bluuebunny/crossref_metadata_2025 huggingface dataset:
Populations and Local Extinctions of Birds on Barro Colorado Island, Panama
Authors: Willis Edwin O.
Published: April 2006

A 1960—71 study of populations of color—banded ant—following antbirds of three species on a tropical—forested lowland reserve, Barro Colorado Island, showed that the small species (Spotted Antbird, Hylophylax naevioides) remained stable at about 20 pairs/km2. A medium—sized species, the Bicolored Antbird (Gymnopithys bicolor), decreased from about 3 pairs to 1.5 pairs/km2. A large species, the Ocellated Antbird (Phaenostictus mcleannani), declined from 1.5 pairs/km2 to near extinction–only one female remained in early 1971. Two of three other species that regularly follow army ants showed relatively stable populations, but a third large species (Barred Woodcreeper, Dendrocolaptes certhia) declined from two pairs to local extinction. Prior to 1960 a very large ground—cuckoo that follows ants had already become extinct there. Thus, the three largest of the seven original species that regularly followed ants were gone or nearly gone by 1970. The decrease in numbers of regular ant—following birds was not made up by increases in occasional followers. Detailed studies of antbirds showed no clear reasons for declines, except that annual mortalities of adults were high in Ocellated Antbirds (about 30%) compared to Spotted Antbirds (15%—17%) and nest losses perhaps higher in the former (96% compared to 91%). Nest mortalities were slightly lower (88%) and adult mortalities intermediate (about 25%) in Bicolored Antbirds. Female Ocellated Antbirds had higher mortalities than males. The antbirds renest repeatedly during long nesting seasons, up to 14 times per year for Ocellated Antbirds. However, to replace females of this species under Barro Colorado conditions 19 nestings per year would be needed. Concurrent listing of all birds of the island showed that 45 species of breeding birds, 22% of the avifauna present when the island was made a reserve, had disappeared by 1970. No new species replaced them. Of the lost species 13 are forest birds, in danger if forests are cut elsewhere. The other species, second—growth and forest—edge birds, have been crowded out by growth of the forest. Loss of species from this tropical reserve, especially the part apparently caused by the small size and isolation of the reserve, poses problems for conservation and ecological studies of tropical biotas. It is suggested that large future reserves have corridor zones to each other, that is, that intensive human use not preempt too much area nor interrupt immigration of animals or plants from one refuge to another.
Interactions between a Sea Pen and Seven of Its Predators
Authors: Birkeland Charles
Published: April 2006

The sea pen Ptilosarcus gurneyi Gray provides a major source of food for at least seven predator species in Puget Sound: the asteroids Hippasteria spinosa, Dermasterias imbricata, Crossaster papposus, and Mediaster aequalis, and the nudibranchs Armina californica, Tritonia festiva, and Hermissenda crassicornis. Ptilosarcus is long—lived (15 yr +), takes 5 or 6 yr to reach sexual maturity, and has a spatially clumped pattern of recruitment. It never grows large enough to avoid predation by four of its predators, and the refuges and escape mechanisms of Ptilosarcus are not sufficient to explain its abundance in the face of this predation. Determination of rates of critical life history processes of species involved in this association, e.g., recruitment, growth, and predation, permitted estimates of the effects of the activities of each species on the others in the association. Particular attention was paid to age structure and to the natural history during the periods shortly following metamorphosis of each species. In dense recruitment patches, 97% of Ptilosarcus mortality occurs during the 1st year because of predation by Hermissenda, Tritonia, and Crossaster. Adult pens are eaten by Hippasteria, Dermasterias, Mediaster, and Armina, which together remove 2 pens/m2°yr, or 3.1% from the 10 adult year classes combined. The latter four predator species selectively prey upon the largest pens available. The time required for capture and consumption increases at a slower rate with prey size than does prey biomass. Recruitment of the long—lived food specialist Hippasteria seems to be impaired by the spatially unpredictable and clumped nature of Ptilosarcus recruitment. Although the adults have an opulent food supply, the young Hippasteria require small Ptilosarcus to feed on and rarely find enough. The generalists, which are not dependent upon one prey during their recruitment and are able to maintain themselves without Ptilosarcus, could in theory increase until they have reduced the sea pen populations to below the level needed by the specialists and thus outcompete them. However, predation on the generalists by a higher order predator, the asteroid Solaster dawsoni, removes each year at least 9.4% of the adult Mediaster and 38% of the adult Crossaster, a rate at least as great as, and usually greater than, the rate of recruitment into the populations of adult prey. The characteristic organization of the Ptilosarcus association and the existence of Hippasteria may depend upon S. dawsoni.
The Hubbard Brook Ecosystem Study: Forest Biomass and Production
Authors: Whittaker R. H. , Bormann F. H. , Likens G. E. , Siccama T. G.
Published: April 2006

