Showing papers in "Ecological Monographs in 1982"

Temporal succession in a desert stream ecosystem following flash flooding

Abstract: Recovery of a desert stream after an intense flash flooding event is described as a model of temporal succession in lotic ecosystems. A late summer flood in Sycamore Creek, Arizona, virtually eliminated algae and reduced invertebrate standing crop by 98%. Physical and morphometric conditions typical of the preflood period were restored in 2 d and the biota recovered in 2—3 wk. Algal communities responded rapidly and achieved a standing crop of nearly 100 g/m2 in 2 wk. Community composition was dominated by diatoms early in succession and by filamentous greens and blue—greens later. Macroinvertebrates also recolonized denuded substrates rapidly, largely by immigration of aerial adults and subsequent oviposition. Growth and development were rapid and several generations of the dominant mayfly and dipteran taxa were completed during the 1st mo of recovery. Invertebrate dry biomass reached 7.3 g/m2 in 1 mo. Gross primary production (Pg) measured as O2 increased in a similar asymptotic fashion and reached 6.6 g°m—2°d—1 in 30 d. Pg exceeded community respiration (R) after day 5 and Pg/R averaged 1.46 for the remainder of the 2—mo sequence. This ecosystem is thus autotrophic and exports organic matter downstream and by drying, laterally. Uptake of nitrate and phosphorus were proportional to net primary production and exhibited a marked downstream decline in concentration during both light and dark periods. Temporal trajectories of various community and ecosystem attributes are compared with those suggested by Odum (1969) to be diagnostic of successional status. Agreement was poor in attributes which are especially modified in open, frequently disturbed ecosystems such as streams.

Fish community structure and function along two habitat gradients in a headwater stream

Abstract: The stream continuum concept suggests that the physical structure of the stream chan- nel coupled with the hydrological cycle and energy inputs results in a consistent pattern of community structure and function along a stream. I evaluated this concept in a headwater fish community along two physical gradients: upstream to downstream and riffle to pool. Habitat diversity and volume increased from upstream to downstream, and from riffle to pool. Temporal variation in habitat diversity was greater upstream. Upstream, and in riffles, habitat volume tended to vary more with time. Fluctuations in rainfall regime caused annual variation in habitat, especially in volume. Benthic insect density was highest from autumn (October-November) through spring (May-June). Following emergence of adults in late spring, invertebrate densities were low in summer in areas with riparian vegetation, but were not lower where riparian vegetation was absent and stable substrates were present. Along a gradient of substrates from silt-sand to gravel-rock, insect production increased, as indicated by adults and pupae in the drift. Peak resource availability for insectivore-piscivore fishes occurred in late summer and autumn, due to increased abundance of young-of-the-year fish. Pool and raceway-pool habitat guilds and insectivore and insectivore-piscivore trophic guilds con- tained the largest number of species. Increases in species richness were primarily associated with the addition of deeper habitats. Species richness of the pool insectivore-piscivore guild was es- pecially variable over time. Biomass in shallow areas consisted predominantly of generalized insec- tivores. In deep, stable habitats, generalized insectivores were replaced as the predominant trophic group by insectivore-piscivores and large benthic insectivores. Immigration of fish occurred in spring and autumn, the periods of highest resource availability. Immigration between midriver and headwater regions primarily involved older age classes (111+), and was associated with changes in flow regime, habitat structure, and seasonal dynamics of the resource base of particular trophic groups. Flow regime and habitat volume appeared to be important factors limiting immigration in autumn, especially in pool species. Habitat diversity (depth, current, and substrate; DCS) was significantly correlated with fish species diversity (FSD). However, considerable variation occurred in the relationship between the two vari- ables, including: (1) FSD decreased, in winter in shallow, less diverse habitats due to emigration, and increased in spring due to recolonization; (2) FSD increased in spring and autumn when resource availability increased; (3) FSD was least predictable from DCS in autumn, when flow regimes were low and large numbers of fish recruits were present; and (4) DCS did not predict FSD as accurately in temporally variable upstream areas where large numbers of small fish dominated the community, especially in areas with human disturbance. Young age groups (0-11) were primarily found in shallow, temporally variable areas upstream and in riffles. Relative growth rates were highest during summer months. However, growth rates did not increase as much between spring and summer as would have been predicted from the seasonal increase in water temperature, suggesting that resource limitation may have occurred during summer periods. Centrarchids had substantially higher growth rates than cyprinids during early life stages. Net pro- duction for age 0-11 fish was highest in upstream and riffle areas because of high densities of young, generalized insectivores. Net production of insectivore-piscivores was highest in downstream and pool habitats. Development of large, stable pools and raceways resulted in decreased fish production due to shifts in age structure toward fewer, large individuals with slower relative growth rates (age III+). Temporal variation in reproductive success and survival of younger age groups (0-I) was associated with random variation in high flow regimes and appeared to be a major factor determining spatial and temporal variation in production. These patterns of fish community structure and function support the qualitative aspect of the stream continuum concept: consistent shifts in community organization are associated with spatial or temporal changes in channel morphology and resource availability. However, for the stream con- tinuum concept to be useful as a quantitative predictor, the ultimate mechanism(s) regulating fish community organization need to be more rigorously established. I hypothesize that in upstream or riffle areas, where younger age classes predominate, recolonization dynamics, the effect of gradual changes in physical conditions on competitive interactions, and temporal variation in reproductive success, are more important than competitive exclusion and predation as determinants of community organization.

