Costs of aggregation: shadow competition in a sit-and-wait predator

The role of top-down (e.g. parasitism) and bottom-up (e.g. resource competition) processes is of fundamental importance for the stability and persistence of insect herbivore populations. Although emphasis has often focused on single regulatory agents, the processes underpinning tri-trophic interactions may actually be more pluralistic. Recently, further complexities involved in the regulation of tri-trophic systems have been highlighted. In particular, life history characteristics may have a concomitant role when coupled with the regulatory effects of resource competition and/or parasitism. Here we present an age-structured model to investigate the effects of larval development period, parasitism and resource competition on the stability and persistence of herbivore–parasitoid interactions. We show that the influence of weak density dependent parasitism is sufficient to stabilise the interaction when the period of host susceptibility to parasitism is short. For longer periods of host susceptibility, parasitism needs to be highly non-linear to overcome the destabilising effects of the time delays. In systems where host development is protracted through the season we predict that resource competition is likely to be the dominant process for herbivore regulation. We use this age-structured approach to explore the population dynamics of two field studies from temperate ecosystems. Predictions from these case studies show 1) that both the strength and type of competition and parasitism are important for the stability and persistence of the particular system, and 2) that the length of the developmental period of the vulnerable host is critical to understand the influence of different regulatory processes. Host demography is of overriding importance in determining whether herbivores show outbreaks, and which particular ecological processes and mechanisms are responsible for generating such overcompensatory dynamics.     

Do larvae of species in macrolepidopteran assemblages share traits that influence susceptibility to parasitism?

Research on how morphology, behavior, and life histories of insects determine their susceptibility to parasitism has primarily focused on the traits of single host species. Very little research has been conducted attempting to determine if similarities or differences in the traits of co-occurring species, on a single plant or in a habitat, influence levels of parasitism imposed on any member of an assemblage. In this study, we use categorical and regression tree analyses to determine which traits of the larvae of macrolepidopteran species are associated with highest levels of parasitism. Of a variety of morphological, behavioral, and ecological traits of 72 species (representing eight families), caterpillar color was the trait that had the greatest influence on susceptibility to parasitism. The highest levels of parasitism were associated primarily with green larvae. These results suggest that the herbivore species composition and, specifically, the traits possessed by the species may influence community or assemblagewide patterns of parasitism. That is, sharing of traits that enhance host finding and successful parasitism may result in associational susceptibility to parasitism, whereas co-occurrence with species that differ morphologically, behaviorally, or ecologically may reduce the likelihood of parasitism and thus result in associational resistance. The concepts of associational resistance and susceptibility have historically been restricted to plant-plant interactions. The extension of these concepts to herbivores is novel. If found to be widespread, these interactions can have significant impacts on our expectations of the effectiveness of biological control agents.     

Resetting our expectations for parasites and their effects on species interactions: a meta-analysis

Despite the ubiquitous nature of parasitism, how parasitism alters the outcome of host–species interactions such as competition, mutualism and predation remains unknown. Using a phylogenetically informed meta-analysis of 154 studies, we examined how the mean and variance in the outcomes of species interactions differed between parasitized and non-parasitized hosts. Overall, parasitism did not significantly affect the mean or variance of host–species interaction outcomes, nor did the shared evolutionary histories of hosts and parasites have an effect. Instead, there was considerable variation in outcomes, ranging from strongly detrimental to strongly beneficial for infected hosts. Trophically-transmitted parasites increased the negative effects of predation, parasites increased and decreased the negative effects of interspecific competition for parasitized and non-parasitized heterospecifics, respectively, and parasites had particularly strong negative effects on host species interactions in freshwater and marine habitats, yet were beneficial in terrestrial environments. Our results illuminate the diverse ways in which parasites modify critical linkages in ecological networks, implying that whether the cumulative effects of parasitism are considered detrimental depends not only on the interactions between hosts and their parasites but also on the many other interactions that hosts experience.     

