Post-dispersal seed predation: consequences for plant demography and evolution

Post-dispersal seed predation is only one of many factors underlying plant demography and evolution. Nevertheless, the generalist feeding habits of many post-dispersal seed predators and the limited ability of plants either to compensate for or to respond to post-dispersal seed losses directly suggest that post-dispersal seed predation may have a considerable impact on plant populations. Seed predators probably have little direct influence on the demography of plants that regenerate exclusively by vegetative means or are buffered by a large active seed bank, but such species are only a minority in most plant communities.In general, ants are significant post-dispersal seed predators in arid and semi-arid ecosystems while they act mainly as seed dispersers rather than as predators in temperate ecosystems. Although studies have probably underestimated the importance of invertebrates and birds as seed predators, rodents appear to have greater potential to influence seed dynamics, and are particularly important in temperate ecosystems. For example, production of mast seed crops is more effective at satiating specialist invertebrate seed predators than generalist vertebrates, and recruitment may be limited by post-dispersal seed predation even during mast years.Both spatial variation in post-dispersal seed predation and differences in predation between species are important elements which facilitate the coexistence of different plant species. Where microsites are limiting, selective post-dispersal seed predators can influence pre-emptive competition for these microsites. Seed size determines the extent of density-dependent predation and the exploitation of buried seed. This suggests that post-dispersal seed predators may also play a role in the evolution of seed characteristics. However, conclusions regarding the ecological and evolutionary impact of post-dispersal seed predators will remain speculative without a more substantial empirical base.     

Trapline foraging by pollinators: its ontogeny, economics and possible consequences for plants

Background

Trapline foraging (repeated sequential visits to a series of feeding locations) has been often observed in pollinators collecting nectar or pollen from flowers. Although field studies on bumble-bees and hummingbirds have clarified fundamental aspects of this behaviour, trapline foraging still poses several difficult questions from the perspectives of both animals and plants. These questions include whether and how traplining improves foraging performance, how animals develop traplines with accumulating foraging experience, and how traplining affects pollen flow or plant reproduction.

Scope

First, we review our previous work performed by using computer simulations and indoor flight-cage experiments with bumble-bees foraging from arrays of automated feeders. Our findings include the following: (1) traplining benefits foragers that are competing for resources that replenish in a decelerating way, (2) traplining is a learned behaviour that develops over a period of hours and (3) the establishment of traplines could be hampered by spatial configuration of plants such as zigzags. Second, using a simulation model linking pollinator movement and pollen transfer, we consider how service by pollinators with different foraging patterns (searchers or trapliners) would affect pollen flow. Traplining increases mating distance and mate diversity, and reduces ‘iterogamy’ (self-pollination caused by return visits) at the population level. Furthermore, increased visitation rates can have opposite effects on the reproductive success of a plant, depending on whether the visitors are traplining or searching. Finally, we discuss possible consequences of traplining for plants in the light of new experimental work and modelling.

Conclusions

We suggest that trapline foraging by pollinators increases variation among plant populations in genetic diversity, inbreeding depression and contributions of floral traits to plant fitness, which should in turn affect the rates and directions of floral evolution. More theoretical and empirical studies are needed to clarify possible outcomes of such a neglected side of pollination.Key words: Artificial flower, Bombus, competition, floral evolution, foraging experience, iterogamy, model, pollen flow, pollinator movement, renewing resource, spatial memory, trapline foraging     

Tri-trophic consequences of UV-B exposure: plants,herbivores and parasitoids

In this paper we demonstrate a UV-B-mediated link between host plants, herbivores and their parasitoids, using a model system consisting of a host plant Brassica oleracea, a herbivore Plutella xylostella and its parasitoid Cotesia plutellae. Ultraviolet-B radiation (UV-B) is a potent elicitor of a variety of changes in the chemistry, morphology and physiology of plants and animals. Recent studies have demonstrated that common signals, such as jasmonic acid (JA), play important roles in the mechanisms by which plants respond to UV-B and to damage by herbivores. Plant responses elicited by UV-B radiation can affect the choices of ovipositing female insects and the fitness of their offspring. This leads to the prediction that, in plants, the changes induced as a consequence of UV damage will be similar to those elicited in response to insect damage, including knock-on effects upon the next trophic level, predators. In our trials female P. xylostella oviposited preferentially on host plants grown in depleted UV-B conditions, while their larvae preferred to feed on tissues from UV-depleted regimes over those from UV-supplemented ones. Larval feeding patterns on UV-supplemented tissues met the predictions of models which propose that induced defences in plants should disperse herbivory; feeding scars were significantly smaller and more numerous – though not significantly so – than those on host plant leaves grown in UV-depleted conditions. Most importantly, female parasitoids also showed a clear pattern of preference when given the choice between host plants and attendant larvae from the different UV regimes; however, in the case of the female parasitoids, the choice was in favour of potential hosts foraging on UV-supplemented tissues. This study demonstrates the potential for UV-B to elicit a variety of interactions between trophic levels, most likely mediated through effects upon host plant chemistry.     

