Showing papers in "Ecological Monographs in 1989"

Comparisons of Treatments After an Analysis of Variance in Ecology

Abstract: The statistical literature on tests to compare treatments after the analysis of variance is reviewed, and the use of these tests in ecology is examined. Monte Carlo simulations on normal and lognormal data indicate that many of the tests commonly used are inappropriate or inefficient. Particular tests are recommended for unplanned multiple comparisons on the basis of controlling experimentwise type I error rate and providing maximum power. These include tests for parametric and nonparametric cases, equal and unequal sample sizes, homogeneous and heterogeneous variances, non-independent means (repeated measures or adjusted means), and comparing treatments to a control. Formulae and a worked example are provided. The problem of violations of assumptions, especially variance heterogeneity, was investigated using simulations, and particular strategies are recommended. The advantages and use of planned comparisons in ecology are discussed, and the philosophy of hypothesis testing with unplanned multiple comparisons is consid- ered in relation to confidence intervals and statistical estimation.

Biological invasion by Myrica faya in Hawai'i: plant demography, nitrogen fixation, ecosystem effects

Abstract: Myrica faya, an introduced actinorhizal nitrogen fixer, in invading young volcanic sites in Hawaii Volcanoes National Park. We examined the population biology of the invader and ecosystem-level consequences of its invasion in open-canopied forests resulting from volcanic cinder-fall. Although Myrica faya is nominally dioecious, both males and females produce large amounts of fruit that are utilized by a number of exotic and native birds, particularly the exotic Zosterops japonica. In areas of active colonization, Myrica seed rain under perch trees of the dominant native Metrosideros polymorpha ranged from 6 to 60 seeds m{sup {minus}2} yr{sup {minus}1}; no seeds were captured in the open. Planted seeds of Myrica also germinated an established better under isolated individuals of Metrosideros than in the open. Diameter growth of Myrica is > 15-fold greater than that of Metrosideros, and the Myrica population is increasing rapidly. Rates of nitrogen fixation were measured using the acetylene reduction assay calibrated with {sup 15}N. Myrica nodules reduced acetylene at between 5 and 20 {mu}mol g{sup {minus}1} h{sup {minus}1}, a rate that extrapolated to nitrogen fixation of 18 kg ha{sup {minus}1} in a densely colonized site. By comparison, all native sources of nitrogen fixation summed to 0.2 kg ha{sup {minus}1}more » yr{sup {minus}1}, and precipitation added < 4 kg ha{sup {minus}1} yr{sup {minus}1}. Measurements of litter decomposition and nitrogen release, soil nitrogen mineralization, and plant growth in bioassays all demonstrated that nitrogen fixed by Myrica becomes available to other organisms as well. We conclude that biological invasion by Myrica faya alters ecosystem-level properties in this young volcanic area; at least in this case, the demography and physiology of one species controls characteristics of a whole ecosystem.« less

The Seasonal Dynamics of The Chesapeake Bay Ecosystem

Abstract: The full suite of carbon exchanges among the 36 most important components of the Chesapeake Bay mesohaline ecosystem is estimated to examine the seasonal trends in energy flow and the trophic dynamics of the ecosystem. The networks provide information on the rates of energy transfer between the trophic components in a system wherein autochthonous production is dominated by phytoplankton production. A key seasonal feature of the system is that the summer grazing of primary producers by zooplankton is greatly reduced due to top—down control of zooplankton by ctenophores and sea nettles. Some of the ungrazed phytoplankton is left to fuel the activities of the pelagic microbial community, and the remainder falls to the bottom where it augments the deposit—feeding assemblage of polychaetes, amphipods, and blue crabs. There is a dominant seasonal cycle in the activities of all subcommunities, which is greatest in the summer and least in the cold season. However, the overall topology of the ecosystem does not ap...

