Spatial associations among algae affect host use in a herbivorous marine amphipod

Host plant choice by herbivores may be constrained by the spatial arrangement of plants with selection of poor quality hosts being more likely if preferred hosts are locally rare. I tested whether the presence of a herbivorous marine amphipod, Peramphithoe parmerong, on a poor quality host could be explained by the relative abundance and spatial arrangement of alternative brown algal hosts. Amphipods strongly preferred the high quality Sargassum linearifolium over the poor quality Padina crassa in laboratory assays with a higher tendency to select Padina crassa when it was relatively more abundant. In the field, however, adults colonised both hosts to equal densities. Rates of colonisation were independent of host quality but strongly affected by the identity of neighbouring hosts with those algae in close proximity to S. linearifolium receiving far greater densities of amphipods than those surrounded by Padina crassa. There was no evidence that the presence of adult Peramphithoe parmerong on Padina crassa was due to local scarcity of S. linearifolium. In contrast, the distribution and behaviour of juveniles were predictable from differences in food quality, with most inhabiting the high quality S. linearifolium and few emigrating from this host. The success of a simple adaptive model based on food quality to predict juvenile, but not adult, behaviour indicates that the relative importance of food quality varies with age, and that other factors are important determinants of adult movements among hosts.     

Effects of an invertebrate grazer on the spatial arrangement of a benthic microhabitat

We demonstrated the effect of an aquatic herbivore on the spatial arrangement of benthic algal biomass within artificial stream channels. Transects of ceramic tiles were exposed to a gradient of snail (Physella) densities in a 30 d experiment. We observed positive effects of snails on the mean abundance of overstory algae (the filamentous chlorophyte Cladophora and associated epiphytes), an important benthic microhabitat in streams. Snails altered several aspects of the spatial arrangement of overstory algae. Snails reduced the strength of downstream gradients in overstory biomass, as well as residual variability around these gradients. Geostatistical analysis revealed that snails also reduced the strength of spatial dependence, and so reduced spatial heterogeneity of the overstory, at small scales (<40 cm). As a result, organisms inhabiting the overstory might experience a more fragmented habitat landscape at high snail densities. In addition, snails increased the scale of spatial dependence in understory algal biomass (algae remaining on tiles after overstory was removed) from 10 cm to 40 cm. Consumer effects on the spatial arrangement of a microhabitat argue for the inclusion of feedbacks between the biota and the environment in spatially-explicit models.     

Stoichiometric impacts of increased carbon dioxide on a planktonic herbivore

The partial pressure of carbon dioxide (pCO₂) in lake ecosystems varies over four orders of magnitude and is affected by local and global environmental perturbations associated with both natural and anthropogenic processes. Little is known, however, about how changes in pCO₂ extend into the function and structure of food webs in freshwater ecosystems. To fill this gap, we performed laboratory experiments using the ecologically important planktonic herbivore Daphnia and its algal prey under a natural range of pCO₂ with low light and phosphorus supplies. The experiment showed that increased pCO₂ stimulated algal growth but reduced algal P : C ratio. When feeding on algae grown under high pCO₂, herbivore growth decreased regardless of algal abundance. Thus, high CO₂‐raised algae were poor food for Daphnia. Short‐term experimental supplementation of PO₄ raised the P content of the high CO₂‐raised algae and improved Daphnia growth, indicating that low Daphnia growth rates under high pCO₂ conditions were due to lowered P content in the algal food. These results suggest that, in freshwater ecosystems with low nutrient supplies, natural processes as well as anthropogenic perturbations resulting in increased pCO₂ enhance algal production but reduce energy and mass transfer efficiency to herbivores by decreasing algal nutritional quality.     

Do distances among host patches and host density affect the distribution of a specialist parasitoid?

