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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Searching for balance : stability and equilibria of food webs
Altena, C. van - \ 2016
University. Promotor(en): Peter de Ruiter; J.A.P. Heesterbeek; Wolf Mooij, co-promotor(en): Lia Hemerik. - Wageningen : Wageningen University - ISBN 9789462576827 - 130 p.
food webs - models - interactions - ecology - biocoenosis - ecological balance - voedselwebben - modellen - interacties - ecologie - biocenose - ecologisch evenwicht

Abstract

How complexity of food webs relates to stability has been a subject of many studies. Often,

unweighted connectance is used to express complexity. Unweighted connectance is

measured as the proportion of realized links in the network. Weighted connectance, on the

other hand, takes link weights (fluxes or feeding rates) into account and captures the shape

of the flux distribution. Here, we used weighted connectance to revisit the relation between

complexity and stability. We used 15 real soil food webs and determined the feeding rates

and the interaction strength matrices. We calculated both versions of connectance, and

related these structural properties to food web stability. We also determined the skewness

of both flux and interaction strength distributions with the Gini coefficient. We found no

relation between unweighted connectance and food web stability, but weighted connectance

was positively correlated with stability. This finding challenges the notion that complexity

may constrain stability, and supports the ‘complexity begets stability’ notion. The positive

correlation between weighted connectance and stability implies that the more evenly flux

rates were distributed over links, the more stable the webs were. This was confirmed by the

Gini coefficients of both fluxes and interaction strengths. However, the most even

distributions of this dataset still were strongly skewed towards small fluxes or weak

interaction strengths. Thus, incorporating these distribution with many weak links via