Fire and landscape diversity in subalpine forests of yellowstone national park

Abstract: Fire history was determined by fire scar analysis in a 73—km2 subalpine watershed in Yellowstone National Park, Wyoming, USA. Evidence was found for 15 fires since 1600, of which 7 were major fires that burned >4 ha, destroyed the existing forest, and initiated secondary succession. Most of the upland forest area was burned by large, destructive fires in the middle and late 1700's. Fires since then have been small and have occurred at long intervals. Fire frequency in this area is partly controlled by changes in the fuel complex during succession. Fuels capable of supporting a crown fire usually do not develop until a stand is 300—400 yr old, and ignitions prior to that time usually extinguish naturally before covering more than a few hectares. Thereafter a destructive crown fire is likely whenever lightning ignites small fuels during warm, dry, windy weather. On the extensive subalpine plateaus of Yellowstone National Park there appears to be a natural fire cycle of 300—400 yr in which large areas burn during a short period, followed by a long, relatively fire—free period during which a highly flammable fuel complex again develops. The 73—km2 study area appears to be about midway between major fire events in this cycle. This, rather than human fire suppression, apparently is the major reason for the small number and size of fires in the area during the last 180 yr. On the basis of the fire history data, the sequence of vegetation mosaics during the last 200 yr was reconstructed for the watershed. Indices of landscape diversity were computed for each reconstruction, treating forest types and successional stages as taxa and incorporating components of richness, evenness, and patchiness. Landscape diversity was highest in the early 1800's following the large fires in the 1700's, then declined in the late 1800's during a 70—yr period when no major fires occurred and the landscape was dominated by even—aged forests developing on the areas burned in the 1700's. Landscape diversity has increased somewhat during the last half—century as a result of two small fires and the effects of the mountain pine beetle. These landscape reconstructions for the last 200 yr suggest that the Yellowstone subalpine ecosystem is a nonsteady—state system characterized by long—term, cyclic changes in landscape composition and diversity. Such cyclic patterns may significantly influence wildlife habitat, streamflow, nutrient cycling, and other ecological processes and characteristics within the Park, and they may be an important consideration in judging whether recent ecological changes are natural or man induced. The landscape reconstructions were also made using a simulation model based on hypothetical fire management policies of total fire exclusion and selective fire control (permitting only small fires to burn). These hypothetical management policies generally reduced the richness and patchiness of the landscape compared to the natural fire regime, but they increased the evenness and reduced the magnitude of periodic fluctuations in overall landscape diversity. At times, overall landscape diversity may actually be higher with a fire control policy than with a natural fire regime. At other times, fire significantly increases landscape diversity, as would be expected.

A Comparative Analysis of Potential Nitrification and Nitrate Mobility in Forest Ecosystems

Abstract: The controls of potential nitrogen mineralization, nitrate production, and nitrate mo- bilization in a wide range of forest ecosystems were investigated through a combination of field and laboratory experiments. Trenched plot experiments were performed in 17 forests, and laboratory incubation studies of potential ammonium and nitrate production were made on soils from 14 of these sites. The site with the greatest potential for nitrate production in the laboratory was a New Hampshire northern hardwoods forest. Several other sites, including New Hampshire balsam fir, Indiana maple- beech, New Mexico aspen, and Oregon western hemlock forests, also had high potential nitrate production. All of these sites also had rapid nitrate movement to below the rooting zone following trenching in the field. Of nine processes which could be important in preventing or delaying solution losses of nitrate from disturbed forests, two appeared most important among the forests we examined. Low net nitrogen mineralization (caused by either nitrogen immobilization or low gross nitrogen mineralization) and lags in nitrification (probably caused by either low initial populations of nitrifying bacteria or the allelochemic inhibition of nitrification) were identified as important in several sites and in different regions. A direct relationship between the amount of nitrogen in annual litterfall and the proportion of forest floor nitrogen mineralized in laboratory incubations was observed, suggesting that refractory organic nitrogen compounds are produced in nitrogen-poor sites. An inverse relationship was found between the amount of nitrogen in litterfall in these and other sites and the carbon:nitrogen ratio of that litterfall, suggesting that the immobilization capacity of litter is increased in nitrogen-poor sites. The presence and length of lags in nitrification were inversely correlated with the mean concentration of mineral nitrogen in mineral soil. These patterns suggest that nitrogen retention within disturbed forest ecosystems can be caused by low nitrogen availability prior to disturbance.

The Demography of Jack‐in‐the‐Pulpit, a Forest Perennial that Changes Sex

Abstract: A size-classified transition matrix model of the demography of Arisaema triphyllum, a perennial herb of temperate deciduous forest, was constructed from 3 yr of field data from two different sites. Arisaema is quite unusual in that individuals change sex in a pattern determined by their sizes: small individuals do not flower, larger individuals reproduce as males, and the largest as females. As plants increase or decrease in size, they also may change sex. Though demographic parameters varied with the site and with the year, some consistent patterns were seen. Mortality rates ranged from 20 to 40%/yr for small, nonflowering plants, but decreased to much lower levels as plants reached reproductive size. For these large plants, mortality rates did not differ between sexes or among size-classes. Because rates of sexual reproduction were severely pollinator limited, levels of seed production were low (averaging 7 seeds female-1 yr-1) and were not correlated with plant size. All plants engaged in clonal reproduction, regardless of their size. Transition matrices based on these data were compared statistically and were found to vary in time, as a result of year-to-year environmental variability. No population had achieved a stable size distribution. The classical method of calculating rate of increase, X, was therefore rejected, and an alternative method incorporating temporal stochasticity substituted. A sensitivity analysis was per- formed to evaluate the effect on population growth of changing various life history parameters. Differences in rates of individual growth and of clonal reproduction were found to be largely respon- sible for the relative success of the two populations, one of which was increasing, the other declining. Arisaema's status as a climax-forest herb suggests that temporally varying transition matrices might be common even in environments traditionally regarded as "stable." The prevalence of tem- poral variability has consequences both for the future direction of demographic theory and for the design and analysis of field studies.