Parasites that change predator or prey behaviour can have keystone effects on community composition

Parasites play pivotal roles in structuring communities, often via indirect interactions with non-host species. These effects can be density-mediated (through mortality) or trait-mediated (behavioural, physiological and developmental), and may be crucial to population interactions, including biological invasions. For instance, parasitism can alter intraguild predation (IGP) between native and invasive crustaceans, reversing invasion outcomes. Here, we use mathematical models to examine how parasite-induced trait changes influence the population dynamics of hosts that interact via IGP. We show that trait-mediated indirect interactions impart keystone effects, promoting or inhibiting host coexistence. Parasites can thus have strong ecological impacts, even if they have negligible virulence, underscoring the need to consider trait-mediated effects when predicting effects of parasites on community structure in general and biological invasions in particular.     

Modeling the cuckoo’s brood parasitic behavior in the presence of egg polymorphism

The common cuckoo Cuculus canorus is a brood parasite that utilizes many host species. These have evolved defense against parasitism to reject cuckoo eggs that look unlike their own and some cuckoos have evolved egg mimicry to counter this defense. Egg phenotype indeed plays a key role for both the cuckoo and its hosts to successfully reproduce. It has been argued that cuckoos should parasitize host nests where egg phenotype matches because this makes parasitism more successful. Details of the cuckoo’s parasitic behavior, however, largely remains unknown if they really parasitize hosts depending on “egg matching”. In this paper, we model a time sequence of parasitic events in which a cuckoo finds host nests and decides to parasitize them or not in the presence of egg polymorphism. We evaluate which strategy is optimal: (1) opportunistic parasitism where cuckoos parasitize hosts irrespective of the phenotype, or (2) non-opportunistic parasitism where cuckoos parasitize hosts where egg phenotype matches. The analysis showed that either of the two strategies can be optimal. Factors not considered in the model, e.g., ecological and evolutionary changes both in the cuckoo and the host side, are discussed to explain apparent contrasts observed in some cuckoo–host interactions.     

Virus mediated trophic interactions between aphids and their natural enemies

Microbial endosymbionts alter the phenotype of their host which may have cascading effects at both population and community levels. However, we currently lack information on whether the effects of viruses on both host phenotypic traits and host population demography can modify interactions with upper trophic levels. To fill this gap, we investigated whether a prevalent densovirus infecting the aphid Myzus persicae (i.e. MpDNV) can modify trophic interactions between host aphids and their natural enemies (i.e. predators and parasitoids) by influencing aphid phenotypic traits (i.e. body mass and defensive behaviours), population demography (i.e. density and age-structure) and susceptibility towards both predation and parasitism. We found that the virus decreased aphid body mass but did not influence their behavioural defences. At the population level, the virus had a minor effect on aphid adult mortality whereas it strongly reduced the density of nymphs and influenced the stage structure of aphid populations. In addition, the virus enhanced the susceptibility of aphids to parasitism regardless of the parasitoid species. Predation rate on adult aphids was not influenced by the virus but ladybeetle predators strongly decreased the number of aphid nymphs, especially for uninfected ones compared to infected ones. As a result, the virus decreased predator effect on aphid populations. By reducing both aphid quality and availability, increasing their susceptibility to parasitism, and modulating predator effect on aphid populations, we highlighted that viral endosymbionts can be prevalent drivers of their host ecology as they modify their phenotypes and interspecific interactions. These virus-mediated ecological effects may have consequences on enemies foraging strategies as well as trophic webs dynamics and structure.     

