Showing papers in "Ecological Monographs in 1984"

Pseudoreplication and the Design of Ecological Field Experiments

Abstract: Pseudoreplication is defined. as the use of inferential statistics to test for treatment effects with data from experiments where either treatments are not replicated (though samples may be) or replicates are not statistically independent. In ANOVA terminology, it is the testing for treatment effects with an error term inappropriate to the hypothesis being considered. Scrutiny of 176 experi- mental studies published between 1960 and the present revealed that pseudoreplication occurred in 27% of them, or 48% of all such studies that applied inferential statistics. The incidence of pseudo- replication is especially high in studies of marine benthos and small mammals. The critical features of controlled experimentation are reviewed. Nondemonic intrusion is defined as the impingement of chance events on an experiment in progress. As a safeguard against both it and preexisting gradients, interspersion of treatments is argued to be an obligatory feature of good design. Especially in small experiments, adequate interspersion can sometimes be assured only by dispensing with strict random- ization procedures. Comprehension of this conflict between interspersion and randomization is aided by distinguishing pre-layout (or conventional) and layout-specifit alpha (probability of type I error). Suggestions are offered to statisticians and editors of ecological j oumals as to how ecologists' under- standing of experimental design and statistics might be improved.

Patch Dynamics and Stability of Some California Kelp Communities

Abstract: This paper considers three concepts of stability as they relate to the dynamics of distinctive patch types of algal canopy guilds in southern and central California kelp communities: (1) persistence of a patch through more than one generation of the dominant species, which was evaluated by using life tables and observations of patch borders; (2) inertia or the resistance of different patches to invasion or disturbance, which was evaluated by artificially enhancing gametophytes by transplanting sporogenic material, by removing canopy, and by evaluating some important disturbance processes; and (3) resilience or recoverability of a patch following a perturbation sufficient to allow invasion of different species, which was studied by defining some of the mechanisms of successful invasion or succession. By working in distinct habitats in southern (Pt. Loma and Santa Catalina Island) and central (Pt. Piedras Blancas) California, we could evaluate different types of physical stresses as they related to these stability concepts. Taller perennial canopy guilds were dominant competitors for light, but were more susceptible to physical wave stress. Dominance hierarchies in the competition for light appeared to be reversed in areas exposed to increasing wave stress. The main causes of mortality at Pt. Loma were entanglement with storm—dislodged Macrocystis plants and, in some areas, sea urchin grazing. Mortality in central California was due to winter storms. In most cases, distinct patches resisted invasion for >10 yr. The mechanisms of resistance involved (1) competition for light and, possibly, nutrients, and (2) limits to spore dispersal. When succession occurred, it was often mediated by many factors, including seasonality of spore production, which coincided with winter storm—related mortalities; mechanisms of kelp dispersal, which were most effective via drifting plants and fragments of fertile material held against the substrate by invertebrates; and survivorship of gametophytes and small sporophytes, which was influenced by local scour and grazing. Appropriate spatial scales, stability, and succession studies in these kelp communities were determined by the size of the disturbed area, which varied from the free space resulting from detachment of single plants to the free space resulting from catastrophies such as overgrazing or unusual storms. Temporal scales were influenced by seasonality of disturbance and algal reproductive condition and aperiodic episodes of cool, nutrient—rich water advected into the patch. There appeared to be conflicting morphological adaptations of the canopy guilds: exploitation of light was enhanced at higher canopy levels, whereas the lower canopy levels were better adapted to tolerate stress from wave surge. The adaptations of the algae appeared to form four distinct groups of tactics: (1) ruderals or plants, such as Nereocystis and Desmarestia, with opportunistic life histories; (2) kelps, such as Macrocystis, adapted to exploitative competition for light and nutrients; (3) kelps (Eisenia, Dictyoneurum) adapted to physical stress such as wave surge; and (4) those algae, such as corallines and Agarum, adapted to heavy grazing. Within any given area, the relative patch stability was determined by biological relationships; between areas, the patch stability patterns were attributable to physical differences.

Invertebrate productivity in a subtropical blackwater river: the importance of habitat and life history'