Distribution Ecology: Variation in Plant Recruitment over a Gradient in Relation to Insect Seed Predation

Abstract: Although predispersal seed predation by insects is common, no test exists of its effect on plant recruitment. This study examines seed predation in the population dynamics of a native, temperate shrub, Haplopappus squarrosus H. and A. (Asteraceae), over an elevational gradient in the coastal sage scrub vegetation of San Diego County, California USA. Frequency and abundance of H. squarrosus increase from coast to mountains. Expected abundance, based on flowers initiated, was highest at the coast and lowest in the interior, the opposite of the observed adult plant distribution. Overall flower and seed predation by insects was high (44-73%) and was greatest at the coast. Insect exclusion experiments at sites along the gradient had three main results. (1) Seedling recruitment was proportional to the number of viable seeds after seed predation. The exclusion plots had significantly higher numbers of seedlings established at all sites than did control plots; the greatest increase was at the coast. (2) Seedling survivorship was independent of both seedling and adult densities. (3) Juvenile recruitment was proportional to seedling establishment. Predation by insects prior to release of seeds played a critical role in the population recruitment of H. squarrosus within and among sites along the gradient. Survivorship of established plants, in addition, was as high at the coast as it was in the interior. These results suggest that indigenous insect seed predators can be a major force controlling the dynamics of a native plant species over its natural distributional range.

The Importance of Predation and Intra- and Interspecific Competition in the Population Biology of Two Infaunal Suspension-Feeding Bivalves, Protothaca staminea and Chione undatella

Abstract: To estimate the effects of varying density on individual and population parameters of two infaunal, suspension—feeding bivalves, Protothaca staminea and Chione undatella, densities of both species were varied experimentally from their natural levels (X) in shallow, subtidal field enclosures in Mugu Lagoon, California. A factorial design with densities ranging from ½X to 8X in two sand habitat matrices of enclosures and from ½X to 4X in one mud habitat matrix permitted tests of both intra— and interspecific effects of density on (1) individual growth rate, (2) recruitment rate, and (3) survivorship rate. After each rate was estimated for each of four consecutive experimental periods spanning 24 mo with replacement of missing and dead clams at each sampling, a subsample of adult clams was sacrificed from each density treatment in the mud matrix and dissected to produce estimates of how reproductive effort (4) varied with density. Short—term (2—wk) migration rates (5) were estimated as a function of density in the two habitats in separate experiments involving no enclosures. Characteristic shell damage and results of a predator exclusion experiment that was sampled four times over 24 mo permitted clam mortality rates (6) as a function of density and size to be partitioned broadly by source for both habitats. For both Protothaca and Chione, growth rates was consistently depressed at high intraspecific densities. In mud, Protothaca's linear growth declined by 49% and Chione's by 38% as intraspecific density increased from ½—1X to 2—4X. Interspecific effects of density on growth were usually nonsignificant and were consistently far smaller than these large intraspecific effects. The dry mass of gonadal material per unit shell volume also declined significantly with increasing intraspecific density. Average percentage of total soft tissue mass found in gonads in June 1977 declined progressively by 38% in Protothaca and by 49% in Chione as intraspecific density was increased from ½—1X to 2—4X. In Protothaca, growth rate decreased at the same rate as reproductive effort with increases in density, demonstrating that Protothaca did not reallocate energy differentially among the competing processes of growth and reproduction as competitive stress changed. The strong effects of intraspecific density in the immediate vicinity of control densities implies intense resource limitation on both growth and reproduction in each species. The general lack of significant interspecific effects of density on both growth and reproductive effort implies ecological segregation. Food seems more likely than space to be the limiting resource which is partitioned between species because these clams occupy the same depth stratum, such that interference competition for space would presumably have interspecific as well as intraspecific components. In sand, recruitment rate of Protothaca was never related to adult density, whereas a single location parameter explained 76% in one matrix and 50% in the other of between—enclosure variance in recruitment. In mud, Protothaca recruitment was occasionally depressed at high adult density, but the decline was only °40% as intraspecific adult density changed from 0—1X to 2—4X. Spatial location still explained 25% of total variance in recruitment in the mud. Chione's recruitment rate was lower by over one order of magnitude and much more variable from year to year than Protothaca's. On the one occasion when its recruitment was high enough to test for density effects, Chione recruited less in the presence of higher densities of adult Protothaca but showed no intraspecific effect. Emigration rate over 2 wk was density dependent in Protothaca in the sand habitat. Protothaca showed reduced mobility in the mud habitat such that the density dependence of emigration rate could not be tested. Chione was so sluggish that no emigration took place at high or at low density in either sand or mud. Annual Chione survivorship was quite high (averaging at least 88—96%) in every matrix over the 2—yr density manipulation experiment; however, in the 2nd yr of the predator exclusion experiment 10 cm of sedimentation from a once—in—100—yr rainstorm caused 100% mortality in the mud habitat. Mortality rate in Chione was never a significant function of density, and predation had little or no effect on the Chione used in this experiment. Annual survivorship of Protothaca was substantially lower (16—61%) than that of Chione in every matrix during each of the 2 yr of the density manipulation experiment. Survivorship of Protothaca was often significantly reduced at higher Protothaca densities in both habitats, although the effect was greater in the mud. The predator exclusion experiments demonstrated that both Protothaca's higher mortality rate and this occasional density dependence of mortality were entirely an effect of predation. In sand and mud, most predation was of the type that removed the shell, though characteristic shell damage of empty shells revealed that Polinices reclusianus in sand and, to a lesser degree, Cancer anthonyi in mud also contributed significantly. The importance of various types of predators changed dramatically between "replicate" years. Predation by sources that removed shells fell most heavily on smaller Protothaca in both habitats; in the sand this decline in risk with size was occasionally compensated by Polinices' preference for larger Protothaca. As with Chione, percent Protothaca mortality from all sources which leave undamaged shells (including starvation) never increased with increasing clam density. Thus, despite evident resource limitation on growth and reproductive effort, competition did not produce mortality effects in the range of densities used. The differing susceptibilities of Protothaca and Chione to predation and the contingent life history differences may be based upon differences in shell thickness, size, and ornamentation and may be a consequence of Chione's more tropical evolutionary history.

