Showing papers in "Ecological Monographs in 1988"

Autogenic succession in a subtropical savanna: conversion of grassland to thorn woodland

Abstract: Dense thorn woodlands occupy what are thought to have been grasslands and savannas prior to settlement of the Rio Grande Plains of Texas. However, the tenet that grasslands have been converted to shrublands and woodlands in recent history is controversial and based largely upon conflicting historical accounts. Our objective was to determine how the presumed physiognomic conversion from grassland or savanna to woodlands might have occurred. Some upland landscapes are dominated by closed-canopy woodlands in southern Texas, whereas others have a two-phase pattern of discrete shrub clusters scattered throughout a grassland. More mesic sites are dominated by closed-canopy woodlands. We hypothesized the two-phase landscapes represented an intermediate stage in the conversion of grassland to woodland. As new shrub clusters were initiated and existing clusters expanded and coalesced, a gradual shift from grassland to savanna to woodland would occur. To address this hypothesis, we inventoried herbaceous interspaces for woody colonizers, quantified the composition and dis- tribution of shrub clusters on upland sites, and compared the structure of clusters to that of adjacent, more mesic areas with continuous woody plant cover. To assess the physiognomic stability of the two-phase landscapes, cluster size, density, and cover were quantified for 1941, 1960, and 1983 from aerial photographs. A lone mesquite (Prosopis glandulosa) plant occurred in > 80% of the upland clusters, where it was typically the largest individual in terms of basal area, height, and canopy area. The number of woody species per cluster ranged from 1 to 15 and was strongly related to mesquite basal diameter (R2 = 0.86). Cluster diversity, evenness, and size were also significantly correlated with mesquite size. The data suggest that mesquite plants invaded grasslands and served as recruitment foci for bird-disseminated seeds of other woody species previously restricted to other habitats. The result was a landscape composed of discrete chronosequences of woody plant assemblages organized about a mesquite nucleus. Within the two-phase portion of the landscape, 50% of the clusters were within 5 m of another and 95% were within 15 m of another. Analysis of the size class distribution of clusters suggested that most had yet to realize their growth potential. Moreover, the herbaceous clearings between clusters contained high densities of woody seedlings, mostly (>70%) mesquite, which occurred in 85% of the clearings, with a mean density of 350 plants/ha. Coalescence will become increasingly probable if new clusters are initiated and existing clusters expand. This phenomenon appeared to be in progress on one portion of the landscape and had apparently already occurred on others. As clusters developed on the two-phase portion of the landscape, their species composition, dominance, and size class structure became increasingly similar to that of adjacent closed-canopy woodlands on more mesic sites. Mean cluster size increased from 494 m2 in 1941 to 717 m2 in 1983. Growth rates of clusters were a function of cluster size and precipitation. During the 1941-1960 period characterized by severe drought, there was a slight decrease in total woody plant cover resulting primarily from the formation of gaps among clusters on the periphery of the site and a 350% decrease in density of clusters 100 m2 were an order of magnitude lower than those of clusters 1,000 M2). Our results indicate (1) mesquite invaded grasslands and served as the nucleus of cluster organization on upland sites; (2) woody plant community development has been highly punctuated by variations in precipitation; (3) clusters >5 m2 in area are persistent features of the landscape; and (4) the present two-phase pattern is moving toward a monophasic woodland as new clusters are initiated and existing clusters expand and coalesce. As a result, (5) shrub clusters on uplands represent an intermediate stage in the conversion of grassland to woodland, and (6) closed-canopy woodlands on more mesic sites appear to represent portions of the landscape where this has already occurred. Because the conversion of grasslands and savannas to woodlands in the Rio Grande Plains is initiated by mesquite, factors regulating its dispersal, establishment, and role as a facilitator of woody community development are emphasized.