Abstract: Habitat and life history are critical elements in assessing the production dynamics of invertebrates and their role in aquatic ecosystems. We studied invertebrate productivity at two sites in a subtropical blackwater river (the Satilla) in the Lower Coastal Plain of Georgia, USA, and found that submerged wooden substrates, or snags, are heavily colonized by aquatic insects. We compared invertebrate productivity on the snag habitat with productivity in the sandy benthic habitat of the main channel, and the muddy benthic habitat of the backwaters. The size-frequency method was applied to individual taxa in order to determine total invertebrate productivity. Emphasis was placed on the importance of the length of larval life, or the cohort production interval, in determining biomass turnover rates. The diversity of taxa was much higher on the snag habitat than in either of the benthic habitats. Filter-feeding caddisflies (especially Hydropsyche spp.) and black flies (Simulium spp.) were the major consumers on the snag habitat. Several species of midges, mayflies, and beetles also were abundant. Total densities, standing stock biomass, and production were very high for primary consumers on snags. Annual production was 51.9 and 67.1 g m-2. yr- I (dry mass per surface area of snag, or effective habitat) for the two sites. Hellgrammites, dragonflies, and stoneflies were the major insect predators colonizing snags, and their production was 5.5 and 5.2 g m-2 yr-t (effective habitat). Annual pro- duction/biomass ratios (P/B) were usually 5-10 for insects that had univoltine or bivoltine life cycles. Annual P/B estimates were very high for midges (> 100) and black flies (>70), since length of larval life was estimated to be very short. The sandy-substrate benthos consisted almost exclusively of very small midges with oligochaetes of lesser abundance. Densities were quite high (>20 000/M2), but biomass was very low (I 100 mg/ m2 or less). Production of primary consumers was >11 g m-2 yr- t with a very high estimate of annual P/B (166-227). The major predators were Ceratopogonidae (biting midges) larvae with an annual production of 1.6-2.6 g.m-2 yr-t. The muddy-substrate benthos consisted primarily of oligochaetes (Limnodrilus) and midges. Annual production was 7-10 g.m-2 yr-t for primary consumers. The major predators were larger Tanypodinae midges. On a substrate surface area basis, standing stock biomass on snags was 20-50 times higher than in the sandy habitat and 5-10 times higher than in the muddy habitat. Production on snags was only 3-4 times higher than production in the benthic habitats, with higher annual P/B in the latter. The production estimates for the snag habitat are among the highest yet reported for lotic ecosystems, and it appears that production on snags is limited by available substrate. Habitat areas per length of shoreline were estimated so that we could approximate relative amounts of biomass and production for a stretch of river. Although the snag habitat accounted for only 6/6% of the effective habitat substrate over a stretch of river, it was responsible for over half of invertebrate biomass, and 15- 16% of production. Taxa within each habitat were categorized to functional feeding groups, and habitat-specific func- tional groupings were evaluated using numbers, biomass, and production. Filtering collectors pre- dominated on snags, and gathering collectors in benthic habitats. When corrected for habitat abun- dance, the distribution of biomass among filtering collectors, gathering collectors, and predators was very close. However, the distribution of production was 12% filtering collectors, 71% gathering collectors, and 17% predators. We suggest that production is the most meaningful parameter to consider in functional group analysis and that the use of numbers or biomass alone can sometimes result in misleading conclusions. As a middle order (5th-6th) stream, the distribution of production or biomass among functional groups in the Satilla River differs considerably from that predicted by the river continuum concept, predicting a high percentage of grazing consumers.

A Study of Avian Frugivores, Bird-Dispersed Plants, and Their Interaction in Mediterranean Scrublands

Abstract: Fruit production and patterns of seed dispersal by birds were studied at two elevations in the Mediterranean scrublands of southern Spain. Fleshy-fruit-producing species represent a very prominent fraction of woody plants in terms of cover (57-76%) and species number (49-66%). Fruit production occurs year round in the lowland site but is confined to August-February upslope. Ripe fruits are most abundant (>105 ripe fruits/ha) in November-December. Fruit abundance fluctuates widely between years at the highland locality but only slightly in the lowlands. In both communities, the dominant species ripen fruits in autumn-winter, display the highest within-plant fruit densities, and tend to have the most lipid-rich fruits. Fruits differ in pulp nutritive value, seediness, and relative amount of pulp among species but are remarkably uniform in size (mostly 5-10 mm transverse di- ameter). Two-thirds of the passerine species at each site eat some fruit. Of these species, 69% (highland) and 26% (lowland) are resident "fruit predators," feeding on either pulp or seeds alone, and damaging the seeds when eating pulp and seeds together. The rest are overwintering or migratory seed dispersers that ingest whole fruits without damaging seeds. Seed dispersers are most common in late autumn- winter, coincident with the peak in fruit abundance and the predominance of lipid-rich fruits. A few small (12-18 g body mass) disperser species (Erithacus rubecula, Sylvia atricapilla, Sylvia melano- cephala) account for most of the frugivory at each site and disperse the majority of seeds. Fruit predators either are relatively scarce or eat fruits infrequently, or fruits represent a negligible fraction of their diets. Fruit removal was very high (89-100% of crops) among species with fruits smaller than the gape width of the abundant small-sized dispersers, and very low among species with fruits larger than gape width. Removal success was negatively correlated with fruit size among species having fruits smaller than dominant dispersers' gape width. No relation has been found between removal success and fruit quality, fruiting time, ripening rate, or within-plant fruit density. The principal dispersers at each site ate mainly the most nutritious fruits, although not to the exclusion of less nutritious fruits. Substantial pairwise plant-bird reciprocity is not common. (The avian species disperses a substantial fraction of a plant's seeds, which in turn provide the bulk of the bird's energy supply.) Current bird-plant seed dispersal interactions are the result of evolutionary, climatic, and geo- graphical factors in the Mediterranean. Mutualistic congruency largely is, in these cases, an epiphe- nomenon of these factors, not resulting necessarily from mutual adaptations (coevolution). It is sug- gested that actual coevolution involving a smaller set of bird and plant species may facilitate the persistence of noncoevolving (or very slowly coevolving) plant species, thus favoring the existence of a chronic "anachronism load" (with regard to dispersal) in the plant community.