The effect of spatial habitat structure and patchiness may differ among species within a multi-trophic system. Theoretical models predict that species at higher trophic levels are more negatively affected by fragmentation than are their hosts or preys. The absence or presence of the higher trophic level, in turn, can affect the population dynamics of lower levels and even the stability of the trophic system as a whole. The present study examines different effects of spatial habitat structure with two field experiments, using as model system the parasitoid Cotesia popularis which is a specialist larval parasitoid of the herbivore Tyria jacobaeae. One experiment examines the colonisation rate of the parasitoid and the percentage parasitism at distances occurring on a natural scale; the other experiment examines the dispersal rate and the percentage parasitism in relation to the density of the herbivore and its host plant. C. popularis was able to reach artificial host populations at distances up to the largest distance created (at least 80 m from the nearest source population). Also, the percentage parasitism did not differ among the distances. The density experiment showed that the total number of herbivores parasitised was higher in patches with a high density of hosts, regardless of the density of the host plant. The percentage parasitism, however, was not related to the density of the host. The density of the host plant did have a (marginally) significant effect on the percentage parasitism, probably indicating that the parasitoid uses the host plant of the herbivore as a cue to find the herbivore itself. In conclusion, the parasitoid was not affected by the spatial habitat structure on spatial scales that are typical of local patches.     

Competitor avoidance drives within‐host feeding site selection in a passively dispersed herbivore

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High nymphal host density and mortality negatively impact parasitoid complex during an insect herbivore outbreak

Insect herbivore outbreaks frequently occur and this may be due to factors that restrict top-down control by parasitoids, for example, host-parasitoid asynchrony, hyperparasitization, resource limitation and climate. Few studies have examined hostparasitoid density relationships during an in sect herbivore outbreak in a n atural ecosystem with diverse parasitoids. We studied parasitization patterns of Cardiaspina psyllids during an outbreak in a Eucalyptus woodland. First, we established the trophic roles of the parasitoids through a species-specific multiplex PCR approach on mummies from which parasitoids emerged. Then, we assessed host-parasitoid density relationships across three spatial scales (leaf, tree and site) over one yeas We detected four endoparasitoid species of the family Encyrtidae (Hymenoptera);two primary parasitoid and one heteronomous hyperparasitoid Psyllaephagus species (the latter with female development as a primary parasitoid and male development as a hyperparasitoid), and the hyperparasitoid Coccidoctonuspsyllae. Parasitoid development was host-synchronized, although synchrony between sites appeared constrained during winter (due to temperature differences). Parasitization was predominantly driven by one primary parasitoid species and was mostly inversely host-density dependent across the spatial scales. Hyperparasitization by C. psyllae was psyllid-density dependent at the site scale, however, this only impacted the rarer primary parasitoid. High larval parasitoid mortality due to density-dependent nymphal psyllid mortality (a consequence of resource limitation) compounded by a summer heat wave was incorporated in the assessment and resulted in density independence of host-parasitoid relationships. As such, high larval parasitoid mortality during insect herbivore outbreaks may contribute to the absence of host density-dependent parasitization during outbreak events.     

Extrinsically and intrinsically generated spatial patterns of algal abundance in an experimental stream

Spatial heterogeneity in species abundance arises from both extrinsic (largely abiotic) and intrinsic (largely biotic) processes. The relative importance of these two types of processes can vary across ecological systems and across temporal and spatial scales. Numerous empirical studies have explored spatial patterns resulting from extrinsic and intrinsic processes, however the interaction of these two types of processes can result in complex patterns that are difficult to test. We used a unique model system consisting of periphytic algae grown on agar in an experimental stream to manipulate an extrinsic and an intrinsic process. We manipulated an extrinsic process by varying the spatial arrangement of nutrients creating both heterogeneous and homogeneous environments for the algae. We manipulated an intrinsic process by introducing a snail herbivore to the system. The resulting spatial algal patterns showed that both types of processes were important in producing spatial abundance patterns and that the patterns occurred at two distinct spatial scales in our system. At the scale of the imposed nutrient heterogeneity, algae “tracked” the differences in nutrient supply rates. The snail herbivores both reduced and promoted spatial patterns in algal abundance at different spatial scales reflecting their species-specific foraging behavior. An ability to detect differences in algal abundance allowed the snails to reduce the power of patterns at the scale of the imposed nutrient heterogeneity; however below a spatial scale of approximately 30 mm the snails could no longer detect differences in algal abundance and so foraged randomly. At this spatial scale the spatial heterogeneity in algal abundance increased and the resulting algal patterns were relatively spatially fixed through time. We suggest that this relative constancy may arise in part from a detected weak Allee effect in algal growth rates.     

