Showing papers in "Ecology in 1989"

Kernel methods for estimating the utilization distribution in home-range studies

Abstract: In this paper kernel methods for the nonparametric estimation of the utilization distribution from a random sample of locational observations made on an animal in its home range are described. They are of flexible form, thus can be used where simple parametric models are found to be inappropriate or difficult to specify. Two examples are given to illustrate the fixed and adaptive kernel approaches in data analysis and to compare the methods. Various choices for the smoothing parameter used in kernel methods are discussed. Since kernel methods give alternative approaches to the Anderson (1982) Fourier transform methods, some comparisons are made.

Nitrogen and lignin content as predictors of litter decay rates: a microcosm test

Abstract: Decomposition rates of leaf litter have been predicted from the leaves' lignin or nutrient (N or P) contents, the C:N ratio, and more recently the lignin:N ratio. But tests of these predictors have been based on groups of substrates each spanning only part of the natural range of lignin contents, and neglecting low—lignin (<10%) species. We allowed leaf litter from eight species of tree, shrub, or herb, ranging in lignin content from 3.4 to 20.5%, to decompose in laboratory microcosms for up to 4 mo (equivalent to 1.5—2 yr decay in the field) to test two hypotheses: (1) that the lignin: nitrogen ratio would have a better correlation with mass loss rates than would the C:N ratio, nutrient content, or other substrate quality indexes, and (2) that correlations of mass loss with initial N content would decrease, while correlations with lignin content would increase, as decay proceeded. Contrary to the first hypothesis, nitrogen content and the C:N ratio were the best predictors of mass loss rate, and were substantially better than the lignin:N ratio. We could find no better predictor of decomposition rate than the C:N ratio, and no better regression model than the simple linear one. However, when regressions were tested using pine needles (lignin content 26.2%), the C:N ratio and N content badly underestimated mass remaining (by 10—16%), while lignin content and the lignin:N ratio overestimated it by <2%. In accordance with the second hypothesis, regressions of initial lignin content or lignin:N ratio on mass remaining improved (higher R2) from 2 to 4 mo decomposition, while those of N content grew worse, illustrating succession of nitrogen to lignin control of decomposition rate. Reported correlations of the lignin: N ratio with decomposition rate for some litter types arise as a special case of this two—phase mechanism of control by nutrients and lignin. For substrates low in lignin, or where a broad range of lignin contents is being considered, the C:N ratio is a better predictor of decomposition rate than the lignin:N ratio.

Canopy Gaps and the Two Major Groups of Forest Trees

Abstract: In all forests there is a cycle initiated by disturbance. We may arbitrarily recognize gap, building, and mature phases (Watt 1947, Cousens 1974, Whitmore 1975, 1978, 1982), thereby designating forests as spatial mosaics of structural phases which change over time as a result of dynamic processes. Gaps, openings in the forest canopy, drive the forest cycle. Very tiny gaps may be filled by lateral ingrowth of surrounding trees. Usually, though, trees grow up from seedlings to form an immature forest of saplings and poles which grow on and develop into mature trees. The mature phase may enter a fourth degenerate phase as trees become senile, but often is destroyed at a stroke by some external factor. Differences in sizes of gaps result in differences in species composition of the next cycle. Present evidence suggests that, in all forests, tree species fall into one or other of two groups. In small gaps seedlings that became established in the shade of the closed forest are released (i.e., commence height growth). By contrast, a quite different group of species colonizes large gaps. Seeds of these species germinate only in the open, so seedlings occur only after formation of gaps. Swaine and Whitmore (1988), while recognizing that there are no universally accepted terms, propose that these groups be called climax (non-pioneer) and pioneer, respectively. Pioneer species only regenerate in large gaps. When a mature canopy of pioneers enters the degenerate phase, small gaps develop, and these are closed by growth of climax species that became established under them. The next cycle is composed of climax species that, unlike the pioneers, can regenerate in situ. The gap phase is thus the most important part of the growth cycle for the determination of floristic composition. Competition among tree species and their different light requirements at the building and mature phases play a lesser role. Intensive study of forest dynamics since the early 1970s suggests this paradigm is applicable to many forests at all latitudes (Whitmore 1988). There are, however, numerous embellishments, as well as many alternatives, and these lead to the great diversity seen even within a single forest biome. A few examples include: clonal spread occurs in some temperate forests; in some tropical rain forests big gaps may be invaded by woody climbers that arrest the next growth cycle; both pioneer and climax species differ in longevity (hence size) and in autecology (e.g., microsite preferences for seedling establishment).

