The impacts of cattle grazing on arboreal reptiles

Nordberg, Eric J. (2018) The impacts of cattle grazing on arboreal reptiles. PhD thesis, James Cook University.

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Habitat disturbances, such as grazing by livestock, can have major direct and indirect impacts on the structure and complexity of both vegetation and faunal communities. Changes to habitat structure and resource availability often lead to changes in biodiversity. While the impacts of livestock grazing has been well studied in vegetation communities, fewer studies have focused on wildlife. Further, terrestrial (ground-dwelling) communities have generally been the focal group of interest, whereas the impacts of grazing on arboreal systems have often been overlooked.

Arboreal reptiles are generally difficult to capture due to their cryptic nature and tendency to flee when encountered. Further, they often shelter under loose, peeling tree bark, or in hollows, making them difficult to capture without damaging their microhabitat. I implemented and used arboreal cover boards (ACBs) as "artificial bark" made from closed-cell foam cover boards to try and increase the capture rate of arboreal lizards. I tested the difference in capture success using standard visual encounter surveys (VES) and the use of arboreal cover boards as survey methods to sample small, cryptic arboreal lizards. Diurnal and nocturnal (spotlight) VES resulted in lower capture success compared to ACB surveys. While nocturnal VES showed a bias towards adult geckos, the use of ACBs captured individuals from all age classes. Further, ACBs were successful in monitoring non-target species, and even broad taxonomic groups. The use of ACBs extends beyond just a useful survey technique, but also provides the potential for long-term monitoring and restoring damaged or degraded microhabitats.

Domestic livestock alter ground-level vegetation structure due to grazing, but little is known about the effects on arboreal habitats. Similarly, the effects grazing has on ground-dwelling wildlife populations has been examined to some extent, yet the potential impacts to arboreal groups have received little attention. I examined the responses of arboreal and terrestrial reptile communities and their respective habitats to domestic livestock grazing. Terrestrial reptiles were generally negatively associated with heavy grazing, as their primary habitat (ground-level) was heavily impacted. Terrestrial reptiles were strongly associated with ground-level habitat complexity, which was greatly reduced in areas with heavy grazing. Alternatively, arboreal reptiles were generally resistant to the impacts of even heavy grazing, and were positively associated with arboreal tree structures, such as peeling bark and tree hollows. While terrestrial and arboreal reptiles showed opposing trends (negative and positive association with grazing, respectively), individual species within these groups showed varied responses. I highlight the importance of retaining trees in rangelands as an overarching management guideline, as trees provide primary habitat for arboreal species, but also provide shade, leaf litter, and woody debris for terrestrial species.

Two species that showed resistance (i.e., no negative effects, or showed positive effects to livestock grazing) were a diurnal skink, the inland snake-eyed skink (Cryptoblepharus australis) and a nocturnal gecko, the Australian native house gecko (Gehyra dubia). Snake-eyed skinks showed no effect of livestock grazing on their abundance, while native house geckos were more abundant in sites with heavier stocking rates. I tested whether food availability was a driving mechanism that allowed these species to persist in areas with high levels of grazing. I quantified the invertebrate prey community available to both lizards using ACBs and light trapping. The invertebrate prey community composition was not significantly different among the four grazing regimes or habitat types. While both lizard species are habitat generalists, they were fairly selective in their diets, consuming prey disproportionally to their availability. Both snake-eyed skinks and native house geckos showed strong selection for beetles, spiders, and scorpions, resulting in high dietary niche overlap. While native house geckos and snake-eyed skinks both occupy similar microhabitats and consume similar prey items, they are temporally segregated by activity time (native house geckos = nocturnal; snake-eyed skinks = diurnal). Although not significantly different, the heavily grazed sites showed the greatest abundance of the most preferred prey items (beetles, spiders, scorpions), which may contribute to the high abundances of native house geckos in these sites.

Predator–prey dynamics play a vital role in shaping population structure and community assemblages. Predator–prey interactions have been of interest to biologists for hundreds of years, yet most studies focus on the perspective of the predator (examining functional and numerical responses), or from the perspective of the prey (examining direct mortality or non-consumptive effects). Few studies have measured predation risk in relation to the abundance of both predators and prey. I tested two competing hypotheses regarding predation risk: i) predation risk is predator-density dependent (i.e., more predators result in greater predation risk); and ii) predation risk follows the alternative prey hypothesis (e.g., predation risk is dependent on alternative prey availability). I used a combination of surveys (for predators, alternative prey (invertebrates), and lizards) and physical models (n = 800) to estimate predation risk on lizards. Predation risk on lizards was greatest in the dry season, when predator abundance and alternative prey populations were lowest. Alternatively, predation risk on lizards was lowest during the wet season, when predator abundance and alternative prey availability was greatest. This suggests that predation risk to lizards is not predator-density dependent, as predation risk was lowest when predator abundance was highest, and vice – versa. Our data also indicates that predation risk follows the alternative prey hypothesis, as predation risk to lizards was greatest when alternative prey populations (invertebrates) were lowest, and vice – versa.

