A Tale of Two Watersheds- A Comparison of Riparian Vegetation Communities


Llano River vs Waller Creek – A tale of two watersheds

This small-scale study was preformed in two locations: the Llano River and heavily urbanized Waller Creek watershed. It is hypothesized that rich and even plant diversity will be present in a healthier watershed, and therefore the heavily urbanized Waller creek will exhibit lower diversity metrics. This experiment is performed under the simplifying assumption that lower plant diversity implies poorer habitat quality as a result of urbanization.

I examined two 30×125 ft transects with the short length along the river edge in order to capture a gradient. I then identified herbaceous and woody vegetation along the riparian zone. I took qualitative notes on other organisms present and concerning water quality, but the study will focus on herbaceous vegetation diversity. While walking in a zig-zag pattern along the transect, I identified species to the genus level, and condensed species within the same genus.

Vegetative species Llano Waller
Ipomopea 29 12
Helenium 4 0
Helianthus 3 0
Parthenocissus 12 19
Smilax 11 2
Melia 3 0
Xanthium 3 0
Ficus 3 1
Symphiotrichum 4 3
Verbesina spp. 9 0
Opuntia spp. 1 0
Populus spp. 1 0
Salix spp. 2 0
Palafoxia spp. 2 0
Ambrosia spp. 0 4
Ruellia spp. 0 158
Toxicodendron spp. 0 23
Carya spp. 0 1
Taxodium spp 0 2
Verbena spp. 2 1
Echinacea spp 0 3
Oxalis spp. 0 3

For the biodiversity calculations I used the formulas as follows:

Richness is defined as total number of species present. Abundance is total individuals present.

Species density is abundance per area.

Simpsons Diversity Index is the sum of individual species counts divided by the total abundance squared. S(n/N)2. Values of Simpson’s D range from 0-1.

Evenness is a metric using Simpson’s D that calculates how well-distributed species are within an ecosystem. It ranges from 0-1 where 1 is the most even.

 Biodiversity Metric Llano Waller
Richness 15 13
Abundance 89 232
Density 0.256 0.667
Dmax 15 13
Simpsons D 6.242 2.066
Reciporacal Index 0.0012 0.0385
Evenness 0.4161 0.1589

The transect along the Llano River had 2 more species present than Waller, which is lower than hypothesized, though the distribution of individuals may indicate further the health of these sites. Both of these sites indicated low species richness, which may be attributed to the presence of aggressive exotic species in the Llano site. This could be attributed to regular removal of invasive species in the Austin site at Waller Creek. For instance, among the species present, there was a higher abundance of individuals, though 158 of them were Ruellia. Ruellia composes of more than half of the individuals present in Waller.  Without Ruellia, Waller abundance is lower than Llano, and the evenness rises, though it is still lower than Llano.

The Simpson’s D is a measure of probability two randomly selected samples will belong to the same species. Simpson’d D ranges from 0-1, with 0 indicating infinite diversity. The higher value in Waller is likely due to the Ruellia, Toxicodendron and Parthenocissus composing nearly 82% of the species present.  This metric implies that Llano has higher diversity than Waller creek.

Finally, evenness was much higher in Llano, meaning the species were more evenly distributed within the transect. All metrics imply that Llano River has higher species diversity than Waller Creek, even though Waller had higher abundance.

All metrics show that Llano had a more diverse and evenly distributed plant community. Poor habitat quality may lead to low plant diversity, which could be tested by examining soil and water quality for both locations. Overall, these metrics imply that Llano’s vegetative community is more diverse as a result of a higher quality environment. This can be likely attributed to some factor in the habitat, be it land use or nutrient availability that is closely tied to the Llano’s rural landscape. This study’s findings suggest that urbanization has a negative impact on the riparian vegetative community.

To perform the experiment, I used Wildflowers of the Texas Hill Country by Marshall Enquist, and Trees, Shrubs & Cacti by James Everitt and  D. Drawe for vegetation ID. Additionally, I used a camera for unknowns in the field for later ID, transect tape, and a field notebook.

By K.T. Strain

Katie Strain is a texan, plant enthusiast, and Ph.D. student studying Evolution, Ecology and Conservation biology.

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