Belowground Carbon Allocation in Forest Ecosystems: Global Trends

Abstract: Carbon allocation to roots in forest ecosystems is estimated from published data on soil respiration and litterfall. On a global scale, rates of in situ soil respiration and aboveground litter production are highly and positively correlated, suggesting that above- and belowground production are controlled by the same factors. This relationship also allows us to predict rates of total soil respiration and total carbon allocation to roots in forest ecosystems from litterfall measurements. Over a gradient of litterfall carbon ranging from 70 to 500 g m-2 yr-1, total belowground carbon allocation increases from 260 to 1100 g m-2 yr-1. The ratio of belowground carbon allocation to litterfall decreases from 3.8 to 2.5 as litterfall carbon increases from 70 to 200 g.m-2 yr-1, but changes little (2.5 to 2.2) as litterfall carbon increases from 200 to 500 g.m-2 yr-1. Use of this relationship permits the construction of simple carbon budgets that can be used to place upper limits on estimates of fine root production in forest ecosystems. Determining live-root respiration rates in forest ecosystems will further constrain the range of possible root production rates.

Effects of Forest Fragmentation on Dung and Carrion Beetle Communities in Central Amazonia

Abstract: This study, one of the first to document the effects of forest fragmentation on insects in the tropics, showed that dung and carrion beetle communities in 1-ha and 10-ha forest fragments differed from those in contiguous forest, even though the fragments had been isolated by <350 m for an ecologically short time (2-6 yr). During 288 pitfall trap days at the Minimum Critical Size of Ecosystems study site 80 km north of Manaus, Brazil, I captured 55 species in 15 genera. Trap days were divided equally between three 1-ha, three 10-ha, three clear-cut, and three contiguous forest areas. Pitfall samples from clear-cut areas separating forest fragments from intact forest in- dicated that beetles rarely moved from intact forest into fragments. The apparent barrier imposed by clearcuts diminished with the invasion of second growth. Except for the four species in the genus Glaphyrocanthon, all species were found more frequently in forested areas than in clearcuts. Glaphyrocanthon constituted 97% of the 717 individuals captured in clearcuts and was never captured in contiguous forest or 10-ha fragments. Forest fragments had fewer species, sparser populations, and smaller beetles than com- parable intact forest areas. The changes in dung and carrion beetle communities help explain the low rates at which dung decomposed in 1-ha fragments. Thus, forest fragmentation not only changes the dung and carrion beetle fauna; its effects ramify through other related community and ecosystem processes, as well.

The Planned Introduction of Genetically Engineered Organisms: Ecological Considerations and Recommendations

