The formation and function of plant volatiles: perfumes for pollinator attraction and defense

https://www.cell.com/trends/plant-science/pdf/S1360-1385(09)00300-8.pdf

The evolutionary context for herbivore-induced plant volatiles: beyond the "cry-for-help"

The concept of the ecological trap, a low-quality habitat that animals prefer over other available habitats of higher quality, has appeared in the ecological literature irregularly for over 30 years, but the topic has received relatively little attention, and evidence for traps remains largely anecdotal. Recently, however, the ecological trap concept has been the subject of a flurry of theoretical activity that is likely to raise its profile substantially, particularly in conservation biology. Ecological trap theory suggests that, under most circumstances, the presence of a trap in a landscape will drive a local population to extinction. A number of empirical studies, almost all of birds, suggest the existence of traps and demonstrate the difficulties of recognizing them in the field. Evidence for ecological traps has primarily been found in habitats modified by human activities, either directly (e.g., through the mowing of grassland birds' nests) or indirectly (e.g., via human-mediated invasion of exotic species), but some studies suggest that traps may occur even in relatively pristine areas. Taken together, these theoretical and empirical results suggest that traps may be relatively common in rapidly changing landscapes. It is therefore important for conservation biologists to be able to identify traps and differentiate them from sinks. Commonly employed approaches for population modeling, which tend to assume a source-sink framework and do not consider habitat selection explicitly, may introduce faulty assumptions that mask the effects of ecological traps and lead to overly optimistic predictions about population persistence. Given the potentially dire consequences of ecological traps and the accumulating evidence for their existence, greater attention from the community of conservation biologists is warranted. In particular, it is important for conservation biologists and managers to incorporate into conservation planning an explicit understanding of the relationship between habitat selection and habitat quality.

/pdf/when-good-animals-love-bad-habitats-ecological-traps-and-the-piorg6a9wo.pdf

When Good Animals Love Bad Habitats: Ecological Traps and the Conservation of Animal Populations

Green leafy volatiles (GLV), six-carbon aldehydes, alcohols, and esters commonly emitted by plants in response to mechanical damage or herbivory, induced intact undamaged corn seedlings to rapidly produce jasmonic acid (JA) and emit sesquiterpenes. More importantly, corn seedlings previously exposed to GLV from neighboring plants produced significantly more JA and volatile sesquiterpenes when mechanically damaged and induced with caterpillar regurgitant than seedlings not exposed to GLV. The use of pure synthetic chemicals revealed that (Z)-3-hexenal, (Z)-3-hexen-1-ol, and (Z)-3-hexenyl acetate have nearly identical priming activity. Caterpillar-induced nocturnal volatiles, which are enriched in GLV, also exhibited a strong priming effect, inducing production of larger amounts of JA and release of greater quantities of volatile organic compounds after caterpillar regurgitant application. In contrast, GLV priming did not affect JA production induced by mechanical wounding alone. Thus, GLV specifically prime neighboring plants against impending herbivory by enhancing inducible chemical defense responses triggered during attack and may play a key role in plant-plant signaling and plant-insect interactions.

https://www.pnas.org/content/pnas/101/6/1781.full.pdf

Airborne signals prime plants against insect herbivore attack

Many plants interact with carnivores as an indirect defence against herbivores. The release of volatile organic compounds (VOCs) and the secretion of extrafloral nectar (EFN) are induced by insect feeding, a response that is mediated by the plant hormone, jasmonic acid. Although VOCs mainly attract predatory mites and parasitic wasps, while EFN mainly attracts ants, many more animal-plant interactions are influenced by these two traits. Other traits involved in defensive tritrophic interactions are cellular food bodies and domatia, which serve the nutrition and housing of predators. They are not known to respond to herbivory, while food body production can be induced by the presence of the mutualists. Interactions among the different defensive traits, and between them and other biotic and abiotic factors exist on the genetic, physiological, and ecological levels, but so far remain understudied. Indirect defences are increasingly being discussed as an environmentally-friendly crop protection strategy, but much more knowledge on their fitness effects under certain environmental conditions is required before we can understand their ecological and evolutionary relevance, and before tritrophic interactions can serve as a reliable tool in agronomy.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2007.02330.x

Indirect defence via tritrophic interactions.

