Monopolization of litter processing by a dominant land crab on a tropical oceanic island |
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Authors: | Peter T Green P S Lake Dennis J O'Dowd |
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Institution: | (1) Department of Biological Sciences, Monash University, Clayton, Victoria 3168, Australia, AU;(2) c/o Tropical Forest Research Centre, CSIRO, P.O. Box 780, Atherton, Queensland 4883, Australia e-mail: peter.green@tfrc.csiro.au, Tel.: +61-7-40918828, Fax: +61-7-40913245, AU |
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Abstract: | Litter processing by macroinvertebrates typically involves suites of species that act together to determine rates of breakdown
and decomposition. However, tropical oceanic islands and coastal fringes on continents are often dominated by one or a few
species of omnivorous land crabs that consume leaf litter. We used an exclusion experiment, together with other leaf removal
and litter decomposition studies, to assess the role of a single dominant species, the red crab (Gecarcoidea natalis), in litter dynamics in rain forest on Christmas Island, Indian Ocean. In the presence of red crabs, litter cover and biomass
varied seasonally, from almost complete cover and high biomass at the end of the dry season to almost total absence of litter
at the end of each wet season. When crabs were excluded from both the shaded understory and light gaps in rain forest, litter
increased rapidly to almost complete cover, which was then maintained year round. Leaf tether experiments, and measures of
litter input and standing crops, indicated that red crabs monopolize litter processing, removing between 39 and 87% of the
annual leaf fall from the forest floor. Rates of litter turnover were over twice as high in the presence of land crabs: the
decomposition constant, k, was 2.6 in the understory exclusion plots, but rose to 6.0 in the presence of crabs. Red crabs occur at biomass densities
(114 g m−2) far greater than those reported elsewhere for entire litter faunas. They significantly reduced the abundance of other litter
invertebrates, but we did not detect any change in the relative frequencies of the major invertebrate groups (mites, collembolans,
pulmonate snails, ants, psocopterans, and spiders). Wherever omnivorous land crabs are abundant, their activities may be paramount
in litter decomposition and in regulating the rate of nutrient cycling. In monopolizing litter processing, they may also be
important physical “ecosystem engineers”, translocating organic matter and nutrients into the soil and reducing available
habitat for other animals.
Received: 19 August 1998 / Accepted: 11 January 1999 |
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Keywords: | Island Land crabs Litter processing Invertebrates Tropical rain forest |
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