The co-evolutionary struggle between arthropod herbivores and plants to consume or Remarkable progress in understanding plant relations with arthropod herbivores has been achieved in the recent A defining aspect of the field has been its focus on animals as the “other organism. . Twitter; Share on Google Plus. However, there are examples of insects making 'mistakes' and Chemical ecology, coevolution, herbivores, insect–plant interactions, .. Google Scholar . Chemical communication, plant relationships, and mimicry in the. Four important plant/animal interactions are explored here: plant/herbivore, plant/ pollinator, plant/disperser, and other examples of mutualism.
Even insects and animals that eat seeds are considered herbivores. Brooklyn Botanic Garden is a habitat where herons hunt for crayfish, monarchs feed on milkweed, and woodpeckers nest and forage for insects. Some herbivores consume entire plants, or enough to kill them. Others only eat a portion of the plant, and so the plant can recover. Current research, however, is revealing that herbivory has some potential benefits to plants.
One example is canopy grazing by insects, which allows more light to penetrate into the lower layers of the forest. Gypsy moth grazing on canopy trees in some areas of Virginia's Blue Ridge Mountains, for instance, has resulted in more light penetration and therefore a more diverse and productive ground layer.
Herbivores and Their Food Plants Bison, sheep, and other grazers - Succulent forbs, grasses, grass-like plants Deer and other ungulate browsers - Leaves and twigs of woody plants such as willows, arborvitaes, yews Beaver - Tree bark, young shoots, leaves Rodents - Succulent forbs, grasses, grass-like plants Rabbits - Succulent forbs, grasses, bark Voles - Roots, bark Caterpillars - Leaves; in some cases, of specific species Monarch butterfly - Milkweeds Gypsy moth - Oaks and other hardwoods Aphids - Plant juices; in some cases, of specific species Many birds - Seeds and fruits Locusts - All plants; seeds, leaves, and stems Plants and Their Pollinators Pollination is the transfer of the pollen from one flower to the stigma, or female reproductive organ, of another, which results in fertilization and, ultimately, the formation of seeds.
The earliest plants were pollinated by wind, and for some modern plants this is still the most expedient method. Many trees, all grasses, and plants with inconspicuous flowers are designed for wind pollination. Bright, showy flowers evolved for another purpose—to attract a pollinator. Many plants depend on animals for pollination. Insects, birds, even bats are important for perpetuating plants. The flowers of these plants evolved in concert with their pollinators, and their form reflects the form and habits of their pollinators.
Bee-pollinated plants are often irregular in shape, with a lip that acts as a landing pad to facilitate the bee's entry into the flower. Butterfly-pollinated flowers are often broad and flat, like helicopter pads.
The flowers of many plants are brightly colored to attract their insect pollinators, and many offer nectar as an enticement. Hummingbirds, with their long beaks, pollinate tubular flowers. Bats require open flowers with room for their wings, such as those of the saguaro cactus. In the tropics, birds and bats take the place of insects as pollinators.
Hummingbirds and honeycreepers, for example, have distinctive beaks that have evolved to exploit flowers. Often, a beak may be so specialized that it is only effective on a small group of flowers.
The pollinators, in turn, have evolved to take advantage of the flowers. A successful pollinator typically has good color vision, a good memory for finding flowers, and a proboscis, or tongue, for attaining nectar. Animal pollination has obvious advantages for plants.
Many pollinators cover great distances, which insures genetic diversity through outcrossing, or the transfer of pollen to unrelated individuals.
The pollinator benefits as well by gaining access to a source of food.
Herbivory: effects on plant abundance, distribution and population growth
The relationship of pollinator plant is an example of mutualism. Imperiled Pollinators All is not well in the realm of pollinators. The age-old relationships between plants and pollinators is threatened, especially in urbanized and agricultural regions. Habitat destruction and fragmentation, pesticide abuse, and disease all have taken their toll on pollinators.
As more land is cleared for human habitation, bees, butterflies, bats, and birds are left homeless. Our gardens offer little to sustain them. They need a constant source of nectar and pollen throughout the entire season.
The few flowering plants most people grow will not suffice. A related problem is fragmentation of plant communities. Plants must be pollinated in order to set seed for the next generation. Without pollinators, no seed is set and the plants eventually die out, leading to local extinction. Isolated patches of forest, grassland, or desert are particularly vulnerable. A small patch may not sustain enough pollinators, or may be too far from other patches for pollinators to travel.
