The Great Diverse North? Flipping the latitudinal gradient.

Every student of Ecology learns that the variety of species declines as you move north or south from the equator. In a new paper led by Dr. Michael Weiser @NEONAnts we show the truth is more delightfully complex. And we do it for the most diverse set of animals on Earth! As always, we thank the National Science Foundation @NSF for their support, as well as our friends in the Department of Biology at the University of Oklahoma @OU_Biology.

A classic “grid cell” approach to quantifying diversity gradients. (Mammal species richness and biogeographic structure at the southern boundaries of the Nearctic region Tania Escalante, Gerardo Rodríguez-Tapia, Miguel Linaje, Juan J. Morrone and Elkin Noguera-Urbano From the journal Mammalia

In a new paper in Oikos, Mike, me, Dr. Cam Siler, Sierra Smith, Dr. Katie Marshall, Dr. Matt Miller, and Dr. Jess Mclaughlin show the diversity of invertebrates from a huge network of pitfall traps *increases* as you go from south to north. To download a copy for your personal use, click here.

Why do we think this study is important? Five reasons.

The scale

The taxa

The community

The methods

The future

The Scale: the paper exploits a series of standardized trapping grids across much of North America. Every terrestrial habitat—from deciduous forest to desert shrub—is sampled the same way. These traps capture all manner of invertebrates that move across the soil surface, adding their data to the large compendium of geographic patterns for trees, mammals, birds…the big stuff. In this way, we give a first, comprehensive look at patterns of diversity measured via passive traps that rely on Activity Density: the rate that critters move across the landscape.

A Large portion of the NEON pitfall trap network, used to quantify the geography of diversity in this study.

Pitfall traps are buried flush with the soil and catch things moving along the surface. In this respect, they measure activity density, a rate by which the organisms sample themselves. In an earlier paper (see link just above) we show how temperature and plant productivity shape the rate that these pitfall traps sample their ecosystem.

The Taxa: The invertebrates are the most diverse group of animals and have thus far been incompletely sampled, with a few groups well sampled nearly everywhere (e.g., ants, butterflies) and most groups known well from far fewer locales. We explore how diversity varies for the summed diversity of all invertebrates plus 12 common taxa, from springtails to grasshoppers to spiders. Collectively, rather than declining toward the poles (the classic “Latitudinal Diversity Gradient” documented at the scale of latitude/longitude grid cells) we show that diversity increases from Puerto Rico through the American South northward, attenuating or dipping only in the Arctic. The pattern is consistent at multiple taxonomic scales (i.e., counting species, genus, family, and orders) and is true for a variety of subgroups.

In short, we flip the latitudinal gradient on its head! Take a look:

Pitfall traps capture a greater variety of invertebrates as you move poleward in North America.
This pattern, with some variation, is consistent across most of the sub taxa, from earthworms to spiders to beetles to springtails.

The Community: Why do our results diverge from one of the oldest ecological patterns in the book? We think the big reason is that we are sampling discrete communities around each trap grid—only a few hundred square meters. Communities differ from grid cells like those at the top of this post. Grid cells encompass, and hence tally, species across a variety of habitat patches (each with their own complement of bugs). Community diversity focuses on a more limited suite of species: those that are living and interacting in the same place.

The science of community ecology is all about the processes that limit the number of critters that can coexist, a suite of processes that follow different rules than those determining the ranges of all those species. A variety of processes—like the existence of food plants, nearby competitors, predators, and mutualists, or local moisture and temperature—all serve to filter that pool to a smaller subset. Our results suggest that those filters are stronger toward the equator. So much so, that even with more species available, communities from the warm subtropics are more rigorous at kicking out species that don’t “fit”.

A second, complementary hypothesis, is that communities closer to the poles have more mobile species (and thus more colonists from the wider species pool coming to visit)…

Or perhaps northern communities are more likely to be disturbed by cold temperatures that knock down populations and open up resources…

Regardless, the flip in the community diversity curve from that often found in the grid cell diversity curve was quite a surprise.

Scrub, tundra, grassland and forest habitats like those sampled by NEON. Image by Karl Roeder.

The Methods: We obtained the NEON pitfall samples from their storage facility at ArizonaState. We use a combo of eDNA (extracting info from the alcohol) and machine learning from images (ditto for pictures) to nondestructively analyze their contents.

A flowchart of our eDNA pipeline. The alcohol used to store the contents of a pitfall trap is extracted for its DNA, sequenced, and those sequence checked against global databases to identify, non-destructively, the critters the trap catches. We also incorporate pictures of the same bugs, to complement the DNA.

The Future: With this pipeline we provide tools to ecologists for monitoring Earth’s invert populations in this era of Insect Declines. We are discovering the rules for insect communities differ from “common knowledge”. Next: the geography of invasive species and size!

A particularly adorable grasshopper.

Oh, and one more thing.

We are proud to publish these cool results in the journal Oikos—rather than the countless spinoffs of journals beginning w ‘Sci’ and ‘Nat’—because we support scientific societies like the Nordic Society Oikos, the British Ecological Society, and the Ecological Society of America that in turn support the ecological community.

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