Nestedness and modularity have been found in many species interaction networks. Despite being con... more Nestedness and modularity have been found in many species interaction networks. Despite being conceptually distinct, negatively correlated and having different causes, these patterns often co‐occur. A realistic but seldom investigated alternative to these simple topologies is hierarchical compound networks, in which the entire network is modular, and modules are internally nested. In compound networks, nestedness is suppressed by modularity at higher network hierarchical levels, but prevails at lower levels, within modules. The aims of this study are (i) to evaluate the prevalence of simple and hierarchical compound topologies in binary and weighted networks describing different kinds of species interactions and (ii) to probe the relationships between modularity and nestedness at different network hierarchical levels. With a procedure that discriminates between simple and compound structures, we re‐analysed the topology of 142 well‐studied binary networks including seed dispersal, host–parasite, pollination and plant–herbivore interactions; 68 of these also had quantitative information. Additionally, we tested the relationship between robustness and topology of binary networks and compared the robustness of networks with compound topologies to different sequences of species removals. Compound topologies were detected in 34% of binary and 71% of weighted networks of all interaction kinds. These results establish the hierarchical compound topology as a widespread network architecture, often undetected without quantitative data. Furthermore, they disentangle an apparent paradox: despite conflicting with overall nestedness, modularity usually co‐occurs with high values of low‐level nestedness. Nestedness progressively decreased, while modularity increased, from seed dispersal to host–parasite, pollination and plant–herbivore networks. There were no consistent differences in the robustness of networks with nested and compound topologies. However, compound topologies were especially vulnerable to removal sequences that accelerate the exclusion of entire modules. Compound topologies improve the depiction of ecological networks and differentiate ecological and evolutionary processes that operate at different hierarchical levels, with the potential to advance our understanding of network dynamics, stability and response to species loss or change. Quantitative data often reveal specialization patterns that are indistinguishable in binary networks, strongly improving the detection of modular and compound topologies.
Hierarchical compound topologies of interaction networks that are segmented into internally neste... more Hierarchical compound topologies of interaction networks that are segmented into internally nested modules have received scant attention, compared to simple nested and modular topologies. This is due to the lack of a theoretical model that encompasses all relevant alternative topologies, and an effective method to differentiate compound from simple topologies. Here we present a framework to address compound topologies in ecological networks. We propose a mechanistic schema of processes that generate modular, nested and compound topologies, based on differences in availability and preferences among species. We show that, in combination, these processes produce a unique signature in the structure of compound topologies. Then, we propose a procedure to identify this signature and hence discriminate among simple and compound topologies. We test the efficiency of this procedure in a set of synthetic matrices and then apply it to an actual plant–herbivore network (Asteraceae and their flowehead feeders in Brazil). In this case, the compound topology is clearly substantiated, demonstrating that these advances are applicable to empirical ecological networks and that compound patterns decidedly belong to the array of topologies to be probed in interaction assemblages at various scales. By including extramodular structures in the analysis of compound topologies, we enhance our understanding of how community‐wide networks are organized and their responses to various drivers.
Among-population variation in host use is a common phenomenon in herbivorous insects. The simples... more Among-population variation in host use is a common phenomenon in herbivorous insects. The simplest and most trivial explanation for such variation in host use is the among-site variation in plant species composition. Another aspect that can influence spatial variation in host use is the relative abundance of each host-plant species compared to all available hosts. Here, we used endophagous insects that develop in flower heads of Asteraceae species as a study system to investigate how plant abundance influences the pattern of host-plant use by herbivorous insects with distinct levels of host-range specialization. Only herbivores recorded on three or more host species were included in this study. In particular, we tested two related hypotheses: 1) plant abundance has a positive effect on the host-plant preference of herbivorous insects, and 2) the relative importance of plant abundance to host-plant preference is greater for herbivorous species that use a wider range of host-plant species. We analyzed 11 herbivore species in 20 remnants of Cerrado in Southeastern Brazil. For 8 out of 11 herbivore species, plant abundance had a positive influence on host use. In contrast to our expectation, both the most specialized and the most generalist herbivores showed a stronger positive effect of plant species abundance in host use. Thus, we found evidence that although the abundance of plant species is a major factor determining the preferential use of host plants, its relative importance is mediated by the host-range specialization of herbivores.
Agromyzids are well documented on plants of economic importance, especially those species whose l... more Agromyzids are well documented on plants of economic importance, especially those species whose larvae leave external marks on the plant, such as mines. They are far less known for non-commercial plants, particularly in tropical countries. In North American compilations of agromyzids, most of the species associated with Asteraceae flower heads belong in the genus Melanagromyza Hendel. Here we record five new seed-eating Melanagromyza species from the Brazilian Cerrado in the state of São Paulo: M. falciformis sp. nov., M. lanciformis sp. nov., M. longomembranacea sp. nov., M. multivora sp. nov. and M. vernonieaevora sp. nov.