Experimental and Mathematical Analyses of Herbivore Movement: Quantifying the Influence of Plant Spacing and Quality on Foraging Discrimination

Abstract: Using mark—recapture experiments, I studied the foraging movements of two crucifer feeding flea beetles, Phyllotreta cruciferae and P. striolata. To determine the influence of distance between host patches, and their quality, on beetle movement, I released marked beetles in experimental arrays of collards (Brassica oleracea). I quantified and interpreted movement behavior by applying a passive diffusion model and a continuous—time Markov model to recapture distributions. These models provided both a standard protocol for interspecific comparisons and a formal tool for translating movements into steady—state foraging patterns. In homogeneous arrays (all collard patches of similar quality), P. cruciferae and P. striolata moved in accordance with a constant—coefficient diffusion model. The dispersal rates of both beetles increased as the distance between patches declined; thus, patches of collards acted as stepping—stones to accelerate beetle dispersal. Beetle movement was hampered by old—field vegetation; beetles moved along cultivated ground in lieu of entering dense stands of goldenrod. Under all experimental conditions, P. cruciferae was consistently more mobile than P. striolata. By differentially fertilizing and pruning collards, I created "lush" and "stunted" patches of collards, which were arranged alternately along linear arrays. To determine the influence of patch dispersion on foraging selectivity, I varied the distance between these patches of unequal quality. The degree of foraging discrimination exhibited by the beetles was greatest where patches were closest together, and declined as distance between patches increased. At comparable spacings, P. cruciferae consistently exhibited a higher degree of foraging discrimination than did P. striolata. I developed a Markov model of patch—to—patch dispersal in which each flea beetle's probability of movement depends on patch quality and distance to neighboring patches. Using 60—min observations of marked flea beetles to estimate model parameters, I found that the steady states of foraging discrimination predicted by the model matched the discrimination levels exhibited by undisturbed flea beetles in comparable control arrays of collard patches. Thus, simple movement rules were extrapolated into expected foraging behavior; the dependence of that foraging behavior on patch dispersion was shown to be mediated by varying levels of mobility. In particular, proximity between heterogeneous patches affords beetles greater opportunity for nonrandom foraging than when patches are widely scattered. Since movement is the mechanism by which beetles respond to heterogeneous environments, rates and patterns of movement determine each species' ability to concentrate its numbers on the best foodplants. P. cruciferae is more mobile than P. striolata and in turn can better adjust to variation in collard quality. The interplay between beetle movement and plant dispersion may have consequences well beyond the simple distribution of feeding activity among heterogeneous host patches.

Experimental Analyses of Structural Regulation in a Marine Sand Community Exposed to Oceanic Swell

Abstract: The development of general theories concerning the origin and maintenance of community organization in marine sedimentary environments will benefit from studies of similar processes in the widest possible range of habitat types. The roles of predation and disturbance by large epibenthos are thought to be significant in many such habitats, but the bulk of recent experimental confirmation comes from shallow areas protected from oceanic swell. This field experimental study examines relationships among demersal predators, predator—caused local disturbance, infauna, and infaunal food resources in an exposed marine sand habitat at 17—m depth in southern California, USA. Manipulation of predator densities with exclusion cages, simulation of biological disturbance, and study of dispersal and habitat selection of infauna showed the importance of recurrent local disturbances by the rays Urolophus halleri and Myliobatis californica, which dig pits to expose prey but clear other infauna in the process. Benthic invert...

