bluuebunny/crossref_metadata_2025 huggingface dataset:
Field Experiments on Desert Ants: Testing for Competition Between Colonies
Authors: Ryti Randall T. , Case Ted J.
Published: May 2006

Our previous observations and short—term experiments on two granivorous ant species in the California desert indicated that established colonies may compete intraspecifically. Here we report the results of 3—yr neighbor—removal and food—addition experiments that tested for the presence of intraspecific and interspecific competition between colonies. We followed the responses of territory use, diet breadth, foraging activity, and reproductive output (alate production) to the treatments. Both territory use and alate production patterns were consistent with the predictions of intraspecific resource competition. We found to evidence for interspecific effects between colonies. Specifically, Veromessor pergandi "neighbor—removed" colonies foraged randomly throughout the treatments, while control colonies tended to forage away from their nearest conspecific neighbor when resource levels were low. Both food—supplemented and neighbor—removed V. pergandei colonies produced proportionately more alates than control colonies. There were no treatment effects on diet breadth or foraging activity at V. pergandei or Pogonomyrmex californicus colonies. Since other experiments on desert granivores have shown significant competitive effects on colony density over the same time scale, the lack of consistent effects of competition was most likely due to: (1) differences between the communities in relation to seed storage and productivity and (2) the fact that our experiments only tested for competition between established colonies and did not consider potential competitive effects of established colonies on alate survival (colony regeneration).
Social Barriers to Pathogen Transmission in Wild Animal Populations
Authors: Loehle Craig
Published: May 2006

Diseases and pathogens are receiving increasing recognition as sources of mortality in animal populations. Immune system strength is clearly important in fending off pathogen attack. Physical barriers to pathogen entry are also important. Various individual behaviors are efficacious in reducing contact with diseases and pests. This paper focuses on a fourth mode of defense: social barriers to transmission. Various social behaviors have pathogen transmission consequences. Selective pressures on these social behaviors may therefore exist. Effects on pathogen transmission of mating strategies, social avoidance, group size, group isolation, and other behaviors are explored. It is concluded that many of these behaviors may have been affected by selection pressures to reduce transmission of pathogens.
Sex Investment in a Social Insect: The Proximate Role of Food
Authors: Deslippe Richard J. , Savolainen Riitta
Published: May 2006

In eusocial Hymenoptera, relatedness asymmetries lead to conflict between parents and offspring over sex investment; workers strive for a 3:1 female—biased ratio of sex allocation, and queens strive for a 1:1 ratio. Many studies support this genetic relatedness hypothesis, but the variation in allocation ratios in natural populations is great and remains mostly unexplained. In this paper, we examined whether food supply determines sex investment of the ant Formica podzolica. We compare reproductive parameters of colonies and populations across habitats, and compare sex allocation of fed and unfed colonies. Nest density, worker size, sex ratio, and sex allocation were all greater along forest edges than in meadows, and these patterns were associated with natural food abundance. Furthermore, there was a strong tendency for individual colonies to produce either all—male or all—female sexuals, and worker size was greatest in colonies producing all females and smallest in those producing all males. Most important, sex investment was greatly affected by a supplemented diet, as the population investment ratio, R (i.e., ratio of males to males plus females), was female biased (0.36) for fed colonies and male biased (0.62) for unfed, control colonies. Finally, investment ratios were more male biased in polygynous than in monogynous colonies as predicted by a genetic relatedness hypothesis. These results demonstrate that food supply has an important proximate influence on sex investment, and may explain much of the natural variation in sex investment in populations of eusocial Hymenoptera.
Parental Host Plant Affects Offspring Life Histories in a Seed Beetle
Authors: Fox Charles W. , Waddell Kim J. , Mousseau Timothy A.
Published: May 2006

Non—genetic effects of maternal environment often have large influences on offspring phenotypes. Some maternal effects may be adaptive in heterogeneous environments, whereby mothers can program a developmental switch in their offspring in response to a predictive environmental cue. For herbivorous insets, maternal rearing hosts (i.e., maternal environment) may have large effects on offspring, including adaptive effects on patterns of host suitability. For example, if maternal rearing host is predictive of local host availability, then it is advantageous for mothers reared on one host to produce offspring that are physiologically 'acclimated' to that host. Previous research has demonstrated that seed beetles, Stator limbatus, collected from Cercidium floridum survived better and developed faster on this host than beetles collected from Acacia greggii or C. microphyllum at either the same site or from a different locality. These results indicate either within— and among—population host—associated genetic differentiation or a non—genetic maternal host effect, in which offspring survive better and develop faster on the host their mother was reared on. We examine the influence of maternal and paternal rearing hosts on the survivorship, development time, and body size in offspring in S. limbatus. We find no evidence that beetles are acclimated to perform better on the host their mother was reared on via non—genetic maternal effects. However, maternal rearing host did affect offspring in other ways; mothers reared on C. floridum produced larger offspring that developed faster than offspring of mothers reared on A. greggii, due to a non—genetic maternal effect (paternal rearing host had no effect). In addition, both maternal and paternal host had a large effect on offspring survivorship; offspring of parents reared on C. floridum survived better than those of parents reared on A. greggii. The symmetry between the maternal and paternal host effects on survivorship suggests that they represent a response to intense selection during the experiment, and suggest the potential for rapid adaptation to this host. Alternatively, non—genetic effects of both maternal and paternal rearing host may simultaneously influence survivorship of S. limbatus offspring. Although paternal effects are generally disregarded as an important source of environmental variation, they are potentially important for S. limbatus; non—genetic paternal effects were detected in this experiment for development time of offspring: fathers reared on C. floridum produced offspring that developed slower on C. floridum, and offspring that developed faster on A. greggii, than fathers reared on A. greggii.
Vector Preference and Disease Dynamics: A Study of Barley Yellow Dwarf Virus
Authors: McElhany Paul , Real Leslie A. , Power Alison G.
Published: May 2006

