Showing papers in "Ecological Monographs in 1993"

Quantitative effects of grazing on vegetation and soils over a global range of environments

Abstract: Multiple regression analyses were performed on a worldwide 236—site data set compiled from studies that compared species composition, aboveground net primary production (ANPP), root biomass, and soil nutrients of grazed vs. protected, ungrazed sites. The objective was to quantitatively assess factors relating to differential sensitivities of ecosystems to grazing by large herbivores. A key question in this assessment was: Do empirically based, broad—scale relationships correspond to ecological theories of plant—animal interactions and conceptual frameworks for management of the world's grazing lands? Changes in species composition with grazing were primarily a function of ANPP and the evolutionary history of grazing of the site, with level of consumption third in importance. Changes in species composition increased with increasing productivity and with longer, more intense evolutionary histories of grazing. These three variables explained >50% of the variance in the species response of grasslands or grasslands—plus—shrublands to grazing, even though methods of measurement and grazing systems varied among studies. Years of protection from grazing was a significant variable only in the model for shrublands. Similar variables entered models of change in the dominant species with grazing. As with species composition, sensitivities of change in dominant species were greater to varying ecosystem—environmental variables than to varying grazing variables, from low to high values. Increase of the dominant species under grazing were predicted under some conditions, and decreases were more likely among bunch grasses than other life—forms and more likely among perennials than annuals. The response of shrublands was different from that of grasslands, both in terms of species composition and the dominant species. Our analyses support the perception of grazing as a factor in the conversion of grasslands to less desirable shrublands, but also suggest that we may be inadvertently grazing shrublands more intensively than grasslands. Percentage differences in ANPP between grazed and ungrazed sites decreased with increasingly long evolutionary histories of grazing and increased with increasing ANPP, levels of consumption, or years of treatment. Although most effects of grazing on ANPP were negative, some were not, and the statistical models predicted increases in ANPP with grazing under conditions of long evolutionary history, low consumption, few years of treatment, and low ANPP for grasslands—plus—shrublands. The data and the models support the controversial hypothesis that grazing can increase ANPP in some situations. Similar to species variables, percentage differences in ANPP between grazed and ungrazed treatments were more sensitive to varying ecosystem—environmental variables than to varying grazing variables. Within levels not considered to be abusive "overgrazing," the geographical location where grazing occurs may be more important than how many animals are grazed or how intensively an area is grazed. Counter to the commonly held view that grazing negatively impacts root systems, there was no relationship between difference in ANPP with grazing and difference in root mass; as many positive as negative differences occurred, even though most ANPP differences were negative. Further, there was a weak relationship between change in species composition and change in ANPP, and no relationship with root mass, soil organic matter, or soil nitrogen. All three belowground variables displayed both positive and negative values in response to grazing. Current management of much of the world's grazing lands based on species composition criteria may lead to erroneous conclusions concerning the long—term ability of a system to sustain productivity.

The Color of Light in Forests and Its Implications

Abstract: Forests exhibit much variation in light environments, and this can affect communication among animals, communication between animals and plants, photosynthesis, and plant morphogenesis. Light environments are caused by, and can be predicted from, the geometry of the light paths, the weather conditions, and the time of day. The structure of forests leads to four major light habitats when the sun is not blocked by clouds: forest shade, woodland shade, small gaps, and large gaps. These are characterized by yellow—green, blue—gray, reddish, and "white" ambient light spectra, respectively. When the sun is blocked by clouds, the spectra of these four habitats converge on that of large gaps and open areas, so the single light environment during cloudy weather will be called open/cloudy. An additional light environment (early/late) is associated with low sun angles (near dawn or dusk); it is purplish. Each light environment is well defined and was found in forests of Trinidad, Panama, Costa Rica, Australia, California, and Florida. Scattered literature references suggest similar patterns elsewhere in North America, Europe, and Java. Perceived colors of animals, flowers, and fruits depend upon the interaction between ambient light color and the reflectance color of the animal or plant parts. As a result, an animal or plant may have a different appearance in each environment, i.e., a color pattern may be relatively cryptic in some light environments while relatively conspicuous in others. This has strong implications for the joint evolution of visual signals and vision, as well as microhabitat choice. Plant growth and form may also be affected by variation in the color of forest light.

