Showing papers in "Ecology in 1986"

Canonical Correspondence Analysis: A New Eigenvector Technique for Multivariate Direct Gradient Analysis

Abstract: A new multivariate analysis technique, developed to relate community composition to known variation in the environment, is described. The technique is an extension of correspondence analysis (reciprocal averaging), a popular ordination technique that extracts continuous axes of variation from species occurrence or abundance data. Such ordination axes are typically interpreted with the help of external knowledge and data on environmental variables; this two—step approach (ordination followed by environmental gradient identification) is termed indirect gradient analysis. In the new technique, called canonical correspondence analysis, ordination axes are chosen in the light of known environmental variables by imposing the extra restriction that the axes be linear combinations of environmental variables. In this way community variation can be directly related to environmental variation. The environmental variables may be quantitative or nominal. As many axes can be extracted as there are environmental variables. The method of detrending can be incorporated in the technique to remove arch effects. (Detrended) canonical correspondence analysis is an efficient ordination technique when species have bell—shaped response curves or surfaces with respect to environmental gradients, and is therefore more appropriate for analyzing data on community composition and environmental variables than canonical correlation analysis. The new technique leads to an ordination diagram in which points represent species and sites, and vectors represent environmental variables. Such a diagram shows the patterns of variation in community composition that can be explained best by the environmental variables and also visualizes approximately the "centers" of the species distributions along each of the environmental variables. Such diagrams effectively summarized relationships between community and environment for data sets on hunting spiders, dyke vegetation, and algae along a pollution gradient.

Environmental impact assessment: "pseudoreplication" in time?'

Abstract: A recent monograph by Hurlbert raised several problems concerning the appropriate design of sampling programs to assess the impact upon the abundance of biological populations of, for example, the discharge of effluents into an aquatic ecosystem at a single point. Key to the resolution of these issues is the correct identification of the statistical parameter of interest, which is the mean of the underlying probabilistic "process" that produces the abundance, rather than the actual abundance itself. We describe an appropriate sampling scheme designed to detect the effect of the discharge upon this underlying mean. Although not guaranteed to be universally applicable, the design should meet Hurlbert's objections in many cases. Detection of the effect of the discharge is achieved by testing whether the difference between abundances at a control site and an impact site changes once the discharge begins. This requires taking samples, replicated in time, Before the discharge begins and After it has begun, at both the Control and Impact sites (hence this is called a BACI design). Care needs to be taken in choosing a control site so that it is sufficiently far from the discharge to be largely beyond its influence, yet close enough that it is influenced by the same range of natural phenomena (e.g., weather) that result in long-term changes in the biological populations. The design is not appro- priate where local events cause populations at Control and Impact sites to have different long-term trends in abundance; however, these situations can be detected statistically. We discuss the assumptions of BACI, particularly additivity (and transformations to achieve it) and independence.

Estimating Uncertainty in Population Growth Rates: Jackknife vs. Bootstrap Techniques

Abstract: Although per capita rates of increase (r) have been calculated by population biologists for decades, the inability to estimate uncertainty (variance) associated with r values has until recently precluded statistical comparisons of population growth rates. In this study, we used two computer- intensive techniques, Jackknifing and Bootstrapping, to estimate bias, standard errors, and sampling distributions of r for real and hypothetical populations of cladocerans. Results generated using the two techniques, using data on laboratory cohorts of Daphnia pulex, were almost identical, as were results for a hypothetical D. pulex population whose sampling distribution was approximately normal. However, for another hypothetical population whose sampling distribution was negatively skewed due to high juvenile mortality, Bootstrap and full-sample estimates of r were negatively biased by 3.3 and 1.8%, respectively. A bias adjustment reduced the bias in the Bootstrap estimate and produced estimates of r and SE(r) almost identical to those ofthe Jackknife technique. In general, our simulations show that the Jackknife will provide more cost-effective point and interval estimates of r for cladoceran populations, except when juvenile mortality is high (at least >25%). Coefficients of variation in the mean of r within laboratory cohorts of D. pulex were one-half to one-third the magnitude of the corresponding coefficients of variation in the mean of total reproduction and in the mean day to death (range of values of cv(r) = 1.6 to 3.8%). This suggests that extremes in reproductive output and survival of individuals tend to be dampened at the population level, and that within-cohort variability in r is not explosive. Moreover, between-cohort variability in r can be much greater than within-cohort variability, as indicated by a statistically significant difference of 30% (P

Elasticity: The Relative Contribution of Demographic Parameters to Population Growth Rate

Abstract: over plants grown without competition (P < .05). Plants suffering from both root and shoot competition were not significantly smaller than those suffering from root competition only, but they showed the greatest size variability. In plants competing both above and below ground, root competition accounted for the reduction in mean plant size, but shoot competition accounted for the increase in size inequality. The results support the hypothesis that competition for light is "asymmetric" and that the observed increases in plant size variability with increasing density may have been primarily due to competition for light. There may be situations in which competition for nutrients is also asymmetric, i.e., situations in which nutrients can be preempted by individuals with larger roots. Because a leaf is shaded only by leaves above it, competition for light appears to be inherently asymmetric.