Endosymbiosis of Chlorella species to the ciliate Paramecium bursaria alters the distribution of the host’s trichocysts beneath the host cell cortex

Each symbiotic Chlorella of the ciliate Paramecium bursaria is enclosed in a perialgal vacuole membrane derived from the host digestive vacuole membrane. Alga-free paramecia and symbiotic algae can grow independently. Mixing them experimentally can cause reinfection. Earlier, we reported that the symbiotic algae appear to push the host trichocysts aside to become fixed beneath the host cell cortex during the algal reinfection process. Indirect immunofluorescence microscopy with a monoclonal antibody against the trichocysts demonstrates that the trichocysts change their locality to form algal attachment sites and decrease their density beneath the host cell cortex through algal reinfection. Transmission electron microscopy to detect acid phosphatase activity showed that some trichocysts near the host cell cortex are digested by the host lysosomal fusion during algal reinfection. Removal of algae from the host cell using cycloheximide recovers the trichocyst's arrangement and number near the host cell cortex. These results indicate that symbiotic algae compete for their attachment sites with preexisting trichocysts and that the algae have the ability to ensure algal attachment sites beneath the host cell cortex.     

Linking herbivore-induced defences to population dynamics

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.     

Effects of food quality on life history of the rotifer Brachionus calyciflorus Pallas

1. Herbivorous zooplankton face considerable temporal and spatial variation in food quality, to which they respond by adapting their life histories. Zooplankton may even take up mineral nutrients directly, and use these to counter the effects of algal nutrient limitation (mineral compensation). This study examined the life history of the rotifer Brachionus calyciflorus fed phosphorus‐, and nitrogen‐limited Scenedesmus obliquus (Chlorophyta), and investigated whether B. calyciflorus was capable of mineral compensation. 2. Both phosphorus‐ and nitrogen‐limited algae gave similar life history responses: somatic growth and reproduction were reduced, whereas lifespan remained unaffected. 3. No evidence was found for mineral compensation in B. calyciflorus in relation to detrimental life history effects, so mineral compensation does not seem to be relevant for this species under field conditions. 4. The similarity in life history responses of B. calyciflorus and the low levels of ω‐3 PUFAs in both phosphorus‐ and nitrogen‐depleted algae suggest that ω‐3 PUFAs were limiting to B. calyciflorus, although other (bio)chemicals or mineral nutrients may also have been important. 5. No trade‐off was observed between life span and reproduction during algal nutrient limitation. Reduced population growth rates of B. calyciflorus were caused by shorter reproductive periods.     

Distribution and colonisation ability of three parasitoids and their herbivorous host in a fragmented landscape

Habitat fragmentation can disrupt communities of interacting species even if only some of the species are directly affected by fragmentation. For instance, if parasitoids disperse less well than their herbivorous hosts, habitat fragmentation may lead to higher herbivory in isolated plant patches due to the absence of the third trophic level. Community-level studies suggest that parasitoids tend to have limited dispersal abilities, on the order of tens of metres, much smaller than that of their hosts, while species-oriented studies document dispersal by parasitoids on the scale of kilometres. In this study the distribution patterns of three parasitoid species with different life histories and their moth host, Hadena bicruris, a specialist herbivore of Silene latifolia, were compared in a large-scale network of natural fragmented plant patches along the rivers Rhine and Waal in the Netherlands. We examined how patch size and isolation affect the presence of each species. Additionally, experimental plots were used to study the colonisation abilities of the species at different distances from source populations.In the natural plant patches the presence of the herbivore and two of the parasitoids, the gregarious specialist Microplitis tristis and the gregarious generalist Bracon variator were not affected by patch isolation at the scale of the study, while the solitary specialist Eurylabus tristis was. In contrast to the herbivore, the presence of all parasitoid species declined with plant patch size. The colonisation experiment confirmed that the herbivore and M. tristis are good dispersers, able to travel at least 2 km within a season. B. variator showed intermediate colonisation ability and E. tristis showed very limited colonisation ability at this spatial scale. Characteristics of parasitoid species that may contribute to differences in their dispersal abilities are discussed.     

