Showing papers in "Ecology in 1985"

Nest Predation in Forest Tracts and the Decline of Migratory Songbirds

Abstract: Nest predation has been suggested as an important cause of the decline of breeding populations of migratory songbirds in small woodlots in eastern North America. I tested this hypothesis by placing artificial nests with fresh quail eggs in forests of different sizes in Maryland and Tennessee. Predation rates were higher in small woodlots than in large tracts. Predation was especially intense in woodlots near suburban neighborhoods compared to woodlots in isolated rural areas. Experimental open-cup nests were more vulnerable to predators when placed on the ground vs. 1-2 m above ground. In either position these open-cup nests were more vulnerable to predators than experimental cavity nests. Since most species of migratory songbirds construct open-cup nests, and several species place them near the ground, migratory songbirds should be strongly affected by higher predation rates in small forest tracts.

Habitat Patch Connectivity and Population Survival

Abstract: We constructed a patch dynamics model which can be used to simulate the changing sizes of resident populations in a series of interconnected habitat patches. We applied the model to white—footed mice (Peromyscus Leucopus) inhabiting patches of forest in an agricultural landscape. The model predicts that mouse populations in isolated woodlots have lower growth rates and are thus more prone to extinction than those in connected woodlots. Field data support this prediction.

Testing For Independence of Observations in Animal Movements

Abstract: Many analyses of animal movements assume that an animal's position at time t + 1 is independent of its position at time t, but no statistical procedure exists to test this assumption with bivariate data. Using empirically derived critical values for the ratio of mean squared distance between successive observations to mean squared distance from the center of activity, we demonstrate a bivariate test of the independence assumption first proposed by Schoener. For cases in which the null hypothesis of independence is rejected, we present a procedure for determining the time interval at which autocorrelation becomes negligible. To illustrate implementation of the test, locational data obtained from a radio-tagged adult female cotton rat (Sigmodon hispidus) were used. The test can be used to design an efficient sampling schedule for movement studies, and it is also useful in revealing behavioral phenomena such as home range shifting and any tendency of animals to follow prescribed routes in their daily activities. Further, the test may provide a means of examining how an animal's use of space is affected by its internal clock.

Gap-phase regeneration in a tropical forest.

Abstract: Gap-phase regeneration of trees was described for the first 5-6 yr of regrowth in 30 treefall gaps (20-705 m2) in tropical moist forest on Barro Colorado Island, Panama Trees were classified as pioneers (saplings found only in gaps) or primary species (saplings found in gaps and in the understory of mature forest) In most of the gaps studied, stem densities rose rapidly after gap formation, then levelled offor declined by years 3-6 This pattern was particularly marked in some large gaps (> 150 m2), where pioneers attained high densities, then experienced heavy mortality Stem density of primary species did not vary with gap size In large gaps the mean rate of growth in height was greater for pioneers than for primary species, size-class distribution broadened more for pioneers than for primary species, and early recruits of both regeneration types grew faster than later ones Gap formation fosters regeneration of pioneer and primary species and, in this forest, produces patches that differ markedly in tree population dynamics, species composition, and growth rate

Individualistic Growth Response of Tundra Plant Species to Environmental Manipulations in the Field

Abstract: In undisturbed arctic tussock and wet meadow tundras we increased air temperature with a plastic greenhouse, increased nutrient availability by NPK fertilization, and decreased light intensity with shade cloth to determine the factors limiting growth of tundra plants. After 2 yr of these manipulations we measured growth of each major vascular species and one moss species. Each species showed a different pattern of growth response to alteration of light, air temperature, and nutrient regimes, indicating that no single factor limits growth of all species in these communities. Growth of canopy species (Betula nana, Ledum palustre, Carex bigelowii, and Eriophorum vaginatum) was reduced by experimental shading more than was growth of understory species (e.g., Vaccinium vitisidaea and Rubus chamaemorus). Species typical of nutrient—rich sites (Betula nana, Rubus chamaemorus, and Polygonum bistorta) generally responded more to nutrient addition than did species typical of nutrient—poor sites (e.g., Empetrum nigrum), although there were species characteristic of fertile sites (Salix pulchra) and infertile sites (Ledum palustre) which did not show this pattern of nutrient response. Species that grow in warm hollows between tussocks showed less growth in response to increased air temperature than did canopy species. We suggest that lack of a single common factor limiting growth of all species in tussock and wet meadow tundras implies that (1) each species is individualistically distributed, as described by the continuum model of community organization, (2) as a result of competition and/or distinct evolutionary histories, the growth of each species is limited by a different combination of environmental factors, and (3) production by individual species varies greatly from year to year, but production by the whole vegetation is more stable, because years that are favorable for growth of some species cause a compensatory decrease in growth of other species.

