Showing papers in "Ecological Monographs in 1983"

Herbivory and defensive characteristics of tree species in a lowland tropical forest

Abstract: Rates of herbivory and defensive characteristics of young and mature leaves were measured for saplings of 46 canopy tree species in a lowland tropical rain forest (Barro Colorado Island, Panama). Grazing rates were determined in the field for sample periods in the early wet, late wet, and dry seasons. Leaf properties such as pubescence, toughness, water, protein, fiber, and phenolic contents explained over 70W% of the variation among plant species in the rates of herbivory on mature leaves. Leaf toughness was most highly correlated with levels of herbivory, followed by fiber content and nutritive value. Phenol content and phenol: protein ratios were not significantly correlated with damage. Mature leaves of gap-colonizing species were grazed six times more rapidly than leaves of shade- tolerant species. Gap-colonizers have less tough leaves, lower concentrations of fiber and phenolics, higher levels of nitrogen and water, shorter leaf lifetimes, and faster growth rates than do shade- tolerant species. Gap-colonizers did not escape discovery by herbivores to any greater extent than shade-tolerant species, as measured by the spatial distribution of plants or by the intraspecific dis- tribution of herbivore damage under natural or experimentally manipulated conditions. In 70W% of the species, young leaves suffered higher damage levels than mature leaves. Although young leaves are more nutritious and less tough and fibrous, they have two to three times the con- centrations of phenols. The temporal appearance of young leaves was not correlated with the distri- bution of herbivory among individuals of a species. Interspecific patterns of defense mechanisms are discussed in terms of current theories of plant apparency, and an alternative model for the evolution of plant defenses is presented.

Changes in the Carbon Content of Terrestrial Biota and Soils between 1860 and 1980: A Net Release of CO"2 to the Atmosphere

Abstract: Changes in land use over the past two centuries have caused a significant release of CO2 to the atmosphere from the terrestrial biota and soils. An analysis of this release is based on amounts of organic carbon within an ecosystem following changes such as harvest of forests; it is also based on rates of changes, such as conversion of forest to agriculture, deduced from agricultural and forestry statistics. A model is used to calculate the net amount of carbon stored or released each year by the biota and soils of 69 regional ecosystems. Some of the changes, such as afforestation, the growth of harvested forests, and buildup of soil organic matter, result in a storage of carbon; others, such as harvest of forests and increase in pasture and agricultural areas, result in a loss of carbon to the atmosphere. According to this analysis, there has been a net release of carbon from terrestrial ecosystems worldwide since at least 1860. Until 1960, the annual release was greater than release of carbon from fossil fuels. The total net release of carbon from terrestrial ecosystems since 1860 is estimated to have been 180 x 1015 g (a range of estimates is 135-228 x 1015 g). The estimated net release of carbon in 1980 was 1.8-4.7 x 1015 g; for the 22 yr since 1958 the release of C was 38-76 x 1015 g. The ranges reflect the differences among various estimates of forest biomass, soil carbon, and agricultural clear- ing. Improvements in the data on the clearing of tropical forests alone would reduce the range of estimates for 1980 by almost 60%. Estimates of the other major terms in the global carbon budget, the atmospheric increase in C02, the fossil fuel release of C02, and the oceanic uptake of C02, are all subject to uncertainties. The combined errors in these estimates are large enough that the global carbon budget appears balanced if the low estimate for the biotic release of carbon given above is used (1.8 x 1015 g released in 1980) with the higher estimates of oceanic uptake. If higher estimates for biotic release are used, then the carbon budget does not balance, and the estimates of oceanic uptake or of other factors require revision.

The role of host plant resistance in the colonization behavior and ecology of bark beetles (Coleoptera: Scolytidae)

Abstract: Unlike most phytophagous insects, the reproduction of primary bark beetles (Coleop- tera: 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. Conse- quently, 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 in- versely 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.

