Acorn Crop Research Articles

Effects of silvicultural disturbance on acorn infestation and removal

Oak (Quercus spp.) regeneration is an important forest management goal in the central hardwood forest region of the United States. Silvicultural methods that target oak regeneration, such as the creation of canopy openings and removal of shade-tolerant midstory trees, are complicated by the impact of pre- and post-dispersal predators of oak acorns like acorn weevils (Curculio spp.) and granivorous small mammals. Understanding the effects of forest disturbance created by silviculture on these predators is important to promote successful oak regeneration. We conducted a 6-year study of acorn production, weevil infestation, and acorn removal at 113 black (Q. velutina) and white oak (Q. alba) trees impacted by three types of silvicultural treatments: adjacent to harvest openings, midstory removal (first-stage shelterwood harvest), and control (no harvest) in south-central Indiana, USA. We observed highly variable acorn production across years, but minimal impact of silvicultural treatments. Levels of weevil infestation and acorn removal by small mammal seed predators also varied greatly, and predation pressure was highest in years when acorns were scarce. Weevil infestation was reduced following midstory removal in shelterwood harvests, but probability of acorn removal by small mammals was unchanged following harvest. Damaged, germinated, and weevil-infested acorns were less likely to be removed by seed predators, suggesting additive effects of pre- and post-dispersal predators. This study emphasizes the importance of considering acorn predators in oak regeneration silviculture, and timing harvests to follow large acorn crops in order to reduce predation pressure and generate a high number of seedlings.

Determinant factors in the seedling establishment of Pasania edulis (Makino) Makino

Pasania edulis (Makino) Makino is one of the dominant Fagucea tree species in evergreen broad-leaved forests in southern Japan, and its regeneration success may have a major impact on the dynamics of evergreen broad-leaved forests. We conducted a field survey on the population process from acorn production to seedling establishment of P. edulis in an evergreen broad-leaved forest in Kagoshima, southern Japan, from 1995 to 2009. The acorn crop varied greatly among the 14 cohorts, with mast cropping being recorded every 3–4 years. The mortality rate of acorns was very high for all 14 cohorts (99.3–100 %). Important mortality factors were failure to mature (empty acorns), attack by Curculio weevil on trees, predation of dropped acorns by Apodemus mice and large or medium-sized mammals (wild boar (Sus scrofa leucomystax), badger (Meles meles anakuma), and raccoon dog (Nyctereutes procyonoides viverrinu)), and attacks on germinated acorns by the acorn borer (Coccotrypes graniceps). Among these factors, predation by Apodemus mice was the greatest contributor to annual fluctuations in total mortality until seedling establishment. Large or medium-sized mammals and the acorn borer also caused severe damage to dropped acorns in some years, but contributed little to annual fluctuations in total mortality. For successful regeneration, mast cropping was essential. However, a small population of Apodemus mice, that is, a low predation pressure, was also required during mast years.

Cues versus proximate drivers: testing the mechanism behind masting behavior

Masting, the intermittent and synchronized production of seeds, is a common and important phenomenon throughout the plant kingdom. Surprisingly, the proximate mechanisms by which populations of masting plants synchronize their seed sets have been relatively unexplored. We examined how temperature influences the acorn crop of the valley oak Quercus lobata, a masting species common in California, USA, over 33 years in order to assess whether temperature acts directly on acorn crop as a cue or whether it acts instead through intermediate steps indicative of a direct mechanistic connection to acorn production. Compared to several alternatives, the difference in temperature during the spring flowering period over the prior two years (Δt) was a good predictor of annual acorn crop in valley oak, as proposed recently by Kelly et al. Significantly, Δt correlates positively with temperatures the previous April, a likely driver of pollination success in valley oak, and negatively with the previous year's acorn crop, which is in turn negatively correlated with the current year's acorn crop, presumably due to resource limitation. Thus, the success of Δt is not as a cue but rather explained by its close relationship to the proximate drivers that have a direct, mechanistic relationship with acorn crop size.