Origin, Composition, and Flux of Dissolved Organic Carbon in the Hubbard Brook Valley

Abstract: The concentration, composition, and flux of dissolved organic carbon (DOC) were measured in the Hubbard Brook Valley, New Hampshire. Data on precipitation, throughfall, soil solution, streamside seeps, stream water, and lake water are presented for 1976-1980. Characterization of DOC included analysis of phenolics, monomeric and polymeric carbohydrates, carboxylic acids, primary amines, and aldehydes. DOC concentrations increased with passage of water through the forest canopy and forest floor, decreased due to abiotic sorption in the mineral soil, and remained relatively low in most downstream ecosystems (streamside seeps, streams, and Mirror Lake). Average flux of DOC is estimated as 17, 47, 263, 54, 23, and 20 kg{center dot}ha{sup {minus}1}{center dot}yr{sup {minus}1} in precipitation, throughfall, soil solution (E, upper B, and B horizons), and streamflow, respectively. The composition of DOC, in particular the relative proportions of carbohydrates, appears to be related to the degree of biological activity at a given site in the landscape. Carbohydrates are particularly high in both absolute and relative terms in throughfall and lake water, which are the areas of highest photosynthetic activity within the terrestrial and aquatic ecosystems, respectively.

Spatial and Temporal Variation in Costa Rican Fruit and Fruit‐Eating Bird Abundance

Abstract: Understory fruit and fruit-eating birds were censused monthly for a year in gaps, intact forest, and second-growth sites of a lowland Costa Rican rain forest. Both fruits and birds displayed significant seasonal variation. Peak fruit abundance corresponded with peak fruit-eating bird abundance. Fruits were most abundant in the mid-to-late rainy season (August-January). Crop sizes were larger on second-growth plants than on either gap or intact forest plants. Also, fruit was much more common in second growth than in gaps and more common in gaps than in intact forest. Fruit-eating birds followed the same general patterns of spatial and temporal variation. They were significantly most abundant in second growth, significantly least abundant in intact forest, and most common from October to January. A large increase in the frugivore population in October was due to an influx of temperate and altitudinal migrants. In addition, populations of some resident frugivore species increased concurrently, suggesting altitudinal migration in some of these species as well. The two most common understory frugivores molted during the period of fruit high and bred during the period of fruit low. I suggest that insect abundance may be more important than fruit abundance in determining breeding cycles of these birds. Given the extent of spatial and temporal variation in fruit and frugivore abundance, and the apparent tracking of fruit resources by birds, large areas of diverse habitats are probably necessary for the long-term maintenance of frugivore populations.

Variation in Algal Dispersal and Recruitment: The Importance of Episodic Events

Abstract: Spatial and temporal variation in dispersal and recruitment of the kelps Macrocystis pyrifera and Pterygophora californica and of filamentous brown algae (FBA) consisting primarily of Ectocarpus siliculosus were examined off southern California, USA. These three taxa of sublittoral brown algae show enormous differences in adult morphology and demography, but release propagules near the bottom that are similar in size, shape, and motility. Their weekly variation in recruitment was measured on replicate frosted glass slides placed at different distances from stands of fertile adults at three sites. For all taxa, significant temporal variation was observed in recruitment at the zoospore source as well as at distances out to 4000 m from the source. For the kelps, this variation was due to variable rates of both zoospore settlement and early postsettlement mortality. In general, recruitment density of both kelps rapidly declined with distance from the adult stand; significantly lower recruitment was observed at as little as 3 m away. In contrast, dispersal of FBA was much greater; no significant decrease in recruitment density was observed out to 500 m. The differences in dispersal between these two algal groups may result from behavioral differences of their zoospores. Unlike those of kelps, zoospores of FBA are positively phototatic. This enables FBA zoospores to remain in the water column longer, thus increasing the distances over which they disperse. The predominant pattern of limited dispersal in the two kelps, however, was not constant over time. Dispersal was greatly expanded during episodic periods of high recruitment that coincided with winter storms. Turbulent water flower associated with these storms may resuspend kelp zoospores normally occurring near the bottom and allow them to be transported greater distances by prevailing currents. Although species characteristics of the adult stand appeared to influence the distribution of recruits, the effect disappeared within 10 m of the stand. At a short—range study site, decreases in spatial variation in recruitment density between the zoospore source and 3 and 10 m away indicated that the distribution of zoospores became more uniform with distance. Likewise at a long—range site, recruitment out to 4000 m appeared to occur uniformly and coincided with recruitment at the zoospore source and at all intermediate distances. This uniformity and coincidence of recruitment indicated that dispersal over longer distances was probably via individual zoospores rather than via clumps of drifting plants or reproductive fragments as suggested in previous studies. Although episodic, such dispersal is probably essential in promoting the persistence of these plants in highly disturbed and unpredictable environments.