Abstract: The ecological and evolutionary aspects of planned introductions of transgenic organisms into the environment are considered in this report. The authors support the timely development of environmentally sound products, such as improved agricultural varieties, fertilizers, pest control agents, and microorganisms for waste treatment, through the use of advanced biotechnology within the context of a scientifically based regulatory policy that encourages innovation without compromising sound environmental management. Economic, social, and ethical concerns also must be weighed along with strictly ecological and evolutionary considerations, but these other issues are beyond the scope of this report. Ecological oversight of planned introductions should be directed at promoting effectiveness while guarding against potential problems. The diversity of organisms that will be modified, functions that will be engineered, and environments that will receive altered organisms makes ecological risk evaluation complex. While we cannot now recommend the complete exemption of specific organisms or traits from regulatory oversight, we support and will continue to assist in the development of methods for scaling the level of oversight needed for individual cases according to objective, scientific criteria, with a goal of minimizing unnecessary regulatory burdens. In this report, we provide a preliminary set of specific criteria for the scaling of regulatory oversight. Genetically engineered organisms should be evaluated and regulated according to their biological properties (phenotypes), rather than according to the genetic techniques used to produce them. Nonetheless, because many novel combinations of properties can be achieved only by molecular and cellular techniques, products of these techniques may often be subjected to greater scrutiny than the products of traditional techniques. Although the capability to produce precise genetic alterations increases confidence that unintended changes in the genome have not occurred, precise genetic characterization does not ensure that all ecologically important aspects of the phenotype can be predicted for the environments into which an organism will be introduced. Many important scientific issues must be considered in evaluating the potential ecological consequences of the planned introduction of genetically engineered organisms into the environment. These include survival and reproduction of the introduced organism, interactions with other organisms in the environment, and effects of the introduced organism on ecosystem function. We encourage the use of small—scale field tests , when justified by previous laboratory and/or greenhouse studies, under conditions that minimize dispersal and under appropriate regulatory oversight. As the biotechnology industry develops, continuing regulatory oversight as well as long—term research and monitoring will be necessary for responsible risk management. Many engineered organisms will probably be less fit than the parent organism, although some important exceptions may arise. Even if an engineered trait reduces an organism's fitness only slightly, may generations may pass before the introduced organisms disappears completely due to decreased fitness. Such persistence is most probable when the turnover rate of populations is very slow. Natural selection will act on genetically engineered organisms, as it does on all others. Selection after the release of the transgenic organism will tend to increase fitness, not decrease it, by reducing the costs associated with the novel traits. If increases in fitness do occur, they will probably increase population growth rate and biological competitiveness, or produce other ecological effects that should be considered in assessing risks. Transfer of engineered genes from the modified organism to other organisms may occur through hybridization in higher organisms, or through conjugation, transduction, or transformation in microorganisms. If lateral transfer occurs, an engineered gene may persist in the natural environment even after the genetically engineered organism itself is no longer present. The available scientific evidence indicates that lateral transfer among microorganisms in nature is neither so rare that we can ignore its occurrence, nor so common that we can assume that barriers crossed by modern biotechnology are comparable to those constantly crossed in nature. Native species, as well as species newly introduced from distant habitats, may become pests. An organism engineered to prosper in a new habitat type, geographic area, or season is effectively an introduced organism in that it will probably enter into new biotic and abiotic interactions. Therefore, regulatory and risk assessment structures that rely on the distinction between ~`native" and ~`non—native" must be used with caution. Concern has frequently been expressed regarding the potential for genetically engineered organisms to displace resident species in the receiving community, particularly microbial species performing key functional roles such as nitrogen fixation of lignin decomposition. Because redundancy of function appears to be common in microbial communities, in many cases there would be little concern over microbial species displacement caused by an introduced transgenic organism. Ecological effects and the geographic ranges of organisms transcend political boundaries; we therefore consider it essential to promote and achieve international coordination of risk assessment and regulation of biotechnology. Special consideration must be given to the protection of rare genetic resources, such as the wild ancestors of domesticated species, and threatened gene pools of other wild species. We urge local, state, national, international cooperation in risk assessment and regulation of the ecological effects of the introduction of transgenic organisms. Evaluating the benefits and risks of biotechnology products requires expertise in many scientific disciplines including molecular biology, genetics, cell biology, evolutionary biology, physiology, population and community ecology, and ecosystem science. For society to realize the full benefits of biotechnology, interdisciplinary research and graduate training programs are needed to expand the expertise of the scientific community at large.

Retention and transport of nutrients in a third-order stream in northwestern california: hyporheic processes'

Abstract: Chloride and nitrate were coinjected into the surface waters of a third—order stream for 20 d to examine solute retention, and the fate of nitrate during subsurface transport. A series of wells (shallow pits) 0.5—10 m from the adjacent channel were sampled to estimate the lateral interflow of water. Two subsurface return flows beneath the wetted channel were also examined. The conservative tracer (chloride) was hydrologically transported to all wells. Stream water was >88% of flow in wells <4 m from the wetted channel. The lowest percentage of stream water was 47% at a well 10 m perpendicular to the stream. Retention of solutes was greater in the hyporheic zone than in the channel under summer low—flow conditions. Nominal travel time (the interval required for chloride concentration to reach 50% of the plateau concentration) was variable by well location, indicating different flow paths and presumably permeability differences in subsurface gravels. Nominal travel time was M 24 h for wells <5 m from the we...

Hot Rocks and Not-So-Hot Rocks: Retreat-Site Selection by Garter Snakes and Its Thermal Consequences

Abstract: Studies of behavioral thermoregulation ofectotherms have typically focused only on active animals. However, most temperate-zone ectotherms actually spend more time sequestered in retreats (e.g., under rocks) than active above ground. We documented retreat-site selection during summer by gravid garter snakes (Thamnophis elegans) at Eagle Lake in northeastern California, USA. To explore the thermal consequences of retreat-site selection, we measured potential body temperatures in retreats and combined these with data on thermal tolerances, thermal preferences, and thermal dependence of metabolism and digestion. Garter snakes at Eagle Lake usually retreated under rocks of intermediate thickness (20-30 cm) even though both thinner and thicker rocks were available. Empirical and biophysical analyses of temperatures under rocks of various sizes and shapes demonstrated that rock thickness had the dominant effect on potential Tb available to snakes and in turn on thermal physiology. Snakes selecting thin rocks ( 40 cm thick) or remaining at the bottom of deep burrows would not experience such extreme Tb, but neither would they warm to Tb in their preferred range. However, snakes selecting intermediate-thickness rocks would never overheat but would achieve preferred Tb for long periods-far longer than if they remained on the ground surface or moved up and down within a burrow. Interestingly, snakes selecting burrows or intermediate-thickness rocks may be able to have either the highest energy gain or the lowest overall metabolic rate, depending on the particular Tb they select. Medium-thickness rocks, the size rocks normally selected by the snakes, offer them a variety of suitable thermoregulatory opportunities.

