bluuebunny/crossref_metadata_2025 huggingface dataset:
The Ecology of Tick Parasitism on Densely Nesting Peruvian Seabirds
Authors: Duffy David Cameron
Published: May 2006

Densely nesting Guanay Cormorants (Phalacrocorax bougainvillii), Peruvian Booby (Sula variegata), and Peruvian Brown Pelican (Pelecanus occidentalis thagus) deserted eggs and young in response to high densities of argasid ticks (Ornithodoros amblus). Alternative causes of desertion such as human disturbance, predation, disease, heat, and food shortage did not fit the evidence. Tick predators were less effective at controlling ticks than were frequent human harvestings of guano. A review of the avian and medical entomological literature indicates that ticks and other ectoparasites may be strong determinants of nesting success and of nest structure and duration of use.
Water Regulatory Efficiency in Heteromyid Rodents: A Model and Its Application
Authors: MacMillen Richard E. , Hinds David S.
Published: May 2006

We have developed and tested a model of water regulatory efficiency for the rodent family Heteromyidae. The model is based on the premise that granivory coupled with increasing aridity and seasonality of rainfall has been the major theme throughout the evolutionary history of the family; it states further that water regulatory efficiency is negatively correlated with body mass in the family, and that member species are largely dependent upon a common resource packet (seeds) to meet both energy and water needs. A test of the model with five genera and 13 species revealed positive results across the entire family, but the two most speciose genera, Perognathus and Dipodomys, showed seemingly conflicting patterns; the smaller (in mass) genus, Perognathus, conformed strongly to the model, while in Dipodomys water regulatory efficiency was fixed at a level equivalent to that of the largest Perognathus spp. (°35—40 g). At this mass in these genera, in addition, occurs the demarcation between strict quadrupedality (Perognathus) and bipedality (Dipodomys), a strict scaling of cheek pouch volume with mass in Perognathus but independence of mass in Dipodomys, and the common use of torpor in Perognathus, with its near absence in Dipodomys. We interpret our findings to indicate that initially selection favored a decrease in mass with a concomitant increase in water regulatory efficiency and reduction in absolute energy need in heteromyids, tracking progressive aridity during the Tertiary. In Dipodomys, the option of bipedality was adopted, apparently freeing them from energetic constraints imposed strictly by mass, coupled with an intermediate and fixed level of water regulatory efficiency that dictates use of seeds with high metabolic water yields. The quadrupedal Perognathus have retained a mass—specific water regulatory efficiency, ensuring maintenance of both water and energy balance on a broad array (with respect to protein: lipid: carbohydrate and metabolic water yield) of seeds, when their more—limited locomotor powers are consistent with seed availability; torpor is the tradeoff, enhancing survival during energetically demanding periods.
Spatial Population Dynamics of a Marine Organism with a Complex Life Cycle
Authors: Possingham Hugh P. , Roughgarden Jonathan
Published: May 2006

This paper presents a modeling framework within which mesoscale features in ocean currents can be integrated with coastal habitat structure to predict the distribution and abundance of a marine organism with a coastal adult phase and a pelagic larval phase. The processes in the model that influence larvae away from shore are advection, diffusion, and mortality; the processes influencing larvae adjacent to the coast are settlement and reproduction by adults. The adults on the coast are influenced by recruitment, mortality, and the availability of suitable habitat. Larvae are passive particles in the ocean whose movement is modeled with a transport equation. The water column is represented by a two—dimensional plane and the coast by a straight line. Age—integrated biological parameters for the barnacle Balanus glandula are used in numerical examples. A one—dimensional model illustrates the interaction of eddy—diffusion and larval mortality in determining larval wastage that, in turn, affects whether a species can persist. In a two—dimensional model (solved using finite difference techniques) the persistence of a population also depends on the length of suitable habitat, the strength and structure of the along—shore flow field, and biological parameters of the species concerned. Initial conditions may have a large influence on the growth and spread of the population, especially in the presence of along—shore flow.
Interspecific Competition between Root‐Feeding and Leaf‐Galling Aphids Mediated by Host‐Plant Resistance
Authors: Moran Nancy A. , Whitham Thomas G.
Published: May 2006

