Edmund W. Stiles

The structural complexity of old field vegetation and the recruitment of bird-dispersed plant species

SummaryThe input of bird-disseminated seeds into four old fields of different structural complexity was examined. Seed input was greatest along the edges of fields. Significantly more seeds were found in a 13 year old field that had structurally complex vegetation, than in a 3-year-old field with a single layer of vegetation. The lower input into the latter field was a function of both low fruit availability and low structural complexity of the field. Similarly, more seeds were found in a 2-year-old field which had artificial structures, simulating saplings, placed in it than in an adjacent control field of the same age. The shape of the structures was not a significant factor in the input of seeds. Timing of seed deposition was correlated with fruit ripening times, relative nutritional value of the fruit and the movements of frugivorous birds. The input of bird-disseminated seeds into fields appears to be directly related to the structural complexity of the vegetation. Woody plants increase the structural complexity of the old fields and serve as recruitment foci for bird-disseminated seeds. Thus, seed deposition by birds influences vegetation pattern, and conversely, the presence of recruitment foci in the vegetation may influence bird dispersal patterns of bird-disseminated seeds.

Links between Microhabitat Preferences and Seed Predation by Small Mammals in Old Fields

Recent studies indicate that seed predation by small mammals in old fields can be intense and, in conjunction with other herbivores, may influence the rate, composition and spatial pattern of tree invasion into these systems. Patterns of seed predation in old fields may be predictable, based on the preferences herbivores exhibit for particular microhabitats. Studies testing the strength of these linkages, however, are rare. The risk of predation for foraging animals can have a considerable impact on decisions about where to feed and what to consume in particular microhabitats. We examined how an experimentally created gradient of predation risk influenced the microhabitat use and foraging behavior a white-footed mice (Peromyscus leucopus) in an old field in central New Jersey, USA. Specifically, we monitored the removal of seeds from dishes placed in the center of replicated treatment plots in which we manipulated ground and canopy cover, as well as distance from the forest edge. We then determined the relative importance of these factors and their interactions in explaining patterns of seed predation by mice. In addition, the distribution and abundance of small mammals were monitored and the vegetation cover in the area immediately adjacent to our experimental plots was quantified to determine how these factors were related to patterns of seed removal. Other factors evaluated included seed species, nocturnal illumination, temperature, precipitation, and time of year. Ground cover, time of day, and distance from the forest edge explained the most variation in patterns of seed predation. Ground cover, particularly proximity to vines such as Rubus allegheniensis, appeared to take precedence over other microhabitat variables in the decisions seed predators made about where to forage. Significantly higher nocturnal rates of seed removal, trap data, feces and seed remains left around seed dishes, reinforced previous findings which suggest that white-footed mice are the dominant seed predators in the northeastern old fields. However, spatial patterns of seed removal did not correspond with the distribution and abundance of mice as indicated by the trapping data. These results suggest that the impact of P. leucopus on the spatial patterns of seed survival is more dependent on the behavioral responses of individual mice than the distributions of the populations they comprise. Overall. there was little variation from general patterns of seed predation for specific tree species, suggesting that in old fields dominated by mice. there may be relatively uniform regions of high and low seed predation based on the microhabitat preferences of this seed consumer. The consequences of the interaction between old field vegetation and patterns of seed predation by mice for tree invasion and establishment in old fields are discussed.

HOW LONG FRUIT-EATING BIRDS STAY IN THE PLANTS WHERE THEY FEED: IMPLICATIONS FOR SEED DISPERSAL

Theories relating foraging behavior of frugivorous birds to patterns of seed dispersal have not considered the effect of prolonged visits by birds to fruit-bearing plants. We present data showing that in New Guinea some obligate frugivores (fruit pigeons and bowerbirds) spend more time in fruit-bearing plants than is required for feeding. Often they remain in the plant between feeding bouts. On the other hand, some insectivorous fruit-eating birds (birds of paradise) visit fruit-bearing plants only long enough to feed. Longer visits by fruit pigeons and bowerbirds may result, in different cases, from greater commitment to frugivory, to predator and competitor avoidance strategies involving crypsis, and to less time-demanding breeding systems. Longer visits may result in a larger number of seeds being dropped beneath food trees.