Landscape heterogeneity and disturbance

Abstract: Section 1 Introduction.- 1. Landscape Ecology: State of the Art.- 2. Scale Effects in Landscape Studies.- Section 2 Disturbances in the Landscape.- 3. Landscape Ecology and Air Pollution.- 4. Parasites, Lightning, and the Vegetation Mosaic in Wilderness Landscapes.- 5. Fire, Grazing, and the Landscape Heterogeneity of a Georgia Barrier Island.- 6. Disturbance by Beaver (Castor canadensis Kuhl) and Increased Landscape Heterogeneity.- 7. Suppression of Natural Disturbance: Long-Term Ecological Change on the Outer Banks of North Carolina.- 8. Vegetation Dynamics in a Southern Wisconsin Agricultural Landscape.- Section 3 Implications for Landscape Management.- 9. Landscape Restoration in Response to Previous Disturbance.- 10. Patch-Within-Patch Restoration of Man-Modified Landscapes Within Texas State Parks.- 11. Progressiveness Among Farmers as a Factor in Heterogeneity of Farmed Landscapes.- Section 4 Conclusion.- 12. The Ethics of Isolation, the Spread of Disturbance, and Landscape Ecology.

A Theoretical Framework for Data Analysis of Wind Dispersal of Seeds and Pollen

Abstract: We compared a variety of models for the analysis of data on the wind dispersal of seeds and pollen. Dispersal distances from a source depend upon such factors as settling velocity, height of release, wind speed and turbulence, and specific morphological adaptations for dispersal. The dispersal curve, which describes the frequency distribution of dispersal distances, usually shows its peak at some distance from a source and falls off with distance. We used the location of that peak as a measure of dispersal, and organized data in terms of the predictions of two models for the dynamics of advection and diffusion. These data are summarized in a seed dispersal diagram in which the modal distance normalized by the height of seed release is plotted against the mean with speed normalized by the falling speed of seeds. Data from 15 studies were used to compare actual dispersal relationships.

Species-Area Relationship for Stream Fishes

Abstract: We sampled riffle and pool habitats of small streams in Minnesota, Illinois, and Panama to examine variation in species-area relationships within and between the respective fish faunas. For six of the seven steams studied, habitat volume was a better predictor of species richness than was habitat area, and number of individuals was a better predictor of species richness than habitat volume. Slopes of species-volume relationships were similar among regions, but the number of species per unit volume was greater in Panama. Multiple regression analyses indicated that knowledge of habitat complexity and volume did not enhance appreciably the capability of linear models to predict species richness from number of individuals in the sample. These results support the hypothesis that species-area relationships may often be epiphenomena stemming from the more com- prehensive community "samples" intercepted by larger habitat patches. Although number of individuals was the best single predictor of species richness, habitat structure and type clearly influenced species' distributions in some streams, thereby indicating that species- area relationships were not strictly sampling phenomena. An index of habitat complexity based on depth, current, and bottom type was correlated with species richness in two Panama streams. Also, the abundance of individual species was more likely to be correlated with habitat volume in Panama than in Illinois or Minnesota, and species relative abun- dances were more similar between years in Panama than in Illinois or Minnesota streams, especially in pools. These patterns suggest that in streams subject to strong seasonal and annual environmental variation, habitat features are poorer predictors of fish distribution and abundance than in streams subject to less environmental variability. We speculate that annual variability in reproductive success and harsh winters interact to maintain imbalance between the fish assemblages and their habitat in Minnesota. Weak relationships between species richness and habitat volume or complexity may be indicative of population vari- ability and the predominance of extinction/recolonization processes in community orga- nization.

Different Respones to Gaps Among Shade‐Tollerant Tree Species

Abstract: Most species of trees in the Eastern deciduous forests of North America can be considered shade tolerant (sensu Whitmore 1975, 1982, Swaine and Whitmore 1988) in that germination and seedling establishment can occur beneath a closed canopy. I propose that the ability to tolerate shade allows a range of responses to disturbances that are qualitatively different from those of intolerant species. Furthermore, quantitative differences exist among shade-tolerant species in their responses to gaps. Differentiation of the responses of shade-tolerant species to gaps has significant implications for general models of forest dynamics.