Colony Defense Strategies of the Honeybees in Thailand

Abstract: The colony defense strategies of the three honeybee species in Thailand were studied to examine the influence of predation on tropical honeybee societies. Each species focuses its defenses upon different stages in the predation sequence of detection—approach—consumption. This radiation in defense strategies apparently reflects each species' preadaptation by worker size (small, medium—sized, or large) and nest site (cavity or tree branch) to a different pattern of colony defense. Wasps, birds, and primates probably have difficulty finding the small, dispersed colonies of Apis florea, whose nests are built low on the branches of dense, shrubby vegetation. Once found, however, they are easily approached and overpowered because their low, exposed nests are accessible and their small workers inflict relatively painless stings. When overwhelmed, the bees quickly abandon their nest; later, they return to salvage wax. Ants find A. florea nests easily and at least one species (Oecophylla smaragdina) easily kills these small bees. However, sticky bands of resin encircling the nests' slender substrate branches prevent ants from invading A. florea nests. Cavity—nesting colonies of Apis cerana are conspicuous with their medium—sized bees streaming in and out of low, clearly visible entrance holes in caves and hollow tress. However, gaining access to A. cerana nests is difficult. Large predators cannot pass through the small entrance opening and small predators are overpowered by entrance guards. But if a large predator can breach a nest cavity's walls, it faces an only moderately powerful stinging defense. Apis cerana colonies are relatively small and their workers are not fiercely aggressive. Predators easily find the large, sometimes aggregated colonies of Apis dorsata, whose nests hang in the crowns of the tallest forest trees. But only skilled fliers and climbers can reach these lofty nests. Those which do face massive stinging attacks from the large colonies of these relatively giant, ferocious bees. Nests of both open—nesting species, Apis florea and A. dorsata, are protected by a three— to six—layer curtain of bees over the comb. Apis cerana colonies lack these curtains but are protected by their nest cavity walls. A curtain of inactive guards requires a large labor force. The high worker: brood ratio in A. florea relative to A. cerana colonies suggests that the age polyethism schedules of the open— and cavity—nesting species are tuned differently to generate the appropriate proportions of guard bees. Each species' colony defense system consists of numerous interwoven lines of adaptation, including nest site, nest architecture, colony population, labor allocation to defense, age polyethism schedule, colony mobility, and worker morphology, physiology, and behavior. Predation has been a pervasive and powerful force in the evolution of these tropical bee societies.

Longevity, life history, and relative body wall size in sea urchins

Abstract: Annual survival rates in 38 populations of 17 sea urchin species in the Indo-West Pacific were related to relative size of the body wall and exposure to the surf. Populations were studied at Hawaii, Enewetak Atok, Queensland, New South Wales and Western Australia, Sri Lanka, Seychelles, Kenya, Zanzibar, and Isaerl (Eilat). Live animals were dissected to determine the size of body components. Parameters of the Richards growth function were determined from animals tagged with tetracycline. Tagged animals were collected after they had been in the field for 1 yr. Growth parameters were used with parameters from size-frequency distributions to estimate Z, the mortality coefficient. Stepwise multiple regression was used to examine the relationship between annual survival probability (p) and two indepencent variables, ..cap alpha.. and E, where E is a subjective measure of exposure to surf (1 = most exposed). Survivorship increases with increased relative size of the body wall and with increased protection from the surf. The positive relationship between survival probability and relative body wall size supports the hypothesis that survival is related to allocation of resources to maintenance. The significane of longevity in urchins probably is that it is related to the predictability of survival of prereproductivemore » individuals. The greater the unpredictability, the longer life must be. Long life requires a greater investment in maintenance mechanisms and hence, among other adaptations, a more massive body wall.« less

A Late-Quaternary Vegetation History from Hanging Lake, Northern Yukon

Abstract: A 403—cm core was recovered from Hanging Lake in unglaciated northern Yukon. Twenty—one radiocarbon dates indicate that the section is at least 25 000 and possibly 33 000 yr old; they permit the calculation of pollen influxes for the full—glacial in eastern Beringia. Numerical methods were used to divide the pollen stratigraphy into five zones. From prior to 33 000 to 18 450 BP, a herb zone was dominant (zone HL 1) with high percentages of Gramineae, Artemisia, and Cruciferae. However, the low pollen influx, ranging from 5—100 grains°cm—2°yr—1, the low organic content of the sediment, and the occurrence of open—ground taxa all indicate that the vegetation cover was sparser than it is today. The arctic—alpine affinities of the herb pollen show that generically the vegetation was akin to modern arctic plant communities. Modern fellfield communities in the northern Yukon and Siberia have a rich and endemic Artemisia flora and they can produce pollen spectra comparable to that of the herb zone. Percent and influx values for spruce, alder, and birch increased slightly during subzone H 1B (21 680—18 450 BP); this subzone probably represents an interstadial. From 18 450 to 14 600 BP, a Salix—Cyperaceae zone (HL 2) occurred, suggesting the development of snowbed and willow scrub communities in sheltered areas. Between 14 600 and 11 100 BP Betula pollen dominated (zone HL 3) indicating the spread of dwarf birches, but the influx data show that this initial increase was modest compared with the subsequent zone and thus dwarf birches were probably restricted at this time to more favorable habitats. The spread of birch together with the increased total pollen influx, the higher organic content of the sediment, and the increased richness of herb pollen indicates that the local flora was more diverse and that the vegetative cover increased. The climate must have warmed. Zone HL 4 (Ericales zone) spanned the period from 11 100 to 8900 BP. Wet heath communities became locally abundant, poplar was more abundant at the beginning of this zone than at any subsequent time, the ranges of Typha latifolia and Myrica gale were greater than today, and pollen influx and sedimentation rate both increased greatly. Spruce became regionally abundant. These varied changes are probably in response to a warmer and wetter climate; they provide evidence for a late Pleistocene to early Holocene warm interval initially recognized by McCulloch and Hopkins (1966). Zone HL 5 (Alnus crispa zone) has been dated from 8900 BP to the present; it reflects the regional expansion of Alnus crispa on organic soils.

