The role of diversity in Sirex noctilio invasion of North American pine ecosystems

Background/Question/Methods

As the number and diversity of species increases in an ecosystem, stability is thought to increase through increasing connections between trophic levels (Diversity-Stability Hypothesis).  Invasive species commonly enter ecosystems with varying levels of diversity. However, the importance of diversity in the success of invasive species introductions is controversial. The invasive sawfly, Sirex noctilio, was not problematic in its native European range, but introductions to Southern Hemisphere pine plantations, where diversity of pine herbivores and their natural enemies are extremely reduced, resulted in epidemic outbreaks causing high mortality (30-80%), presumably due to ‘enemy release’.  The recent introduction of Sirex to the diverse and coevolved pine ecosystems of North America poses the question, will Sirex be relatively benign, as it is in its native range, or become invasive, as in previous introductions? Using a paired treatment design, we evaluated the potential level of interaction from a native red pine (Pinus resinosa) community in North America to the aggressive tree killing behavior of Sirex by emulating Sirex attack symptoms on healthy trees.  Trees of each pairing were attacked by live females, mechanically damaged, or left untouched and monitored over 3 months for diversity and abundance of insects visiting experimental trees.

Results/Conclusions

Using Canonical Correspondence Analysis and permutation tests, we identified a strong response of secondary colonizing (saprotrophic) species to the aggressive attack of pine trees by Sirex (p=0.01). Secondary colonizers are those insects that are unable to kill healthy trees, but readily make use of dead or dying trees. The response among parasitic and predatory guilds was mixed and highly species dependent, but overall not significant. The strong response of saprotrophic species makes intuitive sense, since these species are unable to access new healthy trees as resources and must instead rely on searching out and colonizing often disparate and/or ephemeral dead or dying trees. Secondary colonizers, with their rapid and direct response to aggressive tree attack by Sirex, increased competitive and predatory interactions and may play a key role in moderating the destructive potential of Sirex in North America. This study highlights the potential for diverse native communities to interact with introduced species and supports the hypothesis that diversity plays a role in ecosystem stability relative to species invasions through increasing interactions within trophic levels.