Showing papers in "Ecology in 1983"
TL;DR: Methods to predict the additional mortality expected on a cohort due to a reduction in growth rate are developed, and the potential for predation risk to enforce size—class segregation is discussed, which leads de facto to resource partitioning.
Abstract: We present an experiment designed to test the hypothesis that fish respond to both relative predation risk and habitat profitability in choosing habitats in which to feed. Identical populations of three size—classes of bluegill sunfish (Lepomis macrochirus) were stocked on both sides of a divided pond (29 m in diameter), and eight piscivorous largemouth bass (Micropterus salmoides) were introduced to one side. Sizes of both species were chosen such that the small class of bluegills was very vulnerable to the bass, whereas the largest class was invulnerable to bass predation. We then compared mortality, habitat use, and growth of each size—class in the presence and absence of the bass. Only the small size—class suffered significant mortality from the bass (each bass consumed on average about one small bluegill every 3.8 d); the two larger size—classes exhibited similar mortality rates on both sides of the pond. In the absence of the bass, we found that habitat use of all size—classes was similar and that the pattern of habitat use maximized foraging return rates (Werner et al. 1983). In the presence of the bass the two larger size—classes chose habitats to maximize return rates, but the small size—class obtained a greater fraction of its diet from the vegetation habitat, where foraging return rates were only one—third of those in the more open habitats. The small size—class further exhibited a significant depression in individual growth in the presence of the bass; the growth increment during the experiment was 27% less than that for small bluegills in the absence of the bass. Because of the reduced utilization of more open habitats by the small fish in the presence of bass, resources in these habitats were released to the larger size—classes, which showed greater growth in the presence of the bass than in its absence. We develop methods to predict the additional mortality expected on a cohort due to a reduction in growth rate (because individuals are spending a longer time in vunerable sizes), and discuss and potential for predation risk to enforce size—class segregation, which leads de facto to resource partitioning.
1,637 citations
TL;DR: In this paper, the authors summarized models of estimation and analysis for the measure of consumer food preferences originally derived by Manly et al. (1972) and further developed by Chesson (1978) and showed that this measure does not change with changes in food densities unless consumer behavior also changes.
Abstract: This paper summarizes models of estimation and analysis for the measure of consumer food preferences originally derived by Manly et al. (1972) and further developed by Chesson (1978). Unlike many alternative measure this measure of preference does not change with changes in food densities unless consumer behavior also changes. This measure is shown to relate in a simple way to parameters, such as clearance and attack rates, which appear in certain commonly used models of foraging behavior. See full-text article at JSTOR
970 citations
TL;DR: The role of cereal growth stages and crowding in the induction of alatae in Sitobion a'enae and its consequences for population growth are studied.
Abstract: mals: their productivity and population dynamics, IBP. Volume 5. Cambridge University Press, London, England. Mahmud, F. S. 1980. Alary polymorphism in the small brown planthopper Laodelphax striatellus (Homoptera: Delphacidae). Entomologia Experimentalis et Applicata 28: 47-53. McCoy, E. D., and J. R. Rey. 1981. Alary polymorphism among the salt marsh Delphacidae (Homoptera: Fulgoroidea) of northwest Florida. Ecological Entomology 6:285291. Rey, J. R. 1981. Ecological biogeography of arthropods on Spartina islands in northwest Florida. Ecological Monographs 51:237-265. Stiling, P. D., and D. R. Strong. 1982. The parasitoids of the planthopper Prokelisia marginate (Homoptera: Delphacidae). Florida Entomologist 65:191-192. Varley, G. C., G. R. Gradwell, and M. P. Hassell. 1973. Insect population ecology. Blackwell, Oxford, England. Watt, A. D., and A. F. G. Dixon. 1981. The role of cereal growth stages and crowding in the induction of alatae in Sitobion a'enae and its consequences for population growth. Ecological Entomology 6:441-447.
820 citations
TL;DR: It is suggested that trees with low N and P status retranslocate a similar or smaller proportion of leaf N andP prior to leaf abscission than do trees with more favorable nutrients status.
