A numerical model of wave- and current-driven nutrient uptake by coral reef communities

We developed a numerical model capable of simulating the spatial zonation of nutrient uptake in coral reef systems driven by hydrodynamic forcing (both from waves and currents). Relationships between nutrient uptake and bed stress derived from flume and field studies were added to a four-component biogeochemical model embedded within a three-dimensional (3-D) hydrodynamic ocean model coupled to a numerical wave model. The performance of the resulting coupled physical-biogeochemical model was first evaluated in an idealized one-dimensional (1-D) channel for both a pure current and a combined wave-current flow. Waves in the channel were represented by an oscillatory flow with constant amplitude and frequency. The simulated nutrient concentrations were in good agreement with the analytical solution for nutrient depletion along a uniform channel, as well as with existing observations of phosphate uptake across a real reef flat. We then applied this integrated model to investigate more complex two-dimensional (2-D) nutrient dynamics, firstly to an idealized coral reef-lagoon morphology, and secondly to a realistic section of Ningaloo Reef in Western Australia, where nutrients were advected into the domain via alongshore coastal currents. Both the idealized reef and Ningaloo Reef simulations showed similar patterns of maximum uptake rates on the shallow forereef and reef crest, and with nutrient concentration decreasing as water flowed over the reef flat. As a result of the cumulative outflow of nutrient-depleted water exiting the reef channels and then being advected down the coast by alongshore currents, both reef simulations exhibited substantial alongshore variation in nutrient concentrations. The coupled models successfully reproduced the observed spatial-variability in nitrate concentration across the Ningaloo Reef system.     

Diurnal space utilization in coral reef fish communities

The diurnal use of space by 25 resident species of coral reef fishes was investigated along three depth (10 to 40 m) transects over an 18 month period. Emphasis was placed on the small, cryptic members of the communities. Three aspects of the use of space — temporal utilization, occupancy and time span — are defined and quantified for each of the species. In addition, an estimate of the diurnal home range volume of each species is provided. The territorial pomacentrids showed the highest level of temporal utilization, holding the same territories throughout the study. The consistent presence of the holocentrid species within the reef infrastructure had the potential to influence the space available to other species. Some species residing in tube-worm holes and sponges also exhibited constant use of space.     

Microhabitat utilisation patterns in cryptobenthic coral reef fish communities

The cryptobenthic reef fish communities from four microhabitats at Orpheus Island, central Great Barrier Reef are described. Eighty-four 0.4m² samples yielded a total of 368 individuals from 42 species in eight families, with a mean density of 11 individuals m^–2 (±1.7SE) and 2.9 species 0.4 m^–2 (±0.2SE). Caves contained the highest number of both individuals (120) and species (26), followed by sand/rubble, soft coral, and open reefs. Microhabitat associations included cave and soft coral specialists. Site fidelity in 71 tagged individuals of 4 species was high, with a mean recapture rate of 53% (±8.4SE) remaining within the ~0.4 m² sampling area after a 48-h period. Behavioural observations also reflect this limited movement, with the dominant mode of behaviour in 7 species being a motionless state (67.5% ±11.6SE), followed by feeding (21.8% ±8.7SE), hiding (6.3% ±1.6SE), and swimming (4.4% ±1.5SE). Two distinct behavioural groups are identified: (1) sedentary forms, characterised by long periods of immobility (5 species); and (2) winnowers, characterised by long feeding bouts (2 species). The fine-scale partitioning of microhabitats, restricted home ranges, and sedentary behaviour of many cryptobenthic reef fish species suggest that this reef fish community exhibits similar patterns of habitat utilisation to their larger reef-fish counterparts, but at a much finer scale.     