A small watershed in the White Mountains of New Hampshire bearing mesophytic, cool—temperate, broadleaf—deciduous forests was studied. Acer saccharum, Betula lutea, and Fagus grandifolia are dominant, but toward higher elevations Picea rubens and Abies balsamea also occur and indicate the transition toward subalpine climate. The stands are young (following cutting in 1909—17) but contain older trees; stand composition is thought reasonably representative of the climax. For application of the Brookhaven system of forest dimension analysis, 93 sample trees of major species were cut and roots excavated. Mean dimensions of sample trees, and the constants for the system of logarithmic regressions relating volume, surface, mass, and growth to diameter at breast height and other independent variables, show decrease in tree sizes and height/diameter ratios toward higher elevations. Stand characteristics, based on application of the regressions to forest samples, show trends of decrease for the elevation belts from low to high: stem basal area 26.3, 23.7, and 22.0 m2/ha, weighted mean tree height 16.9, 16.7, and 10.8 m, weighted mean age 124, 95, and 83 yr, stem wood volume 176, 155, and 103 m3/ha, aboveground biomass (dry matter) 162, 152, and 102 t/ha, estimated volume increment 379, 365, and 223 cm3/m2/yr, aboveground net primary productivity (1956—60) 1127, 1041, and 790 g/m2/yr, and leaf area ratio 6.2, 5.7, and 5.5 m2/m2. Biomass (and, presumably, production) of root systems is 18%—21% of that aboveground. Different estimations suggest that a mean climax biomass for the watershed may be around 350 t/ha, aboveground. Net ecosystem production (i.e., addition to the pool of woody biomass in the community) is estimated as 350 g/m2/yr aboveground and 85 belowground for 1956—60, 238 and 52 g/m2/yr for 1961—65. Analysis of stem wood volume increments reveals an abrupt and striking (18%) decrease in volume growth and productivity from 1956—60 to 1961—65. The net primary productivity of the former period, with a weighted mean for the watershed of 1110 g/m2/yr above and below the ground, is thought more nearly normal for the forest. Both drought and effects of increasing air pollution (notably increasing acidity of rainfall) may be responsible for the recent decrease in productivity.
Effects of Fire on Factors Controlling Plant Growth in Adenostoma Chaparral
Authors: Christensen Norman L. , Muller Cornelius H.
Published: April 2006

In the first few years following fire, burned chaparral areas are typified by a flush of seed germination and seedling growth not seen in unburned chaparral. The effects of fire on factors of potential importance to this plant response were examined in recently burned and long unburned chaparral, dominated by Adenostoma fasciculatum, in the Santa Ynez Mountains near Santa Barbara, California. Measurement of soil moisture, soil texture, and light indicated that changes in these factors following fire had little direct effect on postfire germination and growth. Bacteria and fungi were more abundant in burned than in unburned chaparral soil. Thus, the hypothesis that microbial depletion of soil oxygen in unburned chaparral inhibits seed germination appears untenable. Changes in organic matter content, pH, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium were examined in the upper soil layers of both burned and unburned chaparral during the first 18 mo following fire. Mineral addition as ash—fall was also evaluated. Burning increased levels of most mineral elements significantly. Furthermore, a large reservoir of readily available organic nutrients was added in the ash. Causes of low nutrient levels in unburned chaparral were also investigated. Additions of nutrients to unburned chaparral soil in the greenhouse and field resulted in increased growth of species common in burned chaparral areas. Bioassays of aqueous A. fasciculatum leaf washings were performed against 10 plant species common in burned chaparral areas. Significant depression of germination and growth was observed. Rain throughfall collected under the shrubs was also inhibitory. Efforts to isolate and identify the toxins are described. Numbers of small herbivorous mammals were greatly reduced in recently burned chaparral. This factor is shown to have a marked effect on seedling survival. Applications of various heat treatments to unburned chaparral soil and to fresh seeds of species common to burned chaparral indicated that the seed of several species are released from dormancy by heat. Insolational heating of soil in cleared, but unburned, chaparral is sufficient to stimulate germination.
Distribution of Major Forest Species in Southeastern Pennsylvania
Authors: Keever Catherine
Published: April 2006

Vegetation and environment of 82 forest stands in Lancaster, York, and Lebanon Counties in southeastern Pennsylvania were studied to determine the factors controlling the location of major forest species. Plants of some species were transplanted into 12 stands in which certain environmental factors were checked each week for two growing seasons. Thirty—six per cent of the 82 stands were the oak—hickory type, in which Quercus alba, Q. velutina, Q. coccinea, Carya ovalis, and C. tomentosa were among the leading dominants. Representatives of these oak—hickory forests have very low soil moisture for extended periods of time even in a year of normal rainfall. Quercus prinus was among the main dominants in 43% of the stands, representatives of which show excellent soil—moisture conditions even in a year of severe drought. Tsuga canadensis, Quercus rubra, Fagus grandifolia, Betual lenta, and Acer saccharum were among the leading dominants in 27% of the stands; representatives of these mixed mesophytic stands showed intermediate soil—moisture conditions. Tsuga canadensis reproduces only on steep slopes where litter is removed by erosion and where soil moisture is rarely very low. Fagus grandifolia reproduces largely by root sprouts. Quercus alba, Q. coccinea, Carya ovalis, and C. tomentosa reproduce poorly in mature stands, but reproduce much better in open and successional ones. Quercus prinus and Q. rubra reproduce well in mature stands and are judged to be climax species. Vegetation has not yet stabilized following the removal of Castanea dentata as a dominant species.
Consumption of Primary Production by a Population of Kangaroo Rats (Dipodomys Merriami) in the Mojave Desert
Authors: Soholt Lars F.
Published: April 2006

