Introduction

In the following document I intend to provide a reason why we need a better system to define how native a plant is to a certain location. Both the limitations of our knowledge and the current systems people tend to use introduce bias and error. And then I will provide what I believe is a good system to rate how native any given species is to a given locality. If there is another similar system or method, I am unaware of it. Feel free to skip ahead to the methods if the background bores you and feel free to refine this.

Current Systems

Most people judge a plant’s nativeness by political boundaries such as state, a regional grouping of states, or even entire country. I live on the western edge of PA. A plant native to the United States, but only from the Rocky Mountains such as Colorado blue spruce (Picea pungens) isn’t naturally going to be found here. Neither is a native to Pennsylvania, but only from the Atlantic Coastal Plain like Atlantic white cedar (Chamaecyparis thyoides).

The most specific system people use is recorded presence in a given county. While this seems great at first, it only shows what was recorded via collected botanical specimens. Some areas were more heavily collected than others. Some collected specimens have poor location data associated with them. Some records are disputed among botanists for various other reasons, which is why different sources may list different ranges for a given species.

Limitations of Existing Data

While it is debatable who got to the North American continent when, Major impacts started shortly after Columbus sailed here in 1492. Fur trade started in the 1530’s and 40’s and shifted to beaver pelts in the 1500’s. Beavers being a keystone species that creates openings and ponds, etc., this impacted ecosystems as a whole and many other species. As early as the 1500’s, European settlements started. Many of these brought European livestock and crops and by accident European invasives. Many of these early settlements failed, but by the 1600’s they were getting more of a foothold. By the 1700’s European settlement of the east coast was well established.

In 1707, Karl von Linn, better known by his latinized name Carolus Linnaeus, was born. He started developing his binomial classification system that we still use starting in the 1730’s. Other systems existed earlier. His student Peter Kalm visited Pennsylvania and the surrounding area in 1748. Other naturalists followed like Andre Michaux in the early 1800’s. By this time settlement had spread well past the Appalachians and Bison were extinct in most states east of the Mississippi. Most plants and animals native ranges were mapped out based on collections from the second half of the nineteenth and first part of the twentieth centuries. This is roughly 350 to 400 years after the beginning of European settlement and 150 to 200 years after serious impacts from logging, farming, mining, canal building, etc. Even in Peter Kalm’s journal from 1748, the cedar swamps around Philadelphia are all cut down and privet is so widespread that he is uncertain whether it is native or introduced. I could make this a long dissertation with references, but this is all readily available information you can easily Google for yourself. The point is that though the information we have is valuable, it is far from perfect, and we should acknowledge and allow for that.

Ranges Are Dynamic

Even if we had a perfect map of the range of a species in 1492, it would be different today. As the climate cycles, species move around. Seeds get dispersed places where they don’t grow, but if the conditions change a little, they suddenly will grow there. Hotter, cooler, wetter, drier, longer winter, shorter winter and many other climatic conditions are varying all the time. For example; a species may expand tens of miles north after a few mild winters or shrink its range south after a few harsh winters.

Species also change and adapt over time allowing them to spread farther, perhaps out competing and pushing back the range of another species. It has always been a dance with a constant ebb and flow. However, when deciding if something is native there should be a boundary or limit on how far to allow for this.

Method for Rating Nativeness

Regardless of other parameters, localized conditions dictate what grows where. Soil type, drainage, pH, sun exposure, etc., are most important. That said, the question I’m trying to answer is “if some event created these conditions in this spot prior to European settlement, what would live here?” I have gone down many rabbit holes about seed dispersal, geology, etc. but I’ve narrowed my system down to two parameters: county records and USGS Level 3 Ecoregions.

Find a map showing counties and showing Level 3 Ecoregions or make one in GIS.
Find county level range data for the plant (or animal) you are interested in.
Count the number of counties between your location and the nearest county with a record for that species.
1. The county you are in would be zero and the neighboring county would be one if in the same ecoregion.
1. When you cross into the next ecoregion, each county counts for two. If you cross into a third ecoregion, then each county there counts as three.
1. Take the shortest path possible as measured in counties, even if it isn’t a straight line.
1. If records are close on both sides, count to the nearest county in opposite directions. Divide the longer count of the two in half and use the smaller of that measurement or the shorter distance. For example, if you measure 3 counties east and 7 counties west, 3.5 counties is half of the longer distance, but it is greater than 3 so you use 3. But if you measure 5 east and 7 west, 3.5 is less than 5 and you use the 3.5.
1. Go around large barriers such as a large lake or bay.
1. A score of zero is perfect. 1-5 should be reasonably accepted as native. A score of 6-10 can also be acceptable but a bit more of a stretch. Over 11 is unlikely to have been there in the past or to show up on its own. Over 20 flat out not native.

Summary

While I’m sure others can improve on this, I think this approach does a good job of providing guidance without going into detail that exceeds the limits of what information is out there. Ecoregions are much better than political boundaries and Level 3 Ecoregions seems the appropriate level of detail for this task.