Number Of Taxa Vs. Distance Along A Trail

How many taxa do you expect to find for a typical trail vs. distance? If you only go a half mile on a given trail, do you expect that you have seen 50%, 70%, or 90% of all the taxa that you would have seen if you have gone a mile? Are non-native taxa concentrated at the beginning of a trail? Do you have to travel some distance before finding many non-native taxa? How do the answers to any of these questions depend on elevation, trail aspect, number of distinct habitats on the trail, or other parameters?

These are some of the burning questions that can be answered only when we have completed plant lists for many trails. However, it is fun to play with our preliminary data.

The following plots show the number of taxa (total, natives only and non-natives only) vs. distance, for three trails for which our trail lists might be fairly complete. (No guarantees! We need a year of good rainfall to tell us how complete our lists are.)

The trails are:

In terms of all taxa, including both natives and non-natives, the Sunset Ridge Trail is the clear winner. However, most of its numeric advantage is due to non-native taxa, being at low elevation immediately next to the Los Angeles megalopolis. Once the non-native taxa are removed, the trails are all surprising similar:

Perhaps the main cause of the increased number of taxa for the Sunset Ridge Trail past mile 0.5 is that the trail enters a completely different habitat at that point, transitioning from a south-facing slope to a north-facing slope. The other two trails don't have such a dramatic change of habitat beyond mile 0.5.

The model fit to the data for the native taxa vs distance is the usual power law, with an exponent of 0.37. See Introduction and Explanation of Trail Guides and Number Of Taxa Vs. Trail Length for further explanation of the power law.

The non-native plants definitely tend to be concentrated to the beginning of a trail, or wherever a disturbance such as a road crosses the trail. At both BSER and Sunset Ridge, nearly all the non-native taxa are found in the first 0.15 mile of the trail. The Granite Loop Trail has about half of its non-native taxa appearing in the first 0.01 mile of trail, with the rest appearing on the first encounter with grassland at mile 0.20 and near Waterline Road at mile 0.71.

The above plot clearly shows the influence of human activity. In order of the lowest number of non-native plants:

  1. The Granite Loop Trail at the Santa Rosa Plateau is in the most pristine area, and shows the smallest number of non-natives. This is despite the fact that it contains a portion in grassland, the most disturbed California plant community.

  2. The BSER is more disturbed, with the trail actually being a road going through the canyon, and then heading up alongside the artificial dam that created Lake Poway.

  3. The Sunset Ridge Trail has three major sources of disturbance:
    • it has a huge source of non-native plants in a highly-disturbed area immediately adjacent to it (the L.A. basin);
    • it has had nearly a century or more of heavy use by people; and
    • it has had the Forest Service intentionally planting non-natives along the Road which is part of the trail and directly adjacent to the rest of the trail.

Given that the data for the number of taxa vs distance closely follows the power law, it is then easy to calculate the answers to some of the first questions above on average. The following plot gives the % of taxa seen vs. % distance:

For example, if you decide to go only 10% of the total distance along a trail, on average you will have seen about 43% of all the taxa on the trail. If you go half of the total distance, you will have seen on average about 75% of all the taxa.

One can look at these data in a different way, and ask how many more taxa will you see if you decide to go another 10% of the distance you have already gone:

Going another 10% of distance results in only 3.6% new taxa. If you decide to double the distance you have already gone, on average you should see about 30% more taxa. You would have to travel just over 6.5 times more distance to double the number of taxa you have already seen.

Go to Native and Introduced Plants of Southern California

Copyright © 2003 by Tom Chester and Jane Strong
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Last update: 7 February 2003