We have examined the effects of herbivore diversity on parasitoid community persistence and stabi... more We have examined the effects of herbivore diversity on parasitoid community persistence and stability mediated by nonspecific information from herbivore-infested plants. First, we investigated host location and patch time allocation in the parasitoid Cotesia glomerata in environments where host and/or nonhost herbivores were present on Brassica oleracea leaves. Parasitoids were attracted by infochemicals from leaves containing nonhost herbivores. They spent
We analysed the foraging behaviour of two closely related parasitoid species "Cote! sia rubecula ... more We analysed the foraging behaviour of two closely related parasitoid species "Cote! sia rubecula and Cotesia glomerata# with respect to leaving tendencies from patches in di}erent environments [ We investigated how intrapatch experiences like contact with feeding damage and encounters with hosts in~uence patch leaving decisions[ We also estimated the e}ect of experiences in previously visited patches on leaving decisions in the present patch[ 1[ For this analysis we applied the proportional hazards model "Cox 0861# to data collected in three versions of a multiple patch set!up[ These set!ups consisted of di}erent host species or combinations of host species] "0# Pieris rapae\ "1# Pieris brassicae and "2# both P[ rapae and P[ brassicae[ The larvae of these hosts di}er in their spatial distribution on plants] P[ brassicae occur in clusters and the distribution of larvae is heterogeneous^P[ rapae larvae feed solitarily[ 2[ The specialist parasitoid C[ rubecula used a simple strategy] highest leaving tendency on empty leaves\ lower leaving tendency on leaves infested with the non!preferred host P[ brassicae\ lowest leaving tendency on leaves infested with the preferred host\ P[ rapae[ In the environment with both host species\ the leaving tendency only decreased on leaves infested with P[ rapae[ 3[ The generalist C[ glomerata used a more complex set of rules[ "a# Multiple ovi! positions on the present patch decreased the leaving tendency on leaves containing the gregarious host[ "b# Once the parasitoid had encountered two or more hosts\ it had a lower leaving tendency during subsequent patch visits[ "c# The leaving tendency increased with the number of visits on infested leaves[ In environments where the less preferred host P[ rapae was present\ C[ glomerata switched to the same simple type of rule as used by C[ rubecula[ 4[ Neither of the two Cotesia species used a count!down rule\ in which ovipositions increase the leaving tendency[ We discuss how patch exploitation by both Cotesia species compares to the patch exploitation mechanisms as proposed by Waage "0868# and Driessen et al[ "0884#[ 5[ We formulate an {adjustable termination rate| model for patch exploitation in both Cotesia species in multi!patch environments[ Key!words] foraging behaviour\ host distribution\ parasitoids\ phenotypic plasticity\ time allocation[ Journal of Animal Ecology "0887# 56\ 663Ð672 Information processing by two Cotesia species Þ
In contradiction with ®eld observations, theory predicts that the number of coexisting plankton s... more In contradiction with ®eld observations, theory predicts that the number of coexisting plankton species at equilibrium cannot exceed the number of limiting resources, which is called the``paradox of the plankton''. Recently, Huisman & Weissing (1999 showed, in a model study, that the number of coexisting species may exceed the number of limiting resources when internal system feedback induces oscillations or chaos. In this paper, we use the term``supersaturated coexistence'' for this phenomenon. On the basis of these ®ndings, they claimed that the paradox of the plankton is solved. We investigated the prerequisites for supersaturated coexistence in the same model. Our results indicate that supersaturated coexistence is a rare phenomenon in parameter space, requires a very precise parameterization of the community members and is sensitive to the introduction of new species and the removal of the present species. This raises the question of whether supersaturated coexistence is likely to occur in nature. We conclude that the claim by is premature.
Résumé/Abstract We use simple models to examine how the abundances of three trophic levels change... more Résumé/Abstract We use simple models to examine how the abundances of three trophic levels change in response to mortality imposed on each one of the levels. The models contain two factors whose joint effects have not been incorporated into previous analyses: ...