Bottom-up and top-down impacts on freshwater pelagic community structure'

Abstract: For freshwater pelagic ecosystems, the biodiversity and cascading trophic interaction theories both predict that decreased piscivore populations will result in direct, short—term (a few years) increases in planktivore biomass, reductions in crustacean herbivore biomass, and increases in chlorophyll a concentration and phytoplankton biomass. An Alternate view is offered by the bottom—up:top—down theory, which predicts that in eutrophic lakes changes in piscivore biomasses will have strong impacts on planktivore numbers, weaker but observable impacts on zooplankton biomass, and little or no long—term effects on phytoplankton biomass. A partial winterkill at Lake St. George, Ontario, Canada allowed us to test these predictions. The data set comprised measures of: (1) piscivore and planktivore numbers, (2) zooplankton species composition, size structure, and biomass, (3) chlorophyll a concentration and Secchi depth, and (4) water chemistry from 1980 through 1986. Prior to the winterkill of 1981—1982, the piscivore population was high (1000—2000 piscivores/ha), the planktivore population was intermediate (8000—10 000 planktivores/ha), zooplankton biomass was intermediate (2400 µg/L), and chlorophyll a concentration was high (5—12 µg/L). In the year following the winterkill (1982), piscivore and planktivore numbers were low, and zooplankton biomass and chlorophyll a concentration were high. During the next 2 yr (1983—1984) the planktivore population increased rapidly to densities >20 000 individuals/ha, zooplankton biomass density decreased to <1600 µg/L and chlorophyll a concentration decreased. During the final 2 yr of the study, piscivores recruited to near prewinterkill levels, planktivores were reduced to <8000 individuals/ha, zooplankton biomass increased, and chlorophyll a concentration decreased. Over the 7 yr data set, we found a strong negative correlation between numbers of piscivores and planktivores, a weaker correlation between numbers of planktivores and zooplankton biomass, and no between—year correlation between zooplankton biomass and chlorophyll a concentration. There was, however, a positive correlation between total epilimnetic phosphorus and chlorophyll a concentration. These data are consistent with predictions made by the bottom—up:top—down model, and the implication is that at Lake St. George, the trophic cascade uncouples at the zooplankton → phytoplankton link. We speculate that this may be due to the combined effects of lake trophy and Daphnia species composition and size.

Woody seedling dynamics in an east Texas floodplain forest.

Abstract: We monitored woody plant seed deposition, seedling emergence, and the survival and growth of seedlings (i.e., plants ≤0.5 m tall regardless of age) in an East Texas river floodplain forest from 1979 through 1984. In addition, we estimated the relative importance of flooding, drought, fungal attack, herbivory, proximity to a conspecific adult, and shade in causing seedling mortality. Tree species fell into two major groups on the basis of their demographic characteristics and responses to unfavorable conditions. The first group was composed of heavy—seeded species, of which water oak (Quercus nigra) was the primary example. They produced few seeds, but had high seedling survival. Seedlings of these species emerged late in the summer, thereby avoiding peak periods of flooding and damping—off mortality. Seedling survival was little affected by drought, herbivory, or proximity to a conspecific adult. The second group included most of the common tree species (e.g., ironwood, Carpinus caroliniana; sweetgum, Liq...

Ecological, Taxonomic, and Life History Correlates of Seed Mass Among Indiana Dune Angiosperms