Avian seed dispersal of three neotropical gap-dependent plants'

Abstract: In cloud forest at Monteverde, Costa Rica, I investigated the reproductive consequences of avian seed dispersal for three species of gap-dependent plants: Phytolacca rivinoides (Phytolaccaceae), Witheringia solanacea, and W. coccoloboides (Solanaceae). Of six bird species that consumed fruits of these plants, only three (Myadestes melanops (Muscicapidae), Phainoptila melanoxantha (Ptilogonatidae), and Semnornisfrantzii (Cap- itonidae)), dispersed seeds in viable condition. Other species discarded most seeds before swallowing fruit pulp, or destroyed seeds in the gut. I estimated the quality of dispersal service provided by the "legitimate" disperser species by comparing the seed shadows they produced with the spatial and temporal distributions of establishment sites for the plants. I estimated seed shadows from data on gut passage rates of seeds and on movement patterns of radio-tracked birds. Seed shadows produced by all three effective dispersers were extensive, with few seeds deposited near the parent plant, and some seeds moved >500 m. Seeds of the species examined establish in forest gaps formed by treefalls or landslides. Establishment success varies with gap size and age, but the relationship is different for each species; both Witheringia species establish well in gaps as small as 15 m2 or as old as 6 mo, whereas P. rivinoides establishes well only in gaps > 70 m2 or < 4 mo old. Consequently, establishment sites for all three plants are both rare and ephemeral, but to differing degrees. Seeds that are not dispersed to suitable habitat patches can remain dormant in the soil until a gap is formed overhead; seed dormancy experiments showed no significant decrease in viability of seeds buried for up to 27 mo. To determine consequences of dispersal and dormancy for plant reproductive success, I developed a simulation model that uses data on seed shadows, germination requirements, seed dormancy, and forest dynamic processes to estimate reproductive output (total off- spring produced during an individual plant's lifetime) and relative "fitness" (an estimator that discounts the contribution of offspring produced after a long period of dormancy). Results show that (1) dispersal by any of the three legitimate dispersers increases repro- ductive output 16-36 times, even without seed dormancy. (2) Dormancy capabilities up to 2 yr enhance both reproductive output and "fitness," but greater capabilities increase only reproductive output. (3) Without dispersal, dormancy has little effect on either re- productive output or fitness. Thus, both dispersal and dormancy ("dispersal" in time) are essential to these gap-dependent plants, but long-term seed dormancy may be relatively unimportant.