Depth Zonation of Corals in the Gulf of Panama: Control and Facilitation by Resident Reef Fishes

Abstract: The mechanisms that control the distribution and abundance of major benthic organ- isms in subtidal coral reef communities are poorly understood. Through field experimentation and manipulations, this study investigated the factors that account for the vertical zonation patterns found on a fringing reef in the Gulf of Panama (Pacific). In this community, pocilloporid corals form a near-monotypic stand (80-85% live coral cover) in shallow water (0-6 m depth) while massive corals, particularly Pavona gigantea, predominate in low density (- 18% cover) in the deeper areas of the reef (6-10 m depth). The paucity of massive corals in shallow water results mainly from chronic and severe mortality caused by the omnivorous damselfish Eupomacentrus acapulcoensis (Pomacentridae). These highly territorial fish establish algal mats (used as a source of food and nesting site) on hard substrata, including coral. The effect of this activity was found to be differential with respect to (1) species (i.e., Pavona suffered higher mortality compared to Pocillopora) and (2) depth (i.e., the frequency and extent of damaged Pavona colonies were higher in shallow compared to deep areas). Pavona suffered higher mortality due to differences in coral morphology: damselfish are more effective at removing tissue from a massive (open) surface than from a tightly branched pocilloporid coral. Pavona was most severely damaged in shallow water because of depth-related differences in topographic com- plexity. By removing the topographic complexity surrounding colonies with damselfish and by adding complex substratum around colonies without damselfish, it was found that damselfish abandon ter- ritories when substratum complexity is reduced and immigrate to colonies when complexity is in- creased. Immigration eventually led to coral mortality. Hence, shelter is a necessary requisite for the establishment of a damselfish territory. Censuses revealed that shelter site availability surrounding Pavona colonies and damselfish abundance (both overall and per individual Pavona colony) were greater in shallow compared to deep water. Damselfish were conspicuously absent from small colonies (S 1.5 m2 surface area) that lacked holes or crevices within the colony or immediately adjacent to it; such colonies were only present in deep water. Controlled experiments using live coral fragments (simulating 1-1.5 yr old juveniles) attached to hard substratum demonstrated that pocilloporid corals suffer high mortality and substantially reduced growth rates in deep reef areas due to grazing by pufferfish and parrotfish. The effectiveness of this predation/grazing activity was related to the position of the coral fragments relative to defended damselfish territories. Outside the territory, growth and survivorship of fragments were low for Po- cillopora but high for Pavona. Within areas defended by damselfish, Pocillopora and Pavona frag- ments grew equally well both caged and uncaged. On the algal mat itself, however, Pavona fragments suffered high mortality from damselfish; Pocillopora growth under these conditions was only tem- porarily reduced. The data in this study indicate that damselfish territories provide a refuge against juvenile mortality for Pocillopora within defended areas, thus facilitating the establishment of branch- ing corals in shallow water and concomitantly reducing or eliminating its potential space competitor, Pavona. A proposed successional model suggests that Pocillopora abundance may ultimately control the abundance of damselfish. This study indicates that community structure on some eastern Pacific reefs may be controlled by the direct and indirect behavioral interactions between damselfish and transient fish corallivores, and physical factors (availability of shelters) limiting the distribution of damselfish.

Phytoplankton in an Oligotrophic Ocean: Observations and Questions

Abstract: Details of the vertical distribution of oceanic phytoplankton species are investigated by means of four stations occupied during June 1977 and August 1978 near 28°N, 155°W. Two distinct, recurring floral associations, separated by a region of rapid transition near or below 100 m, are identified. The associations are apparently related to the "nutrient limited" and "light limited" physiological regimes described previously. Inverse abundance relationships of congeners within the deeper association are compatible with the hypothesis of disequilibrium competitive interactions and community regulation. Among species of the shallow association congeners are positively associated and predation is postulated to be the important regulating mechanism. See full-text article at JSTOR

Coexistence in a Kelp Forest: Size, Population Dynamics, and Resource Partitioning in a Guild of Spider Crabs (Brachyura, Majidae)