Abstract: The concentrations and pool sizes of nitrogen— and phosphorus—containing chemical fractions were followed seasonally in leaves and young stems of four Alaskan tree species: larch (Larix laricina), black spruce (Picea mariana), birch (Betula papyrifera), and alder (Alnus crispa). We found no major differences in patterns of N and P distributions among major chemical fractions in nutritionally distinct deciduous trees. In the three diciduous species, all N and P fractions were highest in young leaves and declined in concentration through the season, first as concentration was diluted by increasing leaf biomass and later as organic N and P fractions were hydrolyzed and inorganic P and amino acid N were translocated out of leaves. The quantities of nucleic acids and phospholipids hydrolyzed in autumn were equivalent to 40—47% and 26—38%, respectively, of the total P retranslocated from leaves of deciduous species prior to abscission. Protein hydrolyzed and subsequently retranslocated as amino acids was equivalent to 82—91% of the N removed from senescing leaves. Leaching was much less important than retranslocation in removing nutrients from senescing leaves. In buds and stems, P was stored during winter primarily as phospholipid, nonhydrolyzable ester P, and nucleic acid P and was converted to inorganic P in spring. All species stored N primarily as protein and to lesser extent as nucleic acids; larch also stored some N in amino acids. In the evergreen spruce, N and P were stored in the same types of compounds as in deciduous species, but this nutrient storage occurred in leaves and involved no winter translocation to stems. Based on this study and a survey of the literature, we suggest that trees with low N and P status retranslocate a similar or smaller proportion of leaf N and P prior to leaf abscission than do trees with more favorable nutrients status. We conclude that tree have not adapted to nutrient stress through major changes in biochemical use of N and P. However, during the growing season there are important changes in allocation of N and P to different chemical fractions, associated with changing plant requirements.
675 citations
TL;DR: In this paper, the authors studied avian biogeography and habitat selection in forests of southern Wisconsin ranging in area from 3 to >500 ha and found no area-dependent trends in vegetation structure or com- position that seem likely to influence the bird community.
Abstract: We studied avian biogeography and habitat selection in forests of southern Wisconsin ranging in area from 3 to >500 ha. Bird diversity in these woodlots increases with area, due primarily to an increase in the number of forest-dwelling, long-distance migrants. We consider two possible explanations for this pattern: ( 1) area-dependent changes in forest vegetation, or (2) area-dependent change in interactions with competitors, predators, or brood parasites. We first describe vegetation structure and composition, then show that this description comprises important habitat features of forest birds. Bird habitat is characterized in three ways: (1) vegetational structure within bird terri- tories is compared with that at random locations in the same woodlots, (2) structural characteristics of territories of different species are compared, and (3) factors related to species' abundance in different woodlots are analyzed. We found no area-dependent trends in vegetation structure or com- position that seem likely to influence the bird community. However, forest-edge and farmland species increase in density as woodlot area decreases. We suggest that forest-edge and farmland species exclude certain forest-dwelling, long-distance migrants from small woodlots, and that this exclusion influences the bird community more than area-dependent changes in habitat or the degree of woodlot isolation.
498 citations
TL;DR: There was little association between habitat structure and the richness, diversity, abundance, and biomass of small mammals, and new species were added to complex communities primarily through guild expansion rather than guild addition.
Abstract: The relationship between mammal community structure and vertical variation in habitat physiognomy (complexity) and horizontal variation in habitat form (heterogeneity) was examined on five study areas in the llanos of Venezuela. Data on the small mammal ( 38 000 trap nights from 1976—1978. Data on the distribution of large, nonvolant mammals were obtained during 24 mo of field observation. Measures of habitat complexity and habitat heterogeneity were derived using principal components analysis. There was little association between habitat structure and the richness, diversity, abundance, and biomass of small mammals. Abiotic factors, such as the degree of wet—season flooding, probably play an important role in patterns of small mammal distribution and abundance. The total number of mammal species was positively correlated with habitat complexity but not correlated with habitat heterogeneity. Increasing species richness across the complexity gradient was probably accommodated by increasing potential food resources. New species were added to complex communities primarily through guild expansion rather than guild addition.
446 citations
TL;DR: In this article, the authors describe changes in the abundance of shrub species after two fires in 1979 and 1980 on Otay Mountain in San Diego County, California, and conclude that the changes brought about by the 1980 fire will certainly persist for many decades.
Abstract: This study describes changes in the abundance of shrub species after two fires in 1979 and 1980 on Otay Mountain in San Diego County, California. The 1979 fire burned a large area of dense chaparral and coastal sage scrub. The 1980 fire burned a portion of the 1979 fire area that had been seeded with annual ryegrass (Lolium multiflorum) as an erosion protection measure. Changes in the vegetation caused by the 1979 fire alone were similar to those commonly seen in chaparral wildfire, but the reburning of the vegetation in 1980 caused drastic changes in some areas. Ceanothus oliganthus was almost completely eliminated from the area of the 1980 burn. Adenostoma fascicu- latum, the most abundant shrub at the study site, was reduced in density by up to 97%. Even Xylococcuis bicolor, which normally resprouts with complete success after fire, suffered substantial mortality with reburning. It is concluded that the changes brought about by the 1980 fire will certainly persist for many decades. While sudden shifts in vegetation composition probably occurred without human intervention, we believe that human activity, especially after the introduction of aggressive annual grasses 200 yr ago, has caused an increase in the instances of abrupt change.
445 citations
TL;DR: Although Fucus has a grazer—vulnerable stage, it often escapes herbivore control by virtue of spatial and size—related refuges, however; unusually high number of perwinkles can prevent FucUS establishment, even when spatial refugees occur.
