Relationship between capitulum size and pre-dispersal seed predation by insect larvae in common Asteraceae

Abstract

The evolution of a showy floral display as an advertisement to pollinators could simultaneously advertise the availability of resources to pre-dispersal seed-predators. The hypotheses tested here are that the incidence of seed predation by bud-infesting insect larvae in capitula of Asteraceae is positively related to (1) capitulum size among species, (2) capitulum size within species, (3) capitulum lifespan, and (4) the degree of flowering asynchrony on individual plants. Three populations of each of 20 common herbaceous species of Asteraceae from disturbed ground and grassland habitats were monitored for the presence of pre-dispersal, seed-eating insect larvae. Mean capitulum size (receptacle width) of each species was measured. In a sub-set of eight species, individual capitula were tagged to determine their flowering phenology and lifespan (from anthesis to seed shedding). From these data an index of flowering synchrony on individual plants was derived. Among species, the incidence of larval infestation increased with capitulum size. Small-flowered species such as Achillea millefolium were largely free of bud-infesting larvae, whilst large-flowered species such as Arctium minus were heavily infested. In three cases investigated in greater detail, bud infestation was found to increase with capitulum size within species, suggesting a potential for natural selection to favour smaller capitula. No relationship was found between infestation levels and either capitulum lifespan or degree of flowering synchrony, and there was no evidence that the relationship between capitulum size and infestation was confounded by correlations with these other features. The results support hypotheses 1 and 2, but not 3 and 4. It is suggested that the characteristic capitulum size of each species may represent a trade-off between the opposing selection pressures of pollinators and pre-dispersal seed predators.

Considering evolutionary processes in adaptive forestry

“…Since seed predation leads to the eradication of individuals in a population, it plays a crucial role in plant population dynamics with possible genetic drift effects when population size is limited and potentially acts as a selective force driving the evolution of particular plant traits such as flowering synchrony, flowering phenology, inflorescence characteristics, flower size, flower longevity and mast seeding (Janzen 1971;Brody 1997;Fenner et al 2002;Cariveau et al 2004;Rose et al 2005;Strauss and Whittall 2006). Many tree species suffer from large seed losses due to predispersal seed predation, which can have significant effects on recruitment and plant population growth rate (Maron and Crone 2006;Kolb et al 2007).…”

Interaction of pollinators and herbivores on plant fitness suggests a pathway for correlated evolution of mutualism- and antagonism-related traits

“…Through direct or indirect effects on floral displays, flower characteristics, pollen quantity and quality, and plant attractiveness to pollinators, herbivores may ultimately modulate the nature, strength, and fitness consequences of the interaction between plants and their pollinators (6-15). Furthermore, herbivores and pollinators may exert conflicting selective pressures on particular plant traits, and certain characteristics of flowers and inflorescences may reflect compromise adaptations to mutualists and antagonists (5,12,(16)(17)(18)(19)(20).Another possible consequence of the concurrent interaction of plants with herbivores and pollinators that has begun to be recognized recently and the evolutionary origin of which is still poorly understood is the close functional, structural, or phylogenetic association sometimes existing between herbivory-and pollination-related traits. In Hypericum flowers, the same UV pigments play a defensive role in the stamens and ovaries and an attractive role in the petals (21).…”

“…Through direct or indirect effects on floral displays, flower characteristics, pollen quantity and quality, and plant attractiveness to pollinators, herbivores may ultimately modulate the nature, strength, and fitness consequences of the interaction between plants and their pollinators (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). Furthermore, herbivores and pollinators may exert conflicting selective pressures on particular plant traits, and certain characteristics of flowers and inflorescences may reflect compromise adaptations to mutualists and antagonists (5,12,(16)(17)(18)(19)(20).…”

Bottom‐up multitrophic effects in resprouting plants

“…(Asteraceae) is an abundant perennial shrub of the Cerrado, providing critical food and habitat to a variety of insect species specialized in its flower heads (Lewinsohn 1991, Dias et al 2010. Pre-dispersal seed predators are usually very specialized (Prado and Lewinsohn 2004), and are known to be more abundant on larger flower heads both among species and among individuals (Fenner et al 2002). This suggests that these insects are very sensitive to small morphological changes of the flower heads.…”