Abstract: The population dynamics and resource partitioning of five species of spider crabs (Loxorhynchus crispatus, Pugettia producta, Mimulusfoliatus, Pugettia richii, and Scyra acutifrons) were measured in a kelp forest on the central California coast. The distribution of the five species showed zones of peak abundances along a transect from the intertidal zone through the kelp forest to a deep reef. P. richii was the most abundant species with peak densities of 6 crabs/M2 at the middle and inner edge of the kelp forest. M. foliatus was second most abundant with a peak of 3.5 crabs/M2 at the middle and outer edge of the kelp forest. S. acutifrons had peak densities of about 3 crabs/M2 at the outer edge of the kelp forest. P. producta had a peak of 2 crabs/M2 in the intertidal zone. L. crispatus was present in low densities of about 0.5 crabs/M2 from the middle of the kelp forest to the deep reef. The greatest combined density of spider crabs (I I crabs/m2) occurred at the middle of the kelp forest. Densities of all five species in the main study area in the middle of the kelp forest showed a synchronous cycle with a maximum in late summer and fall and a decline in winter to a low in spring. The cycle appeared to be caused by a combination of density-independent mortality from winter swells and storm activity and by slower development and growth rates during the upwelling period of colder water temperatures. Brooding frequencies of all the species remained high year-round. Size- structures of the populations did not change during bimonthly sampling, indicating continuous re- cruitment. Size-frequency analysis of crab populations along the transect indicated that juveniles of Pugettia producta recruit into the intertidal and shallow Phyllospadix zones and migrate out into the kelp forest as they grow, but the other species did not have zones of recruitment separate from adult distribution. The diversity of predators which take spider crabs as a major portion of their diets indicated that the general level of predation pressure is high and may limit the overall population levels of the crabs. Predation by sea otters probably limits the density of P. producta, and the other four species are probably limited by fish predation. The crabs exhibit different color morphs, color change, and decorating behavior, which are adaptive for concealment from predators. Size at maturity of the five species spanned an order of magnitude in carapace width from small Scyra acutifrons (1 cm) to large Loxorhynchus crispatus (10 cm). Mean body size of mature females and mean body size of the entire population of all pairs of species except Mimulus foliatus/Pugettia richii had ratios larger than Hutchinson's (1959) predicted value of 1.28. Body size may reflect limi- tations of the crabs' utilization of crevice refuges in the microhabitat. Partitioning of the microhabitat resource was measured by substrate utilization. Mimulusfoliatus had the largest microhabitat niche breadth and was found on most types of substrates, with an important refuge in kelp holdfasts. Pugettia richii was found primarily on the alga Cystoseira os- mundacea and in coralline algal mats. Scyra acutifrons occurred in the interstices of algal/invertebrate turf, and Loxorhynchus crispatus was found on top of the turf. Pugettia producta occurred upon the kelp plants and had the narrowest microhabitat niche breadth. Analysis of stomach contents showed partitioning of food resources. Pugettia producta was a strict specialist grazing on giant kelp, Macrocystis pyrifera. Mimulusfoliatus and Pugettia richii also had narrow diets of mostly drift M. pyrifera. Scyra acutifrons had major food categories of detritus, sponge, and pieces of M. pyrifera trapped in the algal-invertebrate turf. Loxorhynchus crispatus was a dietary generalist foraging on a broad range of invertebrates and giant kelp. Niche separation in the guild is multidimensional, and similar utilization of one resource is gen- erally complemented by dissimilar utilization of another resource. Graphical analysis of niche sepa- ration for the three dimensions of microhabitat, food, and body size showed that Pugettia producta is an overall specialist and Loxorhynchus crispatus is a generalist. Scyra acutifrons also has a distinct niche within the algal invertebrate turf, but Mimulus foliatus and Pugettia richii exhibit extensive overlap in all of the parameters measured. However, the degree to which a species is a specialist vs. a generalist for a resource did not relate to the amount of niche overlap with the rest of the guild. Microhabitat resources appear to be the most important for niche separation and may also be ex- plained as adaptations for minimizing predation.

Community Structure and Productivity in Ceanothus Chaparral and Coastal Sage Scrub of Southern California

Abstract: Adjacent, 22—yr—old stands of evergreen chaparral and drought—deciduous coastal sages scrub at a low—elevation site near the ocean in the Santa Monica Mountains of California were analyzed. The chaparral is a pure stand of Ceanothus megacarpus and has an aboveground live biomass of 6482 g/m². The coastal sage scrub is co—dominated by Salvia leucophylla and Artemisia californica and has an aboveground live biomass of 925 g/m². Attached and standing dead wood comprise 15% and 21% of the total live and dead biomass in the chaparral and coastal sage scrub, respectively, Mean annual aboveground primary production in Ceanothus chaparral is 1056 g°m— ²°yr— ¹, while litterfall is 801 g°m— ²°yr— ¹. In the coastal sage scrub, mean annual primary production is 355 g°m— ²°yr— ¹ and litterfall, 199 g°m— ²°yr— ¹. The drought—deciduous shrubs begin leaf production 1—3 wk after the initial winter rains and continue for 5—7 mo; thereafter, the shrubs lose most of their leaves in the summer. Twig and flower/fruit production continues for several months after leaf growth has ceased. Artemisia and Salvia have no measurable growth for 2 and 4 mo, respectively. In Ceanothus, leaf production begins 3—4 mo after the first rains of winter, and may or may not continue through the summer. Wood production and flowering/fruiting coincide with foliage production. Ceanothus may show measurable aboveground growth for up to 11 mo of the year. Flower production does not occur each year; however, in a flowering year, the production of all reproductive structures is very high (151 g°m— ²°yr— ¹). The length of the measurable aboveground growing season in the two growth forms does not fully account for the great differences in their annual net primary production. The high net primary production in the Ceanothus chaparral is much greater than other evergreen, mediterranean—type ecosystems and most temperate shrub communities. This high productivity is attributed to a tolerance of low water potentials, high photosynthetic rates, and an efficient use of nutrients in C. megacarpus. The high production in Ceanothus chaparral and the potential for reproduction in the coastal sage scrub suggest that the commonly ascribed senscence of mature shrub stands may not occur in these communities.

An Analysis of Avifauna-Resource Relationships on the Serengeti Plains

Abstract: The objectives of this study were: (1) to test the hypothesis that food was the principal proximate factor limiting the distribution of birds on the Serengeti Plains of Tanzania, and (2) to generate a satisfactory description of the relationships of the Serengeti avifauna to selected habitat factors. I analyzed natural changes in the avifauna among habitats through time and related these changes to variation in avian food and habitat resources. Food limitation was apparently not an important proximate factor affecting short—term variations in bird population densities, although many species were functionally capable of responding to fluctuations in food supply. Habitat partitioning was the principal means of "niche segregation" among the various species. Sites with greatest total avian biomass had low vegetation biomass, low vegetation height, but well—developed vertical vegetation structure. These conditions provided cover along with a great degree of ground—level mobility for most species. Individual bird species were associated with specific types of vegetation structure, while vegetation condition (primarily green biomass) was the main factor associated with increased densities of bird species within vegetation types. Neither conventional competition theory nor Wiens' (1977) "bottle—neck" theory can account for the characteristics of the Serengeti grassland avifauna.