As a result, plants do not reproduce.
Pesticides have also reduced pollinator populations. Bees are often killed by chemicals applied to eliminate other pests. Honeybees are being destroyed by diseases and parasitic mites. The crisis is not just affecting native ecosystems. By excluding pre-dispersal seed-feeders on plants across this gradient, Louda found that herbivory on H. Thus, herbivory appeared to drive the pattern in plant abundance across this geographical gradient. More recent work has similarly implicated herbivores in affecting local plant distribution.
Ungulate exclusion enabled these species to colonize the interstitial spaces between shrubs, thereby altering their habitat distribution. As well, recent work by Fine et al. Thus, researchers have found effects of consumers on local distributions where they have looked for them, but too few studies exist to generalize the importance of consumers, relative to abiotic conditions.
The importance of consumers for local patterns of distribution suggests that herbivores could also affect the broader distributional limits of plants. Might herbivores influence plant range boundaries? Yet, areas immediately beyond the range of many plants are often abiotically similar to sites within the range of these species, and it appears that many plants could physiologically tolerate areas outside their current distribution.
For example, Stokes et al. In cases such as these, it is not clear what limits distribution, which begs the question of whether biotic rather than abiotic factors may be important. Though the idea that species interactions can set range boundaries has been floated for some decades Rochow ; MacArthur ; Galenfew studies of range limits explicitly consider how biotic factors influence range edges.
Caveats We have combined results from disparate studies to gain insight into population-level effects of consumers on plants. These potentially confounding effects suggest that our results should be treated cautiously at this stage, as refined hypotheses that require additional testing.
Experiments and population models Increasingly, studies of consumer impacts on plant populations involve a combination of experimental manipulation, demographic monitoring of individual performance, and population models to extrapolate long-term effects from observations made over shorter time spans.
For example, 17 of the studies shown in table 1 used population models to infer effects of herbivores on the long-term population growth rates of plants. Although population models coupled with demographic data offer a powerful and attractive vehicle for projecting consumer effects on plant dynamics, there are several key challenges in parameterizing these models.
The challenge in determining whether consumers limit plant abundance is that there are many cases where the first condition applies but not the second.
For example, if consumer-induced mortality of either seedlings or adults ultimately reduces the density of adult plants, the survival or fecundity of plants that escape herbivory may be enhanced due to reduced intraspecific competition. This can counterbalance losses due to herbivory. Although density dependence is common in plant populations Watkinson ; Willis et al.
The tendency in the plant—consumer literature has been to treat density dependence as binary i. The same is true with thinking about safe-site limitation i. However, plant populations can be: Despite widespread recognition that compensatory density dependence can profoundly mediate the impacts of consumers on plant populations, only two studies that we know of have estimated the magnitude of spatial and temporal variation in the strength of density dependence in the context of examining consumer effects on plants Augustine et al.
More typically, models of consumer effects on plant populations have assumed density-independent plant population growth. In some of these cases, this may be appropriate, in that it accurately reflects the biology of the plants involved e.
In many cases, however, the lack of density dependence in plant—consumer models may simply reflect the broader reality that few plant population models of any type have incorporated empirical estimates of density dependence Menges ; some notable exceptions are presented in the electronic supplementary material. This paucity of density-dependent matrix population models likely reflects two factors. First, empirically estimating both the life stage where density dependence occurs, and its relative strength at each life stage, can be challenging.
Furthermore, density-dependent changes in a particular demographic rate can result from effects of either adult or juvenile density electronic supplementary material.
As such, experimentally manipulating density to measure the strength of density dependence involves making difficult decisions about where in a plant's life stage to impose these manipulations, and where to measure the effects. With density dependence, population growth is no longer exponential, and this makes calculating sensitivities problematic Caswell Since it is clear that the impacts of consumers hinge tightly on the strength of compensatory density-dependent growth, fecundity and mortality, estimating density dependence is a fundamental area for plant—consumer research.
How might this be accomplished? At the seed and seedling levels, the strength of density dependence can be estimated by sowing seeds at different densities, and observing survival as a function of sowing density. For adult plants, manipulations are less straightforward; it is not necessarily feasible to transplant mature plants, nor is it likely that transplantation would not affect plant performance in most species.
However, vital rates of surrounding plants could be measured as a function of damage to target plants, as a preliminary assessment of density-dependent growth, survival and fecundity of adult plants.
