Showing papers in "Ecological Monographs in 1987"

Secondary Succession and the Pattern of Plant Dominance Along Experimental Nitrogen Gradients

Abstract: In 1982, experimental nitrogen gradients were established on both existing and disturbed (disked) vegetation in three fields (abandoned 14, 25, and 48 yr) and on existing vegetation in native oak savannah. Each of these seven gradients contained five or six replicates of each of nine treatments that differed in the annual rate of nitrogen addition. In none of the fields did plant biomass, height, species richness, or light penetration respond to addition of P, K, Ca, Mg, S, and trace metals. In contrast, plant biomass and height increased significantly, and light penetration and species richness decreased significantly, with added nitrogen along all seven gradients. On average, >60% of the species had been displaced from high—nitrogen treatments by 1985. Nitrogen addition led to a period of transient dominance by certain species. Species that reached peak relative abundance in high—nitrogen treatments in 1982 tended to be rare in all but the low—nitrogen treatments by 1985. In contrast, the relative abundances of most species that dominated the high—nitrogen treatments in 1985 did not increase along the nitrogen gradients in 1982. The relative or absolute abundances of most common species changed significantly along the experimental gradients in at least 1 yr. By 1985, many common species were differentiated in their distributions along the seven gradients. In general, early successional annuals and short—lived perennials and plants of short stature at maturity reached their peak abundance in low—nitrogen plots, whereas plots, receiving high rates of nitrogen addition were dominated by long—lived herbaceous and woody species that are taller at maturity. A survey of 22 old fields at Cedar Creek, Minnesota, showed that total and available soil nitrogen increased during succession and that major species had individualistic, fairly Gaussian distributions along this temporal nitrogen gradient. The distributions along the experimental gradients of most of the common species were consistent with the pattern observed in the old—field survey, demonstrating that nitrogen influences the pattern of secondary succession at Cedar Creek. The major exception was Agropyron repens, and early successional grass that dominated high—nitrogen treatments on six of the seven gradients. Comparisons of species responses on the disturbed plots with those on plots of existing (undisturbed) vegetation showed that, by 1985, most species responded similarly to the nitrogen gradients despite great differences in their initial abundances. For instance, Agropyron repens, an initial dominant of Field A but rare in Fields B and C, was dominant in the high—nitrogen treatments in both the disturbed and undisturbed plots of these three fields. It invaded into and rapidly increased in abundance in numerous high—nitrogen plots from which it was initially absent. Schizachyrium scoparium declined along the nitrogen gradients both in undisked plots in which it was initially dominant and in disked plots in which it was initially rare. Such similarities suggest that the outcome of interspecific interactions among old—field plants is highly dependent on nitrogen supply rates, but fairly independent of initial plant abundances.

Field metabolic rate and food requirement scaling in mammals and birds

Abstract: Field metabolic rates (FMRs or HF), all measured using doubly labeled water, of 23 species of eutherian mammals, 13 species of marsupial mammals, and 25 species of birds were summarized and analyzed allometrically (log10-1og10 regressions). FMR is strong- ly correlated with body mass in each of these groups. FMR scales differently than does basal or standard metabolic rate in eutherians (FMR slope = 0.81) and marsupials (FMR slope = 0.58), but not in birds (FMR slope = 0.64 overall, but 0.75 in passerines and 0.75 in all other birds). Medium-sized (240-5 50 g) eutherians, marsupials, and birds have similar FMRs, and these are - 17 times as high as FMRs of like-sized ectothermic vertebrates such as iguanid lizards. For endothermic vertebrates, the energy cost of surviving in nature is enormous compared with that for ectotherms. Within the eutherians, marsupials, or birds, FMR scales differently for the following subgroups: rodents, passerine birds, her- bivorous eutherians, herbivorous marsupials, desert eutherians, desert birds, and seabirds. Equations are given for use in predicting daily and annual FMR and food requirement of a species of terrestrial vertebrate, given its body mass.