The Evolution of the Life Histories of Four Species of Intertidal Limpets

Abstract: The life histories of four species of intertidal limpets, Cellana tramoserica, Notoacmea petterdi, Patella peroni, and Patelloida alticostata, which occur on the same shore but in different tidal zones, are compared. The population dynamics (recruitment, growth, and mortality) and the causes of mortality of each species are investigated. C. tramoserica and P. peroni have similar rates of growth and mortality, but P. peroni has a higher reproductive effort (the proportion of assimilated energy devoted to reproduction). Pd. alticostata has lower rates of growth and mortality than the above species, but its reproductive effort is similar to that of C. tramoserica. N. petterdi has the lowest rates of growth and mortality, but its reproductive effort is similar to, or perhaps slightly lower than that of C. tramoserica and Pd. alticostata. The causes of mortality vary among species. Predation by wrasses is the main cause of mortality of P. peroni and Pd. alticostata, which occur in the sublittoral and lower littoral zones, respectively. The principal cause of mortality of adult C. tramoserica, a midlittoral species, is starvation. N. petterdi, which occurs in the supralittoral zone, is a very long-lived species and the causes of its mortality could not be determined. Semiquantitative estimates of extrinsic adult mortality rates are obtained for the four species of limpet, and these are shown to be correlated with interspecific differences in reproductive effort. Adults of each species assume different levels of risk during reproduction. The adults of species which occur in seasonal environments appear to take greater risks when breeding than those of species which occupy more constant environments. of predation may be unavoidably high). These two types can only be distinguished by measuring the risks associated with reproduction. Ideally, these risks should be measured by simultaneously estimating the mortality rates of reproductive and nonreproductive adults in the field (Pianka 1976, Stearns 1977). But such experiments have not yet proved possible, and even if it was possible to prevent reproduction (per- haps by withholding mates) we could not be certain that the energy "saved" would be used to maximize the survival of nonreproducing adults. Hirshfield and Tinkle (1975) define reproductive ef- fort as "that proportion of the total energy budget of an organism that is devoted to reproductive process- es" and they hypothesize that the level of reproduc- tive effort should correlate with the rate of extrinsic adult mortality. Their hypothesis has remained un- tested because (1) reproductive effort has seldom been measured by using energy budgets (Hirshfield 1980) and (2) estimates of extrinsic adult mortality rates im- ply a knowledge of the risks associated with repro- duction, and these are difficult to measure. The present paper provides a semiquantitative test of the above hypothesis. The demography and natural history of four species of intertidal limpet, Cellana tramoserica (Sowerby), Notoacmea petterdi (Tennison Woods), Patella peroni (Blainville), and Patelloida al- ticostata (Angas) are considered, and the extrinsic adult mortality rates of each species are estimated. Extrinsic

Ecological History of Lake Valencia, Venezuela: Interpretation of Animal Microfossils and Some Chemical, Physical, and Geological Features

Abstract: The developmental history of Lake Valencia, Venezuela, over the past 12 500 yr was studied by means of animal remains and some chemical and physical variables in three sediment cores. Radiocarbon dates of abandoned beach terraces also provided information for the interpretation of former lake levels. Transects of surficial sediments in the lake served as modern analogues for both animal communities in the sediments and characteristics of the sediment itself. Interpretation of cluster and multiple discriminant analyses led to the recognition of six stratigraphic zones of animal microfossils. The zones were expressions of climatically controlled fluctuations of water levels that have dominated successional processes within Lake Valencia. The animal remains were mostly carapaces of Ostracoda and Cladocera, with fewer head capsules and mouthparts of immature insects and flatworm (neorhabdocoel) egg cases. Study of the variation of animal assemblages with various environmental requirements led to the following interpretation of the zones. The basin was dry from at least 13 000 to 11 000 yr before present (BP). It filled rapidly °10 500 yr BP, and had an outflow for 2500 yr. The level then gradually declined during the time from 8000 yr BP to 2500 yr BP, but was periodically stable during which intervals several terraces were cut. The next 2000 yr included several rises, some to the outflow. As late as 1727 AD the lake was again at the outflow level, after which it began to desiccate. Cultural influences, both indigenous and European, are expressed in the sediments of the past 2000 yr. In 1979 the level was °25 m below the outlet and had been dropping at °0.1—0.2 m/yr.

A population model of the lizard Uta stansburiana, in southern Nevada

Abstract: Population densities, reproduction, and survival of the lizard Uta stansburiana were measured at the Nevada Test Site in southern Nevada, USA, between 1964 and 1974. These data were used to develop a model of the population dynamics of this species. Results of irrigation experiments in 0.4-ha enclosures near Mercury, Nevada, were used to formulate multiple-regression equations predicting frequency and size of clutches laid by two age-classes of females in terms of winter rainfall, March air temperatures and Uta population density. Densities of Uta in these enclosures were manipulated, and age-specific survival modeled in terms of spring densities of Uta. Experiments in which an important predator on Uta (the leopard lizard, Crotaphytus wislizeni) was removed from enclosures were used to estimate the influence of the predator on basic survival rates of hatchling and older Uta. The model was generally developed from data acquired in the small enclosures, but predictions were compared with actual observations of changes in Uta populations in Rock Valley (19 km west of Mercury, Nevada) between 1966 and 1972. The basic model included three density-dependent parameters: clutch frequency, clutch size, and adult survival. It was concluded that processes relating to egg production were modeled more effectively than thosemore » influencing survival, and that improvement of the model will depend on more detailed studies of the impact of predation on age-specific survival rates of Uta.« less