Predation risk and unpredictable feeding conditions: determinants of body mass in birds'

Abstract: The body mass (or fat reserves) maintained by a wintering bird can be viewed as a trade-off between the risk of starvation and the risk of predation. This follows from the fact that fat reserves affect survival in very different ways. From the starvation point of view, a bird should be as fat (or heavy) as possible in order to minimize its probability of starvation during weather-related food unavailability because fatter birds can survive longer without food than leaner birds. From the predation risk point of view, however, a bird should be as lean as possible to minimize its probability of being killed. Leaner birds will incur a smaller cost of existence than heavier birds and thus spend less time feeding, potentially exposed to predators. In addition, lean birds are likely to be more adept at escaping predators once attacked. If a bird is attempting to minimize its probability of death during the winter season, the fat reserves and thus the body mass it maintains will reflect a trade-off between starvation and predation risk. A simple stochastic simulation model developed to explore the nature of this trade-off indicates that body mass should: (1) increase with the frequency and/or harshness of periods of food unavailability (due to snow, ice, wind, etc.); (2) decrease with increasing predation risk; (3) decrease with increasing temperature; and (4) increase with food abundance. The analysis of the model readily explains why wintering birds maintain fat levels lower than those of which they are capable. The tendency for individuals of a given bird species to be heavier in more northerly populations and during midwinter is also readily interpreted in terms of the above trade- off. Implications for foraging behavior and population regulation in wintering birds are discussed.

Applying Metric and Nonmetric Multidimensional Scaling to Ecological Studies: Some New Results

Abstract: Metric (eigenanalysis) and nonmetric multidimensional scaling strategies for ecological ordination were compared. The results, based on simulated coenoplane data showing varying degrees of species turnover on two independent environmental axes, suggested some strong differences between metric and nonmetric scaling methods in their ability to recover underlying nonlinear data structures. Prior data standardization had important effects on the results of both metric and nonmetric scaling, though the effect varied with the ordination method used. Nonmetric multidimensional scaling based on Euclidean distance following stand norm standardization proved to be the best strategy for re- covering simulated coenoplane data. Of the metric strategies compared, correspondence analysis and the detrended form were the most successful. While detrending improved ordination configurations in some cases, in others it led to a distortion of results. It is suggested that none of the currently available ordination strategies is appropriate under all circumstances, and that future research in ordination methodology should emphasize a statistical rather than empirical approach.

Ecosystem Alteation of Boreal Forest Streams by Beaver (Castor Canadensis)

Abstract: Beaver (Castor canadensis) alter the structure and dynamics of aquatic ecosystems with a minimum of direct energy or nutrient transfer. Through dam building and feeding activities, beaver act as a keystone species to alter hydrology, channel geomorphology, biogeochemical pathways, and community productivity. Here we consider the effects of beaver activity on several major ecosystem components and processes in boreal forest drainage networks in Quebec, Canada. The density of dams on the small streams (≤4th order) we studied average 10.6 dams/km; the streams retain up to 6500 m3 of sediment per dam, and the wetted surface area of the channel is increased up to several hundred—fold. Beaver are also active in large order streams (≥5th order), but their effects are most noticeable along riverbanks and in floodplains. Comparative carbon budgets per unit area for a riffle on 2nd order Beaver Creek and a beaver pond downstream show the pond receives only 42% of the carbon acquired by the riffle annually. However, because the pond has a surface area seven times greater than the riffle, it receives nearly twice as much carbon as the riffle per unit of channel length. Carbon in the pond has an estimated turnover time of °161 yr compared to ° 24yr for the riffle. Beaver ponds are important sites for organic matter processing; the stream metabolism index (SMI), a measure of ecosystem efficiency for the utilization or storage of organic inputs, is 1.63 for the pond compared to 0.30 for the riffle; the turnover length (S) for particulate carbon is 1.2 km for the pond compared to 8.0 km for the riffle. Beaver—induced alterations to the structure and function of streams suggest removal of beaver prior to 1900 AD had substantial effects on the dynamics of lotic ecosystems. Our results suggest that current concepts of the organization and diversity of unaltered stream ecosystems in North America should recognize the keystone role of beaver, as drainage networks with beaver are substantially different in their biogeochemical economies than those without beaver.