While the competitive interactions of herbivores are often debated, rarely have the resistance traits of their host plants been considered as an important factor that might determine the outcome of an interaction. In the study reported here, host—mediated interactions were explored for two aphid species that feed on plants of the genus Chenopodium. Pemphigus betae feeds underground on roots, while Hayhurstia atriplicis feeds aboveground where it forms leaf galls. Although they never encountered one another directly, these two aphid species shared a common resource, the phloem sap of their host. We examined the effects of each of these herbivore species on their host plants and on one another through several garden and growth chamber experiments. Four major results emerged. First, the root—feeding aphid P. betae had no significant effects on its hosts. In contrast, leaf galling by H. atriplicis reduced overall host mass by an average of 54% and seed set by an average of 60%. Second, competitive interactions between the herbivore species depended on the level of host resistance to leaf galling by H. atriplicis. On susceptible plants, leaf—galling colonies of H. atriplicis reduced P. betae numbers by an average of 91%, often eliminating the root feeders entirely. In contrast, on plants resistant to galling, H. atriplicis colonies were smaller and did not affect P. betae infesting roots of the same hosts. Third, the interaction was asymmetrical; although H. atriplicis had a strong negative effect on P. betae, the latter showed no measurable effects on H. atriplicis. Censuses indicated that the strong negative effects of H. atriplicis on P. betae, as found in the growth chamber and garden experiments, also take place naturally in the field. Fourth, although one might expect plants resistant to one aphid species to be resistant to other aphids also, resistance to leaf—galling aphids is not correlated with resistance to root—feeding aphids in this system.
Ecophysiology of Secondary Dormancy in Seeds of Ambrosia Artemisiifolia
Authors: Baskin Jerry M. , Baskin Carol C.
Published: May 2006

As buried seeds of Ambrosia artemisiifolia came out of primary and secondary dormancy, they showed similar germination responses. In early spring at simulated habitat temperatures, seeds germinated well in light but not in darkness, but they germinated well in darkness at temperatures characteristic of the habitat in late spring and summer. By late spring, seeds had entered secondary dormancy and did not germinate at any temperature in light (14—h daily photoperiod) or darkness. Prolonged exposure to 5°C in darkness did not decrease germination in either light or darkness at a 12—h alternating temperature regime of 30°/15°C, but seeds subsequently kept at 15°/6°C in darkness for 1 mo required light for germination. Seeds then lost the ability to germinate in light as they were exposed sequentially to 20°/10°, 25°C/15°C, and 30°C/15°C in darkness. Thus, upon exposure to the sequence of natural temperature regimes in spring, induction of secondary dormancy of seeds in darkness occurs in two physiological stages. While buried seeds are at low marginal temperatures for germination they lose the ability to germinate in darkness, and then at higher temperatures they lose the ability to germinate in light.
Reproduction in Polemonium: Competition for Pollinators
Authors: Zimmerman Michael
Published: May 2006

The flowering season of Polemonium foliosissimum Gray (Polemoniaceae) can be divided into two distinct periods. In the first, the bee:flower ratio is relatively high and large numbers of seeds per ovary are set. In addition, no correlation exists between the ollinator visitation rate to flowers and seed—set. In the second period the bee:flower ratio and seed—set are lower and a very strong correlation exists between pollinator service and seed—set. Pollinators are limiting resources during this portion of the season and those individual plants which can best attract pollinators have a strong selective advantage at this time. The evidence suggests that intraspecific exploitation competition for pollinator service might be occurring during this portion of the flowering season.
Fitting Predator‐Prey Models to Time Series with Observation Errors
Authors: Carpenter S. R. , Cottingham K. L. , Stow C. A.
Published: May 2006

Fits of nonlinear difference equations to time series with observation errors were examined by stochastic simulation and analysis of plankton time series from two lakes. Even modest observation errors (e.g., coefficient of variation among replicate samples °10%) cause errors in model identification and bias in parameter estimates. The latter problem can be corrected by estimation techniques that account for observation error, but model identification is difficult unless the state variables are manipulated. Without manipulation, statistical criteria tend to favor linear models, even when data are simulated by nonlinear processes. Methods that account for observation error produced satisfactory fits to time series of edible algae from two lakes over 7 yr. In Paul Lake, which has not been manipulated, the best fitting model included linear growth, a linear functional response for grazing loss, and an autoregressive moving average model for the errors. In manipulated Tuesday Lake, the best fitting model included linear growth and a nonlinear functional response. Experimental manipulations, or other substantial perturbations, may be essential for detection of nonlinearities in ecological interactions.
Plant Compensatory Responses: Bud Dormancy as an Adaptation to Herbivory
Authors: Tuomi Juha , Nilsson Patric , Astrom Marten
Published: May 2006

Some plants can compensate, and even overcompensate, for the loss of productivity caused by herbivory. The presence of latent meristems, or dormant buds, is one of the basic prerequisites of such compensation mechanisms. We present a mathematical model in order to analyze compensation responses in relation to the intensity of herbivory. The model generates a number of qualitatively different kinds of compensation curves when seed production is plotted against the proportion of active meristems lost per grazed plant. The shape of the curves depends on the proportion of dormant buds and their activation sensitivity in relation to meristem loss. Overcompensation is most probable when dormant buds are easily activated. When plants are grazed only once, as assumed in our model, selection favors high bud sensitivity. However, we expect that repeated damage may select for a more restrained pattern of bud activation. When relatively few buds remain dormant, plants can overcompensate for low levels of damage only. On the other hand, when most buds remain dormant, they can overcompensate even for high levels of damage. We consider compensation capacity a potential benefit of bud dormancy when plants are subject to damage. However, bud dormancy may also imply costs on plant productivity and fecundity in the absence of herbivory. Still, intense herbivory may favor bud dormancy in spite of the potential costs. Selection for bud dormancy requires both that the risk of herbivory is high and that herbivores remove a large fraction of active meristems per plant. Consequently, overcompensation is a theoretically plausible possibility, and intense herbivory is a potential selective force that favors bud dormancy. None of these results, however, imply that herbivory is beneficial to plants. In our case, plants with bud dormancy never have higher seed production than plants that have no dormant buds and that are not grazed.
Soil Chemical Change during Three Decades in an Old‐Field Loblolly Pine (Pinus Taeda L.) Ecosystem
Authors: Richter D. D. , Markewitz D. , Wells C. G. , Allen H. L. , April R. , Heine P. R. , Urrego B.
Published: May 2006