Abstract: This study evaluated the ecological and taxonomic correlates of seed mass variation among 648 angiosperm species of the Indiana Dunes region (1 13 families, 507 genera in the original flora). The sample represented 50% of the species, 60% of the genera, and 67% of the families reported from the area. Species were chosen at random from the published flora. Each species was characterized by family membership, habitat, life history, phenological characters, and native vs. alien status, in order to determine the relationship among species between these variables and mean seed mass. Unique to this study are measurements of the effects of phenology and taxonomic family on seed mass. Each species occurred in - 1 of 13 habitat types described in the Indiana Dunes flora. To determine the effect of apparent water and light availability on seed mass, each habitat was assigned to one of four categories representing combinations of inferred water and light availability. Life histories or life forms represented were: annuals, biennials, herbaceous vines, parasites, perennials, short-lived perennials, shrubs, small trees, trees, and woody vines. Two phenological variables were available for most species: the time at which flowering begins (early, middle, or late), and the duration of flowering (short: 3 mo). The mean seed mass of each species was established by weighing samples from herbarium specimens in the United States National Herbarium. The frequency distribution of raw seed mass is highly skewed among species, so mean seed mass for each species was assigned to 1 of 14 seed mass classes based on a log scale (cf. Baker 1972). This transformation achieved a nearly normal distribution and made the data presentation comparable to that of Baker. One-way ANOVAs measured the effects of each factor across all other variables; two-way ANOVAs were conducted to detect significant interactions and strong associations between characters; multi-factorial ANOVAs were performed to measure the effect of each class variable independent of the others and to corroborate associations between characters suggested by the two-way ANOVAs. One-way ANOVAs revealed a statistically significant effect of all ecological and taxonomic factors on seed mass, with the exception of native vs. alien status. Seed mass variance was accounted for as follows: family, 30%; life history, 22%; habitat, 8%; water/light category, 5%; onset of flowering, 5%; and duration of flowering, 4%. The smallest seeds were produced by the Scrophulariaceae and the largest by the Fabaceae, Liliaceae, and Rosaceae. The largest seeded species lived in closed habitats: wooded dunes, thickets, and wet dunes. Small- seeded species were associated with open habitats: wet dunes, aquatic habitats, marshes, and streamsides. Light availability was a better predictor of relative seed mass than was inferred water availability. The strong effect of life history on seed mass was due primarily to the extremely large seeds produced by trees. Significant differences in seed mass were not detected among life forms producing seeds of intermediate mass (seeds produced by annuals, biennials, perennials, shrubs, small trees, and vines were statistically indistinguishable). Early flowering and flowering of short duration were associated with the production of large seeds. Two-way ANOVAs detected eight significant two-way interactions: family x native vs. alien status, family x life history, family x onset of flowering, habitat x life history, water/light category x life history, water/ light category x native vs. alien status, life history x duration of flowering, and onset of flowering x duration of flowering. These were further evaluated to determine the source of the interaction. Multi-factorial ANOVAs provided measurements of the effects of one class variable on seed mass while all other variables were controlled statistically. The statistical effect on seed mass of each variable independent of the others was much lower than in the one-way ANOVAs. Seed mass variance was explained independently by each variable as follows: family, 13%; life history, 13%; habitat, 4%; water/light category, 3%; onset of flowering, 1 %; and duration of flowering, 1%. Two strong associations were detected in these ANOVAs: an association between family and habitat, and between life history and onset of flowering. Although the effects of ecological and life history characters on seed mass evaluated in this study were highly statistically significant, the R2 values associated with these effects were quite low. For example, habitat accounted for only 4% of the variance in seed mass independent of the other effects measured. This suggests that at the geographic scale investigated in this study, there is no primary habitat-specific ecological attribute that determines the seed mass of a habitat's component species. Seed mass of a species is determined by a combination of its

Desert rodent community structure : a test of four mechanisms of coexistence

Abstract: Four mechanisms of coexistence are considered that may contribute to the diversity of desert granivorous rodent communities. In the first, bush/open microhabitat selection, coexistence is possible if there is a trade—off between foraging efficiency in the bush and open microhabitats. In the second, temporal variation in resource abundances, coexistence is possible if there is a trade—off between foraging efficiency and maintenance efficiency. The first species can forage profitably on low resource abundances while the second uses dormancy to travel inexpensively in time between periods of high resource abundances. In the third, spatial variation in resource abundance, coexistence is possible if there is a trade—off between foraging efficiency and the cost of travel. The first species forages patches to a lower giving—up density, (the density of resource at which a forager ceases foraging), while the second can inexpensively travel between patches with high resource abundances. In the fourth, seasonal rotation in foraging efficiencies, coexistence is possible if there is a trade—off between the costs of foraging during different seasons. The species that is the more efficient forager changes seasonally. The first mechanism of coexistence has received much empirical attention and support. The other three have not previously been considered with desert rodents. In a community of four granivorous rodent species, I used artificial seed patches to measure species— and habitat—specific foraging efficiencies and live—trapping to measure population sizes and mean distances between recaptures. Of the four, the fourth mechanism of coexistence best explained the presence of Perognathus amplus, Dipodomys merriami, and Spermophilus tereticaudus in the community. Each species enjoyed a period of the year during which it was the most efficient forager. Furthermore, the annual population densities of these three species fluctuated out of phase. Seasonal changes in species—specific predation risks and body size—dependent metabolic costs may have accounted for these results. The third mechanism of coexistence best explained the presence of Ammospermophilus harrisii in the community. This species preferred to forage a large number of widely spaced patches to a high giving—up density rather than forage a few patches to a low giving—up density.