Home range, time, and body size in mammals'

Abstract: The relationship between home range area and body size of terrestrial mammals is reconsidered in light of the concept of biological time. Biological time is an internal, body—mass—dependent, time scale to which the durations (of rates) of biological events are entrained. These events range from purely physiological (e.g., muscle contraction time) to purely ecological (e.g., time to traverse home range). Evidence is presented that home range size scales linearly to body mass for carnivores as it does for herbivores. This scaling supports the hypothesis that animals select their home range areas to meet metabolic demands integrated over biologically critical periods. Confounding variables in the home range—body mass regression include habitat productivity and methods of location. Data on home ranges derived from telemetry studies of terrestrial carnivores are presented and used to derive allometric equations for home range area. The exponents of these equations are shown to approximate 1.0, although intercept values vary with latitude and, presumably, habitat productivity. Social organization and behavior may also influence the relationship of home range area to metabolic needs for different sex and age categories within a species.

Towards a Unifield Foraging Theory

Abstract: In the broadest sense, foraging adaptations can include problems of finding food, avoid- ing predation while looking for food, and reproducing. In this paper, a theory that treats these three behaviors in a consistent, unified manner, with one common currency, is presented. The theory is called unified foraging theory, although it actually pertains to a wider class of behavioral problems. The theory is based on models using Markovian decision processes and leads to quantitatively testable predictions about behavioral strategies.

Ectoparasitism as a Cost of Coloniality in Cliff Swallows (Hirundo Pyrrhonota)

Abstract: Colonially nesting Cliff Swallows (Passeriformes: Hirundo pyrrhonota) in southwestern Nebraska, USA, are commonly parasitized by hematophagous swallow bugs (Hemiptera: Cimicidae: Oeciacus vicarius) and fleas (Siphonaptera: Ceratophyllidae: Ceratophyllus celsus). We examined to what degree these ectoparasites represent a cost of coloniality for Cliff Swallows. The number of swallow bugs per nest increased significantly with Cliff Swallow colony size. Body mass of nestling swallows at 10 d of age declined significantly as the number of bugs per nestling increased. By fumigating half of the nests in some colonies, killing the bugs, and leaving half of the nests as nonfumigated controls, we showed that swallow bugs lower nestling body mass and nestling survivorship in large Cliff Swallow colonies but not in small ones. Bugs cost nestlings, on average, up to 3.4 g in body mass, and reduced survivorship by up to 50%. Parasitism by fleas showed no consistent relationship with colony size during the nestling period but increased significantly with colony size early in the season, when birds were first arriving in the study area. Fleas did not affect nestling body mass or survivorship and thus, unlike swallow bugs, are probably not important costs of coloniality to Cliff Swallows. Field observations and nest fumigation experiments showed that Cliff Swallows apparently assess which nests are heavily infested with swallow bugs early each spring and select parasite-free nests, leading sometimes to alternate-year colony site usage. Cliff Swallows were more likely to construct new nests (rather than reusing old ones) in large colonies than in small colonies, probably in response to heavier infestations of ectoparasites in the existing nests of large colonies.

Statistical Analysis of Survival and Removal Rate Experiments

Abstract: We consider both parametric and nonparametric statistical analyses of survivorship curves and of removal rates, including assumptions, tests, ecological applications, and the difference between censored and uncensored survival data. For censored data, the Gehan—Wilcoxon test, the longrank test, and the likelihood ratio test and appropriate and are readily available in mainframe computer statistical packages. In cohort analyses, these tests can determine if cohorts of different ages have different age—specific death rates or different death rates over the same time period. See full-text article at JSTOR

Sediment-related mechanisms of growth limitation in submersed macrophytes

Abstract: Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royle grown on 40 different sediments from 17 geographically widespread North American lakes indicated 10— and 20—fold declines in growth with increasing sediment organic matter up to a concentration of °20% dry sediment mass. Poor growth was obtained also on inorganic sediments with a sand fraction exceeding 75% dry sediment mass. Diminished growth on organic sediments occurred at low values of sediment density, and on sands, at high values of sediment density. Differential centrifugation of organic sediment, facilitating an increase in sediment density with no change in organic matter content, stimulated Hydrilla growth, indicating that sediment density rather than organic matter content was most influential in regulating growth. Macrophyte growth and nutrient mass accumulation in shoots were closely correlated, but essentially unrelated to concentrations of nutrients in shoots. The magnitude (r value) and statistical significance of correlations between nutrients in macrophyte shoots and sediments varied appreciably, depending on the form of shoot nutrient data (concentration or accumulation), and the type (interstitial water or total), and basis (mass or volume) of sediment nutrient data. Growth and nutrient accumulation were highly correlated with sediment nutrient concentrations based on volume, indicating interrelationships among growth, nutrition, and sediment density. Additions of P and Fe in combination to organic sediments with N supplied in the overlying solution resulted in significant growth increases in Hydrilla, suggesting that diminished growth on unfavorable sediments was caused by multiple nutrient limitation. Nutrient uptake on low—density organic sediments was apparently limited by long diffusion distances. Limited rates of nutrient diffusion and exchange in coarse—textured sediments, in addition to low nutrient status, may have contributed to their poor ability to support macrophyte growth. Thus, mechanisms of growth limitation on both sands and organic sediments appear to involve nutrition.

