Cascades In Food Webs Research Articles

Intraguild predation dampens trophic cascades in shallow aquatic mesocosms in the subtropics: Implications for lake restoration by biomanipulation

Abstract Intraguild predation (IGP), defined as killing and eating among potential competitors, is commonly observed in shallow lakes and is predicted to dampen trophic cascades and affect ecosystem properties (e.g. phytoplankton biomass or primary production). We tested this hypothesis by manipulating the density of two common lake predators, the small‐sized fish Pelteobagrus fulvidraco (the intraguild predator) and the shrimp Exopalaemon modestus (the intraguild prey), in outdoor mesocosms containing natural phytoplankton and zooplankton communities. In single predator treatments, both predators induced a strong trophic cascade, as evidenced by extinction of the key herbivore, Daphnia pulex, and increasing phytoplankton biomass and chlorophyll a concentration. When the two predators were added together, however, the strength of collective predator effects on zooplankton and the cascading effects on phytoplankton growth were weaker than the sum of the individual predator effects. We attributed this antagonism among predators to intraguild predation as demonstrated by the lower shrimp catch per unit effort in the presence of fish. Our study suggests that common predators in subtropical shallow lakes may act antagonistically and thereby dampen trophic cascades in food webs. Thus, when implementing biomanipulation efforts to suppress algal growth via enhancing zooplankton herbivory, both the quantity and diversity of lake predator assemblies should be considered.

Habitat coupling mediates trophic cascades in an aquatic community

AbstractTrophic cascades and other indirect effects can significantly mediate community interactions. Movement of energy between systems has been shown to be important for trophic cascades in food webs, where coupling between habitats can be important for food web stability and species evenness. To investigate the effects of habitat coupling on the stability and dynamics of chlorophyll a (used as a proxy for phytoplankton biomass), mediated by the abundance and composition of zooplankton and macroinvertebrates, we manipulated habitat use by the predator perch. We show a greater indirect effect of predation on phytoplankton abundance when no habitat coupling occurs, indicating a stronger predation effect and a decrease in zooplankton grazing pressure leading to an increase in phytoplankton biomass. Although we found a significant effect on chlorophyll a between the treatments, this effect was not evident in the abundance of prey resources of perch (zooplankton and macroinvertebrates). Other indirect effects, not measured in this study, such as compositional changes in prey groups, could potentially explain the lack of effect in prey resources. While there is a strong theoretical argument for the stabilizing effects of habitat coupling, empirical evidence is scarce. Our study offers tentative support for these theoretical predictions in a natural system.

Compensation masks trophic cascades in complex food webs

Ecological networks, or food webs, describe the feeding relationships between interacting species within an ecosystem. Understanding how the complexity of these networks influences their response to changing top-down control is a central challenge in ecology. Here, we provide a model-based investigation of trophic cascades — an oft-studied ecological phenomenon that occurs when changes in the biomass of top predators indirectly effect changes in the biomass of primary producers — in complex food webs that are representative of the structure of real ecosystems. Our results reveal that strong cascades occur primarily in low richness and weakly connected food webs, a result in agreement with some prior predictions. The primary mechanism underlying weak or absent cascades was a strong compensatory response; in most webs, predators induced large population level cascades that were masked by changes in the opposite direction by other species in the same trophic guild. Thus, the search for a general theory of trophic cascades in food webs should focus on uncovering features of real ecosystems that promote biomass compensation within functional guilds or trophic levels.

Missing lynx and trophic cascades in food webs: A reply to Ripple et al.

