Ronald H. Karlson

Development and Stability of the Fouling Community at Beaufort, North Carolina

Community development was followed for 2 1/2 to 3 1/2 years on unglazed ceramic tile plates (232 cm2), suspended horizontally beneath the Duke University Marine Laboratory dock, in Beaufort, North Carolina. Series of 3 or 4 plates were submerged at approximately the 1st of each month from May—November 1971 and from April—November 1972. Percentage cover for each species that settled and grew on the lower surface was estimated at 6— to 8—week intervals, using 75 points randomly positioned over the plate area. Samples were nondestructive; plates were resubmerged after each census. Larval recruitment was estimated at 1— to 3—week intervals on newly submerged plates. Temperature and salinity were also measured. Initial community development was relatively unpredictable. Larval recruitment patterns varied markedly from year to year and as a result, different patterns of initial community development were observed both within and between years. Instead of preparing the way for subsequent arrivals, most resident adults strongly inhibited the recruitment and growth of other species. Species varied in their ability to resist subsequent invasion as adults and in their ability to invade occupied substrate as larvae. After an unpredictable initial developmental phase, subsequent changes in species composition depended in part on the degree to which larvae were able to invade existing adult assemblages. This in turn depended on the identity of the resident adults and the identity of the invading larvae. As a result, the direction and rate of community development, dependent on the order of initial invasion and subsequent recruitment, were difficult to predict although an equilibrium number of 8—10 species/plate was often observed. Adult residence time was generally <1 year and the mortality and/or disappearance of these adults produced 20—60% free space on an approximately annual basis. This free space was usually occupied by recruits of a different species than the original occupant. The combined addition of species through larval recruitment and subtraction of species as a result of adult mortality produced dramatic changes in community structure each year. There is no reason to believe these changes will ever cease. We conclude that succession in the classical sense (Odum 1969) does not occur in this system because initial development was variable, residents impeded subsequent development instead of enhancing it, and there was no stable climax. There is good reason to believe similar processes occur in other temperate and subtropical fouling communities. We believe these communities are fundamentally different from terrestrial plant communities, where succession may occur, for 3 reasons: (1) the organisms do not alter the substrate they occupy, i.e., “prepare” it for later arrivals, (2) there is no possibility of “storing” dormant “seeds” of successional species. Colonization of free space is generally by animals which have a short—lived, even nonfeeding larvae, (3) most adults are extremely short—lived. An emerging paradigm of marine benthic community organization postulates the existence of competitive hierarchies in which 1 or a few species win in the absence of disturbances. The fouling community appears to lack such dominants.

Coral communities are regionally enriched along an oceanic biodiversity gradient

Ecological communities are influenced by processes operating at multiple scales. Thus, a better understanding of how broad- as well as local-scale processes affect species diversity and richness is increasingly becoming a central focus in modern community ecology. Here, in a study of unprecedented geographical scope, we show significant regional and local variation in the species richness of coral assemblages across an oceanic biodiversity gradient. The gradient that we sampled extends 10,000 km eastwards from the worlds richest coral biodiversity hotspot in the central Indo-Pacific. Local richness and the size of regional species pools decline significantly across 15 islands spanning the gradient. In addition, richness declines across three adjacent habitats (reef slopes, crests and flats). In each habitat, a highly consistent linear relationship between local and regional species richness indicates strong regional enrichment. Thus, even on the most diverse coral reefs in the world, local coral assemblages are profoundly affected by regional-scale processes. Understanding these historical and biogeographical influences is essential for the effective management and preservation of these endangered communities.

Disturbance, coral reef communities, and changing ecological paradigms

We examine changing ecological theory regarding the role of disturbance in natural communities and relate past and emerging paradigms to coral reefs. We explore the elements of this theory, including patterns (diversity, distribution, and abundance) and processes (competition, succession, and disturbance), using currently evolving notions concerning matters of scale (temporal and spatial), local versus regional species richness, and the equilibrium versus nonequilibrium controversy. We conclude that any attempt to categorize coral reef communities with respect to disturbance regimes will depend on the question being asked and the desired level of resolution: local assemblage versus regional species pool, successional versus geological time, and on the taxonomic and tropic affinities of species included in the study. As with many communities in nature, coral reefs will prove to be mosaics of species assemblages with equilibrial and nonequilibrial dynamics.

Species richness of reef-building corals determined by local and regional processes

Species richness in ecological communities has traditionally been explained in terms of species interactions, especially competition, operating within the local community over relatively short periods of time. Recently, it has become clear that ecological communities can be organized by a variety of processes operating at different spatial and temporal scales. Since local communities are imbedded within larger geographic regions, regional/historical phenomena and local processes may influence local richness jointly and should be analysed simultaneously. Here we perform such an analysis by comparing assemblages of hermatypic scleractinian corals from different regions sampled at over 100 sites around the world. Using multiple regression analysis, we find that local richness is as sensitive to regional richness as it is to each of two variables (depth and habitat) that subsume much of the local variability at a site. Our results indicate that although coral assemblages are believed to be intensely interactive, they are, nevertheless, regionally enriched and show no evidence of saturation. Multi-scale effects on local richness demand further investigation to clarify causal relationships, and to enlighten policy decisions over issues of biodiversity, habitat loss and fragmentation, and ecological restoration.