Grazing Minnows, Piscivorous Bass, and Stream Algae: Dynamics of a Strong Interaction

Abstract: Striking differences in pool-to-pool distributions of an algae-grazing minnow (Campo- stoma anomalum), attached algae (predominantly Spirogyra sp. and Rhizoclonium sp.), and bass (Micropterus salmoides and M. punctulatus) are known to occur in some small Oklahoma streams. This study evaluates the complementarity of bass, Campostoma, and algae at different seasons, and uses in-stream experimental manipulations of bass and Campostoma to determine if the patterns resulted from strong interactions between predators, herbivores, and algae. In a 1-km reach of Brier Creek (south-central Oklahoma), bass and Campostoma distributions in 14 consecutive pools were inversely related in six of seven censuses conducted from 8 November 1982 to 5 September 1983. Bass and Campostoma co-occurred in more than two pools only on two occasions, following the largest floods of the year. Campostoma and algal abundances were inversely related during late summer and in both autumns of this study. This relationship did not hold during the spring, when floods strongly affected algal distributions. During autumn of 1983, we removed bass from a pool, fenced it longitudinally, and added Campo- stoma to one side (1.4 individuals/m2). Over the next 5 wk, standing crop of algae decreased significantly on the Campostoma side but increased on the control side. In a nearby unmanipulated Campostoma pool, standing crop of algae was consistently low. We added three free-swimming bass to a Campostoma pool to evaluate presumptive predator-prey interactions. Within 3 h, the Campostoma moved from the deepest part of the pool to shallow areas. Over the next 5 wk, numbers of grazing Campostoma declined due to behavioral changes, emigration, and (presumably) predation. The standing crop of algae increased significantly 10-13 d after bass addition. In a second bass-addition experiment in June 1984, Campostoma responses were almost identical, and algal standing crop in deeper areas increased significantly after 1 wk. Collectively, our censuses and the experiments indicate that in Brier Creek, biotic interactions strongly influence the pool-to-pool distributions of Campostoma and algae, par- ticularly during long periods of constant low discharge.

Forest Litter Decomposition in Relation to Soil Nitrogen Dynamics and Litter Quality

Abstract: Decomposition and changes in nitrogen and organic—chemical content of six types of forest litter were studied for 2 yr in five adjacent Wisconsin forests. The five forests were floristically dissimilar, being dominated respectively by sugar maple (Acer saccharum), white oak (Quercus alba), bigtooth aspen (Populus grandidentata), white pine (Pinus strobus), and hemlock (Tsuga canadensis), Nitrogen mineralization rates in the five stands ranged from 29 to 125 kg°ha—1°yr—1. Decomposition rates of transplanted sugar maple leaves and red maple (Acer rubrum) wood were not correlated with nitrogen mineralization rates in all five stands, indicating that nitrogen mineralization rates do not affect initial decomposition rates. However, mineralization rates were correlated with decomposition rates of the native dominant foliage litter. Nitrogen first accumulated in all litters, but by the end of the 2—yr incubation period nitrogen release had begun in all foliage litters. Nitrogen concentrations increased approximately linearly with cumulative mass loss but eventually declined in some foliage litters. Neither maximum amount of nitrogen accumulated nor amount accumulated per gram of litter mass loss was related to the rate of soil nitrogen mineralization. Chemical composition of litter affected decomposition rates and patterns. Soluble substances and litters relatively rich in solubles disappeared rapidly during early stages of decomposition. Eventually, slowly disappearing acid—soluble and acid—insoluble substances dominated the pattern of mass loss in all litters.

Fine Roots, Net Primary Production, and Soil Nitrogen Availability: A New Hypothesis

Abstract: The relationships between above- and belowground net primary production and soil nitrogen availability were studied at nine temperate forest sites. Annual allocations of nitrogen (N) and net primary production to leaf litter, perennial tissues (wood + bark), and aboveground biomass all increased significantly (P < .01) in relation to apparent N uptake by vegetation (NJ) as calculated using field measures of net N mineralization (Nj) and other major N fluxes to and from available N pools. Mean annual N content and biomass of fine roots (:<3.0 mm diameter) were both negatively correlated with NU (r= -0.71, P < .05; r -0.63, P < .10, respectively). However, only -50% of Nm at each site could be accounted for by allocation to aboveground litter and perennial tissues. Assuming that mineralized N not accounted for by allocation to these components was taken up by vegetation and allocated to fine roots, annual N allocation to fine roots (Nfr) was a constant fraction of N uptake. Therefore, Nfr increased in absolute terms with both Nm and apparent N uptake. Fine-root N turnover rates (or Nfl/fine-root N content) also increased as Nm and NU increased. Provided that fine-root biomass and N turnover rates were similar within individual sites, allocation of production to belowground biomass also increased relative to increases in soil N availability.Furthermore, the proportion of total net primary production allocated to belowground biomass did not decrease with increased N avail- ability.