Predation, Prey Refuges, and the Structure of Coral-Reef Fish Assemblages

Abstract: We studied the influence of piscivorous fishes and prey refuges on assemblages of fishes occupying 52 model reefs in a large seagrass bed off St. Thomas, U.S. Virgin Islands. We conducted three experiments: two involving 6 reefs each, lasting 2 and 5 yr, and one involving 40 reefs, lasting 1 yr. Each experiment included replicate reefs in various combinations of five structural treatments: holeless controls, 12 and 24 small holes, and 12 and 24 large holes. Tagging studies indicated that the reefs were sufficiently isolated from each other to comprise statistically independent replicates, and that resident piscivores occupied home reefs. We observed 97 species on or near the reefs, representing all major foraging guilds, and each holed reef supported hundreds of individuals. We examined four categories of fish: (1) large reef associates (too large for the small holes; most of these fish were both predators on smaller fish and prey for larger transient piscivores), (2) moray eels (piscivores that could fit into the small holes), (3) small reef associates (potential prey that could fit into the small holes), and (4) juvenile grunts (potential prey that sporadically were extremely abundant). We tested five a priori predictions of the general hypothesis that predation is an important process structuring reef-fish assemblages. The first two predictions dealt with the role of prey refuges. First, if reef holes function as prey refuges, then prey fish should be most abundant on reefs providing holes near their body diameters, because such holes would make the prey fish safest from predation. Seven of eight experimental comparisons supported this prediction, and five of them were statistically significant. Second, if refuge availability limits prey abundance, then prey fish should be more abundant on reefs with 12 holes than those with no holes, and should be more abundant on reefs with 24 holes than those with 12 holes. The first part of this prediction was verified by all nine experimental comparisons, seven of which were statistically significant. However, there were no strong differences between 12-hole and 24-hole reefs. Thus, between 0 and 12 holes per reef, holes limited local prey populations; between 12 and 24 holes per reef, the number of holes was not limiting. Several lines of evidence suggested that the latter pattern was due to temporary saturation of the study area with refuges when we added 40 reefs to 12 existing reefs. The remaining three predictions dealt directly with the community-level role of predation. First, predators should affect local prey abundance either chronically, in which case a negative relationship among reefs is predicted between the average abundances of predators and prey, or sporadically, in which case a negative relationship is predicted between the abundance of predators and the maximum number of co-occurring prey ever observed at each predator abundance. The former prediction was falsified, whereas the latter was verified. Observations of extreme type III survivorship of recruit cohorts on reefs with many piscivores and occasional direct observations of piscivory bolstered the conclusion that this relationship was causal. Finally, we predicted that predators should affect the number of prey species on a reef. We observed a significant negative relationship among reefs between predator abundance and maximum prey-species richness. Comparing species' relative abundances on reefs at the extremes of this regression, piscivores appear to have nonselectively reduced and extirpated both common and rare prey species, although this relationship remains purely correlative. In our model system, high local species diversity appears to have been maintained despite rather than because of predation. We propose a conceptual model where the local abundances of coral-reef fishes are determined by the relative magnitudes of recruitment by larvae, colonization by juveniles and adults, predation, and competition for refuges, each of which varies through time and space. Multifactorial field experiments will be necessary to test such pluralistic hypotheses.

Competition Among Grasses Along a Nitrogen Gradient: Initial Conditions and Mechanisms of Competition

Abstract: We grew four perennial grass species (Poapratensis, Agropyron repens, Agros- tis scabra, and Schizachyrium scoparium) for 5 yr in monocultures and in pairwise com- petition plots on an experimental nitrogen gradient. The gradient contained plots ranging from 100% sand to 100% black soil, plus plots that received additional N fertilizer. To examine the impact of initial conditions on the long-term outcome of interspecific com- petition, three competitive situations were created: seed vs. seed competition (both species planted simultaneously), seed invasions (each species added as seed to year-old monocul- tures of the other), and vegetative invasions (dividers separating adjacent monocultures of two species removed after 1 yr). Extractable soil NO3 and NH4, were measured to test if species differences in the concentration of available soil N in monoculture (i.e., R* for N, Tilman 1982) could predict the long-term outcome of competition. By year 5, Schizachyrium displaced or greatly reduced the biomass of both Poa and Agropyron on the soil mixture gradient (the mixed soils but not the added-N plots) inde- pendent of the wide range of starting conditions. On these soils, Schizachyrium monocul- tures had significantly lower soil concentrations of both NO3 and NH4, than either Poa or Agropyron monocultures. Similarly, Agropyron displaced or greatly reduced the biomass of Agrostis by year 5. Agropyron monocultures had significantly lower concentrations of N03- and NO3 + NH4+, but not NH4+, than Agrostis monocultures. In contrast, no competitive displacement occurred in competition between Poa and Agropyron, and initial differences persisted over 5 yr. Monocultures of these two species did not differ in NO3 concentration, but did differ for NH4+ and NO3 + NH4+. Thus, species differences in ability to deplete soil NO3 successfully predicted the outcome of competition for all four species pairs on the soil mixture gradient. If resource preemption or asymmetric compe- tition had been the mechanism of competition, initial conditions would have affected the long-term outcome of competition. Rather, these results support the R* (i.e., resource reduction) model for competition for soil N. In the added-N fertilizer plots, Schizachyrium had decreased biomass in competition with both Poa and Agropyron. However, neither Agropyron nor Poa appeared to have an advantage when they competed with each other in the added-N plots. For these three species pairs, the 5-yr results of competition in the added-N plots, which had greatly reduced light availability because of increased production and litter accumulation, depended on initial conditions. In the fourth pair, Agrostis was displaced by Agropyron in all competition treatments in the added-N plots. Thus, we cannot reject the hypothesis that resource preemption (i.e., asymmetric competition) is important in light competition.

Multicentury, regional-scale patterns of western spruce budworm outbreaks.