Plant Recolonization and Vegetation Succession on the Krakatau Islands, Indonesia

Abstract: The development of the vegetation and floras of the Krakatau Islands in the Sunda Straits, Indonesia, since their "sterilization" in 1883 is described. Key features of the post—1883 environment, such as pedogenesis, geomorphology (coastal change), human influence, and recent volcanic activity are detailed, and their possible influence on spatial and temporal patterns in both vegetation and flora is discussed. Field work conducted in 1979, 1983, and 1984 has enabled an assessment of the present state of vegetation development, aided by plot—based sampling and analysis of the arboreal component, employing numerical classification (by TWINSPAN) and ordination (by DCA). The coastal communities were established early and have remained little changed, although the distribution of the various components has changed according to the influence of a dynamic coastal geomorphology. Fifty years after forest closure, the forest of the interiors remain species—poor and composed of typical early—seral species. Rakata was dominated inland by Neonauclea calycina and Ficus pubinervis up to ≈550 m altitude, above which a mossy forest of Ficus spp. and Schefflera polybotrya scrub was recorded. The main inland forest types on Rakata Kecil and Sertung were of young, evenaged stands of Timonius compressicaulis (extensive) and older stands of Dysoxylum gaudichaudianum (often with a T. compressicaulis understory). The principal axes of variation within the data were found to be between extremes of the Rakata forest types, with Sertung and R. Kecil stands remaining undifferentiated until lower levels of the analyses. These patterns were ascribed to a mix of environmental variation (coastal and altitudinal factors), chance variations in colonization, and to volcanic action following the emergence of Anak Krakatau in 1927. Disturbance by volcanism (e.g., in 1930, 1934—1936, 1939, 1952—1953, 1961) has resulted in the deflection of vegetation succession on R. Kecil and Sertung into a different pathway from that followed on Rakata, which has remained unaffected by the activity. On Anak Krakatau, volcanic action has prevented successful colonization away from accreting coastal deposits, and has several times eliminated the entire flora. In addition, the major source of propagules for Anak Krakatau is from within the group, and for these several reasons the new island is shown to be a poor analogue for the early recovery phases of the other islands. Complete floral lists are given for each recorded survey for each island in the group, including data from surveys in 1979, 1982, and 1983. These data have been revised from previous publications on the basis of recent herbarium work and literature searches. The data are analyzed according to several different models: cumulative species totals, species totals for particular combinations of surveys, and totals calculated on the assumption of minimum turnover. The early beach spermatophyte assemblages of Rakata have undergone relatively few losses in comparison to the assemblages of the interior, while within the latter there has been a relatively high proportion of losses among the pioneering pteridophytes. The possession of major habitat types has been identified as critical in determining the shape of the overall colonization curve, through the passive sampling of different source pools. The diversity of the floras of the group as a whole and of Rakata and Anak Krakatau has continued to increase. The curves of species present on Sertung and R. Kecil have levelled and fallen respectively, as a result of the volcanic activity of Anak Krakatau. It is shown that the number of animal—transported species on the Krakatau group has continued to increase over the last 50 yr, and that this accounts for the majority of the increase in the size of the spermatophyte flora. The animal—transported species and the early sea—dispersed species appear to be species—stable groups. Later sea—dispersed species included ephemerals and species of temporary habitats, and have experienced a relatively high proportion of losses. Few beach species that have established on all of the three main islands have subsequently become extinct from the group. The number of pteridophytes on the islands has increased over the last 50 yr, mainly through the addition of forest species. A large proportion of plant species has been found only on Rakata, which samples an upland source pool not represented on the other islands. It is argued that the assumptions of "classical" island biogeography are inappropriate to these data and that the pattern in floral recolonization can best be understood as a successional process involving broad habitat and dispersal mechanism determinants. The implication of these findings is that community dynamics are highly significant in determining rates of immigration, colonization (i.e., successful immigration), and extinction, and that the probabilities of each vary among different groups of species and through time.