Winter predation by insectivorous birds and consequences for arthropods and plants in summer

Top-down effects of predators can have important consequences for ecosystems. Insectivorous birds frequently have strong predation effects on herbivores and other arthropods, as well as indirect effects on herbivores’ host plants. Diet studies have shown that birds in temperate ecosystems consume arthropods in winter as well as in summer, but experimental studies of bird predation effects have not attempted to quantitatively separate winter predation impacts from those in summer. To understand if winter foraging by insectivorous birds has consequences for arthropods or plants, we performed a meta-analysis of published bird exclusion studies in temperate forest and shrubland habitats. We categorized 85 studies from 41 publications by whether birds were excluded year-round or only in summer, and analyzed arthropod and plant response variables. We also performed a manipulative field experiment in which we used a factorial design to exclude birds from Quercus velutina Lam. saplings in winter and summer, and censused arthropods and herbivore damage in the following growing season. In the meta-analysis, birds had stronger negative effects on herbivores in studies that included winter exclusion, and this effect was not due to study duration. However, this greater predation effect did not translate to a greater impact on plant damage or growth. In the field experiment, winter exclusion did not influence herbivore abundance or their impacts on plants. We have shown that winter feeding by temperate insectivorous birds can have important consequences for insect herbivore populations, but the strength of these effects may vary considerably among ecosystems. A full understanding of the ecological roles of insectivorous birds will require explicit consideration of their foraging in the non-growing season, and we make recommendations for how future studies can address this.     

Interactions between plants and herbivores: A review of plant defense

Ecologists have long ignored or underestimated the importance of plant–herbivore interactions owing to the diversities of herbivores, plant defensive strategies and ecological systems. In this review, we briefly discussed the categories of herbivores. Then we reviewed the major types of plant defenses against herbivores. Selective forces of herbivore pressures have led to the evolution of various defensive mechanisms in plants, which can be classified into (i) resistance traits that reduce the amount of damage received, including physical, chemical, and biotic traits; (ii) tolerance mechanisms that decrease the impact of herbivore damage, and (iii) escape strategies that reduce the probability of plants to be found by herbivores. These strategies have been studied at different levels from molecular genetics and genomics, to chemistry and physiology, to community and ecosystem ecology. We summarized the development of the methodology for studying plant defenses against herbivores. Particularly, 24 of those hypotheses and models, which are influential in the international community concerning the relationship between plants and herbivores, including the defensive mimicry hypothesis, the compensatory continuum hypothesis, the slow-growth-high-mortality hypothesis, etc, were introduced and grouped into four categories according to plant defense strategies in the present review. Finally, we also reviewed the research progress of plant–herbivore interactions in China, and discussed the perspectives of studies on plant–herbivore interactions.     

Use of plant resources by an omnivorous predator and the consequences for effective predation

Plant-provided food may enhance survival and establishment of omnivorous predators on target crops but on the other hand they may adversely affect predation rates and thus their potential for biological control of target pests. However, it is not known how predation is affected by plant food quality and prey density. The omnivorous predator Macrolophus pygmaeus is commonly used in augmentative releases in greenhouse crops. Experiments have shown its ability to utilize plant resources; eggplant and pepper plant leaves are the most and least suitable, respectively. In this study we searched the effects of floral resources (pollen or flower) of eggplant or pepper plant on the predation rate of M. pygmaeus. We used experiments in dishes (leaves) and cages (plants) under a range of densities of its prey, the aphid Myzus persicae. We did not find evidence that the consumption rates and the type of the functional responses of M. pygmaeus were affected by the plant leaf (eggplant vs pepper plant) or the increase in the spatial scale (leaf vs plant). However, the presence of pollen or a flower of eggplant and to a lesser extent of pepper plant reduced the plateau of the functional response to aphid density and increased the handling time per prey. The extent of prey feeding replacement by flower resources was dependent on the interaction between plant species and prey density. It seems that there is a constant rate of prey consumption replacement at intermediate and high prey densities on eggplant but only at intermediate prey densities on pepper plant. These results indicate the interactions between plant and prey resources in diets of omnivores and may be useful for its efficacy in pest control on eggplant and pepper plant.     

