Does the invader win? Interspecific competition between the ambrosia beetles Xylosandrus germanus and Xyleborinus saxesenii

Imagine a world where tiny, invisible armies are constantly battling for real estate. This isn't a sci-fi movie; it's the hidden drama of ecology, the study of how living things interact with their environment and each other. In this story, the main characters are ambrosia beetles—tiny wood-boring insects that don't eat the wood itself. Instead, they are like master chefs who carry their own personal "starter dough" (a specific fungus) in their pockets. They bore into trees, plant their fungal seeds, and then eat the fungus that grows there. Because these beetles are so good at traveling and planting their fungal gardens, they often invade new countries.

For a long time, scientists and nature lovers have wondered: Why do these invasive beetles take over so easily? The most popular theory was that they are simply the "super-athletes" of the insect world. The idea was that when an invader meets a native species, the invader wins because it is stronger, faster, or a better competitor. It's like assuming a new, flashy sports team always beats the local team just because they are new and exciting. But is that actually true, or is there a more complicated story behind the scenes? Understanding this matters because if we think the invaders are just naturally superior, we might miss the real reasons they are spreading, which could change how we try to protect our forests.

Now, let's zoom in on a specific showdown in Europe between two beetle species: the invasive Xylosandrus germanus (the newcomer) and the native Xyleborinus saxesenii (the local resident). Researchers set up a laboratory arena to see what happens when these two meet. They created two scenarios: one where two beetles of the same species were paired up, and another where one of each species was forced to share a tiny apartment (a tree gallery). They watched closely to see who had more babies, who left home first, and if anyone got eaten by their roommate.

The results were a bit of a plot twist. Contrary to the "super-athlete" theory, the invasive beetle didn't win the fight. In fact, when the two species shared a gallery, the invasive X. germanus actually had fewer babies than when it was with its own kind. The native X. saxesenii, however, didn't seem to mind the company; it kept its reproductive output steady and even packed up and left the gallery earlier than the invader. So, the invader didn't beat the native; the native actually seemed to handle the competition better.

But the story gets even more interesting when we look under the hood. These beetles rely on their fungal partners, Ambrosiella grosmanniae (the invader's fungus) and Dryadomyces sulphureus (the native's fungus). The researchers grew these fungi in a petri dish, pitting them against each other on different types of "food" (sawdust from beech and oak trees) and in different levels of alcohol (ethanol), which beetles use to soften wood. Here, the native's fungus, D. sulphureus, was the clear champion. It grew faster and stronger than the invader's fungus across the board, regardless of the food or the alcohol levels.

This suggests that the reason the invasive beetle struggled in the mixed pairings wasn't because the native beetle was bullying it or fighting it directly. Instead, it looks like the native beetle's fungal partner was simply a better competitor for resources. The paper suggests that the beetle-level outcome (the invader having fewer babies) is likely a side effect of its fungus losing the food fight.

So, does the invader win? According to this study, not because it's a better fighter. The authors propose that the spread of X. germanus in Europe isn't due to it crushing the native species in a head-to-head competition. Instead, its success is more likely due to being a "jack-of-all-trades"—it can handle many different types of trees, it's flexible, and it keeps getting new reinforcements through international trade. The paper challenges the idea that invasive species are always the dominant champions, suggesting instead that their success might come from other advantages, while the native species remains a surprisingly tough competitor in its own backyard.