Tree Seedlings on Logs in Picea-Tsuga Forests of Oregon and Washington

Abstract: Logs are the major seedbed for trees in coastal Picea sitchensis–Tsuga heterophylla forests. Field experiments were conducted at Cascade Head, Oregon, and Hoh River, Washington, to examine pathogens, predation, competition, and standing water as causes for this close seedling–log association. More seedlings survived on log blocks than on soil blocks, regardless of whether the blocks were raised or placed flush with the soil surface. Standing water was therefore an unlikely cause of the seedling–log association. Comparisons of plots protected from and exposed to predation revealed that predation was minor and of equal intensity on soils and logs. Sterilizing soils did not consistently increase seedling survival above controls. Clearing ground–layer vegetation from soil plots significantly increased the survival of conifer seedlings compared with that on uncleared soils. The seed penetration rates through moss mats indicated that <1% of the seedlings germinated within moss mats. Competition with herbs and mosses on the forest floor therefore appears to be responsible for the disproportionate number of tree seedlings found on logs. Recently fallen logs represent sites where competition is low enough for tree seedling recruitment within many Picea–Tsuga forests.

A determination of the energetic equivalence of the risk of predation

Abstract: The influence of predation risk on patch choice was measured by examining the spatial distribution of 10 guppies (Poecilia reticulata) between two feeders, at one of which there was a risk of predation. The distribution was assumed to be ideal free. Nine unique situations were examined using all possible combinations of three risk levels and three diet levels, for each sex of guppy separately. Both sex and diet level influenced the effect of predation risk on patch choice. For the females the effect of risk was highest at the intermediate diet level. However, the males exhibited the opposite response: the effect of risk of predation was lowest at the intermediate diet level. A simple equation was then used to predict how much extra food (representing the energetic equivalent of risk) must be added to the risky patch for the guppies to become indifferent to the risk differences between the two types of patches. This manipulation caused a similar number of guppies to use both the risky and safe feeders, reducing or offsetting the influence of risk of predation. However, the male guppies were less influenced by this manipulation than were the females. The different results for the two sexes are consistent with known differences in their life histories, indicating that a knowledge of an animal's life history will often be necessary to

Choice of Oviposition Site by Hyla Chrysoscelis: Role of Predators and Competitors

Abstract: The role of predators and competitors in the choice of oviposition site by the treefrog Hyla chrysoscelis was examined in a randomized complete block experiment using 90 replicated experimental ponds. Control ponds containing neither predators nor competitors were contrasted with treatment ponds into which one of four species of predators (Ambystoma maculatum larvae, Enneacanthus chaetodon adults, Notophthalmus viridescens adults, Tramea carolina larvae) or one of two species of competitors (Rana catesbeiana, Hyla chrysoscelis) was added. Treatments had significant effects on the mean number of eggs deposited in ponds. Fewer eggs were laid in ponds with Ambystoma, Enneacanthus, or Hyla, as a result of fewer females laying eggs and fewer eggs laid per visit, compared with control ponds. Notophthalmus, Rana, and Tramea had no effect on the number of eggs laid. Ovipositing Hyla discriminated among potential oviposition sites based on the species present. Choice of oviposition site can determine the success of a female's reproductive investment, and it can be a mechanism affecting the structure of ecological communities as well. Our results emphasize the importance of oviposition site choice in the evolution of reproductive patterns and implicate species avoidance by ovipositing females as a mechanism important in generating variability in ecological communities.

A Model of Population Growth, Dispersal and Evolution in a Changing Environment

Abstract: The climatic and biotic conditions at any geographic location will change through time, for example, because of the advance of glaciers. If it is to avoid extinction, a species adapted to a moving habitat must either track its habitat spatially, or adapt genetically to the new environmental conditions. These processes of migration and evolution are important in determining continental biogeographic patterns. We develop a model to explore the relative contributions of adaptation and dispersal as alternative mechanisms whereby a population can respond to changing environmental conditions. In our model the environment to which the species is adapted moves across the landscape at a constant velocity, and a quantitative trait determines each individual's fitness as a function of the local environmental conditions. Local populations are allowed to adapt genetically to the environmental conditions at each point in space, so that a cline develops in the quantitative character. We find that if the rate of environmental movement is slow, the species will track its environment across space, otherwise it will go extinct. Additionally, the higher the genetic variance in the character, the easier it is for the species to maintain itself in a moving environment. Our results generalize previous models that predict a critical patch size of suitable habitat necessary for population persistence.