Interbiome comparison of stream ecosystem dynamics

Abstract: Studies were conducted in four distinct geographic areas (biomes/sites) in northern United States to examine changes in key ecosystem parameters: benthic organic matter (BOM), transported organic matter (TOM), community production and respiration, leaf pack decomposition, and functional feeding-group composition along gradients of increasing stream size. Four stations ranging from headwaters (1st or 2nd order) to midsized rivers (5th to 7th order) were examined at each site using comparable methods. The results for each parameter are presented and discussed in light of the River Continuum Concept of Vannote et al. (1980). The postulated gradual change in a stream ecosystem's structure and function is supported by this study. However, regional and local deviations occur as a result of variations in the influence of: (1) watershed climate and geology, (2) riparian conditions, (3) tributaries, and (4) location-specific lithology and geomorphology. In partic- ular, the continuum framework must be visualized as a sliding scale which is shifted upstream or downstream depending on macroenvironmental forces (1 and 2) or reset following the application of more localized "micro"-environmental influences (3 and 4). Analysis of interactions between BOM and TOM permitted evaluation of stream retentiveness for organic matter. Headwaters generally were most retentive and downstream reaches the least. Estimates of organic matter turnover times ranged between 0.2 and 14 yr, and commonly were 1-4 yr. Both turnover times and distances were deter- mined primarily by the interaction between current velocity and stream retention. Biological processes played a secondary role. However, the streams varied considerably in their spiraling of organic matter due to differences in the interplay between retentiveness and biological activity. Differences in the relative importance of retention mechanisms along the continuum suggest that headwater stream ecosystems may be functionally more stable, at least to physical disturbances, than are their inter- mediate river counterparts.

Predation, Competition, and the Composition of Larval Anuran Guilds

Abstract: Experimental manipulations of densities of the predatory salamanders Notophthalmus viridescens dorsalis and Ambystoma tigrinum significantly altered relative abundances of six species of larval anurans in 22 artificial—pond communities. One competitively inferior anuran, Hyla crucifer, was virtually excluded from predator—free control communities but survived best and occurred at greater relative abundances in communities containing high densities of Notophthalmus. A second competitively inferior species, Hyla gratiosa, survived best at intermediate intensities of predation. Each of four competitively superior species (Scaphiopus holbrooki, Rana sphenocephala, Bufo terrestris, and Hyla chrysocelis) exhibited inverse relationships between relative abundance and Notophthalmus density. Ambystoma eliminated the entire anuran guild from tank communities and had a much greater per capita impact on anuran guild composition than did Notophthalmus. In most anuran species, maximum and mean mass at metamorphosis were positively correlated with predator density, suggesting an inverse relationship between intensities of predation and competition among tadpoles. Low growth rates of most anuran species in the absence of predators were correlated with high abundances of superior competitors. These results indicate that predators mediated interspecific competition among larval anurans. Intermediate values of Notophthalmus density maximized the total production of anuran metamorph biomass. Biomass of metamorphs of each species varied in a species—specific manner with predation. The propensity of Rana sphenocephala for overwintering as tadpoles following a season of growth was also related to predator density. Laboratory feeding experiments demonstrated that at least two competitively superior anurans, Scaphiopus holbrooki and Bufo terrestris, were especially vulnerable to predation by salamanders. This suggested that Notophthalmus may preferentially remove competitively superior anurans from pond communities, allowing competitively inferior anurans to persist and to complete development successfully at moderate to high predator densities.

Serengeti Grassland Ecology: The Role of Composite Environmental Factors and Contingency in Community Organization

Abstract: 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.