Demographic Theory for an Open Marine Population with Space-Limited Recruitment

Abstract: We introduce a demographic model for a local population of sessile marine invertebrates that have a pelagic larval phase. The processes in the model are the settling of larvae onto empty space, and the growth and mortality of the settled organisms. The rate of settlement per unit of unoccupied space is assumed to be determined by factors outside of the local system. The model predicts the number of animals of each age in the local system through time. The model is offered in both discrete and continuous—time versions. The principal result is that the growth of the settled organisms is destabilizing. In the model, there is always a state where recruitment balances mortality. However, growth can interfere with recruitment and can destabilize this steady state, provided also that the settlement rate is sufficiently high. The model suggests that two qualitatively distinct pictures of population structure result, depending on the settlement rate. In the high settlement limit, the intertidal landscape is a mosaic of cohorts, punctuated with occasional gaps of vacant substrate. In the low settlement limit, the intertidal landscape has vacant space and organisms of all ages mixed together and spatial variation in abundance is caused by microgeographic variation in settlement and mortality rates.

Fruit‐Size, Gape Width, and the Diets of Fruit‐Eating Birds

Abstract: In most animals, especially those that must swallow food items whole, prey size is related to predator size. This paper examines gape limitation and the influence of fruit size on diet in fruit—eating birds, drawing on data gathered over a 5—yr period on 70 bird species and 171 plant species in the lower montane forests of Monteverde, Costa Rica. The results suggest that fruit—eating birds face many of the constraints imposed on other gape—limited foragers, but have an unusual minimum—size relationship with their food because of the unique characteristics of fruits. Fruit—eating birds with broad gapes consumed more lauraceous fruit species and a larger mean and maximum size of fruits overall than narrow—gaped birds. However, the size of the smallest fruits eaten was not correlated with gape width; large—gaped species commonly fed on diminutive fruits. Birds effectively selected among individual fruits within a tree on the basis of fruit size, dropping bulky fruits beneath the tree. Effective size selectivity also occurred among trees of different species in the same family and among plant species in various families. The diet of broad—gaped birds was not comprised differentially of large fruit species. For example, Three—wattled Bellbirds favored medium—sized fruits, whereas Long—tailed Manakins took individual fruits in the same proportions as they took fruit species of different mean fruit diameters. Gape limitations and effective size selectivity have obvious consequences for seed dispersal patterns: Plants with large fruits attracted fewer species of birds than plants with small fruits. Moreover, the broad—gaped bird species on which large—fruited plants specialized were those with the most generalized diets.

Use of an Improved Statistical Method for Group Comparisons to Study Effects of Prairie Fire

Abstract: This paper describes an improved method for performing statistical comparisons among experimental groups. This technique, termed multi—response permutation procedures (MRPP), is similar in purpose to the t test and one—way analysis of variance F test. However, in contrast to these, the new method features very relaxed requirements on the data structure, is easily applied to multivariate problems, and makes it possible to relate the analysis visually to the perceived data space. The MRPP test statistic is based on the within—group average of pairwise distance measures between object response values in a euclidian data space. The null distribution of the test statistic is based on the collection of all possible permutations of the objects into groups having specified sizes. For large group sizes, this distribution is approximated by a continuous distribution satisfying three exact moments. The advantages and applications of MRPP are illustrated using both artificial examples and empirical data on total August standing crop in mixed prairie following an October wildfire. The MRPP analyses of the empirical data revealed that there were no differences in standing crop between burned an unburned areas after the first postfire growing seasons, standing crop was significantly greater in the previously burned areas.

Disturbance, nitrogen availability, and nitrogen losses in an intensively managed loblolly pine plantation

Abstract: The interactive effects of harvest intensity, site preparation, and herbicide treatment were evaluated in a clear-cut Piedmont site in North Carolina. Forest harvesting caused increased nitrogen mineralization and nitrification in all treatments, but harvesting without additional treatment had little effect on nitrate-nitrogen pool sizes and losses. The removal of most surface organic material during intensive site preparation led to greatly increased nitrate pool sizes and losses. Treatment with herbicide accentuated this effect, and in combination such removals and herbicide applications led to accumulations of nitrate-nitrogen in surface soil of 27 and 24 kg/ha in the first and second summers following site preparation. Nitrogen losses by leaching, denitrification, and erosion were also greatest in the plots where organic residues had been removed and herbicides applied. These results suggest that microbial immobilization controlled nitrogen pool sizes and losses, and this suggestion was confirmed using 15N. Soils from the plots without residue removal or herbicide treatment immobilized >90% of added 15N in 28 d, while those from the residue removal/herbicide treated plots immobilized < 70%. Microbial activity was the predominant process regulating nitrogen availability and losses following disturbance in this site; site-preparation practices that removed im- portant substrates for microbial metabolism caused elevated nitrogen losses.