Early Successional Patterns of Forest Species: Interactions of Life History Traits and Disturbance

Abstract: Patterns of abundance were examined for vascular plant species during 21 yr of succession in two clear-cut and burned Pseudotsuga forests in the western Cascade Range of Oregon. A majority of forest understory species persisted through disturbance. Most colonizing species established within 2 yr after burning. Individualistic species re- sponses were described by a series of broadly overlapping, unimodal curves of constancy and canopy cover, differing in time of initiation, duration, and magnitude. Thus, early successional change was characterized by gradual shifts in the abundance of generally persistent species. Eleven population patterns (species groups) were identified. Interactions of life history traits and disturbance explain the temporal trends of the most common species. Within the groups of invading species, the timing of initial establishment, as well as the timing and magnitude of peak abundance were related to the origin of propagules, phenological traits, potential for vegetative expansion, and temporal and spatial variation in disturbance. Abundance patterns of invading species were also influenced by stochastic and historical factors. Contrasting responses of species between sites reflected differences in histories of logging and slash burning. Within the groups of residual species, temporal patterns of abundance reflected initial species distributions, resistance to logging and burning distur- bance, mode of reproduction, morphological traits, and spatial variation in disturbance intensity. These observations suggest that early secondary succession in Pseudotsuga forests has a deterministic component, founded in the life history traits of the available species, and

A model of wind dispersal of winged or plumed seeds

Abstract: We derived a micrometeorological model for the dispersal of winged or plumed seeds from a point source. The model is based on six measurable parameters: mean release height, mean and standard deviation of the terminal velocities of seeds, standard deviation of vertical wind velocities, and the mean and standard deviation of the natural logarithms of horizontal wind velocities. Predictions of the model include (1) the distri- bution of the dispersal curve (defined as numbers vs. distance from source) is right skewed; (2) the median and long-range dispersal distances need not be well correlated; and (3) increased variance in flight trajectories for a seed population will place the mode of the dispersal curve closer to the source. Empirical tests of the model showed that it adequately characterizes the observed dispersal curves for experimental releases. It is shown that a simple ballistic equation provides a good estimate of the median dispersal distance (and mean distance if the skew of the dispersal curve is slight) for experimental releases. Tests of the model using natural releases from isolated trees indicated that winged seeds do not detach from the parent randomly with respect to horizontal wind velocity. The need to understand the relationship between the probability of detachment and the frequency distribution of horizontal wind velocities is stressed.

Arrival and Survival in Tropical Treefall Gaps

Abstract: Most tropical tree species require light from a treefall gap at some time during their lives to reach maturity. Responses to light conditions have been dichotomized as shade-intolerant pioneers or shade-tolerant climax species (e.g., Whitmore 1975, 1982, 1989). The former typically have small, widely dispersed seeds from which juveniles establish only in gaps, while the latter typically have larger seeds that can germinate beneath the forest canopy and can persist as suppressed juveniles or grow slowly until a gap forms. According to this framework, a new gap promotes shade-intolerant regeneration through germination and shade-tolerant regeneration through release of suppressed juveniles. Truly shade-tolerant species can grow to maturity beneath the forest canopy, but even these are likely to benefit from any increases in light levels beneath the canopy (Uhl et al. 1988, Canham 1989, Lieberman et al. 1989b, Martinez-Ramos et al. 1989). Although useful, we believe this dichotomy limits views of gap dynamics by implying that each species is constrained to a specific pathway to the forest canopy. In reality, all species recruit to differing degrees from dispersal into new gaps and from release of dormant seed or juvenile banks beneath the canopy (see Martinez-Ramos et al. 1989). The probability that a tree of a given species will enter the forest canopy is a function of the joint probabilities of arriving and surviving in particular habitats. We emphasize three issues that, for any species, define probable regeneration at a given site: (1) pattern of seed arrival in gaps and beneath the canopy, (2) proportion of forest area in gap vs. closed canopy, and (3) survival to reproductive maturity of seeds landing in gaps and beneath the canopy. This view enables recruitment of tropical trees to be interpreted from the perspective of relative advantages of given characteristics within the context of those environments in which individuals with those characteristics are located.

A Strong Relationship Between Oviposition Preference and Larval Performance in a Shoot-Galling Sawfly

Abstract: To investigate the relationship between oviposition preference and offspring performance we tested three hypotheses about the oviposition behavior of Euura lasiolepis: (1) the preference hierarchy hypothesis that Euura had an oviposition preference based on shoot lengths; (2) the larval survival hypothesis that the preference hierarchy corresponded to the suitability of shoots for larval survival; and (3) the flexibility hypothesis that the preference hierarchy changed as resource availability changed. The oviposition preference hierarchy of Euura lasiolepis, a shoot—galling sawfly, for plant shoots of different lengths strongly corresponded with the rank of shoot lengths for larval survival. The oviposition preference hierarchy was related to shoot growth; the more rapidly a shoot was growing the higher the probability that an oviposition site on a shoot would be attacked. Larval survival was highest on long shoots on young ramets. The probability of shoot abscission was negatively related to shoot length, and Euura in galls on abscised shoots invariably died. Intraspecific interactions and willow clone identity, independent of shoot length and ramet age, had relatively small influences on larval survival. The oviposition hierarchy was flexible; if long shoots were not available shorter shoots were accepted for oviposition. Shoots were always attacked in order of length, so that a strong relationship between preference and performance was always maintained.