Seed Germination in a Seasonal Tropical Forest in Panama: A Community Study

Abstract: Seed germination in the seasonal tropical forest on Barro Colorado Island, Panama, was studied at the community level to (1) determine the seasonal timing of germination of the community, (2) identify primary selective factors controlling timing of germination, (3) determine the relative importance of dormancy vs. timing of seed dispersal as mechanisms controlling timing of germination, (4) examine inter— and intraspecific components of variance in length of dormancy, and (5) identify major seed germination syndromes. Three community—level measures of seedling emergence indicated that there was a unimodal community peak in germination within the first 2 mo of the 8—mo—long rainy season. Of the °185 dicot, mostly woody, species germinating each year, the median time of emergence of 75% of the species occurred within the 1st 3 mo. There was a unimodal peak in germination in pioneer tree species, lianas, canopy trees, wind— and animal—dispersed species, and seedlings with and without persistent seed reserves. In contrast, there was no distinct peak period of emergence in understory species and shade—tolerant tree species. Initial seedling height of species emerging sequentially throughout the rainy season did not increase. The early emergence of most species, including three—fourths of the 35 species fruiting in the late rainy season, indicated that the early rainy season was the optimal time to emerge. Maximizing the length of the first growing season was not an important factor selecting for early emergence, because later—emerging species did not compensate for the shorter growing season by being larger at germination or by having seed reserves for rapid growth. Seedling—seedling competition is a primary biotic factor selecting for early emergence in pioneer species growing in the intensely competitive light—gap habitat. In shaded understory habitats, where competition among seedlings is much less intense, understory and shade—tolerant tree species emerged throughout the rainy season. Mean length of dormancy (MLD),the time between sowing and germination, of 157 woody dicot species on Barro Colorado Island ranged from 2 to 370 d. In over half the species, MLD exceeded 4 wk; hence, delays in germination are common. The season in which seeds were dispersed and the dispersal mechanism explained small but significant portions of the variance in MLD among species; life form explained none; but differences among three germination syndromes explained two—thirds of this variance. In the delayed—rainy syndrome (18% of all species) seeds were dispersed in the rainy season but were dormant until the beginning of the next rainy season, 4—8 mo later. Dormancy is the primary mechanism controlling time of germination. In the delayed—rainy syndrome and the intermediate—dry syndrome which follows, the length of the dormant period decreased as the interval between seed dispersal and the beginning of the rainy season decreased. In the intermediate—dry syndrome (42% of all species) seeds were dispersed during the during the dry season and remained dormant until the beginning of the rainy season. Seeds are primarily dispersed 1—2 mo before the beginning of the rainy season, which reduces the number of false germination cues encountered and decreases the length of time seeds are exposed to postdispersal predation while dormancy prevents germination during dry season rains. In the rapid—rainy syndrome (40% of all species) seeds were dispersed in the rainy season and germinated during, but not early in, that season. Dormancy has been replaced entirely by timing of dispersal as a mechanism controlling time of germination. Half of these species germinated in <2 wk, the rest in 2—16 wk.

Relationships among Flowering Phenology, Insect Visitors, and Seed‐Set of Individuals: Experimental Studies on Four Co‐occurring Species of Goldenrod (Solidago: Compositae)