Recruitment-limitation in open populations of Diadema antillarum: an evaluation

SummaryEmpirical evidence from studies of the sea urchin Diadema antillarum suggests that this organism widely disperses its offspring, that both recruitment and mortality rates are independent of local densities, and that local food availability does not regulate local population sizes. These attributes would indicate that local populations are generally open and recruitment-limited. Given that current populations have been devastated by a 1983–1984 mass mortality event which spread throughout the range of this species, we examine current population trends and evaluate the prospects for population recovery under the assumptions of recruitment-limitation and density-independent rates of recruitment and mortality. Specifically, we evaluate the dynamics of five, local populations at Lameshur Bay, St. John, U.S.V.I. in order to 1) determine current rates of recruitment and mortality, 2) predict population densities based on the above assumptions, 3) compare predicted densities against observed 1984–1988 densities, and 4) predict future population densities based on current trends. We estimate current recruitment rates at 0.02–0.11 individuals/m2/yr and per capita mortality rates at 0.27–0.47 deaths/yr. Over the period 1985–1988, predicted densities based on these annual rates did not differ significantly from actual observed densities. Therefore, the assumptions that recruitment and mortality rates are density-independent and that local populations are recruitment-limited are sufficient, at present, to adequately predict current population trends. These trends indicate no recovery towards pre-mass mortality densities. The above description of the dynamics of open, recruitment-limited populations may be appropriate for a wide variety of organisms. We note the prevalence of animals with extensive larval dispersal capabilities. Populations located near the limits of their distribution, in freshwater streams and ponds, mountain tops, or other similarly isolated populations may also be subject to recruitment-limitation. Remote, recruitment-limited populations are likely to be more susceptible to local extinction than less remote populations. Dispersal distances and the scale of the processes controlling recruitment and mortality are important determinants of the degree of openness of local populations.

Coral species richness: ecological versus biogeographical influences

Abstract Species richness in communities varies with habitat area, productivity, disturbance level, intensity of species interactions, and regional/historical effects. All of these factors influence coral richness but their effects vary with spatial scale, position on the reef, and regional location. Species richness of corals along depth gradients shows a unimodal, hump-shaped curve that peaks at intermediate depths. Moreover, the peak of the curve is higher in regions with larger species pools. This “regional enrichment” of the local community appears in line transect samples as small as 10 m in length. The pattern suggests that ecological factors operating over scales of tens of meters and regional/historical factors operating over thousands of kilometers can both affect local richness. Regional factors probably include differences in speciation relative to extinction rates among regions and proximity of local sites to richness hotspots. Plausible factors operating at the local scale are species interactions, disturbance, and productivity which combine in different ways to produce the unimodal pattern. Shallow areas support few species because extinction rates are high due to frequent disturbance or because of environmental extremes. In addition, high productivity encourages rapid growth and thus the potential for intense interspecific competition. In areas where branching acroporids are abundant, exclusion by these dominant competitors is possible. Deep areas may be depauperate because few species can tolerate the low light levels found there. Areas of intermediate depth have the richest communities because they are open for colonization by many species and because extinction rates are low. Several theories may explain this “openness” and species persistence:1. Occasional disturbance coupled with low growth rates results in glacially slow exclusion by the dominant competitor.2. Aggregation of corals creates spatial variation in the intensity of competition and thus refuges from competition within a spatial landscape. Inferior competitors persist because they are superior at dispersal and refuge colonization.3. Specialist predators focus on high-density juvenile populations near the parent, creating ecological space for colonization by non-prey.4. Coral competitive abilities are roughly equal and recruitment into the community is a probabilistic event. The community thus exhibits random drift and exclusion is an extremely lengthy process. Based upon empirical evidence, these theories are listed in order of plausibility, but still need to be rigorously tested.

