Álvaro Gaytán

Climate change is causing major alterations in precipitation regimes and soil water availability, with far-reaching effects on plant traits. Changes in plant chemistry under drought scenarios are particularly relevant, as they can modulate plant interactions with antagonistic organisms. However, how such changes in plant chemistry affect plant susceptibility to biotic damage remains poorly understood. Here we integrated results from a systematic literature review and a field study in Mediterranean mixed forests to advance our understanding of the cascading effects of reduced soil water content on leaf reserves, defences and damage caused by herbivores and pathogens. The literature review revealed that studies linking soil water content with leaf chemistry and damage remain scarce and reported a variety of direct and indirect relationships between the three components. Nevertheless, most studies found higher leaf damage under drier conditions. Results from our field study also supported that leaf damage increased with decreasing soil water content, a relationship largely mediated by changes in leaf phenolic compounds. In Mediterranean forests, phenolics play a major role in protection against desiccation and UV radiation due to their antioxidant properties, yet certain phenolic compounds can also benefit antagonists. Overall, our findings suggest that leaf damage can increase in a drier climate, forcing tree species to face fundamental trade-offs when protecting themselves from the combined stress imposed by abiotic (drought) and biotic (pests and pathogens) components of global change.

Structural equation modelling (SEM) has become a cornerstone tool in ecology for testing hypotheses about causal relationships and disentangling direct and indirect effects. However, its growing use has led to frequent conceptual and methodological misapplications. Here, I clarify the theoretical foundations of SEM and provide practical guidelines for its rigorous implementation in ecological research. SEMs assess whether data are consistent with hypothesised causal structures – they do not prove causation – and every path must be justified a priori by ecological reasoning. I outline common pitfalls, including overfitting, unjustified bidirectional paths, inconsistent data transformations, and theory‐free modelling. Special attention is given to the logic and diagnostics of piecewise SEMs, including Fisher's C test, d‐separation, and model selection. An accompanying R vignette demonstrates transparent, reproducible workflows that integrate model specification, diagnostics and interpretation. Properly implemented, SEMs offer a powerful framework for refining ecological theory and identifying mechanisms linking environmental and biological processes. By explicitly linking SEM specification to ecological theory (e.g. trophic cascades, environmental filtering, and indirect effects in community assembly), this framework facilitates theory‐driven causal inference and the formal testing of complex ecological hypotheses.

Climate aridification and the increasing incidence of exotic pests and pathogens are contributing to a concerning global rise in tree decline and mortality. The analysis of tree elemental composition (the elementome) offers a promising tool to detect physiological disruptions and identify early‐warning signals of decline. However, the role of elemental alterations—particularly in response to exotic pathogens—remains underexplored. Here we analysed the elemental signature of tree decline in Mediterranean savanna‐type ecosystems (dehesas) invaded by the aggressive exotic soil‐borne pathogen Phytophthora cinnamomi . Using a multi‐scale approach (regional and local), we assessed root elemental concentrations and stoichiometry in relation to crown defoliation, soil physicochemical properties and P. cinnamomi abundance in 500 adult Quercus rotundifolia trees across 30 sites in Southern Spain. Our results revealed a distinct chemical signature of tree decline in the root elementome, with patterns varying across spatial scales. At the regional scale, only sodium (Na) emerged as an indicator of site‐level decline. In contrast, the local scale revealed strong and consistent chemical alterations in symptomatic trees, including elevated calcium (Ca) concentrations and reduced levels of key nutrients (potassium [K], phosphorus [P], iron [Fe], zinc [Zn]) and elemental ratios (K:Ca) compared to asymptomatic neighbours. Among all variables, the K:Ca ratio was the best discriminator between asymptomatic and symptomatic trees, showing significant associations with crown defoliation and P. cinnamomi abundance. Elevated Ca uptake in symptomatic trees may reflect a defensive response to pathogen infection but also contribute to nutritional imbalances that exacerbate decline. Synthesis . Our findings highlight the value of multi‐scale elementome analysis for identifying sites and trees vulnerable to decline. Specifically, we showed that nutritional imbalances (particularly the K:Ca ratio) can act as reliable early‐warning signals of pathogen‐driven decline in Mediterranean systems, enabling rapid management responses. We advocate further research on plant elementomes across diverse pathosystems to uncover broader patterns that could inform biosecurity programs aimed at mitigating the growing impacts of exotic pathogens worldwide.

Documenting plant–herbivore interactions at broad spatial scales is crucial for understanding the ecological and evolutionary processes that shape biodiversity. Here I present a continental‐scale interaction matrix encompassing 5152 Lepidoptera species and 3275 vascular plant species across Europe. The dataset compiles records from published literature, monographs, online repositories, and gray sources, with all taxa standardized to current classifications. By resolving associations at the species level, this resource provides a robust basis for investigating host specialization, network structure, and patterns of herbivore diversity in biogeographic and phylogenetic contexts. In addition to supporting fundamental research, the matrix offers applied value for conservation biology—for example, by identifying keystone host plants that sustain diverse herbivore assemblages and by assessing the vulnerability of insect communities to host loss under global change. The dataset is openly available in a machine‐readable format under a CC‐BY 4.0 license, ensuring transparency, reproducibility, and long‐term accessibility for ecological and evolutionary studies.

