Showing papers in "Ecology in 1982"
TL;DR: The effects of initial nitrogen and lignin contents of six species of hardwood leaves on their decomposition dynamics were studied at the Hubbard Brook Experimental Forest by inverse linear relationships between the percentage of original mass remaining and the nitrogen concentration in the residual material.
Abstract: The effects of initial nitrogen and lignin contents of six species of hardwood leaves on their decomposition dynamics were studied at the Hubbard Brook Experimental Forest. Rate con- stants (k) for annual leaf mass loss ranged from -0.08 to -0.47. The rate constants (k) had a negative linear correlation (r2 = .89) with the ratio of initial lignin concentration to initial nitrogen concentra- tion. Decomposition dynamics of the litter materials were described by inverse linear relationships between the percentage of original mass remaining and the nitrogen concentration in the residual material. Initial lignin concentration was highly correlated (r2 = .93) with the slope of the inverse linear relationship for each of the litter types.
2,612 citations
TL;DR: Bluegill sunfish restricted to experimental ponds varying in vegetation density grew better and consumed more prey at intermediate Macrophyte density than fish held at either low or high macrophyte densities, suggesting that feeding rates of predators may be maximized at intermediate structure.
Abstract: Structural complexity of the habitat often reduces predatory efficiency by reducing prey capture rates. Prey density is often positively correlated with habitat structure because it pro- vides food and substrate to the prey as well as a relative refuge from predators. Dense structure inhibits foraging, allowing abundant, highly profitable prey to coexist with predators. Sparse structure allows efficient foraging and generally contains few highly profitable prey. This suggests that feeding rates of predators may be maximized at intermediate structure. If this is true, we might also expect predator growth rates to be higher in intermediate structure habitats. Since diet breadth is thought to be related to rates of encounter with profitable prey, we also expect diets of predators to be narrower at intermediate structure than in either sparsely or densely structured habitats. Bluegill sunfish (Lepomis macrochirus) restricted to experimental ponds varying in vegetation density grew better and consumed more prey at intermediate macrophyte density than fish held at either low or high macrophyte densities. Fish at low macrophyte density had narrower diets than expected due to high initial prey availability relative to prey available at intermediate and high macro- phyte density. Fish at high macrophyte density ate fewer, but larger, prey and thus had a narrower diet than expected. Fish predation reduced total prey biomass as well as mean prey size and altered the prey community structure by removing large active invertebrate predators and herbivores with subsequent release of smaller invertebrate predators and herbivores. These changes in prey com- munity structure were also mediated by habitat structure. Habitat structure-food density interactions may be added to temperature and presence of predators as variables that influence the use of resources by fishes.
1,351 citations
TL;DR: Dynamic models of quantitative (polygenic) characters are more generally applicable in the analysis of life history evolution than are static optimization methods or one and two locus genetic models.
Abstract: Dynamic models of quantitative (polygenic) characters are more generally applicable in the analysis of life history evolution than are static optimization methods or one and two locus genetic models. A dynamic theory of life history evolution is derived by synthesizing population demography with quantitative genetics. In a population under weak selection with a nearly stable age distribution, the relative fitness of individuals with a particular life history phenotype can be approximated as an average of age—specific relative fecundity and mortality rates, weighted respectively by the present productivity and future reproductive value of each age—class. An adaptive topography is constructed showing that, with phenotype— and age—specific fecundity and mortality rates constant in time, evolution of the mean life history maximizes the intrinsic rate of increase of a population. However, the rate and direction of evolution in response to selection are strongly influenced by genetic correlations among characters. Negative genetic correlations among major components of fitness are often obscured phenotypically by positive environmental correlations, but commonly constitute the ultimate constraint on life history evolution, as illustrated by artificial selection experiments. Methods are suggested for measuring selective forces and evolutionary constraints that effect life history characters in natural populations.
945 citations
TL;DR: Two general analytical approaches to the examination of decomposition data are reviewed and single and double exponential models best describe the loss of mass over time with an element of biological realism.
Abstract: The study of plant litter decomposition in terrestrial ecosystems commonly employs litter bags to compare the loss of mass among species, among sites, and under various experimental manipulations, or to investigate the process itself. Analysis of the resulting data is quite variable among investigators, and at times inappropriate. Two general analytical approaches to the examination of decomposition data are reviewed. Analysis of variance is useful if the intent is to compare treatment means, but does not directly test hypotheses regarding decomposition rates. If the intent is to determine rate constants, than fitting mathematical models to data is the more appropriate analysis. Single and double exponential models best describe the loss of mass over time with an element of biological realism. See full-text article at JSTOR
937 citations
TL;DR: The disturbance regimes in the forests studied favored tolerant species but allowed opportunists to persist at low densities, and vegetation within gaps increased in woody species diversity, total basal area, and total number of stems.