The energy requirements for a population of kangaroo rats in the Mojave Desert were estimated indirectly. Energy expenditure for maintenance was 84.1 megacal ha—1yr—1. Total energy expenditure for growth was 1.4 megacal ha—1yr—1, half of which was secondary productivity. Secondary productivity was 0.8% of the total energy flow. This percentage of the total energy flow is lower than for nondesert rodents and may be due to the low productivity of the desert areas in which D. merriami is found. The diet of D. merriami contained an average of 75% seeds. Green herbage formed 35% of the diet during the winter quarter. Kangaroo rats showed a high preference index for Erodium cicutarium seeds, which were larger and had a higher caloric value than the seeds of other annual plants. The average assimilation efficiency was 0.87 for the natural diet. Estimates indicated that kangaroo rats consumed 97.8 megacal ha—1yr—1, 99% of which was from the primary production. Total net primary production (NPP) on the study area was only 1,400 megacal ha—1yr—1; of this, 900 megacal ha—1yr—1 was considered to be available to D. merriami. Kangaroo rats consumed 6.9% of the NPP. This is higher than for any other rodent populations that have been studied and is probably due to the large proportion of available production (AP) in the total primary production (64%). The population consumed 10.7% of AP, which is high when compared to most other rodent species. The high consumption efficiency for D. merriami may be an adaptation to areas of low production, or it may be simply the result of high densities of kangaroo rats in areas of low production. Dipodomys merriami consumed over 95% of the estimated Erodium production and 90% of the total Erodium production. It is estimated that the rats consumed a sufficient number of Erodium seeds in 1970 to reduce the density of that plant by more than 30% in 1971. The D. merriami population was on the brink of overexploitation of its food sources and was potentially food limited. It is probable that desert herbivores in general are food limited since productivity in the desert is so variable.
Growth, Production, and Community Composition of Fishes Inhabiting a First‐, Second‐, and Third‐Order Stream of Eastern Kentucky
Authors: Lotrich Victor A.
Published: April 2006

Fish populations in a first—order, a second—order, and a third—order stream were studied during the summers of 1967—68. The primary food of each fish species was determined. Available food sources included terrestrial invertebrates, aquatic primary production, aquatic invertebrates, aquatic vertebrates, and detritus. Average daily growth rates showed that age—group I fish of most species grew faster in early than in late summer. The growth rate of most age—group II fish varied little between these seasons. Total numbers, total standing crop, and gross production were estimated for each species in each order. Total production was compared among stream orders on a per linear meter basis. Production values were nearly equal in first (2.35 g dry weight per linear meter) and second order (2.36), but increased in third order (3.29). The relative importance of terrestrial invertebrates as an energy source decreased with increasing stream order. Aquatic primary production and aquatic invertebrates began to be utilized as energy sources in second order. The aquatic vertebrates and detritus began to be utilized in third order. The relative importance of aquatic primary production utilization increased in third order, but the relative importance of aquatic invertebrate utilization was approximately the same as in second order. The community structure of the fish populations of each pool was analyzed the information theory and the ratio H/Hmax. Community structure differed between third—order and second—order pools even when the same species complement was present. It is postulated that stream order in most cases represents a biological unit which can be subdivided into microhabitats based on riffle, pool, type of substrate, etc., and addition in most cases proceeds by discrete units of stream order.
Dynamics of Black Cherry (Prunus serotina Erhr.) in Southern Wisconsin Oak Forests
Authors: Auclair Allan N. , Cottam Grant
Published: April 2006

