Effects Of Plant Defenses Research Articles

Adult facilitation becomes competition as juvenile soapberry bugs age.

Intraspecific interactions can change from facilitative to competitive depending on the organism's ontogeny. In plant‐feeding insects, host plant defenses can be strengthened or weakened by insect feeding and can therefore be important for determining whether two insects feeding on the same plant help or harm each other's fitness. Here, I conducted two experiments looking at the direct effect of a physical seed defense and the role of intraspecific facilitation in reducing the effects of that defense for juveniles of the red‐shouldered soapberry bug. I demonstrate that juveniles are severely inhibited by the tough seed coat of their host plant, leading to high mortality early in development. Adults, in contrast, can create holes through which younger individuals could potentially feed. I manipulated whether or not seeds were fed on by adults on two host plant species: a well‐defended native host and a poorly defended introduced host. Survival in the first week of development was dramatically improved by prior adult feeding, and this facilitation was stronger on the well‐defended host plant. However, the benefits of prior adult feeding ceased after the first week of development and shifted to having a negative effect on survival, development time, and body size. These results indicate that ontogeny is a key factor determining the effects of plant defenses and the strength and direction of intraspecific interactions across multiple host plant species.

Effects of plant defenses and water availability on seasonal foraging preferences of the Nubian Ibex (Capra nubiana)

The study of herbivore patch use has implications for herbivore habitat quality assessment, foraging behaviors, species interactions, and coexistence in patchy environments. This research focuses on the comparison of the effects of two qualitatively different plant defenses, mechanical (thorns) and chemical (tannins), on ibex foraging preferences during different seasons of the year. The occurrence of both chemical and mechanical plant defenses were experimentally manipulated in artificial resource patches, in addition to water availability. Ibex foraging preferences were quantified using giving-up densities during four separate fieldwork sessions in each of the seasons of the year at cliff sites overlooking the Zin Valley of the Negev Highlands. Both mechanical and chemical plant defenses significantly hindered ibex food intake overall. Mechanical and chemical defenses acted as substitutable defenses, meaning that their combined effects were not greater than additive. There were strong seasonal patterns of the amount of food consumed by ibex, further corroborated by comparison to rainfall levels. Seasonality also interacted with the effectiveness of plant defenses. Thorns were especially ineffective in summer, whereas tannins were most effective in spring. Decreases in seasonal food availability and increased marginal value of energy for ibex may have resulted in thorn ineffectiveness, while seasonal changes in the emergence of young foliage may have resulted in the greater springtime tannin effectiveness. Water was not found to mitigate the detrimental effects of tannins through dilution. The implications for decreased constraints on selective pressures on ibex due to the substitutability of plant defenses are discussed.

A Coevolutionary Arms Race: Understanding Plant-Herbivore Interactions

Plants and insects share a long evolutionary history characterized by relationships that affect individual, population, and community dynamics (Gatehouse, 2002). Plant-herbivore interactions are a prominent feature of this evolutionary history; it is by plant-herbivore interactions that energy is transferred from primary producers to the rest of the food web (Agrawal, 2004). On average, more than 10% of plant production is consumed by herbivores annually (Coley et al., 1985). This initial interaction between plants and their herbivores establishes the amount of energy available to the rest of the community. Consequently, the nature and extent of plant- herbivore interactions can significantly impact community structure, diversity, and function (see Silliman et al., 2005). Not only are plant-herbivore food webs important to community dynamics, but these webs also constitute a significant portion of the world's biodiversity. A recent study estimated that plant-herbivore food webs represent more than 40% of the global terrestrial biodiversity (Novotny & Basset, 2005). Consequently, it is important to understand the concepts central to these interactions as well as the processes that might influence the evolution of plant and herbivore species. Back Off Bugs! Plant Defenses Plant-herbivore relationships pit the insect against the plant in a race to maximize fitness. Maximizing fitness is the general goal for both plants and insects; however, in the case of plant-herbivore interactions, the insect maximizes its fitness by consuming plant tissues. Reduced biomass and tissue damage can negatively impact plant growth and fecundity, which ultimately decrease plant population growth rate. Individual plants that can somehow prevent or tolerate herbivory have a higher fitness than their more vulnerable neighbors; thus, these individuals contribute more genes to future generations. Not all plant-herbivore interactions are equal; rather, the type and amount of herbivore damage varies greatly among plant species and populations (Coley et al., 1985). This variation in consumption is likely a reflection of variation in plant palatability. Plants employ a variety of mechanical and chemical defenses that affect palatability and deter herbivores (Lerdau et al., 1994; Mauricio, 1998; Gatehouse, 2002; Siemens et al., 2002; Kliebenstein, 2004). Leaf toughness and trichome production are two ways in which plants mechanically protect themselves from herbivores (Feeny, 1970; Mauricio, 1998). These defenses act as physical barriers, preventing or slowing herbivores from accessing and handling plant tissue. In contrast, chemical defenses make it difficult for herbivores to process plant material once it is ingested. Chemicals, including glucosinolates, monoterpenes, and tannins, are generally produced through secondary metabolism (Lerdau, 1994; Mauricio, 1998; Gatehouse, 2002; Siemens et al., 2002; Kliebenstein, 2004). These compounds may have several functions that make them important to the plant's survival. In terms of defense, these compounds are often toxic or lethal to herbivores, causing a suite of problems that deter herbivores from foraging on the defended plant (Gatehouse, 2002). Operation Adaptation: Herbivore Offenses Plant defenses are important to insect herbivore populations because of their effect on insect growth and reproduction (Agrawal, 2004). The direction and magnitude of these effects depend on the evolutionary relationship of the insect herbivore to the plant; in particular, the effects of plant defenses on herbivore populations depend on whether the insect is a specialist or generalist herbivore on a specific host plant (Traw & Dawson, 2002a,b). In general, specialist herbivores tend to be unaffected by chemical defenses, many of which are lethal to generalist herbivores (Agrawal et al., 1999; Agrawal, 2000; Traw & Dawson, 2002). In some cases, chemical defenses are actually positively correlated with feeding by specialist herbivores, indicating that specialist herbivores use the chemical defenses as a cue to locate their preferred prey or exploit the defense in their own interactions with predator species (Ehrlich & Raven, 1964; Agrawal et al. …

Tri‐trophic effects of plant defenses: chickadees consume caterpillars based on host leaf chemistry

Few studies have addressed how plant chemical defenses that directly affect herbivores in turn affect consumption patterns of vertebrates at higher trophic levels. We studied how variable foliar chemistry of trembling aspen (Populus tremuloides Michx.) affects the diet preferences of an avian insectivore feeding on an introduced herbivore, the gypsy moth (Lymantria dispar L.).Black‐capped chickadees (Poecile atricapilla) were offered paired choices of gypsy moth caterpillars feeding on one of three genotypes of aspen that differed in chemical composition. Chickadees chose to eat caterpillars fed aspen foliage with low levels of both condensed tannins and phenolic glycosides, or caterpillars fed foliage with high levels of tannins and low levels of phenolic glycosides, over caterpillars fed foliage with low levels of condensed tannins and high levels of phenolic glycosides. In addition, diet choices of the birds were affected by their previous experience. These findings are consistent with the “extended phenotype” concept, in that genetically‐based chemical traits in an ecologically dominant plant influence the feeding behavior of third trophic level organisms, whose efficacy as regulators of herbivore populations may in turn be modified.