os cerrados amostrados foi comparada a outras 24 listas publicadas para cerrados no Brasil. Embora a maioria das especies mais comuns tenha coincidido, oito especies (11% das especies identificadas) nao constam das listas publicadas. Concluimos que as areas de cerrado sensu stricto estudadas no estado de Sao Paulo encontram-se isoladas, com uma grande parte da flora herbaceo-arbustiva composta por varias especies raras e exclusivas. Diante deste quadro, sugerimos que a manutencao da biodiversidade de Asteraceae depende da conservacao de todo o conjunto de remanescentes de cerrado do estado de Sao Paulo.

Seasonal Patterns in Activity Rhythm and Foraging Ecology in the Neotropical Forest-Dwelling Ant, Odontomachus chelifer (Formicidae: Ponerinae)

ABSTRACT We provide qualitative and quantitative data on the natural history and foraging behavior of the ground-dwelling ant Odontomachus chelifer (Latreille) (Formicidae: Ponerinae) in a forest reserve in southeastern Brazil, with emphasis on colony activity rhythms and diet preferences in relation to seasonal availability of potential food items in the leaf litter. Ant colonies exhibited nocturnal activity throughout the year, and they foraged significantly more intensively in the wet/ warm (November–March) than in the cold/dry season (April–October). As the night begins, small groups of workers disperse and hunt individually on a wide diversity of litter arthropods of variable sizes. At dusk, encounters with foragers of the diurnal ponerine Pachycondyla striata Fr. Smith were conspicuously avoided by O. chelifer, which occasionally had their prey robbed by the former or were even taken as prey themselves. Termites were the preferred prey of O. chelifer, making up 40% of the food items captured in each season. Seasonal comparisons of prey organisms captured by O. chelifer, and of litter-dwelling arthropods sampled in pitfall traps, revealed that the frequency distribution of retrieved prey in each taxonomic group did not differ seasonally, despite the 2.7-fold increase in the overall availability of litter arthropods in the warm/wet period. This result supports foraging theory by showing that preference for certain animal prey types (i.e., taxonomic groups) persists through time despite seasonal fluctuations in the overall availability of potential prey on the forest floor. This study points out to the importance of studying ant foraging ecology and diet preferences in a natural context.

Assemblages of endophagous insects on Asteraceae in São Paulo Cerrados

A survey of the endophagous insects fauna associated to Asteraceae capitula was carried out from 2000 to 2002 in eight cerrado sensu stricto sites located in the Brazilian state of Sdo Paulo. Sixty-four endophagous species of Diptera and Lepidoptera were recorded from 49 asteracean host plants. Approximately half of the species were obtained from a single locality, with a large proportion emerging from a single sample (unicates). Thirty percent of the species were singletons (i.e. only one individual was recorded). The large proportion of rare species suggests a high species turnover among localities. Lepidopteran species were recorded on more host species than dipterans, confirming their more polyphagous food habit, also observed in other Brazilian biomes and in Europe. We conclude that the studied cerrado localities, all within São Paulo State, are isolated with its invertebrate fauna composed of many rare and exclusive species. We suggest that the maintenance of Asteraceae biodiversity and their endophagous insects depend on the conservation of all cerrado remnants in the state.

Conflicting Selection in the Course of Adaptive Diversification: The Interplay between Mutualism and Intraspecific Competition