Food Availability, Sea Urchin Grazing, and Kelp Forest Community Structure

Abstract: Unlike most previously reported kelp forest, which exhibit long—term persistence over large spatial scales, the kelp forest community on the west end of San Nicolas Island, California, occurs as a dynamic patchwork of barren areas characterized by grazing sea urchins and an algal assemblage consisting of upright and encrusting coralline algae, and kelp—dominated areas characterized by high densities of perennial brown algae, including the giant kelp Macrocysits pyrifera. In this study we examine the factor that regulate the grazing of sea urchins (Strongylocentroutus franciscanus), which in turn determines the structure of this community. In the barren area, drift algae sparse, sea urchins were poorly nourished, occupied open, unprotected microhabitats, and actively grazed the substratum. In the kelp—dominated are, drift algae were abundant, sea urchins were well nourished, moved little, occupied cracks and crevices, and probably fed on the drift algae. Early in the study substantial recruitment of brown macroalgae occurred in both sites, and the barren area gradually transformed into a kelp—dominated area. Concomitant with this change, the abundance of drift algae in the barren area increased. Urchins in this area abandoned open microhabitats for protected crevices and pockets, and sea urchin grazing intensity was reduced to levels characteristic of the kelp—dominated area. From these observations we present a qualitative model in which the transformation from one configuration to another is triggered by a behavioral switch in the mode of feeding of red sea urchins. This switch is controlled by the availability of drift algae, which in turn is dependent upon the abundance of attached macroalgae. The adult algal standing stocks appear to be regulated by prevailing hydrographic conditions, which are either favorable or unfavorable for kelp recruitment, survivorship, and growth. In this model grazing intensity is independent of urchin density and therefore provides a new view of how kelp forest communities function.

Early Consequences of Seed Dispersal for a Neotropical Tree (Virola surinamensis)

Abstract: We examined the effects of distance from a fruiting conspecific, light environment, and initial seed size on survivorship using Virola surinamensis seeds and seedlings placed in concentric rings around fruiting Virola trees. Insects and mammals killed 99.2% of seeds and seedlings between fruit drop and exhaustion of parental endosperm at 12 wk after germination. During this time, survival was not influenced by either seed size or light environment, but up to a 44-fold advantage accrued to seeds dropped 45 m from a fruiting tree as compared with those underneath Virola crowns. Seeds that germinated under and near fruiting Virola trees suffered disproportionate attack from a curculionid weevil {Conotrachelus sp. nov.). Both large initial seed size and favorable light environments (assessed by canopy photographs) enhanced seedling vigor at 18 wk of age. Among those seedlings that escaped predation beyond the exhaustion of parental seed stores, a difference of as little as 1% in the amount of canopy open overhead distinguished seedlings with three leaves (3.5% open) from those with two leaves (2.3% open). Our results demonstrate clear advantages to local seed dispersal for a tree ofthe rainforest canopy. They further suggest that dispersal of seeds > 20 m by large birds, such as guans {Penelope purpurascens) and toucans {Ramphastos sulfuratus and R. swainsonii), is more favorable for seedling survival than that of the smaller trogons (Trogon massena) and motmots {Baryphthengus martii), which regurgitate seeds under or near the tree crown.

Size Hierarchies in Experimental Populations of Annual Plants

Abstract: The effects of inter- and intraspecific interference on size hierarchies (size inequali- ties) were investigated in populations of the annual plants Trifolium incarnatum and Lolium multiflorum. Variables experimentally manipulated included plant density, species proportions, soil fertility, and spatial pattern of plantings. Densities were below those for extensive density-dependent mortality. Size inequality always increased with increasing density. Plants grown individually showed very low inequality, while plants grown at the highest density had the most developed hierarchies. Size inequality usually increased with an increase in productivity when interference was occurring. When dominant in mixtures, Lolium showed less size inequality than in monoculture, while the suppressed species, Trifolium, usually displayed an increase in inequality. Spatial pattern appeared to be less important than other factors in causing size inequalities; plants sown in a uniform spatial pattern showed significantly lower size inequality than plants sown in a random pattern in only one out of four cases. Inequality in reproductive output of Trifolium, as estimated by dry mass of flower heads, was always greater than inequality in plant dry mass. The results support a model of plant interference in which large plants are able to usurp resources and suppress the growth of smaller individuals more than they themselves are suppressed. While interference decreases mean plant mass, it increases both the relative variation in plant mass and the concentration of mass within a small fraction ofthe population.