weighted instead of unweighted food web measures can shed new light on classical

theories

Gewapende vrede : beschouwingen over plant-dierrelaties
Schaminée, J.H.J. ; Janssen, J.A.M. ; Weeda, E.J. - \ 2011
Zeist : KNNV uitgeverij (Vegetatiekundige Monografieën 3) - ISBN 9789050113526 - 191
plant-dier interacties - dieren - planten - ecologie - vogels - lepidoptera - biocenose - aquatisch milieu - terrestrische ecologie - plant-animal interactions - animals - plants - ecology - birds - biocoenosis - aquatic environment - terrestrial ecology
Planten en dieren hebben elkaar nodig, maar staan ook op gespannen voet met elkaar. Dieren worden door planten aangetrokken voor bestuiving en het verspreiden van zaden, maar tegelijkertijd moeten deze laatste zich beschermen tegen overmatige vraat en fysiek geweld. Daarover gaat dit boek, een reeks beschouwingen over het fascinerende samenspel van plant en dier in de natuur. Aan bod komen onderwerpen als de relaties tussen een enkele plantensoort en zijn dierlijke partners, het functioneren van levensgemeenschappen in water of op land en de co-evolutie van grassen en grazers. Is er nog toekomst voor weidevogels in ons land en wat zal het effect zijn van klimaatverandering op onze dagvlinders? Welke methoden staan de bioloog ter beschikking om meer inzicht te verkrijgen in de samenhang tussen vegetatie en de daarvan afhankelijke fauna?
Draagkracht en exoten in de Waddenzee
Brinkman, A.G. ; Jansen, J.M. - \ 2007
Den Burg / Yerseke : IMARES (Rapport / Wageningen IMARES nr. C073/07) - 34
biocenose - biotopen - aquatisch milieu - biomassa productie - ecologisch evenwicht - draagkracht - populatiedichtheid - schaaldieren - kokkels - mossels - oesters - ensis - diervoeding - vogels - watervogels - waadvogels - waddenzee - biocoenosis - biotopes - aquatic environment - biomass production - ecological balance - carrying capacity - population density - shellfish - clams - mussels - oysters - animal nutrition - birds - waterfowl - waders - wadden sea
In het kader van het Nationaal Programma voor Zee- en Kustonderzoek is inzicht gewenst in de draagkracht van het gebied. In dit kader is gekeken wat de verwachte toename van exoten binnen het Waddensysteem is. Het betreft hier de Japanse oester en de Amerikaanse zwaardschede
Natuurwijzer water
Nijboer, R.C. ; Verdonschot, P.F.M. - \ 1999
H2O : tijdschrift voor watervoorziening en afvalwaterbehandeling 32 (1999)21. - ISSN 0166-8439 - p. 19 - 21.
oppervlaktewater - zoetwaterecologie - hydrobiologie - biocenose - aquatische gemeenschappen - aquatisch milieu - ecosystemen - biologische indicatoren - waarden - taxatie - natuurlijke hulpbronnen - classificatie - natuur - biologische eigenschappen - aquatische ecosystemen - water - surface water - freshwater ecology - hydrobiology - biocoenosis - aquatic communities - aquatic environment - ecosystems - biological indicators - values - valuation - natural resources - classification - nature - biological properties - aquatic ecosystems
Weer een nieuw meetsysteem voor Nederlandse natuur. Om de natuurwaarde van oppervlaktewateren te meten wordt een natuurwijzer, opgebouwd uit positieve en negatieve biologische indicatoren, voorgesteld. De natuurwaarde wordt afgemeten aan een referentiebeeld, d.w.z. een stabiele, ecologisch optimale situatie zoals o.a. nog in Poolse wateren is te vinden
Effects of a pulsed treatment with the herbicide afalon (active ingredient linuron) on macrophyte-dominated mesocosms. II. Structural responses
Geest, G. van; Zwaardemaker, N.G. ; Wijngaarden, W.R. van; Cuppen, J.G.M. - \ 1999
Environmental Toxicology and Chemistry 18 (1999). - ISSN 0730-7268 - p. 2866 - 2874.
pesticiden - milieueffect - ecosystemen - structuur - biocenose - sloten - nederland - aquatische ecosystemen - pesticides - environmental impact - ecosystems - structure - biocoenosis - ditches - netherlands - aquatic ecosystems
Lake Victoria wetlands and the ecology of the Nile tilapia, Oreochromis niloticus Linne
Balirwa, J.S. - \ 1998
Agricultural University. Promotor(en): W.J. Wolff; P. Denny; R.M.M. Roijackers. - Rotterdam : Balkema - ISBN 9789054104117 - 247
hydrologie - limnologie - meren - plassen - hydrobiologie - biocenose - wetlands - polders - Percidae - rivierbaars - Cichlidae - Tilapia - Lutjanidae - ecologie - diergedrag - gewoonten - Centraal-Afrika - water - reservoirs - aquatische ecosystemen - hydrology - limnology - lakes - ponds - hydrobiology - biocoenosis - perch - ecology - animal behaviour - habits - Central Africa - aquatic ecosystems
