Population dynamics of apparent competition in a host-parasitoid assemblage

Extrinsic Inter- and Intraspecific Competition in Parasitoid Wasps

The diverse ecology of parasitoids is shaped by extrinsic competition, i.e., exploitative or interference competition among adult females and males for hosts and mates. Adult females use an array of morphological, chemical, and behavioral mechanisms to engage in competition that may be either intra- or interspecific. Weaker competitors are often excluded or, if they persist, use alternate host habitats, host developmental stages, or host species. Competition among adult males for mates is almost exclusively intraspecific and involves visual displays, chemical signals, and even physical combat. Extrinsic competition influences community structure through its role in competitive displacement and apparent competition. Finally, anthropogenic changes such as habitat loss and fragmentation, invasive species, pollutants, and climate change result in phenological mismatches and range expansions within host-parasitoid communities with consequent changes to the strength of competitive interactions. Such changes have important ramifications not only for the success of managed agroecosystems, but also for natural ecosystem functioning.

Attack by a common parasitoid stabilizes population dynamics of multi-host communities

We model the population dynamics of two host species attacked by a common parasitoid using a discrete-time formalism that captures their population densities from year to year. It is well known starting from the seminal work of Nicholson and Bailey that a constant parasitoid attack rate leads to an unstable host-parasitoid interaction. However, a Type III functional response, where the parasitoid attack rate accelerates with increasing host density stabilizes the population dynamics. We first consider a scenario where both host species are attacked by a parasitoid with the same Type III functional response. Our results show that sufficient fast acceleration of the parasitoid attack rate stabilizes the population dynamics of all three species. For two symmetric host species, the extent of acceleration needed to stabilize the three-species equilibrium is exactly the same as that needed for a single host-parasitoid interaction. However, asymmetry can lead to scenarios where the removal of a host species from a stable interaction destabilizes the interaction between the remaining host species and the parasitoid. Next, we consider a situation where one of the host species is attacked at a constant rate (i.e., Type I functional response), and the other species is attacked via a Type III functional response. We identify parameter regimes where a Type III functional response to just one of the host species stabilizes the three species interaction. In summary, our results show that a generalist parasitoid with a Type III functional response to one or many host species can play a key role in stabilizing population dynamics of host-parasitoid communities in apparent competition.

Virus and CTL dynamics in the extrafollicular and follicular tissue compartments in SIV-infected macaques

Data from SIV-infected macaques indicate that virus-specific cytotoxic T lymphocytes (CTL) are mostly present in the extrafollicular (EF) compartment of the lymphoid tissue, with reduced homing to the follicular (F) site. This contributes to the majority of the virus being present in the follicle and represents a barrier to virus control. Using mathematical models, we investigate these dynamics. Two models are analyzed. The first assumes that CTL can only become stimulated and expand in the extrafollicular compartment, with migration accounting for the presence of CTL in the follicle. In the second model, follicular CTL can also undergo antigen-induced expansion. Consistent with experimental data, both models predict increased virus compartmentalization in the presence of stronger CTL responses and lower virus loads, and a more pronounced rise of extrafollicular compared to follicular virus during CD8 cell depletion experiments. The models, however, differ in other aspects. The follicular expansion model results in dynamics that promote the clearance of productive infection in the extrafollicular site, with any productively infected cells found being the result of immigration from the follicle. This is not observed in the model without follicular CTL expansion. The models further predict different consequences of introducing engineered, follicular-homing CTL, which has been proposed as a therapeutic means to improve virus control. Without follicular CTL expansion, this is predicted to result in a reduction of virus load in both compartments. The follicular CTL expansion model, however, makes the counter-intuitive prediction that addition of F-homing CTL not only results in a reduction of follicular virus load, but also in an increase in extrafollicular virus replication. These predictions remain to be experimentally tested, which will be relevant for distinguishing between models and for understanding how therapeutic introduction of F-homing CTL might impact the overall dynamics of the infection.

