Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    No evidence of modulation of indirect plant resistance of Brassica rapa plants by volatiles from soil-borne fungi
    Moisan, Kay ; Lucas-Barbosa, Dani ; Villela, Alexandre ; Greenberg, Liana O. ; Cordovez, Viviane ; Raaijmakers, Jos M. ; Dicke, Marcel - \ 2020
    Ecological Entomology 45 (2020)5. - ISSN 0307-6946 - p. 1200 - 1211.
    HIPVs - parasitoids - predators - recruitment

    Upon herbivory, plants emit specific herbivore-induced plant volatiles (HIPVs) that can attract natural enemies of the herbivore thus serving as indirect plant resistance. Not only insect herbivores, but microorganisms may also affect HIPV emission before or after plant colonisation, which in turn can affect behaviour of natural enemies of the herbivore. Yet, it remains elusive whether volatiles from microorganisms influence HIPV emission and indirect plant resistance. In this study, we investigated whether exposure of Brassica rapa roots to volatiles from soil-borne fungi influence HIPV emission and the recruitment of natural enemies of Pieris brassicae larvae. Using a two-compartment pot system, we performed greenhouse and common-garden experiments, and we profiled plant HIPV emission. We found that exposure of plant roots to fungal volatiles did not affect the number of P. brassicae larvae recollected from the plants, suggesting a neutral effect of the fungal volatiles on natural predation. Likewise, in a greenhouse, similar numbers of larvae were parasitised by Cotesia glomerata wasps on control plants as on fungal volatile-exposed plants. Additionally, chemical analysis of HIPV profiles revealed no qualitative and quantitative differences between control plants and fungal volatile-exposed plants that were both infested with P. brassicae larvae. Together, our data indicate that root exposure to fungal volatiles did not affect indirect plant resistance to an insect herbivore. These findings provide new insight into the influence of indirect plant resistance by fungal volatiles that are discussed together with the effects of fungal volatiles on direct plant resistance.

    The use and performance of survey-based pre-recruit abundance indices for possible inclusion in stock assessments of coastal-dependent species
    Pape, Olivier Le; Vermard, Youen ; Guitton, Jérome ; Brown, Elliot J. ; De Wolfshaar, Karen E. Van; Lipcius, Romuald N. ; Støttrup, Josianne G. ; Rose, Kenneth A. - \ 2020
    ICES Journal of Marine Science 77 (2020)5. - ISSN 1054-3139
    coastal nursery - forecast - juvenile habitat - recruitment - stock assessment - survey
    We reviewed the use of survey-based pre-recruit abundance indices in short-term recruitment forecasts for fish species relying on coastal habitats at the juvenile stage and that are assessed by ICES. We collated information from stock assessment reports and from a questionnaire filled out by the stock assessors. Among the 78 stocks with juvenile coastal dependence, 49 use short-term forecasts in stock assessment. Survey-based pre-recruit abundance indices were available for 35 of these stocks, but only 14 were used to forecast recruitment. The questionnaire indicated that the limited use of survey-based pre-recruit abundance indices was primarily due to sampling inefficiency, which may preclude reliable recruitment estimates. The sampling is inefficient because the juvenile coastal distribution is outside the geographical area covered by large-scale surveys or targeted coastal surveys are conducted on limited spatial and temporal scales. However, our analysis of the relationship between survey-based pre-recruit indices and assessment-generated recruitment indices revealed that survey-based pre-recruit abundance indices were sufficiently accurate to provide useful information for predicting future recruitment. We recommend expansion of the use of survey-based indices of pre-recruit abundance in stock assessment and recruitment forecasting, and consideration of how to include
    juveniles in ongoing and future surveys.