Limited spatial response to direct predation risk by African herbivores following predator reintroduction

Predators affect ecosystems not only through direct mortality of prey, but also through risk effects on prey behavior, which can exert strong influences on ecosystem function and prey fitness. However, how functionally different prey species respond to predation risk and how prey strategies vary across ecosystems and in response to predator reintroduction are poorly understood. We investigated the spatial distributions of six African herbivores varying in foraging strategy and body size in response to environmental factors and direct predation risk by recently reintroduced lions in the thicket biome of the Addo Elephant National Park, South Africa, using camera trap surveys, GPS telemetry, kill site locations and Light Detection and Ranging. Spatial distributions of all species, apart from buffalo, were driven primarily by environmental factors, with limited responses to direct predation risk. Responses to predation risk were instead indirect, with species distributions driven by environmental factors, and diel patterns being particularly pronounced. Grazers were more responsive to the measured variables than browsers, with more observations in open areas. Terrain ruggedness was a stronger predictor of browser distributions than was vegetation density. Buffalo was the only species to respond to predator encounter risk, avoiding areas with higher lion utilization. Buffalo therefore behaved in similar ways to when lions were absent from the study area. Our results suggest that direct predation risk effects are relatively weak when predator densities are low and the time since reintroduction is short and emphasize the need for robust, long‐term monitoring of predator reintroductions to place such events in the broader context of predation risk effects.     

Differential induction of plant chemical defenses by parasitized and unparasitized herbivores: consequences for reciprocal,multitrophic interactions

Insect parasitoids can play ecologically important roles in virtually all terrestrial plant–insect herbivore interactions, yet whether parasitoids alter the defensive traits that underlie interactions between plants and their herbivores remains a largely unexplored question. Here, we examined the reciprocal trophic interactions among populations of the wild cabbage Brassica oleracea that vary greatly in their production of defensive secondary compounds – glucosinolates (GSs), a generalist herbivore, Trichoplusia ni, and its polyembryonic parasitoid Copidosoma floridanum. In a greenhouse environment, plants were exposed to either healthy (unparasitized), parasitized, or no herbivores. Feeding damage by herbivores induced higher levels of the indole GSs, glucobrassicin and neoglucobrassicin, but not any of the other measured GSs. Herbivores parasitized by C. floridanum induced cabbage plants to produce 1.5 times more indole GSs than levels induced by healthy T. ni and five times more than uninduced plants. As a gregarious endoparasitoid, C. floridanum causes its host T. ni to feed more than unparasitized herbivores resulting in increased induction of indole GSs. In turn, herbivore fitness parameters (including differential effects on male and female contributions to lifetime fecundity in the herbivore) were negatively correlated with the aliphatic GSs, sinigrin and gluconapin, whereas parasitoid fitness parameters were negatively correlated with the indole GSs, glucobrassicin and neoglucobrassicin. That herbivores and their parasitoids appear to be affected by different sets of GSs was unexpected given the intimate developmental associations between host and parasitoid. This study is the first to demonstrate that parasitoids, through increasing feeding by their herbivorous hosts, can induce higher levels of non‐volatile plant chemical defenses. While parasitoids are widely recognized to be ubiquitous in most terrestrial insect herbivore communities, their role in influencing plant–insect herbivore relationships is still vastly underappreciated.     