A better understanding of immune response dynamics and virus control in HIV infection is an important goal of current research. While measurements are often recorded in the blood, intricate dynamics occur in the lymphoid tissue. Recent data indicate that killer T cell responses, or CTL, show reduced homing to the follicular compartment of the lymphoid tissue, while the majority of the CTL remain in the extrafollicular site, which appears to contribute to the observed unequal distribution of virus load in the two locations. Here, these dynamics are studied with 2-compartment mathematical models. They reproduce previously published as well as newly presented experimental data from CTL depletion studies. Beyond this, the models indicate that so far unknown details of the CTL dynamics, in particular the potential of CTL to undergo antigen-induced expansion in the follicular compartment, can be important determinants of outcome. We find that antigen-induced expansion of CTL in the follicular site can result in more pronounced virus compartmentalization, and essentially in clearance of virus-producing cells from the extrafollicular site. We use the models to predict how experimental addition of engineered, follicular-homing CTL to macaques, influence the overall infection dynamics and level of virus control. Understanding these dynamics is an important step in attempts to improve the level of immune-mediated virus control.

Using the egg parasitoid Anastatus bifasciatus against the invasive brown marmorated stink bug in Europe: can non-target effects be ruled out?

The brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), has been causing massive damage to various fruit and vegetable crops after its arrival in the USA, and more recently in Europe. To provide an alternative control measure to pesticides, the native egg parasitoid Anastatus bifasciatus (Geoffroy) (Hymenoptera: Eupelmidae) was considered as a candidate biological control agent for inundative releases in Europe. In the risk assessment study presented here, all nine heteropteran and 14 out of 19 tested lepidopteran non-target species produced viable A. bifasciatus offspring. The proportion of A. bifasciatus females producing offspring did not differ between non-target and target for 19 out of the 28 non-target species. Larger host eggs corresponded to increased female-biased sex ratio of the offspring as well as an increase in size, particularly for females, with hind tibia lengths varying from 645.5 ± 46 to 1084 ± 28.5 μm. Larger females were also found to have higher offspring production and increased life expectancy. The results of this study confirmed the polyphagous nature of A. bifasciatus and suggest that a number of non-target species, including Lepidoptera of conservation interest, may be attacked in the field. Thus, non-target effects cannot entirely be ruled out, but more information is needed from semi-field and field studies to fully assess potential environmental risks due to inundative releases of this native parasitoid.

Apparent Competition

Most species have one or more natural enemies, e.g., predators, parasites, pathogens, and herbivores, among others. These species in turn typically attack multiple victim species. This leads to the possibility of indirect interactions among those victims, both positive and negative. The term apparent competition commonly denotes negative indirect interactions between victim species that arise because they share a natural enemy. This indirect interaction, which in principle can be reflected in many facets of the distribution and abundance of individual species and more broadly govern the structure of ecological communities in time and space, pervades many natural ecosystems. It also is a central theme in many applied ecological problems, including the control of agricultural pests, harvesting, the conservation of endangered species, and the dynamics of emerging diseases. At one end of the scale of life, apparent competition characterizes intriguing aspects of dynamics within individual organisms—for example, the immune system is akin in many ways to a predator that can induce negative indirect interactions among different pathogens. At intermediate scales of biological organization, the existence and strength of apparent competition depend upon many contingent details of individual behavior and life history, as well as the community and spatial context within which indirect interactions play out. At the broadest scale of macroecology and macroevolution, apparent competition may play a major, if poorly understood, role in the evolution of species’ geographical ranges and adaptive radiations.

Hidden risks and benefits of natural enemy-mediated indirect effects

Polyphagous natural enemies can mediate a variety of indirect interactions between resource populations. Such indirect interactions are often reciprocally negative (i.e. apparent competition), but the sign of effects between resource populations can be any combination of positive (+), negative (-), or neutral (0). In this article we focus on parasitoids to illustrate the importance of natural enemy-mediated indirect interactions in predicting risk and efficacy in biological control. We review recent findings to illustrate how an improved understanding of parasitoid behavioral ecology may increase model accuracy.

