Invasive Grasses Spell Trouble for the Texas Tortoise



Texas Tortoises Invasive Grasses

Authored by Kiley Briggs

After years working as a wildlife technician for a variety of wonderful organizations, including The Orianne Society, I decided to take the next step in my career and moved to South Texas to work on my master’s thesis through the University of Texas Rio Grande Valley. I look back at my time being torn to shreds crawling around in Tamaulipan thornscrub with radio telemetry gear much more fondly than I would have guessed back when I was doing it. While down there I learned a lot about an ecological region I knew very little about, and now I would like to share with you some of what I know about the topic I spent the last three years becoming intimately familiar with.

Texas Tortoises Invasive Grasses

In the early 1900s, after decades of overgrazing led to bare ground and parched earth, cattle ranchers in South Texas were desperate to find a grass to feed their herds that would thrive under immense grazing pressure and that would withstand prolonged periods of drought. Turning to the savannahs of Africa, they found a solution in Buffelgrass. Drought-resistant, fire-tolerant and having co-evolved with large herds of grazing animals, Buffelgrass seemed like a perfect fix to a really big problem. After the first successful introduction of Buffelgrass to Texas in 1947, right in the middle of a severe drought, the species quickly became known as the “wondergrass.” Ranchers throughout the Rio Grande Valley planted Buffelgrass eagerly, and the species spread on its own throughout the region.

Today Buffelgrass occupies roughly five million hectares in South Texas, with millions more occupied by other African grasses, and people are still planting the stuff. Grasses, however, do not make up a large part of the natural environment in South Texas (not counting coastal prairies), and changes in the plant communities brought on by the widespread introduction of African savannah species cause huge problems for wildlife (see the side-by-side comparison photos to the right to get an idea of how drastically invasive grasses change the ground vegetation). One species near and dear to my heart that may have a difficult time adapting to these changes is the threatened Texas Tortoise.

Texas Tortoises are a close relative to one of The Orianne Society’s focal species, the Gopher Tortoise, but the tortoises in Texas are about half the size of the ones in the Southeast and they almost never dig burrows. To understand why invasive grasses spell trouble for Texas Tortoises, there are two things you need to know first. The first is that invasive grasses cause massive reductions in the abundance and diversity of wildflowers (forbs), which are higher in nutritional value than grasses and are an important part of the tortoise diet. The second is that the habitat of Texas Tortoises naturally has lots of bare ground and open space for a tortoise to move around in, but invasive grasses form thick carpets of impenetrable vegetation that can cut tortoises off from resources and funnel them into open paths such as roads. These two basic problems, however, can affect tortoises in a multitude of ways, and I’ll walk you through a few of them.

When tortoises consume a diet rich in forbs, they grow faster. Forbs, generally speaking, have higher concentrations of calcium, phosphorus, magnesium and nitrogen than grasses, and forbs with a high nitrogen-to-potassium ratio (legumes) are of particular importance to tortoise growth. Tortoises can forage selectively and pick forbs from the jungles of grass, but if invasive grasses occupy most of the groundcover in their habitat, a tortoise might be limited in their ability to find, let alone access, the forbs they need for optimal growth. Slower growth might not seem like such a big deal, but that can mean tortoises reach sexual maturity later in life and remain susceptible to predation for a longer period of time.

Texas Tortoises Invasive Grasses

Adult tortoises are more or less immune to predation. The occasional coyote knows how to crack adult tortoises open and other predators might gnaw off a tortoise’s leg from time to time, but predation rates of adult tortoises are extremely low. Hatchling and juvenile turtles, however, are like the Oreos of the animal kingdom: they are kind of hard on the outside, squishy on the inside and everything eats them. From crows and coyotes to snakes and feral hogs, you’ll be hard pressed to find a carnivore that won’t eat a young tortoise. The larger a tortoise gets, the less predators the tortoise is vulnerable to, until a specific point at which predation becomes scarce; the longer a tortoise takes to grow, the longer that tortoise is vulnerable to a larger number of predators. If forbs become scarce in habitats overrun with invasive grasses and tortoise growth decreases, predation rates of tortoises have the potential to increase. I should be clear and point out that this connection is speculative, but in my research I found that female tortoises were larger at sites with more forbs, so I think future research into connections between forbs, invasive grasses, growth rates and predation would be valuable. While an increase in mortality rates is bad on its own, invasive grasses have the potential to affect birth rates (fecundity), too.