Population Dynamics of the Spruce Budworm Choristoneura Fumiferana

Abstract: Using the latest observations, experiments, and theoretical studies, I have reanalyzed spruce budworm data from the Green River Project, and now propose a new interpretation of the population dynamics of the species. Spruce budworm populations in the Province of New Brunswick have been oscillating more or less periodically for the last two centuries, the average period being °35 yr. Local populations over the province tend to oscillate in unison, though their amplitudes and mean levels are not always the same. The local population process in the spruce bud worm is composed of two major parts, a basic oscillation, and secondary fluctuations about this basic oscillation. The basic oscillation is largely determined by the combined action of several intrinsic (density—dependent) mortality factors during the third to sixth larval instars. These factors include parasitoids and, probably, diseases (e.g., microsporidian infection), and, most important, an intriguing complex of unknown causes, which I term "the fifth agent" (a large number of larvae with no clear symptoms died during the population decline in the late 1950s). Other mortality factors, including predation, food shortage, weather, and losses during the spring and fall dispersal of young larvae, are not causes of the basic, universally occurring oscillation. Because of immigration and emigration of egg—carrying moths, the ratio of all eggs laid to the number of locally emerged moths (the E/M ratio, or the apparent oviposition rate) fluctuates widely from year to year but independently of the basic oscillation of density in the local populations that were studied. The fluctuation in this ratio is the main source of the secondary fluctuation in density about the basic oscillation, and is highly correlated with the meteorological conditions that govern the immigration and emigration of moths. The E/M ratio is the major density—independent component of budworm population dynamics. Contrary to common belief, there is no evidence to indicate that invasions of egg—carrying moths from other areas upset the assumed endemic equilibrium state of a local population and trigger outbreaks. Moth invasions can only accelerate an increase in a local population to an outbreak level, but this happens only when the population is already in an upswing phase of an oscillation caused by high survival of the feeding larvae. In other words, the "seed" of an outbreak lies in the survival of feeding larvae in the locality, and moth invasions can act only as "fertilizers." The weight of evidence is against the idea that an outbreak occurs in an "epicenter" and spreads to the surrounding areas through moth dispersal. Rather, the egg mass survey data in New Brunswick since 1952 favor an alternative explanation. If the trough of a population oscillation in a certain area stays comparatively high, as in central New Brunswick in the 1960s, or if the area is more heavily invaded by egg—carrying moths when the populations in that area are in an upswing phase, these populations might reach an outbreak level slightly ahead of the surrounding populations, all of which are oscillating in unison. If a local population oscillates because of the action of density—dependent factors intrinsic to the local budworm system, it may appear to be difficult to explain why many local populations over a wide area oscillate in unison. However, Moran's (1953) theory shows that density—independent factors (such as weather) that are correlated among localities will bring independently oscillating local populations into synchrony, even if weather itself has no oscillatory trend. I illustrate this with a simple time—series model. The same model also illustrates a principle behind the fact that outbreaks occurred fairly regularly in New Brunswick and Quebec during the past two centuries but rather sporadically in other regions of eastern Canada. Finally, I review the commonly accepted theory of outbreaks based on the dichotomy of endemic and epidemic equilibrium states and argue that the theory does not apply to the spruce budworm system.

Compensatory Recruitment, Growth, and Mortality as Factors Maintaining Rain Forest Tree Diversity

Abstract: One general hypothesis to explain how forest tree diversity is maintained is that rarer species are favored over commoner species in their reproduction, growth, and/or mortality. Mechanisms acting in this way would continually compensate for the tendency of some species to increase at the expense of others, and would reduce the chance of local extinction of rare species. Two hypotheses concerning such compensatory mechanisms were tested in subtropical and tropical evergreen rain forests in Queensland, Australia. Hypothesis 1: on the scale of 1—2 ha, commoner species have lower rates of recruitment and growth and higher rates of mortality than do rarer ones. This hypothesis was tested using abundances either of adults or of members of the same size—class, and was rejected for growth and mortality and for recruitment of over—story species, but not rejected for recruitment in subcanopy and understory species in either forest. Hypothesis 2: the close proximity of other individuals is more deleterious (i.e., causes slower growth or higher mortality) if they are the same species than if they are different species. This hypothesis was accepted for growth or survival of nearest neighbors in several of the seedling size—classes in both forests. In contrast, increased densities of the same species in quadrats had no more deleterious effect on growth and survival than did increased densities of different species. At the scale of proximity to adults, hypothesis 2 predicts that young trees have higher mortality nearer conspecific adults than farther away. In both forests, 90% of the species tested showed no such pattern of mortality of seedlings or saplings, nor was the strength or direction of the deviation from equal mortality correlated with the abundance of adults of that species. Field experiments gave the same results. In summary, tests of both hypotheses showed that some compensatory trends occurred and that these were very similar in the two forests. The mechanisms producing these compensatory trends may be attacks by natural enemies (grazers, pathogenic fungi, etc.), interference, or, less likely, competition for resources.