Plant architecture, sectoriality and plant tolerance to herbivores

The evolution of tolerance is one potential plant response to selection imposed by herbivores. Plant architecture, and in turn, sectoriality may influence a plant's ability to tolerate tissue loss. However, each may either constrain or facilitate a plant's ability to compensate following herbivore attack depending on the plant part damaged and the identity of the damaging herbivore.Plants are limited in their ability to respond to localized damage by chewing insects because carbon does not flow freely from damaged to undamaged plant parts, particularly between branches. Thus, defoliation of individual branches invariably results in decreased growth and reproduction of those branches. Within branches, carbon flow via vascular connections between orthostichies may ameliorate the effects of damage restricted within an orthostichy. Local induction of secondary chemicals to spread damage by folivores throughout a plant's canopy, redistribution of resources within and between IPU's, and delaying reproductive activity until resources have been pooled may all alleviate the constraints on response of plants to grazing.In contrast to the effects of damage by grazers, the metameric construction of plants typically ensures points of regrowth from dormant buds when apical meristems are destroyed either by vertebrate browsers or galling insects. Sectoriality constrains the ability of sap-sucking insects to tap the entire resource base of a plant, thus having a positive effect on plant fitness. However, both the site and timing of attack mitigate the degree of limitation imposed by sectoriality. During peak periods of assimilation, photosynthate flow is mainly over short distances (between sources and sinks within the canopy), and thus sap-sucking insects have a small resource base to draw upon. In contrast, when sucking insects tap into vascular elements in which the flow is from roots to leaves and vice versa, resource availability to the insect (and in turn, potential resource loss from the plant) are only limited by the resources present in those vascular elements.Studies of specific traits in species which demonstrate differential tolerance would greatly add to our understanding of herbivore impacts on plant growth and reproduction. In particular, intraspecific variation in tolerance has been documented for individuals within and among populations with different grazing histories. A number of traits related to sectoriality and architecture probably contribute to such variation in tolerance, and because they are easily manipulated and easily quantified, represent potentially profitable avenues of research. These traits include distribution of leaves and buds, ability to release secondary meristems from dormancy, and the timing of resource movement both before and subsequent to damage.     

Plant breeding: importance of plant secondary metabolites for protection against pathogens and herbivores

Summary Chemical protection plays a decisive role in the resistance of plants against pathogens and herbivores. The so-called secondary metabolites, which are a characteristic feature of plants, are especially important and can protect plants against a wide variety of microorganisms (viruses, bacteria, fungi) and herbivores (arthropods, vertebrates). As is the situation with all defense systems of plants and animals, a few specialized pathogens have evolved in plants and have overcome the chemical defense barrier. Furthermore, they are often attracted by a given plant toxin. During domestication of our crop and food plants secondary metabolites have sometimes been eliminated. Taking lupins as an example, it is illustrated that quinolizidine alkaloids are important as chemical defense compounds and that the alkaloid-free varieties (sweet lupins), which have been selected by plant breeders, are highly susceptible to a wide range of herbivores to which the alkaloid-rich wild types were resistant. The potential of secondary metabolites for plant breeding and agriculture is discussed.     

Interactions between above- and belowground insect herbivores as mediated by the plant defense system

Plants are frequently attacked by both above- and belowground arthropod herbivores. Nevertheless, studies rarely consider root and shoot herbivory in conjunction. Here we provide evidence that the root-feeding insect Agriotes lineatus reduces the performance of the foliage feeding insect Spodoptera exigua on cotton plants. In a bioassay, S. exigua larvae were allowed to feed on either undamaged plants, or on plants that had previously been exposed to root herbivory, foliar herbivory, or a combination of both. Previous root herbivory reduced the relative growth rates as well as the food consumption of S. exigua by more than 50% in comparison to larvae feeding on the undamaged controls. We found no effects in the opposite direction, as aboveground herbivory by S. exigua did not affect the relative growth rates of root-feeding A. lineatus . Remarkably, neither did the treatment with foliar herbivory affect the food consumption and relative growth rate of S. exigua in the bioassay. However, this treatment did result in a significant change in the distribution of S. exigua feeding. Plants that had been pre-exposed to foliar herbivory suffered significantly less damage on their young terminal leaves. While plant growth and foliar nitrogen levels were not affected by any of the treatments, we did find significant differences between treatments with respect to the level and distribution of plant defensive chemicals (terpenoids). Exposure to root herbivores resulted in an increase in terpenoid levels in both roots as well as in mature and immature foliage. Foliar damage, on the other hand, resulted in high terpenoid levels in young, terminal leaves only. Our results show that root-feeding herbivores may change the level and distribution of plant defenses aboveground. Our data suggest that the reported interactions between below- and aboveground insect herbivores are mediated by induced changes in plant secondary chemistry.     