Abstract: Four co-occurring species of goldenrod bloom at different times with varying degrees of overlap; in order of peak flowering they are Solidago juncea, S. graminifolia, S. canadensis, and S. nemoralis. All four species are self-incompatible and require an insect vector for successful seed- set. First, the relationship between flowering time and seed-set of individual plants of these four species was determined. All four species of Solidago had significant differences in both the percentage of filled seeds and the total seed-set of clones that flowered at different times. Early-flowering clones had lower seed-set than did late-flowering clones in S. canadensis, S. graminifolia, and S. nemoralis. In contrast, early-flowering ramets of S. juncea had significantly greater seed-set than did late-flow- ering ramets. Secondly, the underlying factors limiting seed-set were investigated by observational and experimental techniques, in order to lend insight into the processes which can select for flowering time in natural plant populations. The abundance of pollinators on goldenrods and other plant species and the abundance of flower predators were monitored over the season. Experimental hand-pollina- tions were performed on individual plants of each species over the entire flowering season, to deter- mine if seed-set was limited by the amount of pollen reaching stigmas or by factors intrinsic to individual plants which flowered at different times. Apis mellifera, the introduced honeybee, is the major pollinator of goldenrods in this system. The major factor influencing the relative abundances of honeybees on these four species of goldenrod seemed to be overlap among the flowering periods of the goldenrods and those of several introduced plant species. Apis began visiting goldenrods at a point when the abundance of flowers in the weedy flora had declined greatly. The flowering period of S. juncea overlapped almost entirely with the flowering periods of several species in the weedy summer flora. This appears to explain the lack of Apis visits to S. juncea, which was visited only by small, native bees and beetles. However, the results from experimental pollinations suggest that seed production of S. juncea was at its potential maximum throughout the flowering season. In early-flowering clones of S. graminifolia, the natural seed-set was significantly lower (by 57%) than the maximum potential seed-set determined from experimental pollinations, but in late-flowering clones there was no difference between actual and potential seed-set. Of the reduction from potential seed-set in early-flowering clones, the low frequency of honeybee visits was estimated to account for most of the loss, and flower predation by the blister beetle, Epicauta pennsylvanica, accounted for the remainder. In S. canadensis, the seed-set of both hand-pollinated and control flowers was greater in late- relative to early-flowering clones of S. canadensis (i.e., late-flowering clones had greater physiological potential for seed production). The differences in the seed-set of clones flowering at different times were due to physiological or microenvironmental differences among clones. There was also some degree of pollen limitation of seed-set (17-34% of the potential maximum seed-set) at all times. Again, pollen limitation of seed-set was due mostly to pollinators, as opposed to flower predators. In S. neinoralis, the late-flowering clones had significantly higher seed-set than early-flowering

Seasonal, Annual, and Among-site Variation in the Ground Ant Community of a Deciduous Tropical Forest: Some Causes of Patchy Species Distributions

Abstract: I examined patterns of species distributions in ground ants on Barro Colorado Island (BCI), Republic of Panama, using baited transect samples and Berlese extraction of litter arthropods. I sampled during wet and dry seasons in 1976 and 1977. All sites sampled were under closed-canopy rain forest at least 75 yr old. More species and more individuals were collected in wet than dry season using either method. Evenness of species abundance at baits also increased during the wet season. Ant activity was higher in 1977 than 1976 at all periods; this was correlated with increased rainfall during the dry season. Measures of species abundance and activity were positively correlated with protection from physical stresses. Over all seasons, fewer species and fewer individuals were collected at drier, sunnier sites. Patterns of seasonal and annual change were similar at bait transects and in Berlese samples. Species that were both active at baits and collected in Berlese samples varied in abundance the same way in both sampling methods. Several lines of evidence suggest that moisture availability is an important contributor to these patterns of among-site and among-season variation. First, ant activity, measured in several different .ways, increased rapidly when moisture availability was increased, either through rainfall or through experimental watering during the dry season. Second, moisture availability may affect the distribution of suitable nest sites. This hypothesis was supported by differences in the number of colonies collected in Berlese samples from sites which differ in moisture availability. Third, the availability of food among sites may be correlated positively with soil moisture content, but there was not enough evi- dence to test this possibility. Fourth, there is some evidence that reduction of army ant raids in drier parts of the island during the dry season may concentrate the effects of these ant predators in moist, sheltered areas of BCI. Establishment and persistence of a given species in an area is a complex function of the availability of suitable food and nest sites and the strength of competitors and predators. Patchy species distri- butions were more the result of changes in the frequency of a species' occurrence among areas, than due to the existence of unique groups of species at separate sites, at least during the periods I sampled. Thus physical conditions were probably at least marginally acceptable to a large fraction of the fauna at all sites. However, differences in abundance and activity of ant species were correlated with changes in moisture availability. I have considered four hypotheses to account for these patterns, but they are by no means exhaustive. Moisture availability, acting through many intermediate steps, has major effects on the small-scale composition of ant faunas, even in a relatively sheltered environment like a tropical forest floor.