Diversity, Patterns of Adaptation, and Stability of Nova Scotian Kelp Beds

Abstract: There are two alternate community states in the rocky subtidal of the Atlantic coast of Nova Scotia, an unproductive sea urchin/coralline alga community, and highly productive kelp beds dominated by Laminaria longicruris. Disease-induced mortality of the sea urchins (Strongylocentrotus droebachiensis) triggered a switch from the first state to the second and provided a unique opportunity to study (1) the ability of L. longicruris to recover its former dominant status, and (2) its stability when competing with other seaweeds and when perturbed by storms and grazers other than urchins. Rates of recolo- nization of L. longicruris depended on the proximity of a refugial source of spores. When reproductive plants were nearby, a closed canopy developed within 18 mo of urchin mortality. When a reproductive population was several kilometres away, there was sparse recolonization for 3 yr, then a massive recruitment occurred with closure of the canopy in the 4th yr. Laminaria is clearly the competitive dominant in the seaweed community. Manipulative experiments showed that the kelp limits the abundance of several understory species, but there was no evidence that the abundant annual seaweeds limited kelp recruitment. When sea urchins were rare, the density and growth rates of Laminaria were influenced mostly by intraspecific competition. When the canopy of adult plants was removed there was a dramatic increase in kelp recruitment, but the recruits that grew in dense patches in the clearings were significantly smaller than those of a similar age that grew more sparsely beneath the canopy. Once the kelp recovered from destructive grazing and formed a mature forest, it was able to maintain its dominance, even in habitats subject to severe nutrient stress for 8 mo of the year. For most ofthe year mortality and erosion of laminae outweighed the effects of recruitment and growth, and the canopy declined, especially during winter when storms were frequent. Erosion was exacerbated by grazing of the gastropod Lacuna vincta. However, in late winter and early spring, recruitment and rapid growth restored the canopy. When severe storm damage was simulated by completely removing Laminaria in patches, the kelp rapidly recolonized and soon outgrew other seaweeds. Unlike the competitive dominants in kelp bed systems in the northeast Pacific, L . longicruris in Nova Scotia manifests multiple patterns of adaptation that enable it to dominate early and late stages of succession in a range of habitats of different levels of nutrient stress and of disturbance from storms and grazers. The principal threat to the stability of the kelp beds is destructive grazing by sea urchins. We suggest that the consid- erable differences between the dynamios of kelp beds in Nova Scotia and those of the northeast Pacific, and the high degree of stability of L . longicruris stands in Nova Scotia, is attributable to the low diversity of kelps and therefore low levels of competition in Nova Scotia, and to the multiple adaptations of L . longicruris that enable it to tolerate several stresses and disturbances. We argue that the dynamics of community organization, and therefore the stability properties of this system are determined primarily by biological interactions and not by physical variables. This differs from the kelp communities in the northeast Pacific, in which both biological and physical factors influence dynamics significantly at a primary level. We offer a qualitative model of the dynamics of community structure in Nova Scotia that may be viewed as a set of deterministic "subroutines," in which each subroutine describes the outcome of a particular biological interaction. The subroutine(s) that predominate at one point in time and space are probably determined mostly by physical hydrographic variables that have a large stochastic component.

An experimental study of the effect of lizards on web-spider communities

Abstract: To determine the effect of lizards on web—spider populations, we conducted an 18—mo field experiment in the Bahamas. Densities of individuals of each common spider species were about three times as high in lizard—removal enclosures as in control enclosures with lizards or in unenclosed plots with lizards; spider densities in control enclosures and unenclosed plots were nearly identical. In the most common spider species, Metepeira datona, lizards reduced juvenile and adult abundance, as well as adult female survivorship and prey consumption. Numbers of spider species were higher where lizards were removed than where they were present; hence, lizards did not promote spider species coexistence in this system. Numbers and biomasses of aerial insects caught in sticky traps were higher in lizard—removal enclosures than in controls; hence, higher spider predation on insects where lizards had been removed did not completely compensate for the lack of lizards. Comparisons between control enclosures and unenclosed plots revealed that the enclosures reduced insect numbers and biomasses. This study and others have demonstrated that lizards play a major role in structuring web—spider communities in the West Indies. Several lines of evidence indicate that predation is the major interaction between lizards and spiders, although some evidence for competition also exists.

