In sessile marine communities, competition for space is thought to be especially fierce and is
believed to have a strong impact on community structure. While organisms compete for space in a
variety of ways, one of the most common means of competing for space is via overgrowth.
Analyzing patterns of overgrowth provides a means of measuring competition and determining the
relative competitive abilities of organisms. In temperate regions, the ranking of competitive
abilities of organisms has often been observed to be hierarchical, however some tropical regions
have shown the existence of transitive networks. A transitive network is a “loop” in a basically
linear ranking of relative competitive abilities in which species A > species B > species C but
species C > species A. Many studies have examined competitive interactions among sessile
organisms, although a large proportion of these studies have been done in temperate and polar
systems with fewer studies done in tropical regions. Of the studies done in tropical systems, the
examination of competitive interactions amongst corals has been especially prominent.
This study examines interspecific competition amongst 85 species of ascidians, 59 species of
sponges, and 27 species of bryozoans living on undersides of coral boulders on the sheltered,
northern, side of One Tree Reef, Great Barrier Reef, Australia. A total of 316 boulders was
randomly sampled at 9 study sites that were located at three different heights on the reef crest.
Digital images were taken of the undersides of each boulder and examined using ImageTool
software. For each competitive interspecific interaction observed, the identity and size (area) of each
species involved was recorded, and the outcomes of the interspecific interactions were documented.
Outcomes of interactions were classified as win (overgrowth), loss (overgrown), or tie (cessation of
growth). Chi-square goodness of fit tests (Siegel 1956) (2 x 3.841, df = 1, p<0.05) were used to
determine which species were significant winners (competitively dominant), losers (competitively
subordinate), or were competitive equals (having no significant difference in competitive abilities).
The potential influence of relative size on the outcomes of competitive interactions was also
There were 1714 total interspecific interactions recorded among species of ascidians, species
of sponges, and species of bryozoans. Ascidians, sponges, and bryozoans were shown to win 82.3%,
23.1%, and 9.7% of their interactions, respectively, when competing between major taxonomic
groups, indicating a clear hierarchy of competitive abilities (ascidians > sponges > bryozoans).
However, the presence of occasional reversals in the outcome of competitive interactions
indicates that while the community was basically hierarchical in structure, there were instances inwhich individual species did not follow this hierarchical order (i.e. some typically inferior species
outcompeted typically superior species). For instance, the 8 species of bryozoans that were
examined in more detail were observed to lose the majority (91.0%) of their 389 interactions when
competing amongst ascidians, while winning only 15 interactions (3.9%). Similarly, the 8 species of
sponges examined more closely lost the majority (83.2%) of their 208 interactions against ascidians
and won only 13 interactions (6.3%).
Examination of the influence of relative size on the outcome of 1705 competitive interactions
showed that larger species won 583 interactions (34.2%), lost 691 interactions (40.5%), and tied 431
interactions (25.3%) when competing against smaller species. Although smaller species
outcompeted larger species more frequently, ascidians accounted for the majority of these wins, and
smaller ascidians were shown to win the majority (49.8%) of their 1197 interactions. In general,
species of ascidians, sponges, and bryozoans tended to be more successful competitively when they
were larger. Relative size clearly had some impact on the outcome of competitive interactions,
although it did not have enough of an impact to alter the hierarchy of relative competitive abilities as
ascidians > sponges > bryozoans.
The hierarchical structure of this assemblage and the relatively few instances of ties and
reversals suggests that competition may not be the primary factor influencing community structure
because a large number of species persist in the community (i.e. there did not appear to be signs of
competitive exclusion by the competitively dominant species). At One Tree Reef, other factors,
including predation and disturbance, may play a stronger role in affecting levels of species diversity,
allowing large numbers of species to co-exist. For instance, the observation of disturbance (wave
action) is likely to shift coral boulders and may keep space open. Unlike many temperate and
tropical systems, large amounts of free space were observed on boulders at One Tree Reef, and
disturbance may be a contributing factor maintaining adequate space for colonisation and the
persistence of a very diverse community.