Here we consider how information transfer shapes interactions in aquatic and terrestrial food web... more Here we consider how information transfer shapes interactions in aquatic and terrestrial food webs. All organisms, whether they are dead or alive, release certain chemicals into their environment. These can be used as infochemicals by any other individual in the food web that has the biological machinery to sense and process such information. Such machinery has evolved in bacteria, plants and animals and has thus become an inextricable part of the mechanisms that underlie feeding relations in food webs.
Realistic functional responses are required for accurate model predictions at the community level... more Realistic functional responses are required for accurate model predictions at the community level. However, controversy remains regarding which types of dependencies need to be included in functional response models. Several studies have shown an effect of very high predator densities on per capita predation rates, but it is unclear whether this predator dependence is also important at low predator densities. We fit integrated functional response models to predation data from 4-h experiments where we had varied both predator and prey densities. Using an information theoretic approach we show that the best-fit model includes moderate predator dependence, which was equally strong even at low predator densities. The best fits of Beddington-DeAngelis and Arditi-Akçakaya functional responses were closely followed by the fit of the Arditi-Ginzburg model. A Holling type III functional response did not describe the data well. In addition, independent behavioral observations revealed high encounter rates between predators. We quantified the number of encounters between predators and the time the focal predator spent interacting with other individuals per encounter. This time ''wasted'' on conspecifics reduced the total time available for foraging and may therefore account for lower predation rates at higher predator densities. Our findings imply that ecological theory needs to take realistic levels of predator dependence into account.
Inducible defenses are dynamic traits that modulate the strength of both plantherbivore and herbi... more Inducible defenses are dynamic traits that modulate the strength of both plantherbivore and herbivore-carnivore interactions. Surprisingly few studies have considered the relative contributions of induced plant and herbivore defenses to the overall balance of bottom-up and top-down control. Here we compare trophic cascade strengths using replicated two-level and three-level plankton communities in which we systematically varied the presence or absence of induced defenses at the plant and/or herbivore levels. Our results show that a trophic cascade, i.e., significantly higher plant biomass in three-level than in two-level food chains, occurred whenever herbivores were undefended against carnivores. Trophic cascades did not occur when herbivores exhibited an induced defense. This pattern was obtained irrespective of the presence or absence of induced defenses at the plant level. We thus found that herbivore defenses, not plant defenses, had an overriding effect on cascade strength. We discuss these results in relation to variation in cascade strengths in natural communities.
European and American populations of the parasitoid Cotesia glomerata show pronounced differences... more European and American populations of the parasitoid Cotesia glomerata show pronounced differences in foraging behavior across plants and leaves. This variation in spatial aspects of foraging behavior was observed about 350 generations after the introduction of C. glomerata from Europe to North America. We used a simulation model to study how these behavioral differences affect lifetime reproductive success in environments that differ in the spatial distribution of hosts. The preferred gregarious host Pieris brassicae occurs in rare large clusters in Europe but is absent in North America. The solitary caterpillars of Pieris rapae are negative binomially distributed across plants during summer in North America, whereas they are Poisson-distributed in Europe, and early and late in the season in North America. Simulations showed that the foraging strategy of American C. glomerata resulted in a higher lifetime reproductive success than did the strategy of European C. glomerata on a Poisson P. rapae distribution, but did not differ on the more clustered negative binomial distribution. American parasitoids spend less time on exploration flights, focusing on the exploitation of P. rapae patches. This suggests that C. glomerata has adapted to the North American environment through the loss of exploration traits necessary for the location of rare clusters of P. brassicae. Lifetime reproductive success of the European strategy was most sensitive to an increase in the giving up time on infested leaves. This behavioral parameter was more than twice as high in the American parasitoids compared with their European conspecifics.
Predation occurs in a context defined by both prey and non-prey species. At present it is largely... more Predation occurs in a context defined by both prey and non-prey species. At present it is largely unknown how species diversity in general, and species that are not included in a predator's diet in particular, modify predator-prey interactions.