Using simple analytical models of the probability of disease transmission and a spatially explicit computer simulation of the spread of the aphid—transmitted barley yellow dwarf virus, we examined the effect of vector preference for diseased or healthy hosts on the spread of an economically important plant pathogen. Our analytical models indicate that the effect of vector preference for diseased plants on the probability of disease spread depends on the frequency of diseased plants in the population. In a non—spatial environment with a high frequency of diseased plants, disease spread is favored by vectors preferring healthy plants. With a low frequency of diseased plants, disease spread is favored by vectors preferring diseased plants. The effect of vector preference depends on the amount of persistence exhibited by the disease. For persistently transmitted diseases, the vector remains infective for a long period after visiting a diseased host. Persistence increases the rate of spread for a vector preferring healthy hosts more than it increases the rate of spread for a vector preferring diseased hosts. Using a Markov chain model of disease transmission, we have shown that an increase in the spatial patchiness of the disease can lead to a decrease in the rate of disease spread by a vector capable of moving only limited distances. The effect of spatial disease structure depends on the preference behavior exhibited by the vector. Our spatially explicit computer simulation explored the effect of frequency, persistence, and spatial structure in a dynamic model. All of these factors were shown to be important in describing the impact of vector disease preference on epidemiology. Many of our results contrast with the assumption found in the agricultural literature that a preference for diseased plants leads to an increase in disease spread. These results may have implications for the evolution of pathogen—modified vector behavior and/or host attractiveness. Explicit knowledge of the interaction between spatial dynamics and vector preference will improve our ability to model epidemics and predict the spread of infectious diseases.
Experimental Analysis of Intermediate Disturbance and Initial Floristic Composition: Decoupling Cause and Effect
Authors: Collins Scott L. , Glenn Susan M. , Gibson David J.
Published: May 2006

The intermediate disturbance hypothesis predicts that richness will be highest in communities with moderate levels of disturbance and at intermediate time spans following disturbance. This model was proposed as a nonequilibrium explanation of species richness in tropical forests and coral reefs. A second model of succession, initial floristic composition, states that nearly all species, including late seral species, are present at the start of succession. This leads to the prediction that richness should be highest immediately following disturbance. We tested these predictions using plant species composition data from two long—term field experiments in North American tallgrass prairie vegetation. In contrast to one prediction of the intermediate disturbance hypothesis, there was a significant monotonic decline in species richness with increasing disturbance frequency, with no evidence of an optimum, in both field experiments. Species composition on an annually burned site was a subset of that of infrequently burned sites. The average number of species per quadrat and the number of grass, forb, and annual species were lowest on annually burned sites compared to unburned sites and sites burned once every 4 yr. The second prediction of the intermediate disturbance hypothesis, however, was supported. Richness reached a maximum at an intermediate time interval since the last disturbance. This contradicts the prediction from the initial floristic composition model of succession. These results also suggest that the two predictions of the intermediate disturbance hypothesis are independent and unrelated. We propose that this may be explained by uncoupling the effects of disturbance as a single, relatively discrete event from system response to disturbance. From this perspective, disturbance becomes an extinction—causing event in these grasslands, where recovery following disturbance is a balance between immigration and extinction.
An Explanation of the High Strain Diversity of a Self‐Fertilizing Hermaphroditic Fish
Authors: Lin Hui-Chen , Dunson William A.
Published: May 2006