Gap light regimes influence canopy tree diversity.

Abstract: In old-growth deciduous forests of the eastern United States, where light levels beneath intact canopies are about 1% of full sunlight (Canham 1988b), gaps result in locally elevated light levels. Such increases in light appear necessary for almost all tree species to attain canopy status (e.g., Barden 1980, Runkle 1981, Canham 1989). While patterns and mechanisms of replacement among the two species most often codominant in these forests, American beech (Fagus grandifolia) and sugar maple (Acer saccharum), have been intensively studied (Cain 1935, Fox 1977, Woods 1979, Donnelly 1986, Canham 1988a), relationships among rates and types of treefall, variability in light levels, and species' responses have not been studied. We have studied populations of tree species in one old-growth Eastern deciduous forest (Warren Woods, Michigan, USA) for the past 15 yr. We have followed the more abundant canopy species under different spatial and temporal patterns of increased light levels associated with different rates of gap formation (Poulson and Platt 1981, 1988, unpublished manuscript). Here we summarize our results to show how size and compass orientation of gaps determine light regimes and how light regimes interact with sapling architecture to influence the diversity of species that reach the canopy. We contrast small, isolated treefalls with small, overlapping treefalls and contrast small, overlapping gaps oriented north-south (N-S) with those oriented eastwest (E-W). In addition, we describe patterns to zonation of species in large, multiple treefalls. We conclude our essay by discussing how changes in latitude influence gap light regimes and, hence, replacement patterns of canopy trees in temperate and tropical forests.

Predation Risk and The Foraging Behavior of Competing Stream Insects

Abstract: Invertebrates that graze periphyton growing on stones in coldwater streams in Michigan are at greatest risk of predation from a benthic-feeding fish (the mottled sculpin, Cottus bairdi) while on the top surface of substrates, where periphyton is most abundant, and during the day, when sculpin feed most actively. We conducted laboratory experiments with larvae of two strong exploitative competitors for periphyton (the mayfly Baetis tri- caudatus and the caddisfly Glossosoma nigrior) to test if and how their foraging behavior was affected by presence of sculpin when sculpin were not allowed to attack prey. We incorporated four experimental factors in a factorial design: sculpin presence/absence, the abundance (low, high) of food that was patchily distributed on the top surface of substrates, grazer hunger level (fed, starved), and time of day (day, night). In feeding trials, Baetis was much more vulnerable than Glossosoma to sculpin pre- dation. Similarly, only the foraging behavior of Baetis was strongly affected by the presence of sculpin. Baetis larvae responded to the presence of sculpin by: (1) significantly reducing the time spent on the top surface of substrates, and their movement rate while moving within and between food patches, and (2) significantly increasing the proportion of time spent in food patches, and the proportion of individuals that left the substrate by drifting downstream. Baetis larvae accepted greater risk of predation (by spending more time on top) when food abundance on top was high and when larvae had been starved for 27 h. Also, Baetis behavior was strongly affected by sculpin presence during the day but not at night. Collectively, these results suggest that Baetis larvae made adaptive compromises between feeding and avoiding sculpin. In contrast, Glossosoma behavior was not affected by the presence of sculpin. Glossosoma responded most strongly to alterations in food density and their own hunger level. Our results suggest that the presence of sculpin, even though the fish consumed no larvae, might indirectly affect the outcome of competitive interactions between Baetis and Glossosoma through the grazers' differential behavioral responses to the predator.

Patterns of plant species richness along riverbanks

Abstract: We studied bank vegetation between the spring high-water level and the summer low-water level along two rivers in northern Sweden. The hypothesis that natural and ruderal species would show different downstream patterns of species richness was tested by sampling species composition and environmental variables along 200 m long stretches of riverbank, 10 km apart. Natural species richness was highest in the midreaches of both rivers, whereas ruderal species showed a significant, monotonic increase down- stream. There are no obvious mechanisms producing the quadratic pattern of natural species richness. The downstream increase in ruderal species suggests a founder effect depending on larger artificial disturbances near the coast, but alternative explanations are also given. Total species richness did not exhibit any interpretable downstream patterns. The only factors significantly correlated with total species richness along both rivers were substrate heterogeneity and substrate fineness. Total species richness increased with substrate het- erogeneity and was at maximum at intermediate levels of substrate fineness. This suggests that co-occurrence of natural and ruderal species is most likely where substrate types are numerous and dominant substrate particle size intermediate. Kep u'ords. natural species; northern Sweden; riparian vegetation; river; riverbank; ruderal species;

Community Structure and Diversity of Coral Reefs: The Role of History

Abstract: [Extract] "Our current evidence on the role of chance and history is circumstantial and the arguments speculative, but such is the state of our knowledge" (Hubbell and Foster 1986:317).