Many organisms have developed defences to avoid predation by species at higher trophic levels. Th... more Many organisms have developed defences to avoid predation by species at higher trophic levels. The capability of primary producers to defend themselves against herbivores affects their own survival, can modulate the strength of trophic cascades and changes rates of competitive exclusion in aquatic communities. Algal species are highly flexible in their morphology, growth form, biochemical composition and production of toxic and deterrent compounds. Several of these variable traits in phytoplankton have been interpreted as defence mechanisms against grazing. Zooplankton feed with differing success on various phytoplankton species, depending primarily on size, shape, cell wall structure and the production of toxins and deterrents. Chemical cues associated with (i) mechanical damage, (ii) herbivore presence and (iii) grazing are the main factors triggering induced defences in both marine and freshwater phytoplankton, but most studies have failed to disentangle the exact mechanism(s) governing defence induction in any particular species. Induced defences in phytoplankton include changes in morphology (e.g. the formation of spines, colonies and thicker cell walls), biochemistry (such as production of toxins, repellents) and in life history characteristics (formation of cysts, reduced recruitment rate). Our categorization of inducible defences in terms of the responsible induction mechanism provides guidance for future work, as hardly any of the available studies on marine or freshwater plankton have performed all the treatments that are required to pinpoint the actual cue(s) for induction. We discuss the ecology of inducible defences in marine and freshwater phytoplankton with a special focus on the mechanisms of induction, the types of defences, their costs and benefits, and their consequences at the community level.
1. Theoretical studies have shown that inducible defences have the potential to affect population... more 1. Theoretical studies have shown that inducible defences have the potential to affect population stability and persistence in bi-and tritrophic food chains. Experimental studies on such effects of prey defence strategies on the dynamics of predator-prey systems are still rare. We performed replicated population dynamics experiments using the herbivorous rotifer Brachionus calyciflorus and four strains of closely related algae that show different defence responses to this herbivore. 2. We observed herbivore populations to fluctuate at a higher frequency when feeding on small undefended algae. During these fluctuations minimum rotifer densities remained sufficiently high to ensure population persistence in all the replicates. The initial growth of rotifer populations in this treatment coincided with a sharp drop in algal density. Such a suppression of algae by herbivores was not observed in the other treatments, where algae were larger due to induced or permanent defences. In these treatments we observed rotifer population densities to first rise and then decline. The herbivore went extinct in all replicates with large permanently defended algae. The frequency of herbivore extinctions was intermediate when algae had inducible defences. 3. A variety of alternative mechanisms could explain differential herbivore persistence in the different defence treatments. Our analysis showed the density and fraction of highly edible algal particles to better explain herbivore persistence and extinctions than total algal density, the fraction of highly inedible food particles or the accumulation of herbivore waste products or autotoxins. 4. We argue that the rotifers require a minimum fraction and density of edible food particles for maintenance and reproduction. We conjecture that induced defences in algae may thus favour larger zooplankton species such as Daphnia spp. that are less sensitive to shifts in their food size spectrum, relative to smaller zooplankton species, such as rotifers and in this way contributes to the structuring of planktonic communities.
Infochemicals are known to play a key role in mediating predator-prey interactions, both in aquat... more Infochemicals are known to play a key role in mediating predator-prey interactions, both in aquatic and terrestrial communities. However, state-dependent variation may exist in how effectively individuals can use this information, depending on genotype, life stage and experience. For our study, we used the predator-prey model system fishwaterflea Daphnia magna Straus (Cladocera, Daphniidae). Adult Daphnia use fish-derived infochemicals, so-called kairomones, as indicators of predation risk, and exhibit a spectrum of morphological, behavioural and life-history responses to the presence of fish kairomones. Here, we investigate whether diapausing eggs, an embryonic resting stage in the life cycle of D. magna, also use fish kairomones and tune their hatching to the risk of fish predation, as reported for diapausing stages of dinoflagellates. In two laboratory experiments, we studied hatching proportion and time until hatching of D. magna diapausing eggs in the absence and presence of fish kairomones. D. magna families differed significantly in their response to the presence of fish kairomones; some families reduced hatching proportion, whereas others increased it. Our results imply genotype-dependent differences in the hatching reactions to fish kairomones as observed for other traits in adult Daphnia.