Abstract: The mid—zone rocky intertidal community at protected and semiprotected areas in New England is characterized by a few, relatively small—bodied herbivore species. The effects of gastropod herbivores, primarily Littorina littorea, on secondary succession in the community were experimentally investigated. These periwinkle snails preferentially consume early successional, ephemeral algae such as Ulva lactuca L., Enteromorpha spp., and Porphyra spp. If not grazed, these early stages inhibit the appearance of a later successional species, Fucus vesiculous L. The effect of the herbivores on succession is to speed up the early successional sequence by releasing Fucus from inhibition by ephemeral species. Although these herbivores effectively control the ephemeral algae, they are usually unable to regulate the establishment of Fuchs. The periwinkles will readily graze small Fucus germlings (<3—5 cm long), and can prevent their successful establishment on smooth rock surfaces. However, substratum microtopography created by small crevices or pits or barnacles provides spatial refuges for small germlings. These plants quickly grow to a size at which they are less susceptible to periwinkle grazing. Thus although Fucus has a grazer—vulnerable stage, it often escapes herbivore control by virtue of spatial and size—related refuges. These escapes are herbivore—density dependent, however; unusually high number of perwinkles can prevent Fucus establishment, even when spatial refuges occur. Although the periwinkles harm young Fucus, they benefit older Fucus plants by grazing epiphytes from the blades, in lieu of the plant itself. The grazers thereby increase the persistence of individual Fucus and stabilize the community that depends upon Fucus for a habitat. The perwinkles thus play a subtle but important role in the community.
430 citations
TL;DR: Predictions of optimal habitat use showed striking correspondence to the actual habitat use of the fish; the bluegills switched from feeding in the open water column to feeding from the sediments within a few days of the authors' predictions.
Abstract: Utilizing optimal foraging theory and laboratory estimates of foraging costs, we predict the choice of foods and use of habitats by fish in the field. These predictions are tested with the bluegill sunfish (Lepomis macrochirus) foraging in three habitats (open water, sediments, and vege- tation) in a pond. Relations describing prey encounter rates in each habitat as a function of prey size, prey density, and fish size were derived from laboratory experiments. These relations permitted us to estimate prey encounter rates based on weekly prey samples in each habitat of the pond. We then determined the optimal diet and profitability (net energy return) for each habitat through time. Predictions of optimal diet exhibited good qualitative correspondence to the actual diet of the fish in the open water and vegetation, although we consistently predicted a slightly narrower diet than the fish were choosing. The model correctly predicted the magnitude of the change in size selection on Daphnia pulex with fish size and with decline in prey density. Predictions of optimal diet in the sediments were considerably in error apparently due to a tendency for late-instar midges to burrow deep in the sediments, thereby becoming unavailable to the fish. In this case habitat profitabilities were computed simply on the basis of the actual observed diet. Predictions of optimal habitat use, i.e., when the fish should switch habitats to maximize feeding rates, showed striking correspondence to the actual habitat use of the fish; the bluegills switched from feeding in the open water column to feeding from the sediments within a few days of our predictions. The actual habitat use pattern differs dramatically from a null model of random habitat use. We indicate how this approach may be useful in studying intra- and interspecific exploitative interactions.
422 citations
TL;DR: Agarwal et al. as mentioned in this paper found that pioneer trees were more concentrated on mineral soil associated with freshly uprooted trees than elsewhere in treefall gaps, partially because large gaps are more often caused by uprooted, thus soil-disturbing, trees.
Abstract: In mature forest on Barro Colorado Island (BCI), Republic of Panama, 0.09% of the ground surface is covered by pits and mounds created by uprooted trees as compared to 14-60% of the ground surface in temperate forests. Newly formed treefall pits on BCI rapidly filled with soil at an average rate of 8 cm/yr. Although pits and mounds rapidly disappeared on BCI, pioneer (gap phase or forest nomad) trees were more concentrated on mineral soil associated with freshly uprooted trees than elsewhere in treefall gaps. There was a large population (742 seeds/M2) of viable buried seeds, primarily of common pioneer tree species, in the top 10 cm of soil in closed forest; these seeds may give rise to many of the pioneers colonizing the disturbed soil. Large treefall gaps are more frequently colonized by pioneer trees than are small treefall gaps, partially because large gaps are more often caused by uprooted, thus soil-disturbing, trees.
TL;DR: The results show that new daughter ramets are physiologically dependent upon resources translocated from their parental clone and that this dependency declines with time, and that establishment and competitive ability are increased equitably among genets, and genets are buffered against localized "patch-specific" selection.