Causes and consequences of migration by large herbivores

Many populations of large herbivores migrate seasonally between discrete home ranges. Current evidence suggests that migration is generally selected for as a means of enhancing access to high quality food and/or reducing the risk of predation. The relative importance of these alternative selection pressures should depend on the demographic circumstances facing a given population. Seasonal migration also has important implications for the structure and dynamics of large herbivore communities. Migrants should tend to be regulated by food availability, while residents should tend to be regulated by predators As a result, migrants should often outnumber residents by a considerable margin - a pattern seen in several tropical and temperate ecosystems. Differences in the mode of regulation could also imply that competition for resources will be weak in purely resident assemblages, but strong in communities dominated by migrants. Continual grazing by resident herbivores can sometimes lead to degeneration of vegetation, while systems supporting migrants are apparently more resilient. This implies that migration can have an important impact on the long-term persistence of plant-herbivore systems, particularly in areas with slow rates of vegetation regeneration.     

Plant defenses against parasitic plants show similarities to those induced by herbivores and pathogens

Herbivores and pathogens come quickly to mind when one thinks of the biotic challenges faced by plants. Important but less appreciated enemies are parasitic plants, which can have important consequences for the fitness and survival of their hosts. Our knowledge of plant perception, signaling and response to herbivores and pathogens has expanded rapidly in recent years, but information is generally lacking for parasitic species. In a recent paper we reported that some of the same defense responses induced by herbivores and pathogens—notably increases in jasmonic acid (JA), salicylic acid (SA), and a hypersensitive-like response (HLR)—also occur in tomato plants upon attack by the parasitic plant Cuscuta pentagona (field dodder). Parasitism induced a distinct pattern of JA and SA accumulation, and growth trials using genetically-altered tomato hosts suggested that both JA and SA govern effective defenses against the parasite, though the extent of the response varied with host plant age. Here we discuss similarities between the induced responses we observed in response to Cuscuta parasitism to those previously described for herbivores and pathogens and present new data showing that trichomes should be added to the list of plant defenses that act against multiple enemies and across kingdoms.Key words: Cuscuta, induced defenses, parasitic plant, jasmonic acid, salicylic acid, phytohormones, hypersensitive response, trichomes, defense signalingSeveral thousand species of plants are parasitic, stealing water and nutrients from other plants through a specialized feeding structure, the haustorium.¹ Haustoria are thought to be modified roots that grow into tissues and fuse with the vascular system of their photosynthetic hosts.¹ Considering that these parasites include some of the world''s most devastating agricultural pests² and are influential, fascinating components of natural communities,^1,3 surprisingly little is known about host defenses induced by parasitic plants. To address this shortcoming, we used a metabolomics approach to track biochemical changes induced in tomato shoots by invasion of C. pentagona haustoria.⁴We found that parasitism induced large increases in both JA and SA beginning about 24 hr after formation of haustoria began, but that production of JA and SA was largely separated in time. Host production of JA was transitory and reached a maximum at 36 hr, whereas SA peaked 12 hr later and remained elevated 5 d later. We also found that C. pentagona grew larger on mutant tomato plants in which the SA (NahG) or JA (jasmonic acid-insensitive1) pathways were disrupted, suggesting that these hormones can act independently to reduce parasite growth. Taken together, these findings suggest the staggered production of JA and SA may be an adaptive response to parasitism—by sequentially activating the JA and SA pathways, tomato plants may minimize the potential for cross-talk between these sometimes antagonistic pathways^5,6 and utilize both signaling molecules.^6,7 Thus, defenses against C. pentagona contain elements characteristic of responses to both herbivores (primarily JA-mediated⁸) and pathogens (primarily SA-mediated⁹)—though it should be noted that some herbivores induce SA¹⁰ and some pathogens JA.¹¹ It is worth noting that parasitism induced predominately cis-JA, the same jasmonate isomer induced by herbivore feeding.¹² Host responses to Cuscuta seem to most resemble that of known plant responses to some pathogens in which a similar sequence of JA and SA production is required to limit disease.¹³C. pentagona also triggered a hypersensitive-like response (HLR) localized around the points of parasite attachment. Using a trypan blue staining technique, we verified host cell death in these parasite-induced lesions. The deposition of eggs by some insect herbivores can elicit the formation of necrotic tissue,¹⁴ but localized cell death is most widely associated with the hypersensitive response (HR) of plants to pathogens. This complex early defense response can restrict the growth and spread of viruses, fungi and bacteria.