Black cherry occupies a key position in the dynamics of oak (Quercus spp.) forests in southern Wisconsin. Its relative importance in sapling and small—tree sizes averaged over 50%, and in some stands it was the only species in these strata. Age analysis of 854 cores showed that 58% of all black cherry stems were established between 1931 and 1941. The appearance of large amounts of black cherry in the 1930 decade was the result of drought and livestock—feed shortages. The latter were especially severe in 1934 and 1936 and resulted in large—scale disturbance of forests through grazing and possibly extensive cutting of forests for foliage. The presence of a strong shrub component exerted a major influence on the dynamics of the oak forest. An inverse relationship existed between overstory tree basal area and the importance of shrubs. Although density and cover of tree seedlings were independent of tree basal area and shrub cover, seedling numbers were insufficient for adequate restocking of most stands. In clear—cut forests the shrub layer was extensive, and regrowth of trees was largely confined to stump sprouts. Shrubs in disturbed forests averaged 38,635 stems/acre, 4,535 square inches basal area/acre, and 145.8% cover. In undisturbed forests, shrub competition (average 16,231 stems/acre, 1,036 square inches basal area/acre, and 51.4% cover) largely precluded seedling establishment by tree species, and maximum basal area levels in these forests were not maintained. Growth and survival of black cherry in the understory depended on the complex interaction of environmental factors. Tree overstory basal area and soil available water were the most significant factors controlling its vigor. Suppressed stems of cherry were as old as 60 years. Growth patterns in stem cores showed that stems responded to release after 39 years of suppression. The capacity of black cherry to develop basal sprouts and the occurrence of light intensities greater than 1,200 ft—c in the understory greatly aided its survival in this stratum. The important role of black cherry in the dynamics of the oak forests was based on opportunistic characteristics including widespread dispersal, delayed germination of seeds, and flexible seedbed requirements. Forest conditions, such as the isolation of forests as woodlots, pioneer environmental conditions, and widespread disturbance, predisposed invasion by opportunistic species. Dynamic processes were characterized by general rather than gradient compositional changes. Patterns of replacement were not orderly and predictable, but irregular as a result of chance dispersal and local catastrophe. Although not uncommon in oak forests in the northeastern United States, invasion by Acer saccharum is not occurring in southern Wisconsin. Succession to Acer—Tilia is confined to specific sites. The overall existing pattern is one of increasing diversity typical of many pioneer communities in an early phase of colonization.
A Phytosociological Analysis of Species‐Rich Tropical Forest on Barro Colorado Island, Panama
Authors: Knight Dennis H.
Published: April 2006

Data on tree species composition and population structure are used as a source of ecological information on the species—rich forest of Barro Colorado Island, Panama Canal Zone (BCI). Thirteen stands, representing both the young and old forest on the Island, were sampled using 10 m ° 20 m quadrats for all individuals °2.5 cm dbh. Over 300 species were encountered and most were identified. The data are evaluated for interpreting late secondary succession (>60 yr), detecting soil—vegetation patterns, and for yielding autecological information. Successional status was estimated by placing species with similar population structure patterns into groups, and then calculating the abundance of species groups that seemed indicative of successional status. Five population structure patterns were recognized. The results suggest that the older BCI forest is not climax after at least 130 yr of succession. Both principal components analysis and a Bray—Curtis type ordination were used to determine whether ecological patterns on BCI are reflected by species composition. Forest age was best reflected by an ordination of the larger tree species. Species found only in the young forest or only in the older forest are identified. Stands with a distinctive gley soil were not segregated on the ordination. Species diversity increases most rapidly during the first 15 yr of succession, but continues to increase slowly after 65 yr. The Shannon—Wiener diversity measure averaged 4.8 (log base 2). Some data suggest that wind—caused canopy gaps are important for the persistence of several species in the older forest, e.g., Cecropia sp.
Ecology of the Water Penny Beetle Psephenus herricki (DeKay)
Authors: Murvosh Chad M.
Published: April 2006

A general theory of distribution suggests that the microhabitat, habitat, and geographic distribution of Psephenus herricki is related to water turbulence. The different life stages typically occur in stream riffles. Adults are found in riffles because they are "attracted" to the moist wave—splashed rocks that stick out of the water. These same rocks provide a mating site and give the female access to the subsurface materials for oviposition. The microdistribution of eggs is a function of the distribution of wave—splashed rocks during oviposition and female oviposition behavior. Oviposition initially accounts for the large numbers of larvae in riffles, but larvae live about 2 years giving them sufficient time to disperse into the runs and pools. Dispersion, however, is negligible due to a thigmokinesis that inhibits vagility when they are wedged in the interstitial spaces of the substrate. This and the distribution of algae on rocks seems to determine the microdistribution of larvae. Riffles contain an abundance of these interstitial crevices due to sorting by stream turbulence. Turbulence and physiography, governing the geographic distribution of riffles, directly influence the geographic distribution of the species.
Organic Matter and Nutrient Dynamics in Forest Floors of Young and Mature Abies amabilis Stands in Western Washington, as Affected by Fine‐Root Input
Authors: Vogt Kristiina A. , Grier Charles C. , Meier Calvin E. , Keyes Michael R.
Published: April 2006

