Rafał Zwolak

Predator satiation is typically viewed as a function of seed abundance, yet plants also vary widely in seed defences that shape handling costs. Whether such traits modify satiation dynamics remains unclear, despite the role that predator satiation plays in the evolution and ecological consequences of masting. We experimentally manipulated seed density and spatial distribution using two oak species that differ in mechanical defence and tracked removal and post‐dispersal fate by scatterhoarding rodents. We also measured the force required to pierce acorn shells to quantify differences in mechanical resistance between species. In the well‐defended species ( Quercus rubra ), key components of seed fate shifted with increasing density: removal and consumption declined and dispersal distance shortened. In the less defended species ( Quercus robur ), these responses showed little change across densities. These contrasting patterns coincided with a nearly fourfold difference in acorn shell mechanical resistance between species. Synthesis . Our results show that predator satiation is not determined by seed abundance alone: mechanical seed defences can shift the density at which consumers become satiated, shaping density‐dependent seed predation and dispersal. Thus, the effectiveness of masting depends on both seed abundance and seed traits.

Foraging animals must balance energy gain with predation risk. In nocturnal animals, this tradeoff is often mediated by moonlight, which helps to find food but also increases exposure to predators. Although most studies suggest rodents avoid moonlight, prior research has largely focused on open habitats and used lunar phase as a crude proxy for illumination. We investigated how fine-scale moonlight brightness affects foraging in yellow-necked mice (Apodemus flavicollis) removing Quercus robur acorns in a broadleaf forest. Using complementary analyses, we examined foraging across nights and the timing of foraging bouts within nights. We expected that foraging would decrease with increasing moonlight, temporal preferences would be less pronounced under high mouse density, and sensitivity to moonlight would vary with residual reproductive value (influenced by sex and body mass) and behavioral type (exploration in open-field tests). Contrary to predictions, seed removal was higher during bright, clear nights. However, within nights, mice concentrated foraging efforts during darker intervals. This pattern is consistent with higher foraging efficiency on bright nights combined with avoidance of high-risk periods within those nights. As predicted, high mouse abundance reduced selectivity for optimal foraging conditions. While behavioral types had no effect, across-nights analyses demonstrated that males and lighter individuals preferred darker nights, while within nights, females showed greater sensitivity to cloud cover, foraging under lower moonlight when skies were clear. Overall, these results indicate that moonlight preferences can vary with the temporal scale of analyses and individual traits, while preference strength declines with increasing conspecific density.

Abiotic factors have traditionally been considered the primary determinants of species' range limits and responses to climate change. However, growing evidence shows that biotic interactions, such as seed predation and dispersal, also play crucial roles in shaping plant distributions and influencing range shifts. We investigated how scatterhoarding rodents affect the recruitment of Swiss stone pine (Pinus cembra) at its upper altitudinal limits in the Swiss Alps. We focused on interactions between the pine and the rodent community, exploring how these interactions can potentially affect the effectiveness of seed dispersal by the spotted nutcracker (Nucifraga caryocatactes). We conducted seed crop estimation, rodent live‐trapping, seed tracking experiments and seedling emergence trials across an altitudinal gradient over 3 years. We tested three hypotheses: (1) rodents cache a considerable portion of removed seeds, including those pilfered from nutcracker caches, in favourable microsites; (2) rodents consume seeds from nutcracker caches, impeding pine recruitment; and (3) the role of rodents varies with seed availability and rodent abundance, shifting between mutualism and antagonism. Rodents acted consistently as seed predators and cache pilferers. In seed tracking experiments, they consumed 97% of the seeds they removed and cached only 3%. In cache pilferage experiments, rodents reduced seedling emergence from simulated caches by two‐thirds. These findings support hypothesis (2), indicating that rodents suppress Swiss stone pine recruitment by reducing the effectiveness of nutcracker‐mediated seed dispersal. The negative impact was most pronounced when rodent populations were high and seed availability was low, aligning with the concept of predator satiation. Our study suggests that climate warming, which is expected to increase rodent abundances at higher altitudes, may intensify the negative effects of rodents on pine recruitment, impeding the upward migration of Swiss stone pine. These findings stress the importance of including biotic interactions, particularly seed predation and interspecific seed pilferage, into ecological models to improve predictions of ecosystem responses to environmental changes. Understanding these dynamics is crucial for developing effective conservation strategies in the face of climate change.

Theoretical models suggest that species abundance plays a crucial role in mutualism; high densities can lead to overexploitation. Additionally, mutualistic benefits are expected to increase under abiotic stress. We investigated the interplay between density dependence and abiotic factors in conditional mutualism, focussing on the interactions between yellow‐necked mice (Apodemus flavicollis) and common oaks (Quercus robur) across 3 years. We controlled for seed abundance, while mouse densities varied. Our approach included monitoring mouse abundance, measuring seed removal, determining the fate of harvested seeds and conducting seedling emergence trials to assess the advantages of seeds caching. We found that mouse abundance exerted a strong influence on all studied aspects of seed dispersal. Higher densities led to increased seed removal and greater dispersal distances. However, they also decreased the probability of seed caching and increased seed consumption, degrading dispersal quality and shifting the interaction towards antagonism. Furthermore, the reliance of seedling recruitment on burial varied over time, likely becoming more critical during dry conditions. This indicates that plants face the worst conditions when high abundance of mice coincides with abiotic stress. Synthesis: Our results supports the notion that increased rodent abundance reduces plant recruitment, revealing the conflicting interests of the interacting species. However, the dynamics of rodent–oak interactions are shaped not only by the density‐dependent foraging decisions of rodents, but also by the benefits of seed burial under stress. These findings illustrate how the interplay between population density and abiotic factors jointly dictate the costs and benefits of mutualistic interactions.