Fire and Other Factors Controlling the Big Woods Vegetation of Minnesota in the Mid‐Nineteenth Century

Abstract: Bearing-tree data from the original land-survey records of 1847-1850 were used to reconstruct the vegetation of the Big Woods and adjacent areas along the prairie-woodland border in south-central Minnesota. The characteristic tree taxa of the Big Woods were elm (Ulmus), basswood (Tilia americana), sugar maple (Acer saccharum), ironwood (Ostrya virginiana), bitternut hickory (Carya cordiformis), butternut (Juglans cinerea), and ash (Fraxinus). The most common tree was elm, which comprised 27% of the bearing trees. A buffer zone of fire-tolerant oaks and aspen generally lay between the Big Woods and prairie. The width of this zone depended on topography and on the presence of additional firebreaks, which in places formed sharp boundaries between the Big Woods and oak-aspen. The prairie-woodland border was characteristically a sharp boundary along firebreaks (water bodies and physiographic breaks). In some places very effective firebreaks formed sharp bound- aries between prairie and the Big Woods, with no intervening oak-aspen zone. The vegetation was most strongly correlated with the fire-probability pattern, which was a function of both abiotic and biotic factors. Soils influenced the probability of fire, but they also were the major factor controlling the vegetation within areas of similar fire probability. Soil drainage was the most important factor controlling vegetation within the units of the overall pattern. Because the locations of firebreaks and the existing pattern of vegetation controlled the fire prob- ability pattern, sites with virtually identical physical characteristics supported qualitatively different types of persistent or stable vegetation.

Disturbance and Recovery in Intertidal Pools: Maintenance of Mosaic Patterns

Abstract: The species composition of pools in the intertidal zone on the coast of Washington State varies greatly from pool to pool and from time to time. While assemblages change somewhat predictably from the low— to the high—intertidal zone (presumably owing to different stress tolerances of the species), the variance among pools at a given tidal height cannot be ascribed to such physical factors. Some pools at each height are dominated by one species that monopolizes space on the rock or in the water column and modifies the pool environment. Each dominant species, once established, can spread rapidly through a pool (either by vegetative growth or by enhanced recruitment of its conspecifics) and is thus potentially self—perpetuating. When abundant, most dominants appear to prevent potential competitors from settling and surviving by monopolization of resources, abrasion of the substratum, and/or collection of sediment. Six such dominants were identified for Washington tidepools: from low to high pools, these are (1) the surfgrass Phyllospadix scouleri, (2) articulated coralline algae, (3) the mussel Mytilus californianus (exposed shores), (4) the cloning anemone Anthopleura elegantissima (more protected shores), (5) the red alga Rhodomela larix, and (6) the green alga Cladophora sp. Colonial diatoms also appear capable of dominating low pools in the absence of wave disturbance. However, each dominant monopolizes only 20—50% of the pools at any height. Disturbances, defined here as a loss of biomass exceeding 10% cover of a sessile species within 6 mo and caused by extrinsic forces, were observed frequently in regularly censused tidepools. Disturbance agents included waves, excessive heat, wave—driven logs or rocks, and unusual influxes of predators and herbivores. Severe disturbances (those affecting a large proportion of the organisms in a pool) tended to occur in high pools in the summer (due to heat stress) and low pools in the winter (due to wave damage). Overall, a disturbance occurred in every pool studied an average of every 1.6 yr. About half of the 231 observed disturbances affected one of the six dominant species. The frequencies of these disturbances ranged from one every 2—5 yr, and recovery of the species to its original level required 3 mo to > 2 yr. Some species (e.g., Rhodomela) were disturbed frequently bu recovered quickly because of rapid vegetative growth. However if asexual propagation was not possible, such as when the entire population of a species was removed from a pool, the slowness and irregularity of recruitment of sexual propagules greatly impeded recovery. Experimental manipulations involving the total removal of dominant species from pools showed that such large disturbances often require > 3 yr for recovery. The irregularity of planktonic recruitment can be compounded by the presence of herbivores, which can remove most settling organisms from the substratum, or by the absence of other organisms that are necessary for the settlement of a dominant (e.g., seed—attachment sites for Phyllospadix). The combination of high disturbance frequency and slow rates of recovery makes it impossible for any dominant to occupy all the pools in its tidal range at any one time. Disturbance is viewed in these habitats as the stochastic factor overlying other, more predictable, community—structuring factors such as tidal height, pool size, wave exposure, and levels of herbivory, predation, and competition. Thus combined deterministic processes and random events operate to produce a complex mosaic of species assemblages in pools in one region. None of the tidepool assemblages is "stable" over many generations; rather, they seem to exist in a dynamic state where disturbances are an integral structuring factor.

