Monopolization of litter processing by a dominant land crab on a tropical oceanic island

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) 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.

Ecological and socioeconomic impacts of invasive alien species in island ecosystems

“…The island has been well known internationally for its dense populations (1 per m 2 ) of native land crabs (Gecarcoidea natalis), which migrate annually to the ocean and are dominant consumers of rainforest detritus (Green et al 1999). The yellow crazy ant was unintentionally introduced between 1915 and 1934, but has spread rapidly in the last decade.…”

Leaf Breakdown in a Tropical Headwater Stream (Puerto Rico): The role of Freshwater Shrimps and Detritivorous Insects

“…Freshwater shrimps and other decapods (e.g., crabs) are important components of streams in tropical ecosystems near coastal areas and on tropical islands (Green et al 1999, Crowl et al 2001, March et al 2001. For example, freshwater shrimps can influence the amount of fine particles suspended in the water column, either by generating particles from coarse organic matter or by removing them through filtration of the water column (Covich 1988, Crowl et al 2001.…”

Recruitment dynamics in a rainforest seedling community: context-independent impact of a keystone consumer

“…On Christmas Island, in the Indian Ocean, forest Xoor dynamics are dominated by a single omnivorous land crab, Gecarcoidea natalis. This generalist consumer occurs across all major forest types, where it includes a wide variety of seeds and seedlings in its diet Lake 1990, 1991) and, in an exclusion experiment at a single site, has been shown to regulate seedling recruitment and litter dynamics (Green et al , 1999. Here we broaden this assessment in three ways.…”