No abstract

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Insect herbivory decreases plant fitness by constraining plant growth, survival and reproductive output. Most studies on the effects of herbivory in trees rely on correlational inter-individual comparisons and could, thus, be affected by confounding factors linked to both herbivory and plant performance. Using the Mediterranean Holm oak (Quercus ilex) as a study model, we followed an experimental approach in which leaf-feeding insects (mainly Lepidoptera caterpillars) were excluded from some shoots in all study trees. Shoots subjected to herbivore exclusion exhibited lower defoliation rates and produced more acorns than control shoots. Defoliation constrained shoot growth throughout the study period, but had no effect on the number of female flowers produced per shoot. Acorn production was, however, lower in control shoots due to their higher abortion rates, and also to their greater mortality risk during summer drought, as shoots with fewer leaves were less likely to survive. Plant reaction to herbivory inhibits certain physiological pathways involved in plant growth, which, together with the effects of physical damage, reduces the amount and efficiency of the photosynthetic tissue. This increases their vulnerability to environmental stresses, such as water deficit, which limit resource assimilation. Defoliation is likely a key factor affecting oak regeneration, as it may be a significant source of seed loss prior to pre-dispersal acorn predation. Further experimental studies could help to elucidate its effects in contrasting environments. In Mediterranean regions, the harsher droughts predicted by climate change models could worsen the effects of insect herbivory on oak reproductive output.

1. Mast seeding is a widespread resource pulse caused by synchronized and intermittent production of a large seed crop by plant populations. The effects of masting on wildlife have been well documented in granivorous vertebrates, but less is known about its impact on population dynamics of insects.2. This study investigated, over 6 years, variation in abundance of a specialist weevil (Curculio elephas) preying on holm oak (Quercus ilex) acorns.3. An immediate bottom‐up effect of seed production on weevil larval abundance was detected, which was driven by an increase in realised fecundity and aggregation at seed‐rich trees. Moreover, trees producing on average more and larger acorns sustained larger weevil populations. However, no correlation was found between current and previous year adult abundance, suggesting that C. elephas did not capitalise on the reproductive bottom‐up effect.4. It was rainfall, not masting, that most strongly shaped the temporal variation in insect abundance. Rainfall facilitates emergence after diapause at underground earth cells and was also responsible for among‐tree synchronisation in adult weevil population dynamics.5. In spite of their trophic specialisation, not only food availability, but also weather affects weevil numbers. The present results indicate that moving beyond bottom‐up effects is required to better understand complex systems that involve masting plants and insects that consume their seeds.

No abstract

Environmental conditions during early life may shape phenotype in adulthood. Early adverse conditions may increase the oxidative stress in adults, which could affect their reproductive output and survival. It has also been hypothesized that the larger the reproductive investment, the higher the oxidative stress. We tested this and the potential influence of early oxidative stress on how individuals respond to a reproductive stimulation. The synthesis of the antioxidant glutathione was inhibited in captive zebra finches (Taeniopygia guttata) during growth. In adulthood, the expression of a carotenoid-based sexual signal, bill redness, increased in both sexes, with females also being heavier than controls. The social context of control and glutathione-inhibited males was then manipulated to stimulate precopulatory reproductive investments. Males were individually caged in front of a female or another male. We predicted that males enduring lower early antioxidant levels and placed close to a female should pay the highest cost of a hypothetical increase in bill redness in terms of oxidative damage. However, early conditions only influenced the male's phenotype via their partners. Males caged with females showed increases in circulating pigment (carotenoid) levels, but only when females endured early low antioxidant values. This was probably related to the higher attractiveness of these females. Nevertheless, the bill redness of males did not differ during the social manipulation. Moreover, males facing females from any early condition group showed lower oxidative damage levels in plasma lipids. This result agrees with some findings in rodents, also in captivity. However, the effect may be due to increased triglyceride levels and body mass in males not facing females, as variation in these traits explained oxidative damage variability. The importance of considering housing conditions and life history when interpreting oxidative stress-related trade-offs is highlighted.

No abstract

The effects of climate change on oaks Quercus spp. constitute a main environmental concern for the conservation of temperate forests. In this context, we assessed the consequences of longer droughts on the interactions between the holm oak Quercus ilex L. and its main acorn pests. Infested acorns were prematurely abscised before reaching their potential size. The volume of the acorns attacked by Cydia fagiglandana (Lepidoptera) was smaller than those attacked by Curculio elephas (Coleoptera); however, their weight did not differ because Curculio larvae consumed more cotyledon. For the same reason, embryo survival likelihood was not lower in Cydia acorns despite their smaller size. Delays of late summer rain reduced infestation by Curculio, as soil hardness hampers adult emergence from their underground cells. By contrast, late and scarce precipitations benefited Cydia; rainfall might hamper adult flight and eggs/L1 larvae survival. There was not a “zero-sum” effect, because the decrease of Curculio infestation rates was not fully compensated by an increase of Cydia. Under the longer droughts projected for the Mediterranean Basin, our results predict lower infestation rates and higher acorn survival likelihood. However, further studies including other environmental factors are needed to better forecast the net consequences for holm oak fitness.