Leaf Damage and Associated Cues Induce Aggressive Ant Recruitment in a Neotropical Ant-Plant

Induced chemical responses following herbivory are common in plants. Plant responses that change the level of physical or biotic defense are less well documented and poorly understood. Many Azteca spp. ants are obligate inhabitants of Cecropia spp. trees. In such ant–plant associations the ants are thought to be analogous to chemical defenses; previous experiments have demonstrated that ant occupation of C. obtusifolia reduced herbivory and plant competition and increased growth. Experiments, conducted over two years, on the dynamics of ant defense demonstrate that leaf damage caused a fivefold increase in the number of Azteca spp. ants on damaged leaves of C. obtusifolia compared to that on disturbed but undamaged control leaves. Ant activity peaked 8–12 min after damage, and differences between damaged and control leaves remained evident for 24 h. Such rapid induction of ant recruitment is likely to be particularly effective against unpredictable and mobile herbivores. The magnitude of the induced ant response to damage was strongly correlated with the number of ants patrolling the leaves before damage occurred. Ant responses to disturbance were not influenced by the presence of damage that had been applied 24 h previously. However, ant responses to subsequent damage, 24 h after initial damage, resulted in greater recruitment than to previously undamaged leaves. Ant recruitment to several other cues associated with herbivory was also tested. Presence of pyralid caterpillars that naturally feed on C. obtusifolia induced a low level of ant recruitment, and most larvae were removed from leaves by the ants within 10 min. Exposure to plant sap collected from damaged conspecifics and a commercially available green leaf volatile (hexanal) commonly released by plants after damage, both resulted in a doubling of ant numbers relative to controls. However, the levels of recruitment in response to these stimuli were insufficient to account for the high numbers of ants and persistence of recruitment observed on experimentally damaged leaves. Experimental wounding of leaves with minimal leaf tissue removal (using pin pricks) revealed that leaf wounds per se can only partially explain the induced ant recruitment following leaf damage. The type of herbivory and size of leaf wounds may be important cues for ant recruitment. Severed C. obtusifolia leaves that were freshly damaged failed to elicit an induced ant response when held adjacent to conspecific leaves with ants. However, induction of ant recruitment on damaged plants did significantly induce a low level of ant recruitment on neighboring conspecifics, providing evidence for interplant communication. Induced ant responses in the Cecropia–Azteca system are the result of multiple physical and chemical cues associated with herbivory. Ant responses to herbivory, although not previously studied in detail, are likely to be common among myrmecophytic plants and are likely to be an important component of antiherbivore defense in such systems.Funding for this study was provided by a Graduate Fellowship from the Organization for Tropical Studies and grants from the Center for Population Biology at the University of California–Davis, the Jastro Shields Research Awards Program (U.C. Davis), the Northern California Chapter of Phi Beta Kappa, and National Science Foundation Dissertation Improvement Grant DEB?9701109

Phylogenetic escalation and decline of plant defense strategies

As the basal resource in most food webs, plants have evolved myriad strategies to battle consumption by herbivores. Over the past 50 years, plant defense theories have been formulated to explain the remarkable variation in abundance, distribution, and diversity of secondary chemistry and other defensive traits. For example, classic theories of enemy-driven evolutionary dynamics have hypothesized that defensive traits escalate through the diversification process. Despite the fact that macroevolutionary patterns are an explicit part of defense theories, phylogenetic analyses have not been previously attempted to disentangle specific predictions concerning (i) investment in resistance traits, (ii) recovery after damage, and (iii) plant growth rate. We constructed a molecular phylogeny of 38 species of milkweed and tested four major predictions of defense theory using maximum-likelihood methods. We did not find support for the growth-rate hypothesis. Our key finding was a pattern of phyletic decline in the three most potent resistance traits (cardenolides, latex, and trichomes) and an escalation of regrowth ability. Our neontological approach complements more common paleontological approaches to discover directional trends in the evolution of life and points to the importance of natural enemies in the macroevolution of species. The finding of macroevolutionary escalating regowth ability and declining resistance provides a window into the ongoing coevolutionary dynamics between plants and herbivores and suggests a revision of classic plant defense theory. Where plants are primarily consumed by specialist herbivores, regrowth (or tolerance) may be favored over resistance traits during the diversification processThis work was supported by the National Science Foundation