Litter production and turnover in basin mangrove forests in southwest florida

Abstract: Litterfall, litter standing crop, and leaf decomposition of basin mangrove sites with mixed associations of Avicennia germinans, Rhizophora mangle, and Laguncularia racemosa adjacent to bay waters were compared to monospecific stands of A. germinans located inland of the mixed associations. The objective was to determine whether basin mangroves with low tidal amplitude are characterized by lower litter productivity, longer litter residence times, and increased nitrogen recycling. Litterfall rates for the two mixed forests averaged 8.10 Mg ha-' yr-I compared to 4.44 Mg ha'- yr- I in the three monospecific sites. Peak litterfall occurred from August to October after increases in soil water salinity during June and July. Seasonal changes in surface litter mass occurred only in the monospecific forests and corresponded with higher leaf decomposition rates when tidal inundation frequency increased. As a result, surface leaf litter in a monospecific site >200 m from the bay had a longer residence time than in two monospecific sites < 100 m inland. The rates of dry mass loss and organic carbon leaching of R. mangle leaves were lower than those for A. germinans, resulting in higher residence times of leaf litter in mixed (0.5 yr) than monospecific (0.2 yr) forests located < 100 m from the bay. Leaf nitrogen decreased during senescence, and absolute nitrogen increased in leaf litter during decomposition on the forest floor. As a result, turnover rates of nitrogen in surface litter were lower than for dry mass. A comparison of litter dynamics and nitrogen recycling among riverine, fringe, and basin mangroves suggests that the relative influence of export and decomposition on litter dynamics depends on the magnitude of tidal inundation. Inland basin mangroves have low nitrogen recycling efficiency in litterfall, but higher internal recycling of litter on the forest floor, due in part to a longer litter residence time, nitrogen retranslocation prior to leaf senescence, and nitrogen immo- bilization during litter decomposition and peat deposition.

Species Competitive Ability and Position Along a Natural Stress/Disturbance Gradient

Abstract: We tested the prediction that plant species that grow in undisturbed, nutrient-rich habitats tend to have higher competitive abilities than those found in disturbed or nutrient-poor habitats. The distributions of seven species (Eriocaulon septangulare, Rhynchospora fusca, Hypericum ellipticum, Juncus pelocarpus, Lysimachia terrestris, Dulichium arundinaceum, and Drosera inter- media) were measured along a gradient of exposure to wave action on the shore of Axe Lake, Ontario. The exposure gradient incorporates disturbance, through the removal of plant biomass, and stress. through the creation of a gradient in sediment organic content, nutrient concentrations, and fine particle sizes. Species distributions on the exposure gradient were quantified by determining the mean sediment organic content ofthe quadrats containing each species. Competitive abilities were measured as relative increase in dry mass per plant, in a field experiment in which species were grown together in all pair- wise combinations (N = 10 replicates). Species had significantly heterogeneous competitive abilities (P < .01). Species found on exposed, nutrient-poor shores (e.g., E. septangulare) had low competitive abilities, while those growing on sheltered, nutrient-rich shores (e.g., D. arundinaceum) had high competitive abilities. Competitive ability was significantly correlated with mean position on the ex? posure gradient.

Antipredator Responses and the Perception of Danger by Mosquito Larvae

Abstract: I conducted a series of laboratory experiments to contrast the responses of two species of mosquito larvae to a common predator, the freshwater bug Notonecta undulata. One prey species, Culex pipiens, commonly co-occurs with Notonecta, whereas the other, Aedes aegypti, shares no evolutionary history with this predator. I tested the evolution-based prediction that Culex should be killed at a lower rate due to stronger and more precise antipredator responses. I then studied the types of cues used by these prey to gauge predation risk by examining prey responses to simple disturbances and to potential chemical cues associated with predation. The results were that (1) both mosquito species responded to Notonecta by moving less and by shifting their microhabitat use; (2) Culex was indeed killed at a lower rate; (3) Culex showed far stronger responses to notonectids; (4) the response of Culex was correlated with actual predation risk even after encounter rate with predators was accounted for; and (5) Culex, but not Aedes, responded to water in which conspecifics had been preyed upon by Notonecta. The differences in the responses of the two species can be explained by a fun- damental difference in the way in which they estimate predation risk. Whereas the behavior of Aedes can be understood as a response to disturbance per se, Culex behavior is also mediated by chemical cues associated with the actual predation act. Since more predation on Culex should produce more alarm chemical, this cue is probably responsible for the observed "precise" (i.e., correlated with risk) responses.

