Geopolítica de un nombre: el control de naturaleza a

2.6.1

Scat counts as a proxy for utilisation of an area

The abundance of scats (dung) is a good proxy for the utilization of an area for feeding by macropods, because they mainly defecate while feeding (Johnson and Jarman 1987). This approach has been used successfully in numerous studies of macropods (Hill 1981; Coulson and Raines 1985; Johnson and Jarman 1987; Vernes 1999; Bulinski and McArthur 2000; Bridle and Kirkpatrick 2001). Compared with such herbivores as deer, macropods produce deposits of relatively few pellets. This often causes difficulty in defining a group; this is eliminated by counting individual pellets rather than groups (Perry and Braysher 1986).

While scat counts are useful for estimating animal abundance (Coulson and Raines 1985; Southwell 1989; Bridle and Kirkpatrick 2001) there is variation in defecation rate with activity (Hill 1978; Johnson and Jarman 1987; Johnson et al 1987).

Macropods defecate rarely while resting in the middle of the day, but do so frequently in the first hour of feeding after rising and with intermediate frequency during the night, then again more frequently while feeding and moving in the morning before lying down (Johnson et al 1987).

This variation in defecation rates creates difficulties when using scats in the assessment of small-scale patterns of habitat utilization, such as the present study, which requires the assumption that defecation rate for macropods is when and where animals are feeding, or at least that defecation rate does not vary markedly with activity. Macropods in any given 24 hour period would be expected to spend more time in the bush compared to the lawn because of the cover the bush provides. Thus scats can be found in habitat where food is sparse.

An example of where macropod scat counts were used to identify preferred habitat is a south-west Queensland study. Here grey kangaroos (Macropus canguru) were

found to prefer a habitat with a thick cover more so than red kangaroos (Megaleia rufa, Desmarest) (Caugley 1964). In another northern Australian study of four species

areas in the moist habitats, but, in the dry rocky habitats, unburnt areas had higher scat abundance. This interactive effect of burning and habitat type on scat abundance was observed immediately (< 4 weeks) following fire, and was still present one year later.

For wombats a different method of estimating abundance than transect or grid square counts is required. Wombat scats are dropped around the entrance to burrows, placed on top of raised objects or around the base of trees. Other behaviours observed include preparing the ground for defecation by scratching away the grass cover or defecating in established latrines (Taylor 1993).

2.6.2

Scat collection methods

Scats were cleared at the start of the experiment (June 2005) from lawn and bush plots at both sites. Scats were collected monthly apart from the start of the experiment when the scats were collected after 10 (August 2005) and 7 (October 2005) weeks respectively. Mean monthly scat counts were calculated for these periods and for analysis and comparison across time were described as scat counts for July-August and September-October 2005.

The Central Plateau lawn site was inundated from August 2005 till February 2006. During this period only bush scats were picked up monthly. Lawn scats were again collected from February – May 2006, though on the May visit the lawn was about one third under water. By June 2006 the lawn was completely submerged and only bush scats were collected (Figure 2.6).

Scats collected were wallaby and pademelon (these were grouped for analysis and will be referred to as wallaby), wombat, rabbit and possum. Possum numbers were small and these were not used in the final analysis.

Initial surveys showed that lawn and bush wombat scat deposition behaviour was different for the two areas. On the lawn three latrines were found (only one of which was in a plot) while in the bush plots, scats were more uniformly distributed making use of any raised log or rock. Thus on the lawn the decision was made to collect the

scats from the whole lawn (80 × 40 m) to include the three latrines but from the bush, scats were collected only from the 20 5 × 5 m plots. For analysis the two areas were compared but with the acknowledgement that as the two areas were different dimensions the bush scat counts may be an underestimation.

Rabbits also generally deposit in latrines but this was not always evident on the lawns. Their scats were generally scattered across the 5 × 5 m plots, probably dispersed by wind or inundation. Rabbit scats were individually picked up and counted.

Figure 2.6. The Central Plateau site inundated October 2005. Square objects on water edge are exclosures covered in snow.

2.6.3

Scat decomposition

The rate of decomposition of scats in the field can vary within and between seasons and sites. To determine if monthly scat counts were a true estimate of animal activity, photo points were established in both bush and lawn plots at both sites. Fresh scats for this experiment were found in the morning following the clearance of scats in the plots the previous day. Photopoints were ‘in situ’ that is, fresh wallaby and wombat scats were left where found within the experimental area, marked and photographed. Several of the initial scat experiments were destroyed by enthusiastic field assistants collecting scats. At the Central Plateau four bush photopoints were established in December 2005 (one of these was accidently cleared), at this point the lawn was inundated and no photopoints were established on the lawn. At the Bangor site two bush scat photopoints and two lawn scat photopoints were established (one of each was lost). Trampling by animals was found to affect decomposition rates, so an

additional scat decomposition experiment using an exclosure (20 × 50 cm), was established on the south eastern edge of the lawn at the Central Plateau and Bangor sites in May 2006. Fresh scats (deposited at night in areas cleared the previous day) of wallaby, wombat, rabbit and possum (Bangor only) were collected and placed under the exclosure in species groupings, photographed and rephotographed June, July and August 2006 and in March 2007 and decomposition rate assessed by comparing number of intact scats, partially decomposed scats and missing scats.

2.6.4

Grazing exclusion experiment

To examine the impact of grazing pressure on woody seedling establishment on the lawn, an exclosure (X) was paired with a control (C) quadrat in each of the 20 randomly located lawn plots (5 × 5 m) and in 10 of the 20 bush plots (5 × 5 m). The exclosures were constructed from wire mesh (25 × 20 mm) with dimensions of 0.5 m

× 0.5 m and 0.2 m high, open on the bottom and held in place with aluminium pegs at each corner and at the midpoint along each side (Figure 2.7). The location of the paired exclosure and control quadrats on the lawn was determined by the presence of woody seedlings. Each plot was searched for woody seedlings which were then marked, numbered and randomly drawn for inclusion in the exclosure or control quadrat ensuring that these were not more than one metre apart. Where no seedlings were found exclosure and control quadrats were placed in the north east corner of the plot with the exclosure or control treatment randomly assigned.

Figure 2.7. Construction of wire exclosures (top left), control plots identified using 20 cm aluminium pegs with hooked top sunk into ground marked with flagging tape (top right). Lower photo, Bangor site showing placement of some of the wire exclosures on the lawn site.

To determine if understorey seedling regeneration was affected by browsing in the bush plots ten plots were randomly chosen to include in paired exclosure and control treatments. The location of the paired quadrats was as close to the north east corner as was physically possible avoiding any vegetation higher than the exclosure.

The experiment was established in June 2005 and species composition and height measured after ten months in March 2006 and after twenty months in February 2007. At the time of establishment the following were recorded in both lawn and bush quadrats: the number, species and height of woody seedlings, percent cover and height of herbaceous species, litter cover and bare ground (defined as ground that has no canopy cover when viewed from above). At the Bangor site Leptospermum lanigerum and L. scoparium, in some instances, appeared to have hybridized and

were difficult to distinguish at the seedling stage, so these were grouped and recorded as Leptospermum spp.

2.6.5

Productivity index and height of Schoenus nitens

An index of plant productivity in exclosure quadrats and control quadrats was calculated using the formula: height × cover, for each species, in each quadrat. The value for each quadrat in 2005 was then subtracted from the 2007 value to give a productivity index for each of the paired quadrats.

Schoenus nitens was a visually dominant species of lawn quadrats at Bangor. To

determine if there was a difference in height between edge and centre quadrats, the height of S. nitens was compared across the three years of sampling (2005, 2006 and

2007).