The impact of weeds and prescribed fire on faunal diversity

Abom, Rickard (2015) The impact of weeds and prescribed fire on faunal diversity. PhD thesis, James Cook University.

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Abstract

Human mediated transport has allowed some species to extend their range beyond their natural ability to disperse. Many exotic annual grasses are highly adaptable and can establish population in their introduced ranges because they can tolerate high variability in local climatic conditions, annual rainfall, and nutrient availability. The most successful invader grasses transform the ecosystems they invade. Invasive grasses can alter the natural fire frequency by increasing local fuel load, and then they flourish under the new conditions they create. This thesis examines the impacts of the introduced weed grader grass (Themeda quadrivalvis) and fire on vertebrate assemblages in tropical savannahs in northern Queensland, Australia.

To determine the effects of weeds and fire, and their interaction, on savannah vertebrates, I conducted a two-year vertebrate fauna survey in tropical savannah woodland at Undara Lava Tubes National Park. My survey sites were carefully chosen to provide me with plots that were not spatially auto correlated, and that included either native grasses, or native grasslands invaded by grader grass. After one year examining the influence of the presence of the weed on vertebrate fauna (reptiles), my sites were burned. I expanded my survey to include more recently burned sites, and continued to survey these through their recovery for 15 months. This allowed me to monitor the recovery of reptile and mammal assemblages after fire. Finally, I conducted an experiment to determine the influence of predation on foraging in mice, using giving-up density experiments. To conduct these experiments, I offered native and introduced mice food items in known quantities in trays, in open and closed environments, and determined the amount of time they were willing to forage in these trays, using the amount of food remaining in the trays as a measure of willingness to forage.

Invasive grasses are among the worst threats to native biodiversity, but the mechanisms causing negative effects are poorly understood. To investigate the impact of an invasive grass on reptiles, I compared the reptile assemblages that used native kangaroo grass (Themeda triandra), and black spear grass (Heteropogon contortus), to those using habitats invaded by grader grass (Themeda quadrivalvis). There were significantly more reptile species, in greater abundances, in native kangaroo and black spear grass than in invasive grader grass. To understand the sources of negative responses of reptile assemblages to the weed, I compared habitat characteristics, temperatures within grass clumps, food availability and predator abundance among these three grass habitats. Environmental temperatures in grass, invertebrate food availability, and avian predator abundances did not differ among the habitats, and there were fewer reptiles that fed on other reptiles in the invaded than in the native grass sites. Thus, native grass sites did not provide better available thermal environments within the grass, food, or lower predator abundance. Instead I suggest that habitat structure was the critical factor driving weed avoidance by reptiles in this system, and recommend that the maintenance of heterogeneous habitat structure, including clumping native grasses, with interspersed bare ground, and leaf litter are critical to reptile biodiversity.

Land managers often use fire as a management tool, to reduce accumulation of fuel, and by extension, the impact of wildfires on flora, fauna and the built environment. Many grassy weeds are tall, and grow in dense stands with high biomass. Grassy weeds often burn at a higher intensity than native grasses, which may alter the influence of fires on fauna. Thus, the response of fauna to fire in weedy environments may be complex. Here I examined reptile and mammal responses to fire in savannah open woodland habitats in native kangaroo and black spear grass habitats, and in habitats invaded by grader grass.

I compared reptile richness, abundance and assemblage composition in a group of replicated habitats that had not been burnt for 2 years, directly after they were burned, and up to 15 months after burning, when grasses had regrown. Reptiles are excellent model systems to examine the influence of fire on fauna, because they respond strongly to habitat structural features, and are only moderately vagile. I found that reptile abundance and richness were highest in unburnt habitats (2 years after burning), and greatly reduced in all habitats immediately after burning, most strongly in grader grass. Abundance and richness recovered in all three habitats one year after burning, but assemblage composition had changed. Three skinks and one monitor lizard were present only in the longest unburnt kangaroo grass sites, and their populations did not recover 15 months after burning. In weedy habitats, reptile abundance was more strongly reduced immediately after fire than in other habitats. Even in fire-prone, often burnt habitats such as these, in which richness and abundance were not strongly influenced by fire, assemblage composition was.