The Dynamics of Habitat Use in a Guild of Ozark Minnows

Abstract: The underlying causes of habitat use in an assemblage of Ozark minnows were investigated in an observational field study. Six habitat variables were used: depth of water, current speed, substrate size, lateral distance from stream edge, vertical position in the water column, and presence in pool, riffle, or raceway. Analysis focused on the six numerically dominant species that used a common set of pool microhabitats and thereby formed a natural habitat guild. Observations on habitat use were conducted so as to record simultaneous habitat use by all species at each sample point. This technique made possible an analysis of interspecific interactions over habitat use. The six species were segregated primarily by the vertical position variable (X overlap = 0.160). Subdivision of pool habitat data into two depth zones yielded subsets that were homogeneous with respect to all habitat variables except vertical position. Within these data subsets, interactions between species pairs over use of vertical position were examined. Species whose distributions shifted significantly toward those of heterospecifics were considered associations while distributions that shifted away were considered dissociations. Of 60 possible vertical interactions, 33 were significant, and of these approximately two—thirds were associations and one—third were dissociations. The subdivision of data also permitted an analysis of horizontal interactions between species pairs over the sample points. This analysis showed that guild members were significantly clumped into multispecies groupings. Analysis of species—pair interactions within these groupings showed that of 60 possible interactions, 37 were significant and of these approximately half were associations and half were dissociations. A pattern of vertical and horizontal interactions was evident between the two depth zones. In the shallower zone, associations were the predominant interaction in the vertical dimension, but the number of associations and dissociations was nearly equal in the horizonal dimension. In the deeper zone, vertical associations still predominated, though to a lesser extent, and associations predominated in the horizontal dimension. Several explanations for these patterns were proposed. Some associative interactions may facilitate opportunistic feeding behaviors, and others may promote interspecific schooling as an antipredation tactic. Dissociations probably promote habitat segregation among guild members. A balance of these two types of interactions may be critical to guild maintenance. The frequent interactions over habitat use indicate that habitat use among guild members is highly variable; the overall pattern of segregation results from a combination of habitat selection as modified by interspecific interactions over time and space. This view contrasts with the more conventional interpretation of segregational patterns in which shifts or differences in resource use are assumed to be relatively static and interactions and competition among species are reduced or eliminated. the dynamic nature of habitat partitioning observed in this minnow assemblage is consistent with the requirement of ecological flexibility in the characteristically unstable and unpredictable environments of streams.

Interaction of Fire and Community Development in Chaparral of Southern California

Abstract: Fire is an ecosystem property rather than an exogenous force in southern California chaparral, and it interacts with processes of drought-mediated canopy devel- opment, production, and mortality to affect stability of community composition. Where species that must reproduce from seed, such as Ceanothus crassifolius or Ceanothus olig- anthus, are predominant, composition can be altered by a single fire with little or no recruitment after initial postfire establishment. Water balance apparently regulates subsequent leaf area development; after 15-22 yr of postfire growth, foliage biomass in monospecific C. crassifolius stands in this study had approached a maximum that was unrelated to incident solar radiation and insensitive to initial population density over a 10-fold range. Thus, establishment success, above that required for canopy closure, should have little effect on the foliage biomass that sustains combustion. After canopy closure, total biomass accumulated at an accelerating rate through at least two decades with aboveground net primary production as great as 12-13 Mg ha- I yr- 1. C. crassifolius mortality was substantially less than predicted from growth rates and the -3/2 power model of Yoda et al. (1963), and there was no approach to a common asymptotic density by stands of disparate initial density. With low deadwood biomass and absence of ground fuels, C. crassifolius cannot sustain burning in the absence of wind, steep slopes, or exceptionally low live-fuel moisture. Increased Ceanothus abundance in multispecies communities with Adenostoma fascicu- latum or Salvia mellifera alters biomass structure and could modify subsequent fire effects even if foliage area fully redevelops in concert with site water balance. Rare, low-intensity fires can devastate Ceanothus chaparral, that reproduces only from seed. Salvia mellifera and Eriogonumfasciculatum can occupy resulting openings in the canopy, and their abun- dant deadwood and compact biomass can readily spread low-intensity fires, thereby per- petuating the degraded community. Productive stands within a chaparral association are probably subject to especially severe fires that limit nutrient accumulation and may also limit subsequent productivity. Copious nitrogen volatilization during burning is promoted by high nitrogen concentrations in foliage and fine woody biomass of Ceanothus and heavy leaf litter of Quercus dumosa and C. crassifolius. The communities most prone to severe fires also accumulate and cycle nitrogen and phosphorus rapidly.