Theoretical studies have predicted that inducible defenses affect food chain dynamics and persist... more Theoretical studies have predicted that inducible defenses affect food chain dynamics and persistence. Here we review and evaluate laboratory experiments that tested hypotheses developed from these theoretical studies. This review specifically focuses on the effects of inducible defenses in phytoplankton-rotifer food chain dynamics. First, we describe the occurrence of colony formation within different strains of green algae (Scenedesmaceae) in response to infochemicals released during grazing by the herbivorous rotifer Brachionus calyciflorus. Then we examined the effects of inducible defenses on the population dynamics of this planktonic system in which algal strains that differed in their defense strategies were used. Simple food chains were composed of green algae (Scenedesmaceae), herbivorous rotifers (Brachionus calyciflorus) and carnivorous rotifers (Asplanchna brightwellii). In this system B. calyciflorus exhibits an inducible defense against predation by developing long postero-lateral spines. Experimental studies showed that inducible defenses, as opposed to their absence, could prevent highamplitude population fluctuations. We discuss the dual effects of induced defenses on extinction probabilities and consider the fit of a theoretical model to experimental data to understand the mechanisms that underlie the observed dynamics.
Macrobenthic deposit feeders and bacteria compete for the same detrital food resources. We hypoth... more Macrobenthic deposit feeders and bacteria compete for the same detrital food resources. We hypothesize that the spatial scale at which food is distributed in the sediment is an important factor determining the outcome of this competition. Macrobenthic deposit feeders are better adapted for fast consumption of food in concentrated patches, whereas diluted food can only be exploited by bacteria. This hypothesis was tested in an experiment in which a fixed quantity of isotopically labeled algal detritus was offered to a natural bacterial community and the polychaete worm Nereis (5Hediste) diversicolor, either as a concentrated patch or mixed through the sediment matrix. Worms dominated food uptake in the concentrated treatment, while bacterial uptake was much greater in the diluted treatment. The experiment demonstrated scale-based niche differentiation between these taxonomically distant groups. It also showed that worms spatially redistributed food and made it available to bacteria in that way. Together, these mechanisms may stimulate stable co-existence through a scale-based partitioning of resources.
11.1 ABSTRACT Recently it has been shown that inducible defenses in species at the bottom and mid... more 11.1 ABSTRACT Recently it has been shown that inducible defenses in species at the bottom and middle of three-species food chains promote stability and typically allow all trophic levels to increase in response to fertilization. However, that past work did not allow ...
We report a 40 year record of eutrophication and hypoxia on an estuarine ecosystem and its recove... more We report a 40 year record of eutrophication and hypoxia on an estuarine ecosystem and its recovery from hypereutrophication. After decades of high inorganic nutrient concentrations and recurring anoxia and hypoxia, we observe a paradoxical increase in chlorophyll-a concentrations with decreasing nutrient inputs. We hypothesise that algal growth was inhibited due to hypereutrophication, either by elevated ammonium concentrations, severe hypoxia or the production of harmful substances in such a reduced environment. We study the dynamics of a simple but realistic mathematical model, incorporating the assumption of algal growth inhibition. It shows a high algal biomass, net oxygen production equilibrium with low ammonia inputs, and a low algal biomass, net oxygen consumption equilibrium with high ammonia inputs. At intermediate ammonia inputs it displays two alternative stable states. Although not intentional, the numerical output of this model corresponds to observations, giving extra support for assumption of algal growth inhibition. Due to potential algal growth inhibition, the recovery of hypereutrophied systems towards a classical eutrophied state, will need reduction of waste loads below certain thresholds and will be accompanied by large fluctuations in oxygen concentrations. We conclude that also flow-through systems, heavily influenced by external forcings which partly mask internal system dynamics, can display multiple stable states.