Abstract: Tree ring chronologies from 24 mixed—conifer stands were used to reconstruct the long—term history of western spruce budworm (Choristoneura occidentalis) in northern New Mexico. Temporal and spatial patterns of budworm infestations (within—stand occurrences) and outbreaks (more—or—less synchronous infestations across many stands) were investigated to identify local—scale to regional—scale forest disturbance patterns. Nine regional—scale outbreaks were identified from 1690 to 1989. One ancient stand of Douglas—fir trees (Pseudotsuga menziesii) exceeding 700 yr in age revealed that budworms and overstory trees can coexist for extraordinary lengths of time. Using spectral analysis we found that the regional outbreak record contained important cyclical components with periods varying from ≈20 to 33 yr. The statistically significant (P < .05) but variable periodicity of regional outbreaks suggests the forest—budworm dynamic is pseudoperiodic (i.e., a stable limit cycle or damped oscillator perturbed by noise). Duration of infestations within stands was ≈11 yr and has not obviously changed in the 20th century; however, infestations tended to be more synchronous among stands in this century than during earlier centuries. Regional budworm activity was low from the mid—1920s to late 1930s and mid—1960s to late 1970s, and the most recent outbreak, beginning in the late 1970s, was unusually severe. These results, and contrasting infestation patterns in mountain ranges with different land use histories, generally support a hypothesis that human—induced changes in Southwestern forests have led to more widespread and intense budworm outbreaks in the late 20th century. Despite human—induced changes in the 20th century, climate variation also appears to have been important to budworm regimes in this century as well as in earlier times. Regional outbreaks in the 20th century tended to occur during years of increased spring precipitation, and decreased budworm activity coincided with decreased spring precipitation. No clear association with temperature was identified. Comparisons of regional outbreak history since AD 1600 with a reconstruction of spring precipitation from limber pine (Pinus flexilis) ring width chronologies also shows that periods of increased and decreased budworm activity coincided with wetter and drier periods, respectively. This finding contrasts with results from shorter time—scale studies conducted in northwestern U.S. and Canada (western spruce budworm) and eastern Canada (spruce budworm C. fumiferana), where low precipitation and/or warmer temperatures were generally associated with outbreaks. Different patterns of budworm population response to changing moisture regimes might be due to differences in regional forest—budworm systems, or to differences in the spatial and temporal scales of observation. We conclude that changes in forest structure in the southwestern U.S. may have shifted the spatial and temporal pattern of budworm outbreaks. The dynamic behavior and statistically significant association between multicentury, regional budworm and climate time series also suggest that complex budworm dynamics are driven by a combination of internal and external factors.

Sociometry and Sociogenesis of Colonies of the Fire Ant Solenopsis Invicta During One Annual Cycle

Abstract: In social insects, colonies as well as individuals have evolving life histories. Identification of the life history tactics of a social insect requires data on colony attributes and their development. To this end a full range of fire ant (Solenopsis invicta) colony sizes was sampled and censused on seven dates throughout 1 yr. Data included: mound volume; the number, dry masses, and fat contents of sexual and worker adults and immatures; stratified nest temperatures; worker distribution within the nest throughout the year; duration of the pupal stages; and respiration rates. Analysis showed: 1. Colonies reached their annual maximum population size in midwinter and their maximum biomass in spring. During the spring sexual production period they declined to a midsummer minimum. Calculations showed that the magnitude of this decline increased with colony size. During January to July, worker mortality exceeded natality, causing colony decline, while from July to December, natality predominated, causing growth. 2. Mound volume was closely related to the total mass of ants in the colony, and varied with season paralleling the mass of ants. 3. The mean size and variability of workers, and the percent major workers, increased with colony size and changed over the year. 4. The fat content (percent fat) of workers increased with worker size and colony size. Worker percent fat was lowest in summer after sexual production, climbed immediately to the annual maximum and then declined gradually through winter and spring. 5. Although sexual male and female pupae were close in mean dry mass (2.55 mg and 3.10 mg, respectively), males gained only 6% during adult maturation while females gained 290%. Females gained fat more rapidly than lean tissue causing their percent fat to increase from 31% to 49%. Mean mass of male and female sexual adults did not change with colony size. 6. The cost of worker maintenance declined from nearly 100% of total colony cost in winter to 46% in late spring when brood production peaked. 7. Production rates peaked in spring, with colonies investing 50% of their daily production in sexuals. This peak production was not sustained through the summer, and was probably fueled by stored worker fat. Worker production dominated in the latter part of the summer. All measures of production rate as well as total annual production increased with colony size, but most did so less rapidly than colony size, resulting in a declining efficiency of production and a declining natality rate. 8. The percent of annual production invested in sexuals increased sharply in colonies of between 20 000 and 50 000 workers, then remained at ≈33% for the remainder of colony growth, showing that the transition from the ergonomic to the reproductive stages is sharp, and that colonies must grow in order to produce more sexuals. 9. Many quantitative colony attributes were related to one another by differential growth, and can thus be seen as isometric or allometric measures. Rules of relative growth may thus constrain the possible combinations of attributes and their evolution. The methods of morphometric size and shape analysis are discussed as tools for understanding suites of colony attributes, and comparing them among species. 10. The sociometric/sociogenic method is discussed as a way to compile, analyze and compare data on social insect colony attributes and their growth and development.