<p>An ecological study of wetlands was undertaken in northern Lake Victoria (East Africa) between 1993 and 1996 with a major aim of characterising shallow vegetation-dominated interface habitats, and evaluating their importance for fish, in particular, for the stocked and socio-economically important <em>Oreochromis niloticus</em> LINNÉ (the Nile tilapia). From field and laboratory experiments, five major habitat types could be defined by the type of the dominant emergent macrophyte at the shore from the more than 40 identified plant species along a 110 km shoreline. These were: <em>Cyperus papyrus</em> L. (papyrus), <em>Phragmites mauritianus</em> Kunth (reeds), <em>Typha domingensis</em> Pers. (bulrush), <em>Vossia cuspidata</em> (Roxb.) (hippo grass), and the alien floating <em>Eichhornia crassipes</em> (Martius) Solms-Laubach (water hyacinth).</p></p>From digital data, considerable long term changes in the shoreline wetland landscape of the lake were discerned and appeared to be primarily associated with increasing human activity (e.g., agriculture, biomass harvests) which had resulted into a 5 % reduction of wetland cover. Inspite of the absence of a well developed euhydrophyte community (e.g., <em>Potamogeton</em> and <em>Ceratophyllum</em> ), and increasing infestations with <em>E. crassipes</em> mats, the width of the littoral zone was established by secchi transparency as being about 50 - 70 m away from the shallow (less than 1 m deep) vegetation fringe sloping to between 2 and 4 m in depth at its outer fringe. Hydrological influences associated with seasonal changes (the alternation of rainy with dry periods) explained most of the observed variation in abiotic (e.g., Si, tot.-P, soluble reactive-P, NO <sub>3</sub> -N, pH, temperature) and biotic (phytoplankton, macrofauna, fish) factors, but there was also significant (p &lt; 0.05) variation due to vegetation, distance from the shore out towards open water and interaction effects between these factors.</p></p>At least 30 species of fish were identified from the shallower (2.5 m) vegetated habitats in contrast to 10 species from the deeper (4 - 8 m) open water habitats. There were other significant (p &lt; 0.05) spatial and temporal differences in habitat use by fish. Species diversity was dominated by haplochromine species but three stocked species (the Nile perch, <em>Lates niloticus</em> L., <em>O. niloticus</em> and <em>Tilapia zilli</em> ) contributed at least 90% of the estimated numerical and biomass densities of which, the Nile tilapia was the most important component making up 45 - 65 % of the biomass of all fish. Season was a major factor in size-related abundance patterns but generally, most of the Nile tilapia biomass was associated with <em>Phragmites-Vossia-Typha</em> -dominated habitats which were also important for small (&lt;15.0 cm) and juvenile fish.</p></p>The shallow vegetated habitats were found to be ecologically important for the Nile tilapia for sheltering and feeding, and, it was also found that Lake Victoria Nile tilapia were significantly more fecund (with 3723 ± 147 eggs.female <sup>-1</SUP>) and had a higher condition index than populations of the species in Lake Kyoga (also stocked) and Lake Albert (a native habitat). It was inferred that these differences could be a result of a better nutritional base in Lake Victoria where the species was shown to be omnivorous (with detrital and animal foods as major dietary items) contrary to the previously believed herbivorous (phytoplankton) habits. Successional patterns associated with water hyacinth and the strong hydrological influences on shallow vegetated habitats imply that basin disturbances could therefore be a major threat to water quality and the fisheries.</p>
Ecology of shallow lakes.
Scheffer, M. - \ 1998
London, UK : Chapman and Hall - ISBN 9780412749209
hydrobiologie - biocenose - meren - reservoirs - plassen - water - aquatische ecosystemen - biologische eigenschappen - hydrobiology - biocoenosis - lakes - ponds - aquatic ecosystems - biological properties
Toetsen van ecosysteemontwikkeling in watersystemen
During, R. - \ 1997
S.l. : RIZA [etc.] (RIZA rapport 97.040) - ISBN 9789036950862 - 50
hydrobiologie - biocenose - waterbeheer - watervoorraden - besluitvorming - bedrijfsvoering - operationeel onderzoek - simulatie - werkschema - lineair programmeren - nederland - hydrologie - aquatische ecosystemen - hydrobiology - biocoenosis - water management - water resources - decision making - management - operations research - simulation - work flow - linear programming - netherlands - hydrology - aquatic ecosystems
On the way to eternity; the success of an aquatic laboratory microecosystem
Kersting, K. - \ 1997
Aquatic Ecology 31 (1997)1. - ISSN 1386-2588 - p. 29 - 35.
biocenose - cladocera - daphnia - modellen - onderzoek - aquatische ecosystemen - biocoenosis - models - research - aquatic ecosystems
Eutrophication Research. State-of-art : inputs, processes, effects, modelling, management : 28-29 August 1997, Wageningen, The Netherlands
Roijackers, R. ; Aalderink, R.H. ; Blom, G. - \ 1997
Wageningen : Wageningen Agricultural University - 364
meren - reservoirs - plassen - water - waterverontreiniging - waterkwaliteit - eutrofiëring - waterbeheer - watervoorraden - hydrobiologie - biocenose - modellen - onderzoek - hydrologie - aquatische ecosystemen - lakes - ponds - water pollution - water quality - eutrophication - water management - water resources - hydrobiology - biocoenosis - models - research - hydrology - aquatic ecosystems
Afscheid Lijklema
Aquatic macroinvertebrate community structure of a Nymphoides peltata-dominated and macrophyte-free site in an oxbow lake
Brock, T.C.M. ; Velde, G. van der - \ 1996
