Two trees diverged in a wood and that made all the difference (for insect impacts)
Written by Bethany Bradley, edited by Nikki Read
Summary
Lack of shared evolutionary history between an introduced species and its recipient ecological community is a key component in several hypotheses about why invasive species succeed. If species have not interacted in recent evolutionary history (i.e. for millions of years), there is a higher likelihood of “ecological surprises” when those species are brought into contact. For example, introduced pests might have evolved mouth parts that enable better sap sucking, but native hosts did not evolve defenses in the absence of a native super-sucking pest. While there are many arguments in invasion ecology for the importance of evolutionary history, its importance for predicting invasion risk is rarely tested. Mech et al. (2019) assessed the impacts of 58 introduced insects that consume sap, leaves, or wood of conifers in North America. Of these, they identified six species (Adelges piceae—Balsam woolly adelgid, Adelges tsugae—Hemlock woolly adelgid, Elatobium abietinum—Green spruce aphid, Gilpinia her‐cyniae—European spruce sawfly, Matsucoccus matsumurae—Redpine scale, and Pristiphora erichsonii—Larch sawfly) that cause high impacts, defined as any level of mortality within populations of the host trees.
Mech et al. (2019) assessed evolutionary history (between host and pest), tree (host) traits, and insect (pest) traits to understand which characteristics best differentiated the six high-impact species from the remaining 52 lower-impact species. They found that evolutionary history was by far the best predictor with divergence times (i.e. when ancestors of the insect and the tree last interacted) of 1.5-5 million years for leaf feeders (folivores) and 10-30 million years for sap feeders leading to the highest risk of high ecological impacts. In contrast, tree traits were weak predictors and insect traits did not predict ecological impact at all. The likelihood of high impact drops to zero for insects and tree hosts whose ancestors last interacted less than ~500,000 years ago. Given that the vast majority of forest insects and trees in the eastern temperate forests have interacted within a much shorter time frame (tens to hundreds of thousands of years through geologically recent glacial/interglacial cycles), it is unlikely that range shifting native insect pests will cause novel interactions leading to “surprise” ecological impacts. Note however that extended growing seasons leading to multiple pest outbreaks as well as the compounding effects of multiple native and/or introduced insect pests and pathogens can still increase risk to forests.
Take Home Points
Leaf feeding pests that diverged (i.e. their ancestors last interacted) from their conifer hosts 1.5-5 million years ago have the highest risk of high impact if those insects introduced
Sap feeding pests that diverged (i.e. their ancestors last interacted) from their conifer hosts 10-30 million years ago have the highest risk of high impact if those insects introduced
Management Implications
Evolutionary history is much more important than insect traits or tree traits when assessing invasion risk from introduced insect pests
Native insects in eastern temperate forests have interacted with native conifers within the last tens or hundreds of thousands of years (through glacial/interglacial periods) and are thus unlikely to have “surprise” ecological impacts as their ranges shift with climate change.
Related Papers
Keywords
Impact studies, risk assessment, invasive insects, conifers, Adelges piceae, Adelges tsugae, Elatobium abietinum, Gilpinia her‐cyniae, Matsucoccus matsumurae, Pristiphora erichsonii