Is top‐down control by predators driving insect abundance and herbivory rates in fragmented forests?

Abstract

The effects of forest fragmentation on ecological interactions and particularly on food webs have scarcely been analysed. There is usually less herbivory in forest fragments than in continuous forests. Here we hypothesize that forest fragmentation enhances top-down control of herbivory through an increase in insectivorous birds and a decrease in herbivorous insects, with a consequent decrease in plant reproductive success in small forest fragments. In the Maulino forest in central Chile, we experimentally excluded birds from Aristotelia chilensis (Elaeocarpaceae) trees in both forest fragments and continuous forest, and analysed herbivore insect abundance, herbivory and plant reproductive success during two consecutive growing seasons. We expected that insect abundance and herbivory would increase, and reproductive success would decrease in A. chilensis from which birds have been excluded, particularly in forest fragments where bird abundance and predation pressure on insects is higher. The abundance of herbivorous insects was lower in the forest fragments than in the continuous forest only in the first season, and herbivory was lower in forest fragments than in the continuous forest throughout the study. Moreover, during the second growing season herbivory was greater in the excluded trees than in the control trees, and as expected, there was a greater difference in the fragments than in the continuous forest, but this was not statistically significant. Exclusion of birds did not affect the reproductive success of A. chilensis. Our results, after 2 years of study, demonstrate that birds affect the levels of herbivory on A. chilensis in the Maulino forest, but do not support our hypothesis of enhanced top-down control in fragmented forests, as the strength of the effect of excluding birds did not vary with fragmentation.

Not all in the same boat: trends and mechanisms in herbivory responses to forest fragmentation differ among insect guilds

“…Such changes in insect herbivory rates could be related to altered resource availability or quality, since habitat fragmentation has been shown to affect plant density (Lienert and Fischer 2003;Valdivia 2011) and leaf traits (Silva and Simonetti 2009;Ishino et al 2012). Furthermore, changes in herbivore populations (Savilaakso et al 2009;Haynes and Crist 2009) or changes in the control of herbivore populations exerted by their natural enemies (Kruess and Tscharntke 1994;De La Vega et al 2012) may lead to altered insect herbivory in fragmented habitats.…”

“…An important consequence of habitat reduction is species loss (Koh et al 2010;Rybicki and Hanski 2013), a pattern that has often been demonstrated for herbivore communities (Barbosa et al 2005;De la Vega and Grez 2008;Cagnolo et al 2009;Haynes and Crist 2009;González et al 2014), with likely consequences on herbivory levels (Savilaakso et al 2009;De La Vega et al 2012). On the other hand, because of their higher trophic-level position, natural enemies may be more vulnerable to fragmentation than herbivores (Tscharntke and Brandl 2004;Cagnolo et al 2009), leading to reduced natural control and higher herbivory levels in small fragments (e.g.…”

“…Although several works have studied the effects of habitat fragmentation on herbivory, few studies have examined the mechanisms involved (Ledergerber et al 2002;Fáveri et al 2008;Silva and Simonetti 2009;Wirth et al 2008). Besides, herbivores from different feeding guilds may show different levels of vulnerability to fragmentation (Golden and Crist 1999), studies have usually pooled different guilds Ruiz-Guerra et al 2010;De La Vega et al 2012) or considered only one type of damage, e.g. chewers (Arnold and Asquith 2002;Skoczylas et al 2007;Savilaakso et al 2009) or leaf miners (Valladares et al 2006;Woodcock and Vanbergen 2008;Klapwijk and Lewis 2012).…”

Zero‐sum landscape effects on acorn predation associated with shifts in granivore insect community in new holm oak (Quercus ilex) forests

“…Similarly, the shorter the time a new patch has been available, the lower the probability may be of plant enemies finding the host (Ruiz-Carbayo et al, 2017). In these cases, the young age or isolation of a patch may diminish insect damage and favour the expansion process of the new habitat, in a sort of "enemy release" effect (De la Vega, Grez, & Simonetti, 2012). Conversely, expansion may take longer if plants established in new and isolated patches experience higher levels of herbivory, owing to the lack of natural enemies of herbivorous insects (Roland, 1993;Terborgh et al, 2001).…”

Using avian functional traits to assess the impact of land-cover change on ecosystem processes linked to resilience in tropical forests

“…However, whether these species help to maintain the top-down control of herbivory in forest remains to be demonstrated [64], and is perhaps unlikely, because nonforest insectivores tend to be generalists with different foraging strategies and a preference for more open microhabitats [65]. Thus, they may fail to capture many types of cryptic herbivorous insects in patches of forested habitat embedded in agricultural landscapes.…”