Abstract: To characterize the disturbance regime of one type of vegetation, study areas in which relatively small-scale disturbance predominates were chosen in several old-growth mesic forests in the eastern United States. Canopy openings covered 9.5% of total land area. New gaps were formed at an average rate of 1% of total land area per year; old gap area closed at a similar rate primarily by sapling height growth. With increased gap size, vegetation within gaps increased in woody species diversity, total basal area, and total number of stems. Stems also showed accelerated growth into larger size classes. As gaps aged, stems grew into larger size classes and basal area increased. Species responses to canopy gaps varied. Some species survived and became established in fairly small gaps (50-100 M2). Although in large gaps (up to 2009 m2 in the present study) these species usually increased in total number of stems and basal area, they declined in importance relative to species which rarely survived in small gaps but grew rapidly in large gaps. The disturbance regimes in the forests studied favored tolerant species but allowed opportunists to persist at low densities.
937 citations
TL;DR: In this article, the authors examined fish assemblage structure, and factors and mechanisms appearing important in the ecological maintence of these structures, were examined for 18 small lakes in northern Wisconsin during summer and winter.
Abstract: Fish assemblage structure, and factors and mechanisms appearing important in the ecological maintence of these structures, were examined for 18 small lakes in northern Wisconsin during summer and winter. The study was focused around the following questions. Are there discrete, repeatable groups of fish assemblages? If so, are they temporally stable? What are the relations between fish assemblage structure and habitat complexity, physical disturbance, biotic interactions and the insular nature of small lakes? A comparative approach was used to generate hypotheses and propose explanations concerning the roles of these factors in structuring the assemblages. Multivariate classification, ordination, and discriminant analyses helped discern two assemblage types: Umbra—cyprinid and centrarchid—Esox. Each had a distinctive species composition and seasonal change in composition. Environmental characteristic of the lakes occupied by each assemblage type also differed consistently. The type of assemblage present in a lake appeared related to oxygen concentrations in winter, interacting with the availability of refuges from either a severe physical environment (low oxygen during winter) or from large pisvivores. Centrarchid—Esox assemblages occurred in lakes with high winter oxygen levels, and also in lakes with low oxygen if a stream or connecting lake could provide a refuge from these conditions in winter. When no refuge was present, low winter oxygen lakes lacked piscivorous fishes, but contained Umbra—cyprinid assemblages. The relationships between species richness in summer and environmental factors were generally similar for the two assemblage types, but the relative importance of individual factors differed. In winter, richness relationships in centrarchid—Esox assemblages for most environmental factors were reversed from those of summer. No significant seasonal change occurred in the Umbra—cyprinid assemblages. Habitat complexity factors, particulary vegetation diversity, were significantly related to summer species richness in both assemblage types. Lake area was also related to summer richness for both types, but the slope of the species—area regression was much steeper for Umbra—cypinid assemblages than for those in centrarchid—Esox lakes. Species richness relationships with winter oxygen concentration were negative in both seasons in Umbra—cyprinid lakes, but the relationship was positive for centrarchid—Esox assemblages in winter. A measure of lake connectedness was related to summer richness in centrarchid—Esox lakes. These patterns suggest that centrarchid—Esox assemblages are in ecological equilibrium but that a disturbance—induced disequilibrium occurs in Umbra—cyprinid assemblages.
566 citations
TL;DR: Previous studies on the mechanisms of honeybee recruitment communication indicate that the foraging strategy of a honeybee colony involves surveying the food source patches within a vast area around its nest, pooling the reconnaissance of its many foragers, and using this information to focus its forager force on a few high-quality patches within its foraging area.
Abstract: To understand the foraging strategy of honeybee colonies, we measured certain tem- poral and spatial patterns in the foraging activities of a colony living in a temperate deciduous forest. We monitored foraging activities by housing the colony in an observation hive and reading its re- cruitment dances to map its food source patches. We found that the colony routinely foraged several kilometres from its nest (median 1.7 km, 95% of foraging within 6.0 km), frequently (at least daily) adjusted its distribution of foragers on its patches, and worked relatively few patches each day (mean of 9.7 patches accounted for 90% of each day's forage). These foraging patterns, together with prior studies on the mechanisms of honeybee recruitment communication, indicate that the foraging strategy of a honeybee colony involves surveying the food source patches within a vast area around its nest, pooling the reconnaissance of its many foragers, and using this information to focus its forager force on a few high-quality patches within its foraging area.
559 citations
TL;DR: In this paper, standing crop, rates of production, mortality, decomposition, and nitrogen dynamics of two size classes of fine roots (0-0.5 mm and 0.0 mm diameter) were estimated for 1 yr in a 53- yr-old red pine (Pinus resinosa Ait.) plantation and in an adjacent 80-yr-old mixed hardwood stand in north-central Massachusetts.
