Although species and their interactions in unison represent biodiversity and all the ecological and evolutionary processes associated with life, biotic interactions have, contrary to species, rarely been integrated into the concepts of spatial β-diversity. Here, we examine β-diversity of ecological networks by using pollination networks sampled across the Canary Islands. We show that adjacent and distant communities are more and less similar, respectively, in their composition of plants, pollinators and interactions than expected from random distributions. We further show that replacement of species is the major driver of interaction turnover and that this contribution increases with distance. Finally, we quantify that species-specific partner compositions (here called partner fidelity) deviate from random partner use, but vary as a result of ecological and geographical variables. In particular, breakdown of partner fidelity was facilitated by increasing geographical distance, changing abundances and changing linkage levels, but was not related to the geographical distribution of the species. This highlights the importance of space when comparing communities of interacting species and may stimulate a rethinking of the spatial interpretation of interaction networks. Moreover, geographical interaction dynamics and its causes are important in our efforts to anticipate effects of large-scale changes, such as anthropogenic disturbances.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2014.2925

Geographical variation in mutualistic networks: similarity, turnover and partner fidelity

The diversity, ecology and evolution of extrafloral nectaries: current perspectives and future challenges.

All mutualistic plant–animal interactions are mediated by costs and benefits in relationships where resources (from plants) are exchanged by services (from animals). The most common trading coin that plants offer to pay for animal services is nectar; the main servers are hymenopterans. Extrafloral nectar (EFN) is produced in almost all aboveground plant parts not directly related with pollination, and their true function has long been an issue of discussion among naturalists and will be our main subject. The protective function of extrafloral nectaries (EFNs) is reviewed and considered with an alternative hypothesis, presenting not only ants, but also spiders and wasps as potential and effective agents in these protective interactions. Despite their likely relevance, the phenological variation (mainly sequential flowering and resprouting) of host plants mediating these interactions have been generally ignored. We discuss how the outcomes of each ant–EFN bearing plant interaction vary depending on physical and biotic changes in interacting organisms (internal factors such as phenology and species identity) as well as in their environments (external factors such as climatic variation), all of which may modify the character of each interaction. We propose that ant–EFN bearing plant interactions serve an excellent and unique model to test the “Geographic Mosaic Theory” of coevolution providing us a more clear view of how evolution has structured these plant–animal ecological networks.

Loss and gains in ant–plant interactions mediated by extrafloral nectar: fidelity, cheats, and lies

Most tropical plants rely on animals for pollination, thus engaging in complex interaction networks. Here, we present a global overview of pollination networks and point out research gaps and emerging differences between tropical and non-tropical areas. Our review highlights an uneven global distribution of studies biased towards non-tropical areas. Moreover, within the tropics, there is a bias towards the Neotropical region where partial networks represent 70.1% of the published studies. Additionally, most networks sampled so far (95.6%) were assembled by inferring interactions by surveying plants (a phytocentric approach). These biases may limit accurate global comparisons of the structure and dynamics of tropical and non-tropical pollination networks. Noteworthy differences of tropical networks (in comparison to the non-tropical ones) include higher species richness which, in turn, promotes lower connectance but higher modularity due to both the higher diversity as well as the integration of more vertebrate pollinators. These interaction patterns are influenced by several ecological, evolutionary, and historical processes, and also sampling artifacts. We propose a neutral–niche continuum model for interactions in pollination systems. This is, arguably, supported by evidence that a high diversity of functional traits promotes greater importance of niche-based processes (i.e., forbidden links caused by morphological mismatching and phenological non-overlap) in determining which interactions occur, rather than random chance of encounter based on abundances (neutrality). We conclude by discussing the possible existence and direction of a latitudinal gradient of specialization in pollination networks.

Plant-Pollinator Networks in the Tropics: A Review

Despite the importance and increasing knowledge of ecological networks, sampling effort and intrapopulation variation has been widely overlooked. Using continuous daily sampling of ants visiting three plant species in the Brazilian Neotropical savanna, we evaluated for the first time the topological structure over 24 h and species-area relationships (based on the number of extrafloral nectaries available) in individual-based ant-plant networks. We observed that diurnal and nocturnal ant-plant networks exhibited the same pattern of interactions: a nested and non-modular pattern and an average level of network specialization. Despite the high similarity in the ants’ composition between the two collection periods, ant species found in the central core of highly interacting species totally changed between diurnal and nocturnal sampling for all plant species. In other words, this “night-turnover” suggests that the ecological dynamics of these ant-plant interactions can be temporally partitioned (day and night) at a small spatial scale. Thus, it is possible that in some cases processes shaping mutualistic networks formed by protective ants and plants may be underestimated by diurnal sampling alone. Moreover, we did not observe any effect of the number of extrafloral nectaries on ant richness and their foraging on such plants in any of the studied ant-plant networks. We hypothesize that competitively superior ants could monopolize individual plants and allow the coexistence of only a few other ant species, however, other alternative hypotheses are also discussed. Thus, sampling period and species-area relationship produces basic information that increases our confidence in how individual-based ant-plant networks are structured, and the need to consider nocturnal records in ant-plant network sampling design so as to decrease inappropriate inferences.

http://www.leci.ib.ufu.br/pdf/Dattilo_et_al(2014)PLoS_One.pdf

Individual-based ant-plant networks: diurnal-nocturnal structure and species-area relationship.

