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with the grass could be examined.     In all cases in which the trans-
planted Halodule perished, rhizomes of Thalassia interdigitated the
sediment. The unprotected plug which persisted was located in a small
clearing and the sediment under it was free of Thalassia rhizomes. The
competitive mechanism thus appears to require root-root interdigita-
tion.
Two sets of disturbance experiments were performed (one inadvertently
and one by design).     The Thalassia bed stations were established 4
meters into the Thalassia bed from the border. Sampling was done on
all visits to the site in 1979 and 1980 at high tide. In August of
1981, the site was visited during a spring low tide (0.06 m below mean
tide level).   A tongue of Halodule was found to extend from the old
border zone 2 meters towards the 1979 station and 4 meters towards the
1980 station.   Apparently, foot-traffic generated by sampling in 1979
and 1980 damaged either the rhizome system and/or the blades suffi-
ciently to allow invasion by Halodule and to cause a noticeable
decrease in the density of Thalassia.
The results   of the herbivory     (cropping) experiment are clear
(Figure 2).     Cropping resulted   in an increase of Halodule in the
Thalassia bed and softened the sharp decline at the border between the
beds. Conversely, the extent of Thalassia in the Halodule bed declined
and the border between the beds sharpened for ThalassiaT.Statistically
(Newmans-Keuls Multiple Range Test at ot = 0.05), there was no differ-
ence between Halodule in control and cropped area in the Halodule bed
while Halodule    numbers in the cropped area were greater than the
control samples in the Thalassia bed.       For Thalassia, there was a
significant difference (a decrease) between the control and the cropped
areas in both beds.     The patterns derived from analysis of biomass
paralleled all of the above and are not presented here.
DISCUSSION
Seagrass distributions have long been of interest.    One prominent
feature of subtropical seagrass beds is the zonation of Halodule
wrightii and Thalassia testudinum.     Just as in algal systems wh-ere
physiological limitations were first postulated to control zonation and
only later were the roles of biological interactions appreciated
(Lubchenco 1980), seagrass zonation patterns have been attributed to
physiological limitations (Strawn 1961).     Suggestions have been made
that the relationship might be competitive (Phillips 1960; den Hartog
1970). As Dayton (1973) has strongly asserted, assuming the operation
of an ecological mechanism (competition, predation, etc.) solely on the
basis of descriptive and correlative data can be highly misleading and
experimentation is often required for verification.
Desiccation has often been identified as the factor controlling
the upper limit of seagrasses (den Hartog 1970; Hunin 1956; Keller &
Harris 1966; Moore 1963; Phillips 1960, 1962).        At Seahorse Key,
desiccation (drying) and head stress are typically paired and all heat
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