The tropical and subtropical estuarine fish Rivulus marmoratus is known for its synchronous, self—fertilizing hermaphroditism. It matures within 3 mo and can produce eggs throughout the year. These features would allow a particular strain to quickly colonize refuge such as crab holes in the high intertidal zone habitat. However, a high strain diversity has been found in the field. In order to explain this observation, we examined the phenotypic responses of R. marmoratus to variation in biotic and abiotic environmental factors over two consecutive generation and employed a modified Euler—Lotka equation to estimate and compare the net reproductive rate, R, among strains. We included five strains of R. marmoratus and manipulated levels of temperature, salinity, and food for the parental generation and levels of food only for the second generation. Low temperature (19°C) inhibited sexual maturity, precluding any egg production during the entire 6—mo study. For individuals kept at high temperature (26°C), salinity and food had a interactive effect on time to maturity, body size at maturity , and number of eggs produced. Using multivariate analysis of variance, we found a significant two—way interactive effect (foods levels for the parental and the second generations) on the five reproductive traits studied in the second generation. When each trait was analyzed separately, we found significant three—way interactions for time to maturity and total length and maturity. We found that the fitness values for each strain varied in different environmental combinations (3 salinites X 3 incubation salinities for eggs X 2 food levels). However, four of five strains had similar overall fitness. Despite a lack of genetic variation within each strain, R. marmoratus demonstrates considerable phenotypic variation in response to environmental changes. This feature and genetic variation among strains may explain how this species can survive and even prosper in an extremely variable and stressful environment.
Functional Organization of Stream Fish Assemblages in Relation to Hydrological Variability
Authors: Poff N. LeRoy , Allan J. David
Published: May 2006

Stream fish assmemblage data for 34 sites in Wisconsin and Minnesota were obtained from archived sources and were used in conjunction with long—term hydrological data to test the hypothesis that functional organization of fish communities is related to hydrological variability. For each of the 106 species present in the data set, six categories of species traits were derived to describe habitat, trophic, morphologica, and tolerance characteristics. A hierarchical clustering routine was used to identify two functionally similar groups of assemblages defined in terms of species presence/absence. Hydrological factors describing streamflow variability and predictability, as well as frequency and predictability of high flow and low flow extremes, were derived for each of the 34 sites and employed to explain differences among the functionally defined groups. Canonical discriminant analysis revealed that the hydrological data could clearly separate the two ecologically defined groups of assemblages, which were associated with either hydrologically variable streams (high coefficient of variation of daily flows, moderate frequency of spates) or hydrologically stable streams (high predictability of daily flows, stable baseflow conditions). Discriminant functions based on hydrological information classified the 34 fish assemblages into the correct ecological group with 85% accuracy. Assemblages from hydrologically variable sites had generalized feeding strategies, were associated with silt and general substrata, were characterized by slow—velocity species with headwater affinities, and were tolerant to silt. Proportions of species traits present at the 34 sites were regressed against an index of hydrological stability derived from a principal components analysis to test the hypothesis that functional organization of assemblages varied across a gradient of hydrological stability. Results were complementary with the discriminant analysis. Findings were in general agreement with theoretical predictions that variable should support resource generalists while stable habitats should be characterized by a higher proportion of specialist species. Several species of fish were identified as indicative of the variable—stable hydrological gradient among stream sites. A taxonomic analysis showed strong geographic patterns in species composition of the 34 assemblages. However, zoogeographic constraints did not explain the observed relationship between stream hydrology and functional organization of fish assemblages. The strong hydrological—assemblage relations found in the 34 midwestern sites suggest that hydrological factors are significant environmental variables influencing fish assemblage structure, and that hydrological alterations induced by climate change (or other anthropogenic disturbances) could modify stream fish assemblages structure in this region.
Effective of Climate on Development of Two Sphagnum Bogs in South‐Central Wisconsin
Authors: Winkler Marjorie Green
Published: May 2006

Aquatic pollen, charcoal, sediment, and accumulation—rate changes from radiocarbon—dated cores from two sites in south—central Wisconsin were studied to explore the evolution of local wetlands since deglaciation. Sphagnum bog evolution in south—central Wisconsin appears to have been directly related to the regional climatic changes during the past 6000 yr (the middle and the late Holocene). Although the two sites had different origins and different early histories, both were deep lakes until 6300 BP when they became shallow ponds dominated by the aquatic macrophyte Brasenia. These wetland changes were concurrent with a regional upland vegetational change from mesophytic forest to Quercus savanna and were caused by a regional decrease in annual precipitation and by an increase in temperature. At °3000 BP, in response to a cooler, wetter climate, both wetlands became Sphagnum—Ericaceae bogs, and the upland vegetation changed to a closed Quercus forest. The paleoecological evidence suggests that bog development is a function of climate and that many bogs have developed in the northern Midwest during the past 3000 yr as effective moisture has increased.
Hydrochory and Regeneration in A Bald Cypress‐Water Tupelo Swamp Forest
Authors: Schneider Rebecca L. , Sharitz Rebecca R.
Published: May 2006