Paleobiogeography of Montane Islands in the Great Basin since the Last Glaciopluvial

Abstract: The existing warm (Larrea) deserts of the Southwest are Holocene expansions replacing late—Pleistocene, evergreen woodlands of low—statured junipers, pinyon pines, and live oaks; these woodlands have been isolated by complementary contraction to the slopes of higher mountains that rise like islands from the modern desert sea. Because pinyon—juniper woodland is now so widespread on the similar fault—block mountains of the Great Basin, even as far north as southern Idaho, it would seem reasonable to suppose that the modern "cold" (Artemisia, Atriplex) deserts were similarly wooded during the last glacial. However, conclusive Neotoma macrofossil evidence (45 14C—dated assemblages are reported here) documents major latitudinal displacement of vegetation that precludes pinyon—juniper woodland in the northern and central Great Basin at that time. On the other hand, the entire Mohave Desert sector (south of °37°N) served as an extensive Pleistocene refugium for pinyon—juniper woodland, as documented by an additi...

Nutrient Dynamics of Aboveground Detritus in Lodgepole Pine (Pinus contorta ssp. latifolia) Ecosystems, Southeastern Wyoming

Abstract: Storage and fluxes of N, P, Ca, Mg, and K in aboveground detritus were measured in six contrasting lodgepole pine (Pinus contorta ssp. latifolia) stands in southeastern Wyoming. Lit- terfall was predominantly leaves (67-80%) in 80-100 yr old stands, while woody litter was more important in an older stand (240 yr old). Leaf litter nutrient concentrations were very low compared with other pine forests, particularly for N (0.40% dry mass). Dry mass loss from decomposing leaf litter was slow (15%/yr in first 2 yr), and summer rates did not differ significantly from winter rates beneath the insulating snowpack. Significant amounts of N, P, and Ca were added to decomposing leaves during the first winter, and N and Ca addition continued for 2 yr. Potassium and magnesium were rapidly lost from decomposing leaves. Rates of mass and nutrient loss from decomposing bark, twigs, and cones were comparable to those observed in other studies of temperate-zone forests. Mass loss from decaying bole wood appeared to be exponential through 40 yr, with an average decay coefficient (k) of 0.016, which is comparable to that in other cold temperate forests. Nitrogen content of decaying boles doubled between 30 and 55 yr following tree death, while smaller additions of P, Ca, K, and Mg also were noted. Relatively large accumulations of organic matter and nutrients were observed in the forest floor, leading to very high steady-state residence times for dry mass (mean = 18 yr), N (54 yr), P (39 yr), Ca (35 yr), Mg (21 yr), and K (18 yr). Deadfall contributed by the present forest generation was a minor component of the aboveground detritus except in an old-age stand and in a dense, self-thinning forest site. In contrast, dead wood inherited from the previous forest generation (killed by fire) was a major detrital storage component, exceeding forest floor mass by several-fold in 80-100 yr old stands. High nutrient immobilization in the dead wood led to storage values which were similar to those of the forest floor in these stands.

Organic Matter and Nutrient Dynamics in Forest Floors of Young and Mature Abies amabilis Stands in Western Washington, as Affected by Fine‐Root Input

Abstract: Seasonal patterns of decomposition and nutrient release from the major litterfall com- ponents 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 decomposing 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 I 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 Key wi~ords: Abies amabilis; abo,'eground litter decomposition; abov'eground litter nutrient re- lease;J'orest floor nutrient residence timne; organic matter residence time; root nutrients; root turnover

The Annual Pattern and Spatial Distribution of Aquatic Oxygen Metabolism in Boreal Forest Watersheds

