Peter T. Green

CONTROL OF SEEDLING RECRUITMENT BY LAND CRABS IN RAIN FOREST ON A REMOTE OCEANIC ISLAND

We tested the hypothesis that an omnivorous land crab determines the dy- namics of seedling recruitment in rain forest on Christmas Island, Indian Ocean. The red crab (Gecarcoidea natalis) occurs at densities of -1 crab/M2 and consumes seeds, seedlings, and leaf litter. A crab-exclusion experiment below closed canopy and in light gaps showed that red crabs markedly reduced seedling recruitment. Overall, seedling emergence was 29-fold and 21-fold greater on the exclusion plots in the understory and in gaps, respectively. In control plots, mean seedling densities never exceeded 15 seedlings/25 m2 in the under- story and 3 seedlings/25 m2 in gaps. Species richness of seedlings was significantly higher in the absence of crabs, averaging >10 and 12 species per plot in the understory and gaps, respectively. On control plots, mean species richness was less than one species per plot. Two-dimensional ordination (using nonmetric multidimensional scaling) showed that red crabs had a large impact on relative species abundances. Analyses of similarity confirmed that significant differences existed between both treatments and habitats. Seed removal and seedling transplant experiments suggested that most rain forest species are rapidly eaten by red crabs. Seedlings of many of these vulnerable species (e.g., Maclura cochinchinensis, Planchonella nitida, Schefflera elliptical were abundant on exclusion plots. However, they were rare on control plots, and none persisted for >2 mo. Persistent seedlings were limited to two species (Inocarpusfagifer and Tristiropsis acutangula), both of which were resistant as seeds and seedlings to crab predation. Although red crabs are the major biotic deter- minants of seedling recruitment, this impact was not reflected in the overhead canopy. Some tree species unable to recruit in the presence of crabs were common as adults. Shifting mosaics in crab densities over space and time may offer the best explanation for recruitment of vulnerable species. Our results show that a dominant consumer has large effects on plant community structure on a remote oceanic island, consistent with the viewpoint that strong control by a single species is more likely in simple communities.

Red crabs in rain forest on Christmas Island, Indian Ocean: activity patterns, density and biomass

The red crab Gecarcoidea natalis is the most abundant land crab on Christmas Island, Indian Ocean, and is found wherever native rain forest is still intact. The largest crabs grow to > 120 mm carapace width and reach > 500 g in mass. Red crabs are diurnally active, and moisture appears to be the most important factor governing surface activity - activity was nil below 77% RH, relatively low at 87% RH, and high above 95% RH. The crabs retreat to the interior of their burrows during dry periods, but emerge rapidly in response to rain. Because of their dependence on moisture for surface activity, red crabs show greater levels of activity during the wet season. Annual breeding migrations and moulting activity decreased the density of surface-active crabs at the main study sites for several weeks at the start of the wet season, and fruit and leaf fall caused dramatic local increases in the density of active red crabs. Density and biomass at the main study sites averaged 1.3 crabs m-2 and 1454 kg ha-, respectively, and 1.2 crabs m-2 and 1137 kg ha- at a total of five sites across the island. Rough calculations suggest that the island-wide population is at least 100 million individuals. Red crabs are the dominant consumer on the forest floor, and consume seeds, seedlings and leaf litter. Because they are both abundant and widespread, these animals have the potential to be the most important determinant of pattern and process in rain forest on Christmas Island.

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 (114u2009gu2009m−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.

Canopy gaps in rain forest on Christmas Island, Indian Ocean: size distribution and methods of measurement

The role of light gaps in the dynamics of tropical forests has received considerable attention in the last 20 years, and has contributed to an understanding of the role of disturbance in shaping natural communities (Platt & Strong 1989). Many studies have focused on the responses of various plant species to gap environments, and how these responses contribute to structure and species diversity in tropical forests [see reviews by Brokaw (1985), Denslow (1987) and Whitmore (1988)]. In all of these studies, the effect of gap size on gap-phase dynamics has been emphasized. In very small gaps, the microclimate is similar to that in the adjacent understorey, whereas in large gaps it approximates conditions above the forest canopy (Whitmore 1990). This variation partly determines the composition and abundance of the seedling pool that can establish in gaps (e.g. AlvarezBuylla & Martinez-Ramos 1992, Barik et al. 1992, Brokaw & Scheiner 1989). Denslow (1987) considered that to the degree that gap size is a good index of resources and microclimate, the gap size-frequency distribution of a forest can be a useful basis for comparing ecosystems. Brokaw (1982a) defined a gap as any hole extending through all layers of the surrounding vegetation to within 2 m of the ground, and measured gap area with distance/direction coordinates from some central position within the gap. This is perhaps the mostly widely used method of measuring gap area, but it

Burrow Dynamics of the Red Land Crab Gecarcoidea Natalis (Brachyura, Gecarcinidae) in Rain Forest on Christmas Island (Indian Ocean)

Abstract This study quantified the burrow dynamics of the gecarcinid land crab Gercarcoidea natalis (Pockock, 1888), the most common land crab in rain forest on Christmas Island, Indian Ocean. Mean burrow density remained relatively stable over a two-year period, fluctuating between 1.4 and 1.6 burrows/m2. Burrows were long-lived, stable structures, with a mean turnover time estimated at least 4.4 ± 1.6 y, and probably greater than 5 y. Burrow entrances were also highly stable in space. Two thirds of burrow entrances were repositioned within an area of 20 cm2 or less, and 83% of entrances moved within less than 60 cm2. Red crabs accumulate litter around their burrow entrances and create nutrient hotspots in the soil around burrow entrances, whose stability and longevity suggested that germinating seedlings may derive some benefit from germinating in the immediate vicinity of crab burrows. The rate of burrow turnover is slow, causing minimal soil disturbance at the study site with little, if any, impact on above-ground plant productivity. Red crabs plugged their burrows intermittently year-round with loose wads of leaves or with soil. Major plugging peaks were recorded in March of both years of the study, midway through the wet season. At this time 49–59% of burrow entrances were sealed with a deep plug of soil, and many burrows remained plugged long enough for the crabs to molt. These data suggest that most red crabs molted annually at the study site.

The breeding distribution and status of Abbott's booby (Sulidae: Papasula abbotti) on Christmas Island, Indian Ocean

Abstract The breeding distribution and population status of the endangered seabird Abbotts booby (Sulidae: Papasula abbotti (Ridgway) Olson & Warheit, 1988) were assessed by an island-wide survey on Christmas Island (Indian Ocean) in 1991. The species is now endemic to the island. A total of 1833 breeding sites were located, and it is estimated that the total breeding population is around 2500 pairs, 600 pairs more than the most recent estimate of 1900 pairs in 1983. We do not interpret the difference as a genuine increase in the breeding population — instead, we attribute it to the discovery of areas on the island where Abbotts boobies have probably always nested, but have never been recorded. Although this result has alleviated immediate concern for the future of P. abbotti , fields cleared for phosphate mining prior to 1987 still pose a continuing threat to the species — 36% of the population still nest near enough to clearings to be adversely affected by them. The Australian Nature Conservation Agency is currently rehabilitating mined fields to reduce their impact on the breeding population, and to eventually create a new breeding habitat for P. abbotti .