Jamie M. Kneitel

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.

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.

RESOURCE AND TOP-PREDATOR REGULATION IN THE PITCHER PLANT (SARRACENIA PURPUREA) INQUILINE COMMUNITY

The effects of top-down and bottom-up forces on species abundance and diversity were quantified in the inquiline communities found in the water-filled leaves of the pitcher plant, Sarracenia purpurea. A press field experiment was conducted in which the abundances of resources (dead ants) and the top predators (larva of the mosquito Wyeomiia smithii) were each maintained at three levels in a factorial design for 23 d. Abundances of mites, rotifers, protozoans, and bacteria increased significantly with prey addition. Protozoan species richness also increased with increasing resource levels, whereas additional resources did not affect bacterial species richness. Increases in top-predator density decreased rotifer abundance but increased bacterial abundance and species richness. Increases in bacterial abundance and richness indicate that a trophic cascade occurs via the larvae, rotifer, and bacteria pathway; omnivory appears to be not strong enough to affect the trophic cascade. Top-down and bottom-up forces both had strong, but different, effects on the abundance and richness of intermediate trophic levels. For most components of the inquiline community, species richness and population sizes are resource limited, whereas predator limitation targets a limited set of groups.

EFFECT OF COMMUNITY STRUCTURE ON INVASION SUCCESS AND RATE

Although invasion has long been recognized as an important ecological pro- cess, there are very few experimental studies of invasion in natural communities and vir- tually no studies that determine how trophic structure affects the probability of invasion. We introduced novel protozoans and rotifers into the natural communities found in the water-filled leaves of the pitcher plant Sarracenia purpurea. The communities were ma- nipulated in a factorial design of removal of predators (larvae of the mosquito Wyeomyia smithii) and addition of resources (dead insects). Three of the six protozoan species suc- cessfully established populations when introduced into pitchers, suggesting that these spe- cies are migration limited. The other three protozoans and a rotifer did not successfully invade established communities, although all four are naturally found in these inquiline communities. Of the three successfully invading protozoans, two were more likely to invade when resources were added and one of those even more frequently when predators were removed. Invasion by the third was unaffected by these experimental manipulations. Similar effects of predators and resources were found on population sizes of these three species. This study is one of very few that have addressed invasion experimentally; its results suggest that a variety of factors, including migration, predation, and resource availability, can have different influences on invasion by fairly similar protozoans.

DISTURBANCE, PREDATOR, AND RESOURCE INTERACTIONS ALTER CONTAINER COMMUNITY COMPOSITION

Species diversity at the local-community scale can be altered by numerous factors, including disturbances, predators, and resource levels. Intermediate levels of these three factors are predicted to enhance coexistence and diversity. However, no study has examined how these factors may interact to alter community composition. The protozoan and rotifer community that colonized containers set in a forest was used to examine the interactions between these local community processes. We conducted a fully factorial mi- crocosm experiment that manipulated disturbance frequency, predator density, and resource levels to examine protozoan and rotifer richness, community composition, and species abundance. Species richness was significantly altered by disturbances and predators, while predator densities interacted with disturbances and resources. Total abundance was signif- icantly affected by each treatment, as well as a disturbance and predator interaction. We found that community composition was altered by each of the treatments and their inter- actions, indicating that different groups of species were present depending on the treatments. These results indicate that strong species sorting occurs in this community. Understanding these factors alone and in concert can provide insight to the potential complexities that underlie community structure and species composition.