The ability of soil to sustain its supply of nutrients to a growing forest is controlled by a complex of biogeochemical processes. Forest soil data are notably absent, however, that describe sustained nutrient supply of nutrient depletion. The objective of this study was to evaluate how exchangeable nutrient cations of a previously cultivated Ultisol responded to the first three decades of pine forest development. On six occasions during the three decades, the upper 0.6 m of soil was sampled from eight permanent plots and chemically analyzed with the same procedures. During this period, KCl—exchangeable acidity (as positive charges of adsorbed H and Al ions) increased by 37.3 kmolc/ha in the upper 0.6 m of soil and positive charges of exchangeable Ca and Mg were depleted by 34.8 and 8.9 kmolc/ha (by 696 and 108 kg/ha), whereas, exchangeable K was reduced by only 0.5 kmolc/ha (19 kg/ha). Depletion of soil exchangeable Ca was on the same order of magnitude as Ca removals (i.e., Ca accumulation in biomass and forest floor plus that lost in soil leaching). Removals of soil Mg also appeared to outpace resupply from recycling, atmospheric deposition, and mineral weathering, but not the same degree as Ca. Over the three decades, soil leaching loss of these divalent cations (from 0.6 m depth) appeared equal to cation accumulation in biomass plus forest floor, with sulfate balancing about half these cations in leachates. In contrast to Ca and Mg, total K removals from the soil exceeded reductions in soil exchangeable K by nearly 20—fold. Exchangeable K was well buffered in surface mineral soils apparently due to a combination of biological recycling via leaching of canopies and forest floor plus mineral weathering release. These nutrient dynamics may be common to many nutrient—demanding forest ecosystems supported by soils with low activity kandic or oxic horizons. Such soils (Ultisols and Oxisols) occur on many hundreds of millions of hectares in temperate and tropical zones.
Biomass Allocation of Montane and Desert Ponderosa Pine: An Analog for Response to Climate Change
Authors: Callaway Ragan M. , DeLucia Evan H. , Schlesinger William H.
Published: May 2006

We measured aboveground biomass allocation of Pinus ponderosa on hydrothermally altered andesite in montane and desert climates. Thus substrates was held constant while climate varied. Trees from montane climates had higher leaf mass per unit cross—sectional area of sapwood (functional conducting xylem) than trees from desert climates, suggesting that a functional response to differences in climate had occurred. Our results also indicate that sapwood mass; leaf mass ratios of P. ponderosa may increase 2°50% with a 5°C change in mean growing—season temperature, approximately the difference between our montane and desert sites. High proportional allocation of biomass to sapwood may improve water relations of P. ponderosa, but because sapwood contains living parenchyma, respiratory costs may be high. Site—specific regression equations were used to calculate aboveground biomass allocation for simulated montane and desert trees with the same diameter at breast height, dbh. Simulated montane trees were 46—52% taller than desert trees, and montane trees 10 cm in dbh had twice the total aboveground mass of desert counterparts. Simulated 50—cm montane and desert trees were almost identical in total mass, even though the montane tree was 46% taller. The predicted proportion of biomass allocated to bole sapwood increased with size for both montane and desert models; however, the 50—cm desert model contained 8% more total sapwood mass than the taller montane model. Total biomass of branches was similar for paired models of all size classes; however, biomass of primary and secondary branches differed considerably. The 50—cm desert model had twice as much biomass in primary branches in comparison to the montane model, whereas the montane model had 3 times more biomass in secondary branches than the desert model. For 10—cm trees of the desert and montane models 29 and 33% of the biomass were leaves, respectively. In larger trees, leaf allocation decreased to 5 and 7% for desert and montane models, respectively. The effects of climate on biomass allocation, such as reported here, and corresponding changes in whole—plant assimilation rates must be incorporated into growth—response models used to predict future fluctuations in forest productivity due to global climate change.
Activity‐Specific Metabolic Rates of Free‐Living Northern Gannets and Other Seabirds
Authors: Birt-Friesen V. L. , Montevecchi W. A. , Cairns D. K. , Macko S. A.
Published: May 2006