We have examined the effects of herbivore diversity on parasitoid community persistence and stabi... more We have examined the effects of herbivore diversity on parasitoid community persistence and stability mediated by nonspecific information from herbivore-infested plants. First, we investigated host location and patch time allocation in the parasitoid Cotesia glomerata in environments where host and/or nonhost herbivores were present on Brassica oleracea leaves. Parasitoids were attracted by infochemicals from leaves containing nonhost herbivores. They spent
We analysed the foraging behaviour of two closely related parasitoid species "Cote! sia rubecula ... more We analysed the foraging behaviour of two closely related parasitoid species "Cote! sia rubecula and Cotesia glomerata# with respect to leaving tendencies from patches in di}erent environments [ We investigated how intrapatch experiences like contact with feeding damage and encounters with hosts in~uence patch leaving decisions[ We also estimated the e}ect of experiences in previously visited patches on leaving decisions in the present patch[ 1[ For this analysis we applied the proportional hazards model "Cox 0861# to data collected in three versions of a multiple patch set!up[ These set!ups consisted of di}erent host species or combinations of host species] "0# Pieris rapae\ "1# Pieris brassicae and "2# both P[ rapae and P[ brassicae[ The larvae of these hosts di}er in their spatial distribution on plants] P[ brassicae occur in clusters and the distribution of larvae is heterogeneous^P[ rapae larvae feed solitarily[ 2[ The specialist parasitoid C[ rubecula used a simple strategy] highest leaving tendency on empty leaves\ lower leaving tendency on leaves infested with the non!preferred host P[ brassicae\ lowest leaving tendency on leaves infested with the preferred host\ P[ rapae[ In the environment with both host species\ the leaving tendency only decreased on leaves infested with P[ rapae[ 3[ The generalist C[ glomerata used a more complex set of rules[ "a# Multiple ovi! positions on the present patch decreased the leaving tendency on leaves containing the gregarious host[ "b# Once the parasitoid had encountered two or more hosts\ it had a lower leaving tendency during subsequent patch visits[ "c# The leaving tendency increased with the number of visits on infested leaves[ In environments where the less preferred host P[ rapae was present\ C[ glomerata switched to the same simple type of rule as used by C[ rubecula[ 4[ Neither of the two Cotesia species used a count!down rule\ in which ovipositions increase the leaving tendency[ We discuss how patch exploitation by both Cotesia species compares to the patch exploitation mechanisms as proposed by Waage "0868# and Driessen et al[ "0884#[ 5[ We formulate an {adjustable termination rate| model for patch exploitation in both Cotesia species in multi!patch environments[ Key!words] foraging behaviour\ host distribution\ parasitoids\ phenotypic plasticity\ time allocation[ Journal of Animal Ecology "0887# 56\ 663Ð672 Information processing by two Cotesia species Þ
In contradiction with ®eld observations, theory predicts that the number of coexisting plankton s... more In contradiction with ®eld observations, theory predicts that the number of coexisting plankton species at equilibrium cannot exceed the number of limiting resources, which is called the``paradox of the plankton''. Recently, Huisman & Weissing (1999 showed, in a model study, that the number of coexisting species may exceed the number of limiting resources when internal system feedback induces oscillations or chaos. In this paper, we use the term``supersaturated coexistence'' for this phenomenon. On the basis of these ®ndings, they claimed that the paradox of the plankton is solved. We investigated the prerequisites for supersaturated coexistence in the same model. Our results indicate that supersaturated coexistence is a rare phenomenon in parameter space, requires a very precise parameterization of the community members and is sensitive to the introduction of new species and the removal of the present species. This raises the question of whether supersaturated coexistence is likely to occur in nature. We conclude that the claim by is premature.
Résumé/Abstract We use simple models to examine how the abundances of three trophic levels change... more Résumé/Abstract We use simple models to examine how the abundances of three trophic levels change in response to mortality imposed on each one of the levels. The models contain two factors whose joint effects have not been incorporated into previous analyses: ...
