Structural changes in vegetation coincident with annual grass invasion negatively impacts sprint velocity of small vertebrates

Abstract

Sagebrush ecosystems in the intermountain west of the United States are being threatened by conversion to the non-native grass, cheatgrass (Bromus tectorum). The dramatic shift in the physical structure of vegetation coincident with cheatgrass invasion likely has negative impacts on animal communities, yet these structural impacts have not been well-studied. In a previous study, dense cheatgrass stems reduced sprint velocity for the flattened, wide-bodied desert horned lizard (Phrynosoma platyrhinos). Here, we asked if a decrease in sprint velocity due to cheatgrass impediment can be generalized to the suite of small vertebrates inhabiting the sagebrush ecosystems of western Utah. We evaluated sprint performance of the common rodent (n = 3) and lizard (n = 4) species on two raceway types, cheatgrass and no-cheatgrass, and hypothesized that body size, body shape, and form of movement are important factors influencing sprint velocity through dense cheatgrass stems. All species showed significant reductions in speed on cheatgrass versus no-cheatgrass raceways, with percent reduction greatest for larger, wider, or hopping organisms compared to smaller, more slender, or running organisms. Of concern, surveys for rodents and lizards at our study areas support a common pattern: lower abundances of small vertebrates, as well as a loss of rodent species richness, in areas infested with cheatgrass compared to intact, native sagebrush communities. By extension, we expect a negative impact on animal communities in other semi-arid regions experiencing dramatic shifts in vegetation structure upon invasion by non-native grasses that are capable of forming dense stands in the interspaces of native desert plants [e.g., Sonoran desert invaded by buffelgrass (Pennisetum ciliare)].

The effects of multiple obstacles on the locomotor behavior and performance of a terrestrial lizard

“…Cluttered habitats prove difficult for maintenance of a straight path of travel and forward speed during movement, requiring a behavioral repertoire to negotiate obstacles (Rieder et al, 2010;Garber and Pruetz, 1995;Tucker and McBrayer, 2012). Quickly maneuvering on or over obstacles can play a vital role in predator evasion in complex habitats (Kohlsdorf and Navas, 2007;Tucker and McBrayer, 2012).…”

“…Many lizard and rodent species have lower escape speeds in dense vegetation compared with open areas (Vasquez et al, 2002;Schooley et al, 1996;Goodman, 2009;Vanhooydonck and Van Damme, 2003). Species utilizing bipedal running may be able to exploit habitat types with uneven substrata because of the increases in visual perception and/or by reaching their maximum speed more quickly (Rieder et al, 2010;Djawdan and Garland, 1988;Rocha-Barbosa et al, 2008;Kohlsdorf and Biewener, 2006). Because two obstacles create the same negative performance effect on stride characteristics and velocity, bipedalism is likely only beneficial when negotiating a single obstacle (Fig.…”

“…dense bunchgrass, loose sand) impede small terrestrial vertebrates' (e.g. mammals and lizards) ability to navigate through, in, on and over such habitat structures (Rieder et al, 2010). Some rock-dwelling lizard species, such as Crotaphytus bicinctores, frequently encounter uneven surfaces and are less sensitive to changes in the degree of substrate unevenness than smaller sand-dwelling species (Collins et al, 2013).…”

Energy flow and functional compensation in Great Basin small mammals under natural and anthropogenic environmental change

“…For example, work at Portal indicates that a precipitation-driven closing of habitats caused a shift in the body size distribution of granivorous small mammals from a mean individual size of 73.4 to 33.7 g over a 24-y time period (3). Reider et al (35) have shown that, although cheatgrass significantly reduces sprint speed for all species, this is disproportionately so for larger saltatory species. Furthermore, Lightfoot et al (36) have recently shown that species affiliated with natural annual grasslands in the Chihuahuan desert exhibit greater reproductive potential, and thus faster population responses to changes in resources, than shrub-affiliated species.…”

An Invasive Grass and a Desert Adapted Rodent: is there an Effect on Locomotory Performance and is it Modified by Prior Experience or Familiarization?

“…In order to elicit maximal running performance, the animals in that study were pursued with a padded yardstick. As previously discussed, Rieder et al (2010) did not thoroughly account for three potentially important factors: a) non-linear running path (such that measurements of velocity as assessed by raceway distance/time rather than path distance/time would potentially underestimate velocity), b) the magnitude (type, frequency, etc.) of the "motivation tactic," or pursuit with the padded yardstick, each animal received, and c) the possibility of habituation to a habitat patch and therefore loss of the detrimental effects imposed by the grass on running performance over time.…”