The relationship between seed size and establishment conditions in tropical woody plants

Abstract: Two approaches were used to examine the relationship between seed size and typical establishment conditions in woody species of tropical rain forests. First we compared seed masses of 36 mature tropical forest tree species with differing light—gap requirements for establishment. The 14 species that become established beneath a closed canopy or in small gaps were found to have higher mean seed masses than the species that require large gaps. Though species were divided into two types of establishment requirements, plotting the mean seed masses produced a continuous unimodal distribution rather than a bimodal distribution, suggesting that either intermediate establishment conditions are most common in tropical rain forests or that other biotic or abiotic conditions confound the relationship between establishment conditions and seed mass. Second, we compared seed masses of 203 early and late successional woody plants in one Peruvian forest. Seed masses of mature forest species were found to be significantly ...

Modifying Lodgepole Pine Stands to Change Susceptibility to Mountain Pine Beetle Attack

Abstract: The premise that mature lodgepole pine forests are susceptible to mountain pine beetle attack when physiologically stressed was supported experimentally by manipulating the canopy density and availability of nitrogen in a 120—yr—old forest exposed to a high population of beetles. Where canopy density was reduced, ether by us or by the insects, surviving trees significantly increased their resistance to attack over a 3—yr period. Increased resistance was reflected by changes in wood production per unit of leaf area (tree growth efficiency). Improved nitrogen nutrition hastened tree recovery but did not prevent attacks by beetles until growth efficiencies exceeded 100 g of wood production per square metre of foliage. Growth efficiency, as here defined, is an index of vigor that may reflect the relative ability of susceptible trees to produce defensive compounds following attack.

Bottomland Vegetation Distribution along Passage Creek, Virginia, in Relation to Fluvial Landforms

Abstract: Persistent distribution patterns of woody vegetation within the bottomland forest of Passage Creek, Virginia, were related to fluvial landforms, channel geometry, streamflow characteristics, and sediment—size characteristics. Vegetation patterns were determined from species presence as observed in transects and traverses on landforms developed along the stream. Distinct species distributional patterns were found on four common fluvial geomorphic landforms: depositional bar, active—channel shelf, floodplain, and terrace. Independent hydrologic characteristics (flow duration and flood frequency) were determined for each of the landforms. Vegetation data were analyzed by binary discriminant analysis, principal components analysis, and detrended correspondence analysis. Results and related field observations suggest that certain species are significantly associated with specific fluvial landforms. Vegetation patterns appear to develop more as a result of hydrologic processes associated with each fluvial landform rather than from sediment—size characteristics. Flood disturbance may be an important factor in maintaining the vegetation patterns, which may therefore be used as indicators for particular hydrogeomorphic site conditions.

Fiddler Crab Regulation of Spartina alterniflora Production on a New England Salt Marsh

Abstract: While a great deal of research has been directed at understanding the high productivity of marsh grasses, little attention has been paid to the potential importance of marsh fauna in influencing marsh grass production. The mud fiddler crab, Ucapugnax, excavates burrows in Spartina alterniflora- dominated salt marshes along the East Coast of North America. Here, I examined experimentally the effect of fiddler crab burrowing on S. alterniflora production. At low intertidal heights, S. alterniflora production was robust and characterized by tall-form plants with roots and rhizomes that penetrate deeply into the substrate. At higher tidal heights, low productivity, short-form S. alterniflora with a dense superficial root mat prevailed. Fiddler crabs differentially affected grass production at different tidal heights. In tall-form S. alter? niflora stands at intermediate tidal heights, experimental reduction of fiddler crab density for a single growing season decreased aboveground production by 47% and increased root mat density by 35%. In contrast, on the lower intertidal soft edge of the marsh and in the high intertidal, dense root mat, short-form S. alterniflora zone, experimental reduction of crab density did not have as marked an effect on cordgrass production. Crab burrows were shown to increase soil drainage, soil oxidation- reduction potential, and the in situ decomposition of belowground plant debris. The relationship between fiddler crabs and S. alterniflora appears to represent a facultative mu- tualism. On the seaward edge of marshes in soft sediment that would not otherwise support burrow structures, the roots, rhizomes, and debris of S. alterniflora provide structural support and consequently facilitate burrowing. At intermediate tidal heights, intensive burrowing activity increases aboveground grass production and prevents the establishment ofa dense root mat, while crabs are largely precluded from burrowing in the dense root mat, short-form S. alterniflora stands at higher tidal heights. There? fore, while initially dependent on S. alterniflora in soft sediments, U. pugnax burrowing activity appears to maintain a habitat suitable for continued burrowing, and as a byproduct, increases cordgrass production and maintains tall-form S. alterniflora stands.

Biogeochemistry of C, N, and P in a Soil Catena of the Shortgrass Steppe

Abstract: Measurements of carbon, nitrogen, and phosphorus content were carried out in the soils of a hillslope of shortgrass steppe. Plant biomass, soil morphology, and soil physical properties were also measured. Soil morphology indicated that the site had undergone several cycles of rapid erosion and deposition. Total mass of C, N, and P increased downslope, following a trend in soil depth, but the summit A horizon had higher C, N, and organic P concentrations than the backslope, reflecting a higher clay content. Laboratory and field incubations showed that N availability increased downslope, while relative N mineralization (N mineralized: total N) decreased. Organic matter content and mineralization rate were closely coupled to physical properties of the soil, which reflect the geomorphic history of the site.

