Mj. Keough et Gp. Quinn, EFFECTS OF PERIODIC DISTURBANCES FROM TRAMPLING ON ROCKY INTERTIDAL ALGAL BEDS, Ecological applications, 8(1), 1998, pp. 141-161
We investigated the ability of an assemblage of animals and plants on
rocky shores in southeastern Australia to resist and/or recover from r
epeated pulse disturbances in the form of trampling. Disturbances of f
our different intensities were applied experimentally over six summers
, with no human access at other times of the year. The dominant intert
idal plant, the brown alga Hormosira banksii, was affected by tramplin
g, but the effects were heterogeneous between sites. At two sites, a s
eries of pulse disturbances produced a series of pulse responses, alth
ough the effect of a given pulse varied among years, possibly related
to the severity of summer desiccating conditions each year. At the thi
rd site, pulse disturbances produced a press response; at high levels
of trampling, Hormosira was almost eliminated within 2 yr, and at two
intermediate levels of trampling, cover was reduced from >90 to 60-70%
, where it remained for 4 yr. Effects of trampling showed little small
-scale spatial variation. Untrampled areas did fluctuate through time,
often as a result of summer burnoff of algae. Natural disturbances oc
curred irregularly through the study, and their effects varied on very
small spatial scales (among plots <30 m apart). Trampling enhanced th
e densities of a range of herbivorous mollusks, especially limpets, an
d reduced the abundance of articulated coralline algae, which were abu
ndant in the understory of Hormosira mats. These effects varied among
sites but showed much less variation on smaller spatial scales. The re
ductions in coralline algae may be a direct effect of trampling, but i
ncreases in mollusk abundance occurred some time after changes to Horm
osira cover, and those changes may be an indirect effect of trampling.
We compared the effects of trampling on areas of the shore that had b
een trampled for two and four summers, to test whether a past history
of disturbance influenced the effect of a new disturbance. No signific
ant effects were found on algae or mobile animals, although a mild sum
mer may have made our test of history relatively weak. Hormosira banks
ii fits the definition of a keystone species or engineer and, as such,
is an appropriate focus for management and as an indicator. Spatially
heterogeneous effects of a constant physical perturbation, however, m
ean that management of these rocky shores requires more complex models
and indicate that caution should be used in adopting this species as
a uniform indicator of environmental change.