Fuzzy questions: How do we define native vs. non-native bees?

Last week, I was talking with Kate LeCroy, a PhD Candidate at the University of Virginia in the Department of Environmental Sciences, about her recent paper on native Osmia declines in connection with the appearance of the non-native Osmia taurus. We got into a really interesting discussion about how to define and categorize native versus non-native bee species (not to be conflated with invasive species, which is a next-level consideration). What seemed like pretty clear boxes to draw around bees at the beginning of the discussion became decidedly less so by the end. Because, as is often the case with discovery, the more you learn about something, the more questions you ultimately have.

To begin with, here are the “official” definitions for both native and non-native species. They come from a 1999 U.S. executive order that created the National Invasive Species Council, and are used by U.S. government agencies like the Fish and Wildlife Service and the National Parks Service.

“Alien species” (or exotic or non-native) means, with respect to a particular ecosystem, any species, including its seeds, eggs, spores, or other biological material capable of propagating that species, that is not native to that ecosystem.

“Native species” means, with respect to a particular ecosystem, a species that, other than as a result of an introduction, historically occurred or currently occurs in that ecosystem.

On the surface, these definitions seem obvious (if not straight forward). But when you start poking at them with real world examples (like abruptly-changing mason bee populations in the mid-Atlantic states), fuzzy questions arise.

Here are three questions I currently have about how we draw the line between native and non-native bees, and some of the thoughts I received from experts in the field when I put the questions to them.

1. Are these the best definitions we have? Do they make sense?

The general consensus among the researchers who I reached out to is that, yes, these definitions make sense. As to whether or not they’re the best ones, that’s a different story.

Daniel Cariveau, assistant professor in the Department of Entomology at the University of Minnesota, says he struggles with these particular definitions. “What is meant by ‘historically occurred’ vs. ‘currently occurs’?” he points out. “We often do not have good records on whether a species might have occurred somewhere in the past.”

Instead, he says, consider the USDA NRCS definition for non-native plants: “A plant introduced with human help (intentionally or accidentally) to a new place or new type of habitat where it was not previously found.”

2. Is human intervention, human introduction, the key element in defining a non-native species?

The consensus is that human introduction is a key element.

“But this element of ‘human intervention’ is not sufficient by itself in defining what a non-native species is,” Kate LeCroy says. “There is also the ecosystem element.” Clearly defining what an ecosystem is – and how one is different from another – is essential for indicating a “new” place that a bee has been moved to.

3. At what point does a non-native species become a native species? Osmia cornifrons was introduced to the U.S. in the 1970s; it has since established wild populations. Does this now count as “historically occurring” and, therefore, becoming native?

All of the researchers agreed that non-native species do not become native.

“I’d say that at least in human timescales, there is no point at which a non-native species becomes a native species,” says Zach Portman, bee taxonomist in the Cariveau Native Bee Lab at the University of Minnesota. Non-native species that establish populations would be considered “naturalized” but, he says, they are absolutely not “becoming native”.

Several researchers referenced honey bees as an even better example. They’ve been in North America for roughly 400 years and have also established wild populations – but they’re not considered native.

Portman also presented another interesting example. The squash bee, Eucera (Peponapis) pruinosa, is a tricky case. Portman has seen it referred to as non-native because its current range is due, in part, to squash cultivation by indigenous people. However, he would consider the species native to North America for several reasons, particularly because its range expansion originated in Mexico.

“I do think that extremes are easy enough,” says Cariveau. “Bombus species are not native to New Zealand. Honey bees are not native to North America.” But what about bee species whose ranges expand, who migrate on their own, as a result of climate change?

Wouldn’t these bees also fit the definition of non-native because they are in a new place as a result of human actions?

A short vacation

Just a quick note to let you all know that the Bee Report and I will be taking a short vacation during the last two weeks of this month. Next week’s Bee Report will be the last one for 2020. We’ll return to your inboxes at the beginning of January 2021!


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One More Thing…

At the intersection of bees and raptors lives the honey buzzard. Two kinds, the Oriental and the European. “A large raptor... True to its name, this species is a raider of wasp and bee nests, although it prefers bee and wasp larvae over their honey.”