Seasonal patterns of decomposition and nutrient release from the major litterfall components were determined using litterbags in young (23—yr—old) and mature (180—yr—old) Abies amabilis stands in western Washington, USA. The time required for each litterfall component to decompose completely was estimated in both stands. Long—term organic matter and nutrient residence times in the forest floor were estimated using forest floor and aboveground litterfall masses. In addition, the effect of root turnover in the forest floor on estimates of organic matter and nutrient residence time was determined. Similar litter substrates lost mass at a significantly faster rate in the young than in the mature stand. An initial rapid mass loss (9—67%) for litter substrates during the winter months coincided with increased immobilization of nitrogen and calcium and mineralization of phosphorus, potassium, and magnesium. However, herb species and epiphytic lichen decomposition in both stands showed no immobilization of N, P, K, Ca, or Mg or increased mass in litterbags at all sampling dates. The remaining litter substrates showed no loss or gain in mass during summer and autumn, with continued immobilization of N and P, while K, Ca, and Mg levels remained the same or decreased. From 36 to 77% of the total 2—yr mass loss of all litter substrates occurred during the initial 4 mo. The various litter components required °6—15 and 2—24 yr for 99% decomposition to occur in the young and mature stands, respectively. The addition of roots into the estimation of organic matter residence time (MRT) in the forest floor reduced the estimate by °75% in both stands. The predicted turnover time of 1 yr's litterfall (11—12 yr) was very similar to the estimated residence time of total forest floor when roots were included in the calculations. The inclusion of roots in the estimation of forest floor MRT decreased the nutrient residence time for N by 74%, for P by 75%—86%, for K by 81—90%, for Ca by 52—55%, and for Mg by 66—85% in both stands. Addition of root nutrient input into estimates of forest floor nutrient MRT increased the annual turnover of forest floor nutrients from 1—2% to 4—16% in the young stand and from 1—4% to 4—8% in the mature stand. The element mobility series resulting from litterbag studies did not give the same pattern of mobility as obtained from the total forest floor with or without root input.
Quantitative Phenology and Water Relations of an Evergreen and a Deciduous Chaparral Shrub
Authors: Gill David S. , Mahall Bruce E.
Published: April 2006

Quantitative phenology and pre—dawn xylem pressure potentials (XPP) of an evergreen shrub, Ceanothus megacarpus, and a deciduous shrub, Salvia mellifera, were compared on pairs of plants in shared microsites in southern Californian chaparral for 2 yr. Shoot elongation of both species occurred at essentially the same time during both growing seasons. Leaf maturation also occurred at essentially the same time for both species during the 1977—1978 season, but commenced ≈1 mo earlier on S. mellifera than on C. megacarpus during the 1978—1979 season. No substantial differences in phenology were found among leaves of C. megacarpus canopy, subcanopy, or short shoots. On S. mellifera, however, leaf maturation was more rapid and was completed earlier in the season on flowering canopy shoots than on nonflowering canopy and subcanopy shoots, and although no mature leaves on flowering canopy shoots persisted beyond June during either year, a substantial number of mature leaves on subcanopy shoots remained well into their second growing season. Leaf curling, associated with changes in XPP, was significant in both species, and allowed rapid adjustment of leaf area without the abscission or production of leaves. Flowering and seed production occurred early in the growing season for C. megacarpus and late in the growing season for S. mellifera. This difference in flowering phenology corresponded to the more rapid vegetative development of S. mellifera flowering canopy shoots. In both species subcanopy or short shoots made up a much larger fraction of the total number of shoots per plant, and consequently contributed relatively more to whole—plant leaf phenology and annual shoot production, than did canopy shoots. There were only subtle differences between species in the timing of increases in whole—plant mature leaf area (MLA). Maximum rates of increase of whole—plant MLA occurred in both species when soil temperature (ST) was ≈13.5°C and XPPs were ≈—0.60 MPa, and these rates declined sharply at STs above 15° and XPPs below —1.5 MPa. Ceanothus megacarpus and S. mellifera had nearly indistinguishable XPPs throughout both years. Leaf shedding by S. mellifera was more related to photoperiod and flowering than to low XPP, and was clearly not a successful drought—avoiding mechanism. Like some evergreen species, S. mellifera tolerated long periods of extremely low XPP. Important differences between the species were that over a 12—mo period the integrated total of MLA—days for S. mellifera was 73% of that for C. megacarpus, and leaves produced during the current growing season contributed 71% of the total MLA—days in S. mellifera and only 37% in C. megacarpus. The differences between the results of this study and several accepted generalizations regarding growth and water relations of deciduous and evergreen chaparral shrubs may be due to the observation of these growth forms in different microsites and the lack of truly quantitative phenological measurements in previous studies.
Structure in the Copepod Community of the North Pacific Central Gyre
Authors: McGowan John A. , Walker Patricia W.
Published: April 2006