Energetics of the Lizard Cnemidophorus Tigris and Life History Consequences of Food- Acquisition Mode

Abstract: Energy budgets for the wide—foraging "arthropodivorous" lizard Cnemidophorus tigris were constructed for the reproductive season using doubly labeled water measurements of field metabolic rate (FMR). Rates of body mass change, clutch sizes, and clutch intervals were also investigated. FMRs of both sexes (males, 298 J · g—1 · d—1; females, 247 J · g—1 · d—1) were greater in the reproductive season than during the postreproductive season. This was not due to differences in resting metabolism, but, instead, was due to higher activity costs during the reproductive season. Although males had significantly higher FMR than females, males and females had similar feeding rates (as reflected by water influx rates). The ratio of energy intake to expenditure was higher in females than in males. Females produced eggs but did not grow. Large males did not grow, but small 1st—yr males similar in size to females did grow. Females laid at least two consecutive clutches during a single reproductive season; clutch interval ...

Population variability and polyphagy in herbivorous insect communities

Abstract: Our purpose in this paper is to determine how the degree of polyphagy of different herbivorous insect species affects their yearly population variability. We assembled data from three studies on herbivorous insects: on British aphids, British moths, and Canadian Macrolepidoptera. Within each data set, we compared estimates of population variability across species, and related these differences to estimates of the degree of polyph- agy. The degree of polyphagy was negatively correlated or uncorrelated with population variability, i.e., highly polyphagous species have a weak tendency to be less variable than host specialists. This result lends some support to MacArthur's (1955) argument that polyphagous species may be less susceptible to fluctuating resource levels. Population variability in monophagous or oligophagous herbivorous insects may, in part, reflect vari- ation in resource levels. However, we have not yet evaluated the possibility that the levels of predation and parasitism suffered may affect variability even more strongly.

The Evolution of Finch Communities on Islands and Continents: Kenya vs. Galapagos

Abstract: I describe the nonbreeding finch community of several habitats in continental Kenya, East Africa, and compare it with previously described communities of granivorous finches on Galapagos islands. The purpose of the comparison is to explore differences in structure between communities that have evolved on a continent and on an isolated archipelago, and to suggest reasons for the differences. The ultimate goal is to infer the factors important in the evolution of finch communities, of which the majority have a continental origin. Four factors that might be expected to influence finch communities differently in Kenya than Galapagos are: a greater diversity of possible competitors, an abundance of predators, movement between localities, and a greater diversity of resources. Differences predicted on the basis of these four factors are compared with actual differences between Kenya and Galapagos, in order to estimate their relative importance. The communities of Kenya and Galapagos were surprisingly different. Despite the greater age of the Kenya fauna, the diversity of beak sizes was less, the range of seed sizes exploited was less, and the overall utilization of seed resources by finches was incomplete. Species in Kenya were more differentiated by habitat, microhabitat, and seed species, and less differentiated by seed size than finches in Galapagos. Diets were on average more taxonomically specialized, and associations between diet and beak and body dimensions were correspondingly weaker. Kenya finches were sensitive to the proximity of trees and shrubs (cover) while foraging. These results, together with observations from a limited number of other studies, suggest that a dichotomy may exist between the finch communities of continents and isolated archipelagos. However, like Galapagos, local finch density in Kenya was related to food abundance, and diet overlaps between species coexisting in the same locality were low. Comparison of results with predictions from hypotheses based on the four factors suggests that competition among the large diversity of granivorous species in Kenya (i.e., finches and other granivorous birds, rodents, and ants) is a major reason for the differences between the finch communities of Kenya and Galapagos. The narrow range of beak and seed sizes used by finch species in Kenya is consistent with competition from other taxa, and competition between the many finch species constrained to a narrow range of seed sizes may explain the more pronounced habitat, microhabitat, and diet specializations. Thus, while the finch communities of continents and isolated archipelagos may differ in structure, there is an indication that in large part they represent alternative outcomes of the same process. Predation risk is the most likely cause of a preference for feeding near cover in Kenya, and it may explain why certain seed resources are not exploited by finches, such as ground seeds in areas of poor visibility (dense grasses). Most species appeared to be similarly sensitive to risk of predation; for example, finch species showed little partitioning along an axis of distance to cover. Thus, the main effect of predation on community evolution in Kenya may have been to further confine the resource spectrum available to finches, a role similar to that of competition from other taxa. There was no evidence that the effects of competition within localities were swamped by an influx of immigrant individuals and species. However, lesser indirect effects of movement may be present; movement may elevate local species diversity and thereby influence community structure by increasing the diversity of potential competitors and predators. The prediction that a greater diversity of resources in Kenya would lead to a greater diversity of morphological forms was not upheld, suggesting that resource differences are less influential than other factors.