Coupling soil water and shoot dynamics in three grass species: A spatial stochastic model on water competition in Neotropical savanna

Savannas are ecosystems known for their high environmental and economic value. They cover at least 20% of the global land surface and, in some cases, can act as a boundary between tropical rainforest and deserts. Water is an important determinant of savanna ecosystems.In this paper, we present a theoretical stochastic model of root competition for water, which couples, soil water availability, phenology, and root and shoot architecture applied to three Neotropical savanna grasses. Soil moisture was simulated using a daily balance, as proposed by Rodriguez-Iturbe et al. [Rodriguez-Iturbe, I., Porporato, A., Ridolfi, L., Isham, V., Cox, D.R., 1999. Probabilistic modelling of water balance at a point: the role of climate, soil and vegetation. Proc. R. Soc. London, Ser. A 455, 3789–3805.]. To simulate rainfall stochasticity, we used daily precipitation data from the airport weather station in the State of Barinas, Venezuela, for the period 1991–2007. Competition among neighbouring plants took into account the spatial distribution of the individuals. As a final step, the model allowed us to calculate the shoot dynamic of the species as a function of soil water availability.Using these data, we compared the behaviour of isolated plants, pairs and trios, and we found below-ground competition to be a fundamental component of global (shoot + root) competition. Finally, our model suggests various circumstances that allow poor competitor plants to coexist in competition for water with more successful competitors. Apparently, this is not only due to transpiration rates, but also to differences in shoot emergence and shoot growth.     

A predator-prey model with satiation and intraspecific competition

We present a new predator-prey model where, except for the prey growth, assumed to be logistic, we endeavor to give some behavioral justification to all elements of the predator-prey interaction. The functional response takes account of predator satiation and predator competition. It is supported by some experimental evidence. We distinguish two contributions to the numerical response: the positive part, proportional to the functional response, is the birth rate of predators; the negative part is the death rate due to hunger.Two outcomes are possible. If the prey are unable to grow fast enough to replace the amount killed by the predators, both species become extinct. In the opposite case, both populations stabilize at a constant population. At this equilibrium level, the prey are not abundant enough to satiate the predators.The predation rate that allows the highest predator population is one half of the ideal prey growth rate. A higher exploitation rate can allow higher populations only temporarily. Evolved predator behavior, reguges for the prey, or other mechanisms can explain this regulation.Two more population behaviors (cycles and predator extinction) can be obtained with a time-lag in one of the responses. This is shown in a separate paper.The model is structurally stable. It can thus withstand small environmental perturbations.     

A spatial model of phytoplankton patchiness

The one-dimensional theory of critical-length scales of phytoplankton patchiness is developed to include phytoplankton growth and herbivore grazing as functions of time and space. The critical-length scale L _c for the pathch is then determined by the initial spatial distribution and concentration of the limiting nutrient and herbivores in addition to the daily averaged values of the growth and loss processes. The response of an initial phytoplankton patch to the stresses of turbulent diffusion, nutrient depletion, light periodicity, and nocturnal or continuous herbivore grazing is investigated numerically for several oceanic conditions. Nocturnal grazing, while less stressful on primary production than continous grazing, results in lower phytoplankton standing stocks. Increase in biomass of vertically migrating zooplankton results in a net loss of nutrient which might otherwise be egested, recycled, and utilized in the euphotic zone under continuous grazing conditions. The Ivlev constant is shown via sensitivity analysis to be a significant parameter ultimately influencing phytoplankton production. It is demonstrated numerically that diffusion of phytoplankton cells from areas of high concentration to low concentration prevents the local extinction of the standing stock, thereby rendering a positive herbivore grazing-threshold unnecessary for ecosystem stability.     