Abstract: Clones of Solidago canadensis were studied to determine the degree of physiological interdependency among interconnected ramets. Ramets severed from their parental clone in the field experienced reductions in growth, survivorship, and flowering, relative to intact controls. The results show that new daughter ramets are physiologically dependent upon resources translocated from their parental clone and that this dependency declines with time. When one of the ramets within each of several connected pairs was shaded to 10%/ of full sunlight, rates of photosynthesis increased in the remaining illuminated ramets. This and patterns of ramet survivorship and growth indicate that during periods of light limitation ramets become reintegrated. Shaded ramets become supported by the translocation of assimilates from other ramets within the clone. There is a tradeoff between rhizome number and length. When ramets become resource limited they produce fewer but longer new rhizomes. This results in the placement of daughter ramets at a location distant from the parent and may improve their probability of encountering more favorable conditions. The physiological integration among intraclonal ramets reduces the establishment risks of new daughter ramets and enables a genet to integrate local heterogeneity in resource availability. The result is that establishment and competitive ability are increased equitably among genets, and genets are buffered against localized "patch-specific" selection. This reduction in genetic deaths may be one mechanism resulting in the maintenance of variation in clonal populations.
TL;DR: The effects of grazing by the herbivorous caddisfly Helicopsyehe borealis on benthic algae and bacteria were experimentally studied in a northern California stream by elevating artificial substrate tiles above the stream bottom and excluding larvae of Helicopsyche, but not other Herbivorous insects, were effectively excluded.
Abstract: The effects of grazing by the herbivorous caddisfly Helicopsyehe borealis on benthic algae and bacteria were experimentally studied in a northern California stream, Big Sulphur Creek. By elevating artificial substrate tiles above the stream bottom, larvae of Helicopsyche, but not other herbivorous insects, were effectively excluded. In three replicate experiments, grazing by Helico- psyche resulted in low amounts of algae (as chlorophyll a, 0.3-1.4 /ug/cm2) and bacteria (0.3-0.7 x 108 cells/cm2) but a high algal turnover rate (02 evolved per unit chlorophyll a = 34 fg.,Ug-i h-i).
TL;DR: In this paper, the authors describe spiralling in Walker Branch, a first-order woodland stream in Tennessee, and measure its uptake from the water and then follow its dynamics in coarse particulate organic matter (CPOM), fine particulate organics (FPOM), aufwuchs, grazers, shredders, collectors, net-spinning filter feeders, and predators over a 6-week period.
Abstract: To describe spiralling in Walker Branch, a first-order woodland stream in Tennessee, we released /sup 32/PO/sub 4/ to the stream water and measured its uptake from the water and then followed its dynamics in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), aufwuchs, grazers, shredders, collectors, net-spinning filter feeders, and predators over a 6-week period. Rates of transfer among compartments and rates of downstream transport were estimated by fitting a partial differential equation model of the ecosystem to the data. With the resulting coefficients, the model was run to steady state to estimate standing stocks and fluxes of exchangeable phosphorus. Phosphorus moved downstream at an average velocity of 10.4 m/d, cycling once every 18.4 d. The average downstream distance associated with one cycle was therefore 190 m (10.4 m/d x 18.4 d). Spiralling length, at steady state, is approximately the ratio of the total downstream flux of phosphorus per unit width of stream (720 mg.d/sup -1/.m/sup -1/) to the rate of P uptake from the water (3.90 mg.m/sup -2/.d/sup -1/). CPOM accounted for 60% of the uptake, FPOM for 35%, and aufwuchs for 5%. Turnover times of P in particulates ranged from 5.6 to 6.7 d, except formore » FPOM, which showed a slower turnover time of 99 d. Of the P uptake from water by particulates, 2.8% was transferred to consumers, while the remainder returned directly to the water. About 30% of the consumer uptake, in turn, was transferred to predators. The small consumer turnover length reflected low consumer uptake of P from particulates and slow downstream drift velocity (0.013 m/d). In spite of the low rate of phosphorus uptake, the combined consumer-and-predator community accounted for 25% of the standing stock of exchangeable P in the stream. The retentiveness of this community is attributable both to the low drift rate and to a long turnover time (152 d) for P within the community.« less
TL;DR: The results demonstrate density dependence and indicate that food is limiting and in short supply in the natural breeding pools of P. triseriata and suggest that population control may be common to other anurans of ephemeral habitats.
Abstract: At the northeast end of Isle Royale, the chorus frog, P. triseriata, breeds nearly exclusively in pools on the exposed rocky shores of Lake Superior. The persistence of these breeding pools increases with size and distance from the lake. Small pools and those next to the lake last less than the 55-83 d required for metamorphosis, and survivorship to metamorphosis in these pools is low. Large pools near the forest edge are permanent, but contain the predators Anax junius and AmnbYstoina laterale. Anaxjunius eliminates P. triseriata if they occur together in the same pool. P. triseriata sustains high densities only in pools at intermediate levels on the shore, where intraspecific competition is suggested by effects of density on initial growth rate and on the length of the larval period. A field experiment using randomized blocks in a factorial design showed significant effects of both food and density on survivorship and growth; in the experiment, tadpoles removed all supple- mental food. The results demonstrate density dependence and indicate that food is limiting and in short supply in the natural breeding pools. Two features of P. triseriata population control may be common to other anurans of ephemeral habitats. First, partitioning of the larval habitat may allow predators to eliminate tadpoles from many pools. yet create refuges from predation in others. Second, in pools free of predation, competition may be intense and exert control on recruitment.