Abstract: Standing crop, rates of production, mortality, decomposition, and nitrogen dynamics of two size classes of fine roots (0-0.5 mm and 0.5-3.0 mm diameter) were estimated for 1 yr in a 53- yr-old red pine (Pinus resinosa Ait.) plantation and in an adjacent 80-yr-old mixed hardwood stand in north-central Massachusetts. Dry matter of live fine roots was higher in the hardwoods (mean = 6.1 Mg/ha; annual range 3.6-8.6 Mg/ha) than in the plantation (mean = 5.1 Mg/ha; annual range 2.5- 7.8 Mg/ha). Dead root mass was similar in the hardwoods (mean = 4.4 Mg/ha) and the plantation (mean = 4.0 Mg/ha). Nitrogen standing crop of live roots in the hardwoods was higher than in the plantation (mean = 65 kg/ha and 42 kg/ha, respectively). Net fine root production was estimated from changes in standing crop. Production estimates ranged from 4.1 to 11.4 Mg-ha-l yr-1 in the hardwoods and from 3.2 to 10.9 Mg ha-l yr-1 in the plantation, depending on the assumptions made in the calculations. Concurrent estimates of total nitrogen re- quirement for this production ranged from 73 to 184 kg ha-l yr-I in the hardwoods and from 44 to 122 kg ha-l yr-I in the plantation. Decomposition, measured as mass loss from buried cloth bags, was -20% in 0.4-mm mesh bags and as high as 47% in 3-mm mesh bags after 1 yr. Integrating production and nitrogen requirements with estimates of decomposition rates and nitrogen mineral- ization for these ecosystems suggests that the lower estimates of production are more accurate.
544 citations
TL;DR: Constraints imposed by the structure of the vegetation and by the types and abundances of prey determine the available foraging opportunities and may affect the foraging traits of birds that can successfully exploit a particular habitat, and hence influence the patterns of bird habitat selection and community structure.
Abstract: The different searching tactics of passerine birds foraging for arthropods among the foliage of a northern hardwoods forest result in the capture of different kinds of prey. Five major searching modes are employed by the 11 foliage-foraging bird species in the Hubbard Brook Exper- imental Forest, New Hampshire. These are distinguished primarily by the rates and distances moved by the searching birds and by the types and forms of their prey-attacking maneuvers. These in turn reflect how large an area is scanned, how thoroughly it is searched, and how the bird moves from perch to perch in its search for prey. Mean searching and prey-attacking flight distances are positively correlated, indicating that birds move just far enough on average to take them into areas they have not previously searched visually. Likewise, birds that move rapidly while searching make significantly more prey attacks per unit time and hence encounter prey more often. Slow searchers scrutinize substrates more thoroughly and seem to take more cryptic and often larger prey. The results suggest that there are limitations on the ways that birds can search for and capture arthropod prey among foliage. We hypothesize that constraints imposed by the structure of the vegetation and by the types and abundances of prey determine the available foraging opportunities. Such habitat parameters may affect, in ecological or evolutionary time, the foraging traits of birds that can successfully exploit a particular habitat, and hence influence the patterns of bird habitat selection and community structure.
511 citations
TL;DR: Mature vegetation from eight of the 12 major vegetation zones in Oregon and Washington was sampled along a transect from the Pacific Coast to the east slopes of the Cascade Moun- tains, finding these systems receive more annual precipitation, and average twice the basal area and biomass.
Abstract: Mature vegetation from eight of the 12 major vegetation zones in Oregon and Wash- ington was sampled along a transect from the Pacific Coast to the east slopes of the Cascade Moun- tains. Six stands were in forests, one in woodland, and one in the shrub-steppe. Aboveground-over- story net primary production (NPP, estimated as the sum of annual stem, branch, and foliage production) ranged from 90% of the variation in leaf area index. Although annual precipitation ranged from 20 cm in the shrub-steppe to 260 cm at the coast, it was a relatively poor predictor of stand structure and produc- tion. Biomass and NPP increased linearly up to a leaf area of =30 ha/ha; above this point, biomass continued to increase while NPP decreased. Except in the coastal forest zones, NPP was less than maximum values reported for other mature systems elsewhere in the world for the same range in leaf area indices. Compared to other forested regions of the temperate zone with the same NPP, these systems receive more annual precipitation, and average twice the basal area and biomass.
467 citations
TL;DR: The degree of avoidance is related to the risk of predation and the possibility that feeding behavior represents an adaptive compromise between the conflicting demands of feeding efficiently and avoiding predators is discussed, and the effects on the predictions of optimal foraging theory of some common mecha- nisms by which notonectids avoid their predators are discussed.