Secondary Production, Emergence, and Export of Aquatic Insects of a Sonoran Desert Stream

Abstract: Aquatic insect secondary production, emergence, and export of adults to the adjacent terrestrial ecosystem were assessed in Sycamore Creek, Arizona, by means of benthic sampling, emer- gence traps, and catch-nets that passively sampled adults falling into the stream. Annual secondary production was 120.9 ? 18.0 g-m-2 yr-t and emergence was 23.1 g.m-2 yr-t (in dry mass units). The ratio of annual emergence to annual production (E/P) varied among taxa and ranged from 2 to 29%. Chironomids comprised 48.2% of production and 59.7% of emergence and mayflies accounted for 45.9 and 19.2%, respectively. Approximately 3% of emergent insect biomass returned to the stream; thus 22.4 g m-2 yr-I was transferred to the adjacent terrestrial ecosystem. The transfer of a significant portion of aquatic insect biomass to the terrestrial habitat reduced insects available to stream insec- tivores while providing prey for insectivores in neighboring terrestrial ecosystems.

Nitrogen Availability and Nitrogen Use Efficiency in Loblolly Pine Stands

Abstract: The effects of nitrogen availability on N cycling and N use efficiency (NUE) were examined in natural and fertilized loblolly pine stands on the upper coastal plain of South Carolina. Indices of N availability, based on potential rates of N mineralization, ranged from 1.6 to 11 kg°ha— ¹ (8 wk)— ¹ in the stands, and concentrations of N in foliage, wood, fine roots, and needlefall increased with greater N availability. Litterfall dry masses, net aboveground production, and litterfall nitrogen were all positively correlated with N availability, while indices of NUE decreased with increased N availability. Mechanisms that could explain increased NUE in low—N sites were examined in the field and in a phytotron study. First, nitrogen retranslocation on a per needle basis did not change significantly across the N—availability gradient, and thus could not account for the change in NUE efficiency. For the stands as a whole, however, substantially more N was retranslocated at the highest levels of N availability. Second, an increase in N uptake efficiency could not account for an increase in NUE with low N availability, since phytotron—grown seedlings fertilized with N had significantly higher rates of N uptake per unite root mass and lower root: shoot ratios than N—limited seedlings. Third, net production per unit N within pine seedlings was significantly higher in the N—limited plants, suggesting that an increase in the amount of carbon fixed per unit of tissue N could account for the observed increased in NUE in loblolly pine stands at low levels of N availability.

The Search for Patterns in the Balance of Nature: Advances and Retreats

Abstract: I predict there will be erected a two-or three-way classification of organisms and their geometrical and temporal environments, this classification consuming most of the creative energy of ecologists. The future principles ofthe ecology of coexistence will then be ofthe form "for organisms of type A, in environments of structure B, such and such relations will hold." This is only a change in emphasis from present ecology. All successful theories, for instance in physics, have initial conditions; with dif? ferent initial conditions, different things will happen. But I think initial conditions and their classifi? cation in ecology will prove to have vastly more effect on outcomes than they do in physics .... Bird censuses in a habitat in successive years or in similar habitats in one year are usually very similar, while insect censuses (to the extent they can be taken) seem often to differ dramatically from place to place and year to year. Thus, plausibly in our classification, insects, at least ofsome kinds, will go into a non-equilibrium category and birds into an equilibrium category. But the classification will be more pervasive than these examples suggest; many morphological, behavioral, and genetic parameters will probably be included. There has been a biological tradition of searching for the best organism to solve a problem?like Drosophila for chromosome genetics and viruses and bacteriafor aspects ofmglecular genetics. The ecologists should resist this temptation.

Recording Devices on Free‐Ranging Marine Animals: Does Measurement Affect Foraging Performance?