    Data from: Body stores persist as fitness correlate in a long-distance migrant released from food constraints
    Dokter, Adriaan M. ; Fokkema, Wimke ; Bekker, Steven K. ; Bouten, Willem ; Ebbinge, B.S. ; Muskens, G.J.D.M. ; Olff, H. ; Jeugd, Henk P. van der; Nolet, B.A. - \ 2018
    Netherlands Institute of Ecology (NIOO-KNAW)
    arctic waterfowl - cultivated grassland - recruitment - GPS tracking - migratory fueling - carry-over effects
    Long-distance migratory birds rely on acquisition of body reserves to fuel their migration and reproduction. Breeding success depends on the amount of body reserve acquired prior to migration, which is thought to increase with access to food at the fuelling site. Here we studied how food abundance during fuelling affected time budgets and reproductive success. In a regime of plenty, we expected that (1) limitations on food harvesting would become lifted, allowing birds to frequently idle, and (2) that birds would reach sufficient fuel loads, such that departure weight would no longer affect reproductive success. Our study system comprised brent geese (Branta b. bernicla) staging on high-quality agricultural pastures. Fuelling conditions were assessed by a combination of high-resolution GPS-tracking, acceleration-based behavioural classification, thermoregulation modelling, and measurements of food digestibility and excretion rates. Mark-resighting analysis was used to test for correlations between departure weight and offspring recruitment. Our results confirm that birds loafed extensively, actively postponed fuelling in early spring, and took frequent digestion pauses, suggesting that traditional time constraints on harvest and fuelling rates are absent on modern-day fertilized grasslands. Nonetheless, departure weight remained correlated with recruitment success. The persistence of this correlation after a prolonged stopover with access to abundant high-quality food, suggests that between-individual differences in departure condition are not so much enforced by food quality and availability during stopover, but reflect individual quality and longer-lived life-history traits, such as health status and digestive capacity, which may be developed before the fuelling period.
    Body stores persist as fitness correlate in a long-distance migrant released from food constraints
    Dokter, Adriaan M. ; Fokkema, Wimke ; Bekker, Steven K. ; Bouten, Willem ; Ebbinge, Barwolt S. ; Müskens, Gerard ; Olff, Han ; Jeugd, Henk P. van der; Nolet, Bart A. - \ 2018
    Behavioral Ecology 29 (2018)5. - ISSN 1045-2249 - p. 1157 - 1166.
    arctic waterfowl - carry-over effects - cultivated grassland - GPS tracking - migratory fueling - recruitment

    Long-distance migratory birds rely on the acquisition of body stores to fuel their migration and reproduction. Breeding success depends on the amount of body stores acquired prior to migration, which is thought to increase with access to food at the fueling site. Here, we studied how food abundance during fueling affected time budgets and reproductive success. In a regime of plenty, we expected that 1) limitations on food harvesting would become lifted, allowing birds to frequently idle, and 2) birds would reach sufficient fuel loads, such that departure weight would no longer affect reproductive success. Our study system comprised brent geese (Branta b. bernicla) staging on high-quality agricultural pastures. Fueling conditions were assessed by a combination of high-resolution GPS tracking, acceleration-based behavioral classification, thermoregulation modeling, and measurements of food digestibility and excretion rates. Mark-resighting analysis was used to test for correlations between departure weight and offspring recruitment. Our results confirm that birds loafed extensively, actively postponed fueling in early spring, and took frequent digestion pauses, suggesting that traditional time constraints on harvest and fueling rates are absent on modern-day fertilized grasslands. Nonetheless, departure weight remained correlated with recruitment success. The persistence of this correlation after a prolonged stopover with access to abundant high-quality food, suggests that between-individual differences in departure condition are not so much enforced by food quality and availability during stopover, but reflect individual quality and longer-lived life-history traits, such as health status and digestive capacity, which may be developed before the fueling period.