Comparative Structure of Harvester Ant Communities in Arid Australia and North America

Abstract: In the Australian arid zone, the species richness of ants is greater and that of mammalian grainvores is less than in North American deserts. This study aimed to determine if the structure of harvester ant communities differs from that seen in North American deserts, focussing on differences related to the paucity of rodents. We tested three hypotheses: (1) because there are fewer rodents, Australian harvester ants should be more abundant and diverse in local habitats than in North American deserts of similar productivity; (2) because the absence of rodents would allow ants to use larger seeds that are preferred by rodents in North America, Australian ant communities should include a larger size range and contain larger workers; and (3) that apart from differences resulting from a paucity of rodents, Australian and North American communities would be convergent in characteristics of community structure resulting from competition. We sampled 19 communities across a climatic gradient in the Australian arid zone and compared the results with data previously obtained for 10 North American communities. Australian harvester ants exhibited similar alpha (within—habitat) diversity but higher beta (between—habitat) diversity between communities. Australian and North American communities were similar in species richness, species diversity, numbers of common species, and abundance of ants, although Australian communities tended to be richer and more diverse at sites with lower precipitation. North American communities increased in species richness, diversity, numbers of common species, and proportions of column—foraging species with increasing precipitation. In contrast, Australian communities did not change regularly across a gradient in precipitation. Thus, the first hypothesis was rejected. The second hypothesis was also rejected because Australian harvester ants were smaller, covered a smaller size range, and tended to be more tightly distributed along the size gradient. Dietary data indicated a lack of correspondence between resource use and availability, suggesting that Australian communities may not generally be in equilibrium with their resource environment. However, neither of the first two hypotheses could be tested unequivocally because it remains possible that evolutionary interaction between seeds and ants in the absence of rodents has allowed seeds to adopt defenses minimizing consumption by ants, or that other granivores (particularly birds) compensate in part for the paucity of rodents. Tests of the third hypothesis were ambiguous. Some results suggest basic similarities between the effects of competition on the communities, such as the similar maximum values for abundance, richness, and species diversity. However, several attributes of Australian communities differed from those in North America: foraging occurred over a wider range of soil temperatures; temporal displacement of foraging among coexisting species was prominent; and individually foraging species occurred with equal frequency across climatic gradients. We discuss various biotic and abiotic features that may explain differences in community structure between continents. We conclude that the paucity of rodents, a potentially competing group of granivores, has not led to predictable changes in harvester ant communities of arid Australia.

Dynamics and Control Mechanisms in a Tropical Zooplankton Community (Lake Valencia, Venezuela)

Abstract: The dynamics of zooplankton herbivores in Lake Valencia, Venezuela, were studied over a 5-yr interval. Lake Valencia is a large, warm-monomictic, eutrophic lake. Copepods were the dominant group; rotifers were more abundant than the cladocerans, which were not present in all years. Variation between years in herbivore biomass bore no statistical relationship to patterns in algal biomass. The average annual dry mass production of the herbivores was high (7.0 Agg L-I d-l), but less than expected given the high primary productivity of the lake. The annual ratios of production to biomass for individual species were well below the maxima expected for steady growth, and thus imply growth suppression. Feeding capacity of Chaoborus, the principal primary carnivore, exceeded herbivore production for 66% of the sampling dates. Annual overturn broke the coupling between predator and prey by causing extensive mortality in both herbivores and carnivores; her- bivores recovered from this suppression more rapidly than carnivores. Herbivores followed three basic strategies for coexistence with their predators in Lake Valencia: (1) two Brachionus species were able to match predation losses by reproductive output; (2) the copepods, which experienced intense predation pressure as adults, were able to sustain the high losses because the adult stage was subsidized by recruitment from earlier, less vulnerable developmental stages; (3) Keratella and the cladocerans were opportunistic in that they became abundant only in response to a decline in the predator population. The opportunistic species were most abundant during the mixing season, which was the period of minimum abundance for Chaoborus. Predation strongly influenced the herbivore populations, but population data indicate that inadequate food resources also played a role. Growth suppression was the result of qualitative rather than quantitative inadequacy in the algal food base. The intensity of control by the two factors was strongly affected by abiotic factors associated with mixing events. Partial mixing provided brief respite from growth suppression for certain species (typically rotifers), but did not alter predation pressure. Annual overturn resulted in im- provement of food quality, but because mortality of herbivores coincided with relaxation of predation, the herbivores were delayed in exploiting qualitative improvements in the food resources. Periods of simultaneous release from growth suppression and predation were infrequent and brief. Herbivores were thus controlled simultaneously by predation and quality of food resource.