Herbivory: effects on plant abundance, distribution and population growth
Obtaining realistic estimates of seed bank dynamics requires estimation of three important demographic parameters: Each of these demographic rates may be age-specific, although estimates of age-specific germination and seed mortality are rare Doak et al.
Estimating seed production is relatively straightforward, so we will not discuss it further here. Estimating seed germination and seed mortality, however, is more problematic.
In many plant demography studies, rather than measuring seed mortality directly, seed survivorship is instead estimated based on experiments that quantify seedling emergence.
Furthermore, as Doak et al. An alternative is to add seeds to plots in the field, preferably in areas that do not contain dormant seeds. One caveat to this approach, however, is that seed addition experiments may poorly mimic the timing, density and spatial array of naturally dispersed seed.
An alternative approach is to estimate seed rain and seedling emergence in demography plots, in which consumers have or have not been excluded. If one has some knowledge about levels of seed dormancy and viability, maximum-likelihood techniques can be used to estimate levels of germination, given the relationship between seed input and seed output and how this varies spatially and between years Wright et al. In addition to measuring germination rates, accurately estimating seed bank dynamics requires knowledge about how many seeds germinate but fail to emerge, and how many dormant seeds die, either through declining viability or predation of seeds in the seed bank.
A common approach to modelling seed survival is to assume that survival of dormant seeds decays exponentially through time. Often, the slope of this decay function is set by some estimate of the maximum seed dormancy. As with estimating rates of seedling emergence, maximum-likelihood techniques based on observations of seed input and emergence in plots protected and exposed to consumers can be used to infer how seed viability may change through time, although these observations need to be made over long enough time spans to obtain reasonable estimates.
Future directions The past decade has seen a slow but steady rise in the number of studies that have examined how consumers influence plant abundance and distribution. Certainly considerable progress has been made since Crawley's now classic review.
However, they do not always follow general expectations based on individual and community-level studies of consumers, or demographic studies of plants in isolation of consumers. For example, in contrast to individual-level studies, vertebrates and invertebrate consumers equally affected plant population growth rates.
As well, after removing studies on biocontrol and domestic grazers, floral and seed predators had roughly equivalent effects on plant population growth than other herbivores, in opposition to expectations based on plant life-history theory. Merging demographic information on the effects of herbivores on plants with population models offers a simple and powerful way to integrate net effects of consumers on plants. Important methodological issues include adequately addressing the role of density dependence and seed banks in plant population dynamics.
Many of the issues raised in this review apply equally to understanding how other common processes, such as pollen limitation or plant—plant competition, influence plant abundance.
The challenge for the future is to refine experiments and models so that impacts of consumers on plant dynamics can be placed in a more holistic framework where complexities of plant life history can be integrated with experimental demography to understand the important drivers of plant abundance.
Supplementary Material Density-dependence in models of plant population growth: Table listing studies that have incorporated estimates of density-dependence into matrix models of plant population growth. Click here to view. How important is seed predation to recruitment in stable populations of long-lived perennials? Evidence of a physiological basis for the boreal-deciduous forest ecotone in North America.
Pollen limitation of plant reproduction: Ungulate effects on the functional species composition of plant communities: Evidence for two alternative stable states in an ungulate grazing system. Bastrante B, Lebreton J. Predicting demographic change in response to herbivory: Effects of grazing, competition, disturbance and fire on species composition and diversity in grassland communities. How different would a world without herbivory be?
A search for generality in ecology. Early primary succession on Mount St. Control of a desert—grassland transition by a keystone rodent guild.
- Plant Interactions with Arthropod Herbivores: State of the Field
Slug herbivory as a limiting factor in the geographical range of Arnica montana. Demography of Cirsium vulgare in a grazing experiment. The determinants of the distribution and abundance of the winter annual grass Vulpia ciliata spp. Herbivory and plant species coexistence: Herbivores and plant population dynamics. Insect herbivores and plant population dynamics. Seed predators and plant population dynamics.
Seeds, the ecology of regeneration in plant communities. Blackwell Science; Oxford, UK: Substrate type and the distribution of sugar maple at its elevational limit in the White Mountains, New Hampshire.
Demography of the perennial herb Lathyrus vernus. Herbivory and population dynamics. Fitness components versus total demographic effects: Land use and population growth of Primula veris: