Background Gut microbial communities play important roles in nutrient management and can change in response to host diets. The extent of this flexibility and the concomitant resilience is largely unknown in wild animals. To untangle the dynamics of avian-gut microbiome symbiosis associated with diet changes, we exposed Parus major (Great tits) fed with a standard diet (seeds and mealworms) to either a mixed (seeds, mealworms and fruits), a seed, or a mealworm diet for 4 weeks, and examined the flexibility of gut microbiomes to these compositionally different diets. To assess microbiome resilience (recovery potential), all individuals were subsequently reversed to a standard diet for another 4 weeks. Cloacal microbiomes were collected weekly and characterised through sequencing the v4 region of the 16S rRNA gene using Illumina MiSeq. Results Initial microbiomes changed significantly with the diet manipulation, but the communities did not differ significantly between the three diet groups (mixed, seed and mealworm), despite multiple diet-specific changes in certain bacterial genera. Reverting birds to the standard diet led only to a partial recovery in gut community compositions. The majority of the bacterial taxa that increased significantly during diet manipulation decreased in relative abundance after reversion to the standard diet; however, bacterial taxa that decreased during the manipulation rarely increased after diet reversal Conclusions The gut microbial response and partial resilience to dietary changes support that gut bacterial communities of P. major play a role in accommodating dietary changes experienced by wild avian hosts. This may be a contributing factor to the relaxed association between microbiome composition and the bird phylogeny. Our findings further imply that interpretations of wild bird gut microbiome analyses from single-time point sampling, especially for omnivorous species or species with seasonally changing diets, should be done with caution. The partial community recovery implies that ecologically relevant diet changes (e.g., seasonality and migration) open up gut niches that may be filled by previously abundant microbes or replaced by different symbiont lineages, which has important implications for the integrity and specificity of long-term avian-symbiont associations.

The trophic interactions between plants, insect herbivores and their predators are complex and prone to trophic cascades. Theory predicts that predators increase plant biomass by feeding on herbivores. However, it remains unclear whether different types of predators regulate herbivores to the same degree, and how intraguild predation impacts these trophic interactions. Despite past syntheses having confirmed cascading effects of vertebrate predators on terrestrial arthropods, we lack a more comprehensive look at the effects of other predators on a global scale. Here we report a meta-analysis of 486 experiments gathered from 157 publications reporting the effect of insectivorous vertebrates (birds and bats) and ants on abundances of predatory (spiders, ants, others) and herbivorous (chewers and others) arthropods; on arthropod richness and plant damage. Generally, the absence of vertebrate predators led to the increase of predatory arthropods by 18%, herbivorous arthropods by 75%, and plant damage by 47%. In contrast, after the removal of ants, the increase in the abundances of other predatory arthropods did not compensate for missing ants, herbivore arthropods increased their abundances by 53%, and plant damage increased by 146%. The effects of ant exclosures were stronger in communities at lower elevations and latitudes, while we did not detect any clear geographical patterns in the effect of vertebrate exclosures. Neither precipitation nor NDVI had a significant impact on most of the measured effects, and the effect of exclosures was robust for both plant growth forms and different habitat types. We found vertebrate insectivores to be the more dominant predators of arthropods, but we detected that the strength of their trophic cascades was weakened by intraguild predation. On the other hand, we found that although ants were relatively less dominant as predators, and their influence was detectable only in the most productive sites, the effect of trophic cascades on plants they caused was stronger than that of vertebrate insectivores.