Here we consider how information transfer shapes interactions in aquatic and terrestrial food web... more Here we consider how information transfer shapes interactions in aquatic and terrestrial food webs. All organisms, whether they are dead or alive, release certain chemicals into their environment. These can be used as infochemicals by any other individual in the food web that has the biological machinery to sense and process such information. Such machinery has evolved in bacteria, plants and animals and has thus become an inextricable part of the mechanisms that underlie feeding relations in food webs.
Realistic functional responses are required for accurate model predictions at the community level... more Realistic functional responses are required for accurate model predictions at the community level. However, controversy remains regarding which types of dependencies need to be included in functional response models. Several studies have shown an effect of very high predator densities on per capita predation rates, but it is unclear whether this predator dependence is also important at low predator densities. We fit integrated functional response models to predation data from 4-h experiments where we had varied both predator and prey densities. Using an information theoretic approach we show that the best-fit model includes moderate predator dependence, which was equally strong even at low predator densities. The best fits of Beddington-DeAngelis and Arditi-Akçakaya functional responses were closely followed by the fit of the Arditi-Ginzburg model. A Holling type III functional response did not describe the data well. In addition, independent behavioral observations revealed high encounter rates between predators. We quantified the number of encounters between predators and the time the focal predator spent interacting with other individuals per encounter. This time ''wasted'' on conspecifics reduced the total time available for foraging and may therefore account for lower predation rates at higher predator densities. Our findings imply that ecological theory needs to take realistic levels of predator dependence into account.
Inducible defenses are dynamic traits that modulate the strength of both plantherbivore and herbi... more Inducible defenses are dynamic traits that modulate the strength of both plantherbivore and herbivore-carnivore interactions. Surprisingly few studies have considered the relative contributions of induced plant and herbivore defenses to the overall balance of bottom-up and top-down control. Here we compare trophic cascade strengths using replicated two-level and three-level plankton communities in which we systematically varied the presence or absence of induced defenses at the plant and/or herbivore levels. Our results show that a trophic cascade, i.e., significantly higher plant biomass in three-level than in two-level food chains, occurred whenever herbivores were undefended against carnivores. Trophic cascades did not occur when herbivores exhibited an induced defense. This pattern was obtained irrespective of the presence or absence of induced defenses at the plant level. We thus found that herbivore defenses, not plant defenses, had an overriding effect on cascade strength. We discuss these results in relation to variation in cascade strengths in natural communities.
European and American populations of the parasitoid Cotesia glomerata show pronounced differences... more European and American populations of the parasitoid Cotesia glomerata show pronounced differences in foraging behavior across plants and leaves. This variation in spatial aspects of foraging behavior was observed about 350 generations after the introduction of C. glomerata from Europe to North America. We used a simulation model to study how these behavioral differences affect lifetime reproductive success in environments that differ in the spatial distribution of hosts. The preferred gregarious host Pieris brassicae occurs in rare large clusters in Europe but is absent in North America. The solitary caterpillars of Pieris rapae are negative binomially distributed across plants during summer in North America, whereas they are Poisson-distributed in Europe, and early and late in the season in North America. Simulations showed that the foraging strategy of American C. glomerata resulted in a higher lifetime reproductive success than did the strategy of European C. glomerata on a Poisson P. rapae distribution, but did not differ on the more clustered negative binomial distribution. American parasitoids spend less time on exploration flights, focusing on the exploitation of P. rapae patches. This suggests that C. glomerata has adapted to the North American environment through the loss of exploration traits necessary for the location of rare clusters of P. brassicae. Lifetime reproductive success of the European strategy was most sensitive to an increase in the giving up time on infested leaves. This behavioral parameter was more than twice as high in the American parasitoids compared with their European conspecifics.
Predation occurs in a context defined by both prey and non-prey species. At present it is largely... more Predation occurs in a context defined by both prey and non-prey species. At present it is largely unknown how species diversity in general, and species that are not included in a predator's diet in particular, modify predator-prey interactions.