Effects of Body Size on the Predator‐Prey Interaction Between Pumpkinseed Sunfish and Gastropods

Abstract: Body size is known to play a critical role in determining patterns of prey selection. In this study, we examined the diets of pumpkinseed sunfish (Lepomis gibbosus) from three Michigan lakes. Pumpkinseeds have highly developed pharyngeal jaws specialized for crushing gastropods, and in our study lakes gastropods consistently contributed >80% of the prey mass in pumpkinseed diets. However, the average dietary composition and prey selection among snail taxa and size classes varied considerably among dates and sites. We hypothesized that this variation was influenced by changes in the size structure of the snail community. We used laboratory studies to quantify the effect of snail (and fish) size on three important components of the predator—prey interaction: encounter rates, attack probabilities, and capture successes. We then used these laboratory data to predict prey selection observed in the field. For most of our field situations, a simple model based on size—specific encouter rates only) explained a large percent (71%) of the observed variation in prey selection. However, in those cases where some of the snails were resistant to predation, due to crushing resistance or gape limitation, this simple model was a very poor predictor of prey selection. A more complex model (based on encounter rates and size refuges) successfully explained 46% of the variance in these cases where snails were relatively invulnerable. Finally, we compared estimates of attack probabilities with predictions from optimal foraging theory and found qualitative agreement in that fish ignored prey of low profitabilities and became more selective as the quality of the environment improved. however, the incorporation of variable attack probabilities into the foraging model resolved only a small part of the observed residual variation in selectivities because snails with low profitabilities were already underrepresented in the diet due to their low encounter rates or capture successes. This study demonstrates that predator—prey interactions in size—structured populations can create apparently complex variation in prey selection, but that this variation can be largely understood within a framework that simultaneously considers the dynamics of prey size distributions and how components of the foraging interaction scale with body size of the predator and prey.

Juvenile Foraging Proficiency, Parental Effort, and Avian Reproductive Success

Abstract: We measured the allocation of time and energy in a population of adult Yellow—eyed Juncos (Junco phaeonotus) and their young (nestlings, fledglings, independent juveniles) throughout the breeding season using concurrent time—activity budgets and doubly labeled water. We constructed energy budgets by extrapolating laboratory measurements of metabolic heat production to field conditions using a linear heat—transfer model and the operative temperature and wind speed experienced by the free—living bird. From our data we calculated daily energy expenditure (HD), the proportion of HD allocated to physical activity vs. maintenance metabolism (basal + thermostatic costs), and foraging efficiency. We examined diet selectivity among parents and their young, and we calculated prey capture rates based on the measured energy content of insect prey. We found that adult juncos feeding young are neither food limited nor working maximally. Adults fulfilled their own energy demands, and those of their four dependent young, while foraging for 75% or less of the daylight hours. In contrast, recently independent young, 4—7 wk after fledging, must forage for >90% of the daylight hours to meet their own energy demands. Juncos younger than this cannot attain energy balance in a 15—h day without supplemental feeding by their parents, even if they forage continuously. The HD of adult juncos (mean mass 19.5 g) remained fairly constant throughout the breeding season, averaging 73.8 kJ/d (n=51), which is 2.1 times their measured nighttime basal metabolic rate. In contrast, HD of fledging juncos increased steadily with age, from 59.7 kJ/d during the 1 st wk out of the nest to 73.8 kJ/d during week 3. This increase resulted partly from an increase in the proportion of time spent foraging. Once their parents quit supplying them with food, the young juncos' HD increased dramatically, peaking at 100 kJ/d in 10—12 wk—old juveniles. The increase in HD of independent young resulted initially from a dramatic increase in time spent foraging, which gradually declined only to be supplanted by an increase in aggressive interactions, especially flights within the juvenile flocks, and a marked increase in energy demand associated with the postjuvenile molt. For adult and young juncos, thermostatic costs represented respectively 20 and 23% of total daily energy expenditure, an amount of energy approaching that devoted to all physical activities. Thermostatic costs would have been even greater than this except that juncos are much better insulated than typical 19.5—g passerine birds (their coefficient of heat transfer is only 76% of that predicted from their mass). For these small birds, adaptations that affect body insulation can have more effect on daily energy budgets than changes in the time allocated to various physical activities. Our data reveal that energy constraints are a major selective force in Yellow—eyed Juncos, operating not through food limitation among adults but rather through the inefficient foraging of young juncos. The critical stage when selection operates most strongly on the efficient use of time and energy occurs during the first 3 mo of a junco's life. The lack of proficient foraging by young juncos explains observed patterns of parental time and energy expenditure and breeding season phenology.