Netherlands journal of aquatic ecology 30 (1996)2/3. - ISSN 1380-8427 - p. 151 - 163.
biocenose - gentianaceae - aquatische ecosystemen - waal - biocoenosis - aquatic ecosystems - river waal
The aquatic fauna in two freshwater biotopes, namely a Nymphoides peltata-dominated site and a macrophyte-free site, were studied quantitatively in a shallow oxbow lake of the river Waal. Water, sediment and macrophyte samples were analysed. Species richness, densities, and biomass of macro-invertebrates were considerably higher in the biotope with macrophytes. Sediment-dwelling fauna constituted a quantitatively important component in the Nymphoides-dominated site. It seems that macrophyte detritus plays an important role in determining the trophic structure of the macro-invertebrate community.
Toetsen van ecosysteemontwikkeling in watersystemen
During, R. - \ 1996
Wageningen : DLO-Staring Centrum - 50
hydrobiologie - biocenose - waterbeheer - watervoorraden - besluitvorming - bedrijfsvoering - operationeel onderzoek - simulatie - werkschema - lineair programmeren - nederland - hydrologie - aquatische ecosystemen - hydrobiology - biocoenosis - water management - water resources - decision making - management - operations research - simulation - work flow - linear programming - netherlands - hydrology - aquatic ecosystems
In het water- en natuurbeleid worden verschillende methoden gehanteerd voor het stellen van ecosysteemdoelen in grote wateren. Verschillen in uitgangspunten leiden tot problemen in de beleidsafstemming en ook in een eenduidige toetsing van de beleidsresultaten. Alle grote wateren vallen in de Ecologische Hoofdstructuur, zodat het integraal waterbeheer voor de taak gesteld is om de nationale en internationale biodiversiteit veilig te stellen. Een aantal acties is geformuleerd om tot een betere afstemming tussen het integrale waterbeleid en het sectorale natuurbeleid te komen.
Cornelis den Hartog: an outstanding aquatic ecologist
Velde, G. van der; Brock, T.C.M. ; Kempers, A.J. - \ 1996
Netherlands journal of aquatic ecology 30 (1996)2-3. - ISSN 1380-8427 - p. 71 - 82.
hydrobiologie - biocenose - biografieën - nederland - aquatische ecosystemen - hydrobiology - biocoenosis - biographies - netherlands - aquatic ecosystems
A survey is given of the work and life of Cornelis den Hartog up to the date in 1996 on which he retired from his position as a professor at the University of Nijmegen. Cornelis (Kees) den Hartog made important contributions to aquatic ecology in the widest sense, e.G. On brackish water typology, meiofauna (microturbellaria), macroinvertebrates, littoral algae, sea-grasses and aquatic macrophytes. He favoured the ecosystem approach in aquatic ecology by studying structure and functioning in an integrated way. He led 31 students to their doctor's degrees (PhD).
Natuurlijk vissen op schol en tong
Rijnsdorp, A.D. ; Ende, K.C.J. van den - \ 1996
H2O : tijdschrift voor watervoorziening en afvalwaterbehandeling 29 (1996)24. - ISSN 0166-8439 - p. 709 - 711.
biocenose - hydrobiologie - bedrijfsvoering - modellen - natuurlijke hulpbronnen - nederland - noordzee - schol - pleuronectiformes - bescherming - herstel - onderzoek - hulpbronnengebruik - solea - duurzaamheid (sustainability) - aquatische ecosystemen - visvangsten - visgronden - biocoenosis - hydrobiology - management - models - natural resources - netherlands - north sea - plaice - protection - rehabilitation - research - resource utilization - sustainability - aquatic ecosystems - fish catches - fishing grounds
Bronnen in Noord- en Midden-Limburg; ligging en globale karakterisering
Verdonschot, P.F.M. ; Schot, J.A. ; Mosterdijk, H.G. - \ 1996
Wageningen : DLO-Instituut voor Bos- en Natuuronderzoek (IBN-rapport 251) - 234
waterbronnen - hydrologie - biocenose - aquatische ecosystemen - noord-limburg - midden-limburg - springs (water) - hydrology - biocoenosis - aquatic ecosystems
Een 'natuurlijk' graassysteem in de Arctis
Oosterveld, P. - \ 1996
Bosbouwvoorlichting 35 (1996)7. - ISSN 0166-8986 - p. 80 - 80.
biocenose - biogeochemie - cervidae - koude klimaatzones - kringlopen - ecosystemen - begrazing - bedrijfsvoering - natuurbescherming - poolgebieden - populatie-ecologie - populatiebiologie - biocoenosis - biogeochemistry - cold zones - cycling - ecosystems - grazing - management - nature conservation - polar regions - population ecology - population biology
Begrazing door rendieren, de aanpassing aan het ongunstige seizoen van herbivoren, klimaat en ondergrond in verband met rendierpopulaties en aandacht voor begrazing en begroeiing
Natuurbeheer en onderzoek in kustwateren
Dankers, N. ; Dijkema, K. ; Brinkman, B. - \ 1996
In: Natuur in het water : van exploitatie naar bescherming / Higler, L.W.G., - p. 19 - 34.
biocenose - hydrobiologie - bedrijfsvoering - natuurbescherming - beleid - aquatische ecosystemen - waddenzee - biocoenosis - hydrobiology - management - nature conservation - policy - aquatic ecosystems - wadden sea
Natuur in het water: van exploitatie naar bescherming
Higler, L.W.G. - \ 1996
Wageningen : DLO-Instituut voor Bos- en Natuuronderzoek (IBN-rapport 249) - 68
