Phenotypic plasticity is considered the major means by which plants cope with environmental heterogeneity. Although ubiquitous in nature, actual phenotypic plasticity is far from being maximal. This has been explained by the existence of internal limits to its expression. However, phenotypic plasticity takes place within an ecological context and plants are generally exposed to multifactor environments and to simultaneous interactions with many species. These external, ecological factors may limit phenotypic plasticity or curtail its adaptive value, but seldom have they been considered because limits to plasticity have typically addressed factors internal to the plant. We show that plastic responses to abiotic factors are reduced under situations of conservative resource use in stressful and unpredictable habitats, and that extreme levels in a given abiotic factor can negatively influence plastic responses to another factor. We illustrate how herbivory may limit plant phenotypic plasticity because damaged plants can only rarely attain the optimal phenotype in the challenging environment. Finally, it is examined how phenotypic changes involved in trait-mediated interactions can entail costs for the plant in further interactions with other species in the community. Ecological limits to plasticity must be included in any realistic approach to understand the evolution of plasticity in complex environments and to predict plant responses to global change.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2007.02275.x

Ecological limits to plant phenotypic plasticity

Several important problems in ecology, evolution and conservation biology are affected by habitat selection in dispersing animals. Experience in the natal habitat has long been considered a potential source of variation in the habitat preferences displayed when dispersers select a post-dispersal habitat. However, the taxonomic breadth of this phenomenon is underappreciated, in part because partially overlapping, taxon-specific definitions in the literature have discouraged communication. Here, we explore the phenomenon of natal habitat preference induction (NHPI) and demonstrate that NHPI has been observed in a broad range of animal taxa. We consider the potential adaptive significance of NHPI, identify implications of its occurrence for problems in evolution, ecology and conservation biology, and encourage further study of this phenomenon.

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The effect of natal experience on habitat preferences

We reviewed the literature on aphid parasitoids to determine the occurrence, nature and outcome of intraguild interactions. Intraguild interactions were described for larval, pupal and adult aphid parasitoids and by the type of natural enemy (fungus, predator, or parasitoid). They appear to be prevalent in most aphid parasitoid systems and, except for parasitoid-parasitoid interactions, they are mostly asymmetric, with aphidophagous predators and pathogens killing parasitoids. The limited experimental evidence from field studies is insufficient to provide a comprehensive pattern of the consequences of intraguild interactions for aphid parasitoid populations in general and, more specifically, for the efficacy of biological control. However, because intraguild interactions are widespread in aphid-natural enemy communities and mostly detrimental to aphid parasitoids, we conclude that intraguild interactions have a primary effect in driving fluctuations in aphid parasitoid populations. Drawing on case studies, we further argue that intraguild interactions can substantially alter the effectiveness of aphid parasitoids as biological-control agents.

/pdf/intraguild-interactions-in-aphid-parasitoids-42qzghzizd.pdf

Intraguild interactions in aphid parasitoids.

Herbivore attack leads to resource conflicts between plant defensive strategies. Photoassimilates are required for defensive compounds and carbon storage below ground and may therefore be depleted or enriched in the roots of herbivore-defoliated plants. The potential role of belowground tissues as mediators of induced tolerance-defense trade-offs is unknown. We evaluated signaling and carbohydrate dynamics in the roots of Nicotiana attenuata following Manduca sexta attack. Experimental and natural genetic variability was exploited to link the observed metabolite patterns to plant tolerance and resistance. Leaf-herbivore attack decreased sugar and starch concentrations in the roots and reduced regrowth from the rootstock and flower production in the glasshouse and the field. Leaf-derived jasmonates were identified as major regulators of this root-mediated resource-based trade-off: lower jasmonate levels were associated with decreased defense, increased carbohydrate levels and improved regrowth from the rootstock. Application and transport inhibition experiments, in combination with silencing of the sucrose non-fermenting (SNF) -related kinase GAL83, indicated that auxins may act as additional signals that regulate regrowth patterns. In conclusion, our study shows that the ability to mobilize defenses has a hidden resource-based cost below ground that constrains defoliation tolerance. Jasmonate- and auxin-dependent mechanisms may lead to divergent defensive plant strategies against herbivores in nature.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.12438

Leaf-herbivore attack reduces carbon reserves and regrowth from the roots via jasmonate and auxin signaling.

1..18 Most organisms represent specialized forms that arose as a result of natural selection and genetic drift to occupy distinct ecological niches. In animals, this process of specialization includes the behaviour of the organisms concerned, honed by locally-induced adaptations to specific host food plants (in herbivores) or prey items (in predators and parasitoids), and possibly reinforced by kairomones, including sex pheromones. The major thrust of evolution is towards ecological specialization as a result of the direct effects of intra- and interspecific competition. Adaptation to new resources lowers such competition and allows survival in new habitats/niches. Other benefits of food resource/habitat switching include ‘enemy free space’. If specialism is the norm for the vast majority of species, what of so-called generalists and generalism, which are widely used terms, but perhaps wrongly so? Does generalism exist or is it a mirage that disappears the closer that it is inspected? We review some of the aspects of specialism and generalism and argue that even apparent generalists are filling distinct ecological niches. Often, generalists are rather specific in terms of food preferences, although they may nevertheless remain opportunistic with an overall broad niche/resource width. When apparent ‘good’ species are examined using molecular (DNA) markers, they are often found to comprise cryptic species. Many generalists may be of this kind. If so, generalism warrants additional investigation to establish its scope and credentials. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 1–18. ADDITIONAL KEYWORDS: cryptic species – DNA – ecology – generalist – molecular markers – niche – specialist.

The evolutionary improbability of 'generalism' in nature, with special reference to insects

The host preference behaviour of the generalist aphid parasitoid Aphidius colemani was investigated using a Y-tube olfactometer. Female A. colemani showed a preference for the host-plant complex on which they had been reared, even though the same aphid host was involved, demonstrating a host plant preference. This preference was not evident when the parasitoids were dissected from their mummies prior to adult emergence. Host plant preference exhibited during host selection appeared to be induced by chemical cues encountered on the mummy case at the time of emergence, but preferences could be changed by subsequent foraging experiences. It is concluded that plant chemical cues play a major role in determining initial preferences through a process of emergence conditioning but that learning processes, involving cues encountered during oviposition in or contact with the host, can modify these initial preferences.

A. Storeck

Papers

The role of plant chemical cues in determining host preference in the generalist aphid parasitoid Aphidius colemani.

Determination of plant foraging preferences in the aphid parasitoid Aphidius colemani : emergence conditioning, learning and memory