Field and activity—specific metabolic rates of 20 free—living Northern Gannets (Sula bassanus; mean mass = 3.21 kg) rearing chicks at Funk Island, Newfoundland, were measured using doubly labeled water and activity timers. Field metabolism (FMR) averaged 4865 kJ/d or 6.6 x basal metabolism (BMR). Regression analyses indicated a metabolic rate of 144 kJ/h while at the nest or on water, 349 kJ/h during flight, and 250 kJ/h during diurnal time at sea. These metabolic rates are high, probably because of costs of thermoregulation and flapping flight. BMR was slightly lower than predicted. Statistical analyses of metabolic rates of free—living seabirds indicated that rates are elevated in seabirds in cold—water regions and in seabirds that use flapping flight, but do not vary with phylogenetic order. FMR was correlated strongly with both BMR and metabolism at the nest. Population energy models may gain accuracy if metabolic rates are either expressed as multiples of metabolism at the nest or calculated from allometric equations appropriate for the species' activity pattern, oceanographic regime, and foraging mode.
Interactions of Factors Affecting Seedling Recruitment of Blue Oak (Quercus Douglasii) in California
Authors: Borchert Mark I. , Davis Frank W. , Michaelsen Joel , Oyler Lyn Dee
Published: May 2006

Acorn germination and seedling recruitment of blue oak (Quercus douglasii) were studied in relation to postdispersal predators, planting depth, oak canopy cover, slope angle and aspect, and herb layer. Removal of acorns and seedling recruitment from surface—sown and buried acorns were measured for 2 yr at two sites, a north—slope forest and a ridgetop savanna. Nested exclosures were used to measure acorn predation by birds, mice, gophers, deer, and cattle. The rate of seedling recruitment (Ps) was related to treatment variables using cluster analysis combined with stepwise logistic regression. Average acorn fall ranged from 3.5 to 58.7 acorns/m2, with germination rates varying from 28 to 85% in different years. 8172 sown acorns yielded 2922 (35.76%) seedlings, but Ps varied from 0.09 to 0.71 among the different treatments. Year, site, and rodents interacted strongly to affect seedling recruitment. Ps averaged 0.48 during a cool wet year compared to 0.23 in drier years. Low correlations between seedling recruitment rates for the same plot in different years were due to annual changes in the distribution of favorable microsites and patchy, unpredictable acorn predation. Recruitment rates for buried acorns were twice those of surface—sown acorns due to improved germination and reduced predation. Pocket gophers (Thomomys bottae) were the major predator of buried acorns. Surface acorns suffered high mortality from dying and overheating, as well as from predation by mice, gophers, and cattle. Seedling recruitment was positively associated with increasing canopy cover and more mesic microsites at the low elevation site, but was negatively associated with these factors at the cooler, high elevation site. Hierarchichal classification combined with stratum—specific logistic regression models was important in revealing strong interactions among seedling recruitment factors.
Effects of Food Availability and Predation on Prairie Vole Demography: A Field Experiment
Authors: Desy Elizabeth A. , Batzli George O.
Published: May 2006

Nutrition, spacing behavior, and predation have been proposed as major factors limiting the population density of microtine rodents (lemmings and voles). However, there is no general agreement as to the relative importance of these factors, nor have their interactions been carefully examined. This paper reports on a factorial experiment designed to test the general hypothesis that predation and the availability of high—quality food act simultaneously and additively to limit densities of microtine rodent populations. Our results showed that supplemental high—quality food significantly increased body growth rates, proportion of adults in the population, adult male body size, reproductive activity, recruitment, and density of prairie vole populations. Food availability had no effect on survival nor on sex ratio in any of the 3 yr of this study. Protection from predation significantly increased adult survival (somewhat sporadically), recruitment, and density, but it did not directly affect growth, population structure, or reproduction. Improved survival of young animals could not be detected, but increased recruitment suggested that it may have occurred, perhaps before the young reached trappable size. Food supplementation and protection from predation generally had additive and equal effects on density. The only significant food—predator interactions were for body growth rates and adult body size. The presence of predators appeared to inhibit the growth response to supplemental food, possibly because of more restricted movement and, therefore, less access to food.
Adaptive Significance of Spination in Estuarine Crab Zoeae
Authors: Morgan Steven G.
Published: May 2006