Temporal and geographic variation in parasitoid attack with no evidence for ant protection of the Melissa blue butterfly,Lycaeides melissa

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Exploitative competition and coexistence in a parasitoid assemblage

Most insect populations are exploited by a complex of different parasitoid species, providing ample opportunities for competitive interactions among the latter. Despite this, resource-mediated competition (i.e., exploitative competition) among insect parasitoids remains poorly documented in natural systems. Here we propose a novel way to infer the presence of competitive interactions from covariance patterns in parasitism levels, and illustrate the use of this approach on a relatively well-defined and simple host–parasitoid system. The parasitism levels caused by three parasitoid species on a shared host showed a highly consistent negative covariance among samples. With the levels of parasitism by one species increasing, the levels of parasitism attributable to the two others decreased. Importantly, negative covariance between parasitism levels by different species appeared at high abundance, but not at low abundance of the phenologically earlier parasitoid species. This as well as several other lines of evidence indicates the importance of competitive interactions in this system. Feeding biology and phenology of the parasitoids suggest that competition in this parasitoid assemblage is primarily resource-mediated rather than occurring through direct interference. The species attacking earlier stages of the host are competitively superior to those attacking their host later in the season. Better dispersal ability and use of alternative host species by the inferior species could contribute to the coexistence of these competing parasitoids.     

Parasites detect host spatial pattern and density: a field experimental analysis

Summary A common assumption in mathematical models of parasitism is that the susceptibility to parasitism of an individual host increases both with host density and the degree of host spatial aggregation. To determine whether this assumption is correct in nature, we developed a factorial field experiment with the parasitic marine isopod Hemioniscus balani and its barnacle host Chthamalus dalli. Our factorial design enabled evaluation of the separate effects on parasitism of the two factors (host density and host spatial pattern) and also to assess the host density-spatial pattern interaction effect. Both host density and spatial aggregation were found to lead to increased parasitism, and the interaction effect was nonsignificant. These findings are the first experimental field demonstration that these processes occur in nature, as widely assumed in ecological theory.     

Host phenology,geographic range size and regional occurrence explain interspecific variation in damselfly–water mite associations

In this study, we tested which host species’ characteristics explain the nature and level of parasitism for host damselfly (Coenagrionidae)–water mite (Arrenuridae) parasite associations. Prevalence and intensity of mite parasites, and mite species richness were examined in relation to geographic range size, regional occurrence, relative local abundance, phenology and body size of host damselfly species. A total of 7107 damselfly individuals were collected representing 16 species from 13 sites in southeastern Ontario and southwestern Quebec, Canada. Using comparative methods, differences in prevalence and intensity of parasitism could be predicted by a host species’ geographic range and phenology. Barcoding based on Cytochrome Oxidase I revealed 15 operational taxonomic units (OTUs) for mite species. The number of mite OTUs known to infest a given host species was explained by a host species’ regional occurrence. Our findings demonstrate the need to measure factors at several ecological scales in order to understand the breadth of evolutionary interactions with host–parasite associations and the selective ‘milieu’ for particular species of both hosts and parasites.     

Errors associated with estimating parasitism using a modified version of Southwood's graphical method

A simulation model is used to examine the errors in estimating parasitized and nonparasitized host densities independently with Southwood's graphical technique. This technique is accurate when parasitoid attack occurs prior to the sampling period (e.g. the previous life stage of the host). When this is not the case, the parasitized host density is estimated accurately, but the unparasitized host density is over estimated by those individuals that are sampled as healthy prior to attack. This error is neglible at low levels of parasitism (<20% parasitized), but increases with increasing parasitism. Of the biological parameters tested, only the parasitoid attack pattern (shape of the parasitoid attack curve) has a significant influence on the magnitude of this error. A generalized simulation model is presented for evaluating errors in estimates of seasonal parasitism for specific host-parasitoid interactions. University of Rhode Island, Agricultural Experiment Station Journal, Article Number 2479.     