Flow regime, juvenile abundance, and the assemblage structure of stream fishes

Abstract: The assemblage of fishes in a second—order stream in east—central Illinois was compared through seine sampling for two years with distinctly different flow regimes. In both years adult (age >0) fish were most abundant in late spring and early summer while juvenile (age 0) abundance peaked in late summer or autumn. Total density of adult fish differed little between years but large changes in juvenile abundance occurred between years in association with differences in hydrologic regime. High stream discharge had little influence on abundance of juvenile suckers and darters, but some minnow species and all sunfish species exhibited large increases in juvenile abundance during stable to low flow conditions. The largest increase in juvenile abundance occurred among species with prolonged breeding seasons: bluntnose minnow (Pimephales notatus) and striped shiner (Notropis chrysocephalus). Increased juvenile abundance during stable flow conditions resulted in increased species richness in small habitat patches and annual changes in species composition of the community. Models of fish assemblages in small streams need to incorporate ontogenetic changes in the role of stochastic environmental variables in regulating the population size of component species.

Recruitment of Coral Reef Fishes: Effects of Distribution of Predators and Shelter

Abstract: Factors affecting recruitment and early survivorship of juvenile coral reef fishes were studied on St. Croix, United States Virgin Islands. The faunal assemblage studied included diurnally active fishes found in the rubble/sand habitat. The most abundant members were: beaugregory {Ste- gastes leucostictus), foureye butterflyfish {Chaetodon capistratus), mahogany snapper {Lutjanus ma- hogoni), surgeonfishes {Acanthurus bahianus and A. chirurgus) and French grunt {Haemulon flavoli- neatum). During the period 1978-1981, sets of experimental reefs constructed from Strombus gigas shells were built in various locations relative to a major reef. Recruitment of juveniles of almost all species in the rubble/sand fish assemblage occurred more heavily on reefs that were built 20-40 m away from the main reef (into the lagoon) than on those built at the edge ofthe backreef. A series of experiments revealed that this is probably due to two factors: differences in available shelter and differences in encounter rates with predators. Seagrass and algae, which provide shelter to very small juvenile fishes, are absent in a "halo" zone close to the reef due to the feeding activities of reef-associated grazers (fishes and urchins). Experiments with artificially produced halos and artificial seagrass and algae showed that part of the explanation for the observed spatial pattern of fish recruitment is the spatial pattern of the shelter provided by seagrass and algae. Experiments in which potential prey fish were tethered demonstrated that the risk of predation for small juvenile fishes was considerably higher close to the reef than it was 20 m away. The results of these experiments suggest that patterns of settlement and juvenile survivorship of coral reef fishes are affected by complex interactions with other reef organisms (in this case, the activities of reef-associated grazers that modify available shelter) and by the frequency of encounters with predators. If they survive, all of the juveniles, regardless of where they initially settle, eventually migrate to a nearby reef. The species studies here differed in the degree to which juveniles use the sea-grass/algal habitat as a refuge from predation; these differences suggest that spatial patterns of recruitment, interacting with predation, may influence both population abundances and species com? position of older juveniles on the reef.

Presettlement regeneration patterns in a southwestern Ponderosa pine stand

Abstract: Tree stems 2 106 yr old (i.e., established before significant European influence in this area) in a 7.3-ha old-growth ponderosa pine (Pinus ponderosa) forest in northern Arizona were aged and mapped. Age structure analysis showed that successful establishment of ponderosa pine was infrequent. The periods without successful establishment could be quite long, as suggested by four consecutive decades in which only two surviving trees were established. The stems were strongly aggregated, as measured with nearest neighbor analysis, and groups were visually distinct in the field. Most of the stems occurred in groups of three or more, with group size ranging from 3 to 44 stems and area occupied by a group ranging from 0.02 to 0.29 ha. Ages of stems within groups were variable, the most homogeneous group having a range of 33 yr and the least having a range of 268 yr. The data are not consistent with the commonly held view that southwestern ponderosa pine occurs in even-aged groups and that each group became established following the demise of the group previously occupying the site. Instead, it seems more likely that seedlings became established when one or two trees within the group died, the additional fuel surrounding the dead trees causing an intensely burned spot in the otherwise low-intensity fires that were frequent in the area. The hot spot would create a potential seedbed for pine by eliminating, at least temporarily, the competing grasses on that small area. This decreased competition, in conjunction with adequate seed production and favorable moisture conditions in the spring and early summer, may well have been critical for pon- derosa pine establishment. The relative infrequency of all these events occuning in the necessary sequence could explain the erratic age structure data from this area.