Abstract: Field and laboratory experiments demonstrated that the degree to which an aquatic insect, Notonecta hoffmanni, shows avoidance behavior towards an important predator is propor- tional to the magnitude of the risk of predation. Laboratory experiments showed that I, II, and III instar juvenile N. hoffmanni can suffer significant mortality due to predation from adult N. hoffmanni whereas IV and V instars suffered no mortality from adults. In particular, the relative risk of predation from adults was I > II > III > IV = V - no risk. Based on these data I predicted that I, II, and III instars should avoid adults whereas IV and V instars should not, and that the relative degree of avoidance should be I > II > III > IV = V = no avoidance. Two measures of predator avoidance were documented: alterations in the use of space, and reductions in movement. In both the laboratory and the field the effects of adults on the behavior of juvenile notonectids were determined by comparing the juvenile's behavior when adults were experimentally removed to that in controls where adults were present. Alterations in spatial patterns were as predicted, i.e., in both the laboratory and the field only I, II, and III instars avoided adults by altering their use of space, and the degree to which spatial patterns were altered was highly correlated with the relative risk of predation. In the laboratory, reductions in movement were also as predicted, i.e., only I, II, and III instars reduced their movement in the presence of adults and the reduction in movement was highly correlated with the risk of predation. However, in the field, only I and III instars avoided adults by moving less; i.e., II instars did not conform to expectations. Accordingly, in the field the correlation between the reduction in movement and the risk of predation was positive but not signif- icant. In the laboratory, the effects of adults on juvenile feeding rates were also documented. Because I and II instars avoided adults by altering their spatial and movement patterns, they suffered a cost in reduced feeding rates. Unexpectedly, III instars altered their feeding behaviors but suffered no reduction in feeding rates. Partial correlation analysis showed that a decrease in the juvenile's use of the central region of water bodies (where prey were four times more dense) resulted in a significant decrease in their feeding rates, but that a reduction in their movement had no such effect. I then discuss the effects on the predictions of optimal foraging theory of some common mecha- nisms by which notonectids avoid their predators. Because the degree of avoidance is related to the risk of predation I also discuss the possibility that feeding behavior represents an adaptive compromise between the conflicting demands of feeding efficiently and avoiding predators. Finally, because the avoidance of predation reduces both the feeding rate of predators on foragers and of foragers on prey, I examine the effects of such reductions on population interactions among predators, foragers, and prey.
TL;DR: Analysis of censuses of breeding birds on islands in Pymatuning Lake, a reservoir at the Pennsylvania-Ohio border, yields the conclusion that for these islands the variation of the number of resident avian species with island size is that which one would expect if the birds were distributed randomly.
Abstract: Thorough censuses have been made of breeding birds on islands in Pymatuning Lake, a reservoir at the Pennsylvania-Ohio border Analysis of the censuses yields the conclusion that for these islands the variation of the number of resident avian species with island size is that which one would expect if the birds were distributed randomly, with the probability of a breeding pair residing on an island proportional to the area of the island and independent of the presence of other pairs This type of random placement of individuals can yield species-area relations which differ from those commonly employed for analysis of biogeographic data
TL;DR: The "biennial" plant species Verbascum thapsus, Oenothera biennis, Daucus carota, and Tragopogon dubius are all found in abandoned agricultural fields, but they differ in their time of first appearance, peak abundance, and length of time populations persist.
Abstract: The "biennial" plant species Verbascum thapsus, Oenothera biennis, Daucus carota, and Tragopogon dubius are all found in abandoned agricultural fields, but they differ in their time of first appearance, peak abundance, and length of time populations persist. In experimental field studies these species showed differences in their abilities to colonize various types of ground cover. Seedling emergence, survival, and growth were highest for all four species in patches of bare ground. The small-seeded species, Verbascum and Oenothera, survived only in bare ground. In contrast, the large-seeded species, Daucus and Tragopogon, survived in several ground cover types. Relative coverage of three ground cover types and differences in seedling performance in these cover types were compared in a 1-yr and 15-yr old field. Verbascum and Oenothera became estab- lished only in the 1-yr old field where bare ground was abundant and not in the 15-yr old field where these patches were rare ( 1-2 yr and are not often present in a newly disturbed area. Once dispersed to and established in an area, they can persist for several generations because their seedlings can survive in several types of ground cover, including those with thick vegetation.
TL;DR: Detailed analysis of differences in invertebrate com- munities among these streams is presented and the relative importance of food and habitat as determinants of abundances of different guilds and taxa is discussed.
Abstract: The relative importance of surrounding riparian vegetation and substrate composition on invertebrate community structure was investigated in six streams in Oregon, USA. We found that canopy type was more important than substrate character in influencing total abundance and guild structure. Streams without shading had higher abundances of invertebrates than did shaded streams. Most guilds were influenced by qualitative differences in food availability rather than quantity of food or substrate composition. Open streams had higher abundances in the collector-gatherer, filter feeder, herbivore shredder and piercer, and predator guilds. Contrary to expectations, shredders were no more abundant in shaded streams than in streams lacking a riparian canopy. Scraper density was inversely related to standing crop of aufwuchs, but biomass was positively correlated with quantity of aufwuchs. Examination of dominance-diversity curves showed that both canopy and substrate influenced ranked abundances of taxa, but neither canopy nor substrate strongly influenced number of taxa. Differences in community structure were not always revealed by analysis of community-level properties, although differences in both the absolute and relative abundances of individual taxa were observed. (algae, high-quality detritus). We did not develop de- tailed analyses of either food sources or invertebrates but presented only those data necessary to explain patterns of vertebrate abundance. We now present a detailed analysis of differences in invertebrate com- munities among these streams and discuss the relative importance of food and habitat as determinants of abundances of different guilds and taxa.
TL;DR: Taking data from censuses published in American Birds, rarefaction is used to ordinate the species richness of communities in terms of samples of equal numbers of individuals and bird species richness is minimal in coniferous forests characterized by high tree density, low canopy, and few species of trees.