Brandtia ciliaticola gen. et sp. nov. (Chlorellaceae,Trebouxiophyceae) a common symbiotic green coccoid of various ciliate species

Many freshwater protists harbor unicellular green algae within their cells and these host‐symbiont relationships slowly are becoming better understood. Recently, we reported that several ciliate species shared a single species of symbiotic algae. Nonetheless, the algae from different host ciliates were each distinguishable by their different genotypes, and these host‐algal genotype combinations remained unchanged throughout a 15‐month period of sampling from natural populations. The same algal species had been reported as the shared symbiont of several ciliates from a remote lake. Consequently, this alga appears to play a key role in ciliate‐algae symbioses. In the present study, we successfully isolated the algae from ciliate cells and established unialgal cultures. This species is herein named Brandtia ciliaticola gen. et sp. nov. and has typical ‘Chlorella‐like’ morphology, being a spherical autosporic coccoid with a single chloroplast containing a pyrenoid. The alga belongs to the Chlorella‐clade in Chlorellaceae (Trebouxiophyceae), but it is not strongly connected to any of the other genera in this group. In addition to this phylogenetic distinctiveness, a unique compensatory base change in the SSU rRNA gene is decisive in distinguishing this genus. Sequences of SSU‐ITS (internal transcribed spacer) rDNA for each isolate were compared to those obtained previously from the same host ciliate. Consistent algal genotypes were recovered from each host, which strongly suggests that B. ciliaticola has established a persistent symbiosis in each ciliate species.     

Pattern analysis of eastern spruce budworm Choristoneura fumiferana dispersal

Dispersal has been proposed as an important mechanism in the broad‐scale synchronisation of insect outbreaks by linking spatially disjunct populations. Evidence suggests that dispersal is influenced by landscape structure, phenology, temperature, and air currents; however, the details remain unclear due to the difficulty of quantifying dispersal. In this study, we used data on the abundance and distribution of spruce budworm Choristoneura fumiferana larvae (potential dispersers) and adult male moths (dispersers) to make inference on the effects of air currents and host‐species abundance on dispersal. Hierarchical‐Bayesian and inverse modeling was used to explore 4 dispersal models: 1) isotropic dispersal; 2) directional‐dispersal; 3) directional‐and‐host‐species dispersal; and 4) host‐species dispersal. Despite their strong dependence on balsam fir Abies balsamea and spruce species Picea spp., the mapped basal area of these host species did not influence the pattern of dispersed moths. The model that best fit the data was the directional‐dispersal model, which showed that the prevailing dispersal direction was from the northwest (328°). We infer that the strong pattern of directional dispersal was due to a prevailing wind from the same direction. Our interpretation was corroborated by independent wind data during the period of active adult male budworm flight, particularly in the region with high larval abundance. Our results indicate that there was a relatively high probability of individuals flying at least 48 km with the wind where larvae abundance at source locations was also high. Such findings emphasize the importance of long‐distance dispersal on spatial distribution of adult male spruce budworms. Insight into the population‐level consequences of such dispersal patterns requires additional research.     

