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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.

En la presente tesis doctoral se presenta un estudio profundo de los ensambles formados por insectos saproxílicos (Coleópteros y Dípteros Syrphidae) ligados a las cavidades arbóreas de quercínas ibéricas (Quercus spp). Las especies saproxílicas (coleópteros y dípteros sírfidos) conforman complejas redes ecológicas conectadas a través de diferentes interacciones de especies insecto-insecto. Nuestros hallazgos recalcan la importancia para la conservación de la comunidad entomológica saproxílica, centrar la atención en el estudio de especies clave que cumplen un papel fundamental en la organización de los ensambles al condicionar la presencia o la ausencia de otras especies o gremios de especies que se desarrollan dentro de cada oquedad (ej. especies “ingenieros del ecosistema”). Ponemos de manifiesto también la importancia que tiene la microbiota dentro la oquedad como recurso trófico de las fases larvarias acuáticas de especies de sírfidos que como adultos, son visitantes florales asiduos de las flores de nuestros bosques mediterráneos.

Saproxylic insect communities inhabiting tree hollows in Mediterranean forests depend on a combination of physical characteristics and interactions occurring between community member species. Despite the need to preserve these organisms, little is known about their interrelationships, in particular those relationships between saproxylic insects and microbiota occurring in these microhabitats. In tree hollows of Quercus rotundifolia Lamark that hold water and contain dead leaves, abundant microbial populations can be found. Developing on them are the larvae of Mallota dusmeti Andréu, 1926 (Diptera: Syrphidae), a vulnerable species (IUCN category: Marcos-García and Quinto 2011). This study provides the first data on the microbiota living inside the gut of the larvae of M. dusmeti, as well as the microbiota in the hollow where these larvae develop. Bacteria were identified by amplification and partial sequencing of the V1–V3 regions and the complete nucleotide sequence of 16S rRNA genes. We found eight species of bacteria living in tree hollows and three species in the gut of M. dusmeti larvae: Bacillus cereus, Bacillus toyonensis, and Lysinibacillus sphaericus. The filter-feeding mechanism characteristic of M. dusmeti larvae is selective in enabling ingestion of bacteria only above 2.1 µm in diameter.

Zopherus chilensis Gray, 1932 (Coleoptera: Zopheridae), is a small size saproxylic beetle called “Maquech”. This Neotropical specie has peculiar connotations into Maya´s culture. In this paper we show the ornamental use of Z. chilensis and our ultimate goal is to claim for a comprehensive conservation of biodiversity of saproxylic beetles from México.

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.

One of the most emblematic mutualistic interactions in the Neotropics is that between Acacia plants and Pseudomyrmex ants. In his first studies, D. H. JANZEN (1969) related the functional defensive system by the ants to allelopathic chemical agents, thus this system can be associated to induced defensive systems. Our study system, the interaction between Acacia cornigera and two Pseudomyrmex species (P. gracilis and P. ferrugineus), includes ant colonization patterns and plant behavior after colonization. Previous research on ant defensive roles had mentioned that P. ferrugineus was a good defensive system for Acacia plants, and that P. gracilis was a so-called parasite of the system. Here we suggest that the subsistence mechanism between the two ant species is based on chemical products, clear postdamage signals which, are intrinsic to the plant and promote recruitment and defense by the two species of Pseudomyrmex.

Extensive research conducted so far suggests that the physiological functionality of myrmecophilous Acacia and Pseudomyrmex ants have evolved through a coevolutionay process. This is one of the most recognized myrmecophilous interactions present in the Neotropics. In our system of A. cornigera and the ant P. ferrugineus and P. gracilis, we first reported a neighborhood effect in the colonization of plants by both ant species, a higher proportion of colonization and larger host plants colonized by P. ferrugineus. However, which is the function of P. gracilis, in classic terms of anti-herbivory, were the two ant species Pseudomyrmex equally efficient in defending their Acacias? A first level of observation is based on an index of herbivory per plant. We also used simulated herbivory and found that P. ferrugineus is more efficient, when present on plants less damage is caused by herbivores, and ants were recruited faster in the presence of a potential herbivore. LARS et al. (2008) suggested that the coexistence of both species and especially the persistence of P. gracilis on a similar system whit Acacia hindsii is because they have fast-growing, low-density colonies, suggesting reproductive winged forms as an r - strategy. However, these authors are not including chemical stimuli, only visual or physical stimuli, so which other mechanisms promote the interaction of P. gracilis with Acacia cornigera. We will find out in the next part (part III).