Ungulates consume plants and seeds together with the insects that feed on them, which have consequences at the ecosystem level. We carried out an experiment to assess the effects of intraguild predation by livestock on the main acorn pest (Curculio spp. weevils) in Iberian oak Quercus spp. dehesas, a widespread traditional agroecosystem. Acorns are a key food source for free‐range livestock (especially pigs), but, if properly managed, livestock could become a pest control agent. In three dehesa farms, we replicated eight experimental trios, each including one tree from which livestock was excluded, another from which all dropped acorns were removed manually (simulating intensified predation by livestock), and a control one in which livestock was allowed at standard densities. Removal of marked infested acorns by livestock, adult and larval weevil numbers, and acorn production/infestation rates were recorded at all trees. Livestock predated most weevil larvae within the prematurely dropped infested acorns before larvae had time to finish development. Hence, in those trees subjected to intensified predation, the local number of larvae decreased. Consequently, adult weevil abundance in the following year was lower than in trees within the exclosures of livestock. The consequence of the decrease in the number of adults was a reduction in the rates of acorn infestation in control oaks and intensified predation compared to those excluded from livestock (8% and 20%, respectively). C. elephas mobility is low, especially when trees are isolated, and acorn infestation is greatly dependent on the number of weevils emerging beneath the canopy of each oak. Therefore, a locally focused elimination of infested acorns may succeed in reducing the negative impacts of the pest. Synthesis and applications: We suggest intensifying livestock predation on prematurely dropped‐infested acorns by allowing livestock foraging from October 1st onward. Pigs are usually released free range in early November, once weevil larvae have completed their development and escaped predation. The proposed management would increase the availability of healthy acorns, thus increasing the farm capacity and the economic profit by up to 20%, tens of millions of euros of additional profits when translated into the prices of the Iberian pork market.

Livestock grazing and trampling have been shown to reduce arthropod populations. Among arthropods, defoliating lepidopterans are particularly important for their impact on trees, the keystone structures of agroforestry systems. This study investigates the impact of livestock on the community of defoliating lepidopterans in agroforestry systems. We conducted both experimental and observational studies in the mid‐west and western regions of the Iberian Peninsula. In our field experiment, we sampled lepidopteran caterpillars in areas with livestock and in areas where livestock had been excluded for short and long periods. To validate our experimental results, we conducted a field survey across seven areas with and without livestock in the western Iberian Peninsula. Our findings revealed that after 2 years from the start of the experiment, the exclusion of livestock led to an increase in the abundance and species richness of lepidopterans, as well as shifts in their community composition. Our experimental findings were corroborated by our field survey. Interestingly, we found that the differences in community composition among exclusions were primarily due to species nestedness. Livestock exclusion consistently favored species that pupate in the ground, suggesting that livestock can alter the lepidopteran community by predating or accidentally trampling these species. This study underscores the significant role livestock play in modifying the community of defoliating lepidopterans in Mediterranean agroforestry systems (oak savannahs), with important implications for food webs and species interactions.

Forecasting the effects of global change drivers on ecosystems is one of the most pressing challenges for scientists worldwide. Particularly, climate change and exotic pathogens might have a large impact on plant community dynamics and ecosystem functioning through changes in carbon uptake and sinks. Nevertheless, we still have a poor understanding of the combined effects of these two drivers on plant communities. Here, we explored the impact of rainfall reduction and exotic pathogens on the carbon balance of Mediterranean tree species. For this, we performed a 3‐year field experiment taking advantage of rainfall exclusion infrastructures (30% exclusion) installed in the southernmost European oak forests invaded by the aggressive exotic pathogen Phytophthora cinnamomi. We measured a set of 10 variables representative of tree carbon sources (photosynthetic rates) and sinks (primary production, reproduction, defence, and reserves) in adult trees of three species in two forest types: closed forests of Quercus suber and Q. canariensis, and open woodlands of Q. suber and Olea europaea. We found a large variability in the sensitivity of the different carbon sources and sinks to the effects of drought and pathogens, from variables highly sensitive to both factors (carbon fixation and reproduction, root chemistry) to variables only responsive to drought (litter production) or totally unresponsive (tree trunk, leaf chemistry). Although negative effects predominated, positive effects of rainfall exclusion were also detected in wet years, likely due to a reduction of pathogen abundance in drier soil. Trade‐offs between carbon sinks appeared in all tree species, but rainfall exclusion only modified trade‐offs in Q. suber, the species most susceptible to P. cinnamomi. Synthesis. We provide evidence on the complexity of the combined effects of abiotic (drought) and biotic (pathogens) global change drivers on carbon source and sinks of adult trees, including both negative direct effects and positive indirect effects. Our results showed that these effects varied among co‐existing species, particularly for carbon sinks directly related to tree demography (reproduction). Therefore, long‐term changes in the structure of Mediterranean mixed forests might be expected towards the dominance of species highly resistant to both drought and pathogens.

Premise: During the last centuries, the area covered by urban landscapes is increasing all over the world. Urbanization can change local habitats and decrease connectivity among these habitats, with important consequences for species interactions. While several studies have found a major imprint of urbanization on plant–insect interactions, the effects of urbanization on seed predation remain largely unexplored.Methods: We investigated the relative impact of sunlight exposure, leaf litter, and spatial connectivity on predation by moth and weevil larvae on acorns of the pedunculate oak across an urban landscape during 2018 and 2020. We also examined whether infestations by moths and weevils were independent of each other.Results: While seed predation varied strongly among trees, seed predation was not related to differences in sunlight exposure, leaf litter, or spatial connectivity. Seed predation by moths and weevils was negatively correlated at the level of individual acorns in 2018, but positively correlated at the acorn and the tree level in 2020.Conclusions: Our study sets the baseline expectation that urban seed predators are unaffected by differences in sunlight exposure, leaf litter, and spatial connectivity. Overall, our findings suggest that the impact of local and spatial factors on insects within an urban context may depend on the species guild. Understanding the impact of local and spatial factors on biodiversity, food web structure, and ecosystem functioning can provide valuable insights for urban planning and management strategies aimed at promoting urban insect diversity.