The importance of hydrochory, or seed dispersal by water, to the regeneration of Taxodium distichum (bald cypress) and Nyssa aquatica (water tupelo) as examined in a forested floodplain of the Savannah River in South Carolina. Seedfall and dispersal by water were quantified for 2 yr using floating seed traps. Water depth, surface velocity, and flow direction were monitored over the same period. Seedfall for bald cypress and water tupelo occurred primarily from early fall throughout the winter, when water levels were rising in the swamp. Extended buoyancy periods for newly released seeds and fruits (bald cypress: 42 ± 37 d; water tupelo: 85 ± 36 d) prolonged dispersal for both species. Flowing water transported experimentally released seeds long distances, but in a uniform direction, and concentrated them nonrandomly against logs, trees, knees, and other emergent substrates. An examination of the soil seed banks in five microsite types supported the results of the seed transport study. Lowest woody seed densities occurred in the open areas, and highest seed densities occurred in sediments adjacent to emergent substrates such as logs. Additionally, elevated water levels of 1—2 m caused by short—term, high—discharge floods scoured seeds of Nyssa sylvatica var. biflora, Quercus spp., Liquidambar styraciflua, Pinus taeda, and other species from adjacent bottomland hardwood communities and transported them into the bald cypress—water tupelo forest. We conclude that elevated water levels, which occur during late fall when seeds are released, influence the distribution of bald cypress and water tupelo seeds and their availability for recruitment. Short—term, deeper floods may also be important for seed transport among bottomland hardwood communities that are spatially separated or differ in species composition.
Plant‐Animal Interactions and Fruit Production in a Neotropical Herb: A Path Analysis
Authors: Schemske Douglas W. , Horvitz Carol C.
Published: May 2006

We used path analysis to estimate the direct and indirect effects of antguards, an ant—tended herbivore, and pollinators, on flower and fruit production in Calathea ovandensis, a neotropical herb. The natural variation in these plant—animal interactions was quantified for individual plants in Veracruz, Mexico in 1984 and 1985. We conclude that variation in plant—animal interactions has significant, and complex, effects on flower and fruit production in this species. Antguards had a positive direct effect on flower number in both years, and because of the positive effect of flower number on fruit number, had a positive indirect effect on mature fruit number. The herbivore had a negative direct effect on flower number and mature fruit number in both years. Thus, direct and indirect effects contribute to the large negative effect of herbivores on mature fruit number. The composition of the pollinator community was very different in the two years, with a marked increase in the number of visits by efficient pollinators in 1985. This change was reflected in between—year differences in the effects of pollinators on number of initiated fruits. In 1984, pollinators had no significant effect on initiated fruit number, while the two pollinators that increased in abundance in 1985 had significant effects on initiated fruit number in that year. These results are consistent with experimental data for 1985, which indicate that supplemental pollination causes a significant increase in the number of fruit initiated. While pollinators may limit the number of fruit initiated in some years, our observational and experimental data suggest that number of mature fruits is primarily limited by resources, not by pollination.
Ecological Studies of Neotropical Ant Gardens
Authors: Davidson Diane W.
Published: May 2006

In a census taken in Peru's Manu National Park, 10 epiphytic angiosperms from seven plant families established principally on arboreal carton—ant nests. These "ant gardens" (AGs) were most often inhabited by parabiotic ants, Camponotus femoratus and Crematogaster cf. limata parabiotica, whose polygynous and polydomous colonies fissioned to form extensive AG aggregations. AGs tended by polydomous but probably monogynous Azteca cf. traili occurred on average in smaller isolates. All three ant species enriched nest gardens with vertebrate feces, but frequencies of occurrence of most AG epiphytes were lower on the less organic carton of Azteca AGs. Interspecific differences in epiphyte abundance and distribution were related to light requirements of plants and to colonizing abilities, as influenced by differences in allocational preferenda and life history. AG aggregations occupied 16—39% of five forest habitat types present and were especially common in frequently flooded habitats and areas of high light intensity. Patchy distribution was explained partly by overrepresentation on resource trees, such as Inga and Calyptranthes (parabiotic ants) and Cordia nodosa (Azteca). Habitat associations did not result from reduction of the terrestrial ant fauna in flooded forests. Other arboreal ants, but not terrestrial ants, were markedly lower in AG aggregations than in areas that lacked AGs, perhaps due to competition from aggressive and dominant AG ants. AGs formed principally by directed dispersal of epiphyte seeds to ant nests, where larvae fed on seed attachments without damaging seeds. AG ants also recognized and retrieved seeds of at least one AG epiphyte from feces of vertebrate fruit dispersers. The preference ranking of epiphyte seeds by Ca. femoratus was not correlated with either obvious differences in quality of seed appendages or long—term resource potential of plants. Seeds of AG epiphytes were rejected by three ants that do not tend AGs but were collected by a fourth such species. Seed attractiveness may depend in part on nonnutritional cues. Preadaptations of plants and ants appear to have been very important to the origin of AGs. Evidence for evolutionary specialization and coadaptation is circumstantial but suggestive.
A Study of the Ecological Bases of Cooperative Breeding in the Harris' Hawk
Authors: Bednarz James C. , Ligon J. David
Published: May 2006