    Data from: Shifts in North Sea forage fish productivity and potential fisheries yield
    Clausen, Lotte W. ; Rindorf, Anna ; Deurs, Mikael van; Hintzen, N.T. - \ 2017
    Technical University of Denmark
    maximum sustainable yield - functional complimentarity - bottom-up effect - small pelagic fisheries - fisheries - regime shift - recruitment - growth
    Forage fish populations support large scale fisheries and are key components of marine ecosystems across the world, linking secondary production to higher trophic levels. While climate-induced changes in the North Sea zooplankton community are described and documented in literature, the associated bottom-up effects and consequences for fisheries remain largely unidentified. We investigated the temporal development in forage fish productivity and the associated influence on fisheries yield of herring, sprat, Norway pout and sandeel in the North Sea. Using principal component analysis, we analysed 40 years of recruitment success and growth proxies to reveal changes in productivity and patterns of synchroneity across stocks (i.e. functional complementarity). The relationship between forage fish production and Calanus finmarchicus (an indicator of climate change) was also analysed. We used a population model to demonstrate how observed shifts in productivity affected total forage fish biomass and fisheries yield. The productivity of North Sea forage fish changed around 1993 from a higher average productivity to lower average productivity. During the higher productivity period, stocks displayed a covariance structure indicative of functional complementarity. Calanus finmarchicus was positively correlated to forage fish recruitment, however, for growth, the direction of the response differed between species and time periods. Maximum sustainable yield (MSY) and the associated fishing mortality (Fmsy) decreased by 33%–68% and 26%–64%, respectively, between the higher and lower productivity periods. Synthesis and applications. The results demonstrate that fisheries reference points for short-lived planktivorous species are highly dynamic and respond rapidly to changes in system productivity. Furthermore, from an ecosystem-based fisheries management perspective, a link between functional complementarity and productivity, indicates that ecosystem resilience may decline with productivity. Based on this, we advise that system productivity, perhaps monitored as forage fish growth, becomes an integral part of management reference points; in both single species and ecosystem contexts. However, to retain social license of biological advice when fish catch opportunities are reduced, it is crucial that shifts in productivity are thoroughly documented and made apparent to managers and stakeholders.
    Abiotic and biotic drivers of biomass change in a Neotropical forest
    Sande, M.T. van der; Peña-Claros, M. ; Ascarrunz, N. ; Arets, E.J.M.M. ; Licona, J.C. ; Toledo, M. ; Poorter, L. - \ 2017
    Wageningen University & Research
    biomass growth - disturbance - ecosystem functioning - functional diversity - mortality - productivity - recruitment - soil conditions - species diversity
    Abiotic and biotic variables and growth, recruitment and mortality for 48 1-ha plots in a moist tropical forest in Bolivia
    FEM growth and yield data - selection forest - Het Oude Trekerbos
    Jansen, J.J. ; Klein, J.P.G. de; Mohren, G.M.J. ; Ouden, J. den - \ 2016
    Wageningen UR
    growth and yield - uneven-aged mixed species forest - selection cut - tree diameter - tree height - recruitment - ingrowth - transitions - monitoring - Douglas fir - Pseudotsuga mensisii - fir Abies spec - spruce - Picea spec. - pine Pinus spec - Japanese larch Larix kaempferi - Common Beech Fagus sylvatica - Common oak Quercus robur - Den Treek
    The current database is part of the FEM growth and yield database, a collection of growth and yield data from even-aged monocultures (douglas fir, common oak, poplar, Japanese Larch, Norway spruce, Scots pine, Corsican pine, Austrian pine, red oak and several other species, with only a few plots, even-aged mixed species forest plots, uneven-aged natural forest, uneven-aged selection forest and roadside plantattions of poplar. The FEM growth and yield data base is currently supervised by Jan den Ouden and Frits Mohren.
