Effect of insect herbivory on plant community dynamics under contrasting water availability levels

Abstract

Plant diversity is impacted by multiple global change drivers but also by altered biotic interactions with antagonist and mutualist organisms that can potentially affect species coexistence. With a 2‐year, outdoor mesocosm experiment in realistic mesic grassland communities, we explored the role of insect herbivory in impacting plant community dynamics under contrasting levels of water availability simulating altered rainfall regimes. We selected a grasshopper species (Calliptamus italicus L.) feeding predominantly on forbs while avoiding grasses. High water availability reduced species coexistence, boosting productivity while decreasing individual plant survival. At the community level, herbivores were not able to promote species coexistence but asymmetrically influenced grasses and forbs, reducing forb biomass under high water availability. Herbivores shaped individual plant responses to both abiotic conditions and individual‐neighbours’ interactions. Herbivores influenced focal plant survival by altering the effect of neighbouring plants, mitigating the negative effect of high neighbour biomass at low water availability and exacerbating it at high level of water availability. Synthesis. Altered rainfall has the capacity to change the relative strength of plant–plant interactions and also to determine the effects of herbivores on grassland communities. The complexity of the interactions between plants and herbivores and the observed context dependence indicate the need to incorporate multiple biotic and abiotic drivers to fully understand the mechanisms underlying plant dynamics and species coexistence in a changing world.

Abiotic factors and plant biomass, not plant diversity, strongly shape grassland arthropods under drought conditions

“…Experiments examining the response of arthropods to precipitation manipulation typically use only one or two levels of rainfall reduction or addition (e.g., Suttle et al 2007, Lee et al 2014, Griffith and Grinath 2018, Tamburini et al 2018), or look at combinations of rainfall frequency (Suttle et al 2007, Grant et al 2014, Mariotte et al 2016, Torode et al 2016). Because climate projections are inexact, our experiment utilized a seven‐level precipitation manipulation gradient in combination with hay harvest.…”

Contrasting response of native and non‐native plants to disturbance and herbivory in mountain environments

“…Previous studies indicate that invasive seedlings seem to be more tolerant to herbivory than natives (Rogers & Siemann, 2002), and that this differential damage can alter community composition in the long‐term (Barton & Hanley, 2013; Relva et al, 2010). Native vertebrate herbivores have been demonstrated to promote non‐native plant invasions (Averill et al, 2018; Christianen et al, 2019; Stokely et al, 2020), but also invertebrates can influence plant dynamics through both direct consumption and many indirect effects that are often difficult to disentangle (Denyer et al, 2010; Maron & Crone, 2006; Tamburini et al, 2018). However, the interaction between invertebrate herbivory and other drivers of plant invasion has been poorly investigated so far (Dostál et al, 2013; Simberloff et al, 2013).…”

Biotic controls of plant coexistence

“…The strategy used to overcome this complexity so far has been relying on the natural history knowledge of the study system to select the most important interactions to test. For example, systems where leaf herbivores has been observed to occur abundantly, one can target the main herbivore as a potential factor to study (Tamburini, Dani, Bommarco, & Marini, ). This is the approach Tamburini et al () used in experimental enclosures manipulating two factors, herbivore pressure and water availability.…”

“…For example, systems where leaf herbivores has been observed to occur abundantly, one can target the main herbivore as a potential factor to study (Tamburini, Dani, Bommarco, & Marini, ). This is the approach Tamburini et al () used in experimental enclosures manipulating two factors, herbivore pressure and water availability. This has the advantage of having tractable systems where only a few factor needs to be manipulated, but ignores other potential important sources of variation.…”

“…This has the advantage of having tractable systems where only a few factor needs to be manipulated, but ignores other potential important sources of variation. In fact, even in this simple two factor scenario, the interactions between factors show complex plant responses (Tamburini et al, ). Interestingly, this approach can also reveal that interactions hypothesized to be important have little impact on species coexistence, as illustrated by the small role that pathogens play in structuring clover populations (Parker & Gilbert, ).…”