Many organisms have developed defences to avoid predation by species at higher trophic levels. Th... more Many organisms have developed defences to avoid predation by species at higher trophic levels. The capability of primary producers to defend themselves against herbivores affects their own survival, can modulate the strength of trophic cascades and changes rates of competitive exclusion in aquatic communities. Algal species are highly flexible in their morphology, growth form, biochemical composition and production of toxic and deterrent compounds. Several of these variable traits in phytoplankton have been interpreted as defence mechanisms against grazing. Zooplankton feed with differing success on various phytoplankton species, depending primarily on size, shape, cell wall structure and the production of toxins and deterrents. Chemical cues associated with (i) mechanical damage, (ii) herbivore presence and (iii) grazing are the main factors triggering induced defences in both marine and freshwater phytoplankton, but most studies have failed to disentangle the exact mechanism(s) governing defence induction in any particular species. Induced defences in phytoplankton include changes in morphology (e.g. the formation of spines, colonies and thicker cell walls), biochemistry (such as production of toxins, repellents) and in life history characteristics (formation of cysts, reduced recruitment rate). Our categorization of inducible defences in terms of the responsible induction mechanism provides guidance for future work, as hardly any of the available studies on marine or freshwater plankton have performed all the treatments that are required to pinpoint the actual cue(s) for induction. We discuss the ecology of inducible defences in marine and freshwater phytoplankton with a special focus on the mechanisms of induction, the types of defences, their costs and benefits, and their consequences at the community level.
1. Theoretical studies have shown that inducible defences have the potential to affect population... more 1. Theoretical studies have shown that inducible defences have the potential to affect population stability and persistence in bi-and tritrophic food chains. Experimental studies on such effects of prey defence strategies on the dynamics of predator-prey systems are still rare. We performed replicated population dynamics experiments using the herbivorous rotifer Brachionus calyciflorus and four strains of closely related algae that show different defence responses to this herbivore. 2. We observed herbivore populations to fluctuate at a higher frequency when feeding on small undefended algae. During these fluctuations minimum rotifer densities remained sufficiently high to ensure population persistence in all the replicates. The initial growth of rotifer populations in this treatment coincided with a sharp drop in algal density. Such a suppression of algae by herbivores was not observed in the other treatments, where algae were larger due to induced or permanent defences. In these treatments we observed rotifer population densities to first rise and then decline. The herbivore went extinct in all replicates with large permanently defended algae. The frequency of herbivore extinctions was intermediate when algae had inducible defences. 3. A variety of alternative mechanisms could explain differential herbivore persistence in the different defence treatments. Our analysis showed the density and fraction of highly edible algal particles to better explain herbivore persistence and extinctions than total algal density, the fraction of highly inedible food particles or the accumulation of herbivore waste products or autotoxins. 4. We argue that the rotifers require a minimum fraction and density of edible food particles for maintenance and reproduction. We conjecture that induced defences in algae may thus favour larger zooplankton species such as Daphnia spp. that are less sensitive to shifts in their food size spectrum, relative to smaller zooplankton species, such as rotifers and in this way contributes to the structuring of planktonic communities.
Infochemicals are known to play a key role in mediating predator-prey interactions, both in aquat... more Infochemicals are known to play a key role in mediating predator-prey interactions, both in aquatic and terrestrial communities. However, state-dependent variation may exist in how effectively individuals can use this information, depending on genotype, life stage and experience. For our study, we used the predator-prey model system fishwaterflea Daphnia magna Straus (Cladocera, Daphniidae). Adult Daphnia use fish-derived infochemicals, so-called kairomones, as indicators of predation risk, and exhibit a spectrum of morphological, behavioural and life-history responses to the presence of fish kairomones. Here, we investigate whether diapausing eggs, an embryonic resting stage in the life cycle of D. magna, also use fish kairomones and tune their hatching to the risk of fish predation, as reported for diapausing stages of dinoflagellates. In two laboratory experiments, we studied hatching proportion and time until hatching of D. magna diapausing eggs in the absence and presence of fish kairomones. D. magna families differed significantly in their response to the presence of fish kairomones; some families reduced hatching proportion, whereas others increased it. Our results imply genotype-dependent differences in the hatching reactions to fish kairomones as observed for other traits in adult Daphnia.