Abstract: The level and diversity of metabolism in lotic ecosystems are largely functions of channel geomorphology and hydrology, making site-specific studies difficult to extrapolate to other parts of the watershed. This paper describes the pattern and distribution of aquatic oxygen metabolism for undisturbed boreal forest watersheds in eastern Quebec, Canada. Metabolism by periphyton, mosses, macrophytes, fine particulate organic matter (FPOM: 0.5 ,um-1mm), and coarse particulate organic matter (CPOM: >1mm) was examined during the ice-free period (April to November) of 1979 and 1980 in First Choice Creek (first order; watershed area: 0.25 km2), Beaver Creek (second order; 1.83 kM2), Muskrat River (fifth order; 207 kM2), Matamek River (sixth order; 673 kM2), and the Moisie River (ninth order; 19871 kM2). As watershed area increases, primary production and the number of autotrophic groups are augmented with moss and macrophyte communities. Total 02 metabolism increases downstream, ranging from .76 in most cases). These data are combined with a geomorphological analysis of the watershed to discern the spatial distribution of aquatic metabolism, and to estimate total aquatic metabolism in the Moisie River drainage network. Mosses, occurring only in streams of fourth or higher order, are the most productive autotrophic component in the watershed (3.9 x 1010 g/yr); by comparison, periphyton produce only 2.1 x 1010 g/yr. FPOM is the most active detrital component (6.6 x 1010 g/yr). Geomorphic analyses show that streams of fourth or higher order comprise only 1.2% of the total number and 12.7% of the total length, but have 76.8% of the lotic surface area, and are responsible for 86.3% of the gross production by the entire lotic drainage network. The surface area of lakes is approximately six times that of streams, but their contribution (phytoplankton) to total aquatic ecosystem metabolism is comparable only to that of lotic periphyton. Results demonstrate that the River Continuum Concept can be extended to the watershed level by combining biological measurements with geomorphological analyses of the drainage network, thus allowing a detailed description of spatial and temporal patterns for specific metabolic components and total ecosystem metabolism to be constructed.

The Role of Benthic Macroinvertebrates in Detritus Dynamics of Streams: A Computer Simulation

Abstract: Detritus dynamics in Big Hurricane Branch, a second—order stream at Coweeta Hydrologic Laboratory in the southern Appalachian Mountains, were simulated with a computer model, using data from a variety of Coweeta stream studies. The model was used to evaluate the role of macroinvertebrates in the stream. Macroinvertebrates accounted for only a small portion of the respiration of detritus; their major role was conversion of benthic detritus into transported detritus. Macroinvertebrates were responsible for 27% of annual particulate organic matter (POM) transport, though when they were removed there was only a 10% reduction in POM transport because of a compensatory increase in storm transport. The contribution of macroinvertebrates to POM transport during nonstorm periods was much more significant, as high as 83% in late summer. Based on an annual budget, macroinvertebrates decrease the efficiency of detritus processing in low—order streams, because they increase transport loss. On a longer time scale, however, macroinvertebrates prevent accumulation of large amounts of detritus in the stream and major losses during infrequent large storms. By stabilizing long—term detritus export dynamics, they provide an important link between low—order and higher—order streams.

Seasonal succession in a natural daphnia assemblage

Abstract: This study uses descriptive data and field experiments to examine a seasonal succes- sion between two planktonic grazers (Daphnia rosea and Daphnia pulicaria) in a small Vermont lake. Each year Daphnia populations hatched from resting eggs during May and both species produced numerous parthenogenetic generations before the appearance of males and ephippia in late September. During the 2 yr of study, D. rosea had an apparent advantage in spring and early summer, but was rapidly replaced by D. pulicaria during late summer and autumn. These shifts in the species balance were associated with reversals in reproductive success. Moreover, the species with the higher birth rate also tended to have a lower mortality rate. An analysis of population structure, reproduction, and mortality suggests that inter- and intraspecific competition, not fish predation, limited population growth. Field experiments in both small (3.8-L) and large ( 12 OOO-L) enclosures demonstrate that the demise of D. rosea during late summer was a direct consequence of competition with D. pulicaria. However, the outcome of field experiments was sensitive to seasonally changing conditions. During spring and early summer, when D. rosea had a reproductive advantage at low densities in the lake, it was also the stronger competitor at high densities within experimental enclosures. The shift in species advantage during summer seems associated with a changing resource base. In spring the food for grazers was composed mostly of highly edible flagellates, while "resistant' algae (mostly gelatinous greens) were predominant during late summer and autumn. Grazing exper- iments and observations on gut contents suggest how D. pulicaria was favored by this change in food conditions. Grazing experiments with a radioactively labeled flagellate (Chlamydomonas) show that the two Daphnia species had very similar feeding rates on this alga. In addition, direct observations on the composition of gut contents demonstrate that they ingested virtually identical proportions of common phytoplankton species. However, experiments with labeled natural algae and observations on the quantity of ingested algae indicate that D. pulicaria was more effective in utilizing "resistant" algae.