Abstract: General patterns of density, species richness, and relative abundance of breeding birds are examined in a wide variety of North American forests. Taking data from censuses published in American Birds, we use rarefaction to ordinate the species richness of communities in terms of samples of equal numbers of individuals. By this criterion young forests in secondary succession and mature deciduous forests can be equally rich in bird species; coniferous forests and dense successional stands having only one or two species of trees have the fewest species of birds. For our data set the density of birds is higher in mature deciduous forests than in successional stands. The species/area relationship (sla) is a function of both this species/individuals relationship (s/n) and the individuals/ area relationship (n/a). According to the Jonckheere-Terpstra statistic there is a statistically significant pattern to the order in which bird communities fall with regard to s/n and n/a. To see these relationships in terms of the structure of the vegetation, we present the positions of 56 stands in a graphic space determined by principal components. Tree species richness and canopy cover dominate the first axis, and variation in canopy height the second. Tree density has a somewhat independent pattern of variation and is expressed by the third axis. In the bivariate space of principal components one and three, a SYMAP contour diagram of the number of bird species expected in 10 ha (sla) shows maximal values in mature deciduous forests, but not in those that have the highest tree species richness, canopy height, or tree density. The number of individual birds/10 ha (n/a) shows a similar pattern except that maximal density occurs at maximal values of tree species richness and canopy height. By this criterion both bird species richness and density are minimal in coniferous forests characterized by high tree density, low canopy, and few species of trees. These patterns are not discernible by the classic bird species diversity/foliage height diversity method proposed by Mac Arthur and Mac Arthur in 1961. They are masked by correlation coefficients, partly because the coefficients are insensitive to nonlinear relationships. We recommend rarefaction, principal components analysis, and contour diagrams to display relationships among communities.
TL;DR: In this article, the Fourier transform is used to estimate the home range in a probabilistic sense, making no assumption about the underlying distribution, and therefore is nonparametric.
Abstract: A new technique for estimating home range size from a sequence of independent observations of animal positions is introduced. This method describes home range in a probabilistic sense, makes no assumption about the underlying distribution, and therefore is nonparametric. The procedure is based upon an existing nonparametric technique for density estimation which uses the Fourier transform (Tarter and Kronmal 1970). A comparison is made with two commonly used methods; the minimum convex polygon method (Southwood 1966) and bivariate normal method (Jennrich and Turner 1969), as well as another recent nonparametric method developed by Ford and Krumme (1979). All these methods have certain problems, making none of them suitable for every situation. The trade—offs among these methods and some fundamental limitations of home range estimation are discussed. Evidence is given which indicates that in some situations the proposed method is preferable to earlier methods.
TL;DR: The rapid recovery illustrated by the total number of individuals, as a measure of community response, support the suggestion that fire and fire frequency have been major factors in producing a fire—adapted, species—rich small—mammal community on this heathland.
Abstract: A species—rich small—mammal community on 7 ha of diverse coastal heath in Myall Lakes National Park, New South Wales, Australia, was studied for 7 mo before the area burned in a wildfire (August 1974) and has subsequently been monitored for 5 yr since the fire. Species regularly trapped were the dasyurids Antechinus stuartii and Sminthopsis murina, the peramelid Isoodon macrourus, and the native murids Pseudomys novaehollandiae and P. gradcilicaudatus, together with the introduced Mus musculus. Pseudomys species, M. musculus, and S. murina reached greater abundance on early seral stages, while the abundance there of both Rattus fuscipes and R. lutreolus was much reduced. Species reach peak abundance in an orderly replacement sequence exhibiting a mammalian secondary succession. A habitat accommodation model is proposed with species entering the succession and reaching peak abundance as externally controlled changes in the vegetation fulfil the habitat requirements of each species. Species leave the succession or are greatly reduced in abundance when these local physical conditions move out of the optimal range for the species. This study shows that a major perturbation of a small—mammal community can produce a variety of responses among species, suggesting that regeneration time may represent a resource axis subdivided by community members in this secondary succession. The rapid recovery illustrated by the total number of individuals, as a measure of community response, support the suggestion that fire and fire frequency have been major factors in producing a fire—adapted, species—rich small—mammal community on this heathland.
TL;DR: The development and duration of the depauperate understory that succeeds canopy closure in southeast Alaska is closely related to the canopy structure of shade—tolerant Tsuga forests with their high foliar biomass.
Abstract: Understory vegetation undergoes successional stages during the 1st 300 yr after logging or fire disturbance in the coastal Picea—Tsuga forests of southeast Alaska. Residual shrubs and tree seedlings increase their growth within 5 yr after overstory removal. Understory biomass peaks at 5 Mg°ha — 1 °yr — 1 °15—25 yr after logging. Shrubs and herbs are virtually eliminated (<0.1 Mg/ha) from the understory after forest canopies close at stand ages of 25—35 yr. Bryophytes and ferns dominate understory biomass during the following century. An understory of deciduous shrubs and herbs is reestablished after 140—160 yr. Thereafter, biomass of the shrubs, herbs, and ferns continues to increase, while bryophyte biomass and tree productivity decline. Departures from this developmental sequence are related to unusual types of stand establishment, soil, microclimate, or disturbance. The development and duration of the depauperate understory that succeeds canopy closure in southeast Alaska is closely related to the canopy structure of shade—tolerant Tsuga forests with their high foliar biomass. In young—growth forests (<100 yr), the decline in understory development immediately after canopy closure is significantly associated with tree basal area and percentage of tree canopy cover. In old—growth forests, in contrast, understory biomass is correlated with mean tree diameter, age, and volume. It is hypothesized that understory development over the chronosequence responds primarily to changes induced in the light environment by developments in the forest canopy. Maintenance of the most productive forests in the aggradation stages of development (0—100 yr) through forest management will minimize the development of a productive vascular understory and thus deprive herbivores of forage during 70—80% of the forest rotation.