Determinants of Pattern in a New England Salt Marsh Plant Community

Abstract: In New England salt marshes, Spartina alterniflora dominates the low–marsh habitat, which is covered daily by tides. The high–marsh habitat, which is not flooded daily, is dominated on its seaward border by Spartina patens, and on its terrestrial border by Juncus gerardi. Each of these vegetation zones has a characteristic suite of physical factors associated with differences in tidal inundation. In particular, substrate redox increases and salinity decreases with decreasing marsh elevation. Although correlations between physical factors and the occurrence of specific marsh plants have been suggested to be causal, a 5–mo transplant experiment suggested that the distribution of perennials across the marsh does not correspond to their potential performance across the marsh in the absence of surrounding vegetation. While the high–marsh perennials appear to be restricted to the high–marsh habitat by harsh physical conditions in the low–marsh habitat, the low–marsh dominant, S. alterniflora, is capable of vigorous growth across the entire marsh and appears to be excluded from the high–marsh habitat by the high–marsh perennials. Throughout the high marsh, two other plant species, Distichlis spicata and Salicornia europaea, are found associated with areas that have been disturbed recently. Physical disturbance, in the form of mats of dead plant material (wrack) rafted by tides onto the marsh, is most severe in the spring and early summer, and decreases with increasing marsh elevation. Differential plant mortality results from short–term disturbance events. D. spicata and S. alterniflora are more tolerant of wrack burial than are the other marsh plants, and short–term disturbance increases the relative abundance of these species in the community. Longer lasting disturbance events kill all the underlying vegetation, leaving discrete bare patches throughout the high marsh. D. spicata rapidly colonizes these patches with vegetative runners, while S. alterniflora and Sa. europaea recruit to these patches by seed. The relative abundance of these plants in recently created bare patches exceeds greatly their relative abundance in the surrounding vegetation. Over time, however, these early colonizers are overgrown and displaced in high–marsh patches by S. patens and J. gerardi, which grow slowly, as dense turfs of roots, rhizomes, and tillers. Physical disturbance and interspecific competition appear to be major determinants of the spatial pattern of marsh plant communities. These processes will need to be considered in relation to edaphic factors in elucidating the underlying mechanisms of salt marsh plant zonation.

Equilibrium and Nonequilibrium Concepts in Ecological Models

Abstract: Mathematical models and empirical studies have revealed two potentially disruptive influences on ecosystems; (1) instabilities caused by nonlinear feedbacks and time-lags in the interactions of biological species, and (2) stochastic forcings by a fluctuating environment Because both of these phenomena can severely affect system survival, ecol- ogists are confronted with the question of why complex ecosystems do, in fact, exist Our study analyzes the basic themes of this research and identifies five general hypotheses that, in recent years, theoretical ecologists have built into models to increase their stability against disruptive feedback and stochasticity To counter feedback instabilities, theoreti- cians have considered (1) functional interactions between species that act as stabilizers, (2) disturbance patterns that interrupt adverse feedback effects, and (3) the stabilizing effect of integrating small-spatial-scale systems into large landscapes To decrease the influence of stochasticity, modelers have hypothesized (4) compensatory mechanisms operating at low population densities, and (5) the moderating effect of spatial extent and heterogeneity We show that modeling based on these ideas can be organized in a systematic way We also show that the stable equilibrium state should not be viewed as a fundamental property of ecological systems, but as a property that can emerge asymptotically from extrapolation to sufficiently large spatial scales

A Reevaluation of the -3/2 Power Rule of Plant Self-Thinning

Abstract: The self—thinning rule predicts that for a crowded even—aged plant population a log—log plot of average plant mass vs. plant density will reveal a straight "self—thinning" line of slope–3/2. The rule is supported by examples from many individual populations, and by the existence of an interspecific relationship that yields a line of slope–3/2 in a log—log plot displaying average mass and density data from many populations of different species. I examined and reanalyzed the evidence to evaluate the strength of support for this widely accepted rule. Some problems in fitting thinning lines and testing agreement with the rule have no truly satisfactory solution, but three improvements on commonly used methods were made: the analysis related stand biomass density to plant density because the alternative of relating average plant mass to plant density is statistically invalid; principal components analysis was used rather than regression, because regression relies on unrealistic assumptions about errors in the data; and statistical tests of hypotheses were used to interpret the results. The results of this reanalysis were that 19 of 63 individual—population data sets previously cited in support of the thinning rule actually showed no significant correlation between stand biomass density and plant density, and 20 gave thinning slopes significantly different (P < .05) from the thinning rule prediction. Four other analyses provided additional evidence against a single quantitative thinning rule for all plants: slopes of the thinning lines were more variable than currently accepted, differed significantly among plant groups, were significantly correlated with shade tolerance in forest trees, and differed among stands of the same species. The same results held for the intercepts of self—thinning lines. Despite the failure of the thinning rule for individual populations, the combined data for all populations are still consistent with an interspecific relationship of slope–3/2; therefore, the existence of the interspecific relationship does not necessarily support the within—population thinning rule. The within—population and interspecific relationships are apparently different phenomena that may arise from different constraints, so the two relationships should be tested and explained separately..