Size-Dependent Growth in Two Zoanthid Species: A Constrast in Clonal Strategies

The widespread occurrence of genet fragmentation among modular, clonal organisms results in size—dependent life history patterns that are often independent of clonal age. In this study the size dependence of clonal growth rates was experimentally evaluated using two common coral reef cnidarians that inhabit shallow reef environments at Discovery Bay, Jamaica. As a result of the turbulent conditions associated with storms, these organisms commonly undergo fragmentation. The growth of aggregations of these clonal fragments in three small size classes (ranging over three orders of magnitude) was statistically evaluated against a null, exponential model that predicts that relative growth rates of small aggregations are size independent. Growth rates for Zoanthus solanderi were consistent with this model. Z. sociatus, on the other hand, exhibited size—dependent relative growth rates. The smallest aggregations of this species had the higher relative growth rates, which were sufficiently high to more than compensate for losses due to mortality. These results are consistent with other life history and distributional differences between these two species. Zoanthus sociatus has a higher rate of mortality, does not undergo sexual reproduction until reaching a larger aggregation size, and commonly has a higher vertical distribution (which may represent a spatial refuge from subtidal predators) than does Z. solanderi. The comparatively rapid relative growth rates of small aggregations of Z. sociatus may be the result of spatial constraints on growth in large aggregations and/or of higher relative energy allocations to growth in small aggregations. The incorporation of fragmentation into the life history strategy of clonal organisms has a range of predicted consequences. Among some organisms, fragmentation and associated adaptations may be rare and of little consequence. Among organisms that frequently fragment as a result of physical disturbances, natural selection should favor repair and regenerative processes as well as resistance to this source of mortality. At the extreme, fragmentation need not be associated with death and injury. Adaptations at the developmental and physiological level may involve genetically programmed production of asexual fragments and size—dependent shifts in energy allocations to growth, sexual reproduction, and energy reserves. The degree of interdependence of the processes controlling the dynamics of genets and fragmented modules may well depend on the relative importance of such adaptations.

SPECIES RICHNESS OF CORAL ASSEMBLAGES: DETECTING REGIONAL INFLUENCES AT LOCAL SPATIAL SCALES

Coral assemblages are generally open to immigration from regional species pools and thus are regionally enriched rather than saturated with species. Previously we have documented the sensitivity of local richness in these assemblages to a number of regional factors including the size of the regional species pool. Here we focus on the local scale used to determine the local-regional richness relationship. We examine the sensitivity of local richness to regional richness and to two environmental variables (depth and habitat type) across a range of locality sizes. In general, local richness is sensitive to the envi- ronmental variables regardless of locality size. In contrast, it is relatively insensitive to regional richness in very small 1-m 2 quadrats but highly sensitive when sampled with 10-m line transects. Thus our analysis suggests a spatial threshold somewhere below 10 m for the detection of regional enrichment in coral assemblages. There are four biological mech- anisms that may allow regional effects to be expressed at this spatial resolution: (1) dis- turbance and slow recovery toward competitive equilibrium, (2) heterogeneous biological interactions involving aggregated resources, neighborhood competition, or nonrandom dis- persal among local neighborhoods, (3) spatially variable predation by specialists, and (4) probabilistic recruitment coupled with equal competitive ability. Empirical tests are now needed to discriminate among these mechanisms.

DENSITY-DEPENDENT DYNAMICS OF SOFT CORAL AGGREGATIONS: THE SIGNIFICANCE OF CLONAL GROWTH AND FORM'

In clonal plants and animals, stolons and runners often promote rapid directional growth and escape from crowded microhabitats. Here we evaluate the effects of density on clonal growth and dispersal by stolons, on colony mortality, and on recruitment in the soft coral Efflatounaria sp. This colonial organism forms dense aggregations on mid-shelf and outer reefs of the Great Barrier Reef, Australia, where it is subjected to frequent physical and biological disturbances. Stolonal growth and asexual recruitment of new colonies (by budding) were enhanced by experimentally reducing local density. Within unmanipulated aggregations of Efflatounaria, per-capita rates of asexual recruitment were higher at low density, but colony survivorship was lower. Furthermore, the effect of density on stolonal growth and dispersal of daughter colonies varied as a function of a colonys history of disturbance. Disturbance was simulated by detaching from the substrate a newly budded colony that was still connected by a stolen to the parent colony. At low density, these pairs of partially detached colonies moved apart, while at high density, stolonally connected colonies moved closer together. Our results suggest that Efflatounaria employs a plastic life-history strategy that promotes recovery from injuries and the formation of dense aggregations. At low density, stolons facilitate rapid directional growth, asexual recruitment, and aggregation. At high density, clonal growth is inhibited, and mortality rates are greatly reduced. Enhanced survival within aggregations provides the adaptive context for interpreting the influence of density and disturbance history on the population dynamics of this clonal organism.

Spatial arrangement of competitors influences coexistence of reef-building corals.

Spatial aggregation among strong competitors has been identified as a putative mechanism promoting the coexistence of weak competitors in intensely competitive communities. With notable exceptions in plant communities, few investigators have tested this hypothesis experimentally. In this study, we manipulated the spatial arrangement of corals to test whether within-patch aggregation of a strong coral competitor enhances the success of a weaker coral competitor. Corals grown in simple aggregated arrangements, where the number and type of competitors were held constant, grew almost twice as much as those in non-aggregated arrangements. These growth results suggest that species coexistence is promoted by aggregation within competitive neighborhoods. Thus spatial aggregation may be one of several important mechanisms contributing to the persistence of weak competitors and species coexistence on coral reefs.