Chapter 1 Allometry of Body Size and Abundance in 166 Food Webs

Summary The relationship between average body masses ( M ) of individuals within species and densities ( N ) of populations of different species and the mechanisms and consequences of this relationship have been extensively studied. Most published work has focused on collections of data for populations of species from a single broad taxon or trophic level (such as birds or herbivorous mammals), rather than on the populations of all species occurring together in a local food web, a very different ecological context. We here provide a systematic analysis of relationships between M and N in community food webs (hereafter simply webs ), using newly collected, taxonomically detailed data from 166 European and North American pelagic, soil, riparian, benthic, inquiline, and estuarine webs. We investigated three topics. First, we compared log( N )‐versus‐log( M ) scatter plots for webs and the slope b 1 of the ordinary‐least‐squares (OLS) regression line log( N ) = b 1 log( M ) + a 1 to the predictions of two theories (Section V.A). The energetic equivalence hypothesis (EEH) was not originally intended for populations within webs and is used here as a null‐model. The second theory, which extends the EEH to webs by recognizing the inefficiency of the transfer of energy from resources to consumers (a trophic transfer correction, or TTC), was originally proposed for webs aggregated to trophic levels. The EEH predicts approximate linearity of the log( N )‐versus‐log( M ) relationship, with slope −3/4 for all webs. The relationship was approximately linear for most but not all webs studied here. However, for webs that were approximately linear, the slope was not typically −3/4, as slopes varied widely from web to web. Predictions of the EEH with TTC were also largely falsified by our data. The EEH with TTC again predicts linearity with b 1 b 1 >−3/4, indicating that populations of larger taxa absorb more energy than populations of smaller ones. Slopes b 1 > −3/4 can occur without violating the conservation of energy, even in webs that are energetically isolated above trophic level 0 (discussed later). Second, for each web, we compared log–log scatter plots of the M and N values of the populations of each taxon with three alternate linear statistical models (Section V.B). Trophic relationships determined which taxa entered the analysis but played no further role except for the Tuesday Lake and Ythan Estuary webs. The assumptions of the model log( N ) = b 1 log( M ) + a 1 + ɛ 1 (including linearity of the expectation) were widely but not universally supported by our data. We tested and confirmed a hypothesis of Cohen and Carpenter (2005) that the model log( N ) = b 1 log( M ) + a 1 + ɛ 1 describes web scatter plots better, in general, than the model log( M ) = b 2 log( N ) + a 2 + ɛ 2 . The former model is also better than the model of symmetric linear regression. Third, since not all of our log–log scatter plots formed approximately linear patterns, we explored causes of nonlinearity and examined alternative models (Section V.C). We showed that uneven lumping of species to web nodes can cause log( N )‐versus‐log( M ) scatter plots to appear nonlinear. Attributes of the association between N and M depended on the type of ecosystem from which data were gathered. For instance, webs from the soil of organic farms were much less likely to exhibit linear log( N )‐versus‐log( M ) relationships than webs from other systems. Webs with a larger range of measured log( M ) values were more likely to appear linear. Our data rejected the hypothesis that data occupy a polygonal region in log( N )‐versus‐log( M ) space.

Constitutive Differences between Natural and Artificial Container Mosquito Habitats: Vector Communities, Resources, Microorganisms, and Habitat Parameters

ABSTRACT Aquatic containers, including tree holes and vehicle tires, harbor a diverse assemblage of mosquitoes capable of vectoring important diseases. Many studies have examined containers as a mosquito breeding site, although no data exist that have simultaneously compared mosquito communities between tree holes and tires, and few have quantified differences in environmental factors or food resources that may be important for explaining population or community differences. At two times (early and late summer 2009) we sampled two tire and two tree hole sites in south-central Mississippi, and for each container we enumerated mosquito larvae and measured several environmental parameters (canopy cover, water volume, and detritus), and biomass and productivity of fungi and bacteria, and species richness and abundance of protozoans. Tree holes held less water but were more shaded compared with tires; however, after correcting for volume differences, tree holes contained more detritus and were higher in some microorganism measures (protozoan richness, bacterial productivity in the water column). Based on community dissimilarity analysis of mosquitoes, strong differences existed between container types and sampling period; Aedes albopictus (Skuse) and Culex quinquefasciatus (Say) were dominant in tires, whereas Ae. triseriatus (Say) and Orthopodomyia signifera (Coquillett) were dominant in tree holes. This study also reports the use of tires by the invasive mosquito Cx. coronator (Dyar and Knab). Tree holes supported a higher density of larvae but fewer species than tires, though there was variation across time. Our work illustrates that detrital inputs and some microorganisms differ in fundamental ways between tires and tree holes, and because of compositional differences in mosquito communities, these small aquatic habitats cannot be considered to be homogeneous mosquito habitats.

