Top-down effects of insect herbivores during early succession: influence on biomass and plant dominance

Abstract

We tested the hypothesis that phytophagous insects would have a strong top-down effect on early successional plant communities and would thus alter the course of succession. To test this hypothesis, we suppressed above-ground insects at regular intervals with a broad-spectrum insecticide through the first 3 years of old-field succession at three widely scattered locations in central New York State. Insect herbivory substantially reduced total plant biomass to a similar degree at all three sites by reducing the abundance of meadow goldenrod, Solidago altissima. As a result, Euthamia graminifolia dominated control plots whereas S. altissima dominated insecticide-treated plots by the third year of succession. S. altissima is the dominant old-field herbaceous species in this region but typically requires at least 5 years to become dominant. Past explanations for this delay have implicated colonization limitation whereas our data demonstrate that insect herbivory is a likely alternative explanation. A widespread, highly polyphagous insect, the xylem-tapping spittlebug, Philaenus spumarius, appeared to be the herbivore responsible for the reduction in standing crop biomass at all three sites. Insect herbivory typically caused little direct leaf tissue loss for the ten plant species we examined, including S. altissima. Consequently, the amount of leaf area removed was not a reliable indicator of the influence of insect herbivory on standing crop biomass or on early succession. Overall, we found a strong top-down effect of insect herbivores on biomass at several sites, so our results may be broadly applicable. These findings run counter to generalizations that top-down effects of herbivores, particularly insects, are weak in terrestrial systems. These generalizations may not apply to insects, such as spittlebugs, that can potentially mount an effective defense (i.e., spittle) against predators and subsequently reach relatively high abundance on common plant species. Our results suggest that insect herbivory may play an important but often overlooked role during early old-field succession.

Insect‐plant interactions on a planet of weeds

“…In early old-field succession in the USA, Carson & Root (1999) found in insect exclusion experiments that insects consistently suppressed plant biomass and changed the relative abundance of species across all three study-sites by the third year of succession. The effect of insects can be attributed almost entirely to one insect species, the cosmopolitan and highly polyphagous meadow spittlebug Philaenus spumarius, which made up 68-94% of the entire herbivore load in the pivotal third year of succession.…”

“…Preference for the symmetrical view may reflect belief in a balance of nature, or it may represent a logical starting point for theoretical models of ecological and coevolutionary dynamics of herbivore-plant, predator-prey, and parasite-host population interactions. More substantively, a growing number of ecological studies have demonstrated that removing herbivores or pathogens often increases plant population size or plant fitness (Bigger & Marvier, 1998;Marquis, 1992), alters the pattern of selection on genetic variation in chemical and morphological characters that have been shown to reduce herbivore damage on plants in the field (Mauricio & Rausher, 1997), and changes the speed and direction of ecological succession (Bach, 2001;Brown & Gange, 1992;Carson & Root, 1999;Fagan & Bishop, 2000). These findings bolster the assumption that insect herbivores impose strong negative effects on plants and support the symmetrical view that insects and plants exert major effects on each other.…”

Herbivore preference drives plant community composition

“…Other theories on how herbivores can promote coexistence require a trade-off between the vigorous growth of plants and their defense against consumers, assuming that defense is costly and constrains investment in other important traits (Coley et al 1985, Herms and Mattson 1992, Viola et al 2010, Kempel et al 2011, Lind et al 2013. If the plants growing most vigorously in a community are also the least defended ones, herbivores promote coexistence by selectively feeding on more vigorously growing and hence less defended plant species (Pacala andCrawley 1992, Carson andRoot 1999), thereby reducing average fitness differences between species, which is considered as an equalizing mechanism (Chesson 2000).…”

Herbivory among habitats on the Neotropical tree Cnidoscolus quercifolius Pohl. in a seasonally deciduous forest

“…Among the insect guilds, the herbivores have an important role in maintaining the diversity and energy flow (Janzen, 1987;Mulder et al, 1999;Belovsky and Slade, 2003;Carson and Root, 2009). Herbivores may prevent high recruitment rates of some plant species allowing coexistence of higher arboreal species diversity in tropical forests (Janzen, 1970;Connell et al, 1984).…”