Non-Linear Interactions between Consumers and Flow Determine the Probability of Plant Community Dominance on Maine Rocky Shores

Although consumers can strongly influence community recovery from disturbance, few studies have explored the effects of consumer identity and density and how they may vary across abiotic gradients. On rocky shores in Maine, recent experiments suggest that recovery of plant- or animal- dominated community states is governed by rates of water movement and consumer pressure. To further elucidate the mechanisms of consumer control, we examined the species-specific and density-dependent effects of rocky shore consumers (crabs and snails) on community recovery under both high (mussel dominated) and low flow (plant dominated) conditions. By partitioning the direct impacts of predators (crabs) and grazers (snails) on community recovery across a flow gradient, we found that grazers, but not predators, are likely the primary agent of consumer control and that their impact is highly non-linear. Manipulating snail densities revealed that herbivorous and bull-dozing snails (Littorina littorea) alone can control recovery of high and low flow communities. After ∼1.5 years of recovery, snail density explained a significant amount of the variation in macroalgal coverage at low flow sites and also mussel recovery at high flow sites. These density-dependent grazer effects were were both non-linear and flow-dependent, with low abundance thresholds needed to suppress plant community recovery, and much higher levels needed to control mussel bed development. Our study suggests that consumer density and identity are key in regulating both plant and animal community recovery and that physical conditions can determine the functional forms of these consumer effects

A dc-link voltage stability analysis technique for hybrid five-phase open-end winding drives

This paper studies the dc-link voltage stability for a hybrid five-phase open-end winding (OeW) drive operating under carrier based (CB) pulse-width modulation (PWM). The drive consists of a five-phase induction machine, supplied using one three-level and one two-level voltage source inverter (VSI). This configuration is analysed for the case of isolated dc-link rails, while dc-link voltage ratio is considered as an additional degree of freedom. It is demonstrated that different dc-link voltage ratios lead to the different overall number of voltage levels across stator windings. Modulation strategies are investigated and their performances are analysed from the dc-link voltages stability point of view. An analytical method for dc-link voltage stability analysis is presented. Results show that the four-level configuration always leads to stable dc-link voltages, regardless of the modulation strategy. On the other hand, if six-level configuration is combined with modulation strategies that lead to an optimal harmonic performance, not all dc-link capacitor voltages will be in balance depending on the operating conditions

Data from: Cascading effects of a top predator on intraspecific competition at intermediate and basal trophic levels

1. Predators can impact competition among prey by altering prey density via consumption or by causing prey to modify their traits or foraging behavior. Yet, differences between these two mechanisms may lead to different cascading impacts on lower trophic levels. 2. Using a crab-snail-barnacle rocky intertidal food chain, we tested the effects of predation risk from crabs (top predators) on intraspecific competition among snails (intermediate consumers) and emergent indirect effects on the density of and competition between barnacles (basal resources). 3. The per capita foraging and growth rates of snails declined with high conspecific density. Predation risk from crabs, which caused even larger reductions in snail foraging and growth, weakened competition among snails, whereas a 45% increase in barnacle density had no detectable effect on snail competition. 4. Intraspecific competition between barnacles, however, depended on the interactive effects of barnacle density, snail density, and crab predation risk. Barnacles developed hummocking morphologies as they grew and competed for space. Hummock formation (a proxy for competition) increased as a result of either greater initial barnacle density or reduced snail foraging pressure, but these effects depended on predation risk. 5. The effects of crab predation risk on snail foraging behavior weakened an otherwise strong relationship between barnacle density and hummock development: hummocking increased with barnacle density in the absence of crabs but remained relatively high when crabs were present. In communities with similar final barnacle densities, hummocking was more common in those with crabs than those without crabs. 6. The extent to which predators can drive trophic cascades by suppressing the foraging rates of their prey is highly context-dependent: the positive trait-mediated indirect effect of predators on basal resource abundance is stronger when many prey respond simultaneously to the threat of predation. However, our results demonstrate that top predators can also enhance competition among basal resources even when their indirect effect on resource abundance is relatively weak. Hence, the cascading effects of predators on competition within lower trophic levels may play an important but underappreciated role in the dynamics of basal resource populations and the communities they support