TL;DR: Mise en evidence d'un leger mais progressif accroissement du 13 C/ 12 C dans les tissus des animaux (muscle) lorsque croit le niveau trophique dans l'etude des biocenoses.
Abstract: L'echantillonnage porte sur une large collection d'invertebres, poissons et mammiferes des eaux cotieres du sud de la Californie et des eaux pelagiques entre Costa Rica et l'ile Cocos. Mise en evidence d'un leger mais progressif accroissement du 13 C/ 12 C dans les tissus des animaux (muscle) lorsque croit le niveau trophique. Interet de ces resultats dans l'etude des biocenoses
TL;DR: The results and those from the literature suggest that fish predation on benthic invertebrates may not be linearly related to decreasing substrate complexity, and there appears to be a complexity threshold below which removal rate is not significantly affected by structure and above which removal rates are significantly reduced.
Abstract: Phytal meiofauna are numerically important members of rocky intertidal communities and known to be prey items for various small fishes. In a series of experiments we demonstrated that substrate complexity is an important factor in reducing predation. The alga Carollina officinalis, the most complex structure used, was the only refuge from blenny (Helcogramma medium) predation for total meiofauna, and the dominant taxon, copepods. This refuge effect was evident whether the prey fauna were originally from Corallina or from another alga. One harpacticoid copepod species (Amphiascus lobatus) was selectively preyed upon, with females preferentially taken over males. Our results and those from the literature suggest that fish predation on benthic invertebrates may not be linearly related to decreasing substrate complexity. Rather there appears to be a complexity threshold below which removal rate is not significantly affected by structure and above which removal rate is significantly reduced. See full-text arti...
TL;DR: In this article, the authors modify Levins' model to allow for a varying difference between the local and regional time scales, and regional competition between two species is analyzed with the new model.
Abstract: There are two implicit assumptions in Levins' (1969) metaphorical model of regional distribution: movements by individuals are possible between any two patches in the region, and the local time scale is much faster than the regional one. These assumptions are frequently incompatible. Levins' model is modified to allow for a varying difference between the local and regional time scales, and regional competition between two species is analyzed with the new model. The main results are that two like species may or may not coexist regionally, depending on the difference between the time scales (a large difference facilitates coexistence) and on the intensity of competition; a species' precompetitive distribution is a good predictor of its competitive success, just as the carrying capacity in the Lotka-Volterra model is a good predictor of success in local competition. Regional priority effect is possible, though unlikely when the time scales are very different. The local time scale is slower than the regional one in some organisms (e.g., orchids, ferns), which may survive regionally, although they may succeed in breeding only seldom. Two possible outcomes of regional competition are (1) a decrease in the fraction of habitat patches occupied by the competing species and (2) an increase in the proportion of regionally rare species, some of which may ultimately go extinct. I ask which of the two outcomes is more frequent in nature and suggest that the answer may depend on the time scales of local and regional population dynamics.
TL;DR: It is equally plausible, especially for the guppies used in these experiments, that growth rate evolved, rather than reproductive effort, and the reported tradeoff between growth and reproduction satisfies this assumption.
Abstract: This study reports on two experiments characterizing the relationship between growth and reproduction in guppies. The first experiment compares the growth of reproducing and nonre- producing siblings raised on controlled levels of food availability. Nonreproducing females have significantly more energy in somatic tissues. The bulk of this energy is in fat tissues. Standard length and "protein" do not differ significantly. The extra energy in nonreproducers is therefore devoted predominantly to energy storage, as opposed to somatic growth. A comparison of total production, however, reveals that most of the energy "saved" by not reproducing does not appear in somatic tissues. The cause of this loss is unknown. The second experiment compares the somatic growth of different stocks of Trinidadian guppies which had previously been demonstrated to devote different quantities of energy to reproduction (Reznick 1980). Increases in the amount of energy devoted to reproduction were matched by decreases in the energy devoted to somatic tissues. Furthermore, all of the stocks have equal total production. These fish are therefore equally efficient in converting food into biomass but differ in how this biomass is allocated to growth and reproduction. A key assumption of the theory predicting the impact of certain forms of selection on life history evolution is that an increase in the resources devoted to current reproduction reduces the future capacity to reproduce. Because fecundity is directly proportional to size in guppies, the reported tradeoff between growth and reproduction satisfies this assumption. However, such theoretical treat- ments concentrate solely on the evolution of reproductive effort. Because growth and reproduction are complementary, it is equally plausible, especially for the guppies used in these experiments, that growth rate evolved, rather than reproductive effort.