Plant genotype and environment interact to shape a diverse arthropod community on evening primrose (Oenothera biennis)

Both an individual's genotype and environment govern its phenotype, and this phenotype may have extended consequences for species interactions and communities. We examined the importance of plant genotype and environmental factors operating at large (habitat) and small (microhabitat) spatial scales in affecting a multitrophic arthropod community on plants. We planted 926 plants from 14 genotypes of Oenothera biennis into five natural habitats that represent the range of environments in which this plant locally occurs. Genotypic differences among plants accounted for as much as 41% of the variation in arthropod diversity (Simpson's diversity index) and also affected arthropod evenness, richness, abundance, and biomass on individual plants. However, the effects of particular plant genotypes on the arthropod community varied across habitats (i.e., there were significant plant genotype?by?habitat interactions). Plant genotype explained more variation in the arthropod community than did environmental variation among microhabitats, but less variation than habitats, as predicted by the scale?dependent hypothesis. Herbivores and omnivores were more strongly affected by plant genetic variation than predators, consistent with the notion that phytophagous insects undergo stronger reciprocal interactions with plants than do predators. We detected heritable variation in arthropod community variables and the ability for the herbivore community to select on plant traits, suggesting that evolution in O. biennis can lead to changes in the arthropod community. Genetic variation in plant size, architecture, and reproductive phenology were the plant traits most strongly correlated with arthropod community variables. Our results demonstrate that genotype?by?environment interactions are a major determinant of arthropod community structure.Our research is funded by Sigma Xi, Mountain Equipment Co?op Environment Fund, and the Natural Sciences and Engineering Research Council of Canada

Defense mutualisms enhance plant diversification

The ability of plants to form mutualistic relationships with animal defenders has long been suspected to influence their evolutionary success, both by decreasing extinction risk and by increasing opportunity for speciation through an expanded realized niche. Nonetheless, the hypothesis that defense mutualisms consistently enhance plant diversification across lineages has not been well tested due to a lack of phenotypic and phylogenetic information. Using a global analysis, we show that the >100 vascular plant families in which species have evolved extrafloral nectaries (EFNs), sugar-secreting organs that recruit arthropod mutualists, have twofold higher diversification rates than families that lack species with EFNs. Zooming in on six distantly related plant clades, trait-dependent diversification models confirmed the tendency for lineages with EFNs to display increased rates of diversification. These results were consistent across methodological approaches. Inference using reversible-jump Markov chain Monte Carlo (MCMC) to model the placement and number of rate shifts revealed that high net diversification rates in EFN clades were driven by an increased number of positive rate shifts following EFN evolution compared with sister clades, suggesting that EFNs may be indirect facilitators of diversification. Our replicated analysis indicates that defense mutualisms put lineages on a path toward increased diversification rates within and between clades, and is concordant with the hypothesis that mutualistic interactions with animals can have an impact on deep macroevolutionary patterns and enhance plant diversity.A.A.A. was supported by Grant 1118783 of the Division of Environmental Biology of the National Science Foundation (NSF) and by the John Templeton Foundation. M.G.W. was supported by the Society for the Study of Evolution’s Rosemary Grant Award and by the NSF (Graduate Research Fellowship and Doctoral Dissertation Improvement Grant)

Consequences of toxic secondary compounds in nectar for mutualist bees and antagonist butterflies

