* To whom correspondence and reprint requests should be addressed. Tel: 886-2-27899519. E-mail:rqjan@gate.sinica.edu.tw In 2000, when the sea anemone, Mesactinia ganesis, was rare on fringing reefs in an embayment in southern Taiwan (21'57°N; 120'45°E) (Jan et al. 2007), bluntsnout gregory Stegastes lividus (Bloch and Schneider) (Fig. 1a) and dusky gregory, S. nigricans (Lacepede) (Jan et al. 2003) had long-established territories on coral branches on the reef flat. An outbreak of the sea anemone has occurred in recent years in this embayment. Colonies of branching corals on the slanting reef surface were eliminated by the spreading M. ganesis. Strong territoriality of the damselfishes has slowed down the elimination process, as remaining coral colonies can still be found within their territories (Fig. 1b). Nevertheless, currently more than 80% of the former damselfish territories have already disappeared from this part of the reef (Fig. 2). Some holders emigrated to the lower part of a near-by reef, competing for new territories on the few subsisting coral colonies (Fig. 1c). In parallel, relic coral skeletons from abandoned damselfish territories have been taken over by Dick's damsel, Plectroglyphidodon dickii (Lienard), as nesting substrate. This observation indicates that the sea anemone outbreak has resulted in drastic changes in the demography of habitat specialist in fishes. While the outbreak has shown no signs of recession, the fish community, along with other fouling organisms, will likely remain unstable in this embayment.
Summary 1. River food webs rely on two major food sources: autochthonous primary production within the river and allochthonous organic matter transferred to the river. We characterised the consumer communities and assessed the food sources of dominant consumers along a subtropical mountainous river (the Lanyang River of north‐eastern Taiwan) at the catchment scale from the headwater to the estuary using natural abundances of stable carbon and nitrogen isotopes. 2. The downstream transport of fine particulate organic matter (FPOM) was two orders of magnitude greater than that of coarse particulate organic matter (CPOM). Transport of both materials increased from the headwater and reached a maximum in the midstream reach. CPOM composition exhibited a gradual shift from leaves and branches in the headwater, an area characterised by high canopy cover, to algae in the midstream reaches and marsh plants in the downstream reaches. 3. Consumer communities can be classified into two regional categories: the upland category in the headwater and upstream and midstream reaches and the lowland category comprised of samples from the downstream reach and estuary. The upland category revealed a clear and gradual seasonal shift in community composition, but a seasonal shift was not apparent for the lowland category. Nutrient concentrations and water temperature were the main factors explaining longitudinal and seasonal variations. 4. The use of sources of organic matter by dominant consumers along the Lanyang River was primarily determined by their availability. Riparian C 3 plants were the major food sources in the headwater, upstream reach and estuary, but the contribution of periphyton increased in the upper midstream reach where the river flows through an agricultural area. In the lower midstream and downstream reaches, the contribution of riparian C 4 plants became dominant. 5. The trophic transfer of organic materials in the Lanyang River may be influenced by the fast current velocity and by sewage nutrient loading in the river, both of which have important implications for predicting how the functioning of subtropical river food webs will respond to human‐related changes in land use.
On the west bank of the intake bay of a nuclear power plant at Nanwan, southern Taiwan, a recent outbreak of sea anemones has eliminated most branching corals at the site. In this habitat currently dominated by sea anemones, the abundance of Dick's damsel, Plectroglyphidodon dickii Lienard 1983, has increased more markedly than the sympatric congener, P johnstonianus Fowler and Ball 1924. To study whether the distribution patterns of the 2 Plectroglyphidodon damselfishes were underlain by food availability and feeding habit plasticity, the reef area in the embayment was divided into 2 different zones, zone A dominated by Acropora corals and zone B dominated by a sea-anemone, and both stomach content and stable isotope analyses were used to delineate feeding habits of these 2 damselfishes in these 2 zones. Stomach contents showed that the major food items for P. dickii in zone A were filamentous algae (33.4%) and coral polyps (22.3%), while in zone B, they were filamentous algae (35.8%) and sea anemones (28.2%). In contrast, coral polyps were the major food item for P johnstonianus in both zones (accounting for 75.5% in zone A and 67.5% in zone B). Analyses of δ 13 C and δ 15 N values concurred that coral polyps were the major nutrient source of P johnstonianus. In addition, the contributions of both coral polyps and sea anemones to P. dickii were verified. Overall, it was postulated that in the sea anemone-dominated habitat, sea anemones were substituted for coral polyps in the diet of P. dickii, but similar plasticity did not occur in P. johnstonianus. Thus the higher feeding plasticity of P dickii might have made it possible to more-successfully immigrate to the newly developed habitat.
To test the effect of food abundance and intruder pressure as determinants of territory size, the dusky gregory Stegastes nigricans were used as subjects on a coral reef in southern Taiwan during November to December 2000. Adults were used as intruders to provoke aggressiveness in a conspecific territory owner. The owner's maximum distance of attack (MDA) was used to delineate the territory size. While the owner of a territory appeared to defend a single boundary against different conspecifics, size variations among territories were evident. The effect of an intruder's identity on territory‐size regulation was not clear because for each territory examined, the MDA was found to be neither linked to the body size of the intruder nor to the amount of algae in the intruder's territory. Moreover, no significant differences were found between the MDAs that the owner maintained against neighbours and non‐neighbours even though, when conspecifics intruded in pairs, the probability was significantly higher for the first attack to be launched on a neighbour than on a non‐neighbour. Also when a neighbour and a non‐neighbour appeared simultaneously near the territory, the bite rate against the neighbour was also significantly higher. An inverse relationship between the amount of algae in the defended territory and the MDA of the owner indicates that food abundance might account for variations among territories. By contrast, territory size was not linked to the body size of the owner.
Recreational scuba diving is a popular activity of the coral reef tourism industry. In practice, local diving centers recommend interesting sites to help visiting divers make their plans. Fish are among the major attractions, but they need to be listed with care because the temporal occurrence of a fish species is difficult to predict. To address this issue, we propose methods to categorize each fish species based on its long-term occurrence and likelihood of being seen. We assume that there are K categories of occurrence of a fish assemblage and propose two methods [an arithmetic-mean method (AM) and a geometric-mean method (GM)] to define the range of species in each category. Experiments based on long term datasets collected at three underwater stations (each having 51–53 surveys and totals of 262–284 fish species) on coral reefs in southern Taiwan showed that when K = 4 (rare, occasional, frequent and common categories), 11–14 species were concurrently assigned to the common category by AM for data sets based on surveys 10, 15, 20, 25, 30, 35, 40, 45, or 51–53 in contrast to the 18–26 species assigned as common by GM. If a similarity index of 0.7 (compared to the total pool of fish species) was the minimum threshold for diver satisfaction, then 20–25 surveys provide sufficient data for listing the common species at a given dive spot. Common fish species, are the most temporally stable, and thus are more appropriate for attracting divers. These can be effectively differentiated by either AM or GM with at least 25 surveys. We suggest regular updating of each fish’s category through periodic surveys to assure the accuracy of information at a particular dive spot.
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