Competitive ability and the potential for lotteries among territorial reef fishes

Effects of competition on the territorial behaviour of a farmer damselfish, Plectroglyphidodon lacrymatus (Perciformes: Pomacentridae)

Aggression and territory size of the farmer damselfish Plectroglyphidodon lacrymatus were examined in the presence of adjacent territories of conspecific and heterospecific farmers that maintain algal turf territories on the reefs of Anda, north-western Philippines. Plectroglyphidodon lacrymatus territories with no adjacent neighbours had a mean size of 1.2 m² whereas those adjacent to conspecifics were 47% smaller. In contrast, the mean size of territories adjacent to a heterospecific farmer (i.e., Pomacentrus burroughi, Pomacentrus chrysurus) did not differ from that of solitary individuals. Aggression towards conspecific farmers (84% probability) was higher than heterospecific farmers (31%). The higher aggression towards conspecific farmers suggests that the energetic costs for defence increased, which may have caused a decrease in territory sizes. Despite the higher aggression towards conspecific farmers, the thickness and percentage cover of algae inside the territories of focal fish did not appear to vary according to type of neighbouring farmer although algal turf cover was higher and juvenile coral occurrence was lower inside than outside the territories. Our results show that the territorial behaviour of P. lacrymatus varies with conspecific and heterospecific farmers, which may influence their distribution and benthic habitat characteristics.

Habitat preferences of two sparrow species are modified by abundances of other birds in an urban environment

Every species has certain habitat requirements, which may be altered by interactions with other co-occurring species. These interactions are mostly ignored in predictive models trying to identify key habitat variables correlated with species population abundance/occurrence. We investigated how the structure of the urban landscape, food resources, potential competitors, predators, and interaction between these factors influence the abundance of house sparrow Passer domesticus and the tree sparrow P. montanus in sixty 25 ha plots distributed randomly across residential areas of the city of Poznań (Poland). The abundance of the house sparrow was positively correlated with the abundance of pigeons but negatively correlated with human-related food resources. There were significant interaction terms between abundances of other urban species and habitat variables in statistical models. For example, the abundance of house sparrow was negatively correlated with the abundance of corvids and tree sparrows but only when food resources were low. The abundance of tree sparrows positively correlated with density of streets and the distance from the city center. The abundance of this species positively correlated with the abundance of corvids when food resources were low but negatively correlated at low covers of green area. Our study indicates that associations between food resources, habitat covers, and the relative abundance of two sparrow species are altered by the abundance of other urban species. Competition, niche separation and social facilitation may be responsible for these interactive effects. Thus, biotic interactions should be included not only as an additive effect but also as an interaction term between abundance and habitat variables in statistical models predicting species abundance and occurrence.

Immigration Rates during Population Density Reduction in a Coral Reef Fish

Although the importance of density-dependent dispersal has been recognized in theory, few empirical studies have examined how immigration changes over a wide range of densities. In a replicated experiment using a novel approach allowing within-site comparison, we examined changes in immigration rate following the gradual removal of territorial damselfish from a limited area within a much larger patch of continuous habitat. In all sites, immigration occurred at intermediate densities but did not occur before the start of removals and only rarely as density approached zero. In the combined data and in 5 of 7 sites, the number of immigrants was a hump-shaped function of density. This is the first experimental evidence for hump-shaped, density-dependent immigration. This pattern may be more widespread than previously recognized because studies over more limited density ranges have identified positive density dependence at low densities and negative density dependence at high densities. Positive density dependence at low density can arise from limits to the number of potential immigrants and from behavioral preferences for settling near conspecifics. Negative density dependence at high density can arise from competition for resources, especially high quality territories. The potential for non-linear effects of local density on immigration needs to be recognized for robust predictions of conservation reserve function, harvest impacts, pest control, and the dynamics of fragmented populations.

The Prevalence and Importance of Competition Among Coral Reef Fishes

Although competition is recognized as a core ecological process, its prevalence and importance in coral reef fish communities have been debated. Here we compile and synthesize the results of 173 experimental tests of competition from 72 publications. We show that evidence for competition is pervasive both within and between species, with 72% of intraspecific tests and 56% of interspecific tests demonstrating a demographically significant consequence of competition (e.g., a decrease in recruitment, survival, growth, or fecundity). We highlight several factors that can interact with the effects of competition and make it more difficult to detect in field experiments. In light of this evidence, we discuss the role of competition in shaping coral reef fish communities and competition's status as one of several processes that contribute to species coexistence. Finally, we consider some of the complex ways that climate change may influence competition, and we provide suggestions for future research.

