Marta Montserrat

HABITAT STRUCTURE AFFECTS INTRAGUILD PREDATION

Intraguild predation is thought to be ubiquitous in natural food webs. Yet, theory on intraguild predation predicts the intraguild prey to persist only under limited conditions. This gap between theory and empirical observations needs scrutiny. One reason might be that theory has focused on equilibrium dynamics and a limited set of species (usually three) that interact in well-mixed populations in unstructured habitats, and these assumptions will often not hold in natural systems. In this review, we focus on the effects of habitat structure on intraguild predation. Habitat structure could reduce encounter rates between predators and prey and could create refuges for prey. In both cases, habitat structure could reduce the strength of intraguild interactions, thereby facilitating species coexistence. A meta-analysis of studies on manipulation of habitat structure shows that intraguild prey indeed suffer less from intraguild predation in structured habitats. This was further confirmed by a meta-analysis in which studies on intraguild predation were classified according to habitat structure. Intraguild predation reduced densities of the intraguild prey significantly more in habitats with little structure than in habitats rich in structure. The effect of intraguild predation on the shared prey was negative, and not significantly affected by habitat structure. We conclude that habitat structure may increase persistence of the intraguild prey by decreasing the strength of the interaction between intraguild predator and intraguild prey.

Intraguild Predation Usually does not Disrupt Biological Control

Intraguild predation is claimed to be ubiquitous in nature. It also occurs among natural enemies in biological control systems, where one natural enemy (the intraguild predator) attacks another species of natural enemy (the intraguild prey), whereas they also compete for the same pest. We review the theory of intraguild predation and its consequences for biological control for two different scenarios. 1. The intraguild predator is the superior natural enemy (

Functional Response of Four Heteropteran Predators Preying on Greenhouse Whitefly (Homoptera: Aleyrodidae) and Western Flower Thrips (Thysanoptera: Thripidae)

i.e.

Predators induce egg retention in prey

reduces the pest population the most). In this case, the intraguild predator will exclude the intraguild prey, thus there will be no intraguild predation in the long term. 2. The intraguild prey is the superior natural enemy. In this case, the intraguild predator and intraguild prey may coexist or the intraguild predator can exclude the intraguild prey. Theory predicts for this scenario that pest numbers will always be lowest when only the intraguild prey is present. Hence, the occurrence of intraguild predation in cropping systems would never result in increased control, but can result in decreased control. We subsequently review experimental tests of the effect of intraguild predation among natural enemies on the population dynamics of pests. Contrary to expectations, we find that intraguild predation often did not result in an increase of pest populations, even when the intraguild predator was the inferior natural enemy. Often, the presence of the intraguild predator had no effect or even resulted in a decrease of pest populations. Although the number of studies was limited, we scanned the literature to identify possible causes for the discrepancy of experimental results with theoretical predictions. We specifically evaluated trends in the effects with respect to the length of the study period, the spatial scale at which experiments were carried out, the number of species involved in the studies and the spatial complexity of the experimental arenas. There was a slight trend towards experiments of longer duration showing less positive effects on pest densities, but no clear effect of spatial scale. All studies that showed positive effects on pest densities were studies with 3 species, but the number of studies with more than 3 species was small. Spatial complexity had mixed effects on experimental results. In conclusion, it is clear that intraguild predation most often does not increase pest densities as was predicted from theory, but more research is needed to reveal why theory does not meet practice.

Patterns of exclusion in an intraguild predator–prey system depend on initial conditions

Abstract The mirid bugs Dicyphus tamaninii Wagner and Macrolophus caliginosus Wagner and the anthocorid bugs Orius majusculus (Reuter) and O. laevigatus (Fieber) are abundant generalist predators in unsprayed vegetable crops of the Spanish Mediterranean coast. We evaluated the functional response of these predators to greenhouse whitefly pupae and western flower thrips larvae (second instar) on cucumber leaf disks in the laboratory. Parameters of the random-predator equation obtained were compared among all predator–prey combinations to determine the potential role of the predators in the biological control of both pests in cucumber. D. tamaninii was efficient at consuming whitefly pupae at high and low densities and thrips at high densities, and took less time to handle either of these prey than did the other predators. Anthocorid bugs were efficient at consuming thrips at low and high densities, but did not performed well as predators of whiteflies. M. caliginosus was less efficient when consuming whiteflies but performed better when preying on thrips. It is concluded that D. tamaninii may have a good action in the control of both greenhouse whitefly and western flower thrips, whereas M. caliginosus and both Orius species may be slower in controlling whitefly and be similarly efficient in consuming western flower thrips.