Does Competition Drive Dispersal

Abstract: What distances would animals be expected to disperse if the only cost to staying on the natal site resulted from competition for resources limiting to reproduction? In such a situation the "best" dispersal distance would be zero, but the constraint of competition would prevent many individuals from achieving this ideal; the actual dispersal distances would be shaped by population demography. The models I describe predict the distribution of dispersal distances as a function of home range turnover probability for animals that move to the first uncontested home range they encounter and no farther. The models provide a good match to data for some, but not all, avian and mammalian data sets, suggesting (1) that competiton for resources limiting to reproduction may be the primary factor driving dispersal, and (2) that the action of inbreeding avoidance, habitat selection, and other processes suggested to influence dispersal distances can be detected as deviations from these model predictions.

Algal Periphyton Growth on Nutrient‐Diffusing Substrates: An in situ Bioassay

Abstract: Differences in nutrient limitation for dominant species within an algal periphyton com? munity were determined using additions of N and P supplied by nutrient-diffusing artificial substrates. Sealed clay flowerpots were filled with 2% agar and one of nine nutrient treatments (all combinations of K2HP04 at 0.0, 0.05, and 0.5 mol/L with NaN03 at 0.0, 0.05, and 0.5 mol/L). The pots were submerged at 0.5 m depth in Douglas Lake, Michigan, and diffused N and P to their outer surfaces in proportion to internal concentrations. After 51 d the pots were scraped and analyzed for attached algae. Total algal biomass as chlorophyll a on the pots ranged from 0.17 ? 0.02 (se) ^g/cm2 for pots without added nutrients to 15.7 + 2.0 /xg/cm2 for pots with K2 HP04 at 0.05 mol/L and NaN03 at 0.5 mol/L. Chlorophyll a on pots containing just P (0.05, 0.5 mol/L) increased 6- to 10-fold over controls. The diatoms Epithemia adnata and Rhopalodia gibba and the blue-green alga Anabaena in? creased significantly on the P-only pots; these species are suspected of N-fixing capability. Chlorophyll a on pots containing just N (0.05, 0.5 mol/L) increased 1.5- to 2-fold, though this increase was nonsignificant; Achnanthes minutissima, Gomphonema tenellum, and Cocconeis placentula showed enhanced growth on these pots. Combinations of N and P caused heavy growths of the filamentous alga Stigeoclonium tenue. Naviculoid diatoms were also most abundant on the N + P pots. Average nutrient levels in Douglas Lake during the study were: NH3, 2.02 /nmol/L; N03, 0.44 Aimol/L; and P04, 0.06 /miol/L. The low ambient concentrations of both N and P, together with results ofthe periphyton bioassay, indicate that the two nutrients may jointly limit overall growth, and that the form of growth limitation differs by species within the periphyton community.

Silica as a Defense against Herbivory and a Growth Promotor in African Grasses

Abstract: Grasses and dung were collected in the Serengeti National Park and analyzed for silica content by wet ashing. Grasses from grasslands differing in the grazing intensities experienced were grown in the laboratory in a factorial experiment to determine factors controlling tissue silicification. Concentrations of silica in tissues of plants collected in the field were higher than have been reported for any other plants abundant in grazing ecosystems. Silica contents in the field were higher in more heavily grazed grasslands and in tissue produced earlier in the growing season. Animal dung contained substantial quantities of silica. Laboratory experiments indicated that tissue silicification was increased by defoliation, was higher in plants from more heavily grazed grasslands, varied in different organs and species in patterns confirming current hypotheses about plant defense, and was affected by the availability of soluble silica in the nutrient medium. Silica in the nutrient medium promoted the yield of unclipped plants substantially. Total yield was 18% higher than that of control plants, although hydroponic experiments with solutions prepared and handled in plastic indicated that silica was not a growth requirement, except, perhaps, at the ecologically unrealistic concentrations that might result from reagent contamination. Yield stimulation by silica was differentially distributed among organs, tending to promote photosynthetically active tissues and crowns. Flowering of one species was promoted by silica. Leaves of silica—fed plants were larger. Leaf blade chlorophyll concentrations were 15% higher in silica—fed plants from the more heavily grazed grasslands. The results suggest that complex patterns of grass silicification had a role in the radiation of grazing animals and grasses and may contribute to maintaining the biotic diversity of contemporary grassland—savanna ecosystems by influencing the partitioning of forage species and organs among grazers. Growth promotion by silica may be due to the substitution of mineral support for carbon—based support associated with the deposition of silica in the intercellular spaces of aerial tissues. Since soils of the Serengeti region commonly have pH levels above neutrality, where the availability of silica is low, silica supply could influence primary productivity and resultant energy and nutrient flow through the trophic web in the native environments of the plants.