Vertebrate-dispersed plants of the iberian peninsula: a study of fruit characteristics'

Abstract: This study examines the characteristics of ripe fruits of 11 1 species (35 families, 62 genera) of vertebrate-dispersed plants (with fleshy fruits) native to the Iberian Peninsula. The sample included 58% of the species, 87% of the genera, and 95% of the families of fleshy-fruit-producing plants occurring in the area. Species were characterized by 23 variables related to the design of the fruit (size, mass of seeds and pulp, number of seeds, seed size) and the organic and inorganic chemical constituents of pulp (water, lipids, protein, fiber, non- structural carbohydrate, ash, Ca, Na, Mg, K, P, Fe, Cu, Zn, Mn). Qualitative variables related to geographical distribution at two different scales (northern temperate vs. southern mediterranean; mediterranean lowlands vs. highlands), growth form, fruit color, and display type of the infructescence, were used to categorize species and perform comparisons of fruit characteristics. Factor analysis (FA) was used to examine trait covariation and identify major independent trends of variation in fruit structure and pulp composition. Factor scores of species were used to examine patterns related to taxonomic affiliation and ecological characteristics of species. With increasing fruit size, pulp mass increased more slowly than seed mass, and thus the pulp/seed mass ratio decreased. The ratio of pulp to seed was greatest in small-seeded fruits. Trees tended to produce the greatest amount of pulp per seed, and herbs the smallest amount. In the species with the smallest seeds the biomass of the pulp exceeded that of the seeds. Major trends of interspecific variation in fruit design included: (1) fruit size (54.5% of variance); (2) inverse relationship between number of seeds/fruit with mass of single seeds and mass of pulp allocated to each seed (23.1%); (3) variation in pulp/seed mass ratio (16.6%). Fruit size, seed size, and pulp/seed mass ratio tended to vary independently of each other. There was significant heterogeneity in factor scores of species differing in geographical distribution, growth form, fruit color, and display type. Interspecific patterns in fruit design were strongly influenced by phylogeny, although no predictable relationship existed between fruit structure and taxonomic affiliation. Water was the dominant constituent of fresh pulp (range = 19.3-91.1°/o, mean = 70.8%). On a dry mass basis, nonstructural carbohydrate (NSC) represented 25.8-93.5% (mean = 67.0°/o), fiber 2.6-52.0% (16.3%), lipids 0.2-58.8% (6.9%), and protein 0.9-27.7% (5.0%). Based on factor analysis, there was a strong negative correlation between NSC and lipids (Factor 1, 46.5% of variance), and a complementary variation of NSC and fiber (Factor 2, 25.5%). Factors 3 (14.8%) and 4 (1 1.5%) corresponded to variation in protein and water, respectively. No significant heterogeneity in factor scores existed among groups of species based on geographical distribution or type of display. Heterogeneity did exist with regard to fruit color and plant growth form. No predictable relationship existed between phylogeny and pulp organic composition, although taxonomic diversity had some influence on interspecific variability. With the single exception of the joint occurrence of Ca and Mg, the concentration of any mineral element in the pulp of a species was not predictably associated with the amounts of other elements in the pulp. Com- position of the inorganic fraction was not significantly related to either geographical distribution of the species or type of fruit display. Species groups based on fruit color or growth form differed significantly in total mineral content. No phylogenetic correlates were apparent in the constitution of the inorganic fraction of fruit pulp. The broadest patterns revealed by this study were of a "null type." The few "positive" ones have rather restricted implications as to the overall constitution of fruits, and only one-third of these seem attributable to the selective action of frugivores on fruit traits. The study failed to reveal any overwhelming influence of the disperser/dispersal environment on the characteristics of Iberian fruits taken as a whole, particularly with reference to interspecific variation. Phylogeny, architectural constraints, and species-specific energy and nutrient allocation patterns, seemed most important in explaining observed variation in fruit constitution.

Subtidal coexistence: storms, grazing, mutualism, and the zonation of kelps and mussels'

Abstract: At exposed sites in the Gulf of Maine, USA, subtidal mussels (AModiolus modiolus) dominated space on upper rock surfaces at intermediate depths (1 1- 18 m), but not at shallow depths (4-8 m), where dominants were the kelps Laminaria digitata and L. saccharina. Observations and experiments were conducted to test the following hypotheses about factors limiting the vertical (depth) zonation of mussels and kelp: (1) that the upward distribution of Modiolus is limited by interference effects of kelp, or (2) that it is limited by the failure of larval recruitment to shallow depths, and (3) that sea urchin grazing controls the downward distribution of kelp. The recruitment failure hypothesis was rejected. Results indicated that storm-generated dislodgement of mussels overgrown by kelp was the mechanism reducing the ability of Modiolus to maintain and hold space in the shallow kelp zone. Dislodgement following kelp overgrowth was the most significant mortality source of large mussels. Removal of sea urchins, Strongylocentrotus droebachiensis, from the lower edge of the kelp zone resulted in the downward shift of kelp to a 12.5 m depth, demonstrating that the lower depth limit of kelp is set by urchin grazing. Storm disturbances varied in frequency and severity of damage to mussel populations, and were an important agent of patch creation in the subtidal zone. Small storms occurred on a monthly basis. Severe storms struck three times in 1982 and twice in 1983; an average of 11 658 mussels with attached kelp were dislodged and cast ashore on monitored beaches during such storms. Storm disturbances were synchronous between coastal and offshore sites. The mean density of patches cleared in mussel beds (2.5 patches/0.25 i2) was highest after fall storms. Mean patch size ranged from 91.0 cm2 (fall) to 122.0 cm2 (winter). With the exception of urchin fronts at 10.5-12.0 m, sea urchin densities were signifi- cantly higher inside Modiolus beds than outside. The hypothesis that urchins increase .Modiolus survivorship by grazing kelp off mussels and decreasing the risk of mussel dis- lodgement was tested by an urchin-removal experiment. Removal of urchins from mussel beds led to rapid kelp recruitment, resulting in a 30-fold increase of mussel mortality (via kelp-induced dislodgement) compared to control beds with resident urchins. Significantly more force was required to pull large urchins out of mussel beds than to remove them from rock surfaces outside the beds, suggesting that urchins are less susceptible to predation and dislodgement-caused mortality while in Modiolus beds. The Modiolus-Strongylocen- trotus interaction is identified as a facultative mutualism that appears to facilitate the coexistence of kelp and mussels at shallow depths. The ability of kelp and mussels to recover from dislodgement disturbance was examined by clearing patches in mussel beds and algal turf. Kelps recolonized and dominated all algal-turf patches and 47% of mussel patches within 7 mo. In contrast, Alodiolus did not close patches by recruitment, or by a leaning response of mussels surrounding the patch, over a 3-yr period. This suggests that the ability of kelps to bounce back from dislodgement disturbance may enhance their competitive superiority. For Modiolus, the rate of storm- generated disturbance exceeded the rate of recovery.

Size-dependent processes underlying regularities in ecosystem structure'

Abstract: The structure of animal communities and the energy flux through them may be characterized by biomass ratios, ecological efficiencies, and production efficiencies of the component organisms. Here, we interpret these ratios in terms of the elementary processes of food intake, specific production rate, and gross growth efficiency that underlie them. Recent information confirms that the magnitude of all these processes is related to the average body mass of the organisms involved. However, our analysis shows that this well-known dependence reflects the influence of two different basic biological properties. One of these is the metabolism-body-size relation of individuals that is familiar from physiology. The other less well-recognized property appears as an ecological population factor reflected in the distribution of particle sizes within animal groups in the community and is probably related to the relative sizes and distributions of predators and their prey. It appears that both the physiological and ecological size relationships have to be recognized as scaling factors in order to transform measures of biological production of various parts of communities into common terms for comparison. Current data on the generality and stability of community structure and production suggest that by using this twofold size scaling, trophic energy flow within the community can be determined from the distribution of body sizes without the necessity of specifying trophic levels of the organisms involved. The ecological size scaling can be seen as an index of the system nature of ecosystems.

Effects of Food Availability and Fish Predation on a Zooplankton Community

Abstract: The effects of fish predation and food availability on population densities and demography of zooplankton were investigated in Dynamite Lake, Illinois, USA, a lake with a high density of size-selective planktivorous fish and low food levels. Fish predators (bluegill sunfish) and food levels (phytoplankton) were manipulated in replicated, factorial- design field experiments during two summers (1980 and 1981). Overall, population densities of zooplankton were affected much more by manipulations of food availability than by manipulations of fish predation. The cladocerans Bosmina longirostris, Ceriodaphnia lacustris, and Diaphanosoma birgei were greatly increased in density by elevated phytoplankton levels in both years, in the presence and absence of fish. Demographic analysis in 1981 revealed that increased densities in response to elevated food levels resulted from both an increase in birth rates (Bosmina, Diaphanosoma) and a decrease in mortality rates (Ceriodaphnia, Diaphanosoma). The rotifers Lecane and Mon- ostyla also increased dramatically in response to elevated phytoplankton densities. Co- pepods were less responsive to manipulations of food levels, but several taxa exhibited increases in density in response to increased phytoplankton abundance. Few species were reduced in density by fish predation. Ceriodaphnia density was reduced by fish more than any other species in the entire community, and the density of even this species was much more affected by food availability. In terms of percent change relative to controls, increased food availability had much more of an effect than fish predation on the density of most zooplankton species and on total zooplankton abundance. Fish predation had several effects on the size structure and life history traits of the cladocerans. All three species attained larger sizes when fish were excluded than when fish were present. Cladoceran individuals also initiated reproduction at a smaller size and produced smaller offspring in the presence of fish. The reductions in mean body size, size at maturity, and offspring size in the presence of fish were most pronounced in Diapha- nosoma and Ceriodaphnia, the two largest species. Smaller body size and size at maturity apparently allow the cladocerans to reproduce before reaching a size at which they become vulnerable to size-selective fish predators. Larger size at first reproduction and larger off- spring size in the absence of fish may be a response to invertebrate predators, which assume more importance in the absence of fish and prey most heavily on smaller size classes. Flexibility in these life-history traits allows the cladocerans to withstand what appears to be intense size-selective predation by planktivorous fish.

Interactions among subtidal australian sea urchins, gastropods, and algae: effects of experimental removals

Abstract: The subtidal region at Cape Banks, New South Wales, Australia, is char- acterized by large areas covered solely by encrusting coralline algae in association with large densities of invertebrate grazers. Orthogonal removals of these grazers: the sea urchin Centrostephanus rodgersii; the limpets Patelloida alticostata, P. mufria, and Cellana tra- moserica; and the turbinids Australium tentiforme and Turbo torquata, were done at two sites and two times to investigate the interactions among these species and their effects on the algal community. These manipulations had large effects on both the algal and animal communities, although the type and intensity of these interactions altered at different temporal and spatial scales. Initially, the removal of urchins and/or all species of limpets resulted in a rapid increase in the amount of noncrustose algae in these areas. Removal of turbinids, however, had no significant effect. Where both limpets and urchins had been removed, the cover of foliose algae continued to increase quickly and was :80-100% after 12 mo. In areas where only urchins were removed, the increase in the cover of algae was slower, and only approached 100% after 18-24 mo. Where only limpets had been removed, 3 mo after an initial increase in the cover of filamentous algae the cover declined to be only slightly more than in control areas. These results were similar at all sites and times. In one series of removals, the effects of some of the species of limpets were separated. In this case, the increase in the cover of algae was found to be fastest when only P. mufria was left (the urchins and other limpets removed), than when all limpets and urchins had been removed. Initially, the removal of the urchins generally resulted in a large increase in the re- cruitment and density of the limpets. Approximately 1 yr following the removal of the urchins, however, the density of adult and juvenile limpets of all species had declined to near zero. Removal of the larger species of limpets, P. alticostata and Cellana, also increased the rate of recruitment of the smaller species, P. mufria. Thus, these invertebrate grazers, especially the sea urchins, appeared to be necessary for the maintenance of the areas of crustose algae. Similarly, the sea urchins were found to be necessary for the continued presence of the limpets within these areas. The strong effects of the urchins on this community, and the application of the situation to theories concerning keystone species and facilitation, are discussed in relation to the important but more subtle effects of the limpets.

Ecomorphology, Locomotion, and Microhabitat Structure: Patterns in a Tropical Mainland Anolis Community

Abstract: According to the habitat—matrix model, arboreal microhabitat specialists are adapted behaviorally and morphologically for locomotion in different subsets of the vegetation, each characterized by its three—dimensional structure, and these adaptations help explain morphological patterns among coexisting species I tested this model's predictions for Anolis lizards near Monteverde, Costa Rica Anolis humilis, A tropidolepis, and A woodi were active at different heights in the shaded forest understory, and A insignis inhabited the canopy Gap specialists, A altae and A intermedius, resembled one another in microhabitat use but were largely separated by elevation Adult males of forest—understory and gap species were active higher above ground than adult females, which averaged higher than juveniles In their different structural environments, species, sexes, and age classes differed in proportional use of locomotor modes: running, jumping, and crawling During field observations of forest—understory and gap species, frequency of crawling was highest for anoles that used slender, widely spaced supports, though much variation in crawling frequency was unexplained Frequency of jumping increased as mean support size and average distance between supports decreased; the latter variable, expressed relative to body length, accounted for most of the variation in jump frequency With enclosure experiments, I assessed proximate effects of microhabitat features on locomotor behavior and removed these effects to test for interspecific differences in intrinsic locomotor tendencies Jump frequency of A altae consistently increased with decreasing distance between supports, whereas the effects of support diameter were more complex, and varied with spacing of supports Support diameter exerted both a surface—area effect (a lower tendency to jump from larger supports compared with smaller ones) and, where supports were widely spaced, a target—size effect (a higher tendency to jump to larger supports compared with smaller ones) Compared in the same array of supports, A tropidolepis, A altae, and A intermedius, although similar in body size, differed in frequency of jumping Thus, differences in locomotor behavior among species reflected not only proximate influences of vegetation structure but also intrinsic tendencies Morphological traits were strongly associated with locomotor behavior and microhabitat specialty Differences in limb proportions of primarily running anoles (A altae and A intermedius), crawling anoles (A insignis), and anoles that jumped frequently (A humilis, A tropidolepis, and A woodi) accorded with predictions from biomechanics Body size was also functionally related to locomotion and was correlated with microhabitat structure Thus, ability to exploit various structural environments may depend not only on body shape but also size Nonrandom patterns of interspecific differences in morphology suggested limiting similarity without revealing the underlying ecological process(es) The mechanistic basis for these patterns,however, appears to lie, at least in part, in the functional relationships between morphology and habitat structure

Tests of density-dependent habitat selection in a patchy environment'

Abstract: A simple regression analysis can be used to assess the response of animal density to differences in habitat quality. The same test can evaluate general predictions of habitat selection theory as well as search for differences in the shapes of habitat suitability- density functions, something previous tests have been unable to do. Combined with de- mographic or other estimates of fitness, regression tests can provide new insights into the evolution of habitat selection. Regression and fitness tests were used to explore the pattern of density-dependent habitat use in two temperate-zone rodents. The intensity of population regulation appeared to be inversely related to a habitat's carrying capacity. Variation in density-dependent habitat choice suggests new and unexpected dispersal strategies that vary with habitat heterogeneity. The predictions of the theory are complicated when habitat quality varies independently of population density. Sweepstake fitness rewards may be reaped by animals that would previously have been assumed to have made a suboptimal habitat choice. At the level of microhabitat, habitat selection models appeared to be much less capable of predicting variation in population density. Such a relation could be due to complex and inverse relationships between microhabitat quality and carrying capacity, or it could simply reflect scaling patterns in habitat selection.

The Impact of Preemption on the Zonation of Two Typha Species Along Lakeshores

Abstract: The objective of this study was to examine the effects of preemption on rates of competitive displacement and on competitive outcomes for two Typha species (T. latifolia, TL, and T. domingensis, TD) growing along gradients of water depth. Evidence from several experiments indicated that the relative competitive abilities of the two species were size dependent. When TL and TD competed as small seedlings, TD was generally favored while TL was the better competitor when the plants were larger before beginning to compete. Because of this effect, initial density was determined to influence strongly the early outcome of competition. When seeds were sown at high densities, TD gained an early advantage; at lower densities TL was the better competitor. At very low densities, such as when the species competed as clones, TL was very much the better competitor. In the absence of preemption, the results showed that differences in germination, seedling growth, and competitive ability act to create an initial segregation of species at the waterline where establishment occurs. TL is upslope and TD downslope initially, but as the species spread downslope, TL progresses faster because of its greater capacity for vegetative spread, and the result is a widespread overlap along the gradient. Studies of the rate of competitive displacement between established plants yielded an estimate of slightly < 6 yr required for complete segregation of the two species starting from maximum overlap. Preemption had a strong effect on early growth and distribution of the species. After the initial effects of temporal disadvantage, delayed plants were able to recover under certain circumstances. Both species recovered better in shallow water than in deep water, and, in general, TL recovered better than did TD. Experiments on spatial preemption found that the species moved towards the expected segregation of TL upslope and TD downslope regardless of the initial conditions. This process of recovery was somewhat slower for established stands of adults than for seedlings. Comparison of aerial photographs of natural populations of TL and TD in PAR Pond (South Carolina) in 1980 with distributions in 1984 also suggests that recovery from even large-scale spatial preemption can occur over only a few years time. These results suggest that extreme temporal advantage by either species can lead to extinction of the other species and that the magnitude of the effect depends upon the initial density of seedlings. If both species survive the initial phase of competition and the habitat remains undisturbed, subsequent vegetative spread and competitive displacement can over- come the initial effects of spatial and temporal preemption, resulting in a predictable