Spines of zoeae of the mud crab Rhithropanopeus harrisii apparently have been selected primarily through predation by small fishes rather than by predatory invertebrates, and do not appear to have evolved either to stabilize the organism or to retard its sinking. The settling velocity and swimming and sinking behavior of spined zoeae vs. those with their spines removed were similar. Sinking slowed when the antennal spines were flared perpendicularly to their resting position, but zoeae do so only when molested. Only 1 of 10 planktonic invertebrates preyed on more despined than spined zoeae, but spines were effective against two fishes that predominate in upper estuaries of the east coast of the United States where larvae of R. harrisii develop. Spines limited the size at which planktivorous silversides, Menidia menidia, and opportunistic killfish, Fundulus heteroclitus, began preying on zoeae, but the effectiveness of spines in preventing predation appeared to diminish with each successive zoeal instar. Gape—limited fishes apparently exert strong selective pressure for crab zoeae to produce spines rather than a large unarmed body. Not only do spines increase the size of the zoeae, but spines are noxious, enhance survival of attacked zoeae, regenerate quickly, and are effective when partially regenerated. Spined first—instar zoeae were preyed upon by silversides less often than despined second—instar zoeae, in much the same numbers as despined third—instar zoeae, and more often than despined fourth—instar zoeae. The sharp multiple spines of zoeae prevented fishes from manipulating their prey for easy ingestion, so that zoeae often lodged in the pharynxes of small fishes and sometimes killed them. Rhithropanopeus harrisii zoeae that were attacked by fishes had the same survival and time to metamorphosis as zoeae that were not attacked, whereas the shorter spines and larger bodies of zoeae of the marsh crab Sesarma reticulatum were less effective at preventing fatalities. Dorsal spines were broken most often during attacks. The partially regenerated spines of third—, but not fourth—, instar zoeae were as effective as unbroken spines in preventing predation by silversides. The effectiveness of spines as an antipredatory adaptation is enhanced, because fishes apparently detected spines and quickly learned to prefer despined to spined zoeae. Zoeae did not attempt to evade or escape attacks by fishes, but instead relied on armor and postcontact antipredatory behaviors for their survival. The differential effectiveness of spines of marine and freshwater zooplankters at deterring predation by fishes and invertebrates is discussed.
Survival of Juvenile Giant Kelp: The Effects of Demographic Factors, Competitors, and Grazers
Authors: Dean Thomas A. , Thies Karl , Lagos Steven L.
Published: May 2006

Patterns of survival of juvenile giant kelp (Macrocystis pyrifera) were examined in relation to grazing by sea urchins, shading by adult kelp, density of recruits, densities of other algal species, and substrate distribution. Survival was poorer in areas where white sea urchins (Lytechinus anamesus) were abundant or where there was an overlying canopy of adults. At other sites, the density of recruits explained the greatest proportion of variability in survival, and the proportion of juveniles that survived was negatively correlated with the number of recruits. Potential algal competitors (Pterygophora californica and Cystoseira osmundacea), red sea urchins (Strongylocentrotus franciscanus), and substrate distributions had no significant effect on survival. The mechanisms of density—dependent survival were similar to "dominance and suppression" models that explain self—thinning in many terrestrial plant populations. In stands where densities of recruits were high, skewed size distributions developed, probably as the result of competition for light. High—density stands had proportionally more small plants than low—density stands, and, during subsequent storms, only the larger plants survived. This led to a more equitable distribution of survivors than would be expected based on the number of recruits. Intraspecific interactions, both between adults and juveniles and among juveniles of the same cohort, appear to be important structuring forces in most giant kelp populations. The parallels in many terrestrial populations. However, in other algal populations, there is little evidence for density—dependent mortality or for the dominance—suppression hypothesis. These differences may relate to competition for light, which is intense in Macrocystis forests and many terrestrial populations, but less so in assemblages of other benthic algae.
Dating Tropical Trees by Means of ^14C From Bomb Tests
Authors: Worbes Martin , Junk Wolfgang J.
Published: May 2006

Stem discs of trees from several tropical stands were macro— and microscopically investigated. The radiocarbon level of individual increment zones was determined. Time series show concurrence of the 14C level of air and wood during the 14C peak from bomb tests and are compared with series from temperate zones. An annual growth rhythm of trees from Amazonian inundation forests and from the Gran Sabana in Venezuela was demonstrated. The increase of 14C levels by internal carbohydrate transfer from sapwood to heartwood is shown for Quercus costaricensis.
Effects of Egg Size on Success of Larval Salamanders in Complex Aquatic Environments
Authors: Semlitsch Raymond D. , Gibbons J. Whitfield
Published: May 2006

Effects of egg size on growth, survival, and metamorphosis of larval salamanders (Ambystoma talpoideum) were examined in varying environments. Pond drying regime and presence vs. absence and an interspecific competitor were manipulated in factorial experimental using artificial ponds to measure the responses of larvae. Females that were 4 yr old produced larger eggs and hatchlings than 1—yr—old females. Differences in body size at hatching persisted through day 49 of the experiment but disappeared by day 129. Drying regime also affected body size at day 49 but not at day 129. Larvae from large eggs, and larvae in constant water level ponds, had higher survival to day 129 than larvae from small eggs, and in drying ponds. There was also a significant interaction between egg size and drying regime. Larvae from large eggs survived better than larvae from small eggs in the constant water level ponds, but not in drying ponds. Interspecific competitors did not affect growth or survival to day 129. More individuals metamorphosed from drying ponds than from constant water level ponds. The growth advantages conferred by larger body size at hatching are transient and may disappear during compensatory growth later in the larval period. Body size advantages early in the larval period, however, probably account for increased survival through size—specific mechanisms at a time when newly hatched larvae are most vulnerable.
Altitudinal Variation in the Life History of the Golden‐Mantled Ground Squirrel (Spermophilus Lateralis)
Authors: Bronson Michael T.
Published: May 2006