Resource Heterogeneity Fosters Coexistence of a Mite and a Midge in Pitcher Plants

Abstract: Resource heterogeneity promotes coexistence between two competing species, Sarraceniopus darlingtoniae, a slime mite, and larvae of Metriocnemus edwardsi, a midge, which live together in pitchers of Darlingtonia californica, the California pitcher plant. Both species depend on resources derived from the decomposing insects captured by the pitchers. Experimentation revealed that midge larvae consumed resources at a greater rate than mites and that the densities of the two species are negatively correlated. These results indicate that M. edwardsi is a potential dominant competitor with S. darlingtoniae. Com- petition theory predicts that the presence of midge larvae should lead to the competitive exclusion of mites, yet these species coexist in field populations. This seeming paradox can be explained by temporal and spatial heterogeneity in resources. Midge larval density is generally associated with expected resource levels of pitchers, whereas mites are found in greatest abundance in pitchers whose resources differ from expected. This pattern suggests that the effects of competition vary and are strongest when actual resource levels equal expected resource levels. When resource levels are above expected levels, coexistence oc- curs. These results support Levins' (1979) model for coexistence of two species on a single resource. The implications of this study to the general debate over the role of interspecific competition in structuring communities are discussed.

PALEOLIMNOLOGY OF A CAPE COD KETTLE POND: DIATOMS AND RECONSTRUCTED pH'

Abstract: Changes in pH over the past 12000 yr in Duck Pond in the Cape Cod National Seashore were reconstructed using a diatom-pH transfer function. The recon- structed pH suggested that the pond has been acid for its entire history with a mean reconstructed pH of 5.3 ? 0.3 (standard error of the predictive equation ? 0.5). There was a brief period during the late-glacial (at - 1 1 500 BP) of higher pH of 6, possibly caused by increased windiness and erosion (inferred from concurrent pollen and sediment changes indicative of an open spruce-Hudsonia parkland), increased leaching of cations from outwash sands of the drainage basin, and/or increased instability of the water levels during lake development. The reconstructed pH history indicates that Duck Pond has become more acid recently (with a mean pH of 5.1 ? 0.1 for the past 150 yr) although the pH has varied both up and down throughout the Holocene with acidity as low as the present at other times in the past 12 000 yr. The significance of these changes in recon- structed pH is problematic because the pH variations lower in the core remain within the standard error of the equation and primarily reflect a long-term naturally acid ecosystem. This finding illustrates the need for more local and regional long-term lake pH histories so that naturally acid lakes can be distinguished from anthropogenically degraded lakes and ecologically sound management decisions can be made. In the pre-European-settlement levels of the core, correlations between charcoal and alkaline Fragilaria diatoms suggest that forest fires in the drainage basin may have affected pond pH. Correlations between charcoal and acid diatoms in post-European-settlement sediments suggest an effect on pond pH from recent increase in windborne industrial acids. Negative correlation between diploxylon Pinus species and the reconstructed pH and pos- itive correlation between haploxylon Pinus and the alkaliphilic diatoms suggest that vege- tation changes, which also reflect the climate and fire history of the region, have also affected the pH of the pond.