⁹ Our work adds to existing evidence¹⁵ that the Cuscuta-induced HLR can play a similar role by preventing or limiting the growth of the parasite.An interesting discovery was that the first attachment by C. pentagona elicited almost no response from young 10-day-old hosts, whereas a subsequent attachment after 10 days induced the wholesale changes discussed above (we also found changes in abscisic acid and free fatty acids). Trials in which we varied the age of the host and parasite indicated that host age, rather than a priming effect on defenses, determined the magnitude of response. We have previously observed that Cuscuta spp. in natural populations germinate very early in the growing season, and hypothesized that this tactic promotes successful parasitism by ensuring the presence of young hosts; recent field work seems to corroborate this.¹⁶ As with the response to Cuscuta parasitism, levels of host plant defenses against insects¹⁷ and pathogens¹⁸ are known to be vary with host age.In an earlier paper we reported that tomato plants parasitized by C. pentagona released greater amounts of volatiles than did unparasitized control plants.¹⁹ The production and release of volatiles is a hallmark of plant responses to feeding by herbivores.²⁰ Herbivore-induced volatiles serve as an indirect plant defense by attracting herbivores'' natural enemies,²¹ repelling herbivores,²² or acting as intra-plant signals that prime systemic responses.²³ Although less well documented, pathogen attack can also induce emissions of volatile compounds,²⁴ some of which are antimicrobial and may serve as a direct defense against infection.²⁵ The same volatile compounds induced by Cuscuta (e.g., 2-carene, α-pinene, limonene, β-phellandrene) were also induced by caterpillar feeding and application of JA.¹⁹ Like herbivores, the JA induced by C. pentagona may regulate the emissions of plant volatiles. Whether or how parasitic plant-induced volatiles might function in defense is unknown, but they presumably could affect host plant choice by Cuscuta seedlings, which use plant volatiles to locate and select hosts.²⁶Following on from our previous studies we examined the potential role of host trichomes in resistance to parasitism by C. pentagona. Plant trichomes have been long appreciated as the first line of defense against insect herbivores^27,28 and more recently pathogens.²⁹ We hypothesized that trichomes could also defend against parasitic plants based on our observations that (1) tomato trichomes become denser with age (Fig. 1), notably on hypocotyls which is the first area contacted by Cuscuta seedlings, and (2) these trichomes can act as a physical barrier to C. pentagona seedlings. To test this hypothesis we allowed seedlings of C. pentagona to attach to 25-day-old tomato plants (Solanum lycopersicum ‘Halley 3155’) in a climate controlled growth chamber. Of 20 trials conducted, in six (30%) the parasite seedling was completely blocked by trichomes and was unable to reach the host stem—the parasite perished in each of these. Type I glandular trichomes, which are several millimeters long with a glandular tip,³⁰ were primarily responsible for the blocking effect. Thus, trichomes can defend against parasitic plants in a manner analogous to herbivores by physically obstructing their movement. Interestingly, the effectiveness of trichomes is also dependent on age of the host since those on younger tomato plants (<20 days old) are too sparse to impede Cuscuta seedlings (Fig. 1).Open in a separate windowFigure 1A newly germinated Cuscuta pentagona seedling encircles and attaches to the hypocotyl of a 10-day-old tomato seedling; the early development of haustoria are visible as nod-like swellings. The trichomes on hypocotyls of young tomato seedlings are not dense enough to affect C. pentagona seedlings, but the increased density of trichomes on 25-day-old plants can act as a physical barrier that blocks parasite seedlings (inset).Considering that the majority of plant defenses are mediated by only a small number of master regulators (e.g., JA, SA, ethylene),⁷ it is not surprising that plant responses to parasitic plants share commonalities with those induced by herbivores and pathogens. These few molecules mediate complex, interacting signaling networks that can be variously activated and modified by plants to tune defenses against a seemingly endless variety of attackers.⁷ Our finding that JA and SA act to defend plants from attack by other plants, further support these phytohormones as ‘global’ defense signals. It is also apparent that constitutive defenses, such as trichomes, can be effective against diverse antagonists (e.g., herbivores and parasitic plants). These new insights into host defenses against parasitic plants suggest many avenues of needed research including the molecular events induced by parasitic plant attack, the parasite-derived cues that elicit responses, and the ways in which JA and SA act to reduce parasite growth. Finally, our findings suggest it might be possible to manipulate induced responses or host plant age by varying planting date to control parasitic plants in agriculture.     