Data from: Moving beyond linear food chains: trait-mediated indirect interactions in a rocky intertidal food web

In simple, linear food chains, top predators can have positive indirect effects on basal resources by causing changes in the traits (e.g. behaviour, feeding rates) of intermediate consumers. Although less is known about trait-mediated indirect interactions (TMIIs) in more complex food webs, it has been suggested that such complexity dampens trophic cascades. We examined TMIIs between a predatory crab (Carcinus maenas) and two ecologically important basal resources, fucoid algae (Ascophyllum nodosum) and barnacles (Semibalanus balanoides), which are consumed by herbivorous (Littorina littorea) and carnivorous (Nucella lapillus) snails, respectively. Because crab predation risk suppresses snail feeding rates, we hypothesized that crabs would also shape direct and indirect interactions among the multiple consumers and resources. We found that the magnitude of TMIIs between the crab and each resource depended on the suite of intermediate consumers present in the food web. Carnivorous snails (Nucella) transmitted TMIIs between crabs and barnacles. However, crab–algae TMIIs were transmitted by both herbivorous (Littorina) and carnivorous (Nucella) snails, and these TMIIs were additive. By causing Nucella to consume fewer barnacles, crab predation risk allowed fucoids that had settled on or between barnacles to remain in the community. Hence, positive interactions between barnacles and algae caused crab–algae TMIIs to be strongest when both consumers were present. Studies of TMIIs in more realistic, reticulate food webs will be necessary for a more complete understanding of how predation risk shapes community dynamics

Fig.5.App.D.Consumer.Dens

These data were used to generate Figure 5 (and the analyses in Appendix D), which portray consumer densities on wave-exposed and sheltered rocky intertidal shores in the Gulf of Maine

Data from: Broad-scale geographic variation in the organization of rocky intertidal communities in the Gulf of Maine

A major challenge facing ecology is to better understand how large-scale processes modify local scale processes to shape the organization of ecological communities. Although the results of ecological experiments are repeatable on local scales, different results often emerge across broad scales, which can hinder the development of general predictions that apply across the geographical range of a community. Numerous studies in the southern Gulf of Maine have shaped our understanding of community organization and dynamics on New England rocky intertidal shores, where consumers strongly control recovery from disturbance on sheltered shores and high invertebrate recruitment and competition for space dictate recovery on wave-exposed shores. It is unclear, however, whether the effects of consumers and recruitment variation on resulting community organization in this region apply more broadly to rocky intertidal habitats throughout the Gulf. We characterized variation in rocky intertidal community structure at 34 sites throughout the Gulf of Maine and experimentally examined the influence of consumers (present, absent) and wave energy (wave-exposed, sheltered) on community recovery from disturbance in the northern and southern Gulf. Our results reinforced previous work in the southern Gulf because consumers dictated the recovery of fucoid algae and mussels on sheltered shores, whereas high barnacle and mussel recruitment and competition for space shaped recovery on wave-exposed shores. However, on sheltered shores in the northern Gulf, neither consumers nor barnacle and mussel recruitment impacted recovery, which was dominated by fucoid algae. Moreover, recovery on wave-exposed shores in the northern Gulf was quite distinct from that observed in the southern Gulf: barnacle and mussel recruitment was negligible and fucoid algae dominated recovery including the long-term establishment of Ascophyllum nodosum, which is largely absent from wave-exposed shores in the southern Gulf. Thus, distinct community types emerged in the northern and southern Gulf despite their sharing many of the same species. These patterns likely emerged because of regional differences in coastal oceanography that dictate the recruitment of barnacles and mussels. Hence, increased attention to regional factors should provide key insight into how rocky shore communities are organized in the Gulf of Maine and elsewhere