TL;DR: Evaluation of the flagellate—Bosmina association and its relationship to Bosmina—Daphnia interactions involved analysis of population responses in large—scale enclosures, in situ grazing experiments with natural and introduced resources resources, and laboratory preference tests.
Abstract: Because phytoplankton succession involves pulses of short duration and a degree of unpredictability, most workers have argued against resource specialization by zooplankters. Yet, in Lake Mitchell, Vermont, two genera of cladocerans (Bosmina and Daphnia) coexist under nearly equilibrium conditions that involve an intriguing conflict. Daphnia can depress Bosmina, but apparently cannot exclude it, even though both share substantial portions of available resources. This coexistence appears mediated by the specialized foraging mode of Bosmina, which deviates markedly from filter feeding in a way that permits efficiency removal of highly edible flagellates, particularly when these items occur at low density. Evaluation of the flagellate—Bosmina association and its relationship to Bosmina—Daphnia interactions involved: (1) analysis of population responses in large—scale enclosures, (2) in situ grazing experiments with natural and introduced resources resources, and (3) laboratory preference tests. See full-text...
TL;DR: This study will demon- strate both the immediate inhibition of certain species and the secondary effects which such a response has on other species, and species with some tolerance to the effects of the chemical will be identified and their role in the changing conditions will be considered.
Abstract: Experimental ponds received single additions of the herbicide atrazine in concentra- tions of 20 and 500 gg/L, and were compared to control ponds for 136 d. Atrazine is an inhibitor of photosynthesis, and both concentrations depressed phytoplankton growth in the ponds within a few days. This was followed by successional changes leading to the establishment of species of phyto- plankton more resistant to inhibition by atrazine. Laboratory studies verified this resistance and verified effects on other species at concentrations of atrazine as low as 1-5 Jig/L. When and to what extent resistant species appeared in the phytoplankton communities differed with treatment. At the atrazine concentration of 500 gg/L, there was a delayed appearance but eventually a greater biomass and persistence of these species. The grazing zooplankton influenced these differences and were in turn affected by them. Natural interactions such as competition and predation among the species of the communities greatly affected their responses to the toxic chemical. The importance of atrazine as an environmental pollutant is suggested by these responses to concentrations of 1-5 gg/L, which are common downstream in many agricultural watersheds, 20 /Ig/L, which is the high level found in these waters, and 500 gg/L, which is the high level found in waters directly adjacent to treated fields. zine in experimental ponds. This study will demon- strate both the immediate inhibition of certain species and the secondary effects which such a response has on other species. Species with some tolerance to the effects of the chemical will be identified in the com- munities, and their role in the changing conditions will also be considered. Atrazine (2-chloro-4-ethylamino-6-isopropylamino- s-triazine) is the single most heavily used pesticide in the United States. In 1976, 41 million kilograms (active ingredient) of this herbicide were applied on farms in
TL;DR: This work uses a simple test to evaluate the null hypotheses that desert rodent communities are composed of species assembled at random with respect to body size and suggests that ability to detect community structure depends to a large extent on thorough knowledge of the organisms and choice of appropriate statistical tests.
Abstract: We use a simple test to evaluate the null hypotheses that desert rodent communities are composed of species assembled at random with respect to body size. This hypothesis is unequivocally rejected for species in the granivore guild: species of similar size (body mass ratios <1.5) coexist less frequently in local communities and overlap less in their geographic distributions than expected on the basis of chance, suggesting that their co—occurrence is precluded by interspecific competition. When granivore species and members of other guilds are combined in the same analysis, the patterns apparent in granivores diminish or disappear. These results indicate that ability to detect community structure depends to a large extent on thorough knowledge of the organisms and choice of appropriate statistical tests. We suggest that recent claims that many communities are indistinguishable from random associations of species are premature.
TL;DR: The role of larval behavior in molding the zonation of the acorn barnacles Balanus glandula and Balanus crenatus is investigated and the data show that the vertical distribution of adults is a reflection of the vertical zonations of cyprids in the plankton and settlement patterns of spat.
Abstract: The nonoverlapping vertical distributions of sessile species in the marine intertidal are usually considered to result from two interacting factors: (1) broadly restricted settlement of juveniles in the vertical horizon, and (2) postsettlement mortality which affects species differently. Investigated here is the role of larval behavior in molding the zonation of the acorn barnacles Balanus glandula and Balanus crenatus. I document the vertical distributions of: (a) adults, (b) newly settled spat, and (c) planktonic cyprid larvae. The data show that the vertical distribution of adults is a reflection of the vertical zonation of cyprids in the plankton and settlement patterns of spat. Although postsettlement mortality and substratum selection occur, presettlement behavior can strongly influence vertical zonation of sessile organisms. See full-text article at JSTOR
TL;DR: The significance of highly rewarding winter fruits for the evolution of some physiological and behavioral traits among overwintering dispersers points to the existence of a closely coevolved system involving the latter and the assemblage of winter-ripening mediterranean evergreens.