Darwin's Finches (Geospiza) On Isla Daphne Major, Galapagos: Breeding and Feeding Ecology in a Climatically Variable Environment

Abstract: The Darwin's finches on Isla Daphne Major, Galapagos, were studied between July 1975 and June 1978. Geospiza fortis and G. Scandens are residents, while G. fuliginosa and G. magnirostris are regular immigrants. The Daphne climate is unpredictably dry. The island has a simple plant community displaying marked annual and spatial variation in the foods which form finch diets. Breeding is stimulated by rain falling irregularly between January and April; G. scandens laid eggs with as little as 16 mm of rain, but G. fortis required 35 mm or more before laying. G. scandens consistently bred prior to the rains, associated with specialized exploitation of dry season Opuntia cactus flowers. The breeding system of both species was similar to that of other Geospiza species: monogamous matings on small, permanent, all—purpose territories. Reproductive output of both species varied. In 1976 single broods were produced by both species at high densities, with modes of three young. In 1977, only 24 mm of rain fell during the breeding season and G. scandens alone bred, with poor success. By 1978, G. scandens populations had declined by 66% and G. fortis by 85%. In 1978, both species laid an average of three clutches per pair, with a mode of four young per brood. During the 1977 drought, the sex ratio became skewed in favor of males in both species, and as a consequence some females bred successively with up to three different males in 1978. The skewed sex ratio retarded population recovery following the drought. The population decline was associated with a decline in seed abundance in the drought. As food disappeared, G. fortis diets widened to include a broad selection of food items, while G. scandens diets contracted to Opuntia seeds. Even in normal years, both species showed pronounced seasonal variation in feeding habits. Both fed heavily on Opuntia flowers in the late dry season, followed by a mixed diet of insects, fresh seed, and other green matter during the breeding season. After breeding, the birds fed primarily on seeds, the two species selecting different proportions of the available range of seed sizes. Overall, G. fortis displayed the feeding, breeding, and population dynamics characteristic of an ecologically generalized species, whereas G. scandens is a highly specialized species. Large variation in food supply, caused by large variation in rainfall, is responsible for the presence of only two species with breeding populations, for large fluctuations in their population sizes, and for their large clutch sizes and opportunistic breeding. Long—term studies are essential for an understanding of communities in such variable environments because some important events are rare.

Nitrogen Budget for a Small Coniferous Forest Stream

Abstract: An annual nitrogen budget is presented for a small stream draining Watershed 10, H. J. Andrews Experimental Forest, Oregon. The role of allochthonous debris in the input, flux, and export of nitrogen is emphasized in the material balance budget. All material entering the stream channel was presumed to enter the water sometime during the year. Material estimates are based on total channel area. The major annual nitrogen input (1974-1975) was subsurface flow (11.06 g/m2) as dissolved organic nitrogen (10.56 g/m2) and nitrate (0.50 g/m2). Biological inputs of nitrogen amounted to 4.19 g/m2 as direct terrestrial inputs of: litterfall (1.35 g/m2), lateral movement (1.78 g/m2), and throughfall (0.30 g/m2). Nitrogen fixation on fine wood debris contributed an additional 0.76 g/m2 based on rates from a nearby watershed. Total nitrogen input was 15.25 g/m2. The nitrogen pool was dominated by large amounts of particulate organic matter. Coarse wood constituted 32% of the nitrogen pool (3.80 g/m2) and fine wood fractions 18% (2.18 g/m2). The coarse wood fraction greatly influenced stream morphology. Fine organic particulates constituted an addi- tional 40% of the nitrogen pool (4.77 g/m2). DON (dissolved organic nitrogen) export (8.38 g/m2) was less than input, presumably due to biological uptake associated with litter mineralization, sorption, and chemical flocculation. Due to effective retention of particulate inputs by debris dams, biological processing in the particulate nitro- gen pool, and uptake and sorption of DON, most particulate organic inputs increased in nitrogen concentration prior to export. Particulate organic nitrogen input (3.13 g/m2) was greater than export (2.53 g/m2). Total annual nitrogen output was 11.36 g/m2, resulting in a gain of 3.89 g m-2 yr-I to the stream. Thus, the stream was not operating on an annual steady state, but on an input-output regime related to the processing of refractory wood debris and resetting by major storms. Although particulate and dissolved nitrogen loss per hectare was small for the 10-ha watershed, these losses passed through or were accumulated in a pool encompassing < 1% of the watershed area. This concentration of N in the stream allowed establishment of a separate ecosystem whose processing efficiency and capabilities for nutrient cycling were related to the retention capacity of the channel and nutrient quality of inputs within the reach.

Density, biomass, productivity, and nutrient-cycling changes during stand development in wave-regenerated balsam fir forests