As above, I also examined mammal richness and abundance in replicated unburnt, burnt, and revegetated native and weedy sites. Mammal abundances were higher in unburnt native grasses than in unburnt weedy sites. The lowest mammal abundances occurred in sites revegetated after fire. All mammals, except rufous bettongs (Aepyprymnus rufescens) and tropical short-tailed mice (Leggadina lakedownensis) were reduced in abundance following fire. Eastern chestnut mice (Pseudomys gracilicaudatus) and common planigales (Planigale maculata) returned with returning grass cover. Over the course of my study, I detected a gradual decline in northern brown bandicoots (Isoodon macrourus). Mammal responses to fire in weeds were idiosyncratic, some species were more abundant in weedy habitats following fire, some less, and some returned to their prior abundance. My study indicated that in, tropical savannahs, a naturally fire-prone habitat, overall mammal abundance, but not richness, decreased with frequent fires (≤ 2 years), in both weeds and native grass, whereas individual species responses varied greatly.

Differential predation risk among habitats, or 'the landscape of fear' can have profound impacts on foraging strategies of prey. Few studies, however, have described the landscape of fear in the wild, in relation to actual predator densities. Using giving up density experiments, and vertebrate surveys, I described the landscape of fear of two rodent species in relation to predator abundances in open savannah woodland. I offered native eastern chestnut (Pseudomys gracilicaudatus) and introduced house mice (Mus musculus) food in the open, and under the cover of grass. When eastern brown snakes (Pseudonaja textilis) were absent, both eastern chestnut and house mice consumed more food items under cover. When snakes were present, eastern chestnut mice consumed more food items in the open than under cover. House mice, on the other hand reduced their foraging activity undercover, but did not increase foraging in the open in the presence of snakes. The abundance of other predators did not correlate with food intake in different habitats. Native mice apparently can adjust their antipredator behaviour to remain successful in the presence of native predators.

In conclusion, my study provides the first insights into the responses of reptile and mammal assemblages to native savannah invaded by grader grass, and the interaction between fire and the presence of grader grass. I describe how fauna respond to habitat modifications after fire, and after vegetation cover had returned to levels similar to prefire. My study found that reptiles and mammal community composition in these naturally fire-prone savannah systems were sensitive to the presence of the weed, and to frequent fires (≤ 2 years), especially in the weedy parts of the habitat. I suggest managers leave longer intervals between prescribed fire in tropical savannahs, which burn frequently anyway, and suggest that fewer fires might help to maintain faunal biodiversity in fire-prone habitats. I also suggest that decisions to burn weeds should include an awareness of the likelihood of enhancing certain species while discouraging others, and conservation decisions should be based on fire sensitive species given a multi-species response.

Item ID: 44635
Item Type: Thesis (PhD)
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Abom, Rickard, Vogler, Wayne, and Schwarzkopf, Lin (2015) Mechanisms of the impact of a weed (grader grass, Themeda quadrivalvis) on reptile assemblage structure in a tropical savannah. Biological Conservation, 191. pp. 75-82.

Chapter 3: Abom, Rickard (2016) Short-term responses of reptile assemblages to fire in native and weedy tropical savannah. Global Ecology and Conservation, 6. pp. 58-66.

Chapter 4: Abom, Rickard, Parsons, Scott A., and Schwarzkopf, Lin (2016) Complex mammal species responses to fire in a native tropical savannah invaded by non-native grader grass (Themeda quadrivalvis). Biological Invasions. (In Press)

Date Deposited: 11 Aug 2016 02:36
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 100%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960505 Ecosystem Assessment and Management of Forest and Woodlands Environments @ 100%
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