Different population responses of three sympatric rodent species to acorn masting—the role of tannin tolerance

Rodent population dynamics are predicted to respond positively to the masting of acorns, but diverging results have been published. This study tested the hypothesis that population responses to acorn masting vary depending on differences in the tolerance to tannins of different rodent species. The effects of acorn abundance on the rodent population densities were analyzed using a dataset obtained in Hokkaido, Japan. Specifically, population fluctuations of three rodent species (Apodemus speciosus, A. argenteus, and Myodes rufocanus) and the abundance of Quercus crispula acorns have been monitored since 1992. Acorn production in previous years had a positive effect on the annual population growth rates of A. speciosus; however, this trend was not clear in the other two species. Tannin tolerance, assessed by body weight changes in an acorn feeding experiment, exhibited clear differences among the rodent species; namely, the body weight of A. speciosus increased, whereas that of the other two species decreased. The observed responses to acorn masting of populations of the three sympatric rodent species reflected tannin tolerance. It suggests that only populations of species with high tannin tolerance positively respond to acorn masting. Previous studies tend to overlook species-specific abilities of coping with tannins in acorns. Our results emphasize the necessity of evaluating the tannin tolerance of rodents and the tannin content of acorns to understand how rodent population dynamics and acorn production are related.     

Foraging patterns of acorn woodpeckers (Melanerpes formicivorus) on valley oak (Quercus lobata N��e) in two California oak savanna-woodlands

Landscape characteristics and social behavior can affect the foraging patterns of seed-dependent animals. We examine the movement of acorns from valley oak (Quercus lobata) trees to granaries maintained by acorn woodpeckers (Melanerpes formicivorus) in two California oak savanna-woodlands differing in the distribution of Q. lobata within each site. In 2004, we sampled Q. lobata acorns from 16 granaries at Sedgwick Reserve in Santa Barbara County and 18 granaries at Hastings Reserve in Monterey County. Sedgwick has lower site-wide density of Q. lobata than Hastings as well as different frequencies of other Quercus species common to both sites. We found acorn woodpeckers foraged from fewer Q. lobata seed source trees (K(g) = 4.1 ± 0.5) at Sedgwick than at Hastings (K(g) = 7.6 ± 0.6) and from fewer effective seed sources (N(em)* = 2.00 and 5.78, respectively). The differences between sites are due to a greater number of incidental seed sources used per granary at Hastings than at Sedgwick. We also found very low levels of seed source sharing between adjacent granaries, indicating that territoriality is strong at both sites and that each social group forages on its own subset of trees. We discovered an interesting spatial pattern in the location of granaries. At Sedgwick, acorn woodpeckers situated their granaries within areas of higher-than-average tree density, while at Hastings, they placed them within areas of lower-than-average tree density, with the outcome that granaries at the two sites were located in areas of similar valley oak density. Our results illustrate that landscape characteristics might influence the number of trees visited by acorn woodpeckers and the locations of territories, while woodpecker social behavior, such as territoriality, shapes which trees are visited and whether they are shared with other social groups.     

Phenotypic variation in seedlings of a “keystone” tree species (Quercus douglasii): the interactive effects of acorn source and competitive environment

Blue oak (Quercus douglasii) is a deciduous tree species endemic to California that currently exhibits poor seedling survival to sapling age classes. We used common garden techniques to examine how genetic variation at regional and local scales affected phenotypic expression in traits affecting oak seedling growth and survival. Between-population variation was examined for seedlings grown from acorns collected from a northern, mesic population and a southern, xeric population. Within-population variation was examined by comparing seedlings from different maternal families within the mesic population. Acorns were planted into neighborhoods of an annual dicot (Erodium botrys), an annual grass (Bromus diandrus), and a perennial bunchgrass (Nassella pulchra). By varying the species composition of herbaceous neighborhoods into which acorns were planted, the interactive effects of competition and acorn germplasm source on phenotypic expression could also be examined. Potential maternal effects, expressed as variation in acorn size, were assessed by weighing each acorn before planting. Probability of seedling emergence increased significantly with acorn size in the xeric population but not in the mesic population. Similarly, the effect of acorn size on seedling leaf area, stem weight, and root weight was also population-dependent. At a within-population level, acorn size effects on seedling traits varied significantly among maternal families. In addition to acorn size effects, rates of oak seedling emergence were also dependent on an interaction of population source and competitive environment. Interactions between maternal family and competitive environment in the expression of seedling leaf characters suggest the possibility of genetic variation for plasticity in traits such as specific leaf area. Using carbon isotope discrimination () as an index of relative water-use efficiency (WUE), higher water use efficiency was indicated for oak seedlings grown in the annual plant neighborhoods compared to seedlings grown in the bunchgrass neighborhood. This trend may represent an adaptive plastic response because, compared to the bunchgrass neighborhood, soil water depletion was more rapid within annual plant neighborhoods.     

Differential patterns of spatial divergence in microsatellite and allozyme alleles: further evidence for locus‐specific selection in the acorn barnacle,Semibalanus balanoides?

We compared patterns of genetic structure at potentially selected (two allozyme loci) and neutral molecular markers (six microsatellite loci) in the acorn barnacle, Semibalanus balanoides from the Gulf of St. Lawrence. Our results confirmed the presence of a geographical shift in alleles MPI and GPI near the Miramichi River. In contrast, no significant patterns of population differentiation among samples located north and south of the river mouth were detected for four of six microsatellite loci. However, analysis of molecular variance (amova) at individual loci revealed that a significant proportion of the total variance in allele frequencies was partitioned among samples located north and south of the river for both the allozyme and the other two microsatellite loci. The two most common alleles at these microsatellites showed frequencies that were highly correlated (r = 0.65-0.74, P < 0.05) with those of the MPI*2 allele, perhaps because of either physical linkage or epistasis. The two allozyme loci were significantly correlated in barnacles located north of the Miramichi River (r = 0.86, P < 0.05). Overall, our results supported the hypothesis that the broad scale pattern of allozyme allelic shifts is maintained by selection. They also indicated that microsatellites may not always behave in a neutral way and must be used cautiously, especially when evidence for genetic structuring relies on only a few assayed loci.     

Does acclimation reduce the negative effects of acorn tannins in the wood mouse<Emphasis Type="Italic">Apodemus speciosus</Emphasis>?

Acclimation as a mechanism of mitigating the damaging effects of acorn tannins was examined in the Japanese wood mouseApodemus speciosus Temminck, 1844. Mice were fed the two types of diet: a control diet (laboratory chow for mice), and acorns ofQuercus serrata (QS), which differ in tannin contents (control=0%, QS=2.7% tannic acid equivalent). Body weight changes and digestive abilities were compared between the first stage (Days 1 to 5; Day 0 was defined as the first day of acorn feeding) and the second stage (Days 6 to 10). The amount of salivary proline-rich proteins (PRPs), which are thought to be defensive products to tannins, was measured before and after the experiment. The responses of the wood mice to QS acorns differed between stages: decreases in body weight during the second stage were less than half those that occurred during the first stage; digestive abilities tended to improve; and the relative amount of PRPs after the experiment increased by a factor of five compared with the value recorded before the experiment in the QS-feeding mice. These results suggest that the negative effects of ingesting acorn tannins may be reduced by acclimation, which may result primarily from the induction of PRP production.