The role of sponge competition on coral reef alternative steady states

Sponges constitute an abundant and functionally important component of coral reef systems. Given their demonstrated resistance to environmental stress, it might be expected that the role of sponges in reef systems under modern regimes of frequent and severe disturbance may become even more substantial. Disturbances have recently reshaped the community structure of many Caribbean coral reefs shifting them towards a state of persistent low coral cover and often a dominance of macroalgae. Using competition and growth rates recorded from Glover's Atoll in Belize, we parameterise a mathematical model used to simulate the three-way competition between sponges, macroalgae and coral. We use the model to determine the range of parameters in which each of the three species might be expected to dominate. Emergent properties arise from our simple model of this complex system, and these include a special case in which heightened competitive ability of macroalgae versus coral may counter-intuitively prove to be advantageous to the persistence of corals. Importantly, we show that even under scenarios whereby sponges fail to invade the system, inclusion of this third antagonist can qualitatively affect the likelihood of alternative stable states - generally in favour of macroalgal dominance. The interplay between multi-species competition and predation is complex, but our efforts highlight a key process that has, until now, remained unexplored: the extent to which sponges dissipate algal grazing pressure by providing generalist fish with an alternative food source. We highlight the necessity of identifying the extent by which this process takes place in tropical systems in order to improve projections of alternative stable states for Caribbean coral reefs.     

Within-group spatial position in ring-tailed coatis: balancing predation, feeding competition, and social competition

A variety of factors can influence an individual’s choice of within-group spatial position. For terrestrial social animals, predation, feeding success, and social competition are thought to be three of the most important variables. The relative importance of these three factors was investigated in groups of ring-tailed coatis (Nasua nasua) in Iguazú, Argentina. Different age/sex classes responded differently to these three variables. Coatis were found in close proximity to their own age/sex class more often than random, and three out of four age/sex classes were found to exhibit within-group spatial position preferences which differed from random. Juveniles were located more often at the front edge and were rarely found at the back of the group. Juveniles appeared to choose spatial locations based on feeding success and not predation avoidance. Since juveniles are the most susceptible to predation and presumably have less prior knowledge of food source location, these results have important implications in relation to predator-sensitive foraging and models of democratic group leadership. Subadults were subordinate to adult females, and their relationships were characterized by high levels of aggression. This aggression was especially common during the first half of the coati year (Nov–April), and subadults were more peripheralized during this time period. Subadults likely chose spatial positions to avoid aggression and were actively excluded from the center of the group by adult females. In the Iguazú coati groups, it appeared that food acquisition and social agonism were the major determinants driving spatial choice, while predation played little or no role. This paper demonstrates that within-group spatial structure can be a complex process shaped by differences in body size and nutritional requirements, food patch size and depletion rate, and social dominance status. How and why these factors interact is important to understanding the costs and benefits of sociality and emergent properties of animal group formation.     

A general two-species competition model with time-varying rates

The classical mathematical model for the behaviour of the populations of two competing biological species has previously been generalized by the author, by assuming that the coefficients of intrinsic growth, self-inhibition and interaction were all functions of time and, for a certain class of the governing differential equations, the exact solutions were obtained: an example was given in which the coefficients were periodic functions. In the differential equations of this model (as well as in the autonomous Lotka-Volterra equations), each of the Malthusian growth-rates was assumed to be diminished by a linear function of the populations of the two species, without there being any rigorous justification for this assumption. We here generalize the differential equations by assuming for these diminution functions general nonlinear forms having time-varying coefficients. The exact solutions are given for four classes of the resulting strongly nonlinear non-autonomous differential equations. Various conclusions about the growth modes of the two populations and their asymptotic behaviour are drawn, both when specific and when arbitrary forms are assumed for the coefficient functions. Cases are examined in which the Competitive Exclusion Principle holds and others in which it does not.     

Influence of corallivory, competition, and habitat structure on coral community shifts

