Thomas E. Miller

Dispersal rates affect species composition in metacommunities of Sarracenia purpurea inquilines.

Dispersal among local communities can have a variety of effects on species composition and diversity at local and regional scales. Local conditions (e.g., resource and predator densities) can have independent effects, as well as interact with dispersal, to alter these patterns. Based on metacommunity models, we predicted that local diversity would show a unimodal relationship with dispersal frequency. We manipulated dispersal frequencies, resource levels, and the presence of predators (mosquito larvae) among communities found in the water‐filled leaves of the pitcher plant Sarracenia purpurea. Diversity and abundance of species of the middle trophic level, protozoa and rotifers, were measured. Increased dispersal frequencies significantly increased regional species richness and protozoan abundance while decreasing the variance among local communities. Dispersal frequency interacted with predation at the local community scale to produce patterns of diversity consistent with the model. When predators were absent, we found a unimodal relationship between dispersal frequency and diversity, and when predators were present, there was a flat relationship. Intermediate dispersal frequencies maintained some species in the inquiline communities by offsetting extinction rates. Local community composition and the degree of connectivity between communities are both important for understanding species diversity patterns at local and regional scales.

Competitive Effects and Responses Between Plant Species in a First-Year Old-Field Community

Competitive interactions involve two separate processes: the effect of a species on others in the community and the response of a species to all others. Five species from a 1 st-yr field were investigated to determine if there is any correlation between competitive effect and response and if the patterns of effect and response can explain the relative success of each species in the full community. Effect and response were measured by observing the growth of individuals in all possible monocultures and two-species mixtures, using ambient densities (Year 1) and a range of densities (Year 2) for each species. Both mono- cultures and two-species mixtures were obtained by removing unwanted individuals at emergence, leaving naturally emerging individuals of the desired species. Competitive effect and response were found to be inversely correlated. The inverse correlation lead to a hierarchy of competitive ability, with Ambrosia artemisiifolia being the competitive dominant, followed by Agropyron repens, Plantago lanceolata, and finally the competitive subordinates Chenopodium album, Lepidium campestre (used in Year 1), and Trifolium repens (used in Year 2). The interactions were generally asymmetric, e.g., Ambrosia artemisiifolia had a large suppressive effect on the other species and demonstrated no response to their presence. The hierarchy and a lack of specificity of the interactions suggest that all the species are limited by, and competing for, the same resource or resources. The hierarchy of competitive ability appears to be a major factor in determining the abundance of each species in the full community.

A Critical Review of Twenty Years’ Use of the Resource‐Ratio Theory

A model of species interactions based on their use of shared resources was proposed in 1972 by Robert MacArthur and later expanded in an article (1980) and a book (1982) by David Tilman. This “resource‐ratio theory” has been used to make a number of testable predictions about competition and community patterns. We reviewed 1,333 papers that cite Tilman’s two publications to determine whether predictions of the resource‐ratio theory have been adequately tested and to summarize their general conclusions. Most of the citations do not directly test the theory: only 26 studies provide well‐designed tests of one or more predictions, resulting in 42 individual tests of predictions. Most of these tests were conducted in the laboratory or experimental microcosms and used primary producers in freshwater systems. Overall, the predictions of the resource‐ratio theory were supported 75% of the time. One of the primary predictions of the model, that species dominance varies with the ratio of resource availabilities, was supported by 13 of 16 tests, but most other predictions have been insufficiently tested. We suggest that more experimental work in a variety of natural systems is seriously needed, especially studies designed to test predictions related to resource supply and consumption rates.

DIRECT AND INDIRECT SPECIES INTERACTIONS IN AN EARLY OLD-FIELD PLANT COMMUNITY

The success of species in a community is determined not only by the direct interactions between species but also by indirect interactions. Indirect effects will occur if an associate species changes the abundance of intermediate species in a chain of interactions that ultimately affect the growth of some focal species. A model of species interactions in a community is presented here and used to quantify direct and indirect effects in a five-species weedy plant community. This method quantifies indirect effects by measuring changes in the abundance of the intermediate species and then estimating how such changes will affect the focal species. The experiments demonstrated that the direct effects in this community were generally negative (competitive) and could be quite large. Indirect effects were positive (facilitative) and negatively correlated with the direct effects, which thus acted to mitigate the usually larger competitive effects. The magnitudes of most species interactions in this community were primarily determined by large direct effects of Ambrosia artemisiifolia and, to a lesser extent, Agropyron repens. The species that exhibited the least competitive release following the removal of a competitor were those that were the most suppressed, because of the nonlinear nature of their response to increasing abundance of competitors.