Adaptive speciation can occur when a population undergoes assortative mating and disruptive selection caused by frequency-dependent intraspecific competition. However, other interactions, such as mutualisms based on trait matching, may generate conflicting selective pressures that constrain species diversification. We used individual-based simulations to explore how different types of mutualism affect adaptive diversification. A magic trait was assumed to simultaneously mediate mate choice, intraspecific competition, and mutualisms. In scenarios of intimate, specialized mutualisms, individuals interact with one or few individual mutualistic partners, and diversification is constrained only if the mutualism is obligate. In other scenarios, increasing numbers of different partners per individual limit diversification by generating stabilizing selection. Stabilizing selection emerges from the greater likelihood of trait mismatches for rare, extreme phenotypes than for common intermediate phenotypes. Constraints on diversification imposed by increased numbers of partners decrease if the trait matching degree has smaller positive effects on fitness. These results hold after the relaxation of various assumptions. When trait matching matters, mutualism-generated stabilizing selection would thus often constrain diversification in obligate mutualisms, such as ant-myrmecophyte associations, and in low-intimacy mutualisms, including plant-seed disperser systems. Hence, different processes, such as trait convergence favoring the incorporation of nonrelated species, are needed to explain the higher richness of low-intimacy assemblages—shown here to be up to 1 order of magnitude richer than high-intimacy systems.

Nestedness across biological scales

Biological networks pervade nature. They describe systems throughout all levels of biological organization, from molecules regulating metabolism to species interactions that shape ecosystem dynamics. The network thinking revealed recurrent organizational patterns in complex biological systems, such as the formation of semi-independent groups of connected elements (modularity) and non-random distributions of interactions among elements. Other structural patterns, such as nestedness, have been primarily assessed in ecological networks formed by two non-overlapping sets of elements; information on its occurrence on other levels of organization is lacking. Nestedness occurs when interactions of less connected elements form proper subsets of the interactions of more connected elements. Only recently these properties began to be appreciated in one-mode networks (where all elements can interact) which describe a much wider variety of biological phenomena. Here, we compute nestedness in a diverse collection of one-mode networked systems from six different levels of biological organization depicting gene and protein interactions, complex phenotypes, animal societies, metapopulations, food webs and vertebrate metacommunities. Our findings suggest that nestedness emerge independently of interaction type or biological scale and reveal that disparate systems can share nested organization features characterized by inclusive subsets of interacting elements with decreasing connectedness. We primarily explore the implications of a nested structure for each of these studied systems, then theorize on how nested networks are assembled. We hypothesize that nestedness emerges across scales due to processes that, although system-dependent, may share a general compromise between two features: specificity (the number of interactions the elements of the system can have) and affinity (how these elements can be connected to each other). Our findings suggesting occurrence of nestedness throughout biological scales can stimulate the debate on how pervasive nestedness may be in nature, while the theoretical emergent principles can aid further research on commonalities of biological networks.

Adaptive Networks for Restoration Ecology

The urgent need to restore biodiversity and ecosystem functioning challenges ecology as a predictive science. Restoration ecology would benefit from evolutionary principles embedded within a framework that combines adaptive network models and the phylogenetic structure of ecological interactions. Adaptive network models capture feedbacks between trait evolution, species abundances, and interactions to explain resilience and functional diversity within communities. Phylogenetically-structured network data, increasingly available via next-generation sequencing, inform constraints affecting interaction rewiring. Combined, these approaches can predict eco-evolutionary changes triggered by community manipulation practices, such as translocations and eradications of invasive species. We discuss theoretical and methodological opportunities to bridge network models and data from restoration projects and propose how this can be applied to the functional restoration of ecological interactions.

Ecology and Evolution of Species-Rich Interaction Networks

The perception that the complexity of tropical ecological interactions is both a product of evolutionary processes and a feedstock for evolution lies at the origin of Evolutionary Ecology. We now have the opportunity to revisit this foundational perception to gain insight into the processes shaping biodiversity structure and ecosystem functioning. Such an opportunity arises from the ongoing theoretical integration between ecological and evolutionary theories, alongside with the application of the network approach to characterize the structure and dynamics of multi-species communities. In this chapter, we focus on the fundamental aspects of ecological, evolutionary, and eco-evolutionary theories underlying the network approach to the study of multi-species systems, such as megadiverse tropical communities. Together, these perspectives illustrate the challenges we shall face in the decades to come in order to take advantage of ongoing theoretical integration, the gradual accumulation of data on tropical interactions, and the availability of robust analytical and computational tools to enlighten the processes shaping biodiversity.