Relationships between Contemporary Pollen and Vegetation Data from Wisconsin and Michigan, USA

Abstract: Scatter diagrams and regression analysis of paired pollen and tree—inventory data show how pollen percentages represent the percent basal area for the major arboreal genera in Michigan and Wisconsin. We show that the relationship between pollen and tree percentages for each taxon is generally similar for two states of comparable size and similar vegetation (Wisconsin and Michigan), but that the relationship is influenced by the size of the pollen—collecting site and the size of the area surveyed for trees around each site. These results provide information concerning the relative size of the pollen—source area for seven arboreal pollen types: Pinus, Quercus, Betula, Tsuga, Ulmus, Fagus, and Acer, listed in descending order of pollen—source area. Moderate—sized lakes (30—150 ha) accumulate significant quantities of Pinus and Quercus pollen produced farther than 30 km away, but accumulate relatively few Fagus grains from >4.5 km, and even fewer Acer grains from >2.3 km. The source areas for Betula, Tsuga, and Ulmus pollen lie within 30 km of each lake, and significant quantities of these grains travel farther than 4.5 km. Regression analysis of data from basins of different size supports the hypothesis that small basins collect their pollen from a smaller area of the surrounding vegetation than do large basins.

Body Size, Food Concentration, and Population Growth in Planktonic Rotifers

Abstract: Population growth rates of eight species of planktonic rotifers were assessed for five to seven food concentrations using daily renewed batch cultures. The food concentration for which population growth rate was zero (the threshold food level) varied by a factor of 17 among species. The log of threshold food concentration was positively and significantly related to the log of body mass. Similarly, a strong positive log-log relationship was found between rotifer body mass and the food concentration supporting one-half the maximum population growth rate (rax/2); this food con- centration varied by a factor of 35 among species. There was a positive relationship between rotifer body mass and maximum population growth rate. Because the smallest species have the lowest threshold food levels and the food levels necessary for them to attain rm,,a/2 are lowest, they appear well adapted to living in food-poor environments. Large species appear to be restricted to food-rich environments but may thrive there because of their high reproductive potential. These conclusions are consistent with observations on species distribution and community structure of rotifers in nature.

The Influence of Predatory Decapods, Refuge, and Microhabitat Selection on Seagrass Communities

Abstract: This research was designed (1) to examine the role of predatory decapod crustaceans as an organizing force in prey distribution and abundance patterns, and (2) to compare the importance of vegetation as a prey refuge from predators with its importance as a microhabitat for epifauna in seagrass meadows. After documenting the predator—exclusion effectiveness of a new cage design by defaunating and monitoring faunal immigration into cages during a 2—mo period of high predator densities, I initiated a 1—mo manipulative field experiment designed to examine the refuge value of vegetation. Replicate predator—inclusion (treatment) and —exclusion (control) cages were established across a vegetation gradient in a Gulf of Mexico, USA, grassbed, and predator shrimp (Penaeus duorarum) were enclosed in treatment cages. While cages successfully excluded larger decapods and fishes, prey species had free access and rapidly colonized cages. A significant predation effect was detected for total densities of all major prey taxa in a simple (low plant biomass) habitat. The general consequence of increasing microhabitat structural complexity on the outcome of predator—prey interactions was a reduction in Penaeus effects on prey densities across the vegetation gradient. However, for some prey, predation effects remained relatively constant across this gradient. These results extend from the laboratory to nature the premise that seagrasses afford protection for certain epifauna. The experiments also reveal that macrophyte structural complexity can account for microhabitat distributions of some grassbed fauna without invoking differential predation as a causal mechanism. Strong nature association of epifauna with seagrasses and macroalgae appear to be net result of predation, refuge, and habitat selection.

The Mechanism of Competition for Pollination between Two Forest Herbs

Abstract: The primary mechanism of competition for pollination between the two forest herbs Stellaria pubera and Claytonia virginica in piedmont North Carolina is interspecific pollen movement. The most common visitor, the bee fly Bombylius major, forages indiscriminately among flowers of the two species. In only one of five experiments did the presence of C. virginica reduce the pollinator visit rate per S. pubera flower, and in other experiments addition of C. virginica enhanced visit rate. Thus these plant species exhibit little or no competition through pollinator preference. In some natural populations, however, visits to S. pubera are frequently immediately preceded by a visit to C. virginica, and a flower receives less conspecific pollen and produces fewer seeds following such an interspecific visit than if the visitor has arrived directly from a conspecific flower. Interspecific pollen movement is responsible for most, if not all, of the reductions in seed set of S. pubera due to pollinator sharing. Although insects deposit a substantial amount of S. pubera pollen on stigmas of C. virginica, little C. virginica pollen is found on S. pubera stigmas. Moreover, application of foreign pollen to the stigma does not influence seed production of S. pubera. The effect of interspecific pollen movement is due to loss of conspecific pollen, not stigmatic interference.