Abstract: Regeneration waves in the high-altitude fir forests of the northeastern United States create a gradient of stands of different ages that are ideal for research on forest developmental processes. Changes in density, biomass, productivity, litterfall, and nutrient content of the vegetation over the course of stand development were documented in a series of these stands on Whiteface Mountain, New York. After a regeneration wave passes through an area and the overstory dies, formerly suppressed seedlings are released, and stand density increases to 10-13 trees/M2 at about age 10 (counting only stems over 25 cm tall). Stand density then begins to decrease, at first slowly but then more rapidly, with a maximum mortality rate of 20-25%/yr between years 20 and 30. Thereafter mortality continues, but the rate decreases over time to t4%/yr at age 55. After about age 20 the relationship between stand age and stand density is p = 0.04 - exp( 115/AGE). For stands that have begun to self-thin, the relationship between stand density and mean aboveground tree mass is log10 mn = 3.94 - 1.24 log1op, with a slope significantly different from - 1.5. Basal area (measured at 25 cm) increases for the first 15-20 yr of stand development, then remains constant at 60-70 m2/ha for the remainder of the life of the stand. Total aboveground biomass of a mature fir stand is z 11.8 kg/M2 (1.5 kg/M2 foliage, 1.7 kg/M2 branches, 0.9 kg/M2 bole bark, and 7.7 kg/M2 bole wood). The biomass accumulation rate is highest during the first decade of stand development (320 g m-2 yr- 1), and declines to z 115 g m-2 yr- I for a 55-yr-old stand. Foliage biomass increases rapidly during the first 10-15 yr of stand development, then remains constant after crown closure. Branch biomass increases until about age 15, declines for about a decade, then increases again for the rest of the life of the stand. Bole bark increases until about age 30, then stabilizes. Bole wood continues to increase throughout the life of the stand. Aboveground net primary productivity for a mature stand is 960 g m-2 yr-I (foliage, 380 g m-2 yr-1; twigs, 150 g-m-2 yr-1; branch wood and bark, 170 g.m-2 yr-1; bole bark, 25 g-m-2-yr-'; and bole wood, 235 g.m-2 yr-1). Productivity remains generally constant as the stand develops, but in the early stages almost 45% of the total aboveground production is devoted to producing bole wood and bark, while in a mature stand this proportion drops to z 25%, with the remainder going to foliage and branch wood production. There is thus a shift from production of boles to branches as the stand ages. This may reflect a shift in the primary stresses on the trees, from intraspecific competition in the younger stands to physical environmental factors (e.g., wind abrasion and rime formation) in the older stands. After the initial increase, woody tissue respiration remains approximately constant throughout the life of the stand, as an increase in the size and height of boles is balanced by death of small trees and a decrease in total bole surface area. A mature fir stand contains nitrogen, potassium, magnesium, and calcium in the amounts 47.7, 18.1, 4.6, and 30.4 g/m2, respectively. All these nutrients accumulate rapidly in the early stages of stand development as the stand accumulates nutrient-rich foliage. This is particularly true for nitrogen; a 9-yr-old stand contains half as much nitrogen as a 60-yr-old one. However, the difference between nutrient uptake by the trees and nutrient release in decomposition is greatest somewhat later in stand development, when the pulse of litter accompanying stand die-off has decomposed but the young stand is still taking up significantly greater quantities of nutrients than it is returning in litterfall. Thus if ecosystem nutrient losses are controlled by nutrient accumulation in living biomass, the period of maximum nutrient conservation should occur in the years immediately after disturbance, when the stand is accumulating nutrients most rapidly. If the critical factor is the balance between uptake and release, then minimum losses should occur later. Increased meteorologic inputs of nitrogen and acid in the high-altitude fir forests may have altered the natural nutrient cycles of the preindustrial era.

Carbon and Nitrogen Dynamics During the Decomposition of Litter and Roots of a Chihuahuan Desert Annual, Lepidium Lasiocarpum

Abstract: Carbon and nitrogen dynamics were analyzed during the decomposition of litter and roots of the desert ephemeral pepperweed (Lepidium lasiocarpum). We treated litter bags with the insecticide chlordane and the fungicides benomyl and captan to eliminate or restrict groups of soil biota. The mass losses of buried litter (51, 39, and 25% for untreated, insecticide—treated, and fungicide—insecticide—treated material, respectively) were higher than those of the respective root treatments (35, 18, and 15%) at 96 d. The mass loss of untreated material was correlated with numbers of detritivorous—fungivorous microarthropods, and only a small percentage of this loss was as CO2: 27 and 42% for litter and roots, respectively. In the absence of microarthropods a higher percentage of mass—loss carbon could be accounted for as CO2: 33 and 76% for litter and roots, respectively, indicating that mass loss was due primarily to litter removal by microarthropod activity and not to mineralization. Litter removal by microarthr...

Comparative Dietary Ecology of Sympatric, Insectivorous Neotropical Flycatchers (Tyrannidae)