The North Pacific central gyre is a large, monotonous, and geologically old system. It (and other such gyres) is probably the nearest oceanic equivalent to an essentially self—regulating, climax ecosystem. We have taken a series of replicated, vertically stratified net tows for macrozooplankton and have replicated measures of a number of habitat variables. Copepods are the most numerous members of the macrozooplankton fraction of the community of the central gyre. It is likely that they are also the largest part of the biomass. There are at least 125 species regularly present and although we do not know what many of them eat, certain species can almost certainly be identified as herbivores or carnivores, and others as omnivores. We show that some species are significantly more frequent parts of each other's biotic environment than other species. These recurrent groups of species tend to occupy different depth zones. The members of the groups show strong concordance of abundance at the depth of high group coherence. Thus there is a dividing up of the water column and a clear tendency for vertical spatial structure in the copepod fauna. Within groups there is a significant constancy of dominance at the group's preferred depth over time. Thus there appears to be a large amount of species structure as well as spatial structure. A highly regulated species equilibrium is implied. There are congeners occurring within the same recurrent groups. The physical and biotic environment of each group differs with respect to both concentration (e.g., chlorophyll α) or magnitude (e.g., temperature) of habitat variables and with respect to the spatial variability of these. Our most diverse group tends to occupy a depth zone ranking high in primary productivity but does not have the greatest total environmental richness or greatest or least heterogeneity, and this seems contrary to what some current community theory predicts. Our evidence indicates that within groups many similar species coexist spatially and temporally. We speculate about the relative roles of competition and predation influencing the structure we see. Aspects of modern community theory, based on competitive equilibrium, seem inadequate to explain our results. Predator regulation of structure seems a more likely explanation. However, existing information indicates a lack of sufficient specialization of what we believe to be the main predators on copepods to account for the observed constancy of copepod species structure. We suggest that a more intensive study of the role of predation is in order.
Water and Nutrient Outflow From Contrasting Lodgepole Pine Forests in Wyoming
Authors: Knight Dennis H. , Fahey Timothy J. , Running Steven W.
Published: April 2006

Factors affecting water and nutrient outflow beyond the rooting zone were studied during a 3—yr period, using data from eight contrasting stands of lodgepole pine (Pinus contorta ssp. latifolia) forest in southeastern Wyoming and the output of a hydrologic simulation model (H2OTRANS) based on tree physiology. Nutrient outflow during a specific time period was estimated by multiplying simulated water outflow times element concentrations in the soil solution, the latter determined from samples collected periodically near the bottom of the rooting zone. Estimates of actual evapotranspiration (ET) for the period from early spring to late fall ranged from 21 to 53 cm, which was 33—95% (x = 73%) of total annual precipitation. For all stands and years, transpiration accounted for 50—61% of ET, and 9—44% of the transpiration occurred during the spring drainage period (vernal transpiration, VT). Estimated VT and outflow varied considerably among the stands, with VT accounting for 4—20% of the snow water. Outflow occurred only during the snow melt period and accounted for 0—80% of the snow water. Snow water equivalent varied annually by 300% or more. Nutrient outflow from the different stands also varied greatly. Ratios between simulated annual outflow and atmospheric inputs (bulk precipitation) were consistently >1.0 for Ca, Na, and Mg; were consistently <1.0 for N; and ranged from 0.3 to 2.0 for P and from 0.2 to 3.3 for K. Much of the variability in water and nutrient outflow can be attributed to the degree of biotic control, with water outflow affected by a different combination of factors than nutrient outflow. H2OTRANS was used to simulate the effects on outflow of different snow water equivalents and different total leaf areas. One result of the simulations was that nitrogen appears to be retained even at the highest levels of water outflow. Another was that increases in water outflow following reduction in leaf area were proportional to the leaf area removed. The results indicate that stands differing in site or habitat type experience different rates of water and element losses at different times during the snow melt season, and contribute differentially to streamwater quality and hydrograph shape. Factors affecting outflow are discussed in the context of successional trends, common perturbations including timber harvest, and hypotheses pertaining to nutrient conservation in terrestrial ecosystems. Nutrient retention in the snow—dominated lodgepole pine ecosystem appears to be primarily dependent on evergreen leaf area, duration of the VT period, and high carbon/nutrient ratios of the forest floor. Net losses of limiting nutrients probably occur primarily in pulses after abiotic perturbations such as fire.
Serengeti Grassland Ecology: The Role of Composite Environmental Factors and Contingency in Community Organization
Authors: McNaughton S. J.
Published: April 2006

