Exploitation Ecosystems in Gradients of Primary Productivity

Oksanen, Lauri; Fretwell, Stephen D.; Arruda, Joseph A.; Niemelä, Pekka

doi:10.1086/283817

Cited by 1,548 publications

(1,574 citation statements)

References 22 publications

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“…At low productivities of the sprat resource (P S b 8.7 g V − 1 day − 1 ) cod does not encounter sufficient food to persist at all, while at high productivities of the sprat resource (P S N 16.0 g V − 1 day − 1 ) cod and sprat coexist. These predictions are in agreement with classic predictions about community structure in a food chain with three trophic levels as a function of system productivity (Hairston et al, 1960;Oksanen et al, 1981). In contrast, however, equilibria with and without cod both represent stable community states in between these two threshold productivities.…”

Section: Model Predictionssupporting

confidence: 87%

“…Most of the observed changes relate to a decrease in top-down effects as a result of the decline of cod as main predator. These changes can hence be explained in terms of a trophic cascade or a classic food chain (Hairston et al, 1960;Oksanen et al, 1981). However, such explanations cannot account for observed declines in cod size at age and cod body condition in the presence of high abundances of prey fish.…”

Section: Discussionmentioning

confidence: 99%

“…From the perspective of the community as a "classic" food chain with three trophic levels (Oksanen et al, 1981) a decrease in predator biomass would be expected to (1) decrease predation mortality on its main prey and hence lead to (2) an increase in prey biomass. Indirectly, the increase in prey biomass will lead to (3) a reduction in zooplankton abundance and hence to (4) increased competition among prey for food.…”

Section: Empirical Observationsmentioning

confidence: 99%

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How cod shapes its world

Leeuwen

Roos

Persson

2008

Journal of Sea Research

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Cod stocks in the North West Atlantic and the Baltic Sea have shown similar dynamics in recent decades with a rapid decline in abundance and a lack of stock recovery following a period of large biomass. We explore whether the lack of recovery can be ascribed to an emergent Allee effect, which is a mechanism intrinsic to the community in contrast to explanations involving environmental factors. We formulate a stage-structured biomass model for the cod-sprat interaction in the Baltic Sea, paying special attention to the size-dependent prey preference of differently sized cod. The model predicts that alternative community states can occur under the same environmental conditions, in which cod is either present or absent. In a stable equilibrium with its main prey cod has a strong effect on the prey size distribution, resulting in larger densities of preferred prey sizes for cod than in the absence of any predation. Cod thus shapes its food environment to its own benefit. Furthermore, in response to increased exploitation cod biomass and yield tend to increase unless a stock collapse is imminent. After a cod stock collapse and the consequent drop in predation the prey size distribution becomes stunted and offers insufficient food for cod to grow and recover. These results are consequences of the indirect effects of predation and harvesting, whereby increased mortality relaxes competition among surviving individuals, leading to an increase in food intake and hence increased somatic growth and reproduction. We review observed community changes following the collapse of the cod stocks in the North West Atlantic and the Baltic Sea in the light of model predictions. In line with our model predictions growth in body size of cod has slowed down after the collapse, despite high densities of prey biomass. Furthermore, estimates of total prey population fecundity in the Baltic Sea identify the emergent Allee effect as a potentially important mechanism contributing to the lack of cod recovery.

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Section: Model Predictionssupporting

confidence: 87%

Section: Discussionmentioning

confidence: 99%

Section: Empirical Observationsmentioning

confidence: 99%

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How cod shapes its world

Leeuwen

Roos

Persson

2008

Journal of Sea Research

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“…Since the landmark paper by Hairston et al (1960), ecologists have been debating whether herbivores are limited by plant defences or by predators (Murdoch 1966;Oksanen et al 1981;Strong 1992). The matter is trivially simple in principle, but in practice, the challenge of experimentally creating predator-free environments in which herbivores can increase without constraint has proven almost insurmountable (Marquis & Whelan 1994;Schmitz et al 2000;Sinclair et al 2000).…”