Survival and causes of mortality in a northern population of western gray squirrels

We studied survival of radio-marked western gray squirrels (Sciurus griseus) and quantified causes of mortality including incidence and severity of notoedric mange in south-central Washington, 1998–2005. We used known-fate models in Program MARK to explore alternative hypotheses on factors related to survival and correlation analysis to investigate parameters that might be related to incidence of mange. The best-supported models indicated that survival varied by year and by sex and that survival of males was lower during the breeding season compared to the non-breeding season. We found little support for differences in survival between juvenile (5–12 months old) and adult squirrels, or for winter severity or size of the acorn crop as significant influences on survival. We determined the likely proximate cause of death for 81 animals; 63% were killed by predators and 37% succumbed to disease, with most disease deaths attributed to mange. Mange was documented in the population during all years and occurred more frequently in animals captured in spring than in animals captured in fall. Counter to our predictions, occurrence of mange was not correlated with 2 measures of winter severity but was strongly correlated with mildness of the preceding winter (number of days with mean air temperature ≥0° C). Sequential use of nests by individual squirrels during mild winters with temperatures conducive to survival of ephemeral, free-living mites may partially explain the periodic epizootics of notoedric mange in this western gray squirrel population. Continued deterioration of squirrel habitat through fragmentation will place additional stressors on the population and may compound the effects of mange on this threatened species. © 2013 The Wildlife Society.

Spatiotemporal variation in acorn production and damage in a Spanish holm oak (Quercus ilex) dehesa

Aim of study. There is a lack of knowledge about spatio-temporal patterns of acorn production in dehesas, especially regarding the influence of different agents causing acorn damage. We examined the spatial and temporal variability on acorn production and damage in four stands within a dehesa farm in 1997, 1998 and 1999.Area of study. The study was carried out in a 1800 ha dehesa farm of Cáceres province, western Spain.Material and Methods. Acorns were sampled by means of seed traps placed in the canopy of six holm oak trees per stand. Acorn collected in it were counted and assessed for damage by Curculio weevils, Cydia moths and the bacterial pathogen Brenneria quercina.Main results. Mean acorn production for the whole study period was 44.60 acorns m-2, which did not vary significantly either among stands or among years. The variability among individual trees was very high (0-300 acorns m-2). The rate of infestation by Curculio was 7.64 ± 10.72 %, by Cydia was 1.76 ± 3.33 %, whereas 10.29 ± 16.12 % of acorns were infested by Brenneria. We found no significant spatial differences, but the rates of acorn loss by insects varied among years. These rates were independent of annual acorn production and there was no correlation among damages by different pests, except between Curculio and Cydia in two crop years.Research highlights. It can be concluded that acorn crops are synchronized at the within-farm level and that the temporal variation in acorn damages can be independent of crop size.Keywords: Acorn production; Brenneria; Curculio; Cydia; dehesa; Quercus ilex; spatio-temporal variation.

北陸地域におけるブナ科樹木の豊凶とクマ大量出没との関係

北陸地域におけるブナ科樹木の豊凶とクマ大量出没との関係

Masting in oaks: Disentangling the effect of flowering phenology, airborne pollen load and drought

Quercus species exhibit an extreme inter-annual variability in seed production often synchronized over large geographical areas (masting). Since this reproductive behavior is mostly observed in anemophilous plants, pollination efficiency is suggested as one hypothesis to explain it, although resource-based hypotheses are also suggested as alternatives. We analyzed the effect of flowering phenology, airborne pollen presence and meteorological conditions in the pattern of acorn production in mixed evergreen-deciduous oak forests (Quercus ilex and Quercus pubescens) in NE Spain for twelve years (1998–2009). In both oaks, higher temperatures advanced the onset of flowering and increased the amount of airborne pollen. Nevertheless, inter-annual differences in pollen production did not influence acorn crop size. Acorn production was enhanced by a delay in flowering onset in Q. ilex but not in Q. pubescens. This suggests that in perennial oaks a larger number of photosynthates produced before flowering could benefit reproduction while the lack of effects on deciduous oaks could be because these species flush new leaves and flowers at the same time. Notwithstanding this effect, spring water deficit was the most relevant factor in explaining inter-annual variability in acorn production in both species. Considering that future climate scenarios predict progressive warmer and dryer spring seasons in the Mediterranean Basin, this might result in earlier onsets of flowering and higher water deficits that would constrain acorn production.