Food profitability and the foraging ecology of crossbills

Abstract: Observations over a two and a half year period in the Northeast United States and adjacent Canada indicate that White-winged Crossbills (Loxia leucoptera) and Red Crossbills (L. curvirostra) shift their diets among the seeds of various conifer species in a seasonal pattern. Both crossbill species forage on white spruce (Picea glauca) in late summer. White-winged Crossbills continue foraging on white spruce or tamarack (Larix laricina) until late autumn or winter, when they switch to black spruce (P. mariana). Red Crossbills switch to white pine (Pinus strobus) in early autumn and to red pine (P. resinosa) or other pines in winter or spring. This pattern of diet shifts is consistent with the hypothesis that crossbills forage to maximize food intake rate. Profitability (milligrams of kernel ingested per second) was measured for crossbills foraging on each of these conifers by observing rates of seed ingestion in the field, and subsequently measuring dry seed kernel masses. Profitability for a given conifer increases as cones and seeds mature, then declines as seeds are shed from the cones. Because conifers differ in the timing of cone ripening, crossbills experience sequential peaks in profitability. Both crossbill species usually foraged predominately on the most profitable conifer species, with switches in conifer use coinciding with shifts in relative profitabilities. However, crossbills often forage on more than one conifer species at a time, even though intake rates might be maximized by foraging on only the most profitable conifer. Predation does not appear to influence diet selection and large-scale patterns of conifer use. Crossbill movements and patterns of abundance, both on local and continent-wide scales, are cor- related with patterns of profitability. Dietary overlap between crossbill species is greatest, often approaching 100%, in late summer when seed is most abundant. Overlap then declines rapidly when white pine cones open, and usually remains negligible most of the remainder of the year. Patterns of dietary overlap are a result of differences in profitability for each crossbill species that are largely inherent in the ripening phenology of conifer cones and seed accessibility, not seed depletion by crossbills. Interspecific competition is most likely between White-winged Crossbills and either the smallest form of the Red Crossbill or redpolls (Carduelis sp.) during the late winter of "invasion" years, which occur every 3-4 yr. Intraspecific competition is also likely to be most intense at these times.

A Quantitative Test of Life History Theory: Thermoregulation by a Viviparous Lizard

Abstract: Pregnant females of the viviparous lizard species Sceloporus jarrovi regulate their body temperatures at a lower level than do males or nonpregnant females. It has been suggested that such a shift in preferred body temperature during pregnancy reflects the presence of divergent optimal temperatures for the female and for development of her young; a pregnant female must compromise between these temperatures in order to maximize her fitness. We examine this hypothesis using a Leslie matrix model of life history that quantitatively predicts the mean body temperature that would optimally compromise between conflicting thermal optima for mother and embryos. The predictions of the model are in close agreement (0.4°C or less) with temperatures observed in the field. According to the model, a pregnant female maintaining the mean body temperature typical of males or nonpregnant females would have ≈13% lower fitness than a pregnant female maintaining the optimal temperature. This is mainly due to increased embryo mortality, but reduced growth and increased mortality of the female also contribute to the loss of fitness. In the model, the optimal mean body temperature depends on the precision of thermoregulation. Thermoregulation by gravid females in the field is imprecise, with a standard deviation of 1.4° during active thermoregulation. If females are (unrealistically) assumed to maintain a perfectly constant body temperature (i.e., a standard deviation of 0°), the optimal mean temperature in the model is ≈2° higher than the mean temperature of gravid females in the field. Although specifically designed to incorporate features of the physiology and life history of Sceloporus jarrovi, the model can be generalized and applied to other situations involving compromise among multiple optima.

