Herbivore Preference for Native vs. Exotic Plants: Generalist Herbivores from Multiple Continents Prefer Exotic Plants That Are Evolutionarily Naïve

Abstract

Enemy release and biotic resistance are competing, but not mutually exclusive, hypotheses addressing the success or failure of non-native plants entering a new region. Enemy release predicts that exotic plants become invasive by escaping their co-adapted herbivores and by being unrecognized or unpalatable to native herbivores that have not been selected to consume them. In contrast, biotic resistance predicts that native generalist herbivores will suppress exotic plants that will not have been selected to deter these herbivores. We tested these hypotheses using five generalist herbivores from North or South America and nine confamilial pairs of native and exotic aquatic plants. Four of five herbivores showed 2.4–17.3 fold preferences for exotic over native plants. Three species of South American apple snails (Pomacea sp.) preferred North American over South American macrophytes, while a North American crayfish Procambarus spiculifer preferred South American, Asian, and Australian macrophytes over North American relatives. Apple snails have their center of diversity in South America, but a single species (Pomacea paludosa) occurs in North America. This species, with a South American lineage but a North American distribution, did not differentiate between South American and North American plants. Its preferences correlated with preferences of its South American relatives rather than with preferences of the North American crayfish, consistent with evolutionary inertia due to its South American lineage. Tests of plant traits indicated that the crayfish responded primarily to plant structure, the apple snails primarily to plant chemistry, and that plant protein concentration played no detectable role. Generalist herbivores preferred non-native plants, suggesting that intact guilds of native, generalist herbivores may provide biotic resistance to plant invasions. Past invasions may have been facilitated by removal of native herbivores, introduction of non-native herbivores (which commonly prefer native plants), or both.

Insect herbivory on native and exotic aquatic plants: phosphorus and nitrogen drive insect growth and nutrient release

“…This will depend strongly on herbivore feeding preferences for certain plant species. There is contrasting evidence: native herbivores preferred exotic aquatic plant species in several studies (Parker & Hay, 2005;Morrison & Hay, 2011), but preferred native plants in other studies (Xiong et al, 2008). We hypothesised that the preference of native herbivores for certain plant species is related to the plant quality.…”

Grouping plant species by shared native range, and not by native status, predicts response to an exotic herbivore

“…Island and to exotic plants from Europe, the Americas, Australia, and Asia. Rabbits could only have had overlapping native ranges with exotic species from Europe, and herbivores often prefer naïve food (Morrison and Hay 2011). This suggests that grouping plant species by continental origin (hereafter origin) may be a better predictor of outcomes of interaction with rabbits than grouping by native status, because exotic plants not native to Europe should be as naïve to rabbits as are native plants.…”

Third-party mutualists have contrasting effects on host invasion under the enemy-release and biotic-resistance hypotheses

“…Some empirical studies have found support for each of these hypotheses while others have been inconclusive, leading to a lack of generality and clarity (Colautti et al 2004;Hill and Kotanen 2009;Carrillo-Gavilán et al 2012;Fan et al 2013). For example, Morrison and Hay (2011) found that apple snails and crayfish prefer non-native plants to their more familiar native confamilials as predicted by the BRH while Tallamy and colleagues (2010) showed that exotic plants were detrimental to the growth of native generalist herbivores as predicted by the ERH. A meta-analysis by Chun et al (2010) found that exotic plants did not always experience enemy release and relatedness to native host species did not predict the behavior of native generalists.…”