Selection on floral morphology and environmental determinants of fecundity in a hawk moth-pollinated violet'

Abstract: This paper presents the results of a 5—yr field study on the determinants of individual variation in maternal fecundity (seed production) in the narrowly endemic violet Viola cazorlensis (Violaceae), at a southeastern Spanish locality. Flowers of this species are characterized by a very long, thin spur and broad morphological variability, and are pollinated by a single species of day—flying hawk moth (Macroglossum stellatarum; Lepidoptera, Sphingidae). The primary aim of this investigation was to answer the question, What are the relative importances, as explanations of individual differences in fecundity, of variability in floral traits and of other fecundity determinants that are of an extrinsic nature, such as microhabitat type and interactions with herbivores? The floral morphology of individual V. cazorlensis plants was characterized by means of both "conventional," linear measurements of the size of flower parts (petals, spur, peduncle), and shape analysis of corolla outline (using thin—plate splines relative warps analysis). Spatial (among substrate types) and temporal (among years) patterns of variation in flower, fruit, and seed production by V. cazorlensis plants are described, with particular emphasis on the comparative effects of floral morphology, herbivory (by mammalian ungulates and two species of lepidopteran larvae), and substrate type (rock cliffs, bare rocks at ground level, and sandy soils), on cumulative seed production at the individual plant level. Cumulative seed production of individual V. cazorlensis plants depended significantly on average floral morphology (both size and shape components), thus revealing the existence of phenotypic selection on the floral morphology of this species at the study population. Among all the floral traits examined, spur length was the only one for which no significant relationship with fecundity was found. Type of substrate largely determined differences between V. cazorlensis plants in the impact of herbivory (plants growing on the soil exhibited the greatest reproductive losses to herbivores), and it also influenced plant size and flower production per reproductive episode. Plant size, in turn, influenced the supra—annual frequency of flowering and the number of flowers produced in each reproductive event. Flower production and herbivory levels significantly influenced (positively and negatively, respectively) fruit number, which was the major direct determinant of seed production. Path analysis revealed that the main determinants of individual variation in cumulative seed production over the study period were, in decreasing order of importance (absolute value of "effect coefficient" in parentheses), cumulative fruit production (0.946), mean flower production per reproductive event (0.868), plant size (0.441), herbivory by ungulates (—0.221), and average score on the first relative warp (0.107), a descriptor of flower shape. After accounting for the effects of substrate type, herbivory, plant size, and flower and fruit production, individual variation in floral morphology (aspects of size and shape) explained a negligible proportion (2.1%) of total individual variation in cumulative fruit production. Phenotypic selection on the floral morphology of V. cazorlensis at the study population, although statistically significant, was therefore almost inconsequential as a source of individual variation in maternal fitness, its effects being heavily "dilute" by the overwhelming influence of other factors. As exemplified by this study, selection on the floral phenotype may often become largely irrelevant in evolutionary terms because other ecological factors are far more important determinants of fitness differences among plants. A realistic assessment of the potential relevance of selection on plant reproductive traits thus requires a quantitative evaluation, in its natural scenario, of the predictable consequences of such selection.

Postglacial Vegetation and Climate of Grand Teton and Southern Yellowstone National Parks

Abstract: Pollen records from northern Grand Teton National Park, the Pinyon Peak Highlands, and southern Yellowstone National Park were examined to study the pattern of reforestation and climatic change following late-Pinedale Glaciation. The vegetational reconstruction was aided by analyses of associated plant macrofossils and the modern pollen rain of the region. Radiocarbon-age determinations and tephrochronology provided a chronologic framework to help correlate pollen records among sites. The fossil records indicate that alpine meadow communities, with Betula and Juniperus, were present between - 14 000 and - 1 1 500 yr BP. This early assemblage implies a lowering of modern upper treeline by at least 600 m and a climate that was 5-60C colder than present. Between 11 500 and 10 500 yr BP, increased temperature and winter precipitation allowed first Picea, and then Abies and Pinus cf. albicaulis to spread into areas underlain by andesite and nonvolcanic bedrock. By 10 500 yr BP, the fossil record in these areas resembled modern spectra from subalpine forest. In contrast, the Central Plateau of Yel- lowstone, which is underlain by infertile rhyolitic soils, was treeless prior to - 10 000 yr BP. The absence of late-glacial subalpine parkland in this area is attributed to the same edaphic factors that limit Picea, Abies, and Pinus albicaulis from the rhyolite plateau today. Between 10 000 and 9500 yr BP, Pinus contorta forest developed throughout the region in response to further warming. Pseudotsuga and Populus were present between 9500 and 5000 yr BP, suggesting a warmer, drier climate than today and more frequent fires. In the last 5000 yr BP mixed forests of Picea, Pinus, and Abies have developed on andesitic and nonvolcanic terrain, and closed forests of Pinus contorta have persisted on rhyolitic sub- strates. The vegetational patterns are attributed to a combination of climatic and nonclimatic controls operating on different spatial and temporal scales. Climatic changes brought about by the retreat of Laurentide ice and the amplification of the seasonal cycle of radiation explain the broad patterns of vegetational change on millennial time scales. On shorter time scales and smaller spatial scales, substrate differences and migration history shaped the vegetational variability.