Response of native parasitoids to a range-expanding host

Abstract 1. As species shift their geographic distributions, new feeding interactions with natural enemies such as parasitoids, and resources such as host plants, may be established, and existing interactions may be severed. 2. The leaf mining moth Phyllonorycter leucographella (Zeller, 1850) (Lep.: Gracillariidae) first colonised the southern United Kingdom in the mid 1980s associated with its ancestral host plant Pyracantha coccinea M. Roem. (Rosaceae), which is widely cultivated in the U.K. The moth has since spread northwards to central Scotland and has been recorded feeding on a novel host plant, Crataegus monogyna L. 3. The combined effects of latitude and time since colonisation on parasitoid community responses to the arrival of this novel host were investigated across its U.K. range. The response of parasitoids to colonisation of C. monogyna was also investigated. 4. Both the observed richness of parasitoid species associated with P. leucographella, and the proportion of P. leucographella parasitised declined with latitude and towards the current range margin. A combination of a latitudinal gradient in parasitoid and alternative host species richness is likely to lead to the trends in species richness and parasitism observed. 5. Experimental host patches exposed to parasitism beyond the current range margin of P. leucographella experienced low levels of parasitism consistent with range‐margin populations, indicating an instantaneous response by native parasitoids to availability of the novel host. Parasitism levels and numbers of associated species in the U.K. were similar to those observed in the species’ native range in Turkey. 6. The host plant switch to C. monogyna was not associated with an altered parasitoid assemblage, but rates of parasitism were significantly higher on the novel host plant. 7. Alterations in the incidence and frequency of victim‐enemy interactions as species shift their geographic ranges may be key in determining rates of range expansion and the impact invading species have on ecological communities.     

Types of parasitism of acarines and insects on terrestrial vertebrates

According to the recent taxonomic revisions, over 40000 species of insects and acarines are parasites or micropredatory blood-suckers of mammals and birds. The largest fraction of them are micropredators and temporary or permanent ectoparasites, the minority being endoparasitic. Some arthropods (blood-sucking dipterans) use the host primarily as a food resource, whereas for others (many astigmatic mites) the host constitutes the entire environment. A number of life forms, or types of parasitism, have arisen in the insects and acarines in the course of their adaptive evolution to parasitism on terrestrial vertebrates. The term “type of parasitism” designates a set of convergently arising morpho-physiological and ecological adaptations (adaptive complexes), demonstrated by different arthropod taxa. A classification of the types of parasitism in arthropods is proposed based on their temporal, spatial, and trophic associations with vertebrates. The following seven types of parasitism are distinguished: micropredatory blood-suckers, nest ectoparasites (nidicoles), temporary ectoparasites with prolonged feeding, permanent ectoparasites, intracutaneous endoparasites, cavity endoparasites, and tissue endoparasites.     

Community-wide patterns of parasitism of a host “generalist” brood-parasitic cowbird

The brown-headed cowbird (Molothrusater) is a generalist obligate brood parasite. Despite intensive study and growing concern over the negative impact of cowbird parasitism on populations of many hosts, very little is known about the factors influencing community-wide patterns of cowbird parasitism. Using systematic nest searches, nest parasitism was studied over two breeding seasons at a study site in northeastern Illinois encompassing grassland, forest-edge, and forest habitat, supporting a diverse avian community. Parasitism was observed for 18 out of 34 altricial bird species found nesting at the study site. A total of 299 cowbird eggs and nestlings were found in 191 of a total of 593 nests. Analyses revealed several ecological and behavioral factors associated with frequency of parasitism and the resulting distribution of cowbird eggs. Much higher frequencies of parasitism were found in edge and forest habitats than in grassland. Within the edge habitat, open nests were parasitized significantly more often than cavity nests. Among open nests in the edge habitat, the two largest species were never parasitized. Host behavior, particularly egg-ejection behavior, was associated with a reduced observed frequency of parasitism, but at least three species known to eject cowbird eggs were sometimes parasitized. For six common hosts capable of rearing cowbirds, we found no correlation between level of parasitism and host nest-survivorship, suggesting that fine-grained assessments of host quality by female cowbirds do not influence patterns of parasitism among acceptable host species, or that differences in host quality are not great and/or predictable enough for such fine-grained assessments. Our results suggest that when a variety of possible nests are available, the level of parasitism on a particular species is a balance between a␣cowbird's preference for a particular species and the effectiveness of host species' defenses. A conceptual model was developed that incorporates the observed correlation of cowbird eggs or nestlings with habitat, nest-type, host species' body mass, and host behavioral defenses. Additional community-wide studies of cowbird parasitism will test if this model is applicable to other avian communities. Received: 20 December 1996 / Accepted: 17 May 1997     