natuurbescherming - bescherming - samenleving - oppervlaktewater - land - hydrologie - hydrobiologie - biocenose - beleid - bedrijfsvoering - nederland - natuurgebieden - aquatische ecosystemen - nature conservation - protection - society - surface water - hydrology - hydrobiology - biocoenosis - policy - management - netherlands - natural areas - aquatic ecosystems
Afscheid WJ Wolff, IBN
Dijken in de Hoeksche Waard: ecologische schakels in wording? Mogelijkhden voor inrichting en beheer van de dijken in de Hoekschge Waard.
Laan, P. van der; Dekker, M. ; Liebrand, C.I.J.M. - \ 1996
Wageningen : Wetenschapswinkel (Rapport / Wetenschapswinkel 114) - 60
dijken - dammen - constructie - landschap - nationale parken - landschapsbescherming - bescherming - landschapsecologie - natuurbescherming - beleid - bedrijfsvoering - biocenose - herstel - Nederland - aquatische ecosystemen - natuur - natuurtechniek - zuid-holland - zuidhollandse eilanden - dykes - dams - construction - landscape - national parks - landscape conservation - protection - landscape ecology - nature conservation - policy - management - biocoenosis - rehabilitation - Netherlands - aquatic ecosystems - nature - ecological engineering
On the ecology of a tropical fish community
Piet, G.J. - \ 1996
Agricultural University. Promotor(en): E.A. Huisman; J. Vijverberg. - S.l. : Piet - ISBN 9789054855347 - 189
hydrobiologie - biocenose - vissen - sri lanka - aquatische ecosystemen - diergemeenschappen - hydrobiology - biocoenosis - fishes - aquatic ecosystems - animal communities
Sri Lanka has more than 12.000 artificial reservoirs and no natural lakes as is true for many other areas in SE Asia. These reservoirs cover a total surface area of around 175.000 ha. They are shallow with maximum depths often less than 5 m and exhibit considerable fluctuations in water level. In general the fish production, estimated from commercial catches, is high in these reservoirs, but lower than would be expected on the basis of the high primary production. This situation changed drastically when around 1952 the exotic tilapias <em>O.</em><em>mossambicus</em> and <em>O.</em><em>niloticus</em> were introduced. From that moment onward the fish yield increased and the proportion of inland fisheries to total commercial fisheries rose from almost 0% up to the current 20%. This freshwater fish yield is dominated by the introduced tilapias. The cause of this successful introduction of the tilapias has long been debated. Two different hypotheses exist. Firstly, it has been suggested that the relatively low fish production (based on the yield) in the reservoirs before the introduction of the tilapias was probably due to the origin of the fish, mainly indigenous riverine carps, which may not be optimally adapted to their new conditions. According to this hypothesis the introduced tilapias were competitively dominant and were able to capture niches formerly occupied by the indigenous fish species. Secondly, the alternative hypothesis assumes that the tilapias are able to occupy an empty niche when introduced in the reservoirs and consequently did not compete with the indigenous fish species. Determination of the correct theory is important, both from a theoretical and applied (fisheries management) point of view. This is attempted by studying, for the first time, the effects of the exotic lacustrine tilapias on the indigenous riverine fish species, within the framework of a tropical fish community.<p>The present study was conducted at Tissawewa reservoir. Tissawewa is a typical shallow lowland irrigation reservoir (average depth = 1.2 m) with a surface area of about 200 ha in the dry SE corner of Sri Lanka. In this study the total biological fish production was estimated at 2430 kg/ha/yr of which about 20% consisted of the exotic tilapias. In contrast, about 80% of the commercial catches of 242 kg/ha/yr consisted of these species. This strongly suggests that the importance of tilapias in terms of biological fish production was markedly overestimated not only in Sri Lanka but most probably in all of SE Asia because all estimates of fish production were based on commercial catches. The gross primary production was estimated at about 13.000 kg C/ha/yr of which 1.9% is transferred into fish. This efficiency is high compared to efficiencies between 0.2% and 1.6% reported for other lakes and reservoirs. This high transfer efficiency of fish production can be explained by the short food chain. Most of the fish biomass (ca 45%) consists of a small indigenous cyprinid, <em>A.</em><em>melettinus,</em> which feeds predominantly on phytoplankton and detritus. The tilapias (which make up ca. 9% of the fish biomass) are also herbivorous. These primary consumers represent about 64% of the biological fish production in the reservoir. The secondary consumers represent 36% of the biologic production while only 0.3% of the fish production is made up by the third trophic level, that of the tertiary consumers.