Effects of plant availability on population size and dynamics of an insect community: diamondback moth and two of its parasitoids

To understand the effect of plant availability/structure on the population size and dynamics of insects, a specialist herbivore in the presence of two of its parasitoids was studied in four replicated time-series experiments with high and low plant availabilities; under the latter condition, the herbivore suffered from some periods of resource limitation (starvation) and little plant-related structural refuges. Population dynamics of the parasitoid Cotesia vestalis was governed mainly by the delayed density-dependent process under both plant setups. The parasitoid, Diadegma semiclausum, under different plant availabilities and different coexistence situations (either +competitor or -competitor) showed dynamics patterns that were governed mainly by the delayed density process (significant lags at weeks 2-4). Both the competing parasitoids did not experience beneficial or costly interferences from each other in terms of their own population size when the plant resource was limited. Variation in the Plutella xylostella population under limited plant availability is higher than that under the other plant setup. For both parasitoids, under limited plant setup, the extinction risk was lower when parasitoids were engaged in competition, while under the unlimited plant setup, the mentioned risk was higher when parasitoids competed. In this situation, parasitoids suffered from two forces, competition and higher escaped hosts.

Pathogen diversity and hidden regimes of apparent competition

Competition through cross-reacting host immune responses, a form of apparent competition, is a major driver of pathogen evolution and diversity. Most models of pathogens have focused on intraspecific interactions to explain observed patterns. Two recent experiments suggested that Haemophilus influenzae, a common nasopharyngeal colonizer of humans, might alter the immune environment in a way that favors otherwise less fit serotypes of another common pathogen, pneumococcus. Using a computational model, we demonstrate that H. influenzae, if it consistently raises the fitness of the less fit serotypes, can strongly promote pneumococcal diversity. However, the effects of H. influenzae are so sensitive to the prevalence of H. influenzae that this species is unlikely to be the main driver of serotype coexistence. Interactions that significantly affect diversity could furthermore be extremely difficult to detect through co-occurrence analysis alone. These results suggest that small differences in strains' adaptations to different immunological regimes, which are shaped by coinfections with other pathogens, can have dramatic effects on strain dynamics and patterns of phenotypic variation. Studies of microbial communities might therefore benefit from the use of varied approaches to infer the presence of indirect interactions.

Apparent Competition and Vector-Host Interactions

Infectious disease influences the dynamics of host populations and the structure of species communities via impacts on host demography. Species that share infectious diseases are well-known to interact indirectly through the process of apparent competition, but there has been little attention given to the role of vectors in these indirect interactions. Here we explore how vector-borne disease and host-vector interactions can drive apparent competitive interactions. We show that different facets of the ecology associated with vector-host-host interactions affect the structure of the three-species assemblage. Crucially, the patterns associated with invasion of alternative hosts, the spread of the infectious disease by the vector, and the dynamics of the community interactions are influenced by the mode of transmission. We highlight the role of alternative hosts on disease amplification, dilution and magnification and discuss the results with reference to recent developments in apparent competition and community structure.

Community structure in ichneumonid parasitoids at different spatial scales

The processes underlying parasitoid community structure are little known. Stochastic niche-apportionment models provide one route to underlying assemblage rules in this and other groups. Previous work has applied this approach to parasitoids found on single host species in single populations. However, parasitoid communities are known to extend across multiple hosts and scales. The patterns of relative abundances generated by five niche-apportionment models were compared to those observed in assemblages of two sub-families of the Ichneumonidae, the Diplazontinae and Pimplinae, at landscape and patch scales, Yorkshire, UK. Three of the five models produced patterns that were significantly different to the observed pattern for all taxonomic levels at both spatial scales. The Diplazontinae fit the random fraction (RF) model at the landscape scale in broadleaved woods. This suggests that hierarchical structuring and biotic interactions may play a role in the structuring of Diplazontinae assemblages at this scale. In contrast the Pimplinae fit the RF model only at the patch scale and only at one site. However, the Pimplini tribe (all chiefly parasitoids of Lepidoptera) fit the random assortment (RA) model at both the landscape and the patch scales, whilst the Ephialtini tribe (wide range of hosts) fit no model at either scale. The ecological interpretation of the RA model suggests that the Pimplini tribe is an unsaturated assemblage, where some of the total available resources are unused. Our results show, through the fit of mechanistic niche-apportionment models, that the processes that may structure ichneumonid parasitoid assemblages are not consistent across taxa and spatial scales.