The species composition of coral communities has shifted in many areas worldwide through the relative loss of important ecosystem engineers such as highly branched corals, which are integral in maintaining reef biodiversity. We assessed the degree to which the performance of recently recruited branching corals was influenced by corallivory, competition, sedimentation, and the interactions between these factors. We also explored whether the species-specific influence of these biotic and abiotic constraints helps to explain recent shifts in the coral community in lagoons of Moorea, French Polynesia. Population surveys revealed evidence of a community shift away from a historically acroporid-dominated community to a pocilloporid- and poritid-dominated community, but also showed that the distribution and abundance of coral taxa varied predictably with location in the lagoon. At the microhabitat scale, branching corals grew mainly on dead or partially dead massive Porites ("bommies"), promontories with enhanced current velocities and reduced sedimentation. A demographic study revealed that growth and survival of juvenile Pocillopora verrucosa and Acropora retusa, the two most common branching species of each taxon, were affected by predation and competition with vermetid gastropods. By 24 months of age, 20-60% of juvenile corals suffered partial predation by corallivorous fishes, and injured corals experienced reduced growth and survival. A field experiment confirmed that partial predation by corallivorous fishes is an important, but habitat-modulated, constraint for branching corals. Competition with vermetid gastropods reduced growth of both branching species but unexpectedly also provided an associational defense against corallivory. Overall, the impact of abiotic constraints was habitat-specific and similar for Acropora and Pocillopora, but biotic interactions, especially corallivory, had a greater negative effect on Acropora than Pocillopora, which may explain the local shift in coral community composition.     

Temporal and spatial variability in coral recruitment on two Indonesian coral reefs: consistently lower recruitment to a degraded reef

Corals are the primary reef-building organisms, therefore it is key to understand their recruitment patterns for effective reef management. Coral recruitment rates and juvenile coral abundance were recorded in the Wakatobi National Marine Park, Indonesia, on two reefs (Sampela and Hoga) with different levels of environmental degradation (12.5 vs. 44 % coral cover with high and low sedimentation rates, respectively) to examine consistencies in recruitment patterns between years and seasons. Recruitment was measured on multiple panels at two sites on each reef (6–7 m depth) and cleared areas of natural reef. Although coral recruitment was twofold higher in 2008–2009 than in 2007–2008, and seasonal differences were identified, consistent significant differences in recruitment rates were found between the two reefs even though they are separated by only ~1.5 km. Recruitment rates and juvenile abundance were lower on the more degraded reef. These patterns are likely a consequence of differential pre- and post-settlement mortality as a result of the high sedimentation rates and degraded conditions and possibly reduced larval supply.     

A hierarchical model for spatial capture-recapture data

Estimating density is a fundamental objective of many animal population studies. Application of methods for estimating population size from ostensibly closed populations is widespread, but ineffective for estimating absolute density because most populations are subject to short-term movements or so-called temporary emigration. This phenomenon invalidates the resulting estimates because the effective sample area is unknown. A number of methods involving the adjustment of estimates based on heuristic considerations are in widespread use. In this paper, a hierarchical model of spatially indexed capture-recapture data is proposed for sampling based on area searches of spatial sample units subject to uniform sampling intensity. The hierarchical model contains explicit models for the distribution of individuals and their movements, in addition to an observation model that is conditional on the location of individuals during sampling. Bayesian analysis of the hierarchical model is achieved by the use of data augmentation, which allows for a straightforward implementation in the freely available software WinBUGS. We present results of a simulation study that was carried out to evaluate the operating characteristics of the Bayesian estimator under variable densities and movement patterns of individuals. An application of the model is presented for survey data on the flat-tailed horned lizard (Phrynosoma mcallii) in Arizona, USA.     