Habitat Selection Attributes of an Avian Community: A Discriminant Analysis Investigation

Abstract: Habitat use characteristics of all bird species occurring in the lower Colorado River valley were studied. Seventy—two transects, each through homogeneous stands of vegetation and together representing all riparian habitat types occurring in the area, were censused three times monthly for 2 yr (1977, 1978) to provide distributional records for all species. From the census data we divided the transects into three groups for each bird species each season: transects where the species was recorded in both 1977 and 1978, transects where the species was absent both years, and transects where the species occurred in only one of the years. We used discriminant analyses to classify the habitat on each transect as suitable or unsuitable for each species, using a set of eight species composition measures, four foliage density measures, and foliage height diversity (FHD). Mean habitat discriminability of all species varied from a low of 86% in summer to a high of 91% in late summer. Most species showed extensive variability among seasons in both the degree of habitat selectivity and the criteria used in habitat selection. This variability reflected both changes in population structure of the species and responses to changing resource and climatic pressures. Ecological attributes of the species were examined to determine which factors shaped the habitat use patterns of each. Feeding guild played only a small role in both the degree and criteria of habitat selectivity, except for highly specialized feeders such as nectivores and frugivores. Species with restricted distributions were usually habitat specialists, but species with widespread distributions included both habitat generalists and specialists. During spring and summer nonresident, short—stay species showed greater habitat selectivity than did extended—stay species, but during nonbreeding seasons both short—stay and extended—stay species showed equivalent habitat selectivity. That both the mix of habitat generalists and specialists and the habitat selectivity of individual species varied seasonally implies that the organization of habitat use of the community is the dynamic product of many interacting factors. The importance of individual habitat attributes was also examined. Species showed preferences for particular habitat attributes significantly more frequently than they avoided habitat attributes. FHD was the most frequently important variable in differentiating areas used from areas not used. Proportions of honey mesquite, cottonwood, and willow in the vegetation were also frequently important variables in habitat selection by avian species, but usually in conjunction with FHD. Foliage density characteristics were less frequently significant, but for species where these measures were important, they tended to be the primary habitat selection criterion. In summer and late summer, habitat selection criteria of individual species seemed independent of the selection criteria of the other species in the community. In other seasons there was evidence of species interactions in these habitat selection patterns. These empirical findings for an entire avian community over all seasons are an advance in our understanding of how habitat use patterns of communities are organized. The extensive seasonal variability in habitat use by avian species and variability of distributions of individual species between years imply that studies of community organization can best be done with large—scale, long—term studies.

Community Organization of Arboreal Birds in Some Oak Woodlands of Western North America