Nestedness and modularity have been found in many species interaction networks. Despite being con... more Nestedness and modularity have been found in many species interaction networks. Despite being conceptually distinct, negatively correlated and having different causes, these patterns often co‐occur. A realistic but seldom investigated alternative to these simple topologies is hierarchical compound networks, in which the entire network is modular, and modules are internally nested. In compound networks, nestedness is suppressed by modularity at higher network hierarchical levels, but prevails at lower levels, within modules. The aims of this study are (i) to evaluate the prevalence of simple and hierarchical compound topologies in binary and weighted networks describing different kinds of species interactions and (ii) to probe the relationships between modularity and nestedness at different network hierarchical levels. With a procedure that discriminates between simple and compound structures, we re‐analysed the topology of 142 well‐studied binary networks including seed dispersal, host–parasite, pollination and plant–herbivore interactions; 68 of these also had quantitative information. Additionally, we tested the relationship between robustness and topology of binary networks and compared the robustness of networks with compound topologies to different sequences of species removals. Compound topologies were detected in 34% of binary and 71% of weighted networks of all interaction kinds. These results establish the hierarchical compound topology as a widespread network architecture, often undetected without quantitative data. Furthermore, they disentangle an apparent paradox: despite conflicting with overall nestedness, modularity usually co‐occurs with high values of low‐level nestedness. Nestedness progressively decreased, while modularity increased, from seed dispersal to host–parasite, pollination and plant–herbivore networks. There were no consistent differences in the robustness of networks with nested and compound topologies. However, compound topologies were especially vulnerable to removal sequences that accelerate the exclusion of entire modules. Compound topologies improve the depiction of ecological networks and differentiate ecological and evolutionary processes that operate at different hierarchical levels, with the potential to advance our understanding of network dynamics, stability and response to species loss or change. Quantitative data often reveal specialization patterns that are indistinguishable in binary networks, strongly improving the detection of modular and compound topologies.
Hierarchical compound topologies of interaction networks that are segmented into internally neste... more Hierarchical compound topologies of interaction networks that are segmented into internally nested modules have received scant attention, compared to simple nested and modular topologies. This is due to the lack of a theoretical model that encompasses all relevant alternative topologies, and an effective method to differentiate compound from simple topologies. Here we present a framework to address compound topologies in ecological networks. We propose a mechanistic schema of processes that generate modular, nested and compound topologies, based on differences in availability and preferences among species. We show that, in combination, these processes produce a unique signature in the structure of compound topologies. Then, we propose a procedure to identify this signature and hence discriminate among simple and compound topologies. We test the efficiency of this procedure in a set of synthetic matrices and then apply it to an actual plant–herbivore network (Asteraceae and their flowehead feeders in Brazil). In this case, the compound topology is clearly substantiated, demonstrating that these advances are applicable to empirical ecological networks and that compound patterns decidedly belong to the array of topologies to be probed in interaction assemblages at various scales. By including extramodular structures in the analysis of compound topologies, we enhance our understanding of how community‐wide networks are organized and their responses to various drivers.
Among-population variation in host use is a common phenomenon in herbivorous insects. The simples... more Among-population variation in host use is a common phenomenon in herbivorous insects. The simplest and most trivial explanation for such variation in host use is the among-site variation in plant species composition. Another aspect that can influence spatial variation in host use is the relative abundance of each host-plant species compared to all available hosts. Here, we used endophagous insects that develop in flower heads of Asteraceae species as a study system to investigate how plant abundance influences the pattern of host-plant use by herbivorous insects with distinct levels of host-range specialization. Only herbivores recorded on three or more host species were included in this study. In particular, we tested two related hypotheses: 1) plant abundance has a positive effect on the host-plant preference of herbivorous insects, and 2) the relative importance of plant abundance to host-plant preference is greater for herbivorous species that use a wider range of host-plant species. We analyzed 11 herbivore species in 20 remnants of Cerrado in Southeastern Brazil. For 8 out of 11 herbivore species, plant abundance had a positive influence on host use. In contrast to our expectation, both the most specialized and the most generalist herbivores showed a stronger positive effect of plant species abundance in host use. Thus, we found evidence that although the abundance of plant species is a major factor determining the preferential use of host plants, its relative importance is mediated by the host-range specialization of herbivores.
Agromyzids are well documented on plants of economic importance, especially those species whose l... more Agromyzids are well documented on plants of economic importance, especially those species whose larvae leave external marks on the plant, such as mines. They are far less known for non-commercial plants, particularly in tropical countries. In North American compilations of agromyzids, most of the species associated with Asteraceae flower heads belong in the genus Melanagromyza Hendel. Here we record five new seed-eating Melanagromyza species from the Brazilian Cerrado in the state of São Paulo: M. falciformis sp. nov., M. lanciformis sp. nov., M. longomembranacea sp. nov., M. multivora sp. nov. and M. vernonieaevora sp. nov.
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