<br/>The structure and fuinctioning of the fish community can be regulated according to several mechanisms such as a deterministic versus a stochastic regulation or, when deterministically regulated, by bottom-up (regulated by primary producers) versus top-down (regulated by predators) control. A deterministically regulated community is generally at an equilibrium, with population levels at carrying capacity determined by resource limitations and coexisting species avoiding competitive exclusion through biotic interactions such as resource partitioning. In contrast, a community is stochastically regulated when the environment is not stable enough to allow an equilibrium to persist. The abundances of species in such a community are determined largely through differential responses to unpredictable environmental changes, rather than through biotic interactions. Based on the disturbances, caused by water level fluctuations, acting on the fish community and community characteristics such as resistance and resilience, the Tissawewa fish community is considered to be deterministically regulated. However, extreme low water levels or the drying up of the reservoir which occur occasionally can cause a relatively brief time of stochastic regulation. Both these phenomena could be studied because during the sampling period, due to a long period of extreme drought, the water level decreased until the reservoir was completely dry. After filling up a different ecosystem had evolved. Before the drought (Period 1) Tissawewa had vegetation only in the shallow, littoral zone and a high turbidity due to resuspension of the thick layer of detritus on the bottom. After the drought (Period 2) vegetation was found all over the reservoir, covering the entire water column, and the water was significantly clearer due to a lower concentration of suspended detritus and phytoplankton. During Period 1 the average fish biomass was about 1700 kg/ha, while during Period 2 this was only 770 kg/ha. The community was at equilibrium at the beginning of the sampling period but as the water level decreased below 1.5 in maximum depth, this equilibrium was disturbed. About one year after the reservoir refilled, a new equilibrium different from the former equilibrium was established. The species mainly affected by these environmental changes was <em>A.</em><em>melettinus.</em> Its share in the total fish biomass decreased from on average 45% during Period I to 9% during Period 2. This was mainly caused by a decrease of its main food source, that of suspended detritus, from respectively 7.3 g C/m <sup><font size="-2">3</font></SUP>to 2.1 g C/m <sup><font size="-2">3</font></SUP>. This also illustrates the importance of bottom-up regulation for the structure of the Tissawewa fish community.<p>The fact that the structure of the Tissawewa fish community is deterministically regulated together with the importance of bottom-up regulation implies that competition and resource partitioning are important mechanisms determining the abundance of the different populations. Therefore the niche occupation of the ten most important fish species was determined along three dimensions: the trophic, the spatial and the temporal dimension. The position along the trophic dimension depends on the diet of the species, the position along the spatial dimension on the distribution over the various habitats in the reservoir and the position along the temporal dimension depends on the time of day the species were actively foraging. Because the body size of fish can increase several orders of magnitude during ontogeny which, in turn, can have considerable consequences for the niche occupation of an individual, each species was subdivided into size-classes for which niche occupation was determined separately. Not only the size-specific niche occupation along each dimension was determined in this study, but also the interaction between the different dimensions. In order to comprehend the impact of these interactions two types of competition should be distinguished: exploitative competition and interference competition. The former describes how species affect each other through the exploitation of the same food sources, the latter deals with a more direct interference through the capturing of space.<p>Exploitative competition mainly depends on the interaction between the trophic and spatial dimension. In other words: what food is taken from where while the time at which this happens is unimportant. Interference competition depends on the interaction between the spatial and temporal dimension: where and when is a certain species active. With regard to the time of day a species is actively foraging, seven diurnal species were observed, two nocturnal piscivores while one species was mainly active during dusk and dawn. When actively foraging the distribution of a fish is determined by a trade-off between foraging profitability and risk of predation. For those species or size-classes subject to predation, avoidance of predation is the main factor determining their distribution even if this results in suboptimal foraging. The juveniles of all species avoid predation by using the cover provided by the vegetation. The main adult prey, <em>A.