Spatial competition among clonal organisms in extant and selected paleozoic reef communities |
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Authors: | Dr J Alfred Fagerstrom Dr Ronald R West Dr Stephen Kershaw Dr Patrick J Cossey |
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Institution: | (1) Dept. of Geology, University of Canterbury, Christchurch, New Zealand;(2) Centre de Recherches et Observatoire de l'Environment-CRIOBE-EPHE, URA CNRS 1453, Moorea, France;(3) Dept. of Geology, Kansas State Univ., 66506-3201 Manhattan, Kansas, U.S.A.;(4) Dept. of Geography and Earth Sciences, Brunel Univ., UB8 3PH Uxbridge, Middlesex, U.K.;(5) School of Sciences, College Rd., ST4 2DE Stoke-on-Trent, U.K. |
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Abstract: | Summary Occurrences of densely packed benthic organisms in extant reefs are of two types: 1) live-live interactions, where two living
organisms interact, and 2) live-dead associations, where only one is alive and uses the other as a substrate. The latter are
common in reef deposits due to biostratinomic feedback, i.e. dense skeletal accumulations provide hard substrates for clonal
recruitment, thus facilitating greater frequency of live-dead encounters than in lower biomass level-bottom communities dominated
by solitary organisms. Differentiating between these two types in ancient reefs is difficult, often impossible.
Most live-live interactions among clones in extant reef communities involve competition for space. Clonal spatial competition
is divisible into four types: 1) direct-aggressive: encrusting overgrowth; 2) indirect-passive: depriving neighbors of resources,
chiefly sunlight, by growth above them; 3) stand-off: avoidance of competition by organisms adopting positions that avoid
or minimize direct polyp/zooid contact; and 4) overwhelming: one clone/ species volumetrically or numerically overwhelms the
other, meeting minimal resistance. Despite class-order level differences in taxa, our results indicate that extant analogs,
based on the arrangement and distortion of skeletons, are valuable for recognizing live-live interactions in Silurian and
Carboniferous reefs and interpreting the types of spatial competition represented.
Comparison of overhead (plan) views of live-live coral competition in Polynesian reefs with vertical sections of Silurian
and Carboniferous sponge-dominated reefs and biostromes suggests that direct-aggressive competition is more common among extant
than among Paleozoic reef-builders. Stand-offs showing clone margin distortion and overwhelming with minor skeletal distortion
are most common in our fossil examples and probably relate to the dominance of these reefs by sponges. Success by extant sponges
in spatial competition is largely due to allelochemical deterrence which may explain the predominance of stand-off and overwhelming
confrontations in fossil sponges rather than tentacle-mesentery based direct aggression among extant corals and bryozoans. |
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Keywords: | Competition Spatial Reefs Ecology Corals Sponges Skeletal Effects Polynesia/Gotland/England/Kansas Extant/Carboniferous/Silurian |
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