Adaptation to Single Resources and the Evolution of Crossbill (Loxia) Diversity

Abstract: I quantitatively test the hypothesis that four taxa or "types" (species or subspecies) of Red Crossbills (Loxia curvirostra) in the Pacific Northwest have diversified morphologically in bill characters in response to alternative adaptive peaks presented by their food: seeds in conifer cones. Hypothetically, each adaptive peak corresponds to one conifer species whose seeds are (1) produced regularly from year to year, (2) held in cones through late winter when seed is most limiting, and (3) protected from depletion by potential noncrossbill competitors. Four such conifers, termed "key conifers," are present (Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Pinus contorta var. latifolia). I use data on foraging efficiency for 31 captive crossbills of four types to determine the optimal bill size and palate structure for foraging on the key conifers. As predicted, if each type is adapted for foraging on a key conifer, the observed morphology of a given type is often the predicted optimal size morphology of foraging on its respective key conifer. Two of the types have mean bill sizes (bill depth) equalling their predicted optimal size. For one of the remaining types, the observed differs from the optimum by 0.4 mm; I was unable to predict an optimal size for the remaining type. Optimal bill size varies with season. Bill sizes corresponded more closely to the optima for winter (lean period) than for summer. Observed mean width of the palate groove, in which crossbills hold conifer seeds while the seeds are being husked, was consistently close to the estimated optimal groove width. Optimal groove width was correlated (r2 = 1.00, n = 4) with seed size (cube root of mass), suggesting optimal groove width is determined by seed size. Overall, each crossbill type has either the optimal bill size or optimal husking groove width, or both, for foraging on their key conifers. Fitness set analyses indicate that there are substantial trade—offs in foraging efficiency. The best phenotype for foraging on one conifer is often only one—half as efficient on other conifers. All four fitness sets are concave, implying selection against intermediate phenotypes. I conclude, first, that reliability of seeds on key conifers during periods of food scarcity is a critical feature in the ecology and evolution of crossbills. Second, optimization of morphological traits occurs even in populations in highly variable environments. Third, disruptive selection against intermediate phenotypes is likely. This should maintain, if not reinforce, the distinctiveness of types. Fourth, the diversity of cone structure and seed size among key conifers is ultimately responsible for the diversification of crossbills.

Disturbance, competition, and herbivory effects on ragwort Senecio jacobaea populations

Abstract: The balance of forces determining the successful control of ragwort Senecio jacobaea by introduced insects was investigated in a field experiment by manipulating the time of disturbance, the level of interspecific plant competition, and the level of herbivory by the cinnabar moth Tyria jacobaeae and the ragwort flea beetle Longitarsus jacobaeae. We used a factorial design containing 0.25—m2 plots arranged as 4 Blocks × 2 Disturbance Time (plots were tilled in Fall 1986 or Spring 1987) × 3 Plant Competition levels (vegetation other than ragwort was Removed, Clipped, or Unaltered) × 2 Cinnabar Moth levels (Exposed, Protected) × 2 Flea Beetle levels (Exposed, protected). The response of ragwort was measured as colonization, survivorship, and reproduction of the first ragwort generation, establishment of juveniles in the second generation, and changes in ragwort biomass from 1987 through 1990. We also made annual measurements from 1987 through 1990 of the allocation of space (the limiting resource in the Unaltered competition treatment) among the categories ragwort, other species, litter, and open space. Natural enemy responses were characterized by relating variation in the concentration of enemies and the concentration of ragwort among patches. We found that abundant buried seed and localized disturbances combined to activate incipient ragwort outbreaks, and that interspecific plant competition and herbivory by the ragwort flea beetle combined to inhibit the increase and spread of incipient outbreaks. Time of disturbance had little effect on the outcome of biological control. Under conditions in the Removed and Clipped treatments (where there was sufficient open space for germination and establishment), reduction in seed production in the first generation caused by cinnabar moth larvae led to a reduction in plant numbers in the second generation, but caused only a weak effect on ragwort cover and no detectable effect on ragwort biomass over the longer term from 1986 through 1990. At the spatial scale examined, inhibition by the ragwort flea beetle and plant competition took the extreme form of elimination of all ragwort individuals except the pool of seed buried in the soil. Our findings lead us to (1) reject the view that successful biological control leads to a stable pest—enemy equilibrium on a local spatial scale, (2) strongly endorse "search and destroy" and weakly endorse "complementary enemies" strategies suggested by Murdoch et al. (1985) as ways to improve control, and (3) emphasize resource limitation in the pest at low density as a key feature distinguishing biological control of weeds from biological control of insects.