The life history of golden—mantled ground squirrels (Spermophilus lateralis) varied across a climatic gradient. I live— and snap—trapped at 4 and 5 elevations, respectively, in the northeastern Sierra Nevada of California for 3 yr. Higher elevation populations were characterized by (1) shorter aboveground active seasons of adults and juveniles, (2) later age at first reproduction in ♀ ♀, (3) greater ♀ survivorship, and (4) lower ovulation rates yielding smaller uterine litters. Analysis of covariance of litter size (N = 284) showed that litter size is partly explained, in decreasing order of importance, be elevation, body weight, and age of the mother. Neither loss of ova nor embryonic mortality contributed to elevational differences in live embryos per litter. Embryonic mortality decreased with altitude. Survivorship of young to 1 yr of age was low (15%—18%) and did not vary with altitude. The sex ratio was even among young—of—the—year (N = 351), but favored ♀ ♀ at 1 yr of age and older (N = 438). Based on tooth wear ages and uterine litter sizes, I constructed fecundity tables for 4 populations. The demographic variation may have arisen from several sources: (1) selection were early winters may have caused variable juvenile mortality (bet—hedging strategy), (2) selection where long winters may have enhanced adult survivorship and diminished the reproductive capability of younger ♀ ♀, or (3) phenotypic plasticity.
Faulty Eclosion in Crowded Suburban Periodical Cicadas: Populations Out of Control
Authors: White JoAnn , Lloyd Monte , Zar Jerrold H.
Published: May 2006

A complete count of exuviae from a yard of °0.1 ha revealed 158,054 emerging periodical cicadas (Magicicada cassini) of which 31% died from faulty eclosion. Mutual interference and/or delay owing to a shortage of vertical perching places appears to be responsible, rather than poor nutrition. We argue that cicadas feeding underground are protected from predation and that they must have a long developmental period because they feed on dilute xylem fluids. The long synchronous life cycle allows Magicicada to escape predatory control by any but a synchronized enemy, and only one of these exists–a fungus disease, Massospora cicadina. In man—disturbed habitats, even this fungus is absent initially, so the cicada population increases until limited by the only remaining factor possible–space for feeding sites. Nearest—neighbor analysis indicates mutual repulsion, i.e., feeding cells were disturbed significantly more uniformly than random expectation. Emergence burrows from previous generations had filled with soil, but these "fossil burrows" were surrounded by soil crevices and not invaded by plant roots; hence, they decrease the amount of space available by for feeding sites. Apparently, periodical cicada nymphs in certain parts of some suburban lawns do indeed occupy all the feeding space available to them, yet the herbivory can be sustained by the trees.
Home Range and Body Weight‐‐A Reevaluation
Authors: Harestad A. S. , Bunnel F. L.
Published: May 2006

Area of home range (H) can be related empirically to body weight (W) by the formulation H = aWk. The computed values of exponent k have generated controversy concerning potential differences between trophic groups and whether they differ from 0.75 (the value expected if area of home range is a function of basal metabolic rate). When large mammals are considered, the empirical relationship assumes the form H = .002W1.02 for herbivores, H = .059W.92 for omnivores, and H = .11W1.36 for carnivores. By treating the animal's energetic requirements and the productivity of its habitat explicitly, empirical values of k > 0.75 are shown to result from declining rates of production of utilizable energy per unit area of habitat with increasing body weight. While trophic status and weight modify the utilizable proportion of energy in the habitat, broad correlations also exist between size of home range and surrogate variables for productivity (precipitation and latitude). Differences in weight alone account for a large portion of the differences between male and female or subadult and adult home ranges. Behavioral phenomena need not be invoked. Differences between herbivores and carnivores are in the direction suggested for birds and mammals. Criticisms regarding inter—class and inter—tropic comparisons appear resolved.
Vegetation Change and Plant Demography in Permanent Plots in the Sonoran Desert
Authors: Goldberg Deborah E. , Turner Raymond M.
Published: May 2006

We report on vegetation changes and population dynamics during a 72—yr period within permanent plots in Sonoran Desert vegetation. A set of plots established in 1906 and supplemented in 1928 at Tumamoc Hill, near Tucson, Arizona, USA, have been mapped at irregular intervals through 1978. Data from the four 100—m2 and one 800—m2 plots censused most frequently (6—8 times since 1906 and 1928) are presented in detail; data from five less frequently censused plots are noted briefly. At each census all woody and succulent plants, including seedlings, were mapped by recording both stem coordinates and canopy outline. There have been no consistent, directional changes in vegetation composition in the Tumamoc Hill plots between 1906 and 1978, despite large fluctuations in absolute cover and density of most species. The relative cover of the dominants was generally similar within a given plot over the entire time sequence. Coverage of most species responded strongly to regimes of extremely wet or extremely dry years; the response of density to climatic extremes was somewhat less strong. Total cover, density,and species diversity have increased more or less continuously in many plots between 1906 and 1978. The species were divided into five groups based on several population dynamic traits. At one end of continuum are species with long maximum lifespan (often greater than the 72—yr study period), high early and long—term (50—yr) survivorship, large size at maturity, low density, erratic recruitment, and age structure skewed toward older plants. At the other end of the continuum are species with short maximum life—span, low early and long—term survivorship, small size at maturity, high density, some recruitment at most censuses, and age structures strongly skewed toward newest recruits. While some of these traits are necessarily related (e.g., long maximum life—span and high long—term survivorship), others are not (e.g., high early survivorship and erratic recruitment), and the relationships deserve further study.
Interindividual Variation in Prey Selection by the Snail Nucella (= Thais) Emarginata
Authors: West Lani
Published: May 2006