Are conspecific and heterospecific opponents assessed similarly? A test in two species of territorial damselfish (Pomacentridae)

Aggression is often a crucial component to interference interspecific competition and yet there are few studies that examine fight behavior when the opponents are different species. To examine conspecific and heterospecific aggression, we used two species of Caribbean damselfish, the dusky (Stegastes adustus) and the longfin (S. diencaeous) with each one serving as the heterospecific opponent to the other species. Our study was confined to whether or not each species measures the body length of the other species as if it were a conspecific intruder. Body length plays an important role in fight outcome in many species of fish and we presumed that both the dusky and the longfin would use it when assessing opponents. Both the dusky and the longfin damselfish were then presented with two individuals that were either greatly different or minimally different in size. When presented with individuals that differed greatly in size, both species spent more time attacking the larger individual, irrespective of species. However, when the size difference was small, the focal dusky and the longfin responded differently; the dusky showed no preference in either conspecific or heterospecific pairs while the longfin continued to show a bias toward the slightly larger individual. Thus, while we were surprised by the species differences, both the dusky and the longfin were internally consistent in how they treated conspecific and heterospecific opponents.

Micro-fabricated polydimethyl siloxane (PDMS) surfaces regulate the development of marine microbial biofilm communities

This study explored an antifouling (AF) concept based on deployment of microfabricated polydimethyl siloxane (PDMS) surfaces with 1-10 μm periodicity corrugated topographies in temperate marine waters. The effect of the surfaces on the development of microbial biofilms over 28 days and during different seasons, including both summer and winter, was examined using confocal laser scanning microscopy (CLSM) as well as terminal restriction fragment (T-RF) analysis for phylogenetic fingerprinting. The microscale topography significantly impacted biofilm development by altering the attachment pattern and reducing microcolony formation on the 1, 2 and 4 μm PDMS surfaces. Also, field deployments over 28 days showed a significant reduction in biovolume on the 4 and 10 μm PDMS surfaces despite altered environmental conditions. The microfabricated PDMS surfaces further significantly impacted on the community composition of the biofilms, as revealed by changes in T-RF profiles, at different stages of development. Moreover, altered biofilm resistance was demonstrated by exposing pre-established biofilms on 10 μm micro-fabricated surfaces to enhanced flagellate predation by a heterotrophic protist, Rhynchomonas nasuta. Pronounced changes in the overall marine microbial biofilm development as well as community composition warrant exploring substratum modification for marine AF applications.

It pays to be pushy: intracohort interference competition between two reef fishes

Competition is often most intense between similar sized organisms that have similar ecological requirements. Many coral reef fish species settle preferentially to live coral at the end of their larval phase where they interact with other species that recruited to the same habitat patch at a similar time. Mortality is high and usually selective and individuals must compete for low risk space. This study examined the competitive interactions between two species of juvenile damselfish and the extent to which interactions that occurred within a recruitment cohort established the disjunct distribution patterns that were displayed in later life stages. Censuses and field experiments with juveniles found that one species, the ambon damsel, was dominant immediately after settlement and pushed the subordinate species higher up the reef and further from shelter. Presence of a competitor resulted in reduced growth for both species. Juvenile size was the best predictor of competitive success and outweighed the effects of short term prior residency. Size at settlement also dramatically influenced survival, with slightly larger individuals displaying higher aggression, pushing the subordinate species into higher risk habitats. While subordinates had higher feeding rates, they also sustained higher mortality. The study highlights the importance of interaction dynamics between species within a recruitment cohort to patterns of growth and distribution of species within communities.

Size-dependent use of territorial space by a rock-dwelling cichlid fish

Territoriality fundamentally influences animal mating systems and patterns of population structure. Although territory ownership is already known to contribute importantly to male reproductive success and the ecological coexistence of African rock-dwelling cichlids, the significance of variation in territory features has received little attention in these fishes. In Lake Malawi, males of Pseudotropheus tropheops "orange chest" defend territories on either of two substrate classes at Harbour Island: flat rock slabs lacking crevices and caves, or structurally complex boulder fields containing cave shelters. Focal watches of this species demonstrated that both territory size and occupancy on either substrate type depend on the size of male residents. Males larger than a threshold size exclusively held the largest and most structurally complex territories. After removal of conspecific residents, more vacant territorial areas on cave-containing substrate were reoccupied by "orange chest" males in full breeding coloration compared to vacant areas on flat substrate. These findings suggest competition among "orange chest" males for complex rocky substrate. Defense of caves was associated with enhanced male courtship rates: the number of caves within a male's territory was a better predictor of courtship activity than was male size or territory area. In addition to territories being crucial for male reproductive success and therefore likely playing a role in sexual selection, male-male competition for caves in rock-dwelling cichlids may be promoted by the ecological advantage of enemy-free space. Smaller "orange chest" males lacking caves tended to move into adjacent boulder fields in the presence of predators, particularly at night. In contrast, males defending caves were more likely to remain on their territories when nocturnal predators were present. The territorial behaviors of P. tropheops "orange chest" that we observed in situ provide an instructive natural framework for testing the roles of substrate and ecology in the mating systems of rock-dwelling cichlid fishes.