Económico ambiental
III. El caso del litio en el Cono Sur
The ‘in situ’ and cage experiments and photopoints confirm that untrampled rates of scat decomposition are such that monthly scat counts provided a reasonable estimate of wallaby, wombat and rabbit activity at both sites. When not protected by an exclosure scats deposited along bush tracks and on the lawn could disappear quickly if trampled. It is estimated that up to 10% is lost to trampling but this would equally affect both lawn and bush plots. Rates of decay were expected to be faster at Bangor given the warmer conditions which presumably would encourage increased invertebrate activity in the decomposition process (Southwell 1989). However, warm conditions may be negated by hot dry conditions ‘baking the scats’ at Bangor which would work against rapid decomposition even after rain. In the caged experiments (May 2006 to March 2007) the Bangor rates of decomposition (Figure 2.15) appeared to be similar to the Central Plateau (Figure 2.14) where the cold conditions were expected to reduce rates of decay. Johnson and Jarman (1987) in their study found that scats in warm, moist conditions in short grass decayed faster than in other habitats, but this was not verified in the present study. It was the trampling of scats that caused the most deterioration.
In spite of the limitations in using scat counts to determine grazing pressure and preferred habitat, the scat data is unequivocal in confirming that the lawn and bush plots are subject to year round grazing pressure by marsupials and rabbits, except when under water. Even under water may not be ‘out of bounds’ for marsupial grazers. At Narawntapu N.P. (see Chapter 4, Figure 4.1 for location), Stewart Blackhall (Wildlife Biologist, Department of Primary Industries and Water, Hobart) reported (pers. com) watching a wallaby wade into an inundated area of the grazing lawn and scoop up with its paws floating vegetation, which it then ate.
2.16.3 Exclosure experiment
The exclosure experiment demonstrated that grazing pressure prevented taller vegetation from dominating lawn vegetation. This was most marked at the Bangor site where shrubs and tree seedlings in the exclosures increased up to 24 cm in height
relative to those outside, while at the same time significantly more seedlings established in the exclosure quadrats than in the controls (Table 2.5). These findings also negate the competition hypothesis that woody species are out-competed by the more aggressive lawn species. There was no increase in the number of seedlings in the bush quadrats where the source of seed would be high, perhaps due to lack of light and to competition with existing trees and shrubs for resources (Table 2.5). Slower growing species, at the higher altitude (1150 m asl), Central Plateau site produced less dramatic results. While the woody seedlings in the exclosure quadrats did not significantly increase, seedlings excluded from grazing grew three times taller than those in control quadrats (Table 2.5).
In March 2007 dozens of Ozothamnus hookeri seedlings were found to have
germinated on one part of the lawn, following a drier spring and hotter summer than the previous year. The fact that the seedlings were able to germinate in the lawn sward, and in such numbers, suggests that competition was not the reason for lack of recruitment on the lawn. O. hookeri is a prolific seed producer and the seeds are
wind-dispersed (personal observation), so unlike Leptospermum lanigerum which
would not easily travel far from the parent bush, O. hookeri could easily disperse to
the centre of the lawn by wind or float across it when inundated.
Explanations for the lack of O. hookeri germination in the exclosures in 2005/6
include viability of O. hookeri seeds is short lived, dormancy has not been broken or
they do not tolerate waterlogging. A major part of the lawn was waterlogged for nearly eight months in the first year of sampling. Seeds released onto the lawn in late autumn of 2005 would not germinate till the following spring. By late spring (Nov. 2005) at Central Plateau the lawn was underwater and a major part of it would remain so till February 2006. Early summer rainfall in late 2005 was the highest ever recorded resulting in extremely wet conditions. It is possible that seeds could float to higher ground to germinate when the water on the lawn rose but this seems unlikely here as the seeds from autumn would have bedded in by the time the lawn became inundated. The lawn area, where woody seedlings were found in March 2007, is slightly elevated compared to the rest of the lawn and had been moist, but not
inundated, in the previous spring, indicating that microsite differences may be important for woody seedling establishment here.
At both sites, the seedlings that successfully established in either the exclosures or controls were in areas immediately adjoining the bush-lawn boundary (zone 1, Figure 2.3 and Figure 2.5) indicating that successful establishment is related to distance from bush edge.
With reference to the first hypothesis (i.e. that vertebrate grazing maintains the boundary), the exclosure experiment unequivocally confirms the impact of grazing on vegetation, through suppression of woody species establishment and growth on the lawns. With grazing removed, woody species were clearly able to flourish. The second hypothesis that environmental factors alone are responsible for the establishment of lawns was not unequivocally supported. While the results of the exclosure and shadehouse experiments discount the hypothesis that environmental variables are solely responsible, the interacting effects of grazing and environmental factors, as explained further below, may be critical in explaining the lawn phenomenon.