Attraction of mutualists and defense against antagonists are critical challenges for most organisms and can be especially acute for plants with pollinating and non?pollinating flower visitors. Secondary compounds in flowers have been hypothesized to adaptively mediate attraction of mutualists and defense against antagonists, but this hypothesis has rarely been tested. The tissues of milkweeds (Asclepias spp.) contain toxic cardenolides that have long been studied as chemical defenses against herbivores. Milkweed nectar also contains cardenolides, and we have examined the impact of manipulating cardenolides in nectar on the foraging choices of two flower visitors: generalist bumble bees, Bombus impatiens, which are mutualistic pollinators, and specialist monarch butterflies, Danaus plexippus, which are herbivores as larvae and ineffective pollinators as adults. Although individual bumble bees in single foraging bouts showed no avoidance of cardenolides at the highest natural concentrations reported for milkweeds, a pattern of deterrence did arise when entire colonies were allowed to forage for several days. Monarch butterflies were not deterred by the presence of cardenolides in nectar when foraging from flowers, but laid fewer eggs on plants paired with cardenolide?laced flowers compared to controls. Thus, although deterrence of bumble bees by cardenolides may only occur after extensive foraging, a primary effect of nectar cardenolides appears to be reduction of monarch butterfly oviposition.The Templeton Foundation and National Science Foundation DEB-1513839 provided financial support

The Benefits of Induced Defenses Against Herbivores

Previous explanations for the evolution of induced resistance of plants to herbivory emphasized arguments based on saving costs when allocations to defense were not needed; these models met with limited empirical support. We offer a novel explanation based on induced resistance providing increased variability in defense. As long as maximal levels of defense are constrained, variability will increase the effectiveness of a given level of investment in defense. We show that variability can decrease herbivore performance if herbivore performance is a concave function of the level of resistance. In particular, if herbivores can choose among different plants and plant tissues, then variability created by induced resistance may benefit plants under attack and hence may be favored by selection. The key assumptions of this model are broadly supported by empirical data from many plant–herbivore systems.This work was supported by USDA NRI 9602065

Evolutionary history predicts plant defense against an invasive pest

It has long been hypothesized that invasive pests may be facilitated by the evolutionary naïveté of their new hosts, but this prediction has never been examined in a phylogenetic framework. To address the hypothesis, we have been studying the invasive viburnum leaf beetle (Pyrrhalta viburni), which is decimating North American native species of Viburnum, a clade of worldwide importance as understory shrubs and ornamentals. In a phylogenetic field experiment using 16 species of Viburnum, we show that old-world Viburnum species that evolved in the presence of Pyrrhalta beetles mount a massive defensive wound response that crushes eggs of the pest insect; in contrast, naïve North American species that share no evolutionary history with Pyrrhalta beetles show a markedly lower response. This convergent continental difference in the defensive response of Viburnum spp. against insect oviposition contrasts with little difference in the quality of leaves for beetle larvae. Females show strong oviposition preferences that correspond with larval performance regardless of continental origin, which has facilitated colonization of susceptible North American species. Thus, although much attention has been paid to escape from enemies as a factor in the establishment and spread of nonnative organisms, the colonization of undefended resources seems to play a major role in the success of invasive species such as the viburnum leaf beetleViburnum phylogenetic studies were supported by National Science Foundation Grant IOS-0842800 (to M.J.D.). This study was supported by US National Science Foundation Grant DEB-0950231 (to A.A.A.) and Federal Formula Funds allocated by the Cornell University Agricultural Experiment Station (to A.A.A.)

Fitness consequences of occasional outcrossing in a functionally asexual plant (Oenothera biennis)