Microbial and Animal Processing of Detritus in a Woodland Stream

Abstract: . . . . . . . . . . . . . . . . . . . . . . ........ ... .. ........ . .. ....... .. . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... ... ....... ... ........ .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... . .. . .. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ......... ...... .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 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Effects of Flow Regime and Cyprinid Predation on a Headwater Stream

Abstract: We used descriptive and experimental approaches on Gould Creek, a first- order tributary of the Mississippi River near Lake Itasca, Minnesota to assess the influence of (1) flow regime on the colonization dynamics and abundance of invertebrates and cyprinids, and (2) cyprinid predation on invertebrates and fishes. Stream flow varied an- nually during the 3-yr (1984-1986) period. One dry year (1984), with few periods of elevated flow, was followed by two wet years (1985-1986), with prolonged elevated (nonscouring) discharge. The density of benthic and drifting invertebrates increased dramatically with elevated flow. Benthic riffle invertebrates in particular increased, from a maximum of 9000 individuals/in in 1984 to 91 000 individuals/i2 in 1985 and 51 000 individuals/i2 in 1986. Larval Hydropsychidae and Simuliidae were the primary groups increasing in abun- dance during elevated flow. To assess directly the influence of flow regime on benthic insect densities, flow was manipulated in six subsections of a riffle and colonization of natural rock substrates monitored. Total insect abundance was higher under elevated (nonscouring) vs. low flow within 6-8 d; after 24 d insect abundance was three times as high under elevated flow. Insect families responded differently to elevated flow, with the largest increase resulting from a pulse of colonization by larval Hydropsychidae in 6-8 d. Cyprinid density in Gould Creek also increased with elevated flow from 0.1-0.3 cyp- rinids/m2 in 1984 to 1-2 cyprinids/m2 in 1985 and 1986. The increased fish density was most pronounced during spawning periods in spring (May-June) and consisted primarily of older individuals. All cyprinids, except the creek chub (Semotilus atromaculatus), de- creased in abundance 4-5 wk after spring colonization, regardless of flow conditions and invertebrate abundance. All cyprinids in Gould Creek selected pool habitats. Experiments in an artificial stream on Gould Creek indicated cyprinids influenced invertebrate abun- dance, but the effect of cyprinid predation was variable among habitats. Invertebrate abundance decreased most in structurally complex pools but exhibited little response to cyprinid predation in shallow riffle and raceway habitats. Because predation intensity varied among habitats, pool-dwelling invertebrates such as Chironomidae and Crustacea decreased more in the presence of cyprinid predation than riffle-dwelling Hydropsychidae and Sim- uliidae. However, if Simuliidae occurred in pool habitats, they were strongly selected by cyprinids, resulting in a significant depression in prey size in pools. Experiments in the artificial stream indicated creek chubs preyed on adult cyprinids, but larger species (adults 70-80 mm) were less susceptible to predation than smaller species (adults 50-60 mm). However, even taxa with small adult size were preyed on at a low rate, and all cyprinids strongly selected pools, with creek chubs having minimal effect on habitat use. These results suggest that (1) the hydrologic regime has broad and pronounced effects on the colonization dynamics and abundance of invertebrates and fishes in headwater streams, and (2) cyprinid predation has weaker but variable effects on the abundance of stream organisms. Predation intensity varies (a) over short temporal scales, because of the dynamic nature of flow regime and the rapid colonizing ability but short post-spawning persistence of cyprinids, (b) over small spatial scales, because of increased abundance of cyprinids in pool vs. riffle habitats, (c) between invertebrate and vertebrate trophic levels because the creek chub is a relatively ineffective piscivore, and (d) between small and large fish because many minnows have a size refuge from creek chubs.

