Interactive effect matters: a combination of herbivory degree and the ratio of generalist to specialist better predicts evolution of plant defense

Herbivory degree and the ratio of generalist to specialist herbivores have long been treated as two important but independent factors in shaping the evolution of plant defense. However, this assumption of independency is poorly supported and has resulted in great controversy in explaining the patterns of plant defense. Here we investigated the possible interaction between herbivory degree and generalist-to-specialist ratio using a cost-benefit model of defense evolution in plants. Our results showed that, with increasing generalist herbivore proportion, plant defense investment increases when herbivory degree is low and decreases when herbivory degree is high. These results provide the first theoretical support for the interactive effect of herbivory degree and ratio of generalist/specialist affecting plant defense, which integrate many of the previous results (e.g. latitudinal patterns of plant defense and defense evolution of invasive plants) and put them into a more general theoretical context.

Neurotoxic alkaloid in pollen and nectar excludes generalist bees from foraging at death-camas, Toxicoscordion paniculatum (Melanthiaceae)

Many plants produce broadly active toxins to which specialist herbivores—typically insects—have evolved counter-adaptations, sometimes spawning co-evolutionary arms races. Many non-social bee species are likewise taxonomic host specialists, but the specialists’ pollen hosts frequently attract diverse floral generalists as well, even to flowers of plants that are otherwise chemically defended. In this study of foothills death-camas, Toxicoscordion paniculatum (Nutt.) Rydberg (formerly Zigadenus), we show that its pollen and nectar both contain zygacine, the steroidal alkaloid responsible for this plant’s notorious mammalian toxicity. Hungry naïve adults of a generalist solitary bee, Osmia lignaria Say (Megachilidae), would briefly drink death-camas nectar or biologically relevant doses of zygacine in syrup, followed by prolonged bouts of irritable tongue grooming; many became paralyzed and some even died. Larvae fed dosed provision masses likewise often ceased feeding and sometimes died. Prolonged irritation and subsequent deterrence of foraging O. lignaria likely illustrates why it and 50+ other vernal bee species were absent from death-camas flowers in a five-state survey. The sole visiting bee, Andrena astragali, foraged exclusively at death-camas flowers for pollen and nectar. Thus, a toxic alkaloid found in death-camas pollen and nectar deters generalist bees from flowers of this pollinator-dependent monocot, restricting visitation to a single specialist bee that tolerates death-camas toxins and is its likely pollinator.

Teleost community composition and the role of herbivory on the intertidal reef of a small isolated island in north-west Australia

Most of the world’s tropical coastal and shelf areas are heavily affected by anthropogenic activities, but the north-west shelf of Australia is considered a ‘very low-impact’ area. The role of herbivory on coral reefs is recognised, but most of that research comes from reefs with considerable land-based impacts. In this study we sampled the teleost community and evaluated herbivory on the reef platform at Browse Island, a small isolated island 200km off north-western Australia, using several approaches: (1) tethering of macroalgae; (2) herbivore exclosures; and (3) video footage. In total, 99 teleost species from 26 families were identified. Turf algal consumption was evident and 18 teleost turf consumers were identified. In contrast, no evidence was found of herbivory on large macroalgae, and browsers, the only group able to consume macroalgae, were represented by just four species all belonging to the genus Naso. The lack of diversity among these specialist herbivores may be a consequence of the small surface area of the reef and the distance to other emergent reefs. Based on a model of top-down control of macroalgae, the reef is potentially vulnerable to disturbance. Small isolated reefs can have low resilience despite having low impacts from land.

Role of Saponins in Plant Defense Against Specialist Herbivores

The diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae) is a very destructive crucifer-specialized pest that has resulted in significant crop losses worldwide. DBM is well attracted to glucosinolates (which act as fingerprints and essential for herbivores in host plant recognition) containing crucifers such as wintercress, Barbarea vulgaris (Brassicaceae) despite poor larval survival on it due to high-to-low concentration of saponins and generally to other plants in the genus Barbarea. B. vulgaris build up resistance against DBM and other herbivorous insects using glucosinulates which are used in plant defense. Aside glucosinolates, Barbarea genus also contains triterpenoid saponins, which are toxic to insects and act as feeding deterrents for plant specialist herbivores (such as DBM). Previous studies have found interesting relationship between the host plant and secondary metabolite contents, which indicate that attraction or resistance to specialist herbivore DBM, is due to higher concentrations of glucosinolates and saponins in younger leaves in contrast to the older leaves of Barbarea genus. As a response to this phenomenon, herbivores as DBM has developed a strategy of defense against these plant biochemicals. Because there is a lack of full knowledge in understanding bioactive molecules (such as saponins) role in plant defense against plant herbivores. Thus, in this review, we discuss the role of secondary plant metabolites in plant defense mechanisms against the specialist herbivores. In the future, trials by plant breeders could aim at transferring these bioactive molecules against herbivore to cash crops.

A test of the evolution of increased competitive ability in two invaded regions

Finding patterns that predict and explain the success of non-native species has been an important focus in invasion ecology. The evolution of increased competitive ability (EICA) hypothesis has been a frequently used framework to understand invasion success. Evolution of increased competitive ability predicts that 1. Non-native populations will escape from coevolved specialist herbivores and this release from specialist herbivores should result in relaxed selection pressure on specialist-related defense traits, 2. There will be a trade-off between allocation of resources for resistance against specialist herbivores and allocation to traits related to competitive ability and 3. This shift will allow more allocation to competitive ability traits.We tested the predictions of EICA in the model plant Mimulus guttatus, a native of western North America (WNA). We compared how well the predictions of EICA fit patterns in two non-native regions, the United Kingdom (UK) and eastern North America (ENA). Coupled with extensive herbivore surveys we quantified genetic variation for herbivore resistance traits and fitness/ competitive ability traits to test adherence to the predictions of EICA in a common greenhouse environment.Herbivore communities differed significantly between WNA, UK, and ENA populations with evidence of specialist herbivore escape in the UK, but not necessarily the ENA plants. Compared to native plants, resistance traits were lower in non-native UK plants with the exception of trichome density, while the non-native ENA plants had equivalent or higher levels of herbivore resistance traits. The UK plants had increased competitive traits than native plants while the ENA plants had equivalent competitive traits to native plants. The UK plants, but not the ENA plants, showed some signs of tradeoffs between resistance traits and fitness/ competitive ability.Synthesis. Plants from the UK conformed to predictions of EICA more closely than those from ENA. The UK invasion is an older, more successful invasion, suggesting that support for EICA may be highest in more successful invasions. The lack of comprehensive conformity of either non-native region to the predictions of EICA also leaves room for other hypotheses that may add to our mechanistic understanding of the success of non-native plant invasions.