Variation in diet among individuals of a population of carnivorous marine snails in a rocky intertidal environment of central California was analyzed. Nucella emarginata drills and eats barnacles, mussels, and limpets. Out of 128 marked snails, 104 were observed through two or more feeding attacks in the field during the 4—mo study. Fifty—one of these snails were observed through five or more sequential feeding attacks. Three major points are illustrated. (1) Within each study site, individuals close to the same size often chose strikingly different diets. (2) There was a range of dietary specialization and generalization among individuals foraging in the same habitat. Individuals showed a high degree of consistency in their diets. While the population of N. emarginata attacked seven prey species at site A and four prey species at site B, no individual snail observed in this study ate more than three species of prey. (3) Food choices made by each individual were not a simple reflection of the relative abundance of the surrounding prey species or the differences in microhabitat prey distribution that each predator encountered. These conclusions remained the same for several different methods of measuring prey availability. I suggest that Nucella emarginata diet patterns have a variety of causes, which may include ingestive conditioning, learning, and genetic variability.
Seedling Survival of Tropical Tree Species: Interactions of Dispersal Distance, Light‐Gaps, and Pathogens
Authors: Augspurger Carol K.
Published: May 2006

This study tested the hypotheses that seed dispersal enhances seedling survival by (1) escape of distance—dependent or density—dependent mortality or both, and (2) colonization of light—gaps. Spatial patterns of seedling survival for 1 yr in shade and in light—gaps and causes of seedling mortality for the first 2 mo were determined for one tree of each of nine species that use wind dispersal on Barro Colorado Island, Panama. Dispersal was advantageous to all nine trees, but for different reasons. The colonization hypothesis was supported by all nine trees. At some time interval during the 1st yr, the escape hypothesis was supported by eight of the nine trees; dispersal away from the parent to shaded conditions lowered the probability of seedling mortality. However, the trees differed greatly in (1) when the dispersal advantage became apparent, (2) whether the advantages persisted through time, (3) the distance at which survival was most enhanced, and (4) the strength of the advantage. By 1 yr only four trees showed a moderate to strong advantage of dispersal to shaded areas; two trees showed a very weak advantage, and three trees showed no advantage. Pathogens caused the largest proportion of deaths among shaded seedlings in six of the nine trees. For the two trees with the strongest support for the escape hypothesis, pathogens caused distance— and/or density—dependent mortality; those trees had high seedling densities near the parent. Four other trees had moderate to high mortality by pathogens, irrespective of dispersal distance; those trees had low seedling densities at all distances from the parent. For these six trees in which pathogens were responsible for the largest proportion of deaths in the shade, this mortality was significantly reduced in light—gaps, even though seedlings occurred in high densities. The study indicates that the location where seedlings survive is greatly influenced by pathogen activity. Only the four trees with moderate to strong support for the escape hypothesis had large numbers of seedlings survive to 1 yr in the shade. Seedling recruitment is expected to occur away from the parent tree, in shaded conditions for trees with distance— and/or density—dependent mortality, and in light—gaps for all trees studied here.
The Natural History of Lianas on Barro Colorado Island, Panama
Authors: Putz Francis E.
Published: May 2006

Liana (woody vine) abundance, height, diameter, and climbing mode were studied in the mature tropical moist forest on Barro Colorado Island, Panama (BCI). Because lianas are capable of extended horizontal as well as vertical growth, sample plots were 100—m2 cylinders extending from the ground up to the treetops. The plots were randomly located in areas representing different stages of regeneration following treefalls. In order to examine canopy lianas closely, trees in or adjacent to the sample plots were climbed freehand or with the aid of mechanical rope ascenders. Lianas are abundant on BCI and play important roles in forest dynamics. A hectare of old—growth forest had 1597 climbing lianas distributed among 43% of the canopy trees. Trees with a least one liana has higher than random probability of having more than one liana, and individual lianas connected an average of 1.56 canopy trees. In the understory, 22% of the upright plants <2 m tall were lianas, and, depending on the species, between 15 and 90% of these plants were vegetative offshoots (ramets) and not true seedlings (genets). Lianas were most abundant in recent treefall gaps and decreased in abundance with time since last disturbance. Trellis availability was found to be a major factor limiting liana access to the forest canopy. Experimental manipulation of supports and experimental planting of Dioclea reflexa seedlings revealed that trellises consisting of small diameter, closely spaced supports are most abundant on the edges of treefall gaps. Tree and liana stems on the edges of treefall gaps provided a major pathway to the canopy for climbing plants. Trees carrying lianas suffered higher mortality rates and upon falling caused more other trees to fall than did liana—free trees. Few lianas died when their host tree fell, and many grew back to the canopy using the abundant trellises that occur on the edges of treefall gaps. Tree sapling growth rates in treefall gaps and Luehea seemannii growth rates in the canopy were slower where lianas were abundant.
Photosynthate Allocation to Floral Nectar: A Neglected Energy Investment
Authors: Southwick E. E.
Published: May 2006