Interplay of Morphology and Development on Size Inequality: A Polygonum Greenhouse Study

Abstract: Increases in size inequalities with time in cohorts of plants have been attributed to two causes: (1) variation in the exponential relative growth rates (RGR) of plants and (2) asymmetry of competitive interactions, whereby large plants can more readily suppress the growth of smaller neighbors than vice versa. It follows, then, that species with a longer exponential growth phase should develop greater size inequality over time. And since asymmetry is thought to be greater when competition is for light than when it is for below—ground resources, competitive interactions should be more asymmetric and size inequality should increase more in response to competition in species whose morphology results in greater between—plant shading. I tested these two hypothesis in greenhouse studies of two annual species, Polygonum arenastrum and P. douglassii, which differ in the duration of exponential growth and the likelihood of between—plant shading as a result of differences in development and morphology. Polygonum arenastrum has a shorter exponential phase of growth because it switches from vegetative growth to reproduction earlier than P. douglassii. It also has a prostrate growth form, more diffuse branching, and smaller leaves than P. douglassii. There should therefore be less shading of one plant by another in P. arenastrum than in P. douglassii. Seedlings from wild—collected seeds were planted in monoculture arrays at four different densities. The plants were harvested at five dates up to 10 wks; growth and branching pattern were measured at each harvest. As expected, in populations of P. douglassii, the species with the longer exponential growth phase, greater size inequality developed over time. In the absence of intraspecific competition, RGR was independent of plant size and size hierarchies were quite permanent. In contrast, P. arenastrum exhibited a period of decelerating growth which resulted in a negative correlation between RGR and plant size, frequent reversals in the size rankings of individuals over time, and declining size inequality through time under noncompetitive conditions. At high densities the morphology and RGRs of plants were size dependent in both species. Large individuals were more branched, had longer branches, and had higher RGRs than small individuals, and these shape and growth—rate differences became more pronounced as competition intensified. Furthermore, the size—dependence of shape and RGR was stronger in P. douglassii at all densities, and became relatively more pronounced as density increased, compared with P. arenastrum. These results are consistent with the expectation of greater competitive asymmetry in the species whose morphology should have led to greater between—plant shading. Size inequality increased with competition in both species. However, contrary to expectation, it did not increase more as density increased in P. douglassii. Reasons for the discrepancy between the morphological, RGR, and size inequality responses of the two species to density are discussed. The link between size inequality and plant development and morphology is examined in light of the effects the latter may have on the size—dependence of RGRs and the asymmetries of competitive interactions.

Demographic Effects of Reproductive Resource Supplementation in a Territorial Frog, Dendrobates Pumilio

Abstract: To determine whether tadpole-rearing sites (bromeliads) or oviposition sub- strates (leaf litter) were the objects of male defense in the poison-dart frog Dendrobates pumilio, I experimentally increased the availability of these two resources in a complete two-factor factorial design for variance analysis and examined the demographic response of the frogs. I also examined the effects of resource addition on sex ratio and age-structure. Mark-recapture data were collected on experimental and control plots during the 7 mo of the treatment period, during which 4754 observations were made on 1208 marked indi- viduals. I used a three-factor repeated measures analysis of variance to determine if resource addition affected density. The addition of oviposition substrate did not result in increased density, but the addition of bromeliads led to an increase in the density of adult males and females on treatment plots relative to the controls. To document which demographic variables were affected, animals were classified as immigrants, recruits, or survivors; all losses were assumed to be the result of emigration. The demographic variables were ana- lyzed with three-factor repeated measures ANOVA. The increase in males resulted from increased survival on treatment plots, whereas the increase in females resulted from in- creased immigration, recruitment, and the accumulation of survivors. Sex ratio was not affected by resource supplementation but age-structure was; litter addition increased the proportion of juveniles on treatment plots. The population-level response suggests that bromeliad availability is limited and this resource is defended by males.

Late-Quaternary History of High-Elevation Vegetation in the White Mountains of New Hampshire

Abstract: The pollen and plant—macrofossil records from four small lakes in the subalpine and alpine zone of the White Mountains, New Hampshire, give a 13 000—yr paleoenvironmental history. The White Mountains were deglaciated before 13 000 yr BP. Downwasting of the continental ice sheet was rapid. The summits projected above the ice as nunataks for only a brief period of time. Residual ice may have existed in Franconia Notch until 11 000 yr BP. From 13 000 to 11 750 yr BP a barren periglacial desert covered the highest altitudes in the White Mountains. Tundra vegetation occupied the lower slopes and valleys. The mean annual temperature was roughly 5°—10°C colder than today. Sparse tundra vegetation surrounded all four high—elevation sites from 11 750 to 10 300 yr ago and several taxa, particularly Artemisia and Caryophyllaceae, indicate disturbance. The summits were subjected to intense periglacial activity. The mean annual temperature was 4—6° lower than present. By 10 300 yr BP shrubs such as willow, juniper, an...

