Invasive plant architecture alters trophic interactions by changing predator abundance and behavior

Abstract

As primary producers, plants are known to influence higher trophic interactions by initiating food chains. However, as architects, plants may bypass consumers to directly affect predators with important but underappreciated trophic ramifications. Invasion of western North American grasslands by the perennial forb, spotted knapweed (Centaurea maculosa), has fundamentally altered the architecture of native grassland vegetation. Here, I use long-term monitoring, observational studies, and field experiments to document how changes in vegetation architecture have affected native web spider populations and predation rates. Native spiders that use vegetation as web substrates were collectively 38 times more abundant in C. maculosa-invaded grasslands than in uninvaded grasslands. This increase in spider abundance was accompanied by a large shift in web spider community structure, driven primarily by the strong response of Dictyna spiders to C. maculosa invasion. Dictyna densities were 46-74 times higher in C. maculosa-invaded than native grasslands, a pattern that persisted over 6 years of monitoring. C. maculosa also altered Dictyna web building behavior and foraging success. Dictyna webs on C. maculosa were 2.9-4.0 times larger and generated 2.0-2.3 times higher total prey captures than webs on Achillea millefolium, their primary native substrate. Dictyna webs on C. maculosa also captured 4.2 times more large prey items, which are crucial for reproduction. As a result, Dictyna were nearly twice as likely to reproduce on C. maculosa substrates compared to native substrates. The overall outcome of C. maculosa invasion and its transformative effects on vegetation architecture on Dictyna density and web building behavior were to increase Dictyna predation on invertebrate prey >/=89 fold. These results indicate that invasive plants that change the architecture of native vegetation can substantially impact native food webs via nontraditional plant --> predator --> consumer linkages.

Plant diversity in a nutshell: testing for small‐scale effects on trap nesting wild bees and wasps

“…In our case however, for all groups except spiders, models with plant composition performed worse than models without. Earlier studies similarly found that the abundances of spiders were strongly affected by the plant community, acting through species composition (Schaffers et al 2008), diversity , or vegetation structure (Pearson 2009. Moreover, the abundances of spiders and of herbivores were strongly positively correlated in our study, reflecting a bottom-up effect.…”

Direct and indirect bottom-up and top-down forces shape the abundance of the orb-web spider Argiope bruennichi

“…Whereas the presence of Malva increased the abundance of our spider, Dipsacus negatively affected A. bruennichi. This is in line with other studies showing that plant composition affects the abundance of spiders (e.g., Pearson 2009;Schaffers et al 2008). However, plant composition for the abundance of A. bruennichi was not anymore a significant factor in our global path analysis.…”

“…The importance of vegetation structure for spiders (McNett and Rypstra 2000;Rypstra and Carter 1995) as well as for arthropods in general (e.g., Perner et al 2005) is well supported. Despite the interest in the relationship between spiders and vegetation structure, few studies have analysed whether this interaction was direct or indirect, acting through its influence on prey communities (Pearson 2009). Here, we found that vegetation structure significantly influenced spider abundance once all other measured variables had been accounted for, but no evidence of an indirect link was detected.…”

“…Species abundance in local communities is the result of a combination of bottom-up and top-down forces, however studies considering both effects simultaneously are much rarer than those focussing on a single direction (Walker and Jones 2001). For example, spider abundance is often seen as bottom-up regulated by prey availability (e.g., Greenstone 1984;Halaj, Ross, & Moldenke 1998;Harwood, Sunderland, & Symondson 2001), or in the specific case of web-building spiders by the physical structure of the vegetation (Halaj et al 1998;McNett and Rypstra 2000;Pearson 2009;Rypstra and Carter 1995;Sanders, Nickel, Grutzner, & Platner 2008;Topping and Lövei 1997). Spiders are also preyed upon by a variety of natural enemies (Foelix 2010), but surprisingly the contribution of top-down processes to their regulation has rarely been assessed (e.g., Askenmo, von Broemssen, Eckman, & Jansson 1977;Spiller and Schoener 1994).…”

Compromising genetic diversity in the wild: unmonitored large-scale release of plants and animals

“…Alien species are recognized as a major threat to biodiversity worldwide and considerable research and political attention has focused on their effects [13]. However, this attention has not stopped widespread use of alien species for commercial purposes, e.g.…”