Does the host matter? Variable influence of host traits on parasitism rates

Parasitism of mammals is ubiquitous, but the processes driving parasite aggregation on hosts are poorly understood, as each system seems to show unique correlations between parasitism and host traits such as sex, age, size and body mass. Genetic diversity is also posited to influence susceptibility to parasitism, and provides a quantifiable measure of an intrinsic unchanging host property, but this link has not been well established. A lack of consistency in host traits predicting parasite heterogeneity may derive from the contribution of environmental factors to parasite aggregation. To evaluate this question, a large dataset was leveraged to explore the relationship between unchanging, intrinsic host traits (heterozygosity and sex), variable host traits (age, length and body mass), and extrinsic factors (sampling date/year and population) and flea presence/absence, abundance and intensity on two species of social burrowing mammal, the black-tailed prairie dog (Cynomys ludovicianus) and the Gunnison’s prairie dog (Cynomys gunnisoni). Prairie dogs experience frequent parasitism by fleas, but the distribution of fleas among individuals is highly skewed. In these systems, intrinsic host traits were nuanced in how they predicted flea aggregation on individual prairie dogs, with sex unimportant to parasitism rates and heterozygosity increasing the probability of infection and influencing the number of fleas in divergent ways. Variable host traits interacted with each other and with environmental or geographic stochasticity to influence flea aggregation. Length and age tended to increase parasitism, whereas the effects of body mass and condition were mediated by date and other host traits to produce both positive and negative effects on parasitism. This finding suggests that the factors affecting ectoparasite infection on individuals are complex, even within species. Importantly, there was no correlation between the number of fleas on an individual in one year and the number of fleas on the same individual the next year, supporting the idea that flea aggregation is not driven by unchanging, intrinsic characteristics of the host. Rather, these findings indicate that host traits influence parasitism in nuanced ways, including interactions with environmental characteristics and stochastic factors.     

Higher gregarine parasitism often in sibling species of host damselflies with smaller geographical distributions

1. This study investigated inter‐specific variation in parasitism by gregarines (Eugregarinorida: Actinocephalidae), among sibling species of damselflies (Odonata: Zygoptera), in relation to relative size of geographical ranges of host species. 2. Gregarines are considered generalist parasites, particularly for taxonomically related host species collected at the same sites or area. Prevalence and median intensity of gregarine parasitism was obtained for 1338 adult damselflies, representing 14 species (7 sibling species pairs) across 3 families within the suborder Zygoptera. Damselflies were collected at three local sites in Southeastern Ontario, during the same periods over the season. 3. Five out of seven species pairs had significant differences in parasitism between sibling species. The less widespread host species was the more parasitised for three species pairs with significant differences in gregarine prevalence, and for two species pairs with differences in median intensity. The more widespread host had a higher intensity of infection as expected, in two species pairs. 4. Future studies on ecological determinants of parasitism among related species should examine robust measures of abundance of species and representation of species regionally.     

Parasitoids on the loose – experimental lack of support of the parasitoid movement hypothesis

Forest fragmentation can disrupt important ecological processes both within and among species. Of particular interest is the extent to which fragmentation decouples economically important pest species from their parasites. In an influential paper, Roland and Taylor (1997) advanced the hypothesis that forest fragmentation at a relatively fine spatial scale would disrupt the ability of parasitoids to aggregate in response to host density, thereby facilitating the build-up of host populations at isolated sites. Our study provides the first experimental test of this notion. We augmented existing densities of forest tent caterpillar individuals at two types of sites: isolated forest fragments and continuous tracts of forest. Of the four parasitoid species studied by Roland and Taylor (1997), two were encountered in our experiment. For the sarcophagid fly Arachnidomyia aldrichi and the tachinid Carcelia malacosomae, we found no difference in parasitism rates among forest fragments and continuous forest tracts. A third tachinid species unique to this study, Lespesia frenchii , is known to be a broad generalist with respect to host choice. In this species, we observed significantly elevated parasitism in forest fragments, and higher parasitism at the forest edge than within the forest. All of these findings suggest complex effects of forest fragmentation on parasitoid movement, and warn against broad generalizations from observational studies. Forest fragmentation may affect movement patterns in a multitude of ways, significantly obscuring or modifying the simplest mechanisms proposed by Roland and Taylor (1997).     