    FEM growth and yield data uneven-aged - Beech-oak
    Ouden, J. den; Jansen, J.J. ; Goudzwaard, L. ; Oldenburger, J.F. ; Mohren, G.M.J. - \ 2016
    Wageningen UR
    growth and yield - uneven-aged mixed species forest - tree diameter - tree height - crown class - coordinates stem positions - recruitment - transitions - monitoring - Boombos - Common beech Fagus sylvatica - Sessile oak Quercus petrea - Common oak Quercus robur - Holly llex aquifolium
    The current database is part of the FEM growth and yield database, a collection of growth and yield data from even-aged monocultures (douglas fir, common oak, poplar, Japanese Larch, Norway spruce, Scots pine, Corsican pine, Austrian pine, red oak and several other species, with only a few plots, even-aged mixed species forest plots, uneven-aged natural forest, uneven-aged selection forest and roadside plantattions of poplar. The FEM growth and yield data base is currently supervised by Jan den Ouden and Frits Mohren.
    FEM growth and yield data selection forest - Kolkbos
    Klein, J.P.G. de; Jansen, J.J. ; Mohren, G.M.J. ; Ouden, J. den - \ 2016
    Wageningen UR
    grown and yield - uneven-aged mixed species forest - selection cut - tree diameter - tree height - coordinates stem positions - recruitment - transitions - monitoring - Douglas fir Pseudotsuga mensisii - Grand fir Abies grandis - Scot pine Pinus sylvestris - Common beech Fagus sylvatica - Common oak Quercus robur - Schovenhorst
    The current database is part of the FEM growth and yield database, a collection of growth and yield data from even-aged monocultures (douglas fir, common oak, poplar, Japanese Larch, Norway spruce, Scots pine, Corsican pine, Austrian pine, red oak and several other species, with only a few plots, even-aged mixed species forest plots, uneven-aged natural forest, uneven-aged selection forest and roadside plantattions of poplar. The FEM growth and yield data base is currently supervised by Jan den Ouden and Frits Mohren.
    Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages
    Caughlin, T.T. ; Ferguson, J.M. ; Lichstein, J.W. ; Zuidema, P.A. ; Bunyavejchewin, S. ; Levey, D.J. - \ 2015
    Proceedings of the Royal Society. B: Biological Sciences 282 (2015)1798. - ISSN 0962-8452 - 9 p.
    spatial-patterns - rain-forest - recruitment - consequences - neighborhood - defaunation - habitat - uncertainty - diversity - abundance
    Overhunting in tropical forests reduces populations of vertebrate seed dispersers. If reduced seed dispersal has a negative impact on tree population viability, overhunting could lead to altered forest structure and dynamics, including decreased biodiversity. However, empirical data showing decreased animal-dispersed tree abundance in overhunted forests contradict demographic models which predict minimal sensitivity of tree population growth rate to early life stages. One resolution to this discrepancy is that seed dispersal determines spatial aggregation, which could have demographic consequences for all life stages. We tested the impact of dispersal loss on population viability of a tropical tree species, Miliusa horsfieldii, currently dispersed by an intact community of large mammals in a Thai forest. We evaluated the effect of spatial aggregation for all tree life stages, from seeds to adult trees, and constructed simulation models to compare population viability with and without animal-mediated seed dispersal. In simulated populations, disperser loss increased spatial aggregation by fourfold, leading to increased negative density dependence across the life cycle and a 10-fold increase in the probability of extinction. Given that the majority of tree species in tropical forests are animal-dispersed, overhunting will potentially result in forests that are fundamentally different from those existing now.
    Positive shrub-tree interactions facilitate woody encroachment in boreal peatlands
    Holmgren, M. ; Lin, C.Y. ; Murillo, J.E. ; Nieuwenhuis, A. ; Penninkhof, J.M. ; Sanders, N. ; Bart, T. van; Veen, H. van; Vasander, H. ; Vollebregt, M.E. ; Limpens, J. - \ 2015
    Journal of Ecology 103 (2015). - ISSN 0022-0477 - p. 58 - 66.