Abstract: Observations of foraging patterns are essential to understanding the energetic ecology of marine animals. However, direct observations are not often feasible in the field because most foraging takes place below the sea surface or at great distances from land. Attachment of data-recording devices to animals can greatly facilitate the collecton of meaningful data. Devices are being used to an increasing extent on marine mammals (Kooyman et al. 1976, Ray et al. 1978), birds (Kooyman et al. 1971, 1982, Adams and Brown 1983, Lishman and Croxall 1983, Wilson and Bain 1984a, b), reptiles (Stoneburner 1982), and fish (Voegeli and Piucock 1980, Priede 1983a). These devices may supply data upon recovery (e.g., Kooyman et al. 1971, 1982) or through telemetry to local receivers (e.g., Voegeli and Piucock 1980, Priede 1983a, b) or to orbiting satellites (Stoneburner 1982). The effects of instrument mass and harness attachment on land animals are considered to have the potential of affecting the outcomes of experiments and observations (Dumke and Pils 1973, Gilmeretal. 1974, Perry 1981, Perryetal. 1981). However, the effects of devices on marine animals, which live in a much more viscous medium than air, have not been evaluated. In this study we investigated the effects of attached instruments on the foraging activity of the African Penguin (Spheniscus demersus). We show that instrument size may adversely affect foraging behavior, but that data obtained using devices of varying sizes can be used to back-calculate true foraging parameters of free-swimming penguins without devices. Data-recording devices are especially important in studies of free-swimming penguins because these birds are inconspicuous on the surface of the water and are not visible during their frequent traveling and foraging dives. Instruments have been constructed to record speed (Wilson and Bain 1 984b), foraging range (Wilson and Bain 1984b, Wilson and Achleitner 1985), dive time (Trivelpiece et al., in press), dive frequency and depth (Kooyman et al. 1982, Wilson and Bain 1984a). Results show that African Penguins are capable of sustained speeds of at least 7.5 km/h (Nagy et al. 1984) and that other penguin species forage at depths exceeding 200 m (Kooyman et al. 1982). Measurements of foraging distance have also been used to estimate the energy cost of swimming and to construct energy budgets (Nagy et al. 1984). Devices may affect performance in two ways. The mere presence of a device may modify behavior. However, in our experience with penguins this does not appear to be important; after a short period of adjustment, African Penguins with devices continued their normal nesting activity and entered the sea to forage with frequencies similar to control birds (Wilson and Bain 1984a, b). Second, the mass or drag of a device may retard swimming speed, thus reducing dive depth, foraging range, and the number of prey encountered. Instrument mass in itself probably does not adversely influence foraging behavior because attached devices are usually .05) which suggests that the smallest meters had virtually no effect on foraging behavior. On the other hand, two penguins fitted with an electronic distance meter 14.6% of penguin

The Role of Life History Processes in Primary Succession on an Alaskan Floodplain

Abstract: The pattern of primary succession on the floodplain of the Tanana River in interior Alaska resulted largely from interactions between stochastic events and life history traits of the dom? inant species. Seed rain by willow {Salix alaxensis), alder {Alnus tenuifolia), poplar {Populus balsam- iferd), and spruce {Picea glauca) varied substantially among years but was highest in the successional stage dominated by that species. Some seeds of each species arrived in all stages, and seedlings of all species were initially present in early successional sites. The copious, wind-dispersed seeds and rapid seedling growth rates of willow and poplar resulted in their abundant establishment on early succes? sional vegetated-silt bars. Heavier alder and spruce seeds were less widely distributed, yet rapid growth rates of alder resulted in dense alder thickets within 20 yr of silt bar formation. We found no evidence of buried seed of the four study species. Sown seeds of willow, alder, and poplar established only in early successional vegetated-silt and willow sites. Spruce established in these same sites and in alder sites. Spruce was the only species that naturally colonized mid and late successional sites. Removal ofthe litter and forest floor enabled all species to germinate in all sites. Flooding resulted in substantial mortality of seedlings in early successional sites. A combination of short life span, herbivory by hares, and shade intolerance eliminated willow from mid-successional alder-dominated sites. Thereafter differences in longevity explained successional change from alder to poplar to spruce. Facilitative interactions among species did not appear essential to explain changes in species composition in this primary successional sequence.

Indirect Interactions Between Temporally Separated Herbivores Mediated by the Host Plant

Abstract: I tested the hypothesis that early season herbivory by leaf-chewing insects affects dis- tribution, densities, and survivorship of late-feeding, leaf-mining insects; early chewing may physically and chemically alter quality of leaves for later leaf miners on the shared host plant, Quercus emoryi. Proportions of intact leaves and leaves damaged by native leaf chewers on six control and six experimental trees were determined over two growing seasons. I manually damaged - 50% of leaves on experimental trees to increase the total fraction of damaged leaves to z 75%. Leaf-miner densities, distribution, survivorship, and mortality were monitored on control and experimental trees and in intact and damaged leaves within trees. Leaf miners occurred more frequently than expected by chance on intact than damaged leaves for both control and experimental trees. Leaf-miner densities did not differ between control and experi- mental trees. This result suggests ovipositing leaf miners did not discriminate between damaged and undamaged trees, but selected leaves within trees. More leaf miners survived in intact than damaged leaves in both growing seasons. Survivorship was less in damaged leaves because of significantly increased rates of parasitism on these leaves. However, death from other causes (including bacterial and fungal death) was significantly less for leaf miners on damaged leaves. This positive effect did not compensate for overall lower survivorship of leaf miners in damaged leaves due to increased parasitism. Survivorship and mortality did not differ between control and experimental trees when damaged and intact leaves were pooled within treatments. However, in 1982-1983, survival was lower and parasitism greater for leaf miners in control-damaged leaves than experimental-damaged leaves. Survival and parasitism were not different between control- intact and experimental-intact leaves. These results suggest that (1) the effect of early season herbivory on leaf miners was inversely density dependent and, (2) occurrence of leaf miners on intact leaves may be caused by avoidance of damaged leaves rather than preference for intact leaves. Early season herbivory caused localized changes in photochemistry within trees, and these chemical alterations were consistent with observed distributions and survivorship of leaf miners. Both early season herbivory and experimental damage resulted in higher levels of condensed tannins and lower protein content in damaged leaves within trees, but herbivory had no effect on between-tree chemistry differences. Leaf miner distributions corresponded to these localized chemical changes: leaf miners avoided damaged leaves within trees but showed no between-tree preferences. Parasitism of miners on damaged leaves was higher, possibly because parasitoids used physical and chemical changes as cues to locate leaf-miner hosts, or because exposure of leaf miners to para- sitoids was prolonged. However, when many leaves were damaged (experimental trees), the negative effect of parasites was mitigated because physical and chemical cues associated with damaged leaves were less effective. Lower leaf-miner mortality from other causes may be related to the bactericidal or fungicidal properties of increased tannins in damaged leaves. This study demonstrates that temporally separated guilds can interact subtly at low levels of herbivory through changes in the host plant. Current theories of within- and between-guild organization of phytophagous insects may need to be re-evaluated if such interactions are common.

