Are non-native plants “ecological traps” for birds? — Part II
We previewed this post last week in our response to Doug Tallamy’s belief that some bird species require caterpillars for their survival. We are continuing our critique of a claim that non-native plants are “ecological traps” for birds.
In “Nonnative plants: Ecological Traps,” Amanda Rodewald is quoted as saying that non-native honeysuckle significantly reduces the nesting success of cardinals by increasing nest predation:
“Typically in the wild, male cardinals that are in the best condition grab the best territories and nesting spots and breed earliest in the year. They also successfully rear more young than their less-fit competitors—an example of natural selection at work. But this pattern changes when honeysuckle invades a forest. Because honeysuckle leafs out sooner in spring than most plants, the fittest cardinals rush to mate and nest in the shrubs’ dense foliage. But instead of a gain in reproductive success, these birds pay a price. The early nesters in honeysuckle rear 20 percent fewer young than those that nest in native plants.” (1) (emphasis added)
She speculates that the probable reason for this reduced success is nest predation which she believes is greater earlier in nesting season because there are fewer nests.
We don’t know if Ms. Rodewald was accurately quoted in her interview or if she has changed her mind. However, Ms. Rodewald has published a study which says exactly the opposite:
“…these results provide no evidence that urban forests were acting like ecological traps for cardinals. Instead, cardinals in urban and rural forests had similar numbers of nesting attempts, young fledged over the breeding season, and apparent annual survival rates. Thus, these findings do not support the idea that urban forests in Central Ohio represent ecological traps for synanthropic understory birds” [birds that live in artificial habitats created by humans]. (2)
Urban forest sites in her study contained far more exotic vegetation than rural forest study sites: “Understory woody vegetation was over 50% more dense, with nearly 3 times greater numbers of exotic shrub stems than rural forests.” Exotic vegetation in this study was described as predominantly honeysuckle and multiflora rose. There was no statistical relationship between the number of nesting attempts and the composition of the landscape: “There were no significant differences in either the number of nesting attempts among years or between [urban and rural] landscapes.” (2)
This study offers several possible explanations for the reproductive success of cardinals in urban forests dominated by exotic shrubs:
- “…urban forests in this study contained denser understory vegetation than rural forests.”
- Therefore, there is “…increased cover provided by exotic shrubs in the urban forest.”
- “…winter microclimates may be particularly important for resident birds…cities may be favorable thermal climates for birds because cities may act as ‘heat islands.’”
- “Urban sites…probably provided more food sources for wintering birds. Urban forests had nearly 3 times more fruit and nearby birdfeeders than rural forests. Cardinals… were regularly seen feeding on fruits of exotic shrubs (e.g., honeysuckle, multiflora rose). Previous studies have indicated that supplemental food sources…can improve overwinter survival rates, body mass, and nutritional condition…Such changes in winter food and microclimate may explain increases in species richness and abundance of birds wintering in urbanizing landscapes around Columbus. Ohio.”
In her published study, Ms. Rodewald also contradicts her statement that greater nest predation in honeysuckle is the cause of reduced reproductive success of cardinals. In her published study, she says, “…high rates of nest predation frequently documented in urban landscapes do not necessarily translate to reduced productivity or survival.”
Ms. Rodewald’s statement that early nesting in honeysuckle is more likely to result in predation is also contradicted by another study. (3) Although this study was conducted in Ithaca, New York, the study site was also dominated by honeysuckle. This study found that the most fit cardinal males bred earliest in the nesting season and their nesting success for the entire nesting season was therefore greater than pairs starting later in the season: “These results confirm that an earlier breeding date is associated with producing more offspring in a season regardless of any possible effect vegetation density may have on nest initiation date.” (3)
Ms. Rodewald’s published study is consistent with a similar study: “Predation on Northern Cardinal nests: Does choice of nest site matter?” This study was also conducted in Ohio in a mixed landscape of both native and non-native shrubs. Non-native shrubs were predominantly honeysuckle and multiflora rose, as they were in Ms. Rodewald’s study. This study was trying to find a relationship between predation and the location of nests. Many hypotheses were tested relative to these variables: height of nest, concealment of nest, accessibility of nest, common vs. rare shrub location, proximity to habitat edge, distance to human activity. It found no such relationship: “Several hypotheses to explain differences between the location of the successful and failed nests were tested. None of these explained why the contents of particular nests were taken.” (4)Although cardinals had the choice of nesting in native or non-native shrubs in this 80 hectare bird sanctuary (Aullwood Audubon Center and Farm), 65% of the 121 nests in this study were in non-native honeysuckle or multiflora rose. The study tested for an association between plant species and probability of nest success by dividing all nests into two categories, one for the two dominant species of non-native plants and the other for all other plant species: “The probability of success was not associated with [plant] species category.” (4)
Only 25% of the 121 nests were successful. Although that sounds like a low success rate, it is consistent with other sources of information about reproductive success of cardinals: “Northern Cardinals have a very low nesting success rate with only 15–37% of their nests succeeding in fledging young.” (5)
This publication (4) had a very interesting theory about why nest location is unrelated to nesting success. The cardinal is unusual in having a very long nesting season from April to late-September. They have as many as 6 broods. When a nest fails, they start building a new nest in another location within 5-7 days. The female chooses the nest. Essentially, she is rolling the dice. She makes a nearly random choice of nest site and her long-term odds of nest success is primarily because she rolls the dice many more times than other species of birds: “We propose that a high incidence of predation by a rich guild of nest predators precludes the existence of predictably safe nests for cardinals. Instead, the cardinals appear simply to be well-adapted to renest rapidly in response to the near randomness of nest predation.” There are trade-offs for every potential nest location. For example, a low nest is more accessible to ground-dwelling predators such as snakes or rodents while a nest high in a tree is more accessible to nest parasites such as cowbirds. So, the cardinal mom takes her chances by making different choices and her eventual success is largely a question of luck.
Looking for “ecological traps”
We have been unable to find evidence that non-native plants are “ecological traps” for birds. When we find such claims, we turn to scientific literature for evidence. We find many studies begin with hypotheses which predict ecological harm done by non-native plants. But we have yet to find empirical studies that reach that conclusion.
The article we have critiqued in this post is a case-in-point. It starts with the report that the population of cardinals has exploded where honeysuckle has invaded. Despite that fact, the article concludes that the cardinal population is somehow harmed by honeysuckle. One wonders how the authors of that article reconcile this contradiction. Doesn’t the increased population speak for itself?
In any case, in the absence of evidence of harm, we do not believe that the destruction of existing landscapes can be justified, particularly since doing so requires large amounts of herbicide. Given the probability that the herbicides are harmful to animals, particularly soil microbes and insects, how can such destruction be justified?
(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) Lionel Leston, Amanda Rodewald, “Are urban forests ecological traps for understory birds? An examination using northern cardinals, Biological Conservation, 131 (2006), 566-574
(3) L. LaReesa Wolfenbarger, “Red coloration of male northern cardinas correlates with mate quality and territory quality,” Behavioral Ecology, Vol. 10 No. 1 (1999), 80-90
(4) Tamatha Filliator, Randall Breitwisch, Paul Nealon, “Predation northern cardinal nests: Does choice of nest site matter?,” The Condor, 96, 761-768, 1994