Population Dynamics of Weddell Seals (Leptonychotes Weddelli) in McMurdo Sound, Antarctica

Abstract: Population dynamics of Weddell seals in McMurdo Sound, Antarctica, were studied from 1970 to 1984 using mark-recapture estimation, aerial surveys, age structure and magnitude of harvests, and direct counts of pup production. Similar data from earlier studies were used to reconstruct the history of the population during the period of human presence after 1956. Jolly-Seber estimates of population size indicated a general decline from 1970 to 1976, low numbers in 1976 and 1978, and relative stability from 1979 to 1984 at - 1500 adult seals. From 1970 to 1983, mean annual survival of adult female and male Weddell seals was estimated at 0.85 and 0.76. Counts made in aerial surveys in the summers of 1982/1983 and 1983/1984 were substantially below similar survey counts in the 1960s. Pup production in the breeding colonies also declined from 1967 to 1976, but has recovered and been stable since 1979. Collections spanning 28 yr indicated significant shifts in age structure characteristic of populations undergoing a rapid increase and then decline. The age structure of both sexes indicated a constant age of recruitment to the adult population at 5 yr, which corresponds closely with age of maturity in females. Generalized least squares and standard regression analyses failed to detect any correlation of adult survival or female reproductive estimates with population size or ice conditions, but there were significant trends in survival estimates from 1970 to 1983 that may be due to shifting age structure. The most plausible interpretation of these data is that heavy harvests of seals to feed dog teams in the mid-1950s severely depleted the resident population of adult Weddell seals. When population studies began in the early 1960s the population was expanding rapidly, probably as a result of immigration by juveniles. The population declined to low levels in 1976-1978 and has been fairly stable since 1979, probably at a level lower than before harvesting began. Survival and reproductive parameters of these Weddell seals are low relative to other pinnipeds, suggesting that the adult population is at an equilibrium with its environment.

Environment and Variation in Life History Traits of the Chuckwalla, Sauromalus Obesus

Abstract: Life history attributes for the western chuckwalla, Sauromalus obesus, and environmental variables, e.g., rainfall and vegetational growth, were recorded simulta- neously during a 7-yr investigation in the Colorado desert of southeastern California. The timing of rainfall was most critical to the growth of annual plants, and the standing crop of winter annuals was greatest when substantial rainfall occurred early (October-January). Chuckwallas preferred to eat winter annuals during the spring. However, broad and op- portunistic feeding habits were observed, and the persistence of relatively mild winters and frequent summer rainfall enabled individuals to feed throughout most of the year. Males typically achieved reproductive maturity at 125 mm snout-vent length (SVL) and 2 yr of age. Females achieved reproductive maturity at 125 mm SVL and 2-3 yr only when optimal environmental conditions prevailed. Clutch size was highly correlated with body size, and the mean for the study period was 6.9 eggs. Mean values for other repro- ductive attributes were: egg mass, 8.4 g; relative clutch mass (RCM), 0.343; and expenditure per progeny (EPP), 0.053. Clutch size, egg mass, RCM, and EPP did not differ significantly among years for a given body size. Mean annual frequency of reproduction was 52%, but ranged from 0 to 95% during the study period. Mean lst-yr survivorship was 38% but showed considerable year-to-year variability. Egg mortality had the greatest impact on 1 st-yr survivorship. Mean survivorship for chuck- wallas older than 1 yr approached 75% and values for males and females were not signif- icantly different. Adults of both sexes appeared to be considerably more susceptible to predation after achieving 165 mm SVL. Consequently, large adult individuals were never common. Annual recruitment was 20% and was largely the result of reproduction. Popu- lation densities showed nearly a twofold annual variation, ranging from 15 to 30 individ- uals/ha. Life table analysis showed that younger females (3-6 yr) contributed to 50% of the replacement rate; mean generation time was 8.2 yr, and the life expectancy was - 15 yr. Year-to-year consistency of various reproductive attributes (clutch size, egg mass, and RCM), indicative of "boom or bust" iteroparity, is possibly a typical adaption for extremely long-lived lizard species. Relatively mild winters and occurrence of summer rainfall in the Colorado desert greatly promoted early maturity and frequent reproductions. In addition, summer rainfall improved the survivorship of eggs, probably by providing adequate mois- ture for incubation. Such conditions were responsible for relatively high densities and predation rates. However, under drier conditions maturity was delayed, reproduction less frequent, and egg survivorship much lower; such conditions are typical in the Mojave desert. The relative stability of clutch size, egg mass, and RCM, but apparent variability of age at reproductive maturity and reproductive frequency in this study, may indicate that the life history of chuckwallas evolved under a variable but predictable environmental setting.