Canopy interception data from 105 grassland stands in the Serengeti National Park in Tanzania and the Masai Mara Game Reserve in Kenya were used to evaluate community organization within an ecosystem defined by the annual movements of large herds of nomadic ungulates. Other studies tested hypotheses about factors influencing species composition and diversity of the communities. The herbaceous vegetation was principally C4 grasses, which commonly contributed over 90% of total abundance. Ninety—nine stands were classified into 17 communities, using a numerical clustering method. Sixteen communities were principally perennial grasses; one was dominated by an annual species. Short grasslands predominated below 700 mm mean annual rainfall and in heavily grazed sites on hilltops at higher—rainfall locations. Much of the savanna understory at intermediate rainfall levels was medium—height Themeda triandra grassland; six variants were recognized by clustering. Tall grasslands dominated by Hyparrhenia filipendula occurred as a savanna understory in locations with >900 mm of annual rainfall. Two ordination axes revealed a shortgrass continuum that graded into a medium—to tallgrass continuum, and an outlying floodplain tall grassland dominated by Echinochloa haploclada. The latter and the short grasslands dominated by Andropogon greenwayi were very different in species composition from the other communities. The first ordination axis was related to grazing intensity, and the second to soil texture. Spatial heterogeneity was an important attribute of the grasslands from the level of local stands to the level of landscape regions. Spatial pattern, rather than point diversity, was the major vegetation property correlated with α—diversity. Higher α—diversity was accompanied by low dominance concentration and more clumped species distributions. Individuals of rare species were more aggregated than members of common species. β—diversity also was a significant feature of the vegetation, leading to pronounced γ—diversity and a variety of different grassland types in local areas. Coenoclines were evident only in more arid locations. γ—diversity was associated with topographic and geological heterogeneity. Animals, soils, the overlying tree canopy in savannas, and fire influence α—diversity through their contribution to creating spatial pattern. Animals have particularly complicated effects, in addition to defoliating areas. Grazing is a major environmental factor affecting the grasslands, but it is a composite factor interacting in complex ways with other environmental factors.
Dynamics and Control Mechanisms in a Tropical Zooplankton Community (Lake Valencia, Venezuela)
Authors: Saunders James F. , Lewis William M.
Published: April 2006

The dynamics of zooplankton herbivores in Lake Valencia, Venezuela, were studied over a 5—yr interval. Lake Valencia is a large, warm—monomictic, eutrophic lake. Copepods were the dominant group; rotifers were more abundant than the cladocerans, which were not present in all years. Variation between years in herbivore biomass bore no statistical relationship to patterns in algal biomass. The average annual dry mass production of the herbivores was high (7.0 mg · L—1 · d—1), but less than expected given the high primary productivity of the lake. The annual ratios of production to biomass for individual species were well below the maxima expected for steady growth, and thus imply growth suppression. Feeding capacity of Chaoborus, the principal primary carnivore, exceeded herbivore production for 66% of the sampling dates. Annual overturn broke the coupling between predator and prey by causing extensive mortality in both herbivores and carnivores; herbivores recovered from this suppression more rapidly than carnivores. Herbivores followed three basic strategies for coexistence with their predators in Lake Valencia: (1) two Brachionus species were able to match predation losses by reproductive output; (2) the copepods, which experienced intense predation pressure as adults, were able to sustain the high losses because the adult stage was subsidized by recruitment from earlier, less vulnerable developmental stages; (3) Keratella and the cladocerans were opportunistic in that they became abundant only in response to a decline in the predator population. The opportunistic species were most abundant during the mixing season, which was the period of minimum abundance for Chaoborus. Predation strongly influenced the herbivore populations, but population data indicate that inadequate food resources also played a role. Growth suppression was the result of qualitative rather than quantitative inadequacy in the algal food base. The intensity of control by the two factors was strongly affected by abiotic factors associated with mixing events. Partial mixing provided brief respite from growth suppression for certain species (typically rotifers), but did not alter predation pressure. Annual overturn resulted in improvement of food quality, but because mortality of herbivores coincided with relaxation of predation, the herbivores were delayed in exploiting qualitative improvements in the food resources. Periods of simultaneous release from growth suppression and predation were infrequent and brief. Herbivores were thus controlled simultaneously by predation and quality of food resource.
Partitioning Herbivory and Its Effects on Coral Reef Algal Communities
Authors: Carpenter Robert C.
Published: April 2006

Coral reef herbivores were classified into three functional groups according to criteria describing the frequency and intensity of disturbance to the algal community created by their herbivory. These herbivore groups included: microherbivores, which have small foraging ranges (1—100 cm2) and a high frequency of grazing any point within their range; the echinoid Diadema antillarum, which has intermediate—size foraging ranges (0.5—1 m2) and intermediate frequencies of grazing any one area within an individual's range; and herbivorous fishes, with large foraging ranges (up to 0.5 ha) and the lowest grazing frequencies at any one site. Algal communities on experimental coral—settling plates were subjected to treatments designed to detect the effects of grazing by representatives of each functional herbivore group on algal biomass, community structure, and primary productivity in a backreef/reefcrest habitat in St. Croix, United States Virgin Islands. Abundances of microherbivores and herbivorous fishes (mainly juvenile scarids) fluctuated over time, while Diadema antillarum abundance remained relatively constant. Grazing intensity (as measured by the rate of algal biomass removal and algal standing crop under different grazing regimes) was greatest for D. antillarum; grazing by D. antillarum resulted in low—biomass, highly productive algal turfs (O2 release per unit chlorophyll a per unit time: 11.3—22.4 mg · mg—1 · h—1). Algal turfs grazed by D. antillarum were dominated by epilithic and endolithic filamentous and crustose species. Grazing by herbivorous fishes led to algal communities with algal turfs of higher biomass than in D. antillarum—grazed treatments; these communities became dominated by a single algal species (Sphacelaria tribuloides). Fish—grazed algal communities had lower biomass—specific productivity rates than algal turfs grazed by D. antillarum (2.4—8.4 mg · mg—1 · h—1). Microherbivore—grazed and ungrazed treatments had the lowest biomass—specific productivity rates and eventually became dominated by a few macroalgal species. Although algal biomass in sea urchin—grazed treatments was only 25—50% of the biomass values in treatments not grazed by sea urchins, productivity per unit area was not significantly lower in the former treatments, suggesting a positive effect of sea urchin grazing on algal productivity. This positive effect is a result of the reduction of algal self—shading when grazing is intense and/or the input of regenerated nutrients from D. antillarum excretions. Coral reef plant—herbivore interactions are similar to those in terrestrial grassland—herbivore systems. As in many terrestrial systems, plant—herbivore interactions result in higher overall ecosystem primary productivity and apparently facilitate the flow of energy and materials in this case from the highly productive algal turf component to higher levels in the reef trophic web.
Proximal Causes of Natal Dispersal in Belding's Ground Squirrels (Spermophilus Beldingi)
Authors: Holekamp Kay E.
Published: April 2006