Abstract: This study compares and contrasts diets, based on 2834 prey individuals from 126 stomachs, of 16 insectivorous, aerially foraging Neotropical flycatcher species (Tyrannidae) that are seasonally sympatric in the humid Caribbean lowlands of Costa Rica. Dietary parameters examined are prey type, diversity (breadth) of prey types, heterogeneity of prey types among individuals of a species, "patchiness" of morphologically indistinguishable prey within individual stomachs, and number of prey items per stomach. An R—type factor analysis of prey taxa in flycatchers' stomachs delimited four interpretable factors (axes) based on differences in prey detectability, location, escape behavior, and substrate where caught. A Q—type cluster of flycatcher species, based on factor scores from the analysis of prey variables, tended to juxtapose congeners. Noncongeners that clustered closely included a pair of species (Todirostrum sylvia and Oncostoma cinereigulare) that tend to replace each other geographically. Some flycatchers failed to cluster with any other species. "Saturation curves" of prey—type diversity against number of stomachs sampled showed (1) adequate sampling effort for most species with 4—10 stomachs, and (2) species differences in breadth of prey types eaten. The flycatchers that did not cluster with any other species on the basis of prey taxa had the largest and smallest diet breadths of all species studied. Two species are exceptional and consistent specialists: the Ruddy—tailed Flycatcher (Terenotriccus erythrurus) ate 94% Homoptera (largely Fulgoroidea), and the Long—tailed Flycatcher (Colonia colonus) ate 67% stingless bees (Trigona species). Flycatcher species that had the most "patchy" diets (i.e., many individuals of the same prey species within a stomach) tend to hawk flying prey and/or reside in open country or forest canopy. Colonia colonus had extremely patchy stomach contents, but unlike other flycatchers that hawk flying prey, it had an exceptionally homogeneous diet. "Patch feeders," compared with other flycatchers, tended to have many more items per stomach, suggesting the selection of many, relatively small prey per unit time. Significant variation in numbers of items per stomach, even among congeners, suggested differences in feeding rate and perhaps prey size. Comparison of species with repect to these dietary parameters helps identify a "food resource" for each species. The identification of the food resource (1) helps interpretation of foraging behavior of the predators, (2) facilitates discussion of prey—type and prey—size selection, (3) delineates potential ecological and evolutionary routes of species interactions, and (4) focuses attention on the nature of the food supply available to each species. The assumption that species with patchy and heterogeneous diets feed opportunistically on relatively ephemeral prey permits the following conclusions: (1) many tropical flycatchers are not opportunistic, (2) many guilds, including tropical ones, are composed of species with a variable degree of opportunism, (3) migrants are more opportunistic while wintering (in Caribbean Costa Rica) than syntopic year—round residents, and (4) open—country and canopy flycatchers tend to be more opportunistic than forest—interior species. Variation in flycatcher diets with respect to all parameters examined necessitates multiple explanatory hypotheses, and warrants a pluralistic approach to questions of community structure in these birds.

The Development and Field Test of a Tactical Model of the Planktivorous Feeding of White Crappie (Pomoxis Annularis)

Abstract: A computer model was developed to predict the feeding selectivity of planktivorous white crappie (Pomoxis annularis) from a known distribution of zooplankton. The model was based on the assumption that each predation event could be subdivided into a series of independent steps: prey location, pursuit, attack, and retention. The probability that white crappie successfully completed each step was determined for potential zooplankton prey species in a series of laboratory experiments. The four steps were then incorporated into a stochastic model where the probability of a particular prey type being consumed is equal to the product of the probabilities of the individual steps. The model was field tested by sampling fish, zooplankton, and physical parameters from discrete depth strata in a small reservoir on nine dates from October 1978 through November 1979. The model proved to be very accurate at predicting the species and size distribution of the ingested prey across the range of light intensities, turbidities, temperatures, and zooplankton densities encountered. Prey consumption could not be characterized as simply size selective; rather, it reflected the selectivity expressed at each step in the feeding cycle.

Ecological Energetics and Foraging Behavior of Overwintering Bald Eagles

Abstract: The ecological energetics and foraging behavior of overwintering Bald Eagles (Haliaeetus leucocephalus) were studied in 1978—1979 and 1979—1980 on the Nooksack River in northwestern Washington and in the laboratory. We investigated eagle energy requirements and energy relationships in a winter environment, and adaptations to winter energy and food (chum salmon [Oncorhynchus keta]) stress. We developed a model to predict the energy metabolism of free—living eagles. Information of food consumption and energy requirements of captive Bald Eagles was supplemented with laboratory and field data to determine energy costs of free existence. Basal metabolic rate, as measured by oxygen consumption on four eagles, was 11.595 kJ°g— ¹°h— ¹; a lower critical temperature of 10.6°C was recorded. Standard metabolic rate increased linearly with decreasing temperature. Thermal conductance was 0.347 kJ°g— ¹°h— ¹°°C— ¹. The diel range in body temperature was from 38.9° to 41.2°. Energy metabolism was measured in response to artificially induced rainfall. During 40 trials of 4 h of continuous rain, metabolism increased up to 9 and 21% at 6.1 and 22.2 cm/h rain levels, respectively. Effects of rain on heat loss by wild eagles on the Nooksack River were negligible even though precipitation was high. Data on metabolic responses were incorporated in a black—body heat—budget model which predicted energy costs of free existence. Ambient temperature, wind velocity, long—wave radiation, and rainfall data collected at three microhabitats used by eagles also were incorporated into the heat—budget model to determine heat production by wild birds. Activity budgets were assessed by tracking four radio—tagged eagles for 38 d. Flight activity occurred only during 1% of the 24—h day, and energy costs of flight (110 kJ/d) were included in the model. The daily energy budget (total energy metabolized), daily energy consumption (total food energy required), and daily food (salmon) consumption (total mass of food required) were 1703 kJ, 2068 kJ, and 489 g, respectively, for a 4.5—kg Nooksack eagle. Energy and food needs of free—living eagles were °10% more than for captive eagles. Most eagles roosted in coniferous rather than deciduous forest even though they expended more energy to travel there. By roosting in conifers they accrued a net daily energy savings of 61 kJ, or °5% of the daily energy budget after accounting for energy costs of flight to and from the roost. Energy savings in this protected microclimate were afforded by higher ambient temperature and long—wave radiation levels, and lower wind velocity; reduced rainfall had little effect. Social interactions by feeding eagles were quantified during 46 d of observation. Adult eagles were dominant over younger birds and were more successful at kleptoparasitizing (stealing) food. Kleptoparasitism was the primary means by which food was acquired. Interaction frequency averaged 0.27 interactions/min during feeding and was positively correlated with the size of the feeding group. Juveniles and subadults were less effective at feeding than adults. They consumed 410 and 459 g°bird— ¹°d— ¹ of salmon, respectively, while the adults ingested 552 g°bird— ¹°d— ¹. Young eagles failed to acquire the needed 489 g°bird— ¹°d— ¹ of salmon. The effects of this socially mediated food deprivation resulted in a less than optimal use of time and energy by young birds. When food is limited, young eagles probably incur the highest mortality rates. Our data indicate that overwintering Bald Eagles are effective at exploiting and conserving energy. They maximize energy gain by foraging in groups, gorging, and assimilating more energy during cold stress. They minimize energy loss by becoming sedentary, seeking protective microclimates, reducing nocturnal body temperatures, and reducing foraging costs by living in groups. Because Bald Eagles are suspected to be food—limited, protective management policies that reduce energy stress could reduce overwinter morality.