Abstract: The hypothesis is formulated that, among vertebrate-dispersed plants, species ripening fruits at different times of year should differ in the nutritional properties of their fruits in such a way as to match the seasonally changing demands of their major dispersers. This was tested for a sample of 62 species of southern Spanish bird-dispersed plants, and results fully conform to expectations. Species ripening fruits during the dry mediterranean summer, when water demands of dispersers are highest, have the most watery fruits. Species producing fruits in winter, when energy needs of avian frugivores are at a maximum, possess fruits with the highest lipid content. No marked seasonal trend in protein yield of fruits was found, which is consistent with the fact that protein demands of avian dispersers appear to be fairly constant during the summer-through-winter period considered. Coupling between fruit quality and frugivores' needs is shown to be most likely related to coevolution between plants and birds, and not to fortuitous coincidence over time of fruit quality and disperser needs. The significance of highly rewarding winter fruits for the evolution of some physiological and behavioral traits among overwintering dispersers points to the existence of a closely coevolved system involving the latter and the assemblage of winter-ripening mediterranean evergreens. It is concluded that the seasonal gradient in plant-bird coevolutionary adjustment has been concurrently brought about by (1) seasonally changing demands of dispersers, and (2) the differential coevolutionary potentials open to the plant-bird system through changing spatio-temporal asymmetry in relationships between vege- tation and avifauna.
TL;DR: Variation in the number of fruit matured per inflorescence seemed to be related to variation in precipitation and plant survival through the fruit-filling period, and the data support both the second and third hypotheses.
Abstract: This study examines the factors that limit female reproductive function (fruit and seed production) in the annual plant, Cassia fasciculata Michx. (Leguminosae), an hermaphroditic herb that inhabits prairies, open woods, and waste places in the eastern USA. Removal of competitors and addition of water to plants in the field showed that resource levels prior to fruit filling influence plant size which, in turn, is positively related to the number of fruit matured. Manual addition of pollen to flowers in the field did not increase the number of fruit matured, suggesting that fruit production was not pollinator limited. Removal of rapidly growing fruit resulted in growth and mat- uration of fruit that normally would not have matured, indicating that resources limited fruit produc- tion. In four populations studied, 24-70% of initiated fruit matured, 12-40% aborted or died, 3-31% were still developing at the time of plant death, and 6-12% were lost to predators. In fruit that matured, between 23% and 73% of initiated seeds reached maturity, 22-43% were aborted, and 0- 49% were lost to seed predators. These data indicate that seed production was limited by resources and seed predators. It is hypothesized that overinitiation of fruit in C. fasciculata is an adaptive response to either: (I) the loss of juvenile fruits to seed predators, (2) variation in fruit quality coupled with the ability of the plant to selectively mature high-quality fruit, or (3) unpredictability of resource levels and survival during fruit filling. The generally low levels of predation on juvenile fruits make the first hypothesis unlikely for C. fasciculata. Plants did, however, selectively mature fruit with large num- bers of developing seeds. Furthermore, variation in the number of fruit matured per inflorescence seemed to be related to variation in precipitation and plant survival through the fruit-filling period. Thus, the data support both the second and third hypotheses.
TL;DR: The potential contribution of mycorrhizal fungi (as sporocarps and mycor rhizal sheaths) to total ecosystem biomass and turnover and nutrient distribution and turnover was examined in a 23-yr-old and a 180- yr-old Pacific silver fir stand in western Washington.
Abstract: The potential contribution of mycorrhizal fungi (as sporocarps and mycorrhizal sheaths) to total ecosystem biomass and turnover and nutrient distribution and turnover was examined in a 23-yr-old and a 180-yr-old Pacific silver fir (Abies amabilis) stand in western Washington While mycorrhizal fungi contributed roughly 1% to total ecosystem biomass in both stands, the percentage of net primary production (NPP) in the mycorrhizal fungal component was roughly 14% in the younger stand and 15% in the mature stand Mycorrhizal fungi plus conifer fine roots contributed :45% of NPP in the young stand and :75% in the mature stand Sclerotia pro- duction (1700 kg' ha-t yr-t) contributed the largest proportion of total mycorrhizal fungal production in both stands followed by mycorrhizal sheath production (,350 kg ha-' yr-t and 430 kg'ha-1 yr-t in the young and mature stands, respectively) Potentially, 17 kg'ha-t'yr-1 of N is cycled through sporocarps in the young stand and 41 kg'ha-1'yr-t in the mature stand Annually, fine roots (including mycorrhizae) cycle at least 60, 10, 10, 30 and 10 kg'ha-1tyr-t of N, P, K, Ca, and Mg, respectively, in the young stand In contrast, 110, 10, 20, 30 and 10 kg'ha-1tyr-t of N, P, K, Ca and Mg, respec- tively, are cycled through fine roots in the mature stand
TL;DR: It is shown here with simulated community data that ordi- nation selectively recovers patterns affecting several species simultaneously in early ordination axes, while selectively deferring noise to late axes, which helps to explain the observation that ordinations of field data are frequently useful even when the percentage of variance accounted for by the first few Ordination axes is small.