Costs of reproduction in Nyssa sylvatica : sexual dimorphism in reproductive frequency and nutrient flux

SummaryWe examined the influence of differential reproductive frequency between the sexes on tertiary (phenotypic) sex ratios in the the dioecious tree Nyssa sylvatica (Nyssaceae). Reproduction was evaluated in relation to sex, size and canopy exposure using flowering data collected from 1229 marked trees over a four year period. For subsets of each population we used data on flower number, fruit crop size, fruit/flower ratios, and individual flower and fruit mass to compare biomass invested in reproductive structures of males and females. We also examined seasonal changes in stem nitrogen and soluble carbohydrate content in relation to flower and fruit production for trees of each sex. Our results indicate that: 1) Male-biased tertiary sex ratios could be explained by more frequent reproduction by male trees; 2) Estimated secondary sex ratios based on sums of all known males and females were not significantly different from 1:1; 3) Flowering frequency of males and females was significantly related to plant size (DBH) and exposure of the canopy to light; 4) Estimtes of reproductive biomass allocation ranged from 1.36 to 10.8 times greater for females relative to males; 5) Flower production was related to stem nutrient status for both sexes, but nutrient depletion and its effect on subsequent flowering was much more pronounced for female trees. We conclude that less frequent flowering by female trees may result from depletion of stored reserves, and that differential flowering frequency in N. sylvatica may ultimately reduce apparent sexual differences in the costs of reproduction.

Dispersal of salt marsh seeds on the feet and feathers of waterfowl

Dispersal of seeds by adhesion to animals, known as epizoochory, is often referred to as a significant mode of dispersal. Few studies have quantitatively examined this method of dispersal; those that do often concentrate on mammalian rather than avian dispersers. To measure the extent of this form of seed dispersal, we studied the external seed loads on the feet and feathers of four waterfowl species found in a salt marsh on the New Jersey shore. More than 75% of the birds were found to be carrying seeds, mostly on their feathers. The seeds of twelve plant species, predominantly from salt marsh plants and particularly salt grass (Distichlis spicata) and cordgrass (Spartina alterniflora), were identified in the samples. All but one species has seeds with potentially adhesive structures. Results indicate that for many salt marsh plant species, seed dispersal by adhesion to waterfowl may be a significant method of seed transport.

Living off the wax of the land bayberries and yellow rumped warblers

Yellow-rumped Warblers (Dendroica coronata) and Tree Swallows (Tachycineta bicolor) are among a small group of birds in temperate North America that regularly eat waxy fruits. During the autumn, winter, and spring, these species feed extensively on fruits of the bayberry (Myrica spp.). Covering the pulp of these fruits is a solid, waxy material consisting primarily of saturated long-chain fatty acids. For most animals, saturated fatty acids are poorly assimilated ( 80%) of bayberry wax when fed berries recoated with radioactive wax tracers. Efficient fatty-acid assimilation extends to berries coated with cetyl palmitate, a common marine, saturated wax ester (> 90%). The fatty-alcohol moiety of the marine wax was assimilated with a much lower efficiency (<50%). A beeswax coating of the berries is assimilated with an efficiency of approximately 50%. Similar assimilation efficiencies of each wax are recorded for Tree Swallows feeding on recoated bayberries. Yellow Warblers (D. petechia) rejected recoated bayberries and exhibited little (<5%) lipid assimilation of radiolabeled lipids. Yellow-rumped Warblers possess several gastrointestinal traits that permit efficient saturated-fat assimilation. Among these are an apparent retrograde reflux of intestinal contents to the gizzard, elevated gall-bladder and intestinal bile-salt concentration, and a slow gastrointestinal transit of dietary lipids. These gastrointestinal traits permit efficient assimilation of saturated fatty acids on bayberry fruits and may allow these small passerines to maintain more northerly wintering ranges than closely related species. Received 24 May 1991, accepted 5 November 1991.