Acorn viability following prescribed fire in upland hardwood forests

Restoration of structure and function of mixed-oak (Quercus spp.) forests is a focal issue of forest land managers in the eastern United States due to widespread regeneration failure and poor overstory recruitment of oaks, particularly on productive sites. Prescribed fire is increasingly used as a tool in oak ecosystem restoration, with the goal of reducing competition, and creating light and seedbed conditions conducive to germination and growth of oak seedlings. Yet, oak seedling establishment is dependent on the presence of viable acorns, which may be vulnerable to prescribed fire. We assessed the effect of prescribed burning and fire temperature on the viability of white oak and northern red oak acorns placed on the leaf litter surface, in the duff, or in the mineral soil during five winter prescribed burns in southern Appalachian upland hardwood forests. Fire temperatures varied among acorn plots, ranging from <79 to <371°C. After the burns, acorns were planted in trays with water-saturated vermiculite, exposed to 14 continuous hours of light daily, and maintained at 27°C on germination beds in a greenhouse. After three weeks we recorded the proportion of acorns germinating and the proportion of germinants with shoots. Our study indicated that patchy, low-intensity dormant season prescribed fire in upland hardwood forests reduced viability of white oak and northern red oak acorns located on the leaf litter surface, but did not generally affect acorns in the duff or mineral soil. Germination rates of both northern red oak and white oak acorns on the leaf litter surface decreased with increasing fire temperature. Shoot production by northern red oak germinants from acorns on the leaf litter surface (and less so in the duff), also decreased with increasing fire temperature. Acorns of both species on the leaf litter surface burned at temperatures ⩾204°C showed high mortality levels, with mortality virtually 100% at temperatures ⩾260°C. Fall burns, especially after a heavy acorn crop, could result in high acorn mortality, potentially impacting oak regeneration from seedlings for many years given erratic acorn production patterns among years and species. Frequent burning that reduces litter and duff depth could compromise availability of ‘safe sites’ where acorns are insulated from high fire temperatures. When oak ecosystem restoration is a goal, land managers should consider the timing and size of acorn crops, as well as the forest floor condition when determining the timing and frequency of prescribed burning.

The coexistence of acorns with different maturation patterns explains acorn production variability in cork oak

In dry areas such as Mediterranean ecosystems, fluctuations in seed production are typically explained by resource (water) availability. However, acorn production in cork oak (Quercus suber) populations shows a very low relationship to weather. Because cork oak trees produce acorns with different maturation patterns (annual and biennial), we hypothesized that acorn production in coexisting individuals with a different dominant acorn maturation type should respond differently to climatic factors and that disaggregating the trees according to their acorn-maturation pattern should provide a more proximal relation to weather factors. We assessed acorn production variability in fragmented cork oak populations of the eastern Iberian Peninsula by counting the total number of acorns in 155 trees during an 8-year period. An initial assessment of acorn production variability in relation to weather parameters yielded very low explained variance (7%). However, after the trees were grouped according to their dominant acorn maturation pattern, weather parameters were found to account for 44% of the variability in acorn crops, with trees with annual acorns exhibiting mast fruiting in years with reduced spring frost and shorter summer droughts and trees with biennial acorns showing the opposite pattern. Thus, conditions that negatively affect annual production could be beneficial for biennial production (and vice versa). The results highlight the importance of the resource-matching hypothesis for explaining acorn production in Quercus suber and suggest that different seed maturation types within a population may allow the species to deal with highly variable weather conditions. They also emphasize the importance of understanding acorn maturation patterns for interpreting masting cycles.

Acorn production prediction models for five common oak species of the eastern United States

AbstractAcorn production varies considerably among oak (Quercus) species, individual trees, years, and locations, which directly affects oak regeneration and populations of wildlife species that depend on acorns for food. Hard mast indices provide a relative ranking and basis for comparison of within‐ and between‐year acorn crop size at a broad scale, but do not provide an estimate of actual acorn yield—the number of acorns that can potentially be produced on a given land area unit based on the species, number, and diameter at breast height (dbh) of oak trees present. We used 10 years of acorn production data from 475 oak trees to develop predictive models of potential average annual hard mast production by five common eastern oak species, based on tree diameter and estimated crown area. We found a weak (R2 = 0.08–0.28) relationship between tree dbh and acorn production per unit crown area for most species. The relationship between tree dbh and acorn production per tree was stronger (R2 = 0.33–0.57). However, this is because larger‐dbh trees generally have larger crowns, not because they have a greater capacity to produce more acorns per unit crown area. Acorn production is highly variable among individual trees. We estimated that dbh of at least 60 dominant or codominant oak trees per species should be randomly sampled to obtain an adequate representation of the range of dbhs (≥12.7 cm dbh) in a given forest area, and achieve precise estimates when using these equations to predict potential acorn production. Our predictive models provide a tool for estimating potential acorn production that land managers and forest planners can apply to oak inventory data to tailor estimates of potential average annual acorn production to different forest management scenarios and multiple spatial scales. © 2011 The Wildlife Society.