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

Tree hollows offer an ideal niche for saproxylic insects in mature Mediterranean forests, where Diptera and Coleoptera are the richest groups. Co-occurrence is frequently observed among many species of both groups in these microhabitats, and some of these species have been considered to facilitate the presence of other species by acting as ecosystem engineers. One of the systems that is found in Mediterranean tree hollows is formed by cetonid (Coleoptera: Cetoniidae) and syrphid (Diptera: Syrphidae) larvae. Here, cetonid larvae feed on wood and litter and produce a substrate that is easier to decompose. To assess the possible role of these larvae as facilitating agents for the saproxylic guild, we studied whether the presence of saprophagous Syrphidae inside tree hollows is associated with the activity of cetonid larvae. Furthermore, in laboratory conditions, we tested whether cetonid larvae activity can improve the development and fitness of the saprophagous syrphid species. Our results show that “cetonid activity” was the variable that best explained the presence of saprophagous syrphid species in natural conditions. Myathropa florea (L., 1758) was one of the species most influenced by this activity. The laboratory experiment gave similar results, demonstrating that an enriched substrate with Cetonia aurataeformis Curti, 1913 larval feces improves syrphid larval growth rate and fitness of adults (measured as longer wing length) of M. florea.

/pdf/facilitation-among-saproxylic-insects-inhabiting-tree-4qpga5wpz3.pdf

Facilitation among saproxylic insects inhabiting tree hollows in a Mediterranean forest: the case of cetonids (Coleoptera: Cetoniidae) and syrphids (Diptera: Syrphidae).

We analysed the effect of a hurricane on a plant–ant network and on vegetation cover. Plant cover was sampled using linear sampling in several vegetation types: deciduous forest, a dry forest, sand dune pioneers, sand dune scrub, ecotone of freshwater marsh, deciduous forest and dune scrub, and mangrove forest. We sampled ant–plant interactions and vegetation cover before and after Hurricane Karl hitting (September 2010) the central coast of the state of Veracruz, Mexico. The pre-hurricane network consisted of 16 plant and 25 ant species in 52 associations. The post-hurricane network consisted of 17 plant and 20 ant species in 56 associations. We found a significant decrease in the total linear cover of EFN-bearing plants between October 2009 (646 m, no hurricane effect) and October 2010 (393 m, after hurricane Karl) (total sample length 2025 m). Both networks were significantly nested (0.999 and 0.973, P < 0.001), suggesting that network topology remained similar. Our results show changes in several network characteristics and species proportions. The number of plant species that contributed to nestedness vs. idiosyncratic species did not differ significantly in the pre-hurricane network, while the number of plant species that contributed to nestedness vs. idiosyncratic species did differ significantly in the post-hurricane network. The number of ant species that contributed to nestedness vs. idiosyncratic species differed significantly in the pre-hurricane network, and also in the post-hurricane network. Differences in nestedness contributions of species before and after the hurricane reflect an alteration from a generalized, highly nested, more stable pre-disturbance network, to a more low-degree or specialized network (i.e. fewer interactions among generalist species, those species with the most associations). The maintenance of important core components of the network after a huge disturbance, suggests a short-term resilience typical of mutualistic networks.

Effect of Hurricane Karl on a plant–ant network occurring in coastal Veracruz, Mexico

Several studies have shown that qualitative (binary) ant-plant networks are highly nested in tropical environments, in which specialist species (with fewer interactions) are connected with generalists (with the most interactions) in cohesive subgroups. Interactions occur in both qualitative and quantitative networks, however, how their frequency may structure the nestedness in ecological networks involving these organisms is, we believe, unknown. Based on this perspective, we used nestedness analysis to address the effect of interaction frequency on ant-plant networks (n = 14 networks). Unlike binary networks, quantitative networks are often significantly non-nested. In addition, species with a higher interaction frequency have a higher number of links, indicating that these species are possibly more abundant and/or competitive. Moreover, different biological parameters can change the nature of ant-plant interactions, as a plant can be a good resource for one ant and a ‘bad’ resource for another. Thus, this suggests a new perspective for the study of interaction networks in the tropics, since species with lower interaction frequency are not necessarily subsets of species with higher frequency, and consequently generate the non-nested pattern in quantitative networks.

/pdf/importance-of-interaction-frequency-in-analysis-of-ant-plant-1h8650ahdf.pdf

Importance of interaction frequency in analysis of ant-plant networks in tropical environments

Tree hollows are complex microhabitats in which a variety of abiotic and biotic factors shape the community assembly of saproxylic insects. Detecting non-random species co-occurrence patterns is a fundamental goal in ecology in order to understand the assembly mechanisms of communities. We study association patterns of species of Coleoptera and Diptera (Syrphidae), belonging to different trophic guilds, on 72 tree hollows at a local and regional scale in three protected areas in Mediterranean forests using a fixed-fixed null model. Our matrix-level analysis shows a tendency for segregation in species association (species exclusion) at the regional and site levels. However, the high complexity of tree-hollow habitats, offering different resources for a more or less specialized fauna, makes it difficult to prove competition interactions. Indeed, pairwise analysis shows a dominance of non-random aggregation patterns (species coexistence) at the local and regional levels. Both aggregation and segregation of non-random patterns were more common among species from different trophic guilds than within the same guilds, with predators being a common denominator for a high percentage of the inter-guild pairs. Our results suggest that predation and facilitation interactions, together with habitat segregation, are the main factors shaping tree-hollow assemblages, while competition seems to be less important. We conclude that species interactions take an important part of the process of assemblage structuration and special attention should be paid to 'ecosystem engineers' and threatened species in the conservation of tree hollow assemblages.

Ingrid R Sánchez-Galván

Papers

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

Facilitation among saproxylic insects inhabiting tree hollows in a Mediterranean forest: the case of cetonids (Coleoptera: Cetoniidae) and syrphids (Diptera: Syrphidae).

Effect of Hurricane Karl on a plant–ant network occurring in coastal Veracruz, Mexico

Importance of interaction frequency in analysis of ant-plant networks in tropical environments

Unraveling Saproxylic Insect Interactions in Tree Hollows from Iberian Mediterranean Forest