Section: Resultsmentioning

confidence: 99%

Vegetation dynamics of predator‐free land‐bridge islands

et al. 2006

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Summary 1We tested the 'green world' hypothesis of Hairston, Smith and Slobodkin by monitoring vegetation change on recently created predator-free land-bridge islands in a huge hydroelectric impoundment, Lago Guri, in the State of Bolivar, Venezuela. 2 Our results affirm the green world hypothesis and expose the operation of a strong top-down trophic cascade that negatively impacted nearly every plant species present, implying that community stability is maintained through the action of predators. 3 To test the hypothesis, we monitored vegetation on nine predator-free islands and compared demographic parameters to those observed at control sites supporting complete or nearly complete suites of predators. 4 Herbivore abundance was high on 'small' ( ≥ 0.5, < 2 ha) islands, moderate on 'medium' islands (> 3, < 15 ha) and low on the 'large' landmasses that served for reference. 5 Small sapling densities on small islands were only 37% of controls in 1997 (after 11 years of isolation), and when recensused in 2002, had fallen to 25% of controls. High mortality and, especially, low recruitment contributed to the decline in sapling cohorts. 6 Sapling decline occurred earlier on small islands, although recruitment failure had become equally pronounced on medium islands by the end of the monitoring period. 7 Several mechanisms could potentially account for suppressed sapling recruitment, but the weight of evidence points to herbivory on seedlings and small saplings by leaf-cutter ants ( Atta spp. and Acromyrmex sp.). Exposure to prevailing trade winds (windward vs. leeward slopes of islands) had no detectable effect on the density or diversity of seedlings or saplings.

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“…Eventually, a trophic level may contain insufficient energy to support additional ones. The number of trophic levels in a system may thus depend on energy received by the base trophic level ; additional energy may enable extra trophic levels to arise, which support novel consumer species, generating positive species-energy relationships (Oksanen et al, 1981;Fretwell, 1987 ;Schoener, 1990;Kaunzinger & Morin, 1998).…”

Section: (6 ) More Trophic Levelsmentioning

confidence: 99%

Species–energy relationships at the macroecological scale: a review of the mechanisms

2005

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Correlations between the amount of energy received by an assemblage and the number of species that it contains are very general, and at the macro-scale such species-energy relationships typically follow a monotonically increasing curve. Whilst the ecological literature contains frequent reports of such relationships, debate on their causal mechanisms is limited and typically focuses on the role of energy availability in controlling the number of individuals in an assemblage. Assemblages from high-energy areas may contain more individuals enabling species to maintain larger, more viable populations, whose lower extinction risk elevates species richness. Other mechanisms have, however, also been suggested. Here we identify and clarify nine principal mechanisms that may generate positive species-energy relationships at the macro-scale. We critically assess their assumptions and applicability over a range of spatial scales, derive predictions for each and assess the evidence that supports or refutes them. Our synthesis demonstrates that all mechanisms share at least one of their predictions with an alternative mechanism. Some previous studies of species-energy relationships appear not to have recognised the extent of shared predictions, and this may detract from their contribution to the debate on causal mechanisms. The combination of predictions and assumptions made by each mechanism is, however, unique, suggesting that, in principle, conclusive tests are possible. Sufficient testing of all mechanisms has yet to be conducted, and no single mechanism currently has unequivocal support. Each may contribute to species-energy relationships in some circumstances, but some mechanisms are unlikely to act simultaneously. Moreover, a limited number appear particularly likely to contribute frequently to species-energy relationships at the macro-scale. The increased population size, niche position and diversification rate mechanisms are particularly noteworthy in this context.

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Exploitation Ecosystems in Gradients of Primary Productivity

Cited by 1,548 publications

References 22 publications

How cod shapes its world

How cod shapes its world

Vegetation dynamics of predator‐free land‐bridge islands

Species–energy relationships at the macroecological scale: a review of the mechanisms

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