    scots pine - sphagnum - bogs - growth - mire - communities - recruitment - transitions - sylvestris - ecosystems
    1. Boreal ecosystems are warming roughly twice as fast as the global average, resulting in woody expansion that could further speed up the climate warming. Boreal peatbogs are waterlogged systems that store more than 30% of the global soil carbon. Facilitative effects of shrubs and trees on the establishment of new individuals could increase tree cover with profound consequences for the structure and functioning of boreal peatbogs, carbon sequestration and climate. 2. We conducted two field experiments in boreal peatbogs to assess the mechanisms that explain tree seedling recruitment and to estimate the strength of positive feedbacks between shrubs and trees. We planted seeds and seedlings of Pinus sylvestris in microsites with contrasting water-tables and woody cover and manipulated both shrub canopy and root competition. We monitored seedling emergence, growth and survival for up to four growing seasons and assessed how seedling responses related to abiotic and biotic conditions. 3. We found that tree recruitment is more successful in drier topographical microsites with deeper water-tables. On these hummocks, shrubs have both positive and negative effects on tree seedling establishment. Shrub cover improved tree seedling condition, growth and survival during the warmest growing season. In turn, higher tree basal area correlates positively with soil nutrient availability, shrub biomass and abundance of tree juveniles. 4. Synthesis. Our results suggest that shrubs facilitate tree colonization of peatbogs which further increases shrub growth. These facilitative effects seem to be stronger under warmer conditions suggesting that a higher frequency of warmer and dry summers may lead to stronger positive interactions between shrubs and trees that could eventually facilitate a shift from moss to tree-dominated systems.
    Do plant traits explain tree seedling survival in bogs?
    Limpens, J. ; Egmond, E. van; Li, B. ; Holmgren, M. - \ 2014
    Functional Ecology 28 (2014)1. - ISSN 0269-8463 - p. 283 - 290.
    sphagnum mosses - picea-mariana - water-table - scots pine - boreal - growth - peatlands - recruitment - establishment - carbon
    1.Moss-dominated peat bogs store approximately 30% of global soil carbon. A climate induced shift from current moss-dominated conditions to tree-dominated states is expected to strongly affect their functioning and carbon sequestration capacity. Consequently, unraveling the mechanisms that may explain successful tree seedling establishment in these ecosystems is highly relevant. 2.To assess the role of drought on early tree seedling establishment and the relative importance of plant traits in tree seedling survival, we conducted a factorial glasshouse experiment with seven conifer species. 3.Our results show that drought inhibits moss growth, thereby increasing survival of tree seedlings. Survival success was higher in Pinus than in Picea species, ranking Pinus banksiana > Pinus sylvestris > Pinus nigra > Picea mariana > Picea glauca, Picea sitchensis > Picea rubens. We found that those species most successful under dry and wet conditions combined a fast shoot growth with high seed mass. 4.We conclude that plant traits contribute to explaining successful early tree seedling establishment in bogs
    Growth of juvenile blue mussels (Mytilus edulis) on suspended collectors in the Dutch Wadden Sea
    Jacobs, P. ; Beauchemin, C. ; Riegman, R. - \ 2014
    Journal of Sea Research 85 (2014). - ISSN 1385-1101 - p. 365 - 371.
