Dispersal of sweet pignut hickory in a year of low fruit production, and the influence of predation by a curculionid beetle

The rate at which fallen hickory nuts are removed from beneath the parent tree, and the effect on this rate of the seed predator Conotrachelus affinis , was studied in an oak-hickory forest in southeastern Michigan, USA, during a year in which few nuts were produced. The trees responded to Conotrachelus , which destroyed half the nut crop, by aborting inviable nuts during the summer. The seed dispersers, mostly gray squirrels, removed fallen nuts rapidly, showing the ability to distinguish viable nuts and remove them preferentially. The number of nuts removed in a week varies directly with the number available, and removal rate increases when many viable nuts are falling. The death of most seeds before dispersal, and the squirrels' efficiency at foraging on nuts and recovering them after burial, imply that successful hickory reproduction takes place only in years of heavy nut production.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47717/1/442_2004_Article_BF00751606.pd

Diversity in insect seed parasite guilds at large geographical scale: the roles of host specificity and spatial distance

[Aim]: Host specificity within plant-feeding insects constitutes a fascinating example of natural selection that promotes inter-specific niche segregation. If specificity is strong, composition of local plant parasitic insect guilds is largely dependent on the presence and prevalence of the preferred hosts. Alternatively, if it is weak or absent, historic and stochastic demographic processes may drive the structuring of insect communities. We assessed whether the species composition of acorn feeding insects (Curculio spp. guilds) and their genetic variation change geographically according to the local host community. [Location]: An 800 km transect across California, USA. [Methods]: We used DNA taxonomy to detect potential Curculio cryptic speciation and assessed intra-specific genetic structure among sampling sites. We monitored larval performance on different hosts, by measuring the weight of each larva upon emerging from the acorn. Our phylogenetic and spatial analyses disentangled host specificity and geographical effects on Curculio community composition and genetic structure. [Results]: DNA taxonomy revealed no specialized cryptic species. Californian Curculio spp. were sister taxa that did not segregate among Quercus species or, at a deeper taxonomic level, between red and white oaks. Curculio species turnover and intra-specific genetic differentiation increased with geographical distance among localities irrespective of local oak species composition. Moreover, larval performance did not differ among oak species or acorn sizes when controlling for the effect of the locality. [Main conclusions]: Historical processes have contributed to the structuring of acorn weevil communities across California. Trophic niche overlapped among species, indicating that ecologically similar species can co-exist. Acorn crop inter-annual variability and unpredictability in mixed oak forests may have selected against narrow specialization, and facilitated co-existence by means of an inter-specific time partitioning of the resources. Wide-scale geographical records of parasitic insects and their host plants are necessary to understand the processes underlying species diversity.This work was financed by the projects: CONSOLIDER-MONTES CSD2008-00040 MICINN, PII1C09-0256-9052JCCM and ESF, AGL2014-54739-R (MINECO), PPII-2014-01-PJCCM ESF and CGL2008-00095 ⁄ BOS (MICINN). A.M.was funded by a Juan de la Cierva contract and R.B. by acontract of the Atracción de Talento Investigador Programme (Gobierno de Extremadura TA13032). J.O. wasfunded by Severo Ochoa (SEV-2012-0262) and Ramón y Cajal (RYC-2013-12501) research fellowships.Peer reviewe

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

Foraging patterns of acorn woodpeckers (Melanerpes formicivorus) on valley oak (Quercus lobata Née) 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 (Kg = 4.1 ± 0.5) at Sedgwick than at Hastings (Kg = 7.6 ± 0.6) and from fewer effective seed sources (Nem* = 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

Climatically stable landscapes predict patterns of genetic structure and admixture in the Californian canyon live oak

[Aim]: We studied which factors shape contemporary patterns of genetic structure, diversity and admixture in the canyon live oak (Quercus chrysolepis). Specifically, we tested two alternative hypotheses: (1) that areas with high habitat suitability and stability since the Last Glacial Maximum (LGM) sustain higher effective population sizes, resulting in increased levels of genetic diversity; and (2) that populations from areas with lower habitat stability show higher levels of genetic admixture due to their recurrent colonization by individuals originating from genetically differentiated populations. Furthermore, we analysed the relative importance of past and current habitat suitability and their additive effects on contemporary patterns of genetic structure. [Location]: California, USA. [Methods]: We sampled 160 individuals from 33 localities across the distribution range of the canyon live oak in California and then combined information from 13 nuclear microsatellite DNA markers and climate niche modelling to study patterns of genetic variation in this species. We used Bayesian clustering analyses to analyse geographical patterns of genetic structure and admixture, and circuit theory to generate isolation-by-resistance (IBR) distance matrices. [Results]: We found that the degree of genetic admixture was higher in localities with lower inferred population stability, but that genetic diversity was not associated with habitat suitability or stability. Landscape genetic analyses identified habitat stability as the primary driver of population genetic differentiation. [Main conclusions]: This study shows that habitat stability can be a major factor shaping genetic variation in wind-pollinated trees and supports the idea that stable regions contribute to genetic connectivity across different climatic periods. To our knowledge, this study is the first to report an association between patterns of genetic admixture and stability of local habitat.During this work J.O. was supported by Juan de la Cierva (MICINN), José Castillejo (ME) and Severo Ochoa (EBD) research fellowships. P.F.G. received post-doctoral support from a UCLA research award to V.L.S. This workreceived financial support from grants CGL2011-25053(MICINN) and UNCM08-1E-018 (FEDER).Peer reviewe