Measurements of photosynthetic assimilation and nectar production provide data for the calculation of energy allocated to the production of floral nectar in the common milkweed, Asclepias syriaca. Four to 37% of daily photosynthate assimilated during blossoming is secreted as nectar sugar. Amortization of nectar cost over the seasonal lifetime of the plant showed that as much as 2.8% of photosynthetically assimilated carbohydrate is routed through the nectaries. In comparison, in a field crop of alfalfa, Medicago sativa, nectar is an energy sink containing as much as 20% of the amount of energy contained in the harvested alfalfa forage, or almost twice the energy yield in the total seed crop. The relative costs of nectar production and other costs such as flowering, seed production, and growth and maintenance are discussed.
Demographic Models Based on Size, Not Age, For Organisms with Indeterminate Growth
Authors: Kirkpatrick Mark
Published: May 2006

In conventional demographic theory fecundity and survival are determined by the age of the individual, but for many organisms size is more important than age in determining life history characteristics. Continuous—time demographic models presented here explicitly account for growth, including individual variability in growth rate, and assume that size alone determines the birth and death rates. Growth and mortality processes are introduced that result in a normal distribution (on some scale of measurement) of the sizes of individuals within a cohort; this permits the cohort to be described in terms of only three variables; mean size, variance in size, and proportion of individuals surviving. Using general formulae for the intrinsic rate of population increase and the stable size distribution, applications of the models are presented illustrating the effects of individual growth rate variance and size—specific mortality on characteristics of the population.
Tree Growth, Mortality, Recruitment, and Canopy Gap Formation during a 10‐year Period in a Tropical Moist Forest
Authors: Lang Gerald E. , Knight Dennis H.
Published: May 2006

All trees @>2.5 cm dbh were censused on a 1.5—ha tract of 60—yr—old tropical moist forest in 1968 and again in 1978 to determine rates of tree mortality, recruitment, dbh increment, and canopy gap formation. Species composition changed very little. The pioneer of gap species Cordia alliodora, Luehea seemanii, and Spondias radlkoferi had no recruitment and accounted for most mortality in the larger size classes. Ninety percent of all mortality was for stems <10 cm dbh. Total tree density decline 11% (from 3112 to 2781 trees/ha), but basal area increased 22% (from 25.7 to 31.4 m2/ha). Growth in diameter was highly variable, both among species and among size classes. Trees in the 30—50 cm dbh class had a mean dbh increment of 0.9 cm/yr. Gaps occurred over an area equal to 7.3% of the plot during the 10—yr period, suggesting that about 137 yr would be required for the 1.5—ha plot to be affected by tree falls.
Experimental Ecological Genetics in Plantago: A Structural Equation Approach to Fitness Components in P. Aristata and P. Patagonica
Authors: Maddox G. David , Antonovics Janis
Published: May 2006

A modified path—analysis technique, maximum likelihood linear structural equaton modelling, is applied to the comparison of fitness components in the annuals Plantago aristata and P. patagonica, grown under controlled environment conditions. The technique is used to determine the magnitude and significance of causative relations in a model scheme where "fitness," an unmeasured construct of two measured variables, seed size and seed number, is determined by direct and indirect (via reproductive structures) paths from the photosynthetic structures represented by different "leaf areas," unmeasured constructs of leaf number and leaf size, during development. The model provides a reasonable fit to the observed data, and the two species do not differ significantly with regard to the parameter estimates. Inflorescence number and capsule number have large significant effects on fitness, but the effect of number of seeds per capsule is not significant. Leaf areas during midlife influence fitness indirectly via production of reproduction structures; during late—life they have a direct effect on fitness via seed filling. The analysis of fitness components using the method of structural equation model fills an intermediary role between simple morphometric analysis and more explicit modelling of physiological processes.
Some Observations of the Use of Discriminant Analysis in Ecology
Authors: Williams Byron K.
Published: May 2006

The application of discriminant analysis in ecological investigations is discussed. The appropriate statistical assumptions for discriminant are illustrated, and both classification and group separation approaches are outlined. Three assumptions that are crucial in ecological studies are discussed at length, and the consequences of their violation are developed. These assumptions are: (1) equality of dispersions, (2) identifiability of prior probabilities, and (3) precise and accurate estimation of means and dispersions. The use of discriminant functions for purposes of interpreting ecological relationships is also discussed. It is suggested that the common practice of imputing ecological meaning to the signs and magnitudes of coefficients be replaced by an assessment of structure coefficients. Finally, the potential and limitations of representation of data in canonical space are considered, and some cautionary points are made concerning ecological interpretation of patterns in canonical space.