A Model of Competition Incorporating Plasticity through Modular Foliage and Crown Development

Abstract: The model of competition for light presented here uses modular autonomy to incorporate plasticity in plant growth under competition. Once plants are characterized as composed of modules, then model structure for competition changes in a fundamental way. Interactions between the plant module and its local resource environment must be modeled rather than the traditionally viewed interactions between whole plants and their neighbors. We assume that a plant module interacts with its local resource environment regardless of whether this environment was altered by a neighbor or by the same plant. Two spatial processes are considered: resource acquisition and growth. The spatial pattern of resource acquisition by a module determines a growth and allocation pattern, e.g., the elongation of branches into a gap. The spatial structure of a module and its connection to the whole tree then determines the pattern of resource distribution and resource acquisition of the next time step.Plasticity of plant growth is incorporated by variation in both the efficiency of resource capture of modules and patterns of resource allocation for individuals of different canopy positions and results in individuals in the community having different spatial structures. The model simulates the three—dimensional development of tree crown structure over time. It is applied to the 30—yr development of a dense, spatially aggregated stand of Abies amabilis beginning with an initial pattern of seedlings. The importance of incorporation of plasticity is apparent when the model output is compared to observed height distribution and crown structure data. Simulations indicate that asymmetrical crown development, one form of plasticity, is advantageous to stand productivity and becomes more advantageous as the degree of spatial aggregation in the initial spacing of trees increases.

Interspecific antagonism and species coexistence in a diverse guild of larval trematode parasites

Abstract: The salt marsh snail Cerithidea californica is first intermediate host to a diverse guild of larval trematode parasites. In Bolinas Lagoon, in central California, the site of this study, at least 15 species of trematodes infect snail populations. This study investigated patterns of interspecific association and interaction among members of this parasite guild. Seven to 19 host subpopulations were sampled annually at each of two sites in the lagoon from 1981 to 1988. Mixed—species infections constituted only 2.5% of the 5025 infections examined in the study. A Monte Carlo simulation procedure demonstrated that the numbers of such infections were often less than would be expected by chance, especially when the overall prevalence of infection was high. Patterns of association between particular pairs of species depended on whether the species' life histories include redial or only sporocyst larvae. Species that develop as rediae were predominantly negatively associated with other redial species and with most species that develop only as sporocysts. There was weak evidence of positive interspecific association between a few redial and sporocyst—only species, while members of other such pairs were distributed independently. Associations between sporocyst—only species were either weakly positive or neutral. Snails carrying known infections were marked, released, and recaptured at both study sites. During their exposure in the field, some initial infections were invaded by another parasite species that often excluded the first parasite. The vulnerability of a parasite species to invasion and replacement by another differed among the tested species. Infections of the largest redial species, Parorchis acanthus, were especially resistant to replacement, while those of the smallest redial species, Euhaplorchis californiensis, were the most frequently excluded. Four other species were invaded or replaced at intermediate rates. The two largest redial species, P. acanthus and Himasthla rhigedana, were responsible for >90% of the invasions or exclusions. Direct observations showed that the rediae of these species prey on the larval stages of other species, as do the rediae of Echinoparyphium sp. This direct form of interspecific antagonism is probably the primary mechanism by which such species exclude others from host snails, as has been widely demonstrated in similar freshwater snail—trematode systems. While hierarchical, negative interactions prevent the coexistence of species at the level of the individual host, the mark—recapture study showed that rates of exclusion are low for most subordinate species, with the exception of Euhaplorchis californiensis. At the level of the host subpopulation, the assemblage of larval trematodes is diverse, and its composition is temporally and spatially variable. There is no trend toward dominance of the assemblage by large redial species as the level of infection rises within aging cohorts or subpopulations of hosts. These patterns of guild structure within host cohorts and subpopulations are consistent with the hypothesis that recruitment processes rather than interspecific interactions primarily determine the composition and relative abundance of species at this regional level. Several characteristics of snail—trematode systems that may promote regional coexistence of such a large number of potentially interacting parasite species are the isolated and subdivided nature of the host resource, the aggregated distributions of larval stages, and the differential exploitation of different—sized hosts. Many features of this system are consistent with Price's (1980) non—equilibrial view of parasite communities.

Factors Controlling Plant Distributions: Drought, Competition, and Fire in Montane Pines in Arizona