Climate Change and Eco-Evolutionary Dynamics in Food Webs

To prevent predation on their eggs, prey often avoid patches occupied by predators. As a result, they need to delay oviposition until they reach predator-free patches. Because many species allocate energy to egg production in a continuous fashion, it is not clear what kind of mechanism prey use to delay oviposition. We used females of the phytoseiid mite Neoseiulus cucumeris to study these mechanisms. Females were placed in patches with pollen, a food source they use for egg production, and they were exposed to another phytoseiid mite, Iphiseius degenerans, which is an intraguild predator of N. cucumeris juveniles. We found that the oviposition of N. cucumeris females on patches with the predator was lower than on patches without the predator. Cues left by the intraguild predator were not sufficient to elicit such behaviour. Females of N. cucumeris reduced oviposition when exposed to the predator by retaining the egg inside their body, resulting in a lower developmental rate once these eggs were laid. Hence, females are capable of retaining eggs, but the development of these eggs continues inside the mother’s body. In this way, females gain some time to search for less risky oviposition sites.

To be an intra‐guild predator or a cannibal: is prey quality decisive?

1. When intraguild (IG)-prey are superior to IG-predators in competing for a shared resource, theory predicts coexistence of the IG-prey or the IG-predator with the resource depending on the productivity level: (a) resource and IG prey coexist when productivity is low; (b) IG-predator and resource coexist at high productivity; (c) if IG-prey and IG-predators can coexist, it is only at intermediate productivity levels. 2. We tested the existence of productivity-dependent regions of coexistence using an experimental system of two predatory mites and a shared food source (pollen). 3. At high levels of pollen supply (i.e. high productivity), the IG-predator excluded the IG-prey in most, but not all, cases. The same pattern of exclusion was observed at low productivity, at which the IG-prey was expected to exclude the IG-predator. Therefore, species composition could not be predicted by productivity levels. Instead, our results show that initial conditions affected strongly the outcome of the interaction. 4. We emphasize the need for theory on IG-predation that takes the effects of stage structure, initial conditions and transient dynamics into account.

Eco-Evolutionary Spatial Dynamics: Rapid Evolution and Isolation Explain Food Web Persistence

Abstract The analysis of the causes of selection is in essence a problem in ecology Wade and Kalisz 1990 Past evolution determines the genetically determined available phenotypes in populations which affect ecological dynamics in communities, shaping in turn the selective pressures that further model phenotypes. Because an increase in temperature increases metabolic rates and encounter rates, climate change may have profound eco-evolutionary effects, possibly affecting the future persistence and functioning of food webs. We introduce a semi-spatially explicit individual-based model (IBM) framework to study functional eco-evolutionary dynamics in food webs. Each species embedded in the web includes 13 genetically determined and multidimensionally variable traits (the G matrix), 4 of which are flexible physiological and behavioural (personality) traits that respond to temperature. An increase in temperature and stronger correlation among traits leads to stronger trophic cascades but higher stochasticity, with higher probability of extinction for some trophic levels. A combination of the abiotic (temperature) and biotic (predators’ presence/absence) matrix of selective agents (the O matrix) generates differential selection for activation energies for metabolic rates and several instances of correlational selection (selection in one trait changes with the levels of another), suggesting how global warming might favour certain trait combinations. Our results and the future prospects of this IBM approach open new avenues for climate change research.

Intraguild predation between phytoseiid mite species might not be so common

Abstract 1. Many cannibalistic species are also intra‐guild predators. Such predators will often face the decision whether to consume a conspecific or a heterospecific prey from the same guild. This decision may depend on the relative quality and abundance of the prey but also on other factors such as relatedness by descent, prey‐specific defence and the probability of the victim harbouring shared diseases.

Whether ideal free or not, predatory mites distribute so as to maximize reproduction

Abstract One of the current challenges in evolutionary ecology is understanding the long-term persistence of contemporary-evolving predator–prey interactions across space and time. To address this, we developed an extension of a multi-locus, multi-trait eco-evolutionary individual-based model that incorporates several interacting species in explicit landscapes. We simulated eco-evolutionary dynamics of multiple species food webs with different degrees of connectance across soil-moisture islands. A broad set of parameter combinations led to the local extinction of species, but some species persisted, and this was associated with (1) high connectance and omnivory and (2) ongoing evolution, due to multi-trait genetic variability of the embedded species. Furthermore, persistence was highest at intermediate island distances, likely because of a balance between predation-induced extinction (strongest at short island distances) and the coupling of island diversity by top predators, which by travelling among islands exert global top-down control of biodiversity. In the simulations with high genetic variation, we also found widespread trait evolutionary changes indicative of eco-evolutionary dynamics. We discuss how the ever-increasing computing power and high-resolution data availability will soon allow researchers to start bridging the in vivo–in silico gap.