Theoretical studies have predicted that inducible defenses affect food chain dynamics and persist... more Theoretical studies have predicted that inducible defenses affect food chain dynamics and persistence. Here we review and evaluate laboratory experiments that tested hypotheses developed from these theoretical studies. This review specifically focuses on the effects of inducible defenses in phytoplankton-rotifer food chain dynamics. First, we describe the occurrence of colony formation within different strains of green algae (Scenedesmaceae) in response to infochemicals released during grazing by the herbivorous rotifer Brachionus calyciflorus. Then we examined the effects of inducible defenses on the population dynamics of this planktonic system in which algal strains that differed in their defense strategies were used. Simple food chains were composed of green algae (Scenedesmaceae), herbivorous rotifers (Brachionus calyciflorus) and carnivorous rotifers (Asplanchna brightwellii). In this system B. calyciflorus exhibits an inducible defense against predation by developing long postero-lateral spines. Experimental studies showed that inducible defenses, as opposed to their absence, could prevent highamplitude population fluctuations. We discuss the dual effects of induced defenses on extinction probabilities and consider the fit of a theoretical model to experimental data to understand the mechanisms that underlie the observed dynamics.
Macrobenthic deposit feeders and bacteria compete for the same detrital food resources. We hypoth... more Macrobenthic deposit feeders and bacteria compete for the same detrital food resources. We hypothesize that the spatial scale at which food is distributed in the sediment is an important factor determining the outcome of this competition. Macrobenthic deposit feeders are better adapted for fast consumption of food in concentrated patches, whereas diluted food can only be exploited by bacteria. This hypothesis was tested in an experiment in which a fixed quantity of isotopically labeled algal detritus was offered to a natural bacterial community and the polychaete worm Nereis (5Hediste) diversicolor, either as a concentrated patch or mixed through the sediment matrix. Worms dominated food uptake in the concentrated treatment, while bacterial uptake was much greater in the diluted treatment. The experiment demonstrated scale-based niche differentiation between these taxonomically distant groups. It also showed that worms spatially redistributed food and made it available to bacteria in that way. Together, these mechanisms may stimulate stable co-existence through a scale-based partitioning of resources.
11.1 ABSTRACT Recently it has been shown that inducible defenses in species at the bottom and mid... more 11.1 ABSTRACT Recently it has been shown that inducible defenses in species at the bottom and middle of three-species food chains promote stability and typically allow all trophic levels to increase in response to fertilization. However, that past work did not allow ...
We report a 40 year record of eutrophication and hypoxia on an estuarine ecosystem and its recove... more We report a 40 year record of eutrophication and hypoxia on an estuarine ecosystem and its recovery from hypereutrophication. After decades of high inorganic nutrient concentrations and recurring anoxia and hypoxia, we observe a paradoxical increase in chlorophyll-a concentrations with decreasing nutrient inputs. We hypothesise that algal growth was inhibited due to hypereutrophication, either by elevated ammonium concentrations, severe hypoxia or the production of harmful substances in such a reduced environment. We study the dynamics of a simple but realistic mathematical model, incorporating the assumption of algal growth inhibition. It shows a high algal biomass, net oxygen production equilibrium with low ammonia inputs, and a low algal biomass, net oxygen consumption equilibrium with high ammonia inputs. At intermediate ammonia inputs it displays two alternative stable states. Although not intentional, the numerical output of this model corresponds to observations, giving extra support for assumption of algal growth inhibition. Due to potential algal growth inhibition, the recovery of hypereutrophied systems towards a classical eutrophied state, will need reduction of waste loads below certain thresholds and will be accompanied by large fluctuations in oxygen concentrations. We conclude that also flow-through systems, heavily influenced by external forcings which partly mask internal system dynamics, can display multiple stable states.
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Papers by Matthijs Vos