Abstract: Recent models suggest that a trade—off in plants between tolerance of water limitation vs. tolerance of light limitation results in changes in dominant species over productivity gradients of increasing soil moisture and decreasing forest—floor light. With increasing elevation (1568—2296 m) in the Chiricahua Mountains in southeastern Arizona, soil moisture and plant cover increased and, as a result, mean forest—floor light levels decreased, in accordance with the models. The light—moisture trade—off hypothesis predicts that, over this gradient, (1) shade tolerance and drought resistance should be negatively correlated, (2) decreasing light and lack of shade tolerance (i.e., tolerance of light competition) should control upper elevational limits of species distributions, and (3) low soil moisture availability and lack of drought resistance should control lower elevational limits. With increasing elevation, however, fire frequency and litter depth also increased and soil temperature decreased. I tested the trade—off hypothesis and the role of these additional factors in controlling upper elevational limits of three pine species distributed along this gradient. Consistent with the trade—off hypothesis, results suggested that water stress controlled lower elevational limits of all three species. Seeds of each species germinated with the summer rains in experimental plots below their respective lower elevational limits, but all seedlings died by the end of the following May—June drought, apparently from water stress. In contrast, seedlings were still alive in experimental plots within each species' range after 2 yr. Furthermore, with decreasing elevation, seedlings of the three species increasingly occurred in microsites with relatively low light, low soil temperature, and deep litter, all reflecting high soil moisture compared to random microsites. From the middle to the lower portion of each species' range, recruitment, seedling survival, and seedling abundance decreased but height growth increased. Thus, dry season water stress appeared to control lower elevational limits by causing high mortality of young seedlings, rather than by curtailing seed germination or the performance of older seedlings. Inconsistent with the trade—off hypothesis, upper elevational limits were not controlled uniformly across species by light limitation. In Pinus leiophylla, the middle elevation species, low light and deep litter appeared to control the upper elevational limits. In a field experiment, P. leiophylla emergence and survival were significantly lower above its upper elevational limit than in plots within its range, removal of litter increased emergence, and removal of canopy increased seedling survival. In a greenhouse experiment, P. leiophylla was significantly less shade tolerant than higher elevation pine species. In contrast, P. discolor, the low elevation species, low light, deep litter, and low soil temperature appeared not to influence distribution. Emergence and survival were actually higher at high than middle elevations in the field experiment. Litter removal and canopy removal did not increase P. discolor emergence and survival, respectively, even at high elevation. In the highest elevation plots, P. discolor seedlings occurred in microsites slightly lower in light, higher in litter depth, and equivalent in soil temperature to random microsites, contrary to expectations if these variables were limiting. Finally, in greenhouse experiments, P. discolor was more shade tolerant than higher elevation species, including P. leiophylla. Two tests supported the hypothesis that the upper elevational limits of P. discolor were controlled by the high fire frequency found at higher elevation. First, P. discolor exhibited slow juvenile growth rates, thin bark, and other traits suggesting a lack of fire resistance compared with the two higher elevation pine species. Second, in two wild fires, survival of P. discolor stems was significantly lower than that for the other two species. This conclusion is corroborated by the observation that juvenile P. discolor occurred commonly at much higher elevations than did adults, into plots with very low light and soil temperature levels and very deep litter, a pattern likely resulting from fire suppression. Results for a third species, P. engelmannii, were equivocal, showing weak support for control of upper elevational limits by light. The lack of a light—soil moisture trade—off in these species may result from P. discolor's strategy of exploiting nurse tree sites at low elevation and the apparent fire—associated regeneration of the other two species. Nevertheless, control of P. discolor upper elevational limits by fire may, in part, be a result of constraints imposed by drought resistance on maximum growth rate and height. These results suggest that fire, or other agents of selective mortality correlated with soil resource gradients, can exert strong control over plant distribution and community composition, and should be incorporated into the proposed general models relating plant strategies to community structure.

Herbivore effects on phytoplankton succession in a eutrophic lake

Abstract: Herbivory can potentially affect the speed and direction of plant succession by favoring the development of a community dominated by grazing-resistant species. This idea was tested experimentally by examining the effects of the planktonic herbivore, Daph- nia, on phytoplankton succession in a naturally eutrophic lake. Phytoplankton succession was characterized by two major transitions in community structure. Algal dominance shifted from small diatoms and chlorophytes during the spring bloom to cryptophyte flagellates during the clear-water phase. After the clear-water phase, dominance shifted to filamentous blue-greens (cyanobacteria). Algal species positions in the successional se- quence were repeatable from year to year, despite interannual shifts in the timing of species' peak abundances associated with a drastic change in the food web. In addition, evidence is presented to suggest that Daphnia-induced changes in water clarity may have fostered increases in epilimnetic mixing, which, in turn, may have stimulated brief blooms of large- celled algae after clear-water periods. Daphnia manipulation in large enclosures, and whole-lake observations before and after a fish kill, showed that intense grazing promoted the transition from edible, spring-bloom species to similarly edible, cryptophyte flagellates. In contrast, Daphnia grazing retarded further succession to grazing-resistant, filamentous blue-greens. Thus, the effects of her- bivory on algal succession were not predictable from the relative susceptibilities of these algal species to grazing mortality. These results underscore the importance of indirect effects in the herbivore-plant interactions of planktonic communities. The observation that a single species of herbivore had opposite effects during two successional transitions implies that caution should be exercised when extrapolating grazer effects beyond the time scale of an experiment.