Texas Tortoises Invasive Grasses

In reptiles, fecundity is very closely tied to diet. For example, Timber Rattlesnakes in the North (another Orianne favorite) only give birth every three or four years, but when rodent populations are high, there is a surge in rattlesnake birth rates the following year. Roughly a third of female Texas Tortoises do not lay eggs in any given year, and the availability of high-quality foods likely plays a role in how often those tortoises reproduce. More certain is that larger tortoises lay more eggs than smaller tortoises, so a reduction in growth caused by invasive grasses limiting higher-quality foods might reduce egg production. My observation that female tortoises were larger at sites with more forb cover provides evidence that this could be a real problem. Research comparing reproductive rates at sites with varying exotic grass and forb cover would be very interesting, but to the best of my knowledge, it has not been conducted. There is, however, one very direct and extremely clear link between invasive grasses and a big problem for Texas Tortoises: death by mower.

Invasive grasses grow thick and high and can quickly overrun ranch roads and crowd the edges of highways unless they are mowed. The problem is that tortoises are attracted to roads because roads provide paths through the grass for tortoises to travel along. That mowing knocks back the grass enough to allow forbs to establish along roadsides, likely drawing the tortoises in, as well. After my time walking ranch roads and crawling through thornscrub in the lower Rio Grande valley, I can tell you that many roads are littered with the bones of tortoises, but the land surrounding the roads is not. In my study sites I identified a total of about 200 Texas Tortoises and found only one tortoise that appeared to have recently died due to natural causes. I found about 10 tortoises recently run over by mowers though, and all of them were on roads in areas thick with invasive grass. Another tortoise was found recently crushed in a tire track where invasive grasses completely blocked sight of the ground (the poor thing was using the tire tracks to cross a dense patch of Guinea Grass), and when that road was finally mowed a week later, I found the mangled bodies of another six tortoises in the mower’s wake.

Texas Tortoises Invasive Grasses

About five percent of the tortoises I observed were freshly killed by mowers when I found them. Five percent might not sound too bad, but tortoises are long-lived animals with low reproductive rates, and their reproductive strategy relies on the survival of adults. Texas Tortoises typically lay only two or three eggs a year, but mortality rates of young are high and predators take a large number of tortoise eggs long before they hatch; that’s just how it works. The survival of adults is critical so they can breed and lay eggs year after year for decades on end, with only a couple of their offspring surviving to adulthood and having young of their own. There are all sorts of ways invasive grasses might harm tortoises by reducing availability of high-quality foods, but in my personal opinion, a much larger problem is that invasive grasses necessitate mowing, and mowers kill adult tortoises.

Invasive grasses are not a problem unique to South Texas, but are a common problem in just about every part of the earth. Indeed, many of the same grasses plaguing South Texas are causing the same problems in Australia. I wish there was a clear solution to the problem of invasive grasses, not just for the sake of the tortoises but also for the countless other animals native to the area, many of which are rare and found nowhere else in the country. Unfortunately, demand for solutions to the problem of invasive species across the country, and globe, far exceeds supply, and eradication of most invasive species is an unrealistic goal. Learning how to manage ecosystems to lessen the impact invasive species have, however, is much more practical. Understanding the intricate ways in which the effects of invasive species ripple through ecosystems will allow us to set realistic goals in habitat restoration efforts.

Despite invasive grasses, which are just one problem on a list of many, I am hopeful that tortoises will remain an iconic part of the lower Rio Grande Valley ecosystems long into the future, and I look forward to seeing them during my next visit. Until then, I’ll always have the photos and scars to remember them and their extremely thorny habitat.