AbstractRipple et al. (2011) proposed a hypothesis that the recovery of gray wolves (Canis lupus) may positively affect the viability of threatened Canada lynx (Lynx canadensis) populations in the contiguous United States through indirect species interactions. Ripple et al. (2011) proposed 2 key trophic linkages connecting wolf restoration with lynx recovery. First, recovering wolf populations may benefit lynx through reduced interference and exploitative competition with coyotes (C. latrans). Second, recovering wolf populations may benefit lynx through reduced exploitative competition among ungulates and snowshoe hares (Lepus americanus), the primary prey of lynx. Both proposed linkages have weak or contradictory empirical support in the available literature on lynx–hare ecology, casting doubt on the utility of Ripple et al.'s (2011) hypothesis. Debate over Ripple et al.'s (2011) hypothesis demonstrates the importance of experimental or comparative documentation when proposing trophic cascades in complex food webs. In this case, publishing unsupported opinions as hypotheses that concern complex trophic interactions is a potential disservice to lynx conservation through misallocated research, conservation funding, and misplaced public perception. © 2012 The Wildlife Society.

Dominance and species co-occurrence in highly diverse ant communities: a test of the interstitial hypothesis and discovery of a three-tiered competition cascade

The role of competitive exclusion is problematic in highly diverse ant communities where exceptional species richness occurs in the face of exceptionally high levels of behavioural dominance. A possible non-niche-based explanation is that the abundance of behaviourally dominant ants is highly patchy at fine spatial scales, and subordinate species act as insinuators by preferentially occupying these gaps--we refer to this as the interstitial hypothesis. To test this hypothesis, we examined fine-scale patterns of ant abundance and richness according to a three-tiered competition hierarchy (dominants, subdominants and subordinates) in an Australian tropical savanna using pitfall traps spaced at 2 m intervals. Despite the presence of gaps in the fine-scale abundance of individual species, the combined abundance of dominant ants (species of Iridomyrmex, Papyirus and Oecophylla) was relatively uniform. There was therefore little or no opportunity for subordinate species to preferentially occupy gaps in the foraging ranges of dominant species, and we found no relationship between the abundance of dominant ants and nondominant species richness at fine spatial scales. However, we found a negative relationship between subdominant and subordinate ants, a negative relationship between dominant and subdominant ants, and a positive relationship between dominant and subordinate ants. These results suggest that dominant species actually promote species richness by neutralizing the effects of subdominant species on subordinate species. Such indirect interactions have very close parallels with three-tiered trophic cascades in food webs, and we propose a "competition cascade" where the interactions are through a competition rather than trophic hierarchy.

Fluctuations in density of an outbreak species drive diversity cascades in food webs

Patterns in food-web structure have frequently been examined in static food webs, but few studies have attempted to delineate patterns that materialize in food webs under nonequilibrium conditions. Here, using one of nature's classical nonequilibrium systems as the food-web database, we test the major assumptions of recent advances in food-web theory. We show that a complex web of interactions between insect herbivores and their natural enemies displays significant architectural flexibility over a large fluctuation in the natural abundance of the major herbivore, the spruce budworm (Choristoneura fumiferana). Importantly, this flexibility operates precisely in the manner predicted by recent foraging-based food-web theories: higher-order mobile generalists respond rapidly in time and space by converging on areas of increasing prey abundance. This "birdfeeder effect" operates such that increasing budworm densities correspond to a cascade of increasing diversity and food-web complexity. Thus, by integrating foraging theory with food-web ecology and analyzing a long-term, natural data set coupled with manipulative field experiments, we are able to show that food-web structure varies in a predictable manner. Furthermore, both recent food-web theory and longstanding foraging theory suggest that this very same food-web flexibility ought to be a potent stabilizing mechanism. Interestingly, we find that this food-web flexibility tends to be greater in heterogeneous than in homogeneous forest plots. Because our results provide a plausible mechanism for boreal forest effects on populations of forest insect pests, they have implications for forest and pest management practices.