Carbon Dioxide and Methane Exports from a Southeastern Floodplain Swamp

Abstract: Patterns and rates of generation of CO2 and CH4 by aerobic and anaerobic soil respiration are a significant gap in knowledge of floodplain carbon dynamics Gaseous and hydrologic exports of CO2 and CH4 from the forested floodplain of the Ogeechee River in Georgia, USA, were studied from July 1987 to September 1989 Net emissions to the atmosphere were measured with short (10-20 min) static chamber incubations CO2 emis- sions were highly seasonal, with largest rates during summer, and were strongly correlated with soil temperatures Annual total CO2 emissions were similar in both years of the study, and averaged 919 g/m2 The contribution of live root respiration to this total was estimated with in situ incubations of attached roots excavated from the soil Over 55% of the total CO2 flux appeared to arise from live roots rather than mineralization of soil organic matter Significant atmospheric CH4 fluxes were found only at flooded sites CH4 emissions were highly variable, with high rates of release of methane carbon (up to 271 mg im-2- d-l) occurring irregularly during the warmer months The temperature effect on CH4 emissions appeared to be a step function No significant CH4 emissions occurred when soil temper- atures were below 1 5'C; during warmer periods emission rates were generally positive, but showed no additional correlation with temperature This effect is hypothesized to represent an interaction of the rates of oxygen consumption and replenishment in the soil Spatial variability in CH4 emissions was also large Over 90% of the total floodplain CH4 emissions came from the 30% of the floodplain that was most frequently inundated Annual total fluxes of CH4 carbon from low habitats averaged 17 g m-2 yr-1 Methane oxidation rates were estimated in floodplain surface waters by in situ bottle incubations Roughly half of the CH4 that entered the water column was consumed without reaching the atmosphere Hydrologic exports of CO2 and CH4 via surface and groundwater were small, repre- senting 1-2% of total export for both species, and were much less important than fluvial exports of organic carbon Overall, floodplain detrital carbon processing was dominated by aerobic respiration and gaseous CO2 export, although methanogenesis did constitute up to 20% of total soil respiration at some sites

Variation of dispersal phenology in a bird-dispersed shrub, cornus drummondii

Abstract: Variation in the timing of seed dispersal among individuals of Cornus drummondii was marked at two sites in east—central Illinois. Individuals whose fruit crops were removed most quickly shared few characteristics. Fruit removal rates, dispersal efficiency (percent of total crop removed), and dispersal success (estimated percent of dispersed seeds attributable to each individual) were not closely related to characteristics of the fruits or the immediate environment of the parent plant. Although small fruit crops were often depleted proportionately faster than large crops, dispersal success relative to others in the population was positively related to crop size; i.e., plants with larger crops contributed more seeds to the total pool of dispersed seeds than plants with smaller crops. Over the 4—yr study, individual plants often exhibited between—year differences in ripening time, crop size, and pulp characteristics, but between—year correlations of traits were only sporadically significant. Crop size showed the most consistent between—year correlation (i.e., plants with large crops tended to make large crops in all years), but even this trait was not always correlated between years. Dispersal success was usually correlated between years, but dispersal efficiency was not. Annual (and sometimes site) differences in most dispersal—related plant traits and in dispersal efficiency and dispersal success were common. No plant traits except crop size had consistent effects on dispersal success, although we found some evidence of selective foraging. We suggest that fruit removal may be more a function of avian biology (flocking and social behavior, frequency of foraging interruptions, sun—seeking in cold weather, etc.) than of most characteristics of the fruiting plant. The ecological consequences of differences in dispersal phenology were variable between years. Postdispersal seed predation and germination success showed no consistent seasonal trends. The most important consequence of failure of early fruit dispersal may be a high probability of nondispersal (falling, rotting, being eaten by a predispersal seed predator or poor dispersal agent). Great variation in space and time in the outcome of bird/fruit interactions means that studies directed at the evolution of the interactions need to have both larger samples sizes and longer duration than has been feasible so far.

The genetic component in plant size hierarchies: norms of reaction to density in a polygonum species'

Abstract: An important motivation for the study of variability in size and reproductive output in plant populations is its potential relation to natural selection. However, very few data are available to assess the genetic component of fitness-related traits in competing plant populations, or the differential performance of plant genotypes at different densities. To address these issues we conducted an experiment using 25 genotypes of a colonizing herbaceous annual, Polygonum pensylvanicum. These genotypes were randomly sampled from a natural population and cloned by axillary meristern enhancement. Cloned plants were grown in a glasshouse at three densities spanning the range encountered in the natural population (from individually grown to 850 individuals/i2). The growth and fate of a total of 1400 individuals were followed over the course of a 10-wk growing period. Variability in size and reproductive output (as measured by the coefficient of variation of vegetative and reproductive dry mass) increased with density. Early plant size measures were positively correlated with subsequent relative growth rates in dense populations, but not among individually grown plants. These observations indicate the likely importance of asymmetric or "one-sided" competition in the dense populations. The proportion of variance in final size and reproduction explained by genotype was generally higher for individually grown plants than for plants grown under crowded conditions. We suggest that this may result from asymmetric competitive interactions working to amplify early size differences resulting primarily from environmental and developmental "noise." The same genotypes were not superior across all densities. Qualitative ("cross-over") interac- tions for fitness-related characters were observed in comparing genotype performance be- tween the individually grown vs. the low and high density treatments. Genotypes with an early size advantage were predictably favored in dense populations, but the genetic cor- relation between early and final performance was weaker among individually grown plants. In sum, density increased relative variation in fitness correlates such as reproductive bio- mass, but decreased the heritability of these traits. The response of selection is the product of these two opposing forces. Applying our results to some elementary quantitative genetic models suggests that the potential for natural selection would increase with population density, while the potential for genetic drift would decrease. Such patterns may be of particular evolutionary importance in colonizing annuals, whose life histories imply an alternate exposure of genotypes to high and low densities.