Modularity and robustness of a plant-frugivore interaction network in a disturbed tropical forest

<p>Tropical forests are undergoing a biodiversity crisis including defaunation processes. Structure and function of biotic communities in disturbed ecosystems can be assessed with network analyses of interspecific interactions. In a disturbed tropical forest we studied the network of interactions between fruiting plants and three groups of frugivorous animals, determining the structure, modular roles of species and the robustness of the network under simulated extinction scenarios based on species strength (SS) (relevance), species role (SR) (connectivity) and body size (vulnerability). We recorded 5,347 interactions between 63 frugivore and 60 plant species. The network had a modular structure with four modules: (1) plants with exposed fruits consumed by bats; (2) plants with juicy fruits consumed by larger birds; (3) plants with dry high-lipid-content fruits consumed by migratory birds; and (4) generalist plants consumed by several frugivores (three guilds). Robustness of the network was lower when species with high SS or important connectivity roles (i.e., module hubs) were lost first; when larger animals were lost first the network was more robust than in the other scenarios. Our results suggest that modularity may protect disturbed communities against species loss. We provide bases for developing ecologically-sound measures for the conservation of ecological interactions in anthropogenic landscapes.</p

Results of the repeated measures analysis of variance (ANOVA) for the effect of census and vegetation type on structural network parameters (A) and the plant community attributes (B).

<p>Significant values of P (≤0.05) are in bold and italics.</p><p>Results of the repeated measures analysis of variance (ANOVA) for the effect of census and vegetation type on structural network parameters (A) and the plant community attributes (B).</p

Vegetation type and seasonality effect on network parameters and plant community attributes.

<p>Vegetation types (PIO: pioneer dune vegetation, DUN: coastal dune scrub, FFW: flood forest with a wetland ecotone, SFY: recently established tropical lowland subdeciduous forest, SFO: tropical lowland subdeciduous forest in an advanced stage of succession). Census (DS1: Dry season 1, DS2: Dry season 2, RS1: Rainy season 1, RS2: Rainy season 2). Network parameters (H²: network specialization, M: Modularity, C: Connectance). Plant community attributes (TPR: Total plant richness, HPR: Host plant richness, HPC: Host plant cover). (A) Network parameters by vegetation type. (B) Network parameters by census. (C) Plant community attributes by vegetation type. (D) Plant community attributes by census. Comparisons are valid only within each network parameter (panels A and B, for vegetation types and census, respectively) and within each plant community attribute (panels C and D, for vegetation types and census, respectively). Bars indicate standard error of the mean. Different letters represent significant differences resulting from post-hoc test (P<0.05).</p

NMDS ordination of lepidopteran herbivore species composition showing the spatio-temporal variation in sub-networks.

<p>The plus signs (+) indicate different herbivore species upon which the sub-network ordination was performed. The vectors on the ordination represent the gradient in network parameter values across all sub-networks (for habitats abbreviations see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0110430#pone-0110430-g001" target="_blank">Figure 1</a>): H²: network specialization, M: Modularity, NM: Number of modules, C: Connectance. The arrow points to the direction in which the linear change in network parameters was strongest, and the length of the arrow is proportional to the correlation between these variables and the ordination of herbivore species composition.</p

Relative modularity (Mr) based in z-score values estimated for the vegetation types and censuses (mean ±SD) in plant-herbivore sub-networks.

<p>Different letters among vegetation types and censuses represent significant differences in post-hoc test at P≤0.05. See Methods section for vegetation types and censuses abbreviations.</p><p>Relative modularity (Mr) based in z-score values estimated for the vegetation types and censuses (mean ±SD) in plant-herbivore sub-networks.</p

NMDS ordination of network parameters showing the spatio-temporal variation of plant-herbivore sub-networks.

<p>The plus signs (+) indicate the network parameters upon which the sub-network ordination was performed (H²: network specialization, M: Modularity, C: Connectance, IE: interaction evenness, V: vulnerability, G: generality). The vectors on the ordination represent the gradient in HPR (Host plant richness) and HPC (Host plant cover) for all sub-networks. The arrow points to the direction in which the linear change in HPR and HPC was strongest, and the length of the arrow is proportional to the correlation between these variables and the sub-network ordination. Arrows were plotted only for variables with significance of P≤0.05. Factors fit in the ordination (season and habitat type) are not included. For habitats abbreviations see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0110430#pone-0110430-g001" target="_blank">Figure 1</a>.</p

Appendix A. Species codes and identities.

Species codes and identities

Appendix B. Dissimilarity dendrograms for ant species in Networks 1990 and 2000.

Dissimilarity dendrograms for ant species in Networks 1990 and 2000

Network attributes modeling tree hollow-saproxylic interacting patterns per woodland site.

<p>DO: deciduous oak; RA: riparian ash; SO: sclerophyllous oak; <i>N</i> (NODF): nestedness using <i>NODF</i> estimator; <i>WNODF</i>: weighted nestedness; <i>M</i> (SA): modularity index using the simulating annealing procedure; <i>L/S</i>: links per species; <i>C</i>: connectance; <i>LD</i>: linkage density; <i>V-ratio</i>: variance ratio; R RE: robustness for a random extinction of tree hollows; R DE1: robustness for a directed extinction from the least to the most connected tree hollows; R DE2: robustness for a directed extinction from the most to the least connected tree hollows.</p><p>Network attributes modeling tree hollow-saproxylic interacting patterns per woodland site.</p

Appendix C. Dissimilarity dendrograms for plant species for Networks 1990 and 2000.

Dissimilarity dendrograms for plant species for Networks 1990 and 2000