Relation of Michigan's Presettlement Pine Forests to Substrate and Disturbance History

Abstract: Little is known of the factors that engendered the classic pine forests of Michigan in the period before settlement by Europeans. Some of the forests were relatively open savannas of jack pine and red pine. Other stands were composed of large red and white pines, occasionally with an admixture of hemlock and hardwoods. The early Government Land Office Survey records of Ros- common and Crawford Counties were used to determine the composition and the disturbance history of the pine forests of northern lower Michigan. The abundance of pine (mainly Pinus banksiana, P. resinosa, and P. strobus) in this area that was between lobes of the Wisconsin Laurentide ice sheet was correlated with the presence of coarse-textured soils derived from outwash and ice-contact deposits. These soils promoted a vegetation type which was extremely susceptible to fire. Records suggest the average return time for severe crown fires ranged from 80 yr in the case of the jack pine forest type to 120-240 yr for the mixed pine type to 1200 yr for the hemlock-white pine-northern hardwoods type. Blowdowns had a much longer return time and reached their greatest frequency in the swamp conifer and hemlock-white pine-northern hardwoods types. A three-dimensional diagram summarizes the role of fire, windthrow, substrate, and topography in the formation of Michigan's pine forests.

Vegetation Change and Plant Demography in Permanent Plots in the Sonoran Desert

Abstract: We report on vegetation changes and population dynamics during a 72-yr period within permanent plots in Sonoran Desert vegetation. A set of plots established in 1906 and supplemented in 1928 at Tumamoc Hill, near Tucson, Arizona, USA, have been mapped at irregular intervals through 1978. Data from the four 100-M2 and one 800-M2 plots censused most frequently (6-8 times since 1906 or 1928) are presented in detail; data from five less frequently censused plots are noted briefly. At each census all woody and succulent plants, including seedlings, were mapped by recording both stem coordinates and canopy outline. There have been no consistent, directional changes in vegetation composition in the Tumamoc Hill plots between 1906 and 1978, despite large fluctuations in absolute cover and density of most species. The relative cover of the dominants was generally similar within a given plot over the entire time sequence. Coverage of most species responded strongly to regimes of extremely wet or extremely dry years; the response of density to climatic extremes was somewhat less strong. Total cover, density, and species diversity have increased more or less contin- uously in many plots between 1906 and 1978. The species were divided into five groups based on several population dynamic traits. At one end of a continuum are species with long maximum life- span (often greater than the 72-yr study period), high early and long-term (50-yr) survivorship, large size at maturity, low density, erratic recruitment, and age structure skewed toward older plants. At the other end of the continuum are species with short maximum life-span, low early and long-term survivorship, small size at maturity, high density, some recruitment at most censuses, and age structures strongly skewed toward newest recruits. While some of these traits are necessarily related (e.g., long maximum life-span and high long-term survivorship), others are not (e.g., high early survivorship and erratic recruitment), and the relationships deserve further study.

Variable Functional Responses of a Marine Predator in Dissimilar Homogeneous Microhabitats

Abstract: Adult soft-shelled clams {Mya arenaria) persist at low densities in Chesapeake Bay sandy habitats despite intense predation by blue crabs {Callinectes sapidus). Clam persistence may be a consequence of variation in blue crab foraging rates as a function of clam density and sediment composition. In laboratory aquaria, we measured the functional responses (prey consumption per predator as a function of prey density) of large blue crabs to six densities of adult soft-shelled clams buried at natural depths in two sediment types (mud and sand). Functional responses in sand and mud were differentiated statistically by analyses of (1) residuals and residual sums of squares of discrete and continuous-time models, and (2) the exponent (3 of a general functional response model. Crab predation rates were significantly higher in mud than sand, and functional responses differed significantly between these two substrates. Blue crabs displayed type III (sigmoid) density-dependent functional responses in sand, and type II (decelerating rise to an upper asymptote) inversely density- dependent responses in mud. Risk of mortality for clams decreased sharply in sand at low densities similar to those observed in the field near the end of the annual period of active predation. These observations (1) suggest that variable blue crab functional responses result in microhabitat-specific mortality rates of benthic prey, and (2) indicate that functional responses can differ significantly according to the physical properties of topographically simple habitats.

