Rodent acorn selection in a Mediterranean oak landscape

Grazing hinders seed dispersal during crop failure in a declining oak woodland

Masting, the synchronized production of variable quantities of seeds, is a global phenomenon in diverse ecosystems, including treed grazing systems where trees and grazing animals coexist. This phenomenon can be interspersed with years of extreme crop failure, whose frequency and unpredictability are increasing. Yet, the combined impact of crop failure and grazing on seed dispersal and seed-to-seedling transition remains poorly understood. To address this concern, we investigated rodent-mediated cork-oak (Quercus suber) acorn predation, dispersal, and seedling emergence in cattle grazed and non-grazed areas in central Portugal during years with contrasting masting seasons. We found that the acorns supplied in the crop failure year were dispersed more rapidly and over longer distances than those supplied in the crop success year when other acorns were naturally available. The crop failure year also had 83 % more dispersal events and 84 % more predated acorns than the reproductive success year. However, the higher acorn predation was offset by a 2.4-fold higher percentage of unpredated dispersed acorns recruiting into seedlings. Both years ended up recruiting a similar number of seedlings. Acorns emerged seedlings 3.4 times farther in the crop failure year than in the crop success year. Cattle grazing was the main constraint on seed dispersal distance by rodents, reducing it by 53 %. Our study provides empirical evidence that cattle grazing modulates how an extreme crop failure year can surprisingly be an opportunity for the few existing acorns to have seedlings established farther apart than in a crop success year. If we are to better manage and preserve the high conservation and socio-economic value of Mediterranean cork oak woodlands in the face of climate change, we must prioritize fecund trees and carefully manage seed dispersal factors such as cattle grazing, particularly during years of crop failure.

Food preference strategy of four sympatric rodents in a temperate forest in northeast China

Rodents are well known as both seed predators and dispersers of various plant species in forest ecosystems, and they play an important role in the regeneration of vegetation. Thus, the research on seed selection and vegetation regeneration by sympatric rodents is an interesting topic. To understand the characteristics of preferences of rodents for different seeds, a semi-natural enclosure experiment was performed with four rodent species (Apodemuspeninsulae, Apodemusagrarius, Tscherskiatriton, and Clethrionomysrufocanus) and the seeds of seven plant species (Pinuskoraiensis, Corylusmandshurica, Quercusmongolica, Juglansmandshurica, Armeniacasibirica, Prunussalicina, and Cerasustomentosa) to investigate the differentiation in niches and patterns of resource utilization of sympatric rodents. The results showed that all the rodents had consumed many seeds of Pi.koraiensis, Co.mandshurica, and Q.mongolica but differed significantly in how they selected the different seeds. The rate of utilization (R_i) of Pi.koraiensis, Co.mandshurica, and Q.mongolica exhibited the highest values. The E_i values indicated that the rodents tested exhibited differences in their priorities used to select the seeds from different plant species. All four species of rodents exhibited obvious preferences for certain seeds. Korean field mice preferentially consumed the seeds of Q.mongolica, Co.mandshurica, and Pi.koraiensis. Striped field mice favor the seeds of Co.mandshurica, Q.mongolica, P.koraiensis, and Nanking cherry. Greater long-tailed hamsters prefer to consume the seeds of Pi.koraiensis, Co.mandshurica, Q.mongolica, Pr.salicina, and Ce.tomentosa. Clethrionomysrufocanus likes to eat the seeds of Pi.koraiensis, Q.mongolica, Co.mandshurica, and Ce.tomentosa. The results supported our hypothesis that sympatric rodents overlap in food selection. However, each rodent species has a marked preference for food selection, and different rodent species differ in their food preferences. This reflects the role of distinct food niche differentiation in their coexistence.