Modern Pollen Assemblages and Vegetation in the Myrtle Lake Peatland, Minnesota

Abstract: Pollen grains and spores in moss polsters are compared with vegetation along transects across vegetational gradients in a large peatland in northern Minnesota. The distribution, percentage values, and source of 135 pollen and spore types are traced and related to the broad—scale vegetation pattern in the midwestern United States, and to the fine—scale vegetation pattern in the peatland. The pollen assemblages of taxa presently not growing in the peatland constitute the regional pollen deposition, with nearly constant proportions of pollen types distributed across the study area. Regional pollen values are compared for several peatlands in northern Minnesota and with the regional forest composition. The fine—scale pattern of peatland vegetation types corresponds well with the spatial pattern of the local pollen assemblages. Peatland vegetation types are characterized on the basis of these pollen assemblages. Conclusions on development of peatland vegetation can be drawn from local pollen assemblages preserved in sediment cores. Pollen transport for wind—pollinated Abies, Pinus, Betula, Alnus, Fraxinus, and Typha is discernible in the peatland only within 200 m from their source in local plant populations. The dispersal pattern of Abies pollen from an Abies balsamifera stand is related to wind speed and direction and sky cover for as much as 5 yr preceding sample collection. Most peatland pollen types are restricted in distribution to their corresponding vegetation types.

Plant-soil processes in eriophorum vaginatum tussock tundra in alaska: a systems modeling approach

Abstract: The Arctic Tundra Simulator (ARTUS) is a computer—based simulation model of Eriophorum vaginatum tussock tundra ecosystems found in north central Alaska. ARTUS simulates the annual patterns of heat and water balance, carbon fixation, plant growth, and nitrogen and phosphorus cycling. ARTUS runs in 1—d time steps for a growing season from 1 May to 17 September and is intended to run for several years. The abiotic section of ARTUS encodes the seasonal input of the environmental driving variables and calculates the resultant thermal and water regimes to define the heat and water environments for the tussock tundra system. The primary driving variables are daily total solar radiation, air temperature, precipitation, surface albedo, wind, and sky conditions. The soil compartment contains three organic horizons, which are recognized by their state of physical and chemical decomposition, and one mineral horizon. Six vascular plant species and four moss species are simulated. The model has seven compartments for each vascular plant species: total nonstructural carbohydrates, total nitrogen, total phosphorus, leaves grown in the current season, leaves grown in previous years, conducting and storage stems plus roots, and absorbing roots. In ARTUS the functional unit of the plant is the shoot system or ramet. Each shoot system consists of leaves, stems, fine roots (which do not have secondary growth and have a limited life—span), and larger roots, which have secondary growth and an extended life—span. Although plant processes are based on individual shoots, the ARTUS model as a whole is based on a square metre of ground. Values per square metre are calculated from the values per shoot by multiplying by the shoot density of each species. The model was validated by comparing calculated and measured peak season biomasses and nutrient contents, and the seasonal progression of environmental processes, biomass, carbohydrate contents, and nutrient contents. ARTUS successfully simulated the seasonality of the physical environment, but simulated thaw depths were deeper than those measured at all sites. The simulated value for total vascular plant production was 77% of the measured value. The simulated values for ecosystem respiration for Eagle Creek were within the range of measured values. Simulations with ARTUS indicated different patterns of growth and different storage—carbohydrate levels in deciduous shrubs, evergreen shrubs, and graminoids. The simulated seasonal course of net primary production of vascular plants and mosses was similar to the pattern measured at Eagle Creek. Sensitivity analysis using ARTUS indicated that the tussock tundra is more sensitive to external environmental factors, such as increased temperature, than to internal ecosystem variables. The development of ARTUS was limited by the unavailability of data on whole—plant carbon balance including root and stem respiration. More data are also needed on decomposition processes and nitrogen and phosphorus cycling. Adequate climatological data for northern Alaska are needed for extensive validations of the model. While caution should be used in basing managerial decisions on model simulations, ARTUS can be used to identify and quantify the magnitude and direction of plant responses to changes in state variables in the model.