12165 research outputs found
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Valuable: Indicators for valorisation
No full textHet rapport Waardevol – Indicatoren voor valorisatie biedt kennisinstellingen, wetenschapsfinanciers en kennisgebruikers een gemeenschappelijk kader voor valorisatie. De methode werkt met valorisatiekaarten waarmee betrokken organisaties systematisch kunnen monitoren hoe groot de valorisatie-inspanningen zijn, wat de opbrengsten zijn en welke mogelijkheden voor verbetering er zijn. Het gebruik van valorisatiekaarten zal helpen om het inzicht te vergroten in wat publieke kennisinstellingen, wetenschapsfinanciers en bedrijfsleven nu al doen aan valorisatie.
Invasive crayfish threaten the development of submerged macrophytes in lake restoration
No full textSubmerged macrophytes enhance water transparency and aquatic biodiversity in shallow water ecosystems. Therefore, the return of submerged macrophytes is the target of many lake restoration projects. However, at present, north-western European aquatic ecosystems are increasingly invaded by omnivorous exotic crayfish. We hypothesize that invasive crayfish pose a novel constraint on the regeneration of submerged macrophytes in restored lakes and may jeopardize restoration efforts. We experimentally investigated whether the invasive crayfish (Procambarus clarkii Girard) affects submerged macrophyte development in a Dutch peat lake where these crayfish are expanding rapidly. Seemingly favourable abiotic conditions for macrophyte growth existed in two 0.5 ha lake enclosures, which provided shelter and reduced turbidity, and in one lake enclosure iron was added to reduce internal nutrient loading, but macrophytes did not emerge. We transplanted three submerged macrophyte species in a full factorial exclosure experiment, where we separated the effect of crayfish from large vertebrates using different mesh sizes combined with a caging treatment stocked with crayfish only. The three transplanted macrophytes grew rapidly when protected from grazing in both lake enclosures, demonstrating that abiotic conditions for growth were suitable. Crayfish strongly reduced biomass and survival of all three macrophyte species while waterfowl and fish had no additive effects. Gut contents showed that crayfish were mostly carnivorous, but also consumed macrophytes. We show that P. clarkii strongly inhibit macrophyte development once favourable abiotic conditions for macrophyte growth are restored. Therefore, expansion of invasive crayfish poses a novel threat to the restoration of shallow water bodies in north-western Europe. Prevention of introduction and spread of crayfish is urgent, as management of invasive crayfish populations is very difficult.
Was Lates Late? A Null Model for the Nile Perch Boom in Lake Victoria
No full textNile perch (Lates niloticus) suddenly invaded Lake Victoria between 1979 and 1987, 25 years after its introduction in the Ugandan side of the lake. Nile perch then replaced the native fish diversity and irreversibly altered the ecosystem and its role to lakeshore societies: it is now a prised export product that supports millions of livelihoods. The delay in the Nile perch boom led to a hunt for triggers of the sudden boom and generated several hypotheses regarding its growth at low abundances – all hypotheses having important implications for the management of Nile perch stocks. We use logistic growth as a parsimonious null model to predict when the Nile perch invasion should have been expected, given its growth rate, initial stock size and introduction year. We find the first exponential growth phase can explain the timing of the perch boom at the scale of Lake Victoria, suggesting that complex mechanisms are not necessary to explain the Nile perch invasion or its timing. However, the boom started in Kenya before Uganda, indicating perhaps that Allee effects act at smaller scales than that of the whole Lake. The Nile perch invasion of other lakes indicates that habitat differences may also have an effect on invasion success. Our results suggest there is probably no single management strategy applicable to the whole lake that would lead to both efficient and sustainable exploitation of its resources.
Plant-animal interactions in two forest herbs along a tree and herb diversity gradient
No full textBackground: Plant diversity can influence numerous ecosystem processes, including plant–animal interactions, which, in turn, will affect plant growth and fitness. At present, little is known on how plant–animal interactions in forests respond to gradients in tree and herb-layer diversity.
Aims: To quantify how invertebrate herbivory, pollination-dependent seed production and post-dispersal seed predation vary along a gradient of tree and herb diversity in a semi-natural temperate deciduous forest.
Methods: Potted individuals of the understorey herbs Lathyrus vernus and Primula elatior were exposed in 40 forest plots along a natural gradient of tree and herb diversity for 3 months to record seed production and leaf damage caused by invertebrate herbivores. In half the plants, pollinators were experimentally excluded to test if seed production depended on insect pollination. In Lathyrus vernus linkages between below- and above-ground herbivory were tested by inoculating plants with root-feeding nematodes. To study seed predation, we measured seed removal from seed depots that were selectively accessible to different seed predator groups.
Results: Herbivore damage decreased with increasing tree diversity in P. elatior. In L. vernus, above-ground herbivory was higher in nematode-treated plants than in control plants. Seed production in L. vernus, which strongly depended on insect pollination, showed a positive relationship with tree diversity. Seed predation was positively related to herb diversity in L. vernus, but only weakly so in P. elatior. While both vertebrates and invertebrates acted as seed predators of L. vernus, seeds of P. elatior were mainly predated by invertebrates.
Conclusions: The fitness of understorey plants is linked to tree and herb diversity via changes in plant-invertebrate interactions. However, species-specific responses of study plants underline the importance of species identity effects in addition to effects of biodiversity per se.