Ungulate presence and predation risks reduce acorn predation by mice in dehesas

Foraging decisions by rodents are key for the long-term maintenance of oak populations in which avian seed dispersers are absent or inefficient. Decisions are determined by the environmental setting in which acorn-rodent encounters occur. In particular, seed value, competition and predation risks have been found to modify rodent foraging decisions in forest and human-modified habitats. Nonetheless, there is little information about their joint effects on rodent behavior, and hence, local acorn dispersal (or predation). In this work, we manipulate and model the mouse-oak interaction in a Spanish dehesa, an anthropogenic savanna system in which nearby areas can show contrasting levels of ungulate densities and antipredatory cover. First, we conducted a large-scale cafeteria field experiment, where we modified ungulate presence and predation risk, and followed mouse foraging decisions under contrasting levels of moonlight and acorn availability. Then, we estimated the net effects of competition and risk by means of a transition probability model that simulated mouse foraging decisions. Our results show that mice are able to adapt their foraging decisions to the environmental context, affecting initial fates of handled acorns. Under high predation risks mice foraged opportunistically carrying away large and small seeds, whereas under safe conditions large acorns tended to be predated in situ. In addition, in the presence of ungulates lack of antipredatory cover around trees reduced mice activity outside tree canopies, and hence, large acorns had a higher probability of survival. Overall, our results point out that inter-specific interactions preventing efficient foraging by scatter-hoarders can reduce acorn predation. This suggests that the maintenance of the full set of seed consumers as well as top predators in dehesas may be key for promoting local dispersal.

Effects of season and food on the scatter-hoarding behavior of rodents in temperate forests of Northeast China

To explore the differences in hoarding strategies of rodents for different seeds in various seasons, we labeled and released the seeds of Pinuskoraiensis, Corylusmandshurica, Quercusmongolica and Prunussibirica in temperate forests of Northeast China and investigated the fate of the seeds in spring and autumn. The analysis showed that the hoarding strategies of the rodents varied substantially between seasons. The seeds were consumed faster in the spring than in the autumn. More than 50% of the seeds in the two seasons were consumed by the 16^th day. It took 36 days to consume 75% of the seeds in the spring and 44 days in the autumn. The rate of consumption of the seeds in the spring was greater than in the autumn, and the rate of spread of the seeds was greater in the autumn. The distances of removal for the consumption and dispersal of seeds in the spring (3.26 ± 3.21 m and 4.15 ± 3.52 m, respectively) were both shorter than those in the autumn (3.74 ± 3.41 m and 4.87 ± 3.94 m, respectively). In addition, the fate of different seeds varied significantly owing to differences in hoarding strategies. The seeds of the three preferred species, P.koraiensis, C.mandshurica, and Q.mongolica, were quickly consumed. More than 90% of the seeds of these species were consumed. Only 21% of Pr.sibirica seeds were slowly consumed, and the two seasons had the same seed consumption rate patterns: the consumption rate of P.koraiensis seeds was the highest, followed by C.mandshurica, then Q.mongolica, and finally Pr.sibirica. The median removal times of the two seasons were different, but the rules were the same: P.koraiensis was the shortest, followed by C.mandshurica, and the third was Q.mongolica. In both seasons, the most predated in situ seeds were those of P.koraiensis; the most hoarded seeds were those of C.mandshurica, and the most unconsumed seeds were those of Pr.sibirica.

Fire benefits flower beetles in a Mediterranean ecosystem

Despite the abundance of plants that benefit from fire in Mediterranean ecosystems, little is known about the possible presence of fire-favoured insects (other than bark beetles). For two years we sampled invertebrates after two large wildfires in eastern Spain and demonstrate that two flower beetle species, Protaetia morio and P. oblonga (Cetoniidae), show a pyrophilous behaviour. These beetles were much more numerous after the fires than in unburnt plots around the fire perimeter; in addition, these species tended to increase in number with the distance from the fire perimeter and with fire recurrence, especially P. morio. These results were maintained for the two postfire years sampled. The results for the beetles do not support the hypothesis of postfire colonization, but that local populations survived the fire as eggs or larvae protected in the soil (endogenous persistence). We propose that the increase in population size (compared with unburnt zones) could be driven by the reduction of their predator populations, as vertebrates that feed on these beetles were disfavoured by fire. That is, the results suggest that these flower beetle species benefit from fire because fire disrupts antagonistic interactions with their predators (predation release hypothesis). Given the omnipresence of small mammals, soil insects, and fires, the processes described here are likely to be general but unexplored.

Ecological significance of seed desiccation sensitivity in Quercus ilex

BACKGROUND AND AIMS

Several widespread tree species of temperate forests, such as species of the genus Quercus, produce recalcitrant (desiccation-sensitive) seeds. However, the ecological significance of seed desiccation sensitivity in temperate regions is largely unknown. Do seeds of such species suffer from drying during the period when they remain on the soil, between shedding in autumn and the return of conditions required for germination in spring?