Food web structure of three guilds of natural enemies: predators, parasitoids and pathogens of aphids

1. Most communities of insect herbivores are unlikely to be structured by resource competition, but they may be structured by apparent competition mediated by shared natural enemies. 2. The potential of three guilds of natural enemies (parasitoids, fungal entomopathogens and predators) to influence aphid community structure through indirect interactions is assessed. Based on the biology, we predicted that the scope for apparent competition would be greatest for the predator and least for the parasitoid guilds. 3. Separate fully quantitative food webs were constructed for 3 years for the parasitoid guild, 2 years for the pathogen guild and for a single year for the predator guild. The webs were analysed using standard food web statistics designed for binary data, and using information-theory-based metrics that make use of the full quantitative data. 4. A total of 29 aphid, 24 parasitoid, five entomopathogenic fungi and 13 aphid specialist predator species were recorded in the study. Aphid density varied among years, and two species of aphid were particularly common in different years. Omitting these species, aphid diversity was similar among years. 5. The parasitoid web showed the lowest connectance while standard food web statistics suggested the pathogen and predator webs had similar levels of connectance. However, when a measure based on quantitative data was used the pathogen web was intermediate between the other two guilds. 6. There is evidence that a single aphid species had a particularly large effect on the structure of the pathogen food web. 7. The predator and pathogen webs were not compartmentalized, and the vast majority of parasitoids were connected in a single large compartment. 8. It was concluded that indirect effects are most likely to be mediated by predators, a prediction supported by the available experimental evidence.

How adaptive is parasite species diversity?

Has species diversity in parasites evolved as a by-product of adaptive diversification driven by competition for limited resources? Or is it a result of gradual genetic drift in isolation? One can move closer to answering these questions by evaluating the ubiquity of host switching, the key stage of adaptive diversification. Studies dealing with evolutionary role of host switching suggest that this process is extremely common in the wild, thus pointing at adaptive nature of parasite species diversity. However, most of these studies are focused on the evidence that may or may not have emerged as a consequence of host switching, - an approach potentially associated with a degree of uncertainty. After an overview of the data I am making an attempt to get a clearer view on host switching by focusing on factors that cause this phenomenon. In particular, I review theoretical work and field observations in order to identify the type of genetic host-use variance and the type of dispersal that underpin host switching. I show that host switching is likely to require generalist modifier alleles which increase the host range of individual genotypes and is likely to be promoted by wave-like patterns of dispersal. Both factors appear to be common in parasites. I conclude by outlining key areas for future research, including: (i) direct testing for divergence with gene flow, the main "footprint" of adaptive speciation; (ii) investigating the association between demography, dispersal potential and the potential to colonise novel habitats; and (iii) determining the genetic mechanisms underpinning host range variance in parasites.

Habitat shape, metapopulation processes and the dynamics of multispecies predator-prey interactions

1. The effects of habitat shape, connectivity and the metapopulation processes of persistence and extinction are explored in a multispecies resource-consumer interaction. 2. The spatial dynamics of the indirect interaction between two prey species (Callosobruchus chinensis, Callosobruchus maculatus) and a predator (Anisopteromalus calandrae) are investigated and we show how the persistence time of this interaction is altered in different habitat configurations by the presence of an apparent competitor. 3. Habitat structure has differential effects on the dynamics of the resource-consumer interaction. Across all habitat types, the pairwise interaction between C. chinensis and A. calandrae is highly prone to extinction, while the interaction between C. maculatus and A. calandrae shows sustained long-term fluctuations. Contrary to expectations from theory, habitat shape has no significant effect on persistence time of the full, three-species resource-consumer assemblage. 4. A stochastic metapopulation model for a range of habitat configurations, incorporating different forms of regulatory processes, highlights that it is the spatially explicit population dynamics rather than the shape of the metapopulation that is the principal determinant of interaction persistence time.