Many clonal organisms occasionally outcross, but the long?term consequences of such infrequent events are often unknown. During five years, representing three to five plant generations, we followed 16 experimental field populations of the forb, Oenothera biennis, originally planted with the same 18 original genotypes. Oenothera biennis usually self fertilizes, which, due to its genetic system (permanent translocation heterozygosity), results in seeds that are clones of the maternal plant. However, rare outcrossing produces genetically novel offspring (but without recombination or increased heterozygosity). We sought to understand whether novel genotypes produced through natural outcrossing had greater fecundity or different multigenerational dynamics compared to our original genotypes. We further assessed whether any differences in fitness or abundances through time between original and novel genotypes were exaggerated in the presence vs. absence of insect herbivores. Over the course of the experiment, we genotyped >12,500 plants using microsatellite DNA markers to identify and track the frequency of specific genotypes and estimated fecundity on a subset (>3,000) of plants. The effective outcrossing rate was 7.3% in the first year and ultimately 50% of the plants were of outcrossed origin by the final year of the experiment. Lifetime fruit production per plant was on average 32% higher across all novel genotypes produced via outcrossing compared to the original genotypes, and this fecundity advantage was significantly enhanced in populations lacking herbivores. Among 43 novel genotypes that were abundant enough to phenotype with replication, plants produced nearly 30% more fruits than the average of their specific two parental genotypes, and marginally more fruits (8%) than their most fecund parent. Mean per capita fecundity of novel genotypes predicted their relative frequencies at the end of the experiment. Novel genotypes increased more dramatically in herbivore?present compared to suppressed populations (45% vs. 27% of all plants), countering the increased competition from dandelions (Taraxacum officinale) that resulted from herbivore suppression. Increased interspecific competition likely also lead to the lower realized fitness of novel vs. original genotypes in herbivore?suppressed populations. These results demonstrate that rare outcrossing and the generation of novel genotypes can create high?fecundity progeny, with the biotic environment influencing the dynamical outcome of such advantages.This study was supported by a grant to A. A. Agrawal from NSF DEB-0950231. M. T. J. Johnson received funding from NSERC. J. L. Maron was supported by NSF DEB-1553518

Phenotypic Mismatches Reveal Escape from Arms-Race Coevolution

Because coevolution takes place across a broad scale of time and space, it is virtually impossible to understand its dynamics and trajectories by studying a single pair of interacting populations at one time. Comparing populations across a range of an interaction, especially for long-lived species, can provide insight into these features of coevolution by sampling across a diverse set of conditions and histories. We used measures of prey traits (tetrodotoxin toxicity in newts) and predator traits (tetrodotoxin resistance of snakes) to assess the degree of phenotypic mismatch across the range of their coevolutionary interaction. Geographic patterns of phenotypic exaggeration were similar in prey and predators, with most phenotypically elevated localities occurring along the central Oregon coast and central California. Contrary to expectations, however, these areas of elevated traits did not coincide with the most intense coevolutionary selection. Measures of functional trait mismatch revealed that over one-third of sampled localities were so mismatched that reciprocal selection could not occur given current trait distributions. Estimates of current locality-specific interaction selection gradients confirmed this interpretation. In every case of mismatch, predators were “ahead” of prey in the arms race; the converse escape of prey was never observed. The emergent pattern suggests a dynamic in which interacting species experience reciprocal selection that drives arms-race escalation of both prey and predator phenotypes at a subset of localities across the interaction. This coadaptation proceeds until the evolution of extreme phenotypes by predators, through genes of large effect, allows snakes to, at least temporarily, escape the arms race

A Field Experiment Demonstrating Plant Life-History Evolution and its Eco-Evolutionary Feedback to Seed Predator Populations

The extent to which evolutionary change occurs in a predictable manner under field conditions and how evolutionary changes feed back to influence ecological dynamics are fundamental, yet unresolved, questions. To address these issues, we established eight replicate populations of native common evening primrose (Oenothera biennis). Each population was planted with 18 genotypes in identical frequency. By tracking genotype frequencies with microsatellite DNA markers over the subsequent three years (up to three generations, approximate to 5,000 genotyped plants), we show rapid and consistent evolution of two heritable plant life-history traits (shorter life span and later flowering time). This rapid evolution was only partially the result of differential seed production; genotypic variation in seed germination also contributed to the observed evolutionary response. Since evening primrose genotypes exhibited heritable variation for resistance to insect herbivores, which was related to flowering time, we predicted that evolutionary changes in genotype frequencies would feed back to influence populations of a seed predator moth that specializes on O. biennis. By the conclusion of the experiment, variation in the genotypic composition among our eight replicate field populations was highly predictive of moth abundance. These results demonstrate how rapid evolution in field populations of a native plant can influence ecological interactions