We compared ecological variables of pairs and larger groups of Harris' Hawks (Parabuteo unicinctus) in southeastern New Mexico in an effort to test several ecologically based hypotheses proposed to explain cooperative breeding. Univariate and multivariate analyses of 21 habitat variables revealed no differences (P > .1) between the nest areas of pairs and groups. Habitats not occupied by hawks had significantly (P < .05) fewer and smaller mesquite trees and greater litter cover than occupied areas. Densities of lagomorph fecal pellets (an index of abundance of the primary prey species) was slightly greater (P > .1) in group areas than pair areas in summer, but the pattern was reversed in winter. Pair ranges had significantly (P < .05) fewer woodrat (Neotoma, an alternate prey) nests than group areas in all seasons. Rabbit femurs from prey remains of groups were slightly, but not significantly, longer than those from pair remains. Although general hypotheses cannot be fully evaluated in studies of single species, the data we obtained do not provide clear support for any of several current hypotheses concerning cooperative breeding in the Harris' Hawk. Instead, group living in this hawk appears to be related to dependence on large, elusive prey in a manner analogous to the social system of wolves (Canis lupus) and wild hunting dogs (Lycaon pictus). We conclude that this species' social system is based on (1) extended parental care, where young birds remain with older group members during their first year or years, and (2) cooperation, where group hunting and prey—sharing provide a critical advantage to each individual.
Demographic Theory for an Open Marine Population with Space‐Limited Recruitment
Authors: Roughgarden Jonathan , Iwasa Yoh , Baxter Charles
Published: May 2006

We introduce a demographic model for a local population of sessile marine invertebrates that have a pelagic larval phase. The processes in the model are the settling of larvae onto empty space, and the growth and mortality of the settled organisms. The rate of settlement per unit of unoccupied space is assumed to be determined by factors outside of the local system. The model predicts the number of animals of each age in the local system through time. The model is offered in both discrete and continuous—time versions. The principal result is that the growth of the settled organisms is destabilizing. In the model, there is always a state where recruitment balances mortality. However, growth can interfere with recruitment and can destabilize this steady state, provided also that the settlement rate is sufficiently high. The model suggests that two qualitatively distinct pictures of population structure result, depending on the settlement rate. In the high settlement limit, the intertidal landscape is a mosaic of cohorts, punctuated with occasional gaps of vacant substrate. In the low settlement limit, the intertidal landscape has vacant space and organisms of all ages mixed together and spatial variation in abundance is caused by microgeographic variation in settlement and mortality rates.
Wax Blooms in Tenebrionid Beetles of the Namib Desert: Correlations with Environment
Authors: McClain Elizabeth , Seely Mary K. , Hadley Neil F. , Gray Vivian
Published: May 2006

Twenty—six species of adult Namib tenebrionid beetles belonging to the tribes Adesmiini and Zophosini have an extracuticular wax bloom covering either part or all of their body surface. These species occur in the dunes, dry river bed, and adjacent gravel plains of the central Namib. The percentage of species possessing a wax bloom increases from the coastal fog desert to the inland hot dry desert. The amount of bloom and its distribution over the integument also increase in a similar manner along the climatic gradient. Despite considerable diversity in coloration and patterns of the wax blooms, the chemical composition of the lipid components showed little variation. Hydrocarbons were the predominant lipid class, with the size of constituent molecules ranging from 19 to 40 carbon atoms. In all species tested, n—alkanes were more abundant than branches alkanes. Odd—numbered carbon chains (e.g., C27, C29, C31) comprised the bulk of the hydrocarbon fractions. The possible functional roles of these blooms in water balance, thermoregulation, predator avoidance, and chemical communication are discussed.
Demography of a Townsend Ground Squirrel Population in Southwestern Idaho
Authors: Smith Graham W. , Johnson Donald R.
Published: May 2006

The demography of a Townsend ground squirrel (Spermophilus townsendi) population was investigated during a 7—yr study in southwestern Idaho, where this species serves as a major food source for raptors and other predators. Squirrels were identified as residents, transients, or immigrants based upon the frequency of capture. Immigration peaked in late February and early March, shortly after the termination of the breeding season. Squirrels identified as transient appeared throughout the trapping season (February though June). A majority of the yearling females and nearly all of the adult females produced a single litter annually. Intersexual differences in dispersal and perhaps in mortality accounted for a significantly greater survival of juvenile and adult females. Survival rates of yearling males and females were similar. This ground squirrel has evolved a reproductive strategy adapted to its unpredictable desert environment. Successful reproduction is related to the availability of a fresh growth of grasses and ultimately to the amount of rainfall in the preceding fall and early winter. With suspension of reproduction during the drought in 1977, the study population entered torpor 4—6 wk earlier than usual. Body mass at immergence was not affected. Population density was reduced by more than one—half as a result of the drought. An increase in the pregnancy rate of yearling females was the only process operating during the post—drought period to restore an equilibrium density. There were no significant changes in immigration rate, litter size, or overwinter survival rate in response to density reduction. Both dispersal and predation operated to reduce density. In addition, disease in the form of sylvatic plague (Yersinia pestis) was present within the study population. A plague—caused reduction in density over a wide region would have a significant effect on the reproductive success of a variety of reptilian, avian, and mammalian predators.
Bumble Bee Foraging: Energetic Consequences of Using a Threshold Departure Rule
Authors: Hodges Clayton M.
Published: May 2006