Costs of Flower and Fruit Production in Tipularia Discolor (Orchidaceae)

Abstract: The cost of reproduction may be an important constraint on the evolution of life history traits, yet it has seldom been adequately measured in plants. To demonstrate a net cost of reproductive structures one must show that their production has a negative effect on future growth. Using a summer—deciduous orchid, we compared subsequent growth of plants with no inflorescence, no fruits, few fruits ( 10). Greater investment in reproduction led to increasingly negative effects on future corm size and leaf area. The cost of an inflorescence without fruits was about one—half the cost of an inflorescence with few fruits. Each fruit cost the plant °2% of its total leaf area the next year. Natural pollination resulted in a mean of seven fruits per plant. Hand—pollination led to a dramatic increase in fruit set (up to 25 fruits per plant), and a large decrease in subsequent growth. Plants with many fruits were less likely to propagate vegetatively than those with few fruits. Also, plants with many fruits were less likely to flower the following year, probably because they were smaller than those with few fruits. The potential benefits of greater pollinator visitation are unclear because larger "clutch size" may not result in greater lifetime fecundity.

The Seed Bank of a Freshwater Crustacean: Copepodology for the Plant Ecologist

Abstract: The spatial and temporal aspects of dormancy in a freshwater copepod, Diaptomus sanguineus, were investigated during three consecutive years in Bullhead Pond, Rhode Island. Patterns of diapausing egg production and deposition were monitored with plankton sampling and settling traps. Vertical and horizontal distributions of diapausing eggs in sediments were investigated by taking core samples. Diapausing eggs removed from sediments were tested in the laboratory for hatching ability. The long—term spatial and temporal patterns of emergence from the diapausing egg stage were documented in the field using inverted plastic funnel traps sampled weekly. The field data along with an estimated annual budget for diapausing eggs in the pond suggest that D. sanguineus has an egg bank, analogous to the seed banks of plants, that allows it to survive through harsh environmental periods. See full-text article at JSTOR

Role of debris dams in the structure and functioning of low-gradient headwater streams

Abstract: The importance of debris dams to organic matter dynamics and the mac- roinvertebrate community of two low-gradient, headwater streams on the Coastal Plain of Virginia was examined through sampling of natural dams and by experimental manipu- lation of dam abundance. Buzzards Branch had permanent flow and a sand substrate; Colliers Creek had intermittent flow and an organic substrate. Dam abundance varied from 8 to 13 dams/100 m of stream length and covered only 1-3% of channel surface. Annual mean storage of organic matter (ash-free dry mass; particles >0.15 mm) in debris dams was 922 g/m2 and 3356 g/m2 in Colliers Creek and Buzzards Branch, respectively. Wood > 16 mm constituted 73-80% and fine particulate organic matter (FPOM; particles 0.15-1 mm) < 1% of total organic matter in the streams. Dams stored 21% and 85% of the coarse particulate organic matter (CPOM) on the channel surface in Colliers Creek and Buzzards Branch, respectively. Storage was highly seasonal, with highest concentrations following autumnal leaf fall and only 5-17% of December storage present in February. Dams were net exporters of large amounts of small particles of CPOM and FPOM. Macroinvertebrate density and biomass in dams was correlated with changes in organic matter storage both over seasons and between streams. Annual mean densities were 8915 individuals and 22 302 individuals/, and biomass was 0.3 g/m2 and 3.2 g/m2, in Colliers Creek and Buzzards Branch, respectively. Densities were at least 10 times, and biomass 5 times, as great in dams as on sediment. Increasing the abundance of dams increased organic matter storage, macroinvertebrate abundance, and the relative contribution of shredders to biomass, both in dams and on sediment. Increasing dam abundance also increased retention of leaves during base and especially storm flow. Widespread removal of riparian vegetation throughout the south- eastern Coastal Plain, resulting in a decrease in the number of dams in streams, no doubt has altered the ecological characteristics of these streams relative to historical, pristine conditions. In particular, the transport of both energy and nutrients to downstream riverine areas likely is greater, and production of higher consumer levels probably is lower under present conditions.