[Written in response to reviews of Jennifer Owen, Wildlife of the Garden: A 30 Year Study, claiming that insects prefer exotic plants]
What Dr. Owens (or the reviewer) is claiming is contrary to scientific consensus, since Ehrlich and Raven first published in 1964. That’s not to say she is wrong, but it does require that she support her claim through rigorous, repeatable scientific inquiry.
I have not read her book, so I can only comment on the reviews that were sent to me. There are several things I would need to see before I took her results seriously.
First, plant/ insect interaction theory describes how insect herbivores should respond to host plants. In essence, the theory claims — and is supported by thousands of studies — that most insects (an estimated 90%) adapt to plant chemical defenses by developing specific physiological mechanisms, behaviors, and life histories that enable them to circumvent a particular type of defense.
Once they have developed this ability, they can eat any plant that uses that particular chemical as a defense. Typically, a plant lineage (genus, sometimes family) shares a common defense. This means the lineage of insect that adapted to that particular defense specializes on the plant lineage that makes that defense. It also means that that insect cannot eat plants it did not adapt to.
Note, we are talking about insects that eat plants. Not pollinators, and not predators. So the first thing I would look for in her study is the type of insect she was counting on her native and alien plants.
One review says she found 533 species of parasitoids. I am sure she collected these when they were seeking nectar at flowers. That is, they are not herbivores, so they don’t count if we are challenging accepted theory.
She also counted 23 species of butterflies. If they were counted as adults, they don’t count. Same with the 375 species of moths. Did she survey them as larvae when they were eating leaves, or as adults when they were sipping nectar? Bees and wasps are also pollinators at flowers (wasps are predators when not at flowers), while lacewings and ants are predators. All of those insects were mentioned in the reviews, but are simply seeking sugar water. Nearly all flowers, alien or native make nectar.
When we focus only on insect herbivores, it turns out that Dr. Owens’s results are actually not contrary to predictions. She found that 46 species of moths fed on 40 native plants in the garden, while 75 alien plants provided food for 38 species of moth. That is, if she had planted a garden of only the 40 native plants, she would have supported 46 species of moths. If she had planted only aliens, she would have supported 38 species of moths.
So the review’s claim that aliens support more insects is incorrect. Note that if these plant numbers represent the abundance of plants in her garden, she had twice as many species of aliens as natives. In a true comparison of insect preference, you would need the same biomass of both alien and native plants.
The real confusion comes from her study of the 4 species of generalist moths. Remember, if 90% of insect herbivores are specialists with a narrow host range, 10% are generalists with a broad host range. Here she found that the generalists preferred alien host plants.
John Parker has found similar results here in the US, so there does seem to be something going on between generalists and well-defended native hosts and this has stimulated new research as to why one out of ten insect herbivores can circumvent alien plant defenses better than native plant defenses.
But let’s be careful with our overall conclusions. Missing from Dr. Owens’s garden are most of the specialist herbivores. She recorded 46 species of specialist moths, but how many never showed up at all, because their host plant has been eliminated from surrounding suburban ecosystems?
One review says she found ¼ of the species in Britain. If she only recorded ¼ of the caterpillar species in her area, ¾ of the species never made it to her garden, and my guess is that most of those were specialists on native plants that have been lost to her suburban ecosystem because of the bias toward alien plants in suburban gardens.
Through no fault of her own, her study was filtered to include only those species that could survive in a sea of alien plants: the generalists. What is surprising is that so many specialists were able to locate their native hosts when she did supply them.
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