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Recruitment Failure (recruitment + failure)

Distribution by Scientific Domains

Life Sciences	80%

Selected Abstracts

A new role for MSY in single-species and ecosystem approaches to fisheries stock assessment and management

FISH AND FISHERIES, Issue 1 2001

In 1977, Peter Larkin published his now-famous paper, ,An epitaph for the concept of maximum sustained yield'. Larkin criticized the concept of single-species maximum sustained yield (MSY) for many reasons, including the possibility that it may not guard against recruitment failure, and the impossibility of maximising sustainable yields for all species simultaneously. However, in recent years, there has been a fundamental change in the perception of the fishing mortality associated with MSY (FMSY) as a limit to be avoided rather than a target that can routinely be exceeded. The concept of FMSY as a limit is embodied in several United Nations Food and Agriculture Organization (FAO) agreements and guidelines, and has now been incorporated into the US Magnuson,Stevens Fishery Conservation and Management Act. As a result, the United States now requires the development of overfishing definitions based on biological reference points that treat the FMSY as a limit reference point and must also define a lower limit on biomass below which rebuilding plans with strict time horizons must be developed. This represents a major paradigm shift from the previously mandated (but often unachieved) objective to simply maintain fishing mortalities at levels below those associated with recruitment overfishing. In many cases, it requires substantial reductions in current fishing mortality levels. Therefore, the necessity of the new paradigm is continually questioned. This paper draws on examples from several fisheries, but specifically focuses on the recent US experience illustrating the practical difficulties of reducing fishing mortality to levels below those corresponding to MSY. However, several studies suggest that even more substantial reductions in fishing mortality may be necessary if ecosystem considerations, such as multispecies interactions, maintenance of biodiversity and genetic diversity, and reduction of bycatch and waste, are taken into account. The pros and cons of moving beyond single-species assessment and management are discussed. A US plan for improving stock assessments indicates that even a ,basic' objective such as ,adequate baseline monitoring of all managed species' may be extremely costly. Thus, the suggestion of Larkin (1983, 1997) that the costs of research and management should not exceed 10,20% of the landed value of the catch may preclude comprehensive ecosystem management. More importantly, neither single-species nor ecosystem-based fisheries management is likely to improve appreciably unless levels of fishing capacity are aligned with resource productivity, as is currently being promoted by FAO and several individual nations. [source]

Vegetation dynamics of predator-free land-bridge islands

JOURNAL OF ECOLOGY, Issue 2 2006
JOHN TERBORGH
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. 2Our 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. 3To 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. 4Herbivore 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. 5Small 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. 6Sapling decline occurred earlier on small islands, although recruitment failure had become equally pronounced on medium islands by the end of the monitoring period. 7Several 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. [source]

MARINE PROTECTED AREA PERFORMANCE IN A MODEL OF THE FISHERY

NATURAL RESOURCE MODELING, Issue 4 2002
USSIF RASHID SUMAILA
ABSTRACT. What bio-economic benefits can be expected from the implementation of marine protected areas (MPAs) in a fishery facing a shock in the form of recruitment failure, and managed jointly compared to separately? What are the optimal sizes of MPAs under cooperation and non-cooperation? I explore these questions in the current paper by developing a computational two-agent model, which incorporates MPAs using the North East Atlantic codfishery as an example. Results from the study indicate that MPAs can protect the discounted economic rent from the fishery if the habitat is likely to face a shock, andfishers have a high discount rate. The total standing biomass increases with increasing MPA size but only up to a point. Basedon the specifics of the model, the study also shows that the economically optimal size of MPA for cod varies between 50 70% depending on (i) the exchange rate between the protectedandunprotectedareas of the habitat; (ii) whether fishers behalf cooperatively or non-cooperatively; and(iii) the severity of the shock that the ecosystem may face. [source]

Spatial distribution and prediction of seed production by Eucalyptus microcarpa in a fragmented landscape

AUSTRAL ECOLOGY, Issue 1 2010
PETER A. VESK
Abstract Woodlands worldwide have been greatly modified by clearing for agriculture, and their conservation and restoration requires understanding of tree recruitment processes. Seed production is one possible point of recruitment failure, and one that the spatial arrangement of trees may affect. We sampled 118 Eucalyptus microcarpa (Myrtaceae) trees to compare and analyse the determinants of seed production in this dominant tree of modified, fragmented temperate grassy woodlands, which extend over much of southeastern Australia. Fecundity was estimated as the seed crop measured on leaf mass and whole tree bases and was compared between categories of tree configuration. We also modelled fecundity using boosted regression trees, a new and flexible tool. Fecundity on a leaf mass basis was predominantly influenced by environmental factors (topographic ,wetness', slope, soil type), rather than by local tree density and configuration. Fewer seed per unit leaf mass were produced on flat and topographically wet sites, reflecting poor tolerance of waterlogging by E. microcarpa. By contrast, whole tree fecundity was little influenced by environmental factors. Local tree density and configuration did influence whole tree fecundity, which was high in solitary and woodland-spaced trees and reduced under high local density. We found little evidence for reduced fecundity of E. microcarpa in solitary trees. This points to the importance of scattered trees as sources of seed for tree recruitment and for natural regeneration of landscape level tree cover. Considerable uncertainty remains in modelled seed supply, and may be reduced with sampling across multiple years and greater environmental and spatial domains. [source]

Forest Elephants: Tree Planters of the Congo

BIOTROPICA, Issue 4 2009
Stephen Blake
ABSTRACT The abundance of large vertebrates is rapidly declining, particularly in the tropics where over-hunting has left many forests structurally intact but devoid of large animals. An urgent question then, is whether these ,empty' forests can sustain their biodiversity without large vertebrates. Here we examine the role of forest elephant (Loxodonta africana cyclotis) seed dispersal in maintaining the community structure of trees in the Ndoki Forest, northern Congo. Analysis of 855 elephant dung piles suggested that forest elephants disperse more intact seeds than any other species or genus of large vertebrate in African forests, while GPS telemetry data showed that forest elephants regularly disperse seeds over unprecedented distances compared to other dispersers. Our analysis of the spatial distribution of trees from a sample of 5667 individuals showed that dispersal mechanism was tightly correlated with the scale of spatial aggregation. Increasing amounts of elephant seed dispersal was associated with decreasing aggregation. At distances of<200 m, trees whose seeds are dispersed only by elephants were less aggregated than the random expectation, suggesting Janzen,Connell effects on seed/seedling mortality. At the landscape scale, seed dispersal mode predicted the rate at which local tree community similarity decayed in space. Our results suggest that the loss of forest elephants (and other large-bodied dispersers) may lead to a wave of recruitment failure among animal-dispersed tree species, and favor regeneration of the species-poor abiotically dispersed guild of trees. [source]