Future directions in the ontogeny of plant defence: understanding the evolutionary causes and consequences

Plant defence often varies by orders of magnitude as plants develop from the seedling to juvenile to mature and senescent stages. Ontogenetic trajectories can involve switches among defence traits, leading to complex shifting phenotypes across plant lifetimes. While considerable research has characterised ontogenetic trajectories for now hundreds of plant species, we still lack a clear understanding of the molecular, ecological and evolutionary factors driving these patterns. In this study, we identify several non‐mutually exclusive factors that may have led to the evolution of ontogenetic trajectories in plant defence, including developmental constraints, resource allocation costs, multi‐functionality of defence traits, and herbivore selection pressure. Evidence from recent physiological studies is highlighted to shed light on the underlying molecular mechanisms involved in the regulation and activation of these developmental changes. Overall, our goal is to promote new research avenues that would provide evidence for the factors that have promoted the evolution of this complex lifetime phenotype. Future research focusing on the questions and approaches identified here will advance the field and shed light on why defence traits shift so dramatically across plant ontogeny, a widespread but poorly understood ecological pattern.     

Fear factor: prey habitat selection and its consequences in a predation risk landscape

Predation risk influences prey use of space. However, little is known about how predation risk influences breeding habitat selection and the fitness consequences of these decisions. The nest sites of central-place foraging predators may spatially anchor predation risk in the landscape. We explored how the spatial dispersion of avian predator nests influenced prey territory location and fitness related measures. We placed 249 nest boxes for migrant pied flycatchers Ficedula hypoleuca , at distances between 10 and 630 m, around seven different sparrowhawk nests Accipiter nisus . After closely monitoring flycatcher nests we found that flycatcher arrival dates, nest box occupation rates and clutch size showed a unimodal relationship with distance from sparrowhawk nests. This relationship suggested an optimal territory location at intermediate distances between 330 and 430 m from sparrowhawk nests. Furthermore, pied flycatcher nestling quantity and quality increased linearly with distance from sparrowhawk nests. These fitness related measures were between 4 and 26% larger in flycatcher nestlings raised far from, relative to those raised nearby, sparrowhawk nests. Our results suggest that breeding sparrowhawk affected both flycatcher habitat selection and reproductive success. We propose that nesting predators create predictable spatial variation in predation risk for both adult prey and possibly their nests, to which prey individuals are able to adaptively respond. Recognising predictable spatial variation in perceived predation risk may be fundamental for a proper understanding of predator-prey interactions and indeed prey species interactions.     

Relationships among Key deer, insect herbivores, and plant quality

Deer can have severe effects on plant communities, which in turn can affect insect communities. We studied the effects of Key deer herbivory on the incidence of insect herbivores that occur within deer habitats in the lower Florida Keys, within the National Key Deer Refuge (NKDR). We analyzed plant chemistry (tannins, nitrogen) and surveyed for the occurrence of insects (above the browse tier) among plant species that were either deer-preferred or less-preferred. Results indicated higher levels of foliar tannins on islands with fewer Key deer and larger amounts of foliar nitrogen on islands with a high density of Key deer. Consequently, leaf miners were significantly more abundant on islands with high deer density, irrespective of deer-preference of plant species. On islands with a high deer density, incidence of leaves damaged by chewing insects was lower on preferred plant species but greater on less-preferred species than on islands with fewer deer. No apparent patterns were evident in the distribution of leaf gallers among plant species or islands with different deer density. Our results imply that plant nutrition levels—either preexisting or indirectly affected by deer deposition—are more important than plant defenses in determining the distribution of insect herbivores in the NKDR. Although high densities of the endangered Key deer have negative effects on some plant species in the NKDR, it seems Key deer might have an indirect positive influence on insect incidence primarily above the browse tier. Further research is warranted to enable fuller understanding of the interactions between Key deer and the insect community.