I studied the proximal factors influencing dispersal from the natal site in two free—living populations of Belding's ground squirrels (Spermophilus beldingi) in the Sierra Nevada of California. I tested 10 hypotheses, each suggesting a different social, ecological, or ontogenetic factor as a proximal cause of natal dispersal in this species. Using discriminant analysis I also examined effects on dispersal of several independent variables concurrently. Data from marking and live—trapping studies over three field seasons (1979—1981) failed to support hypotheses suggesting resource shortage, ectoparasite load, social facilitation, conspecific aggression, or avoidance by conspecifics as proximal causes of natal dispersal in S. beldingi. Furthermore, dispersal is apparently not caused by changes in juveniles' response thresholds to conspecific aggression or by juveniles' attempts to avoid members of their family units, nearest neighbors, or other members of their local populations. My data supported an "ontogenetic switch" hypothesis that suggests natal dispersal by male S. beldingi is triggered by attainment of a particular body mass or body composition, or some combination of these two variables. Other data indicated that this ontogenetic switch initiates not only dispersal behavior per se, but also a larger syndrome o behaviors that appear to be functionally related to dispersal. Specifically, this syndrome involves changes in exploration, responses to frightening stimuli, and locomotor behavior exhibited by dispersers. I discuss possible relationships among variables operating at different proximal levels (sensu Tinbergen) to cause natal dispersal in S. beldingi and other mammals. I conclude that a complex suite of variables probably operates at genetic, physiological, and socioecological levels to proximally cause natal dispersal in each species. The ultimate (evolutionary) hypotheses most consistent with my data from S. beldingi are that dispersal: (1) reduces nuclear family incest, (2) optimizes inbreeding, and (3) improves access to mates.
The Role of Host Plant Resistance in the Colonization Behavior and Ecology of Bark Beetles (Coleoptera: Scolytidae)
Authors: Raffa K. F. , Berryman A. A.
Published: April 2006

Unlike most phytophagous insects, the reproduction of primary bark beetles (Coleoptera: Scolytidae) is contingent on host mortality. Consequently, there have been intense selective pressures on trees for properties which confer resistance to attack, and likewise, on the insect for increased behavioral complexity by which to overcome these defenses. In this study, we examined the relationship between the physiology of Pinus contorta var. latifolia and the behavior of the bark beetle, Dendroctonus ponderosae. Host mortality is a discrete outcome which is contingent on the quantitative interaction between host resistance and beetle numbers. At low attack densities, trees respond by confining beetles and their associated fungi within necrotic lesions containing toxic or inhibitory compounds. Beyond a critical "threshold of attack," however, the defensive capacity of the tree is exhausted, and mortality occurs. This threshold occurred at °40 galleries/m2 in our experimental stands. The reproductive success of the bark beetle is directly related to the depletion of host defenses through concentrated attack. At sufficiently high attack densities, °80 galleries/m2, the potential suppressive effect of the host on brood development is not manifested. Increased beetle numbers, however, cause an exponential decline in brood production due to intraspecific competition. Consequently, there is an optimal density, °62 attacks/m2 at which reproduction and survival of the beetle is maximized. Dendroctonus ponderosae utilize defensive secretions as precursors and synergists of aggregation pheromones, and thereby, elicit maximum responses from flying beetles while the host's production of toxins is at its maximum. This mechanism also ensures that the attack will terminate once the tree has been rendered suitable for brood development and, thus, minimizes the deleterious effect of intraspecific competition. Such an interaction, in which all trees could theoretically be colonized regardless of their resistance capacity, cannot result in a stable host—parasite system. Conifers, however, seem to be able to interfere with bark beetle communication. Pheromone synthesis within the hindgut and emission from the entrance site are distinct events, and beetles contained within a copious flow of resin seem unable to elicit attraction. This ability to interrupt the beetles' communication sequence, however, is inversely related to the number of beetles initiating localized attacks. Consequently, the colonization sequence reflects the outcome of a dynamic interaction between the tree and the initial "pioneer" beetles.