Brooding, provisioning, and compensatory care in the cooperatively breeding acorn woodpecker

We analyzed reproductive investment in parental care (brooding and the provisioning of nestlings) in the acorn woodpecker (Melanerpes formicivorus), a cooperatively breeding species in which both polygynandry and helping-at-the-nest are common. As predicted based on the strategies pursued by birds of different sex and status, breeders generally invested more in parental care than helpers, and breeder females invested more than breeder males. Contrary to expectations, however, the degree to which individuals reduced their effort with increasing group size (i.e., patterns of load lightening or compensatory care) did not match overall investment. Instead, as group size increased, there was no significant difference in compensation in either brooding or provisioning among the different categories of birds. Compensation, at least by breeders, was significantly lower during the first week of the nestling period than later on, supporting the hypothesis that compensatory reduction in care is less likely when brood reduction is more common and was not affected by the acorn crop, which had no significant effect on the incidence of brood reduction despite being important to overall reproductive success. Our results offer support for the hypothesis that levels of compensation are influenced by the relative importance of brood reduction. More theoretical work, however, will be needed in order to understand the basis for patterns of compensation among individuals of different sex and status.

Variable Helper Effects, Ecological Conditions, and the Evolution of Cooperative Breeding in the Acorn Woodpecker

The ecological conditions leading to delayed dispersal and helping behavior are generally thought to follow one of two contrasting scenarios: that conditions are stable and predictable, resulting in young being ecologically forced to remain as helpers (extrinsic constraints and the habitat saturation hypothesis), or that conditions are highly variable and unpredictable, leading to the need for helpers to raise young, at least when conditions are poor (intrinsic constraints and the hard life hypothesis). We investigated how variability in ecological conditions influences the degree to which helpers augment breeder fitness in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus), a species in which the acorn crop, territory quality, and prior breeding experience all vary in ways that have important effects on fitness. We found that the relationship between ecological conditions and the probability that birds would remain as helpers was variable but that helpers generally yielded greater fitness benefits when ecological conditions were favorable, rather than unfavorable, for breeding. These results affirm the importance of extrinsic constraints to delayed dispersal and cooperative breeding in this species, despite its dependence on a highly variable and unpredictable acorn crop. Our findings also confirm that helpers can have very different fitness effects, depending on conditions, but that those effects are not necessarily greater when breeding conditions are unfavorable.

Interspecific and Intraspecific Pollination Patterns of Valley Oak,Quercus lobata, in a Mixed Stand in Coastal Central California

Pollination patterns within Quercus lobata and interspecific hybridization between Q. lobata and Quercus douglasii were studied in a coastal central California mixed woodland. We first identified hybrids by means of microsatellite genotyping and assignment tests. Hybrids were rare, both among adults (4 of 190, 2.1%) and among acorns collected from Q. lobata trees (6 of 392, 1.5%). These low rates of hybridization at both early and late life history stages suggest that fertility barriers, rather than natural selection against hybrids, limit hybridization between these two species. However, hybrid adults, although rare, may facilitate gene flow between the two species. Acorns collected from a hybrid tree had both Q. lobata pollen donors (11 of 30, 37%) and Q. douglasii or hybrid pollen donors (19 of 30, 63%). After removing hybrid acorns from the analysis, we used paternity assignment to track pollination patterns within Q. lobata. Of 108 acorns, only 32 (30%) were assigned to candidate pollen donors within 200 m of the maternal tree, indicating that the majority of effective pollen travels more than 200 m. Individual trees had acorn crops with many different sires and an average effective number of pollen donors (Nep) of 219 per tree. Indirect methods using correlated paternity estimated mean pollination distances of ∼100 m and mean Nep of 5.2 per tree, values much lower than those derived directly from paternity assignments.