The role of phylogeny and ecology in experimental host specificity: insights from a eugregarine-host system

The degree to which parasites use hosts is fundamental to host-parasite coevolution studies, yet difficult to assess and interpret in an evolutionary manner. Previous assessments of parasitism in eugregarine-host systems suggest high degrees of host specificity to particular host stages and host species; however, rarely have the evolutionary constraints on host specificity been studied experimentally. A series of experimental infections were conducted to determine the extent of host stadium specificity (larval vs. adult stage) and host specificity among 6 tenebrionid host species and 5 eugregarine parasite species. Eugregarines from all host species infected both the larva and adult stages of the host, and each parasite taxa colonized several host species (Tribolium spp. and Palorus subdepressus). Parasite infection patterns were not congruent with host phylogeny, suggesting that host phylogeny is not a significant predictor of host-parasite interactions in this system. However, the 2 host stages produced significantly different numbers of parasite propagules, indicating that ecological factors may be important determinants of host specificity in this host-parasite system. While field infections reflect extant natural infection patterns of parasites, experimental infections can demonstrate potential host-parasite interactions, which aids in identifying factors that may be significant in shaping future host-parasite interactions.     

Monarch–parasite interactions in managed and roadside prairies

Roadsides cover an extensive area within the United States, are actively managed, and have been considered potential areas of habitat for several taxa. For monarch butterflies (Danaus plexippus), roadsides may act as important habitat along their migration route by providing nectar and host plant resources, which is especially important considering the loss and fragmentation of monarch habitat throughout their breeding range. However, the interactions between monarchs and their parasites may be altered in these areas by management regimes. Monarchs are infected by Ophryocystis elektroscirrha (OE), an obligate, spore-forming protist of monarchs and queens, and Lespesia archippivora, a generalist tachinid fly parasitoid. Roadsides could increase parasitism by concentrating monarchs in certain areas or decrease parasitism by modifying habitat (e.g., the roadside management practice of mowing could reduce the availability of OE spores by removing the above ground portion of host plants and generating re-growth), including the distribution and abundance of host plants. In this study, we compared the proportion of infected monarchs between roadside prairies and managed prairies to evaluate the potential of roadside prairies as habitat for monarch butterflies. Our results suggest that the proportion of infected monarchs does not differ between roadside prairies and managed prairies. Thus, roadsides may provide habitat for monarchs that is similar in quality (at least in terms of parasitism rates) to managed prairies. The role of roadsides as habitat for monarchs should be considered when developing roadside management strategies.     

Evolution of dispersal in metacommunities of interacting species

Theoretical studies on the evolution of dispersal in metacommunities are rare despite empirical evidence suggesting that interspecific interactions can modify dispersal behaviour of organisms. To understand the role of species interactions for dispersal evolution, we utilize an individual‐based model of a metacommunity where local population dynamics follows a stochastic version of the Nicholson–Bailey model and dispersal probability is an evolving trait. Our results show that in comparison with a neutral system (commensalism), parasitism promotes dispersal of hosts and parasites, while mutualism tends to reduce dispersal in both partners. Search efficiency of guests (only in the case of parasitism), dispersal mortality and external extinction risk can influence the evolution of dispersal of all partners. In systems composed of two host and two guest species, lower dispersal probabilities evolve under parasitism as well as mutualism than in one host and one guest species systems. This is because of frequency‐dependent modulations of dispersal benefits emerging in such systems for all partners.