    spat collectors - macoma-balthica - temperature - larvae - recruitment - predation - responses - winters - seed
    In The Netherlands, fishing for juvenile blue mussels (Mytilus edulis) on wild beds is gradually replaced by harvesting of seeds from suspended collectors. Both the relaxation of fishing as well as the up-scaling of the number of seed collectors are expected to result in an increase in the number of mussels in the Wadden Sea. Consequently, an enhanced mussel population will cause an additional filtration impact on the system. The question is raised to what extent collectors can be used without negatively affecting the carrying capacity of an ecosystem. Therefore, a monitoring programme was initiated to study the growth of juvenile mussels on suspended collectors. This growth was related to food availability, measured as chlorophyll-a, and temperature both before and after settlement. Findings will serve as input for mathematical models predicting the carrying capacity for mussel seed collectors in this area. The results for 2010 and 2011 are presented. In 2011 settled mussels achieved a higher growth rate, while phytoplankton concentrations after settlement were lower. This contradicts the general agreement that higher phytoplankton concentrations result in higher growth rates. A positive relation between chlorophyll-a concentrations during the larval period and the growth rate of settled mussels was found. The number of settled larvae was higher in 2011. Results from existing studies on settlement and recruitment on tidal flats combined with estimated settlement date in the current study led to the hypothesis that the number of settled mussels on rope collectors is inversely related to the duration of the larval period (determined by water temperature). Our results indicated that in the Wadden Sea, the intra-annual differences in chlorophyll-a and temperature did not have an impact on the juvenile growth rate, while the interannual differences did. This is an indication that the larval stage is strongly discriminative in terms of juvenile growth rates. Modelling growth of juvenile mussels on collectors should thus include conditions before settlement.
    A bio-economic analysis of harvest control rules for the Northeast Arctic cod fishery
    Eikeset, A.M. ; Richter, A.P. ; Dankel, D.J. ; Dunlop, E.S. ; Heino, M. ; Dieckmann, U. ; Stenseth, N.C. - \ 2013
    Marine Policy 39 (2013). - ISSN 0308-597X - p. 172 - 181.
    reaction norms - gadus-morhua - management - stock - economics - model - recruitment - maturation - evolution - fecundity
    Harvest control rules (HCRs) have been implemented for many fisheries worldwide. However, in most instances, those HCRs are not based on the explicit feedbacks between stock properties and economic considerations. This paper develops a bio-economic model that evaluates the HCR adopted in 2004 by the Joint Norwegian-Russian Fishery Commission to manage the world's largest cod stock, Northeast Arctic cod (NEA). The model considered here is biologically and economically detailed, and is the first to compare the performance of the stock's current HCR with that of alternative HCRs derived with optimality criteria. In particular, HCRs are optimized for economic objectives including fleet profits, economic welfare, and total yield and the merging properties are analyzed. The performance of these optimal HCRs was compared with the currently used HCR. This paper show that the current HCR does in fact comes very close to maximizing profits. Furthermore, the results reveal that the HCR that maximizes profits is the most precautionary one among the considered HCRs. Finally, the HCR that maximizes yield leads to unprecautionary low levels of biomass. In these ways, the implementation of the HCR for NEA cod can be viewed as a success story that may provide valuable lessons for other fisheries
    Size-based species interactions shape herring and cod population dynamics in the face of exploitation
    Denderen, P.D. van; Kooten, T. van - \ 2013
    Ecosphere 4 (2013)10. - ISSN 2150-8925
    north-sea cod - clupea-harengus - intraguild predation - prey - model - recruitment - recovery - biomass - management - fisheries
    Size-specific competition and predation interactions often link the population dynamics of fish species in their response to exploitation. The effects of harvesting on interacting fish species is of increasing relevance as more and more fish populations worldwide are reduced by fishing. When stocks are harvested, effects of harvesting may percolate to populations of other species with which it interacts through competition, predation, etcetera. When multiple species are exploited, this can lead to interactions between fisheries, mediated by ecological interactions. Nevertheless, most fish stocks are managed using a single-species framework. We studied how single-species explanations of historical population dynamics work out when size-based interactions between harvested species are taken into account. We have taken as a case study the dynamics of cod (Gadus morhua) and herring (Clupea harengus) in the North Sea. These dynamics are generally considered to be shaped by fishing pressure on and food availability to single species. Our results indicate that the explanatory power of these factors is maintained with the inclusion of species interactions, but the processes leading to the observed patterns are altered as the fates of the species are interdependent. The sign and magnitude of the interaction between the species depends on the state of the populations, their exploitation history and environmental factors such as resource productivity. This context-dependent response to changing fishery intensity has important ramifications for management. We show that management plans for the exploitation of either one of these species, or for the recovery of North Sea cod, which do not account for these subtle interactions, may fail or backfire. Hence, such interactions link the fate of these species in complex ways, which must be taken into consideration for successful management of their exploitation, including harvesting at maximum sustainable yield, as we move towards an ecosystem-based management of marine fisheries.