The phenomenon of site fidelity (i.e. remaining faithful to sites where an individual has bred successfully in the past) has been documented for many taxa, especially birds. It has been suggested that individuals may use a simple rule of thumb: stay (or return) if breeding was successful, or switch to a new site if breeding was unsuccessful (win-stay/lose-switch; WSLS). Using simulations, I examined the evolutionary dynamics of the WSLS strategy in competition with two alternative strategies: stay-always (SA; total site fidelity) and two forms of an ideal-free settlement strategy (IFS; site indifferent). I considered two habitats identical in all respects except that one habitat (low quality) had a higher level of nest predation than the other. Between breeding seasons, females: (1) remained in a habitat if they successfully bred or otherwise dispersed to the alternative habitat (WSLS); (2) remained in the habitat regardless of their breeding success (SA); or (3) settled within habitats such that the quality of the highest ranking unoccupied territory was equalized, when possible, across habitats (IFS). The WSLS strategy invades and replaces either of the alternative strategies. I then examined the consequences of the WSLS strategy on population density and mean fecundity across two habitats that had contrasting levels of nest predation or brood parasitism. Unlike contrasting predation rates between habitats, higher levels of brood parasitism in the low-quality habitat rapidly drained away individuals from the better habitat. This result, however, depended both qualitatively and quantitatively on differences (or the lack thereof) in predation between the two habitats. A second invasibility analysis conducted on populations experiencing contrasting brood parasitism between habitats indicated that the WSLS resists invasion by the SA strategy but not the IFS strategy. The IFS strategy resists invasions by either alternative. Thus cowbird brood parasitism may rapidly drain individuals away from high-quality habitat because birds cannot discriminate between lowand high-quality habitats. Furthermore, parasitism alters the evolutionary dynamics of competing site fidelity strategies.

/pdf/site-fidelity-in-habitats-with-contrasting-levels-of-nest-2eje6o7a57.pdf

Site fidelity in habitats with contrasting levels of nest predation and brood parasitism

We develop mathematical and graphical models of diet selection incorporating recent advances in digestive physiology – that is, the adaptive modulation of active nutrient transport and gut retention time in response to changes in diet composition – into an explicitly ecological context based on consumer–resource dynamics and optimal foraging theory. The models indicate that gut modulation causes the consumer to treat two resources that are perfectly substitutable (the benefit derived from consumption of one resource is a constant fraction of the benefit derived from consumption of the second resource) as if they are antagonistic resources (the benefit derived from consumption of either resource alone is greater than the benefit derived from joint consumption of both resources). This will tend to favour diet switching and specialization, and also suggests a definitive (laboratory or field) test of the models. The models also suggest that modulation ultimately leads to more efficient use of resources, although it incurs an initial cost. We further cast gut modulation in three ecological scenarios. In the first, the consumer species does not deplete its resources and the optimal modulation strategy is determined by the standing crop of resources. In the second, a fixed population size of consumers results in resource depletion and the standing crop of resources results from a dynamic equilibrium between resource renewal and resource consumption. Under this scenario, the optimal gutmodulation strategy is determined by this dynamic equilibrium between renewal and consumption. In the third, we let resource renewal, depletion and consumer population sizes equilibrate. In this last scenario, the optimal gut modulation strategy emerges from the combined effects of resource renewal and the intersection of the depletion trajectory with the consumer’s zero net growth isocline. We conclude that forging tighter links between gut physiology and foraging ecology will lead to greater understanding and predictability of diet selection and its ecological consequences.

/pdf/linking-consumer-resource-theory-and-digestive-physiology-4ek72m4oyz.pdf

Linking consumer-resource theory and digestive physiology: Application to diet shifts

We derived unique sets of predictions from patch use theory to differentiate apparent, exploitative and interference competition. Armed with these predictions, we used field enclosures to determine whether white-footed mice (Peromyscus leucopus) perceived meadow voles (Microtus pennsylvanicus) as apparent competitors. The enclosures were sited at forest-field ecotones. Within them, we manipulated vole density, food patch microhabitat (i.e. predation risk) and distance from the forest edge. We studied the effect of these variables on giving-up densities of white-footed mice. Giving-up densities increased in response to higher vole activity. Under higher vole activity, mice increased foraging costs associated with risky, open microhabitats. These results were consistent only with apparent competition. Thus shared predators may be contributing to the pattern of habitat selection between mice and voles along forest-field ecotones.

/pdf/voles-competing-with-mice-differentiating-exploitative-11v1aqq2jq.pdf

Kenneth A. Schmidt

Papers

Site fidelity in habitats with contrasting levels of nest predation and brood parasitism

Linking consumer-resource theory and digestive physiology: Application to diet shifts

Voles competing with mice: differentiating exploitative, interference and apparent competition using patch use theory