Effects of asynchronous acorn production by co-occurring Quercus trees on resource utilization by acorn-feeding insects

To clarify the effects of asynchronous seed production among tree species on the population of seed predators, we investigated the relationship between the annual variation in production of mature acorns and the insect damage in those acorns of two sympatric oak species, Quercus variabilis Blume and Quercus serrata Thunb. ex Murray, over 4 years at two study sites. The annual variation in acorn production was noticeable, with a coefficient of variation (CV) at the two sites of 1.05 and 0.80 for Q. variabilis and 0.87 and 0.73 for Q. serrata. Annual fluctuation in acorn production by Q. serrata was synchronized between the two sites. Since annual fluctuation in acorn production was not synchronized between the two species, the CVs for the total acorn production by both oak species (0.83 and 0.62 at the two sites) were lower than those for Q. variabilis and Q. serrata alone. The rate of predation by the specialist predators (Curculio weevils) on the acorns of both species was not related to the annual acorn crop size. Prolonged diapause of Curculio weevils might stabilize their populations. The rate of acorn predation by the generalist predators (tortricid moths) was also not related to the annual crop size. Asynchronous acorn production by the two oak species would help to stabilize the population.

Do ecological and silvicultural factors influence acorn mass in Holm Oak (southwestern Spain)?

The effect of orientation, location, year, and pruning treatment on mean acorn mass in two open woodland experimental locations (Huelva, South-Western Spain) of Holm Oak (Quercus ilex L. subsp. ballota [Desf.] Samp.) was analyzed during 2001–2006. Data were analyzed by using a mixed linear model, where the covariates were tree size parameters (circumference at 80 cm height, crown size), topographic position (wet index and flow length), intraspecific competition, total acorn crops per tree (weight and number), and climatic parameters. Average estimated acorn mass was 3.17 g acorn−1 in the Calañas location and 4.12 g acorn−1 in the San Bartolomé location, with significant differences between locations, among individual trees and across years. Acorns from the south sides of trees were significantly heavier than those at other positions. The effect of pruning treatment and the interactions location × treatment and treatment × year were not significant. Our study provides no evidence that pruning affects acorn mass. None of the analyzed covariates related to tree size, topographic position and intraspecific competition explained the difference among trees, suggesting that heritable factors play an important role. Specific climatic parameters during September, the most important month for acorn growth, may explain the effects of year and location × year on acorn mass. In addition, there appeared to be a “trade-off” between acorn mass and number of acorns.

Increased drought reduces acorn production in Quercus ilex coppices: thinning mitigates this effect but only in the short term

Summary In order to explore the effects of climate change on Mediterranean regenerating forests, we experimentally assessed the effects of increased drought on the reproductive attributes of Quercus ilex over a 4-year period (2005 – 2008). We also investigated whether traditional thinning (selection of one to a few stems per stump) could mitigate the consequences of increased drought in oak coppices. Increased drought reduced the number of reproductive trees, mean number of female fl owers produced and acorn crop size, although most of these effects appeared only in the last 2 years of the experiment. In a different way, thinning enhanced all reproductive attributes, but its main effects were transient and covered only 1 or 2 years after the application of the treatments. Our results indicate that a moderate reduction in rainfall (15 per cent ) reduces the reproductive ability of Q. ilex . This may have long-term negative consequences for recruitment as well as for the fauna feeding on acorns. Although traditional thinning may mitigate the consequences of increased drought, it has a remarkably short-term effect. This highlights the need to re-examine traditional forestry practices as potential adaptive strategies for coping with climate change in Mediterranean regenerating forests.

Trade-offs between vegetative growth and acorn production in Quercus lobata during a mast year: the relevance of crop size and hierarchical level within the canopy.

The concept of trade-offs between reproduction and other fitness traits is a fundamental principle of life history theory. For many plant species, the cost of sexual reproduction affects vegetative growth in years of high seed production through the allocation of resources to reproduction at different hierarchical levels of canopy organization. We have examined these tradeoffs at the shoot and branch level in an endemic California oak, Quercus lobata, during a mast year. To determine whether acorn production caused a reduction in vegetative growth, we studied trees that were high and low acorn producers, respectively. We observed that in both low and high acorn producers, shoots without acorns located adjacent to reproductive shoots showed reduced vegetative growth but that reduced branch-level growth on acorn-bearing branches occurred only in low acorn producers. The availability of local resources, measured as previous year growth, was the main factor determining acorn biomass. These findings show that the costs of reproduction varied among hierarchical levels, suggesting some degree of physiological autonomy of shoots in terms of acorn production. Costs also differed among trees with different acorn crops, suggesting that trees with large acorn crops had more available resources to allocate for growth and acorn production and to compensate for immediate local costs of seed production. These findings provide new insight into the proximate mechanisms for mast-seeding as a reproductive strategy.