TL;DR: It is suggested that the common practice of imputing eco- logical "meaning" to the signs and magnitudes of coefficients be replaced by an assessment of "struc- ture coefficients."
Abstract: The application of discriminant analysis in ecological investigations is discussed. The appropriate statistical assumptions for discriminant analysis are illustrated, and both classification and group separation approaches are outlined. Three assumptions that are crucial in ecological studies are discussed at length, and the consequences of their violation are developed. These assumptions are: (1) equality of dispersions, (2) identifiability of prior probabilities, and (3) precise and accurate estimation of means and dispersions. The use of discriminant functions for purposes of interpreting ecological relationships is also discussed. It is suggested that the common practice of imputing eco- logical "meaning" to the signs and magnitudes of coefficients be replaced by an assessment of "struc- ture coefficients." Finally, the potential and limitations of representation of data in canonical space are considered, and some cautionary points are made concerning ecological interpretation of patterns in canonical space.
TL;DR: In this article, the authors tested several hypotheses relating community structure to environmental disturbance, by testing for changes in the zonation patterns of lakeshore vegetation, and found that the range of water depths tolerated by individual species (realized niche width) changes with exposure.
Abstract: This study tests several hypotheses relating community structure to environmental disturbance, by testing for changes in the zonation patterns of lakeshore vegetation. Twenty—five transects were examined at different positions along an exposure gradient. The range of water depths tolerated by individual species (realized niche width) changes with exposure. Some (Lobelia dortmanna, Utricularia cornuta) reached their maximum on exposed shores, others (Drosera intermedia, Cladium mariscoides) at intermediate exposure, and others (Pontederia cordata, Triadenum fraseri) on sheltered shores. Species richness peaked significantly at intermediate levels of exposure (P < .01). In spite of changes in both species composition and richness, mean niche width did not change with exposure. The distribution of the upper and lower limits of species along the gradient was examined using measures of boundary clustering. The lower and upper boundaries of species were both significantly clustered (P < .001), suggesting discrete communities exist on the shoreline gradient. As exposure increased, upper boundaries became more cluster (P < .002); lower boundaries were unaffected. The distribution of boundaries shifted landward with increasing exposure. This fact is possibly related to an identical landward shift in the lower boundaries of shoreline shrubs, which appear to have a major influence on the distribution of herbaceous shoreline vegetation.
TL;DR: Visual censuses of terrestrial predators and sweep samples of aquatic predators established that more individual terrestrial pred- ators, more terrestrial predator species, and more individual aquatic predators per cubic metre exist in permanent than in temporary ponds, suggesting that predators may prevent temporary-pond species from using permanent ponds.
Abstract: Several anuran species in central New Mexico have access to both permanent and temporary ponds yet typically use only one or the other pond type. Visual censuses of terrestrial predators and sweep samples of aquatic predators established that more individual terrestrial pred- ators, more terrestrial predator species, and more individual aquatic predators per cubic metre exist in permanent than in temporary ponds, suggesting that predators may prevent temporary-pond species from using permanent ponds. Eight predator species consumed more temporary- than permanent- pond tadpoles in 17 of 20 pairwise choice experiments. Temporary-pond tadpoles were almost con- tinually in motion, while permanent-pond tadpoles were almost always stationary. This differential movement may account for the vulnerability of temporary-pond tadpoles to predation. The nonrandom distribution of predators in this tadpole community and the inability of some tadpoles to avoid pre- dation apparently restricts some anurans to temporary ponds.
TL;DR: The outcomes of these perturbations support a nonequilibrial view of reef fish communities, rather than hypotheses that emphasize the importance of competition among populations usually at carrying capacity.
Abstract: The spatial distributions of two common damselfishes, Potnacentrus wvardi and P. flavicauda, suggest that they compete for common resources within lagoons of the southern Great Barrier Reef. Both fishes are herbivores that defend territories on rocky substrata covered with shallow water, but are segregated by microhabitat. When young P. flavicauda recruit into the habitat where P. w'ardi are most numerous, they have lower persistence than counterparts on reefs where conspecifics predominate. Several experiments were done, using as replicates the large patch reefs populated by P. wiardi, to determine what effects territory-holding individuals have on the distribution and abundance of conspecifics, P. flatvicauda, and other grazing fishes. Total and selective removal of the numerically dominant P. wardi revealed that these reefs are mostly repopulated by larval fishes (recruitment), rather than by already recruited fishes (immigra- tion). P. wardi are the most frequent recruits. The removals also showed that resident P. wtnardi do not determine the numbers of nonspecific larvae invading their territories, and do not reduce their survival after recruitment (at least for 300-400 d). They do determine the growth of such juveniles. Despite this interaction, there was no evidence that either density or biomass is usually regulated in these populations. Both measures vary greatly among replicate reefs. The lack of numerical equilib- rium, within the study period, was caused by a patchy and limiting supply of larval recruits. Manipulations also showed that resident P. bxardi do not limit the recruitment or reduce the postrecruitment survival of P. flavicauda on large patch reefs. An unknown density-independent cause is implicated in the observed habitat segregation and in the failure of P. flavicauda to reach breeding size on such reefs. The removal of P. wardi produced different results for the fish grazers from other families, de- pending upon their feeding strategy. For example, solitary acanthurids concentrated their grazing activities on reefs with fewer damselfishes, but a schooling parrotfish did not. The outcomes of these perturbations support a nonequilibrial view of reef fish communities, rather than hypotheses that emphasize the importance of competition among populations usually at carrying capacity.