Abstract: Community organization of the arboreal birds in two oak woodlands of western North America was examined during the course of two breeding seasons. A "natural experiment" comparing the arboreal avifaunas inhabitating oak woodlands of similar tree structure in northeastern Sonora, Mexico (referred to as Purica), and in the central coastal mountains of California, USA (referred to as Carmel), was used to address questions concerning (1) variables that account for variation in foraging behavior, (2) overall similarity in foraging behavior of the two avifaunas, (3) partitioning of resources within each avifauna, (4) guild structure, (5) foraging heights, (6) changes in foraging be- havior between sites, and (7) complementarity in resource use during foraging. At Purica there were 15 breeding species, and at Carmel 10 species. Foraging technique, portion of the tree, perch substrate and height, and the substrate from which food was derived during foraging were quantified with the aid of an audio metronome. The first three axes in a Principal Components Analysis (PCA) accounted for -75% of the variation in foraging behavior at both sites, and were associated with technique, food site, and height. The rank order of the PCA loadings for all foraging variables on each PCA axis was highly correlated between Purica and Carmel, indicating that the overall pattern of foraging at both sites was very similar despite differences in avifaunal composition, density, and consuming biomass. Analysis of species breadth and overlap during foraging indicated that at both sites foraging spe- cialization does not connote a decreased potential for species interactions. In addition, estimates of community breadth and overlap indicated that technique and food site were the most important foraging parameters in community organization at both sites. Three methods were used to develop the guild classification used in this paper: reciprocal averaging ordination, cluster analysis, and discriminant function analysis. At Purica five guilds were recognized: foliage gleaners, bark gleaners, bark probers, air salliers, and ground salliers. At Carmel four guilds were recognized: foliage gleaners, bark probers, air salliers, and ground salliers. In terms of relative guild consuming biomass (CB), foliage gleaners and bark gleaners at Purica together constitute 34% of the community. Interestingly, at Carmel a distinct bark gleaning guild is absent, and foliage gleaners constitute 32% of the community's CB. The proportion of community CB contributed by the other guilds was significantly different between sites. Although the possible functional roles performed within each community were similar (based on the kinds of guilds present), their guild structures were significantly different (based on the distribution of guild CB within each avifauna). The foraging space used by each of the guilds and the entire community was examined in detail. Foraging height of the birds was generally not related to the tree profile, although at both sites bark probers derived most of their food from upper heights, foliage gleaners mostly from intermediate to upper heights, and ground salliers perched at intermediate heights. In contrast, air salliers at Purica usually perched at lower heights, while at Carmel they used upper heights most frequently. At Purica 50% of the compensatory shifts in foraging were consistent with predictions of competition-based hypotheses, whereas at Carmel none of the predictions was verified. Interspecific complementarity of resource use was demonstrated only marginally in air salliers at Purica. It is concluded that (1) the foraging behavior of some species at Purica, especially air salliers, is consistent with predictions from competition theory, although a lack of interspecific complementarity indicates that the competitive forces are rather weak, (2) at Carmel, opportunism appears to be more important in affecting the pattern of foraging behaviors, (3) other factors, especially ground cover, shrub density, and the local weather, have influenced foraging behavior and community structure at both sites, (4) similarity between sites in avian community structure is a consequence of the similarity in tree structure, whereas differences between sites probably result from short-term behavioral flex- ibility in microhabitat use in response to differences in prey distribution and abundance, and (5) interspecific competition is not a prerequisite for resource partitioning.

Population Dynamics and Habitat Suitability of the Intertidal Sea Anemones Anthopleura elegantissima and A. xanthogrammica

Abstract: Population dynamics of the intertidal sea anemones Anthopleura elegantissima (Brandt) and A. xanthogrammica (Brandt) were investigated by long—term monitoring and experimental manipulation of populations in Washington State. Sizes of individual A. xanthogrammica and A. elegantissima fluctuated seasonally, increasing during the spring and early summer and shrinking or reaching a pleateau the rest of the year. Data from marked individual A. xanthogrammica showed that adults (>6.5 cm) moved very little and may persist for at least several decades. Only 3 of 160 mapped adult anemones disappeared over 2 yr. Juveniles moved more often and thus appeared and disappeared at much higher rates. Growth of A. xanthogrammica in control and experimental removal areas was compared by a new technique. One—year growth increments were used to compare growth in each size class by analysis of variance and were also used to plot long—term growth trajectories and expected reproductive success (habitat suitability, Fretwell 1972...