Apparent competition, quantitative food webs, and the structure of phytophagous insect communities

Phytophagous insects and their natural enemies make up one of the largest and most diverse groups of organisms on earth. Ecological processes, in particular negative indirect effects mediated by shared natural enemies (apparent competition), may be important in structuring phytophagous insect communities. The potential for indirect interactions can be assessed by analyzing the trophic structure of insect communities, and we claim that quantitative food webs are particularly well suited for this task. We review the experimental evidence for both short-term and long-term apparent competition in phytophagous insect communities and discuss the possible interactions between apparent competition and intraguild predation or shared mutualists. There is increasing evidence for the importance of trait-mediated as well as density-mediated indirect effects. We conclude that there is a need for large-scale experiments manipulating communities in their entirety and a greater integration of community and chemical ecology.

Indirect effects and spatial scaling affect the persistence of multispecies metapopulations

Quantifying the role of space and spatial scale on the population dynamics of ecological assemblages is a contemporary challenge in ecology. Here, we evaluate the role of metapopulation dynamics on the persistence and dynamics of a multispecies predator-prey assemblage where two prey species shared a common natural enemy (apparent competition). By partitioning the effects of increased resource availability from the effects of metapopulation structure on regional population persistence we show that space has a marked impact on the dynamics of apparent competition in multispecies predator-prey assemblages. Further, the role of habitat size and stochasticity are also shown to influence the dynamics and persistence of this multispecies interaction. The broader consequences of these processes are discussed.

The prevalence of asymmetrical indirect effects in two-host–one-parasitoid systems

Empirical studies of indirect effects mediated by shared enemies have been characterized by several puzzling features: (a) there exist far fewer documented cases than for interactions via shared resources; (b) the majority of empirical studies have measured indirect effects where one of the two reciprocal effects could not be distinguished from zero; (c) there is a lack of documented positive effects mediated by a shared enemy, in spite of several mechanisms that could produce such effects. One potential explanation is that these are statistical expectations over the range of potential species characteristics. We systematically examine the indirect interactions between two hosts with a shared parasitoid across all potential parameter values, using a family of simple models. By including a detection limit for nonzero interspecific effects, we demonstrate that (-,0) indirect interactions between hosts are the most common type for many variants of the model. However, the absence of positive indirect effects in empirical studies constitutes a puzzling inconsistency between the empirical and theoretical literatures.

Spatial mechanisms for coexistence of species sharing a common natural enemy

We examine the conditions under which spatial structure can mediate coexistence of apparent competitors. We use a spatially explicit, host-parasitoid metapopulation model incorporating local dynamics of Nicholson-Bailey type and global dispersal. Depending on the model parameters, the resulting system displays a plethora of asynchronous dynamical behaviors for which permanent or transient coexistence is observed. We identify a number of spatially mediated tradeoffs which apparent competitors can utilize and demonstrate that the dynamics of spatial coexistence can typically be understood from consideration of two and three patch systems. The phase relationships of species abundances are different for our model than for some other mechanisms of spatial coexistence. We discuss the implications of our findings relative to issues of community organization and biological conservation.