Effects of emergence time on survival and growth in an early old-field plant community

SummaryThe time at which plants emerge from the soil is shown to be correlated with both survival and growth in each of four years in a plant community emerging after yearly plowing. For all seven species investigated, earlier emerging individuals generally had both a higher biomass and probability of survival. There were differences among species in the effect of emergence time on biomass, with the growth of upright, annual species being more suppressed by later emergence than the growth of other species. No significant differences were found among species in the effects of emergence time on survival. It was expected that differences among species in the effect of emergence time on fitness might lead to a correlation between the patterns of emergence time and species characteristics such as growth form or lifespan. Mean emergence times did vary significantly among both the seven species and the four years of the study. However, there was no correlation between emergence time and species lifespan, growth form, abundance, or competitive ability. The lack of a correlation between the selection pressure on species and their emergence time could be due low heritability, insufficient time for selection and evolution, and/or selection on correlated characters.

The Evolutionary Role of Indirect Effects in Communities

Indirect effects occurring through chains of direct species interactions have been shown to be important in many communities. If indirect effects have strong ecological effects, then they must also have strong evolutionary effects through natural selection. We consider how indirect effects, both positive and negative, combine with positive and negative direct effects to influence evolution between interacting species. Any one species can evolve in response to direct interactions with other species by changing its “response” to the presence of the other species or changing its “effect” on the species. We argue that the direction (positive or negative) of the direct and indirect effects will determine whether traits related to “response” or “effect” are most likely to be subject to natural selection. Further, covariance patterns of species in the same community should increase when direct and indirect effects between species act in the same direction, resulting in greater compartmentalization of such communities.

Catastrophic decline of a top carnivore in the gulf of california rocky intertidal zone.

The predatory sun star, Heliaster kubiniji, once the commonest rocky intertidal asteroid of the Gulf of California, has been rare throughout this region since summer 1978 when a devastating disease outbreak occurred. This unprecedented phenomenon and several other exceptional ecological events in marine communities of the northeastern Pacific appear to be linked to large-scale climatic changes that occurred during 1977 and 1978. Implications of the decline in Heliaster kubiniji are discussed.

Local Density Variation may Mimic Effects of Asymmetric Competition on Plant Size Variability

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THE EFFECTS OF SURROUNDING VEGETATION AND TRANSPLANT AGE ON THE DETECTION OF LOCAL ADAPTATION IN THE PERENNIAL GRASS ARISTIDA STRICTA

1 A reciprocal-transplant experiment tested the hypothesis that two populations of a perennial grass were locally adapted. Because population differentiation might be due to differential adaptation to the presence of surrounding vegetation, plants were transplanted into undisturbed areas and areas from which surrounding vegetation had been removed. As an indirect investigation of the effects of temporal environmental heterogeneity on the detection of local adaptation, three age-classes of plants were included in the experiment. 2 Seeds collected from the two populations were sown into soil from both sites in the greenhouse. Seedlings of three age-classes were transplanted reciprocally between the two field sites into both undisturbed quadrats and quadrats from which vegetation had been removed. Seedling survival was recorded for six months, and dry weight was determined for all seedlings that survived to the end of this period. 3 Comparisons of seedling survival and biomass revealed evidence for local adaptation, and the expression of local adaptation was affected very little by the presence of surrounding vegetation. Results for vegetation-removal quadrats closely paralleled those for unmanipulated quadrats. This result does not support the hypothesis that the two populations are differentiated with respect to performance in the presence of vegetation. 4 The detection of local adaptation depended strongly on the age of transplants. Home-site survival and final biomass advantages were greater for older age-classes. The effect of transplant age on the detection of population differentiation suggests that evidence of population differentiation may not always be apparent because of temporal variation in environmental conditions.

Morphological variation in Sarracenia purpurea (Sarraceniaceae): geographic, environmental, and taxonomic correlates.

Geographic variation in morphology reflects phenotypic responses to environmental gradients and evolutionary history of populations and species and may indicate local or regional changes in environmental conditions. The pitcher plant (Sarracenia purpurea) illustrates these principles. At local scales, its morphology reflects nutrient availability. At points along its broad geographic range (from Florida to northern Canada) morphology has been used to distinguish subspecies and varieties, but there has been no detailed study of the continuum of morphological variation across this entire range. Patterns of morphological variation in S. purpurea were characterized as a function of climatic and environmental conditions at 39 sites spanning its range. Differences in pitcher size and shape were strongly correlated with temperature, annual precipitation, and availability of ammonium and calcium in peat pore water. Pitcher shape (lip width, mouth diameter, and pitcher width) in Florida panhandle populations differed significantly from pitcher shape of all other populations, even after accounting for environmental correlations. In contrast, the northern and southern subspecies of S. purpurea (the latter exclusive of the Florida panhandle populations) cannot be distinguished based on these morphological measurements alone. These results support a recent proposal that identifies the Florida populations as a distinct species, Sarracenia rosea.