METHODS

To test this hypothesis, the Mediterranean holm oak (Quercus ilex) forest was used as a model system. The relationships between the climate in winter, the characteristics of microhabitats, acorn morphological traits, and the water status and viability of seeds after winter were then investigated in 42 woodlands sampled over the entire French distribution of the species.

KEY RESULTS

The percentages of germination and normal seedling development were tightly linked to the water content of seeds after the winter period, revealing that in situ desiccation is a major cause of mortality. The homogeneity of seed response to drying suggests that neither intraspecific genetic variation nor environmental conditions had a significant impact on the level of desiccation sensitivity of seeds. In contrast, the water and viability status of seeds at the time of collection were dramatically influenced by cumulative rainfall and maximum temperatures during winter. A significant effect of shade and of the type of soil cover was also evidenced.

CONCLUSIONS

The findings establish that seed desiccation sensitivity is a key functional trait which may influence the success of recruitment in temperate recalcitrant seed species. Considering that most models of climate change predict changes in rainfall and temperature in the Mediterranean basin, the present work could help foresee changes in the distribution of Q. ilex and other oak species, and hence plant community alterations.

Acorn crop size and pre-dispersal predation determine inter-specific differences in the recruitment of co-occurring oaks

The contribution of pre-dispersal seed predation to inter-specific differences in recruitment remains elusive. In species with no resistance mechanisms, differences in pre-dispersal predation may arise from differences in seed abundance (plant satiation) or in the ability of seeds to survive insect infestation (seed satiation). This study aimed to analyse the impact of pre-dispersal acorn predation by weevils in two co-occurring Mediterranean oaks (Quercus ilex and Quercus humilis) and to compare its relevance with other processes involved in recruitment. We monitored the patterns of acorn production and acorn infestation by weevils and we conducted experimental tests of acorn germination after weevil infestation, post-dispersal predation and seedling establishment in mixed forests. Monitoring and experimental data were integrated in a simulation model to test for the effects of pre-dispersal predation in recruitment. In both oaks pre-dispersal acorn infestation decreased with increasing acorn crop size (plant satiation). This benefited Q. ilex which exhibited stronger masting behaviour than Q. humilis, with almost a single and outstanding reproductive event in 6 years. Acorn infestation was more than twice as high in Q. humilis (47.0%) as in Q. ilex (20.0%) irrespective of the number of seeds produced by each species. Although germination of infested acorns (seed satiation) was higher in Q. humilis (60%) than in Q. ilex (21%), this could barely mitigate the higher infestation rate in the former species, to reduce seed loss. Conversely to pre-dispersal predation, no inter-specific differences were observed either in post-dispersal predation or seedling establishment. Our results indicate that pre-dispersal predation may contribute to differences in seed supply, and ultimately in recruitment, between co-existing oaks. Moreover, they suggest that seed satiation can barely offset differences in seed infestation rates. This serves as a warning against overemphasising seed satiation as a mechanism to overcome seed predation by insects.

Acorn dispersal estimated by radio-tracking

Bird-dispersed seeds are difficult to track, especially in the case of long-distance dispersal events. To estimate the oak dispersal distance and the seed shadow generated by the European jay (Garrulus glandarius), we inserted radio-transmitters in 239 acorns, placed them in bird-feeders and then located them by radio-tracking. Using this methodology we located the exact caching site of 94 Quercus ilex and 54 Q. suber acorns and determined the caching habitat characteristics (vegetation type, distance, spatial distribution). The results show that: (1) there is no differences in the dispersal distance distribution between the different acorn species or sizes, (2) dispersal distances range from approximately 3 m up to approximately 550 m (mean = 68.6 m; median = 49.2 m), (3) recently abandoned fields and forest tracks were the sites preferred by jays to cache acorns, whereas fields and shrublands were avoided and (4) seed shadows showed acorn aggregation zones (i.e. clusters of caches) close to the feeder as well as isolated caches at longer distances. The results also suggest that radio-transmitters are a cheap and reliable way to determine seed shadows and quantify both seed dispersal and post-dispersal seed predation for medium to large seeds.