Testing the environmental prediction hypothesis for mast-seeding in California oaks

We analyzed 29 years of acorn production by five species of California oaks (genus Quercus ) to test the hypothesis that trees produce large seed crops prior to wet years, conditions facilitating seedling germination and survival. The mean crop of three of the species correlated positively and nontrivially with the following year’s rainfall, but none was statistically significant. Including the acorn crop 1 and 2 years earlier yielded several significant relationships between the acorn crop and future rainfall, but none held up when applied to a second, independent site. Across individuals, acorn production by 7% of trees correlated significantly with subsequent rainfall. Although these trees differed from other trees in several characters, differences were not sufficient to discriminate between trees that correlated significantly with subsequent rainfall from those that did not. We conclude that acorn production by California oaks does not forecast wet years and does not support the environmental prediction hypothesis.

Multiple pathways for tree regeneration in anthropogenic savannas: incorporating biotic and abiotic drivers into management schemes

Summary 1. Oak savannas are biodiversity‐rich landscapes allowing sustainable livestock production throughout the world. The long‐term persistence of these ecosystems critically depends on the regeneration of the tree layer. Nevertheless, studies addressing the mechanisms involved for conservation planning are of limited value because they tend to focus on single explanatory factors. 2. We evaluated the combined effect of biotic and abiotic factors on recruitment of holm oak Quercus ilex in the Mediterranean savannas of western Spain. Transition probabilities from flower to seed to the established seedling were estimated in grazed, shrub‐encroached and cropped plus fenced habitats in two consecutive years. 3. Trees in cropped habitats produced more female flowers and larger acorn crops in both years. The physiological condition of trees was better in cropped habitats and worst in shrub‐encroached plots. Overall, resource‐mediated effects overrode the effects of biotic damage on tree fecundity in all habitats. 4. Acorn survival and seedling establishment were higher in cropped and shrub‐encroached plots, though in cropped plots saplings are predictably destroyed by subsequent grazing and/or by mechanical treatment used to restart the cropping cycle. 5. Complete regeneration failure was found in 6 out of 24 possible management scenarios, mostly in the presence of large vertebrate herbivores. However, even low positive cumulative transition probabilities between life stages exceeded a safe threshold for early regeneration. 6. Synthesis and applications. Natural early recruitment of oak savannas can be achieved through various management regimes. These include cereal cropping in fenced plots (provided established saplings were not subsequently destroyed) or shrub encroachment in undergrazed or livestock‐excluded plots. Among these, natural recruitment after encroachment is a cost‐effective tool as compared to artificial plantation.

Pre‐dispersal acorn predation in mixed oak forests: interspecific differences are driven by the interplay among seed phenology, seed size and predator size

Summary1. Pre‐dispersal seed predation (PSP) often occurs in multi‐host–predator systems (e.g. several plant species exposed to a common array of granivorous insects). However, whether the interaction among seed phenology, seed size and predator size accounts for interspecific differences in PSP remains elusive.2. We studied PSP in a mixed‐oak forest with two oaks (the larger‐seeded Quercus humilis and the smaller‐seeded Q. ilex), both depredated by two acorn weevils (the smaller Curculio glandium and the larger C. elephas). We intensively monitored acorn production and infestation phenology and we identified the weevil species depredating acorns by means of DNA taxonomy.3. The minimum acorn size required for infestation was lower for C. glandium than for C. elephas, in accordance with their different body sizes. This resulted in an earlier infestation phenology in C. glandium and the ability of this species to infest both smaller and larger acorns. Above a minimum acorn size threshold, no selection for larger acorns by weevils was observed.4. Initial acorn crop size was similar in the two oaks. Nonetheless, the earlier acorn phenology and the production of larger acorns in Q. humilis favoured the earlier infestation by C. glandium and the predation by both small and large weevils. Smaller acorns of Q. ilex almost excluded infestation by the larger C. elephas.5. Although larger acorns of Q. humilis could better survive infestation (preserve the embryo), higher PSP in this species finally resulted in a lower mature acorn crop size than in Q. ilex.6. Synthesis. In a multi‐host–predator system, smaller‐seeded species may benefit from a reduced PSP because they exclude larger granivorous insects, but also by means of a ‘free‐rider effect’, if larger‐seeded heterospecifics earlier reach a critical size to be depredated. These results also highlight the benefits of a small body size in granivorous insects to depredate seeds earlier and to forage on a wider range of seed sizes. Whether the advantage of ‘being small’ in this antagonistic plant–animal interaction is offset by other processes, or whether it results in a pressure towards seed and insect size reduction, deserves further attention.