Stable‐Carbon Isotope Variability in Tree Foliage and Wood

Abstract: This study documents variation of stable-carbon isotope ratios (13C/'2C) in trees of genera Juniperus and Pinus under field conditions. Results are from cellulose analysis on leaves, twigs, and wood from a number of localities in the southwestern United States. Substantial variability, typically 1-3%oc, exists among leaves, within wood (radially, vertically, circumferentially), and between individuals at a site. These results may help guide sampling in tracer-type studies with stable-carbon isotope ratios and aid in the interpretation of isotopic results from such studies.

Wind Effects on Needles of Timberline Conifers: Seasonal Influence on Mortality

Abstract: In the Rocky Mountains, USA, year—round wind exposure has been correlated with widespread needle dehydration and death in winter for alpine timberline conifers. Needle death may be due solely to the drying effects of winter wind or may also result from predisposition to winter injury associated with suboptimal summer growth conditions. Our experiments showed that in the lower timerbline ecotone in southwest Wyoming, needle mortality was primarily due to winter wind and cuticle abrasion; death was frequent only in wind—exposed needles of flagged trees and surface needles of krummholz mats. At higher elevations where only krummholz mats and a few flagged trees exist, mortality averaged ≥75% for needles unprotected by snow, regardless of wind exposure. Snow covered needles had low mortality throughout the timberline ecotone. Winter death of naturally wind—exposed needles of Picea engelmannii occurred at ≤—4.5 MPa water potential (≤60% relative water content). Cuticular resistance of wind—exposed needles declined from 100—250 ks/m in autumn to ≤30 ks/m by midwinter. Experimentally reversing the windward—leeward orientation of small, flagged trees in early winter resulted in lower xylem pressure potentials and needle viabilities for newly wind—exposed (originally leeward) compared to newly sheltered (originally windward) shoots. Also, sheltering exposed branches from winter wind on flagged trees in the lower timberline ecotone (3200 m) increased mean overwinter needle survival from near 0 to ≥50% in both Picea engelmannii and Abies lasiocarpa. Scanning electron micrographs of dehydrate wind—exposed needles collected in March at this site showed little cuticular surface wax, probably because of windborne ice crystal abrasion. However, winter dehydration and death in both experimentally and naturally wind—sheltered needles at higher elevation may have been due to inadequate needle maturation during summer, which could act to exclude flagged trees from the upper timberline ecotone.

Sulfur and Carbon Isotopes as Tracers of Salt‐Marsh Organic Matter Flow

Abstract: Stable isotopes of sulfur and carbon were used to trace the dominant flows of organic matter from producers to macroconsumers in Great Sippewissett Salt Marsh on Cape Cod. Spartina alterniflora and sulfur-oxidizing bacteria were found to assimilate isotopically light sulfides produced via sulfate reduction, and this light sulfur was detected in consumers. In contrast, phytoplankton and upland plants assimilate isotopically heavier SO42- with little or no fractionation. A dual-isotope approach using both 613C and 634S showed that Ilyanassa obsoleta and Fundulus heteroclitus depend very heavily on Spartina detritus, while filter feeders such as Crassostrea virginica and Geukensia demissa depend on a mixture of plankton and Spartina detritus. Spartina detritus and plankton were both shown to be much more important as organic matter sources for marsh macroconsumers than either sulfur-oxidizing bacteria or organic matter derived from terrestrial inputs.

Resource Regulation by a Stem‐Galling Sawfly on the Arroyo Willow

Abstract: We studied the impact of the stem—galling tenthredinid sawfly, Euura lasiolepis, on the growth and branch age structure of the arroyo willow, Salix lasiolepis. Normally, as willows age they become less susceptible to galling, but heavy Euura galling maintains willows at a young, relatively susceptible juvenile stage. The Euura form more galls and long shoots. Long shoots are found on clones with young branches. Heavy galling stunts or kills growth distal to the gall, stimulating sprouting by indefinitely dormant buds located near branch bases. The resulting young branches keep the clone susceptible to further galling. In contrast, increasing branch age of lightly galled clones confers resistance to galling. We term this model of gall—induced resource maintenance the resource regulation hypothesis. Resource regulation is the maintenance or increase of high—quality resources by an herbivore species that impacts immediately subsequent generations of the same herbivore species on the same plant. See full-text...