Response of a stream diatom community to top predator manipulations

I investigated the effects top predator manipulations have on the community structure of benthic diatoms in a small, third-ordered stream ecosystem. Following the concept of trophic cascades in food webs, changes in top predator abundance should, via herbivore elimination, influence community metrics and species composition of stream periphyton. Using in situ stream enclosures, I varied the density of two common stream predators, sculpin and a perlid stonefly larva. Over three weeks, in three seasons, I sampled both invertebrate prey and diatoms from tiles placed in each enclosure. Diatoms did respond to predator manipulations. Changes in diversity, species richness, evenness, and dominance varied with treatment and between seasons. Cluster analysis showed pronounced assemblage dissimilarities before and after predator manipulations. However, species composition did not change between treatments, though rankings of the ten most abundant species did shift. Top predators can alter the community composition and structure of stream benthic algae indirectly through grazer suppression and directly by disruption due to foraging activity. This study demonstrates that top predators can impact nonadjacent trophic levels and that predator type and abundance should be considered when comparing diatom communities temporally and spatially.

IMPACT OF A DETRITAL SUBSIDY ON TROPHIC CASCADES IN A TERRESTRIAL GRAZING FOOD WEB

Theory predicts that allochthonous energy subsidies can modify the strength of trophic cascades in food webs. We tested the hypothesis that a detrital subsidy would enhance trophic cascades in the grazing food web of an agroecosystem in which generalist predators, wolf spiders and carabid beetles, feed both on detritivores and herbivores. We crossed detritus-addition with predator-removal treatments in replicated 8 × 8 m open and fenced plots of cucumbers and squash. Collembola, a major detritivore, became 2–4 times more abundant in detritus-addition than control plots; densities of other detritivores also increased. Although the detrital subsidy initially increased ground beetle numbers and produced a 2–3-fold increase in wolf spider densities, we found no consistent evidence of strong trophic cascades measured as changes in fruit yield in either detritus-addition or predator-removal treatments. This result contrasts with predator-removal experiments in previous years in the same study area, which revealed strong cascades affecting cucumber yield. We hypothesize that (1) competition between cucurbits and fungi for nitrogen in the detritus-addition plots, (2) unusually high herbivore densities in the spring cucumber crop, and low densities on the summer squash crop, (3) changes in patterns of intraguild predation in the detritus-addition plots, and (4) perhaps a switch by generalist predators to prey of the detrital food web all may have contributed to preventing the detrital subsidy from benefiting primary producers via an enhanced trophic cascade.

TROPHIC CASCADES AND COMPENSATION: DIFFERENTIAL RESPONSES OF MICROZOOPLANKTON IN WHOLE-LAKE EXPERIMENTS

Food webs in three lake basins were manipulated by altering fish commu- nities to either reduce or increase the abundance of Daphnia. These basins were subsequently fertilized with nitrogen and phosphorus for two years. We tested three predictions about the response of heterotrophic flagellates, ciliates, and rotifers (collectively, microzooplank- ton) derived from prior studies. We predicted that (1) microzooplankton would increase with lake fertilization, (2) lakes with abundant Daphnia would have lower increases in microzooplankton, and (3) both increases in resource availability and suppression by Daph- nia would determine microzooplankton dynamics. Contrary to the first prediction, micro- zooplankton did not increase with fertilization relative to the reference lake, except in the low-Daphnia system. The second prediction was supported, as Daphnia prevented micro- zooplankton from increasing in the fertilized lakes with the strength of the Daphnia effect being greater than anticipated. Because of this strong effect, microzooplankton dynamics were in all but one case most strongly related to suppression by Daphnia rather than to a combined effect of resources and suppression. The microzooplankton communities were differentially affected by the trophic cascade. Heterotrophic flagellates appeared to be limited by a variety of predators. Even in the low-Daphnia fertilized lake, mortality was apparently high. Ciliates and rotifers increased in the low-Daphnia fertilized lake and were strongly suppressed otherwise. These experiments indicate that small-scale, short-term ex- periments and larger-scale comparative analyses may be inadequate for assessing the strength of trophic interactions. The potential for community-level responses, not well assessed except at the ecosystem scale, may alternatively dampen or enhance the impacts of trophic cascades in food webs.