Herbivory and patch dynamics on rocky reefs in temperate Australasia: The roles of fish and sea urchins

Sea urchins are widely considered to be the major grazers in temperate subtidal systems, with herbivorous fish being browsers of minor importance. This paper reviews spatial and temporal patterns in these herbivores on rocky reels in temperate Australasia, with the aim of assessing their relative impacts on patch structure and dynamics. Herbivorous fishes are widespread and make up a significant numerical component the reel fish fauna. Sea urchins are also abundant, but not all geographic locations support actively grazing species. Both fish and sea urchins exhibit distinct patterns of distribution among depth strata. Within depth strata, all herbivores are restricted to (sea urchins) or forage preferentially in (fish) particular habitat patches, causing a mosaic of different feeding activities. These patches are either related to specific features of the habitat (e.g. Kelp patches, topography) or behavioural interactions. Foraging by sea urchins and demersal-nesting damselfishes is intense and persistent, whereas in the kelp-feeding fish Odax cyanomelas, foraging reaches greatest intensity at predictable locations during a few months of every year. Many fish and sea urchins consume some algae in preference to others. However, feeding preferences may determine the nature of the impact only in fishes. For sea urchins, preference may occasionally determine the order in which algae are consumed, but at high densities they consume all available macroalgae. Impacts of both types of herbivore on the abundance of algae have been recorded. Some sea urchins (e.g. Evechinus chloroticus, Centrostephanus rodgersii) appear to severely modify biogenic habitat structure by maintaining ‘barrens’ (areas devoid of macroalgae) over long periods. In contrast to this, the effects of fishes may be more transitory (e.g. seasonal impact of Odax cyanomelas on brown algae) or occur at smaller spatial scales (e.g. nest sites maintained by male Parma victoriae) Herbivorous and other fishes appear to respond to spatial patterns in algal distributions, rallier than having it major impact upon them. The relative effects of fish and sea urchins on the long-term dynamics of kelp forests are unknown, hut temporal patterns in herbivore abundance and behaviour, and algal demography arc urgent targets for research.     

Does colonization contribute to spatial patterns of common invertebrates in coralline algal turf?

Abstract The potential of colonization to contribute to the spatial patterns of six common invertebrates in coralline algal turf was investigated on a rocky shore near Sydney, Australia. The species, which included two amphipods (Elasmopus warra, Hyale spp.), a small bivalve (Lasaea australis), a fly larva (Limonia marina), and two microgastropods (Amphithalamus incidata and Eatoniella atropurpurea), had a range of dispersal modes (larval dispersal, crawling, swimming, rafting, and passive transport). Field sampling between May 1997 and November 1999 demonstrated that the amphipods were more abundant in low‐shore areas, the fly larvae and bivalves were more abundant in mid‐shore areas, and the abundances of gastropods did not vary with tidal height. Furthermore, abundances of all species varied among patches separated by tens of metres at one time or another. To test whether rates of colonization could contribute to established patterns of abundance, habitat mimics were deployed for 2‐week periods. The supply of new individuals matched long‐term patterns of abundance at different tidal heights for E. warra and L. marina. Colonization rates also differed among patches separated by tens of metres for three of the six species. Overall, there was little evidence to suggest that common species in coralline turf are limited by colonization on local scales, regardless of their major mode of dispersal. However, the potential for colonization to determine patterns of abundance varied from species to species.     

Dispersal abilities of riverine freshwater mussels influence metacommunity structure

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Population structure of a vector‐borne plant parasite

