Reading:  Economy of Nature, pp. 459-462.

Wild ginger and banana slugs

Wild ginger plants, Asarum caudatum, vary in growth rates, seed production, and palatability to banana slugs, Ariolimax dolichophallus.

In habitats with low slug densities (and therefore low rates of herbivory),
        plants grow faster,
        plants produce more seeds, and
        plants are more palatable to slugs (in a laboratory)
compared to wild ginger plants from habitats with high slug densities.

These results suggest that plants from habitats with high predation risks mount greater defenses than do plants from habitats with low predation risks. However, mounting a chemical defense has costs in reduced growth rates and reduced numbers of seeds produced.

Plant Responses Following Herbivory
        Compensatory responses may occur:
                Preservation of balanced root/shoot ratio
                Redistribution of photosynthate to undamaged and intact tissues
                Increased rate of photosynthesis in intact tissues
                        (unit leaf rate increases)
                Compensatory regrowth by dormant buds
                        modification of flower and fruit abortion

However, compensation has its limits. Herbivory always represents a cost to the prey plant.

Plant Responses Following Herbivory
        Induction of chemical defenses may occur:
                Occurrence of rapidly-inducible defenses is not always conclusive.

The individual growth, survival, and reproduction of herbivores on prey plants subsequent to herbivory have been the clearest evidence of rapidly-inducible chemical defenses.

Cotton plants and mites (Karban and Carey, 1984)
        Cotton plants exposed to mites subsequently support smaller total mite populations (Tetranychus urticae), have fewer mite eggs, fewer immature mites, and fewer female mites compared to control plants that had no prior exposure to mites (Ricklefs, 1993, p 335, Fig. 18.14). Abbreviation key: T=total population, E=eggs, I=immatures, M=adult males, and F=adult females.

Disproportionate Effects

Bark-ringing, tree girdling
        Consumption of cambial tissues and phloem by herbivores will result in root starvation, but only a small fraction of the total plant biomass is consumed.

Grass meristem consumption by slugs

Herbivore Vectors

Scolytid beetles (bark beetles) are herbivores that incidentally carry Dutch elm fungus between prey elm trees.

The cactus moth, Cactoblastis cactorum, feeding on prickly pear cactus, Opuntia, leave feeding scars on the cactus. The feeding scars are the invasion sites for bacteria that rapidly kill the cactus.

Combination Effects

Competition and Herbivory
        When barley and oats are grown together in a replacement series, seed yields of each species are influenced by the abundance of the other species. There is a degree of reciprocal competitive inhibition, but in the absence of an oat herbivore, oats compete very strongly against barley. The root feeding nematode, Heterodera avenae, attacks oats but not barley. In the presence of the nematode and in the absence of barley, oat seed yields are unaffected, but in the presence of barley competitors, the combination of herbivory and competition has a strong negative effect on oat seed yields (after Begon, Harper and Townsend, 1990, p 285, Fig. 8.4).

Competition and Predation

Predatory newts, Notophthalmus viridescens, influence the competitive interactions between tadpoles of Scaphiopus, Bufo, and Hyla changing the growth and survival of tadpoles to metamorphosis.

Relative weight at metamorphosis and survival to metamorphosis by tadpoles for the three anuran species changes with predatory newt density (Ricklefs, 1996, p 442, Fig. 19.16).

Number of Predatory newts per pond

In absence of predatory newts, Hyla suffers from competition: poor survival and low metamorphosis weight.

In presence of predatory newts, which prefer Scaphiopus and Bufo prey, survival of Scaphiopus and Bufo decreases and survival of Hyla increases. Competition between tadpoles decreases. Growth of surviving tadpoles increases for all three species.

Air Pollution and Herbivory
        There is some evidence of greater levels of herbivore damage when plants are exposed to sulfur dioxide gas, and greater rates of herbivore growth in the presence of sulfur dioxide or nitrogen dioxide gas pollution. These findings suggest that these forms of air pollution may have indirect damaging effects on plants. There is no apparent direct plant damage due to either sulfur dioxide or nitrogen dioxide gas pollution, but these gases increase plant susceptibility to herbivore damage.

Effects on Prey Populations
        Herbivores have a negative effect on plants, particularly herbivores that specialize on flowers, fruits, and seeds. However, co-evolution-mutualism between pollen and nectar consumers and plants (pollinators), and between fruit or seed consumers and plants (seed dispersers) is an important outcome of what began as a predator-prey interaction.

The effect of herbivory on an individual prey is always negative, but population effects are not always clear because:
        prey individuals are not a random sample and
        responses of survivors may compensate for losses due to predation.

Compensation at the population level may mask the obvious negative individual effects, but compensation has limits and is not perfect.

Effects on Consumer Populations

Predator individuals benefit from consumption with increased rates of growth and development, increased birth rates, and/or decreased death rates.

A minimal consumption level may be required for net positive production in herbivore populations. There is a threshold below which a predator is at maintenance or worse. There are also consumption thresholds for normal individual growth and reproduction. A maximum consumption level is also possible beyond which there are no additional changes in growth, fecundity, and survival.

Consumption rates become independent of supply at high food availability levels, and then benefit becomes independent of supply. This is the satiation level of food supply. Some plants, particularly tree species, produce mast crops (seed masting) which is the production of very large quantities of seeds at one time.

Seed masting is an anti-herbivore tactic by plants whose seeds are consumed by predators with long generation times. Predators are unable to respond rapidly to a mast crop, so some proportion of seeds escape predation (see Ricklefs, 1996, p 422, Fig. 18.19). Similar tactics may occur among animals with distinct population-wide breeding seasons, synchronous nesting, synchronous spawning, and synchronous calving.

Copyright 1999 Northern Arizona University
ALL RIGHTS RESERVED