12165 research outputs found
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Huren of kopen? Een kwestie van gezin, baan, gezondheid en geld
No full textDe meeste huiseigenaren willen bij verhuizing kopen en de meeste huurders willen blijven huren. Maar er zijn ook overstappers: kopers die willen huren en vooral huurders die willen kopen. Dat is interessant voor een overheid die eigenwoningbezit wil stimuleren. Wie zijn die kopers die geen koopwoning meer willen en welke huurders willen dat juist wel? En wat willen de starters op de woningmarkt? Relatie, baan en gezondheid bepalen de uitkomst, en het geld natuurlijk.
Strong effects of drying on water clarity and cyanobacterial blooms in cool tropical reservoirs
No full textKeywords:
Ethiopia;
fish biomass;
Microcystis;
reservoir ecology;
water clarity
Summary
1.In semi-arid regions, the construction of small reservoirs is important in alleviating water shortage, although many have poor water quality with high turbidity and dense blooms of algae and cyanobacteria, and there are large differences in the ecology of such reservoirs.
2.We took advantage of two exceptionally dry years in northern Ethiopia to study the effect of a dry period and the associated fish kills on reservoir ecology and water quality. We studied 13 reservoirs, seven of which dried up in 2009. Four of the latter dried up again in 2010. We monitored the ecology of these reservoirs from 2009 to 2011, hypothesising that the pattern of reservoir drying would explain ecological differences among them.
3.Reservoirs that refilled after drying had a significantly lower fish biomass, lower biomass of phytoplankton (expressed as chlorophyll-a) and cyanobacteria (Microcystis), clearer water, greater macrophyte cover and lower nutrient concentrations than reservoirs that did not dry. Although the differences in water quality were most striking in the wet season after a drying event, there were persistent effects on reservoir ecology. The three categories of reservoirs we distinguished, based on their behaviour in 2009 and 2010, also showed differences in 2004, a year during which none of the reservoirs dried out. While drying evidently results in better water quality, we could not disentangle the effects of drying per se from that of reductions in fish biomass. The total combined effect was highly significant in all 3 years, whereas the separate effects of drying and loss of fish were only significant in 2004.
4.Our results suggest that differences in water quality and ecology among reservoirs depend on their propensity to dry out. Drying might be used as a restoration measure to reduce potentially harmful cyanobacterial blooms in reservoirs.
Effects of previous intrusion pressure on territorial responses in nightingales
No full textIn territorial animals, establishing and defending a territory against rivals is commonly a prerequisite for successful reproduction. Yet, often, non-territorial males that are seeking to establish their own territory may intrude into occupied territories and persistently challenge residents in order to test their resource-holding potential. Such challenges may have long-term consequences for the territorial behaviour of the residents. Here, we tested whether territorial Nightingales (Luscinia megarhynchos) use information on the intrusion pressure of rivals in territory defence. Using multi-loudspeaker playback experiments, we simulated rivals that either sang from different song perches within occupied territories (persisting intruders) or that immediately left territories after previous intrusions (retreating intruders). In a final playback 1 h later, we then simulated the same rivals singing from a different location at the territory boundary. During this final playback, residents approached the loudspeaker closer, changed their song perches more often, sang more songs in close proximity to the loudspeaker, and sang more trills when rivals were previously simulated as having stayed in their territory without retreating. Our findings show that songbirds can integrate the level of threat from rivals differing in their intrusion behaviours in subsequent territorial encounters. This study emphasises the importance of considering how territorial defence is affected by previous intrusion pressure from rivals.
Zooplankton, fish communities and the role of planktivory in nine Ethiopian lakes
No full textFish and zooplankton populations of nine Ethiopian freshwater lakes were quantitatively sampled along a North–South gradient. Differences in altitude and latitude resulted in a temperature gradient from North to South. We tested three hypotheses: (1) the degree of zooplanktivory decreases with water temperature, i.e. from North to South; (2) the degree of zooplanktivory increases with the abundance of large-bodied zooplankton; and (3) the pattern of zooplanktivory in eutrophic Ethiopian water bodies differs from other tropical and temperate water bodies. Proportions of zooplanktivory in the fish communities did not show a geographical trend, but mainly depended on fish species, zooplankton density and the availability of large-bodied cladocerans. The degree of zooplanktivory in eutrophic Ethiopian water bodies differs from other eutrophic water bodies, both temperate and tropical. In Ethiopia, the degree of zooplanktivory can be both low and high, in contrast with other tropical water bodies where zooplanktivory is generally low and with temperate eutrophic water bodies where it is generally high. As a result, predation pressure on zooplankton by fish varies dramatically amongst Ethiopian water bodies.
Convergence and Divergence in Direct and Indirect Life-History Traits of Closely Related Parasitoids (Braconidae: Microgastrinae)
No full text[KEYWORDS: Cotesia Development Egg load Microplitis Phylogeny Reproduction]
Closely related species in nature often show similarities in suites of direct and indirect traits that reveal aspects of their phylogenetic history. Here we tested how common descent affects trait evolution in several closely related parasitoid species in the genera Cotesia and Microplitis (Hymenoptera: Braconidae: Microgastrinae) by comparing development, resource use and allocation into reproduction and maintenance. Parasitoids in these genera exhibit traits, like haemolymph feeding as larvae and external pupation that are rare in most parasitoid lineages. The growth of parasitized hosts was reduced by 90 % compared with healthy hosts, and maximum host size depended to a large extent on adult parasitoid size. Development time was longer in the more generalist parasitoids than in the specialists. Adult body mass was sexually dimorphic in all Cotesia species, with females being larger, but not in Microplitis spp. In contrast, in one of the Microplitis species males were found to be the larger sex. Egg load dynamics during the first 6 days after emergence were highly variable but egg number was typically higher in Cotesia spp. compared to Microplitis spp. Longevity in the various species was only greater in female than in male wasps in two Microplitis sp. There was a clear inverse relationship between resource use and allocation, e.g. maximum egg load and longevity, in these parasitoids. Our results reveal that adaptation to constraints imposed by host quality and availability has resulted in trait convergence and divergence at the species, genus and subfamily level.
Biochars produced from individual grassland species differ in their effect on plant growth
No full textBiochar, pyrolyzed biomass, has been shown to be a promising way to improve plant productivity and soil quality. Biochar characteristics and its effect on plant performance depend strongly on the type of feedstock from which it is made. However, whether biochars produced from individual grassland species differ in their characteristics and effects on plant growth when applied to soil is poorly understood. The aim of this study was to examine how soil application of pyrolyzed and non-pyrolyzed biomass originating from different grassland species influences plant performance.
We measured the growth of the forb Jacobaea vulgaris in soil amended with pyrolyzed or non-pyrolyzed biomass of seven different plant species, and in control soil without amendments.
The characteristics (nutrient content, C:N) and effects on plant growth of both pyrolyzed and non-pyrolyzed biomass differed significantly between species from which the biomass originated (‘feedstock species’). For most feedstock species there was no relationship between the effects that the pyrolyzed and the non-pyrolyzed biomass had on plant performance. Our results show that pyrolyzed grassland species differ in their characteristics and their effect on plant growth when amended to soil. This shows that it is important to test what the effect of pyrolysing a chosen feedstock is on a species before applying it on a larger scale and that potentially biochar with predefined effects could be designed for specific purposes.
Remarkable recovery and colonization behaviour of methane oxidizing bacteria in soil after disturbance is controlled by methane source only
No full textLittle is understood about the relationship between
microbial assemblage history, the composition and function of
specific functional guilds and the ecosystem functions they
provide. To learn more about this relationship we used methane
oxidizing bacteria (MOB) as model organisms and performed
soil microcosm experiments comprised of identical
soil substrates, hosting distinct overall microbial diversities
(i.e., full, reduced and zero total microbial and MOB diversities).
After inoculation with undisturbed soil, the recovery of
MOB activity, MOB diversity and total bacterial diversity
were followed over 3 months by methane oxidation potential
measurements and analyses targeting pmoA and 16S rRNA
genes. Measurement of methane oxidation potential demonstrated
different recovery rates across the different treatments.
Despite different starting microbial diversities, the recovery
and succession of the MOB communities followed a similar
pattern across the different treatment microcosms. In this
study we found that edaphic parameters were the dominant
factor shaping microbial communities over time and that the
starting microbial community played only a minor role in
shaping MOB microbial community