Density dependence, precipitation and biological control agent herbivory influence landscape‐scale dynamics of the invasive Eurasian plant Linaria dalmatica

Abstract

Summary1. Resource availability and natural enemies are among the most commonly cited mechanisms affecting competitive ability of invasive plants, but their simultaneous effects on plant dynamics are seldom evaluated in the field. Understanding how endogenous and exogenous factors affect invasive plant abundance is essential when evaluating the impact of classical weed biological control agents because misinterpretations of the mechanisms regulating plant demography may bias inference of herbivore impact. 2. In this study, we report results from a citizen-science monitoring programme initiated to evaluate the effects of the stem-mining weevil Mecinus janthiniformis on the Eurasian invasive weed Dalmatian toadflax Linaria dalmatica. We used a discrete model of population dynamics to evaluate the relative importance of endogenous and exogenous processes affecting changes in ramet density of L. dalmatica at 38 spatially replicated sites in Idaho, USA. 3. Analysis of per-capita population growth rates based on ramet density indicated the presence of direct density dependence, which was most likely due to intraspecific competition. Changes in ramet density were not influenced by the abundance of the resident plant community. 4. Precipitation had a strong, positive effect on ramet density of L. dalmatica, while the abundance of M. janthiniformis had a weaker, but significant, negative effect after accounting for the density dependence. There was no support for an interactive effect between precipitation and herbivory. 5. Synthesis and applications. Our results indicate that biological control is an important factor affecting weed population growth at the landscape scale, but they also suggest that biological control impact may vary considerably on local infestations due to site-specific variation in rainfall and density-dependent processes. We recommend that invasive plant management strategies integrate precipitation and biological control agent monitoring into their programmes to estimate expected biological control efficacy. Alternative control methods should be prioritized in areas where herbivore impact is expected to be low.

Native insect herbivory overwhelms context dependence to limit complex invasion dynamics of exotic weeds

“…The effects of herbivores on individual plant performance are well‐established (Crawley ) and there are examples of insects being used as effective biocontrol agents of weedy plants (Landis et al . ; Weed & Schwarzländer ), but whether insect herbivores generally limit plant population abundance remains a persistent question (Maron & Crone ; Myers & Sarfraz ). Answering this question requires scaling from individual‐ to population‐level effects (Halpern & Underwood ; Maron & Crone ) and incorporating density‐dependence, which can modify population‐level impacts of herbivory (Underwood & Halpern ).…”

“…Furthermore, effects of herbivory can be mediated by environmental context, including both abiotic (Mitchell et al . ; Weed & Schwarzländer ) and biotic conditions (Mitchell et al . ).…”

“…From the classic Green World Hypothesis to current studies of invasive plants, ecologists have studied and debated the role of insect herbivores in plant population dynamics (Hairston et al 1960;Janzen 1970;Harper 1977;Colautti et al 2004). The effects of herbivores on individual plant performance are well-established (Crawley 1989) and there are examples of insects being used as effective biocontrol agents of weedy plants (Landis et al 2003;Weed & Schwarzl€ ander 2014), but whether insect herbivores generally limit plant population abundance remains a persistent question (Maron & Crone 2006;Myers & Sarfraz 2017). Answering this question requires scaling from individual-to population-level effects (Halpern & Underwood 2006;Maron & Crone 2006) and incorporating density-dependence, which can modify population-level impacts of herbivory (Underwood & Halpern 2012).…”

“…Answering this question requires scaling from individual-to population-level effects (Halpern & Underwood 2006;Maron & Crone 2006) and incorporating density-dependence, which can modify population-level impacts of herbivory (Underwood & Halpern 2012). Furthermore, effects of herbivory can be mediated by environmental context, including both abiotic (Mitchell et al 2006;Weed & Schwarzl€ ander 2014) and biotic conditions (Mitchell et al 2006). Understanding how herbivory and density interact and how these effects vary across heterogeneous landscapes would advance understanding of biotic resistance to plant invasions (D'Antonio et al 2001) and address the larger question of the role of consumers in plant population dynamics (Katz 2016).…”

Measuring and evaluating ecological outcomes of biological control introductions

“…Furthermore, variability in local conditions will lead to a patchwork with failure in one region or habitat and success in another using the same or different agents (Hoddle et al, 2013;Center et al, 2014;Weed and Schwarzländer, 2014), but what constitutes success under these circumstances? Furthermore, variability in local conditions will lead to a patchwork with failure in one region or habitat and success in another using the same or different agents (Hoddle et al, 2013;Center et al, 2014;Weed and Schwarzländer, 2014), but what constitutes success under these circumstances?…”