Parasites are among the most diverse groups of life on Earth, yet complex natural histories often preclude studies of their speciation processes. The biology of parasitic plants facilitates in situ collection of data on both genetic structure and the mechanisms responsible for that structure. Here, we studied the role of mating, dispersal and establishment in host race formation of a parasitic plant. We investigated the population genetics of a vector‐borne desert mistletoe (Phoradendron californicum) across two legume host tree species (Senegalia greggii and Prosopis velutina) in the Sonoran desert using microsatellites. Consistent with host race formation, we found strong host‐associated genetic structure in sympatry, little genetic variation due to geographic site and weak isolation by distance. We hypothesize that genetic differentiation results from differences in the timing of mistletoe flowering by host species, as we found initial flowering date of individual mistletoes correlated with genetic ancestry. Hybrids with intermediate ancestry were detected genetically. Individuals likely resulting from recent, successful establishment events following dispersal between the host species were detected at frequencies similar to hybrids between host races. Therefore, barriers to gene flow between the host races may have been stronger at mating than at dispersal. We also found higher inbreeding and within‐host individual relatedness values for mistletoes on the more rare and isolated host species (S. greggii). Our study spanned spatial scales to address how interactions with both vectors and hosts influence parasitic plant structure with implications for parasite virulence evolution and speciation.     

Effectiveness of Mistletoe Seed Dispersal by Tyrant Flycatchers in a Mixed Andean Landscape

The dependence of mistletoes on few dispersers and the directed dispersal they provide is well known, yet no recent work has quantified either the effectiveness of these ‘legitimate’ dispersers, or the extent of redundancy among them. Here, I use the seed dispersal effectiveness (SDE) framework to analyze how birds (Mionectes striaticollis and Zimmerius bolivianus) contribute to mistletoe (Struthanthus acuminatus and Phthirusa retroflexa) infection in traditional mixed plantations within a humid montane forest in Bolivia. I calculated SDE for each bird–mistletoe pair and for the disperser assemblage, by estimating both the quantity and the quality of dispersal. The quantity of dispersal was measured as: (1) disperser abundance; (2) frequency of visits; and (3) number of seeds dispersed per visit, and the quality of dispersal was measured as: (1) germination percentage and speed of germination of seeds regurgitated by birds; and (2) the concordance of deposited seeds and seedling distribution patterns with adult mistletoe distribution at three scales (habitat, host, and microhabitat). Dispersers were not redundant: the more generalist species M. striaticollis dispersed more seeds, but provided lower quality seed dispersal, whereas the mistletoe specialist Z. bolivianus provided low‐quantity and high‐quality seed dispersal. Whereas S. acuminatus benefited more from the SDE of Z. bolivianus, P. retroflexa benefited from the complementary seed dispersal provided by both birds. These results demonstrate how sympatric mistletoes that share the same disperser assemblage may develop different relationships with specific vectors, and describe how the services provided by two different dispersers (one that provides high‐quality and one that provides high‐quantity dispersal) interact to shape spatial patterns of plants.     

The role of herbivore‐ and plant‐related experiences in intraspecific host preference of a relatively specialized parasitoid

Parasitoids use odor cues from infested plants and herbivore hosts to locate their hosts. Specialist parasitoids of generalist herbivores are predicted to rely more on herbivorederived cues than plant-derived cues. Microplitis croceipes (Cresson)(Hymenoptera: Braconidae) is a relatively specialized larval endoparasitoid of Heliothis virescens (F.)(Lepidoptera: Noctuidae), which is a generalist herbivore on several crops including cotton and soybean. Using M. croceipes/H. virescens as a model system, we tested the following predictions about specialist parasitoids of generalist herbivores:(i) naive parasitoids will show innate responses to herbivore-emitted kairomones, regardless of host plant identity and (ii) herbivore-related experience will have a greater influence on intraspecific oviposition preference than plant-related experience. Inexperienced (naive) female M. croceipes did not discriminate between cotton-fed and soybean-fed H. virescens in oviposition choice tests, supporting our first prediction. Oviposition experience alone with either host group influenced subsequent oviposition preference while experience with infested plants alone did not elicit preference in M. croceipes, supporting our second prediction. Furthermore, associative learning of oviposition with host-damaged plants facilitated host location. I terestingly, naive parasitoids attacked more soybeathan cotton-fed host larvae in two-choice tests when a background of host-infested cotton odor was supplied, and vice versa. This suggests that plant volatiles may have created an olfactory contrast effect. We discussed ecological significance of the results and concluded that both plant- and herbivore-related experiences play important role in parasitoid host foraging.