    Variability and connectivity of plaice populations from the Eastern North Sea to the Western Baltic Sea, and implications for assessment and management
    Ulrich, C. ; Boje, J. ; Cardinale, M. ; Hintzen, N.T. ; Miller, D.C.M. ; Rijnsdorp, A.D. - \ 2013
    Journal of Sea Research 84 (2013). - ISSN 1385-1101 - p. 40 - 48.
    pleuronectes-platessa l. - marine fishes - skagerrak - kattegat - atlantic - flatfish - stocks - size - microsatellites - recruitment
    An essential prerequisite of sustainable fisheries is the match between biologically relevant processes and management action. Various populations may however co-occur on fishing grounds, although they might not belong to the same stock, leading to poor performance of stock assessment and management. Plaice in Kattegat and Skagerrak have traditionally been considered as one stock unit. Current understanding indicates that several plaice components may exist in the transition area between the North Sea and the Baltic Sea. A comprehensive review of all available biological knowledge on plaice in this area is performed, including published and unpublished literature together with the analyses of commercial and survey data and historical tagging data. The results suggest that plaice in Skagerrak is closely associated with plaice in the North Sea, although local populations are present in the area. Plaice in Kattegat, the Belts Sea and the Sound can be considered a stock unit, as is plaice in the Baltic Sea. The analyses revealed great heterogeneity in the dynamics and productivity of the various local components, and suggested for specific action to maintain biodiversity.
    Is age structure a relevant criterion for the health of fish stocks?
    Brunel, T.P.A. ; Piet, G.J. - \ 2013
    ICES Journal of Marine Science 70 (2013)2. - ISSN 1054-3139 - p. 270 - 283.
    cod gadus-morhua - northern north-sea - atlantic cod - spawning stock - fisheries sustainability - reference points - climate-change - baltic cod - egg size - recruitment
    The age and size structure of exploited fish stocks is one of the criteria for Good Environmental Status of commercial fish. However, two underlying assumptions to this criterion remain to be tested: first, that a well-balanced age structure is indeed indicative of a “healthier” stock, and second, that managers can exert a control on the age structure, independently from the regulation of stock abundance. This study investigates these questions using simulations from a population model in which recruitment is based on egg production, which is more sensitive to age structure variations than the traditionally used spawning stock biomass (SSB) and that was parameterized to represent the population dynamics of North Sea cod, plaice, and herring. Our results show that (i) the age structure is highly dependent on the selection pattern, as well as on the level of fishing mortality; (ii) the selection pattern determines the ability of fish stocks to withstand, and recover from, external perturbation; and (iii) the selection pattern determines the output of the fishery providing the management option to balance stable but relatively low yields vs strongly fluctuating high yields. Therefore, we propose to make the selection pattern, for which clear management targets can be set, a policy goal instead of the age structure that is currently in place
    Natural born indicators: Great cormorant Phalacrocorax carbo (Aves: Phalacrocoracidae) as monitors of river discharge influence on estuarine ichthyofauna
    Dias, E. ; Morais, P. ; Leopold, M.F. ; Campos, J. ; Antunes, C. - \ 2012
    Journal of Sea Research 73 (2012). - ISSN 1385-1101 - p. 101 - 108.
