Trophic trickles rather than cascades: Conditional top‐down and bottom‐up dynamics in an Australian chenopod shrubland

Dawes-Gromadzki, Tracy Z.

doi:10.1046/j.1442-9993.2002.01210.x

Cited by 23 publications

(9 citation statements)

References 76 publications

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“…Consequently, after 2 years of treatment, bird exclusion gave way to a significant decrease in average leaf size. These patterns concur with other studies suggesting that trophic cascades might be conditional on the timing and extent of rainfall (Dawes‐Gromadzki 2002; Ovadia & Schmitz 2004).…”

Section: Discussionsupporting

confidence: 92%

Variable strength of top‐down effects inNothofagusforests: bird predation and insect herbivory during an ENSO event

Mazía¹,

Chaneton²,

Kitzberger³

et al. 2009

Austral Ecology

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Predators are thought to play a key role in controlling herbivory, thus having positive indirect effects on plants. However, evidence for terrestrial trophic cascades is still fragmentary, perhaps due to variation in top-down forces created by environmental heterogeneity. We examined the magnitude of predation effects on foliar damage by chewing insects and mean leaf size, by excluding birds from saplings in 'dry' and 'wet' Nothofagus pumilio forests in the northern Patagonian Andes, Argentina.The experiment lasted 2 years encompassing a severe drought during the La Niña phase of a strong El Niño/Southern Oscillation event, which was followed by unusually high background folivory levels. Insect damage was consistently higher in wet than in dry forest saplings. In the drought year (1999), bird exclusion increased folivory rates in both forests but did not affect tree leaf size. In the ensuing season (2000), leaf damage was generally twice as high as in the drought year. As a result, bird exclusion not only increased the extent of folivory but also significantly decreased sapling leaf size.The latter effect was stronger in the wet forest, suggesting compensation of leaf area loss by dry forest saplings. Overall, the magnitude of predator indirect effects depended on the response variable measured. Insectivorous birds were more effective at reducing folivory than at facilitating leaf area growth. Our results indicate that bird-initiated trophic cascades protect N. pumilio saplings from insect damage even during years with above-normal herbivory, and also support the view that large-scale climatic events influence the strength of trophic cascades.

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Section: Discussionsupporting

confidence: 92%

Variable strength of top‐down effects inNothofagusforests: bird predation and insect herbivory during an ENSO event

Mazía¹,

Chaneton²,

Kitzberger³

et al. 2009

Austral Ecology

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“…Full factorial manipulations are necessary to quantitatively assess the direction of control for community biomass distribution. Although the relatively more pervasive influence of predation compared to system productivity has been found in individual empirical studies (e.g., Dawes-Gromadzki 2002, Boyer et al 2003, our study quantifies this relationship and highlights the generality of this finding across ecosystems. In addition, our results demonstrate that the decoupling in community dynamics between herbivores and plants observed in marine systems (Menge 2000) is general to freshwater, marine, and terrestrial communities.…”

Section: Discussionsupporting

confidence: 63%

Asymmetry in Community Regulation: Effects of Predators and Productivity

Borer

Halpern

Seabloom

2006

Ecology

126

135

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Eutrophication and predator additions and extinctions are occurring in ecosystems worldwide. Although theory predicts that both will strongly alter the distribution of biomass in whole communities, empirical evidence has not been consolidated to quantitatively determine whether these theoretical predictions are generally borne out in real ecosystems. Here we analyze data from two types of trophic cascade studies, predator removals in factorial combination with fertilization and observed productivity gradients, to assess the role of top-down and bottom-up forces in structuring multi-trophic communities and compare results from these analyses to those from an extensive database of trophic cascade studies. We find that herbivore biomass declines and plant biomass increases in the presence of predators, regardless of system productivity. In contrast, while plants are increased by fertilization, this effect does not significantly increase herbivores in either the presence or absence of predators. These patterns are consistent among marine, freshwater, and terrestrial ecosystems and are largely independent of study size and duration. Thus, top-down effects of predation are transferred through more trophic levels than are bottom-up effects of eutrophication, showing strong asymmetry in the direction of control of biomass distribution in communities.

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“…Closer examination of the studies that have not uncovered bottom-up cascades could reveal that, like the Piper system, indirect positive effects are not possible due to an increase in plant defense. For example, Dawes-Gromadzki (2002) investigated the thermodynamic bottom-up model for arthropods of a chenopod shrubland community and found only weak direct effects of enhanced nutrients on herbivores and no effects on upper trophic levels. Plants in the Chenopodiaceae are rich sources of both carbon-based (e.g., flavonoids, terpenoids, and steroids) and nitrogenbased (alkaloids) defenses (Aljaber et al 1992, Alsaleh et al 1997.…”

Section: Discussionmentioning

confidence: 99%

Ecological Causes and Consequences of Variation in Defensive Chemistry of a Neotropical Shrub

Dyer

Letourneau

Dodson

et al. 2004

Ecology

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Prominent models of herbivore regulation focus on predators, low plant biomass, or poor resource quality as factors that limit herbivore populations. We examined predictions of these tritrophic models for herbivores on the understory shrub, Piper cenocladum, which is defended by mutualistic ants (Pheidole bicornis) and three amide secondary metabolites. To examine sources of variation in P. cenocladum amide content and to compare the effects of amides vs. ants on herbivores, we used three linked experiments in the field and in shadehouses. We manipulated light, nutrient availability, and presence of symbionts for experimental plant fragments and shrubs and then quantified leaf amide concentration. We also examined relationships between amide content and damage by the three most common groups of folivores on P. cenocladum: generalist orthopterans, specialist coleopterans, and specialist lepidopterans. For all experiments, enhanced resources and absence of symbionts caused higher levels of amides. These increased chemical defenses had strong effects on generalist herbivores in this system, while the ant predators were more effective at depressing herbivory by specialists. The negative effects of amides on specialist lepidopterans were small, suggesting that these herbivores are adapted to chemical defenses in their host plant. It is possible that our results are part of a more general trend where top-down effects are stronger against specialist herbivores, while chemical defenses are more effective against generalists. We concluded that different models of herbivore regulation were supported by components of the P. cenocladum arthropod community, depending on resource availability and on the portion of the web examined.

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Trophic trickles rather than cascades: Conditional top‐down and bottom‐up dynamics in an Australian chenopod shrubland

Cited by 23 publications

References 76 publications

Variable strength of top‐down effects inNothofagusforests: bird predation and insect herbivory during an ENSO event

Variable strength of top‐down effects inNothofagusforests: bird predation and insect herbivory during an ENSO event

Asymmetry in Community Regulation: Effects of Predators and Productivity

Ecological Causes and Consequences of Variation in Defensive Chemistry of a Neotropical Shrub

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