TL;DR: The structure of 40 real food webs, representing aquatic and terrestrial communities from all latitudes, was found to be markedly affected by the degree of variability of the physical environment as discussed by the authors.
Abstract: The structure of 40 real food webs, representing aquatic and terrestrial communities from all latitudes, is found to be markedly affected by the degree of variability of the physical environment. In particular, food webs in fluctuating ecosystems are characterized by a significantly lower connectance than webs representative of more constant systems. This is interpreted within the context of stability theory as a means to optimize feeding in the face of increasing disturbance. It is shown further than the nature of the habitat itself imposes additional constraints on food web structure in intertidal, pelagic, estuarine, and forest ecosystems. See full-text article at JSTOR
TL;DR: The response of a stream ecosystem to disturbance in its watershed was investigated by comparing mass balances of dissolved organic carbon (DOC) for a stream draining an undisturbed watershed with a stream that was clear-cut 2 yr before the study began as mentioned in this paper.
Abstract: The response of a stream ecosystem to disturbance in its watershed was investigated by comparing mass balances of dissolved organic carbon (DOC) for a stream draining an undisturbed watershed with a stream draining a watershed that was clear-cut 2 yr before the study began. These second-order streams are in the Coweeta Hydrologic Laboratory, North Carolina. Both streams had similar, elevated DOC concentrations (from <1 to 5 mg/L) during storms. Rising and falling limbs of the hydrograph also had similar DOC concentrations. During the growing season DOC concentration increased from headwater seep to the weir in the undisturbed stream under baseflow conditions. No significant longitudinal change was observed in the stream draining the clear-cut watershed. Hence concentration was consistently lower in baseflow samples during the growing season in the stream draining the clear-cut watershed. As a result annual DOC export from the clear-cut watershed was less (9.8-11.5 kg/ha) than from the reference watershed (14.6-15.1 kg/ha). The lower DOC export was partly due to reduced DOC inputs from throughfall and leaching of fresh litter, but most impor- tantly to lower DOC inputs in subsurface water and probably also less in-stream generation of DOC. The rate of recovery of this stream from disturbance is therefore dependent on the rate at which the terrestrial system recovers.
TL;DR: The historical evidence for the Indians' burning the forests of the northeastern United States is reevaluated in this article, where it is concluded that the frequent use of fires by the Indians to burn the forests was probably at most a local occurrence.
Abstract: The historical evidence for the Indians' burning the forests of the northeastern United States is reevaluated. Of 35 documents that describe vegetation or Indian life in the 16th or 17th centuries, only half mention any use of fire except for cooking. Only six purportedly first—hand accounts might refer to purposeful, widespread, and frequent use of fire. These six are all consistent with use of fire only locally near camps or villages, or with accidentally escaped fires. It is concluded that the frequent use of fires by the Indians to burn the forests was probably at most a local occurrence. The Indians' presence in the region and their use of fire for many purposes did, however, increase the frequency of fires above the low levels caused by lightning, and thus had some effect on the vegetation; for example, grasses characterized the ground cover at small, local, frequently burned sites.
TL;DR: The variety of activity patterns exhibited by birds on moisture and vegetation gradients clearly demonstrates the complexity of dynamics that affect assemblage attributes and undermines the recent perception of a dichotomy between equilibrium (deterministic) and nonequilibrium (stochastic) assemblages.