    double-crested cormorants - australian estuary - fish farm - food-web - diet - seabirds - variability - recruitment - management - abundance
    The ecological traits of piscivorous marine birds have been acknowledged to reflect ecosystem changes. We used the great cormorant as our indicator species in the Minho estuary (NW-Iberian Peninsula, Europe) to assess the temporal variation of their diet and the factors that could influence that variation. Pellets were collected in a night roost, located centrally in the estuary, during two consecutive wintering periods (2005–2006 and 2006–2007). The great cormorant population showed a high degree of feeding plasticity and most of the variation in cormorants' diet was attributed to river discharge fluctuations. Overall, during periods of increased river discharge, marine and marine opportunistic species disappeared from diet, whereas freshwater species increased. The cormorants in this study were using a roost in the middle of the estuary, so they were facing a changing food base over time, in accordance to variation in river discharges. The birds did not keep their diet constant but rather took what became locally available, notwithstanding their broad foraging range. Therefore, we suggest that great cormorants may be considered good samplers of local ichthyofauna and thus, temporal variation in the local prey can be followed by analyzing cormorants' diet.
    Directed seed dispersal towards areas with low conspecific tree density by a scatter-hoarding rodent
    Hirsch, B.T. ; Kays, R. ; Pereira, V.E. ; Jansen, P.A. - \ 2012
    Ecology Letters 15 (2012)12. - ISSN 1461-023X - p. 1423 - 1429.
    chipmunks tamias-amoenus - tropical forests - spatial-patterns - dependent survival - cache protection - fox squirrels - recruitment - pilferage - abundance - ecology
    Scatter-hoarding animals spread out cached seeds to reduce density-dependent theft of their food reserves. This behaviour could lead to directed dispersal into areas with lower densities of conspecific trees, where seed and seedling survival are higher, and could profoundly affect the spatial structure of plant communities. We tested this hypothesis with Central American agoutis and Astrocaryum standleyanum palm seeds on Barro Colorado Island, Panama. We radio-tracked seeds as they were cached and re-cached by agoutis, calculated the density of adult Astrocaryum trees surrounding each cache, and tested whether the observed number of trees around seed caches declined more than expected under random dispersal. Seedling establishment success was negatively dependent on seed density, and agoutis carried seeds towards locations with lower conspecific tree densities, thus facilitating the escape of seeds from natural enemies. This behaviour may be a widespread mechanism leading to highly effective seed dispersal by scatter-hoarding animals.
    Thieving rodents as substitute dispersers of megafaunal seeds
    Jansen, P.A. ; Hirsch, B.T. ; Emsens, W.J. ; Zamora-Gutierrez, V. ; Wikelski, M. ; Kays, R. - \ 2012
    Proceedings of the National Academy of Sciences of the United States of America 109 (2012)31. - ISSN 0027-8424 - p. 12610 - 12615.
    palm astrocaryum-standleyanum - tropical forests - spatial-patterns - acorn dispersal - rain-forest - agouti - recruitment - predation - tracking - survival
    The Neotropics have many plant species that seem to be adapted for seed dispersal by megafauna that went extinct in the late Pleistocene. Given the crucial importance of seed dispersal for plant persistence, it remains a mystery how these plants have survived more than 10,000 y without their mutualist dispersers. Here we present support for the hypothesis that secondary seed dispersal by scatter-hoarding rodents has facilitated the persistence of these large-seeded species. We used miniature radio transmitters to track the dispersal of reputedly megafaunal seeds by Central American agoutis, which scatter-hoard seeds in shallow caches in the soil throughout the forest. We found that seeds were initially cached at mostly short distances and then quickly dug up again. However, rather than eating the recovered seeds, agoutis continued to move and recache the seeds, up to 36 times. Agoutis dispersed an estimated 35% of seeds for >100 m. An estimated 14% of the cached seeds survived to the next year, when a new fruit crop became available to the rodents. Serial video-monitoring of cached seeds revealed that the stepwise dispersal was caused by agoutis repeatedly stealing and recaching each other's buried seeds. Although previous studies suggest that rodents are poor dispersers, we demonstrate that communities of rodents can in fact provide highly effective long-distance seed dispersal. Our findings suggest that thieving scatter-hoarding rodents could substitute for extinct megafaunal seed dispersers of tropical large-seeded trees.
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