</em><em>melettinus, H. gaimardi</em> and <em>R.</em><em>daniconius</em> avoid predation by evading the bottom layer of the reservoir where the piscivores reside. For those individuals which are not foraging their distribution is aimed at avoiding interference competition and predation. This is realised by all fish through a migration upward along the vertical plane. Environmental changes affecting the risk of predation and foraging rate are reflected in the distribution patterns of the fish species involved, confirming the ability of species to facultatively respond to these factors. Because fish density and consequently interference competition was lower after the drought, the upward migration also decreased.<p>For the Tissawewa fish community a correlation was observed between the morphology of a fish and its diet. This diet was apparently determined by only a few morphological characters. The gape width constrained the maximum size of the food particles, the gut length was related to the potential of utilising vegetable matter and the orientation of the mouth and the presence of barbels were correlated with the position of the food along the vertical gradient. Assuming functional relationships, the potential niches of the species were established, and within each species the main ontogenetic differences could be explained. The realised trophic niches of each population both before and after the drought coincided with what could be expected from the relative positions of the potential niches of these populations along the trophic resource dimension assuming niche segregation.<p>It was observed that size-specific measures of calculating niche breadths and niche overlaps, in which ontogenetic changes can be incorporated, present considerable advantages to the conventional measures when studying intra- and interspecific competition. For example, niche breadth which can be considered an indicator of the flexibility of a species to adjust to changes in food availability is markedly affected by ontogenetic changes; species with large ontogenetic changes would be considered more flexible then they actually are. Species of which the juvenile stages appeared to compete for resources would not be considered potential competitors based on the conventional measures. Also, possible competition for food was observed between the zooplanktivorous juvenile stages of several species with non- zooplanktivorous large adults, and the adult zooplanktivores: the juvenile competitive bottleneck. The abundance of a species is determined by the combination of intra- and interspecific competition. The calculated niche overlap measures were compared: 1) within one species between ontogenetic stages; 2) between species; 3) for the entire fish community between the periods before and after the drought. This strongly suggested that: 1) intra-specific competition is lower for species with large ontogenetic changes; 2) inter-specific competition is lower for specialists; and 3) competition for resources exists. The latter was concluded because during Period 1 when fish biomass was highest, niche segregation was highest and consequently average niche overlap was lowest. For the Tissawewa fish community segregation along the trophic dimension was more important than along the spatial or temporal dimension.<p>Based on niche overlap along three resource dimensions, the main potential indigenous competitor of the exotic introduced tilapias is the small pelagic cyprinid, <em>A</em> . <em>melettinus.</em> However, a more detailed analysis of the foraging behaviour of these potential competitors shows that <em>A.</em><em>melettinus</em> mainly feeds on the fine particulate detritus and phytoplankton, suspended in the water column, while the tilapias graze the deferral aggregate from the bottom. This implies that <em>A</em> . <em>melettinus is</em> not a true competitor of the tilapias and might even benefit from the tilapias breaking down the flocculant material from the bottom into smaller, better suspendable, particles. Therefore, the successful introduction of the lacustrine tilapias can be explained from the ability of these species to occupy a previously unfilled niche, and as a consequence the indigenous fish community was most probably not harmed by this introduction.<p>The acquired knowledge of the Tissawewa ecosystem pertaining to the effects of, among others, competition, predation and water level fluctuations on the fish community is used to discuss the consequences of various management measures on the fish production and fish yield. It was concluded that an additional exploitation of van the small pelagic fish species, either directly, or through introduction and exploitation of a large (controllable) piscivore, will most probably enhance the existing fishery on tilapias. Based on the Tissawewa case study it is concluded that at least for Tissawewa, but most probably for all similar tropical waterbodies a markedly higher utilisation efficiency (yield/ production) can be achieved than the 10% that is currently assumed.
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