This paper examines the energetic consequences of queen bumble bees (Bombus appositus) using a relatively simple threshold volume rule to allocate time among multiflowered Delphinium nelsonii plants. Threshold volumes that maximize a forager's rate of net energy intake (optimal threshold volumes) were calculated, using an estimate of nectar availability in naturally occurring plants. Optimal threshold volumes are near median standing—crop volumes in flowers, and increase both with standing—crop volumes and with the degree that standing—crop volumes are correlated among flowers within plants. Estimates of threshold volumes used by naturally foraging bumble bees are very similar to optimal threshold volumes. Moreover, bees efficiently realign threshold volumes to near—optimal levels as nectar availability varies seasonally. Occasional violations of the threshold departure rule prevent bees from maximizing rates of net energy intake. Nonetheless, average rates of return adjusted for the violations were found to be higher than rates from any of three random plant—departure rules. The hypothesis that foragers violate the threshold departure rule to reduce variability in the rate of net energy intake was not supported. It is argued that violations of the departure rule result, in part, from foragers having to sample flowers occasionally to update estimates of threshold volumes as standing—crop volumes vary. Sampling is most frequent when the need for accurate estimates of threshold volumes is highest and the energetic cost of obtaining this information is lowest.
Periodicity in Extinction: The Role of Systematics
Authors: Patterson Colin , Smith Andrew B.
Published: May 2006

The case for periodicity in extinction, with a mass extinction every 26 x 106 yr since the mid—Permian, rests on analyses of the fossil record of marine families and genera. We have checked the echinoderm and fish family extinctions which make up °20% of the data, and find that only 25% of our sample is signal (extinctions of monophyletic groups correctly dated to stage). The 75% noise component includes pseudoextinctions of non—monophyletic groups, and spurious data from other sources. The signal/noise ratio is virtually the same in echinoderms and fishes. Conspicuous peaks in our total family data correspond to five of the eight periodic extinction peaks that have been identified, but these five peaks are a feature of the noise component, not of the signal. We have also checked a randomly selected 20% of the generic extinctions in echinoderms and fishes since the Permian. The proportion of our data reflecting plausible extinctions is much lower among genera (31%) than among families (47%), whereas the proportion that samples peaks of fossilization rather than of extinction is much higher among genera (32%) than among families (16%). As with our families, peaks corresponding to periodic extinctions are restricted to the noise component of the generic data, but here the signal component is too small to bear much significance. We question the expectation that studies of global extinction will be more reliable the lower taxonomic level investigated.
Algal Structure Defenses: Form and Calcification in Resistance to Tropical Limpets
Authors: Padilla Dianna K.
Published: May 2006

Growth form and calcification in marine macrophytes have been suggested to act as structural defenses against grazers. Support for this hypothesis has been largely indirect, consisting of negative correlations of plant and herbivore abundance and the observation that some herbivores prefer less calcified food plants. The hypothesized defensive role of calcification and growth form has generally not been directly tested. Experiments were conducted to test this hypothesis directly for an important group of grazers, the docoglossan limpets, and a large variety of tropical algae with varying degrees of calcification and different thallus forms. This was accomplished by mimicking the mode of feeding of the herbivore using its feeding apparatus (the radula) and directly measuring the forces required to remove tissue. The results were opposite to the predictions. Neither calcification nor the thallus forms that had been predicted to be structural defenses resulted in increased amounts of force to remove tissue. For these particular herbivores, and possibly others, these morphological features are not acting as simple structural defenses. Other explanations for observed correlations of herbivore and plant abundance and the food preferences of herbivores must be considered.
Population Dynamics in Loblolly Pine Stands: Changes in Skewness and Size Inequality
Authors: Knox Robert G. , Peet Robert K. , Christensen Norman L.
Published: May 2006

We determined the changes in size inequality and asymmetry that occurred over a 50—yr period in 16 even—aged, naturally seeded populations of Pinus taeda. Initial densities ranged from 618 to 28 861 stems/ha. We used these data to test four predicted trends in size structure. We found that (1) in young stands size inequality was greater at higher plant densities, (2) skewness of tree volumes was significantly greater at higher initial densities, whereas skewness of diameters was not, (3) size inequality increased significantly prior to the onset of self—thinning, then decreased during self—thinning, and (4) skewness did not increase before self—thinning or significantly decrease during self—thinning. Numerous measures of size distribution have been employed to assess competition in plant populations, including various measures of inequality and skewness, as well as parameters of fitted distributions such the Weibull distributions. We found the two—parameter Weibull distribution only moderately effective at capturing the forms of the distributions observed. Correlations showed very close linear relationships among the Gini coefficient, the coefficient of variation, and the shape parameter of the Weibull distribution–both within and across different measures of tree size. All three statistics were correlated with initial density. Skewness coefficient for distributions of diameter, height, and volume were not closely correlated with each other population characteristics. We recommended measures of inequality as more suitable for summarizing biologically important properties of tree populations than measures of skewness or skewness—sensitive parameters of probability distributions.
Fifty Years of Change in the Arboreal Vegetation of Heart's Content, An Old‐Growth Hemlock‐White Pine‐Northern Hardwood Stand
Authors: Whitney Gordon G.
Published: May 2006