Selection on plant fruiting traits by brown capuchin monkeys: a multivariate approach

We present the use of a recently developed multivariate method (Lande and Arnold, 1983) to estimate selection on plant fruiting traits caused by seed dispersal via wild brown capuchin monkeys (Cebus apella). There was significant selection on some traits influencing the quality of individual fruits (energy content per fruit, carbohydrate and protein concentrations) as well as on spatial aspects of fruit displays (crown size, tree density and distribution). In several cases, the results of this multivariate analysis differed markedly from those of univariate analyses on the same data, in which the effects of correlated variables can obscure the selection coefficients. The feeding behavior of capuchins and birds appears to be affected differently by various plant fruiting traits, but no direct comparison can be made without a comparable multivariate analysis of seed crop removal by birds.

Co-Occurrences of Foods in Stomachs and Feces of Fruit-Eating Birds

Short-term dietary mixing by American Robins (Turdus migratorius) and other fruit-eating birds in eastern North America was examined using 5,697 records of stomach contents for 11 bird species and 3,618 avian fecal droppings from New Jersey. Avian seed budgets were estimated by using fruit morphological data to relate foraging observations to seed counts from stomachs and feces. Remnants of multiple taxa of foods were found commonly in individual feces and stomachs although these samples held only 0.25-2 times the seeds consumed during a typical feeding bout. Depending on bird taxon, seeds from different fruit species were mixed in 1.5-39.6% of feces and 4.2-41.6% of stomachs, and fruit and animal material were mixed in 24-59% of stomachs. Frequency of mixed seeds was positively correlated with proportion of fruit in the stomach and with seed concentration of fruits in the diet. For birds overwintering in the United States, seed mixing in stomachs peaked in winter when birds were most dependent on fruit, not in fall when fruit abundance and diversity were greatest. Thus, estimates of dietary mixing may be biased by seasonal or habitat-related trends in avian fruit dependence or fruit morphology. Furthermore, mixing was no less common in feces than in comparable stomachs even though stomachs contained three to four times more seed mass. This finding along with our observations of seed treatment by birds suggested that mixing was amplified by shuffling of seeds and fruits in the upper gut and by variability of seed transit times through the intestines.

Seed predation by the bean weevil Acanthoscelides obtectus on Phaseolus species: consequences for seed size, early growth and reproduction

We examined potential implications of seed predation by the common bean weevil, Acanthoscelides obtectus for seed size variation in cultivated beans (Phaseolus vulgaris L., P. coccineus L. and P. lunatus L.) grown in common gardens. Significant seed size variation existed among different plants, among pods of differing seed number within plants, among successive seed crops within plants, and among seeds within pods. Within-plant sources of seed size variation were equally or more important than among-plant sources. Initial seed size strongly influenced early growth under laboratory and field conditions, and the advantage in early growth due to large seed size carried over to enhance subsequent plant reproduction in the field. Predationrelated shifts in seed and seedling size distributions apparently occurred without active selection by the bruchid. Oviposition that was evenly distributed among pods, but clumped within one or a few seeds within pods, allowed a greater proportion of seeds to escape from many-seeded pods. This, coupled with negative size/number relationships within and among plants, resulted in shifts toward smaller mean seed size. Due to nearly ubiquitous embryo damage by bruchid larvae, seed size of host plants was not related to tolerance to predation. We conclude that indirect sizeselective seed predation can have significant effects on maternal plant fitness both by altering the number of progeny produced, and by altering the fitness of surviving progeny.

Effects of fruit crop size on intensity of fruit removal in Viburnum prunifolium (Caprifoliaceae)

We tested the effect of crop size on fruit removal from an early successional shrub, Viburnum prunifolium (Caprifoliaceae), by comparing the «survival» distributions of fruit from plants with large crops to those with small crops. We found that crop size does not affect rate of fruit removal. Our findings differ from previous researchers who demonstrated correlations between large crop size and higher removal rates from Sambucus pubens and from Viburnum dentatum shrubs. Differences between our findings and those of other researchers may be due either to differences in statistical analyses or type of avian dispersers (solitary versus flocking species). These data do not support the fruit crop size hypothesis