Do non-native plants contribute to insect declines? Tallamy, Narango & Mitchell 2020

Compendium Volume 5 Number 1 July 2021

The widespread distribution of plants outside of their native range due to human activity is a significant yet underrecognized cause of global insect decline, according to this article. To illuminate the issue, the authors: “examine the evidence for and against the hypothesis that long term changes in the species composition of plant assemblages have contributed to local and global declines in the abundance and diversity of the insect communities dependent upon those assemblages” [Tallamy 2020: 2].

To be sure, insect conservationists have long noted the importance of habitat containing appropriate native host plants, but the widespread replacement of native host plants with non-native species has yet to penetrate the growing literature on insect declines in any meaningful way [Tallamy 2020: 1].

It is not simply the absence of native plants harms plant-eating insects, however, but also the presence of non-natives. While some insects feed successfully on non-native plants, this is the minority. Most either avoid non-native plants, or do use them and are killed or malnourished by doing so. For example,

Swallowworts (Vincetoxicum spp.) are confamilials of milkweeds (Asclepias spp.) and have become invasive in parts of the northeastern United States. Similar phytochemistry between swallowworts and milkweeds can lead monarch butterflies (Danaus plexxipus) and milkweed beetles (Chrysochus auratus) to fatally mistake these chemically protected plants as hosts. The degree to which Vincetoxicum act as ecological traps for these taxa is likely to become more pronounced as the plants become dominant and displace milkweeds in the landscape [Tallamy 2020: 3].

Species that share a particular environment over hundreds or thousands of years evolve in relation to one another. For plant-eating insects, adapting to certain plants meant developing “traits to detect and tolerate plant defenses over time” [Tallamy 2020: 2]. Most herbivorous insects adapted to only a particular set of plants, specializing in feeding on those plant hosts.

The diet of most insects is constrained to a single plant family in any one habitat or location, with dietary specialization even narrower both in many temperate lineages and hyper-diverse tropical lineages. In fact, diet specialization increases with decreasing latitudes, concurrent with theories of increased plant and animal diversity in the tropics [Tallamy 2020: 2].

When native plants are displaced in the landscape by non-native species, phytophagous [plant-eating] insects typically do not recognize the novel host for feeding or oviposition [egg laying], or may be unable to overcome novel plant defenses. The concurrent loss of native plant hosts and dominance of non-native plants can lead to local extirpation of phytophagous insects and thus to changes in the composition and structure of local food webs [Tallamy 2020: 2].

The most likely successful substitute for a native plant is a non-native plant in the same genus or family.

Non-native congeners [members of the same genus] or confamilials [members of the same family] that are similar in foliar chemistry and nutrition, phenology, and morphology, may occasionally serve as novel hosts for herbivorous insects and support higher diversity and abundance than non-native, non-congeners. However, novel use of congeners may increase larval mortality, extend development or pupation time, reduce biomass, and reduce fitness compared to that of native hosts [Tallamy 2020: 3].

The narrower the native plant diet an insect species has, the less likely to tolerate novel, non-native food sources. However, there are more species of specialist insects than of generalists, meaning a larger proportion of susceptible species. Adaptability to exotic host plants also depends on an insects’ feeding habits.

Insects with chewing (mandibulate) mouthparts are typically more susceptible to defensive secondary metabolites contained in leaf vacuoles than are insects with sucking (haustelate) mouthparts that tap into poorly defended xylem or phloem fluids. Thus, sucking insects find novel non-native plants to be acceptable hosts more often than do chewing species [Tallamy 2020: 4].

Considering that there are more than 4.5 times as many mandibulate insect herbivores as haustelate species, there is reason for concern when non-native plants replace native hosts; the largest guild of insect herbivores is also the most vulnerable to non-native plants and the most valuable to insectivores [Tallamy 2020: 5].

“The dispersal and spread of invasive plants has been driven by global trade networks and colonialism” [Tallamy 2020: 6] and, more specifically, from agroforestry, forestry, agriculture, and horticulture.

Although plants have always distributed themselves around the globe, the increased temporal and spatial mobility of humans has resulted in an extraordinary increase in the rate of plant movements and most species’ introductions have happened in the last 200 years. Habitat is rapidly being converted from coevolved native ecosystems into novel assemblages of plants and animals, making the conversion of native plant communities into plant assemblages dominated by non-native species one of the most ubiquitous threats to biodiversity today. The introduction of non-native plants has completely transformed the composition of present-day plant communities in both natural and human-dominated ecosystems around the globe and the magnitude of introductions is staggering. An estimated 13,168 plant species (about 3.9% of global vascular flora) have been introduced and naturalized beyond their native ranges as a result of human activity [Tallamy 2020: 6].

Tallamy, Douglas W., Desiree L. Narango & Adam B. Mitchell, 2020, Do non-native plants contribute to insect declines? Ecological Entomology, 

For the full PDF version of the compendium issue where this article appears, visit Compendium Volume 5 Number 1 July 2021