Are non-native plants “ecological traps” for birds?

One of the reasons why native plant advocates want the managers of our public lands to destroy non-native plants and replace them with native plants is that they believe native plants provide superior habitat for birds.  However, empirical studies do not support this belief, as we have explained in earlier posts.  Today we will examine an article recently published in an advocacy magazine, making the claim that non-native plants are “ecological traps” for birds:  “Nonnative Plants: Ecological Traps Offering alluring habitat for songbirds, exotic plants may actually decrease the animals’ long-term survival and fitness” (1)

The article begins auspiciously with the good news that populations of some bird species have increased significantly in recent decades because of the spread of non-native plant species which are valuable sources of food:  “…a 2011 paper, published in the journal Diversity and Distributions, concluding that the number of fruit-eating birds such as cardinals, robins and catbirds tripled during the past three decades in parts of central Pennsylvania due to the spread of nonnative honeysuckles.”  (1)  And then the article attempts to contradict this good news by turning to the usual nativist caveats.

Generalists vs. Specialists

Nativists claim that the animal kingdom is divided into generalists and specialists.  The generalists are theoretically omnivores—they have a varied diet—and so depriving them of native plants will not prevent their survival.  Specialists, on the other hand, are dependent upon a narrow range of plant or animal species for survival.  We are expected to believe that specialists far outnumber generalists and that we doom them to extinction when one particular species of native plant or animal is unavailable to them.

Doug Tallamy is the purveyor of the generalist vs. specialist overstatement.  We have critiqued his assumptions in an earlier post.  In a nutshell, there are few mutually exclusive relationships in nature because they are a risky evolutionary strategy.  The plant or animal that is dependent upon one other species is significantly less likely to survive in the long term than an animal with more dietary options.  The perception that there are immutable relationships between insects and plants also underestimates the speed of adaptation and evolution, particularly of insects with large populations and short lifespans.

For example, a bird that eats insects usually eats all manner of insects as well as spiders.  They are not dependent solely upon caterpillars as Mr. Tallamy seems to believe:  “…warblers and chickadees rely on caterpillars for 90 percent of their diet during the breeding season, eating hundreds per day. ‘That’s a lot of insects,’ Tallamy says. ‘If you don’t have those insects, you don’t have the birds.’” (1)

According to Cornell Ornithology Lab–America’s most prestigious research institution for birds–warblers and chickadees have a much more varied diet than Mr. Tallamy believes.  (We chose specific species with ranges and abundant populations in Delaware where Mr. Tallamy lives.  However, the diet of all species of chickadees and warblers are similar.)

• Black-capped Chickadee:  “In winter Black-capped Chickadees eat about half seeds, berries, and other plant matter, and half animal food (insects, spiders, suet, and sometimes fat and bits of meat from frozen carcasses). In spring, summer, and fall, insects, spiders, and other animal food make up 80-90 percent of their diet. At feeders they take mostly sunflower seeds, peanuts, suet, peanut butter, and mealworms.” (2)
• Orange-crowned Warbler:  “insects and spiders.” (2) Most insects are not caterpillars and many are not herbivores.

No evidence that insects require native plants

Mr. Tallamy is focused on caterpillars because they are herbivores, that is, they eat plants.  Just as he believes that the birds need native plants, he also believes that plant-eating insects need native plants.  However, Mr. Tallamy disproved his own theory about an immutable relationship between native plants and insects when he supervised a graduate student whose thesis concluded: 

“Erin [Reed] compared the amount of damage sucking and chewing insects made on the ornamental plants at six suburban properties landscaped primarily with species native to the area and six properties landscaped traditionally.  After two years of measurements Erin found that only a tiny percentage of leaves were damaged on either set of properties at the end of the season….Erin’s most important result, however, was that there was no statistical difference in the amount of damage on either landscape type.” (3)

This empirical study, supervised by Mr. Tallamy, was unable to find evidence that there are more plant-eating insects in a native garden than in a landscaped garden of non-native cultivars.  Yet, Mr. Tallamy continues to claim that insects require native plants and birds require those insects for their survival:  “Tallamy’s research shows that birds also may be harmed indirectly because nonnative plants affect insects. He has found that the number and diversity of plant-eating insects, especially caterpillars, drops dramatically when exotic plants invade…[Tallamy said,] ‘My prediction is that birds that specialize on insect herbivores will take a bigger hit than those that eat other insects,’” (1)

The study by Mr. Tallamy’s student about the relationship between native plants and insects is not the only empirical evidence that his assumption is incorrect.  We have published several articles about local studies that have found no such relationship:

• Many native butterflies use non-native plants and some depend upon them.  Since caterpillars are the larvae stage of butterflies, this study casts more doubt on Mr. Tallamy’s belief that caterpillars require native plants.
• Equal numbers of insect species were found in oak woodlands and eucalyptus forests in Berkeley, California.
• Equal numbers of benthic microorganisms were found in the leaf litter of native woodlands and eucalyptus forests in riparian corridors in the East Bay.

Native plant advocates have also offered “evidence” of insect populations in the local eucalyptus forest.  UCSF produced a video to promote their original plan to destroy most of the eucalypts on Mount Sutro (now on hold indefinitely).  An arborist shows us eucalyptus leaves that have been chewed by insects.  He claims that a drastically thinned forest will be healthier because it will have fewer insect predators.  So, there are insects in the eucalyptus forest when it suits native plant advocates’ purposes and there are no insects in the eucalyptus forest when it does not.  They want more insects when they are advocating on behalf of birds and they want fewer insects when they are demanding that trees be destroyed.  It’s rather confusing.

Insects ARE important to birds

We agree with Mr. Tallamy that insects are very important to birds because they are a major source of food, especially during the nesting season when their high-protein content is vital to nestlings.  Therefore, we believe that Mr. Tallamy should join us in making climate change our highest environmental priority.  Because insects are cold-blooded, they are particularly vulnerable to the extreme weather conditions associated with climate change.  They cannot adjust their body temperature as warm-blooded animals can in response to such fluctuations in temperature.  A recent study predicts devastating consequences for insect populations in coming decades:  “Our predictions are that some species [of insects] would disappear entirely in the next few decades, even when they have a fairly wide distribution that currently covers hundreds of kilometers.” (4)

We believe that a single-minded focus on native plants is misguided because in a rapidly changing climate the entire concept of “native” becomes meaningless.  Just as insects are unlikely to survive radical changes in temperature, the ranges of native plants must change if species are to survive.

Stay tuned for Part II

In our next post, we will continue our critique of the article that theorizes that non-native plants are “ecological traps.”  We will tell our readers about the published research that contradicts statements in the article about predation of cardinal nests in non-native honeysuckle.  The author of one of the studies is quoted in this article, saying something completely different than her own published study.  It’s an intriguing contradiction.

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(1)    John Carey, “Nonnative Plants: Ecological Traps Offering alluring habitat for songbirds, exotic plants may actually decrease the animals’ long-term survival and fitness,” National Wildlife Federation, January 14, 2013

(2)    Cornell Ornithology Laboratory, Guide to Birds

(3)     Tallamy, Doug, “Flipping the Paradigm:  Landscapes that Welcome Wildlife,” chapter in Christopher, Thomas, The New American Landscape, Timber Press, 2011

(4)    “Extreme weather caused by climate change decides distribution of insects, study shows,”  Science Digest, February 20, 2014