The Effects of Enrichment on the Dynamics of Apparent Competitive Interactions in Stage‐Structured Systems

In the absence of other limiting factors, assemblages in which species share a common, effective natural enemy are not expected to persist. Although a variety of mechanisms have been postulated to explain the coexistence of species that share natural enemies, the role of productivity gradients has not been explored in detail. Here, we examine how enrichment can affect the outcome of apparent competition. We develop a structured resource/consumer/natural enemy model in which the prey are exposed to attacks during a vulnerable life phase, the length of which depends on resource availability. With a single prey species, the model exhibits the "paradox of enrichment," with unstable dynamics at high levels of resource productivity. We extend this model to consider two prey species linked by a shared predator, each with their own distinct resource base. We derive invasion and stability conditions and examine how enrichment influences prey species exclusion and coexistence. Contrary to expectations from simpler, prey-dependent models, apparent competition is not necessarily strong at high productivity, and prey species coexistence may thus be more likely in enriched environments. Further, the coexistence of apparent competitors may be facilitated by unstable dynamics. These results contrast with the standard theory that apparent competition in productive environments leads to nonpersistent interactions and that coexistence of multispecies interactions is more likely under equilibrial conditions.

The role of variability and risk on the persistence of shared-enemy, predator-prey assemblages

The role of indirect effects such as apparent competition in structuring predator-prey assemblages has recently received empirical attention. That one prey species can be excluded by the impact of a shared-enemy contrasts with the known diversity of multispecies predator-prey interactions. Here, the role of predator foraging among patches of two different prey species is examined as a mechanism that can mediate coexistence in multispecies prey-predator assemblages. Specifically, models of host-parasitoid interactions are constructed to analyse how different types of aggregative behaviour (generated by host-dependent and host-independent responses) affect persistence of the assemblage. How the distribution of hosts and the response of the parasitoid to these distributions can influence coexistence is shown. A generic explanation for coexistence suggests that it is the variability rather than the precise functional relationship that is critical for coexistence under shared-enemy interactions.

Gene flow between arrhenotokous and thelytokous populations of Venturia canescens (Hymenoptera)

In the solitary parasitoid wasp Venturia canescens both arrhenotokously (sexual) and thelytokously (parthenogenetical) reproducing individuals occur sympatrically. We found in the laboratory that thelytokous wasps are able to mate, receive and use sperm of arrhenotokous males. Using nuclear (amplified fragment length polymorphism, virus-like protein) and mitochondrial (restriction fragment length polymorphism) markers, we show the occurrence of gene flow from the arrhenotokous to the thelytokous mode in the field. Our results reinforce the paradox of sex in this species.

The effects of metapopulation structure on indirect interactions in host-parasitoid assemblages

The interaction between two species that do not compete for resources but share a common natural enemy is known as apparent competition. In the absence of other limiting factors, such three-species interactions are impermanent, with one species being excluded from the assemblage by the natural enemy. Here, the effects of metapopulation structure are explored in a system of two hosts that experience apparent competition through a shared parasitoid. A coupled-map lattice model is developed and used to explore species coexistence and patterns of patch occupancy at the metapopulation scale. Linking local and regional dynamics favours coexistence by uncoupling the dynamics of the three species in space. Coexistence is promoted by the inferior species being either a fugitive or a sedentary species. The occurrence of these two mutually exclusive mechanisms of coexistence is influenced by the relative dispersal of the inferior apparent competitor.

Adaptive host preference and the dynamics of host-parasitoid interactions

Models of two independent host populations and a common parasitoid are investigated. The hosts have density-dependent population growth and only interact indirectly by their effects on parasitoid behavior and population dynamics. The parasitoid is assumed to experience a trade-off in its ability to exploit the two hosts. Three alternative types of parasitoid are investigated: (i) fixed generalists whose consumption rates are those that maximize fitness; (ii) "ideal free" parasitoids, which modify their behavior to maximize their rate of finding unparasitized hosts within a generation; and (iii) "evolving" parasitoids, whose capture rates change between generations based on quantitative genetic determination of the relative attack rates on the two hosts. The primary questions addressed are: (1) Do the different types of adaptive processes stabilize or destabilize the population dynamics? (2) Do the adaptive processes tend to equalize or to magnify differences in host densities? The models show that adaptive behavior and evolution frequently destabilize population dynamics and frequently increase the average difference between host densities.