The structure and composition of Heart's Content, an old—growth hemlock—white pine—northern hardwood stand in northwestern Pennsylvania, were studied in 1929. The original plots, established by H. J. Lutz, were relocated and remeasured in 1978. Heavy deer browsing over the past 40 yr has dramatically altered the diameter size—class distribution of the stand, creating a gap in the smaller size—classes of some species. The rapidly growing, shade—intolerant species have the largest gap, while the slower growing, more tolerant species are still represented in the smaller size—classes.
Modeling a Disease‐Caused Patch Disturbance: Oak Wilt in the Midwestern United States
Authors: Menges Eric S. , Loucks Orie L.
Published: May 2006

Sparse data and inadequate methodology have limited our ability to characterize disease—caused forest disturbances and their effects on plant communities. We describe the formulation of an empirically derived, stochastic simulation model that predicts tree—by—tree mortality due to oak wilt, an important disease in central United States hardwood forests. Oak wilt has two means of spreading: through root grafts between adjacent trees, and by insect vector over longer distances. The model describes the level of spread from outside the woodlot, the level of within—plot vector spread, and the probabilities of root graft transmission between pairs of trees. Also included are the time between consecutive tree deaths and the spatial characteristics of patch spreading. The model generates stochastic mortality which is highly variable even in runs having identical parameters. As parameters are varied, average mortality is found to be highly sensitive to changes in parameters specifying insect vectors spread in general, and new introductions to a woodlot in particular. This supports the hypothesis that oak wilt dynamics are constrained by the rare occurrences of woodlot introductions. Highly variable damage, as seen both in the field and in model results, follows from this constraint and the positive feedback between the two types of local spread. The model results also explain regional differences in the amount of damage to oaks and average size of disease pockets (foci). Simulations using data from southern Wisconsin stands predict serious damage in both dry and dry—mesic forests, but in view of the advance reproduction, oak wilt disturbances seems likely to shift species in only dry—mesic forests.
Foraging Ecology of the Long‐Nosed Bat, Glossophaga Soricina, With Respect to Resource Availability
Authors: Lemke Thomas O.
Published: May 2006

The relationship between foraging behavior and resource availability for the long—nosed bat (Glossophaga soricina) and five species of plants it fed on in northern Columbia was investigated in a 10—mo study. Glossophaga employed two foraging tactics: (1) territorial defense of concentrated food sources, and (2) trapline foraging along regularly used feeding routes. The tactic used depended on availability of Agave desmettiana nectar and the individual bat's ability to dominate conspecifics. When both hunger and nectar levels were highest, bats concentrated their feeding activity on A. desmettiana. Aggressive behavior was displayed by dominant male and female bats during the first 3—5 h of feeding. Territorial behavior ceased suddenly when nectar levels were depleted by 50% or more at 2.5—5 h after sunset. Later in the evening, bats foraged briefly from one plant to another at several species along regularly used routes. Bats that maintained early—evening territories had increased access to nectar—rich agave flowers and had significantly smaller total foraging areas than nonterritorial bats. The feeding territories of female Glossophaga were shared with their immature offspring. Territorial behavior at agave was attributed in part to defensibility of its flowers. Foraging experiments demonstrated that G. soricina used a feeding strategy that was efficient with respect to choice of food patch and allocation of time. This is the first study that used marked individuals to document feeding territories and trapline foraging for nectar—feeding bats.
Responses of Honey Bees (Apis Mellifera) to Amino Acid Solutions Mimicking Floral Nectars
Authors: Inouye David W. , Waller Gordon D.
Published: May 2006

Amino acids are probably ubiquitous components of floral and extrafloral nectars. Although it seems likely that they serve, as sugars do, to reward nectarivores, little is known about the responses to amino acids by flower visitors. The experiments reported here were conducted to determine the responses of honey bees (Apis mellifera) to single amino acids at relatively high concentrations dissolved in an aqueous solution of 30% sucrose. Bees' responses to 24 L—amino acids, 2 DL mixtures, 2 phenolics, and a flavonoid were tested with artificial—flower feeders. The bees' responses were classified as: (1) consumption that generally declined as amino acid concentration increased (although weak solutions of amino acids were sometimes preferred over a sucrose control), (2) a preference for the strongest concentration offered (phenylalanine), (3) no significant difference between the amino and acid and control. There were no apparent correlations between characteristics of the amino acids and the bees' reactions to them, and no obvious difference between responses to essential and nonessential amino acids.
Variation Among Individual Seed Masses in Lomatium Grayi (Umbelliferae) Under Controlled Conditions: Magnitude and Partitioning of the Variance
Authors: Thompson John N.
Published: May 2006

