Relation between flower head traits and florivory in Asteraceae: a phylogenetically controlled approach

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PREMISE OF THE STUDY

While much research has examined the relation between leaf traits and herbivory, very little is known about the interaction between floral traits, particularly biochemical traits, and florivory. We investigated patterns between floral traits and florivory across multiple species using phylogenetic comparative approaches to enhance our understanding of the evolution of plant-florivore interactions.•

METHODS

The relation between the intensity of florivory and five biochemical traits (concentrations of carbon, nitrogen, phosphorus, water, and total phenolics) and two morphological traits (diameter and number of flower heads) were investigated in wild individuals of 18 native species of Asteraceae. The phylogenetic signals in the morphological traits and intensity of florivory were also tested.•

KEY RESULTS

We found that species with higher nitrogen, water, and total phenolics and lower phosphorus concentrations in the flower heads and species with a large number and diameter of flower heads tended to be attacked by florivores. In addition, we found significant phylogenetic signals in florivory and morphological traits.•

CONCLUSIONS

Our results clearly show that biochemical traits also play important roles in plant-florivore interactions, as previously shown in plant-leaf herbivore interactions. The positive relationship between florivory and total phenolics implies that phenolic compounds in flower heads may not act as a defense in the species. In addition, the observed pattern of signals in florivory might not be solely explained by the signals of the measured traits and other plant traits may also play significant roles in plant-florivore interaction in these species.

Predispersal seed predation is higher in a rare species than in its widespread sympatric congeners (Astragalus, Fabaceae)

PREMISE OF THE STUDY

Rare plants may be rare, in part, because they are more susceptible to damage by predispersal insect seed predators than widespread congeneric species; thus, seed predation may be an important determinant of plant rarity. Scant evidence exists to either support or reject this hypothesis, limiting our ability to predict herbivore effects on plant rarity. We used a comparative framework to test this hypothesis in rare-common, sympatric congeners.

METHODS

We compared seed consumption by insect type and seed production among a rare Astragalus (Fabaceae) species and two sympatric, widespread congeners. We measured plant traits and tested whether traits were correlated with seed damage within and among rare-common species.

KEY RESULTS

Seed predation was significantly higher in a rare species than in common congeners over 2 yr. Seed production per pod was significantly lower in the rare species. Seed weevils exclusively consumed seeds of the rare species. Higher seed predation in the rare species is related to a combination of factors: plant phenology, dispersal ability, and vigor; seed predator identity; and insect phenology.

CONCLUSIONS

Our results support the hypothesis that a rare plant is more susceptible to seed predators than two common, sympatric congeners. Seed predator reduction is a promising management tool to increase seed-set, recruitment, and survival of the rare species. Further studies that compare rare-common, sympatric congeners are greatly needed to broadly evaluate the hypothesis concerning rarity and susceptibility to seed predators and to inform conservation plans for rare species.

Advertising to the enemy: enhanced floral fragrance increases beetle attraction and reduces plant reproduction

Many organisms face challenges in avoiding predation while searching for mates. For plants, emitting floral fragrances to advertise reproductive structures could increase the attraction of detrimental insects along with pollinators. Very few studies have experimentally evaluated the costs and benefits of fragrance emission with explicit consideration of how plant fitness is affected by both pollinators and florivores. To determine the reproductive consequences of increasing the apparency of reproductive parts, we manipulated fragrance, pollination, and florivores in the wild Texas gourd, Cucurbita pepo var. texana. With enhanced fragrance we found an increase in the attraction of florivores, rather than pollinators, and a decrease in seed production. This study is the first to demonstrate that enhanced floral fragrance can increase the attraction of detrimental florivores and decrease plant reproduction, suggesting that florivory as well as pollination has shaped the evolution of floral scent.

Invasive competitor and native seed predators contribute to rarity of the narrow endemic Astragalus sinuatus Piper

The conservation of rare plant species hinges on our ability to identify the underlying mechanisms that limit rare plant populations. Theory on rarity suggests that both predispersal seed predation and competition can be important mechanisms influencing abundance and/or distribution of rare plant populations. Yet few studies have tested these interactions, and those that have evaluated each mechanism independently. Astragalus sinuatus Piper (Whited's milkvetch) is a narrow endemic plant species restricted to eight populations within a 10-km2 area in eastern Washington. We used experimental and observational methods to test the effects of native insect predispersal seed predators and an invasive grass (Bromus tectorum L. [cheatgrass]) on seed set and population density of A. sinuatus. We quantified per capita seed production and pod predation rates across four sites and among four years. Seed predation rates were high across four sites (66-82%) and all years (65-82%). Experimental reduction of predispersal seed predators significantly increased per capita seed set of A. sinuatus (164-345%) at two experimental sites. Concurrently, two seed addition experiments demonstrated the effect of seed loss and presence of B. tectorum on seedling recruitment and establishment of A. sinuatus over four growing seasons. In the first seed addition experiment, we found no difference in recruitment and establishment between low (40) and high (120) seed addition levels. In the second addition experiment (one level of addition; 40 seeds), we found that recruitment and survivorship increased 200% in plots where B. tectorum was removed compared to plots where B. tectorum was present. Thus, seed addition had no impact in the presence of B. tectorum; conversely, in the absence of B. tectorum, seed addition was highly effective at increasing population numbers. Results suggest that, in areas where B. tectorum is present, recruitment is site limited, and it is seed limited when B. tectorum is absent. We recommend that managers reduce B. tectorum in an effort to increase population growth of A. sinuatus; in areas where B. tectorum is absent, short-term reduction of insect predators should be considered as a strategy to increase population growth of this rare species.