Abstract: Both physical and biological processes shape species assemblages (communities) For birds, vegetation structure has long been assumed to be the dominant factor in habitat selection, especially along successional gradients While vegetation may be important as a proximate factor, detailed knowledge of ultimate factors governing habitat selection is required Gradients of microcli- mate, especially temperature and moisture, may be such an ultimate factor through direct physiolog- ical pressures on birds or indirectly through distribution and availability of food resources We doc- umented the existence of gradients of both vegetation structure and microclimate in the undergrowth of seasonally humid forest in central Panama To assess the relative importance of these gradients in shaping local avian distribution, birds were netted in undergrowth (up to 3 m above ground) during 2-wk periods in dry (March) and wet (July) season for 4 yr (1979-1982) A total of 3037 captures of 95 species was recorded during that period Although patterns of species richness and capture rate are relatively simple, species composition and other assemblage attributes are complex and difficult to interpret without careful evaluation of the dynamics of individual species The variety of activity patterns exhibited by birds on moisture and vegetation gradients clearly demonstrates the complexity of dynamics that affect assemblage attributes Overall, birds are more active at dry sites and at sites with intermediate shrub density However, activity levels change with time, suggesting that avian activity reflects a dynamic process of habitat selection Though no food resource data are available at present, the general pattern of habitat use on diurnal time scales for several guilds suggests that birds track microclimatic optima for physiological reasons Habitat selection processes are both ex- tremely complex and variable in time and space for the avifauna of tropical forest undergrowth Indeed, the scale of study in space and time is important in determining the conclusion of a study The recent perception of a dichotomy between equilibrium (deterministic) and nonequilibrium (stochastic) assemblages is undermined by these results Habitat selection in birds produces dynamic (nonequilibrium) assemblages in space and time However, these are not stochastic assemblages Each species seeks habitat optima in the context of current environmental conditions on diurnal, seasonal, and between-year time scales Thus, instead of nonequilibrium and stochastic assemblages, the avifaunas of tropical forest undergrowth are nonequilibrium but relatively predictable from knowl- edge of current environmental conditions
TL;DR: Daphnia parvula grew equally well when fed either yeast or amended sediments, and dissolved organic matter can be made available for daphnid growth by the mechanism of adsorption.
Abstract: Reservoirs built on rivers draining agricultural watersheds are often turbid with sus- pended sediments. Filter-feeding zooplankton, e.g., members of the genus Daphnia, seem to be as abundant in such reservoirs as in natural lakes. Using controlled laboratory conditions, we have investigated the potential role of suspended sediments in the nutrition of Daphnia from a turbid reservoir. In three sets of experimental procedures we measured: (1) the physical effect of sediments on the ingestion and incorporation rates of algae by daphnids, (2) the ingestion rates of two sizes of clay mineral sediment particles by daphnids, and (3) the growth and survival of daphnids fed yeast and sediments with and without organic matter adsorbed onto the particle surfaces. Increases in suspended sediment concentration from 0.0 to 2451 mg/L decreased ingestion rates of 4C-labeled Chlorella vulgaris by Daphnia parvula and D. pulex by ~95% and decreased incor- poration rates by 99%o. Sediment concentrations of 50-100 mg/L reduced the algal carbon ingested by daphnids to potential starvation levels. 15Zn-labeled fine and coarse (1.88 and 4.65 Am mean diameter) clay mineral sediment particles were fed to D. parvula, D. pulex, and D. similis. Ingestion rates of sediments by daphnids were limited at particle concentrations of :5.0 x 10" particles/mL and were dependent on particle size, daphnid species, and body size. Fine particles were ingested at greater rates by D. pulex and D. parvala than were coarse particles, while large D. similis ingested coarse and fine particles at similar rates. In addition, D. pulex had higher ingestion rates in each particle size suspension than did D. parvula or D. similis. The differential ability to ingest these particles suggests a mechanism of regulating daphnid species composition if the organic matter ad- sorbed to these sediment particles is useful as food. To evaluate this mechanism, we adsorbed dis- solved organic protein to sediment particles. When D. pulex were fed the amended sediment suspen- sion, they grew larger than when fed control sediments, but not as large as when fed yeast. D. parvula grew equally well when fed either yeast or amended sediments. Thus, dissolved organic matter can be made available for daphnid growth by the mechanism of adsorption.
TL;DR: Age—class distributions of major tree species further substantiated that major physiognomic changes have occurred on the plateau since the arrival of European man.
Abstract: Major differences were found between the vegetation structure of ponderosa pine—dominanted communities on the Horse Pasture Plateau and those on the nearby but isolated Church and Greatheart Mesas in Zion National Park. The Horse Pasture Plateau was heavily grazed by livestock in the late 19th and early 20th centuries, while the mesas were never grazed. Conditions on the mesas now approximate the pre—European situation of the region as described in the earliest written accounts. Pine, oak, and juniper sapling density and cover were much higher on the formerly grazed plateau than on the relict mesas. Herbaceous species dominated the groundlayer in mesa ponderosa pine savanna stands, while grass and forb cover was low on analogous sites of the plateau. Age—class distributions of major tree species further substantiated that major physiognomic changes have occurred on the plateau since the arrival of European man. Analysis of fire scars showed that prior to 1881, the mean fire—free interval for ponderosa pine stands on the plateau was 4 to 7 yr, while the interval for Church Mesa was 69 yr. Since there were no recorded fires on Church Mesa between 1892 and 1964, and yet no corresponding increase in sapling density, the increased understory density of plateau stands should not be attributed primarily to cessation of fires. Instead, heavy grazing by livestock and associated reduction of the herbaceous groundlayer promoted the establishment of less palatable tree and shrub seedlings, Fire, however, played an important secondary role in maintaining savanna and woodland communities.