Environmental Correlates of Abundances of Mosquito Species and Stages in Discarded Vehicle Tires

ABSTRACT Discarded vehicle tires are a common habitat for container mosquito larvae, although the environmental factors that may control their presence or abundance within a tire are largely unknown. We sampled discarded vehicle tires in six sites located within four counties of central Illinois during the spring and summer of 2006 to determine associations between a suite of environmental factors and community composition of container mosquitoes. Our goal was to find patterns of association between environmental factors and abundances of early and late instars. We hypothesized that environmental factors correlated with early instars would be indicative of oviposition cues, whereas environmental factors correlated with late instars would be those important for larval survival. We collected 13 species of mosquitoes, with six species (Culex restuans, Cx. pipiens, Aedes albopictus, Cx. salinarius, Ae. atropalpus, and Ae. triseriatus) accounting for ≈95% of all larvae. There were similar associations between congenerics and environmental factors, with Aedes associated with detritus type (fine detritus, leaves, seeds) and Culex associated with factors related to the surrounding habitat (human population density, canopy cover, tire size) or microorganisms (bacteria, protozoans). Although there was some consistency in factors that were important for early and late instar abundance, there were few significant associations between early and late instars for individual species. Lack of correspondence between factors that explain variation in early versus late instars, most notable for Culex, suggests a difference between environmental determinants of oviposition and survival within tires. Environmental factors associated with discarded tires are important for accurate predictions of mosquito occurrence at the generic level.

Ecosystem-phase interactions: aquatic eutrophication decreases terrestrial plant diversity in California vernal pools

Eutrophication has long been known to negatively affect aquatic and terrestrial ecosystems worldwide. In freshwater ecosystems, excessive nutrient input results in a shift from vascular plant dominance to algal dominance, while the nutrient-species richness relationship is found to be unimodal. Eutrophication studies are usually conducted in continuously aquatic or terrestrial habitats, but it is unclear how these patterns may be altered by temporal heterogeneity driven by precipitation and temperature variation. The California vernal pool (CVP) ecosystem consists of three distinct phases (aquatic, terrestrial, and dry) caused by variation in climatic conditions. The purpose of this study was to test the hypothesis that resource addition during the aquatic phase results in increased algal abundance, which reduces vascular plant cover and richness of the terrestrial phase upon desiccation. We used mesocosms layered with CVP soil, in which treatments consisted of five levels of nitrogen and phosphorous added every 2 weeks. Resource addition increased available phosphorus levels and algae cover during the aquatic phase. Increased algal crusts resulted in decreased vascular plant percent cover and species richness. Few significant patterns were observed with individual plant species and total biomass. The phosphorus-plant richness relationship was not significant, but species composition was significantly different among the low and high treatment comparisons. These results highlight a neglected effect of eutrophication in seasonal habitats. Interactions among ecosystem phases clearly require more attention empirically and theoretically. Management and restoration of temporally heterogeneous habitat, such as the endemic-rich CVP, need to consider the extensive effects of increased nutrient input.

Ecosystem-level effects of bioturbation by the tadpole shrimp Lepidurus packardi in temporary pond mesocosms

An example of ecosystem engineering gaining attention in aquatic systems is bioturbation, the disruption of sediment at the water–sediment interface due to burrowing and foraging. One consequence of bioturbation can be increased turbidity from suspended sediment, which generally inhibits macrophyte growth and reduces ecosystem functioning. Conversely, bioturbation may promote invertebrate species richness by unearthing dormant cysts. Temporary-pond crustaceans are not widely regarded as agents of bioturbation, but on the basis of aquaria observations we hypothesized that certain taxa can disturb the sediment and create highly turbid water. We tested this hypothesis by removing crustaceans from mesocosms lined with vernal pool soil. Compared to this treatment group, mesocosms containing crustaceans had extremely high turbidity from suspended sediment, as well as reduced total macrophyte cover. We also found clear compositional differences in macrophyte communities between treatments, driven largely by differences in water physicochemistry, including turbidity. Regression analysis linked most of the bioturbation to the endangered notostracan Lepiduruspackardi Simon 1886, which was a strong predictor of turbidity in our mesocosms. We also found a trend toward increased crustacean species richness in our mesocosms in the presence of this taxon. An analysis of published data from King et al. (1996) suggests that this trend may extend to natural vernal pools. Overall, our results suggest that L. packardi may have large effects on vernal pool communities, likely mediated in part through its disturbing of the sediment.