Hydrology, Community Structure, and Productivity Patterns of a Dystrophic Carolina Bay Wetland

Abstract: A shallow, 5.4-ha dystrophic Carolina bay wetland was studied between 1974 and 1978 to test the premise that biomass and production are constrained by the stagnant hydrology and dilute, acidic chemistry of bay wetlands. Our objectives were to evaluate: (1) surface and subsurface hydrology, (2) sources of production, (3) community change along a depth gradient, and (4) seasonal community patterns. The hydrology study compared surface water levels to groundwater levels in four adjacent wells. A cylinder enclosure and total harvest procedure and 24-h dissolved oxygen curves were used for 2 yr in a 1-ha sampling area to measure spatial and temporal biomass patterns and organic production of community components.

Community Responses to Variable Predation: Field Studies With Sunfish and Freshwater Macroinvertebrates

Abstract: I studied the impact of variable predation by bluegill sunfish on macroin- vertebrate prey in a North Florida lake. Underwater time-lapse cinematography and cen- suses of bluegill abundances in shallow, middepth, and deep habitats permitted estimation of predation intensity and variability within and among lake habitats. I then incorporated predation rates typical of the middepth zone in caging experiments where predation fluc- tuated in one treatment and remained constant in another. Prey community structure was subsequently monitored for 1 yr under variable, constant, ambient, and no predation regimes. Patchy, temporally variable predation characterized middepth and deep lake habitats, whereas in the shallow zone predation was relatively constant and homogeneous. Predation varied significantly every 2-4 wk in the middepth zone, but varied little between consecutive weeks or days. Caging experiments revealed that variable predation altered prey community composition and increased the mean size and size range of some prey (e.g., Odonata) as compared to the constant predation treatment. Prey abundances also appeared more het- erogeneous among cages (patches) and varied more temporally under a variable predation regime. However, total prey abundance, species abundance, and within-patch spatial het- erogeneity did not differ among predator treatments. In general, the macroinvertebrate community exposed to variable predation more closely approximated the natural middepth zone community than that from the constant predation regime. Previous studies of fish predation on macroinvertebrate communities have concentrated on shallow littoral hab- itats, but these results suggest that conclusions drawn from shallow habitats may not be representative of all lake zones. Variable predation may occur in many systems, and may contribute substantially to the spatial heterogeneity, temporal inconstancy, and species composition of prey communities.

Variation in Reproductive Strategy Among Clones of the Bryozoan Celleporella Hyalina

Abstract: To test alternative hypotheses concerning life history flexibility in variable environments I focused on the genotypic and environmental components of variation in Sex Ratio and Reproductive Allocation in clones of a benthic modular animal, the bryozoan Celleporella hyalina, grown in different flow regimes through all seasons. Clones of this bryozoan growing in autumn and winter varied significantly in growth rate, modular sex ratio, and relative investment in sexual reproduction. Experimental reduction of ambient water flow significantly reduced growth rate but did not affect Reproductive Allocation or Sex Ratio. Clonal differences in Reproductive Allocation were associated with a trade-off between somatic and sexual functions: clones investing more heavily in sex reached a smaller somatic size. In spring and summer, Celleporella clones showed an inconsistent and less pronounced growth response to flow regime. Significant genetically based variation was found for all measured characters except Reproductive Allocation. In both seasonal groups, clonal Sex Ratio was genetically canalized, and insensitive to the experimental treatments. The presence of clonal variability in Reproductive Allocation in autumn/winter but not in spring/summer may arise from seasonal differences in lifespan on natural substrata. These may select for maximal sexual investment in short-lived generations, but allow scope for alternative strategies if potential longevity is greater. The genetic constraint on clonal Sex Ratio may not conflict with models predicting flexible sex expression in variable environments, since the seasonal fluctuations experienced probably do not affect sex-specific reproductive success. The small Genotype x Treatment interactions and the positive correlations between clonal performance in differing treatments and seasons, suggest that the relative fitness of clones does not vary, a result at odds with assumptions underlying current hypotheses for the adaptive basis of sexual reproduction.