When walking through a patch of habitat a key question obsesses ecologists: “How many species are there?”. An ornithologist or mammalogist can usually get a number within a reasonable range, in part because those critters are well studied. What happens when you dip into the realm of the hyper-diverse? In a new open access paper in Ecology and Evolution, our team uses a network of pitfall traps and eDNA to explore the balance of native and non-native insects in North American insect communities.

Insect communities (and by that, I’ll mean all invertebrates) are a challenge to study as they are hyper-diverse, with each taxon studied by a handful of taxonomists–the high priests of identification. As a result, a simple answer to the question “How many species are there, and how does that answer vary as you move from place to place?” is rather sticky (but see our recent contribution highlighted here).” And we need that answer before we can get to the trickier question, namely, “Why?”.

For example, individuals of a novel species are thought to arrive in a patch of habitat with some regularity but fewer “stick” and leave offspring, reflecting their ability to grow and thrive. The schema below captures some of the key ideas. You can increase the number of new species (shorthand “non-natives”) by increasing the community’s Capacity—its ability to support all species, native and non-native alike—or by increasing the likelihood of Establishment once arrived. But first, of course, you need some data to evaluate.

Enter NEON, the National Ecological Observatory Network, the US’s distributed network of monitoring sites that are running pitfall traps from the Arctic Circle to Puerto Rico. Each of these sites samples all the local available habitats, for a total of 51 communities of ground dwelling insects. Our other ace in the hole (methodologically speaking) was extracting DNA from the ethanol in which the bugs were stored. We identified the bugs from their leaky DNA. A longer explanation of the techniques can be found here.

Our first surprise was just how few non-native species (typically < 5%) were found in most North American communities for most major groups. The big exceptions are rather notorious—earthworms and isopods—as much of eastern North America is awash in European worms and sowbugs.

The second satisfying result was using current theory to account for the geograpny of non-invasives. The first is the Capacity hypothesis that diversity begets diversity: communities that support more native species—for all the myriad possible reasons—are able to support more non-natives. We estimate about one non-native species for every 14 native species.

Once we account for local native diversity, other hypotheses more clearly express themselves, accounting for a total of about 2/3rds of geographic variation in successful species invasions. One novel result was the 3-fold variation in fraction of invasives across North American habitats. Deciduous forests, wetlands, and pastures and hayfields all supported more non-natives than grasslands, shrub steppe and evergreen forests. This corresponds with predictions that high levels of disturbance and resource pulses enhance opportunities for establishment.

Likewise, areas surrounded by faster traffic appear better able to promote non-native establishment (good reason to keep cars out of our parks and preserves!). Finally, even for a given native diversity, more productive ecosystems support proportionally more non-natives. A new non-native species was added for every 250 g/C/m² of NPP over a 35-fold gradient of productivity

The big picture?

The US’s National Ecological Observatory Network is designed to monitor ecosystems and populations across North America over the next 30 years. Using eDNA—the DNA from critters suspended in storage ethanol—we can non-destructively monitor the presence/absence of populations over time. At the same time, long-term studies of abundance remain rare and such distributed networks are vital. We cannot reliably detect—and reverse—insect declines without them.

Cover illustration by Brittany Benson.