Although predator–prey dynamics are important in shaping wildlife community composition, predation events are rarely observed. In many cases, our perception of what constitutes "predators" or "prey" may be biased towards anecdotal observations or studies that have documented predation events. In many ecosystems, vertebrates constitute a majority of what we consider "predators", but large invertebrates exist across much of the world, especially in tropical regions, that may be formidable predators to small vertebrate groups. I conducted 500 man-hours of visual searches, compiling observations of in situ predation events, and deployed 800 model lizards to measure attack frequency and identify potential predators. Observing a predation event in situ was rare: I observed 9 predation events in total (4 instances of vertebrates eating another vertebrate; and 5 instances of invertebrates eating a vertebrate). This suggests that while observing any predation event is rare, invertebrate predators (e.g., large spiders, mantids) may contribute to similar levels of predation to more traditional vertebrate predators (e.g., snakes, birds). Further, large invertebrates (predominantly huntsman spiders) contributed to up to 23% of attacks on model lizards. I highlight the potential importance of large predatory invertebrates as predators to small vertebrates, especially herpetofauna, in comparison to more "traditional" predator groups, such as birds and snakes. Predatory invertebrates have largely been overlooked in the literature as potential predators, with exception to anecdotal observations. Their importance as predators in shaping community assemblages should be reconsidered.

Environments with greater habitat complexity support high biodiversity because they have abundant and diverse resources, which may reduce competition among species. Habitat disturbances, such as livestock grazing, often reduce habitat complexity and structure, resulting in a simplified, or homogenized environment. I tested if reduced competition, as a result of a homogenization from livestock grazing, acted as a mechanism that allowed native house geckos to thrive in heavily grazed environments. I compared the habitat utilization of three co-occurring arboreal geckos, native house geckos (G. dubia), northern velvet geckos (Oedura castelnaui), and eastern spiny-tailed geckos (Strophurus willsiamsi). All geckos displayed resource partitioning among tree species and tree structural characteristics. I found evidence of interspecific competition between geckos, in which native house geckos shifted their preferred habitat of dead trees, to Silver-leaf ironbark trees (Eucalyptus melanophloia) and complex trees with peeling bark in the presence of velvet geckos. Native house geckos were more resistant to the negative effects of grazing (low habitat complexity) than either velvet or spiny-tailed geckos. Native house geckos were more abundant in heavily grazed areas. In contrast, velvet and spiny-tailed geckos were rarely found in heavily grazed sites, and were more abundant in areas with lower grazing pressure. My data suggests that grazing by livestock homogenized the environment, and in turn, homogenized the arboreal reptile community. A lack of competition allowed native house geckos, a microhabitat generalist, to persist and even increase in abundance, where microhabitat specialists, such as velvet and spiny-tailed geckos, declined.

Habitat disturbances, such as livestock grazing, have detrimental effects on plant and animal communities. While many studies focus on species that decline in response to disturbance, it is just as important to understand how and why some species respond positively. This thesis is focused around the positive response of an arboreal lizard, G. dubia, to try and understand what mechanisms were responsible in allowing them to persist in environments where other species declined. I tested a series of potential mechanisms, including food availability, habitat availability, predation pressure, and competition, to try and identify their effects on native house gecko populations. While my data indicates that a reduction in competition as a result of habitat simplification is a major contributor, it is likely that no stand-alone mechanism is responsible. Rather, a combination of these mechanisms contribute to their success in heavily grazed environments.

Item ID: 52961
Item Type: Thesis (PhD)
Keywords: active searches, agriculture, alternative prey hypothesis, arboreal, arthropods, Australia, birds, cattle grazing, diet shift, diet, disturbance, electivity, feeding ecology, habitat fragmentation, habitat selection, herpetofauna, homogenization, lizards, methods, niche, off-reserve conservation, predation, predator–prey dynamics, predator–prey, prey-switching, rangelands, reptiles, response mechanisms, spiders, spotlighting, trapping
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Nordberg, Eric J., and Schwarzkopf, Lin (2015) Arboreal cover boards: using artificial bark to sample cryptic arboreal lizards. Herpetologica, 71 (4). pp. 268-273.

Chapter 3: Neilly, Heather, Nordberg, Eric J., VanDerWal, Jeremy, and Schwarzkopf, Lin (2018) Arboreality increases reptile community resistance to disturbance from livestock grazing. Journal of Applied Ecology, 55 (2). pp. 786-799.

Date Deposited: 22 Mar 2018 22:57
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 50%
05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050102 Ecosystem Function @ 50%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960504 Ecosystem Assessment and Management of Farmland, Arable Cropland and Permanent Cropland Environments @ 100%
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