铁限制对浮游植物生长和群落组成的影响研究综述

Fe是浮游植物生长必需的微量元素,其供应对水体中浮游植物的生物量、生长速率以及种群组成均具有重要的影响。从Fe在浮游植物新陈代谢中的作用出发,重点对近年来国内外有关Fe对浮游植物光合作用、营养盐吸收利用以及浮游植物群落组成的影响等方面的研究进展作了较详细的介绍,认为：（1）Fe通常是HNLC海域中初级生产力的主要限制因子之一,Fe限制会导致浮游植物细胞色素密度以及相应的光合叶绿素减少,从而降低其光合作用速率,导致生长缓慢;（2）Fe限制还能改变浮游植物吸收营养盐（N、P、Si等）的速率和比例,影响浮游植物的种间竞争,进而影响其群落结构;（3）Fe加富能够改变水体中浮游植物的优势种群,在海洋中表现为硅藻占优势,而在淡水湖泊中则表现为绿藻和细菌占优势;（4）在淡水湖泊中总Fe含量普遍较高,但浮游植物可利用Fe的含量受水体中有机配体的种类和数量的影响,在一定条件下Fe也可能成为浮游植物生长的限制因子,这对进一步认识湖泊水华暴发的机制具有一定的意义;（5）有学者提出将施加Fe作为一种生态手段,来提高HNLC海域的初级生产力,从而缓解由于大气中CO2浓度增加而导致的全球变暖的趋势;相反地,将降低浮游植物可利用Fe含量作为一种生态手段,来控制富营养化湖泊中藻类的生物量及群落结构,从而缓解日益严重的水华问题,值得进一步研究和探讨。     

Analysis of the influence of substrate variables on coral reef fish communities

A simple field technique to obtain a gross estimate of the surface area of a quadrat on a coral reef is described. This measure, termed the substrate rugosity index, was determined, in conjunction with two other substrate variables (vertical relief and coral species richness), in a series of 4 quadrats (10 to 40 m depth) along 4 transects. The mean substrate rugosity and vertical relief of a quadrat were highly correlated. A correlation analysis was made of the substrate variables and several reef fish community parameters (species richness, number of fishes and diversity). Species richness was highly correlated with substrate rugosity. This relationship was tested in two experimental quadrats and the results were generally in accord with those predicted. Stratification of the fish communities by body size revealed that the correlation with substrate rugosity was scale-dependent. The fish community parameters were poorly correlated with percentage substrate cover by corals (ramose and glomerate) and by sand. A significant area effect was determined for two species of sand-dwelling goby.     

Coral weight increment in situ. A new method to determine coral growth

An underwater weighing apparatus is described. This consits basically of a float, the buoyancy of which is neutralized by a known amount of lead weights. The standard error of the method is ±0.1g. The entire weighing procedure takes place on the reef using SCUBA diving gear. The advantages are that the animals are not harmed by experimental treatment, and that the pure calcium-carbonate increment is registrated. Some results obtained by this method show growth of Montastrea annularis and Madracis mirabilis.     

Safety in numbers and the spatial scaling of density-dependent mortality in a coral reef fish

In coral reef fishes, density-dependent population regulation is commonly mediated via predation on juveniles that have recently settled from the plankton. All else being equal, strong density-dependent mortality should select against the formation of high-density aggregations, yet the juveniles of many reef fishes aggregate. In light of this apparent contradiction, we hypothesized that the form and intensity of density dependence vary with the spatial scale of measurement. Individual groups might enjoy safety in numbers, but predators could still produce density-dependent mortality at larger spatial scales. We investigated this possibility using recently settled juvenile bluehead wrasse, Thalassoma bifasciatum, a small, aggregating reef fish. An initial caging experiment demonstrated that juvenile bluehead wrasse settlers suffer high predation, and spatial settlement patterns indicated that bluehead wrasse juveniles preferentially settle in groups, although they are also found singly. We then monitored the mortality of recently settled juveniles at two spatial scales: microsites, occupied by individual fish or groups of fish and separated by centimeters, and sites, consisting of approximately 2400-m2 areas of reef and separated by kilometers. At the microsite scale, we measured group size and effective population density independently and found that per capita mortality decreased with group size but was not related to density. At the larger spatial scale, however, per capita mortality increased with settler density. This shift in the form of density dependence with spatial scale could reconcile the existence of small-scale aggregative behavior typical of many reef fishes with the population-scale density dependence that is essential to population stability and persistence.     