Intraspecific variation plays a pivotal role in shaping ecological dynamics. As the dispersal of seeds of most woody plants is mediated by animals, individual variation within the animal dispersers holds considerable implications for plant population and ecology. We explored how individual traits (such as sex, body mass and exploration levels) of yellow‐necked mice (Apodemus flavicollis) impact the dispersal of common oak (Quercus robur) acorns. Over 3 years, we collected data on seed fate and the specific mice responsible for their dispersal. The relationship between individual traits and seed dispersal was not static, but influenced by yearly environmental conditions. Heavier individuals tended to carry seeds farther, yet contrary to our expectations, sex had no effect on the distance of seed dispersal. Moreover, the exploration rate showed an inconsistent association with seed consumption and dispersal distance, while it positively impacted the distance of dispersal from the nearest tree. Synthesis. Our findings suggest a more nuanced role of individual traits in seed dispersal than often assumed, with noticeable annual variation significantly influencing these impacts. Consequently, it appears there is no single, universally beneficial individual type to ensure maximal benefits to plants. Rather, the traits conferring advantages in seed dispersal are dynamic, subject to change over time in response to environmental context. Read the free Plain Language Summary for this article on the Journal blog.

Summary: Conditional mutualisms involve costs and benefits that vary with environmental factors, but mechanisms driving these dynamics remain poorly understood. Scatterhoarder–plant interactions are a prime example of this phenomenon, as scatterhoarders can either increase or reduce plant recruitment depending on the balance between seed dispersal and predation. We explored factors that drive the magnitude of net benefits for plants in this interaction using a mathematical model, with parameter values based on European beech (Fagus sylvatica) and yellow‐necked mice (Apodemus flavicollis). We measured benefits as the percentage of germinating seeds, and examined how varying rodent survival (reflecting, e.g. changes in predation pressure), the rate of seed loss to other granivores, the abundance of alternative food resources, and changes in masting patterns affect the quality of mutualism. We found that increasing granivore abundance can degrade the quality of plant–scatterhoarder mutualism due to increased cache pilferage. Scatterhoarders are predicted to respond by increasing immediate consumption of gathered seeds, leading to higher costs and reduced benefits for plants. Thus, biotic changes that are detrimental to rodent populations can be beneficial for tree recruitment due to adaptive behavior of rodents. When scatterhoarder populations decline too drastically (< 5 individuals ha⁻¹); however, tree recruitment may also suffer.

Mast seeding causes strong fluctuations in populations of forest animals. Thus, this phenomenon can be used as a natural experiment to examine how variation in host abundance affects parasite loads. We investigated fleas infesting yellow‐necked mice in beech forest after 2 mast and 2 non‐mast years. We tested 2 mutually exclusive scenarios: (1) as predicted by classical models of density‐dependent transmission, an increase in host density will cause an increase in ectoparasite abundance (defined as the number of parasites per host), versus (2) an increase in host density will cause a decline in flea abundance (“dilution,” which is thought to occur when parasite population growth is slower than that of the host). In addition, we assessed whether masting alters the relationship between host traits (sex and body mass) and flea abundance. We found a hump‐shaped relationship between host and flea abundance. Thus, the most basic predictions are too simple to describe ectoparasite dynamics in this system. In addition, masting modified seasonal dynamics of flea abundance, but did not affect the relationship between host traits and flea abundance (individuals with the highest body mass hosted the most fleas; after controlling for body mass, parasite abundance did not vary between sexes). Our results demonstrate that pulses of tree reproduction can indirectly, through changes in host densities, drive patterns of ectoparasite infestation.

Theory predicts that risk taking should be influenced by external (e.g., season) and internal (e.g., breeding condition, sex, and body mass) conditions. We investigated whether these factors are associated with a potentially risky behavior: exploration of a novel environment. We conducted repeated open‐field tests of exploration in a common forest rodent, the yellow‐necked mouse Apodemus flavicollis. Contrary to expectations, the exploration did not vary with the season (spring vs. fall) or the reproductive status of the tested animals. Also unexpectedly, there was an inverted U‐shaped relationship between body mass and exploration: animals with intermediate body mass tended to have the highest exploration tendencies. Males were more exploratory than females. Finally, even after adjusting for the effects of body mass and sex, individuals exhibited consistent, repeatable differences in exploration tendencies (“behavioral types” or “personalities”). The discrepancies between certain broad generalizations and our results suggest that risk taking depends on details of species‐specific biology.

Significance: Masting, or synchronous production of large seed crops, is widespread among plants. The predator satiation hypothesis states that masting evolved to overwhelm seed predators with an excess of food. Yet, this popular explanation faced few rigorous tests. We conducted a meta-analysis of studies that related the magnitude of seed production to the intensity of seed predation. Our results validate certain theoretical notions (e.g., that predator satiation is more effective at higher latitudes) but challenge others (e.g., that specialist and generalist consumers differ in the type of functional response to masting). We also found that masting is losing its ability to satiate consumers, probably because global warming affected masting patterns. This shift might considerably impair the reproduction of masting plants.