Abstract: Field data on the species content of plant and animal communities are noisy. Variation in community samples partly reflects interesting variation in environmental and historical factors, and partly reflects random fluctuations in species abundances. Routinely community data are analyzed by eigenvector ordination techniques, such as principal components analysis, reciprocal averaging, and detrended correspondence analysis. It is shown here with simulated community data that ordi- nation selectively recovers patterns affecting several species simultaneously in early ordination axes, while selectively deferring noise to late axes. Eigenvector ordinations thus appear to be effective for reducing noise. This result helps to explain the observation that ordinations of field data are frequently useful even when the percentage of variance accounted for by the first few ordination axes is small. A related conclusion is that rounding of the abundance values of community data sets has little effect on results from ordination, and consequently fairly crude field data are entirely adequate for ordination purposes.
Journal Article•
TL;DR: The number of apparently specific adaptations of both organisms, with mutual advantages, suggests evolution toward a coevolved interdependency.
Abstract: The population density of the limpet Acmaea testudinalis is highest on the crustose coralline alga Clathromorphum circumscriptum in both tide pool and subtidal environments in the Gulf of Maine. Juvenile limpets recruit to C. circumscriptum and both juveniles and adults preferentially feed on this species (not its epibionts) over a choice of other corallines, foliose algae, microalgae (diatoms), and detritus. Abundances of the predator (limpet) and the prey (coralline) are positively correlated, and the fitness of both organisms may be increased by the association. Acmaea testudinalis possesses a radula apparently adapted to eat efficiently C. circumscriptum, a food of low caloric value. The coralline is not only an abundant and dependable food throughout the geographic range of the limpet but also provides a superior holding surface for it. Clathromorphum circumscriptum requires grazing to remove potentially lethal epiphytes, but is not harmed by limpet grazing since it possesses a uniquely thick protective tissue over the region of growth. Reproductive structures of C. circumscriptum develop in winter and are buried below the surface so the effects of grazing on them are minimal. Over a wide range of depths, the rate of cell removal by limpets matches the rate of cell production by Clathromorphum. The number of apparently specific adaptations of both organisms, with mutual advantages, suggests evolution toward a coevolved interdependency. Trophic specialization of small organisms with low mobility (limpets) on larger, longer lived prey (coralline algae and kelps) is more likely than in other herbivore/prey interactions. By specializing on a long—lived and predictable food source, A. testudinalis may have evolved a strategy of economy by minimizing its cost of foraging (risk to predation and energy expenditure) relative to its absolute intake of calories. This foraging strategy may have been reinforced over time since it also improves the survivorship of the preferred prey.
TL;DR: Dendraster excentricus larvae capable of metamorphosis presented with various sub- strates show a significant preference for adult-associated sand, which should result in increased larval and juvenile survival.
Abstract: Dendraster excentricus larvae capable of metamorphosis presented with various sub- strates show a significant preference for adult-associated sand. Adult D. excentricus produce a chem- ical cue, possibly a small peptide (molecular mass < 10,000), that is sequestered by some component in the sand and that is stable for at least 7 wk. Thus, larval settlement occurs within or adjacent to existing sand dollar beds which often contain several hundred adults per square metre. Experimental evidence indicates survival of newly metamorphosed D. excentricus is significantly reduced by an extremely abundant, tube-building predator, Leptochelia dubia (Crustacea: Tanaidacea). Tanaidacean and possibly other micropredators, however, are excluded from sand dollar beds by the reworking activities of adult sand dollars. Thus, preferential settlement of D. excentricus larvae near adults of the species should result in increased larval and juvenile survival.
TL;DR: In this article, the maximum likelihood estimator is derived and shown to be readily calculated using an iterative procedure that starts with the Mayfield (1975) estimate as a trial value.
Abstract: Statistical methods for estimating and comparing constant survival rates are developed here for sampling designs in which survival of a subject is checked at irregular intervals. The maximum likelihood estimator is derived and shown to be readily calculated using an iterative procedure that starts with the Mayfield (1975) estimate as a trial value. Sampling distributions of this estimator and of the product of two or more estimates are skewed, and normalizing transformations are provided to facilitate valid confidence interval estimation. The sampling distribution of the difference between two independent estimates is found to be sufficiently normal without transformation to allow valid use of conventional normal theory procedures for testing differences and determining sample size for specified power. Statistical validity under the variable intensity sampling design does require that the duration of intervisit periods vary independently of observer perceptions concerning the survival status of the subject and, in order to achieve robustness with respect to the assumption of constant survivorship, sampling intensity must vary independently of any temporal changes in the daily survival rate. Investigators are warned no, to return earlier than planned to subjects thought to have died, as this observer behavior may cause serious bias in the survivorship estimate.