A dynamic programming implemented resource competition game theoretic model

Resource competition is commonly occurred in animal populations and studied intensively by researchers. Previous studies have applied game theoretic model by finding Nash equilibrium to investigate this phenomenon. However computation of the Nash equilibrium requires an understanding of the payoff matrix that allocates the rewards received by players when they adopt each of the strategies in the game. In our study we present a dynamic programming implemented framework to compute 2 × 2 intraspecific finite resource allocation game's payoff matrix explicitly. We assume that two distinct types of individuals, aggressive and non-aggressive, are in the population. Then we divide the entire animal development period into three different stages: initialization, quasilinear growth and termination. Each stage for each type of players is specified with their own development coefficient, which determines how resource consumption could convert into strength as reward. Each player has equal and finite resource at the beginning of their development and fights against other players in the population to maximize its own potential reward. Based on these assumptions it is reasonable to use backward induction dynamic programming to compute payoff matrix. We present numerical examples for three different types of aggressive individuals and compute the payoff matrices correspondingly. Then we use the derived payoff matrices to determine the Nash equilibrium and Evolutionary Stable Strategy. Our research provide a framework for future quantitative studies on animal resource competition problems and could be expanded to n-players interspecific stochastic asymmetric resource allocation problem by changing some settings of dynamic programming formulation.     

A mathematical model of competition and colonization in a community of marine benthic algae

A mathematical model has been constructed for the algal community on the rocky shores of a Norwegian fjord. We report here on the studies of competition and colonization along a vertical transect from the upper intertidal to the sublittoral habiats. Results on species abundance and distribution (patterns of zonation) and time to reach maturity have been compared to observations both in the fjord area and in other rocky shore areas.Competition coefficients for the algae were inferred from plant morphology and shown to be in agreement with observations of algal abundance and their zone-forming ability. Competition restricts the distribution of the species, especially at the lower elevations, but does not alter their relative position. However, increasing uniform competition prolongs the time in which zone-forming can occur, and it also decreases the overall biomass which an area can sustain. Colonization by a single species may create transient stages in community development of the same order of magnitude as algae longevity, and probably also alters the zonation pattern to some degree.The simulation results indicate that the large-scale algal distribution pattern in the Hardangerfjord area results from global stability of the rocky shore community.     

Annual density banding in massive coral skeletons: result of growth strategies to inhabit reefs with high microborers’ activity?

Porites and Montastraea are the major reef-building massive coral genera in the Indo-Pacific and Atlantic oceans, respectively. They are also the most commonly used genera in sclerochronological studies. Despite the marked differences in the way these genera use calcareous material to construct their skeletons (growth strategies) and in their skeletal architectural structure, they form annual high and low density bands in their skeletons, that result from the positive relationship of coral calcification rate with sea surface temperature and seasonal changes of the latter. Evidence in the literature suggests that the different growth strategies allow these organisms to construct denser skeletons far from terrigenous inputs, on reefs where microborers’ activity is high. It seems quite probable that this has consequences for the evolution, diversity, distribution and abundance of reef corals.     

Fractal properties of spatial distribution of intertidal benthic communities

We studied the spatial distribution of intertidal macrozoobenthos, microphytobenthos (diatom algae) and sediments at scales from decimeters to kilometers using an index of spatial homogeneity, D _I. Sediments were found to be randomly distributed, making up a mosaic of silty and sandy sites. On the contrary, the estimated spatial variability of macrofauna within all the scales up to 5500 m depended upon neither extent (total area covered) nor grain (finest spatial resolution) but only their ratio. We treat this as evidence of statistical self-similarity (fractal property) of the pattern. For diatoms, spatial heterogeneity of community structure was also self-similar in the range from 0.25 to 75 m (within a single bight). At larger scales, microalgae showed a combination of patchy structure with pronounced gradient along the shoreline from brackish-water to marine flora. Thus, fractal properties of both groups became manifested at scales corresponding to their mean body size. The ranges of fractal patterns were approximately equal to 10³–10⁵ if measured in body size units. We suggest that fractal-like spatial structures may be a general feature of communities, and speculate on the nature of such patterns. Received: 16 February 1999 / Accepted: 30 December 1999