Although seed masses are generally believed to be fairly constant within plant populations and important in plant demography, there have been no studies that quantified the magnitude and partitioning of variation in individual seed masses within and among total seed crops of uncultivated plants grown under similar conditions. In lomatium grayi (Umbeliferae), mass of seeds with endosperm varied 15.8—fold among 2—yr—old plants that were grown from seedlings under similar conditions. Within individual plants, seed masses varied 2.6—fold to 8.1—fold. Forty—four percent of the variance was among seeds within umbels, 16% was between umbels within plants, and 40% was between plants. Plants varied in mean seeds mass from 3.7 to 8.3 mg, a 2.2—fold variation in means. The distribution of means around the grand mean was not skewed but was significantly leptokurtic. Coefficients of variation among the means averaged 26% (range: 16—48). These results together with the few other studies from natural populations in which large numbers of seeds have been weighed individually show much more variability in individual seed masses than is indicated by the common ecological impression (based mostly on comparison of means) that seed masses tend to be fairly constant.
The Problem of Pattern and Scale in Ecology: The Robert H. MacArthur Award Lecture
Authors: Levin Simon A.
Published: May 2006

It is argued that the problem of pattern and scale is the central problem in ecology, unifying population biology and ecosystems science, and marrying basic and applied ecology. Applied challenges, such as the prediction of the ecological causes and consequences of global climate change, require the interfacing of phenomena that occur on very different scales of space, time, and ecological organization. Furthermore, there is no single natural scale at which ecological phenomena should be studied; systems generally show characteristic variability on a range of spatial, temporal, and organizational scales. The observer imposes a perceptual bias, a filter through which the system is viewed. This has fundamental evolutionary significance, since every organism is an "observer" of the environment, and life history adaptations such as dispersal and dormancy alter the perceptual scales of the species, and the observed variability. It likewise has fundamental significance for our own study of ecological systems, since the patterns that are unique to any range of scales will have unique causes and biological consequences. The key to prediction and understanding lies in the elucidation of mechanisms underlying observed patterns. Typically, these mechanisms operate at different scales than those on which the patterns are observed; in some cases, the patterns must be understood as emerging form the collective behaviors of large ensembles of smaller scale units. In other cases, the pattern is imposed by larger scale constraints. Examination of such phenomena requires the study of how pattern and variability change with the scale of description, and the development of laws for simplification, aggregation, and scaling. Examples are given from the marine and terrestrial literatures.
Biogeochemistry of Adjacent Conifer and Alder‐Conifer Stands
Authors: Binkley Dan , Sollins Phillip , Bell Randy , Sachs Don , Myrold David
Published: May 2006

To determine the long—term effect of alder on soil fertility, biogeochemical fluxes were measured and calculated for two pairs of adjacent, 55—yr—old stands dominated by conifers, primarily Douglas—fir (Pseudotsuga menziesii), and by conifers and nitrogen—fixing red alder (Alnus rubra). At a low—fertility site in the Wind River Experimental Forest in southwestern Washington, biomass of the alder—conifer stand (289 Mg/ha) exceeded that of the conifer stand (171 Mg/ha), and the aboveground net primary production (ANPP) of the alder—conifer stand (10.3 Mg.ha—1.yr—1) was more than twice that of the conifer stand (4.8 Mg.ha—1.yr—1). At a more fertile site in the Cascade Head Experimental Forest in western Oregon, both biomass and ANPP were higher than at Wind River, and biomass and ANPP were higher in the conifer stand (584 Mg/ha and 19.2 Mg.ha—1.yr—1) than in the alder—conifer stand (342 Mg/ha and 10.7 Mg.ha—1.yr—1). Nitrogen accretion in the alder—conifer stand at Wind River averaged 54 kg.ha—1.yr—1 for the 52 yr since stand establishment, with a current rate of N fixation of °75 kg.ha—1.yr—1. For the alder—conifer stand at Cascade Head, N accretion averaged 73 kg.ha—1.yr—1 for 55 yr, with a current N—fixation rate of 85 kg.ha—1.yr—1. The cycling of all nutrient appeared very malleable under the influence of alder. At Wind River, return of nutrients in fine litterfall in the alder—conifer stand ranged from 1.5 (P) to 7.9 (N) times those in the conifer stand; whereas at Cascade Head, these ratios ranged from 1.7 (S) to 4.2 (N). Nutrient—use efficiencies (kilograms of ANPP per kilogram of nutrient uptake) were generally lower for the alder—conifer stands at both sites. Denitrification appeared negligible (<0.3 kg.ha—1.yr—1) in all stands. Leaching of organic plus inorganic N ranged from °5 kg.ha—1.yr—1 for the conifer stand at Wind River, to 50 Kg.ha—1.yr—1 for the alder—conifer stand at Cascade Head.