Intraspecific competition in Tridacna crocea,a burrowing bivalve

Summary Intraspecific competition for space and light occurred when Tridacna crocea burrowed into coralline substratum of boulders on leeward coral reefs in the central Great Barrier Reef near Townsville, Australia. Intensity of competition was linearly related to clam density. Above about 200 clams/m², all clams physically contacted one another and all shells sustained damage. Mortality in isolated populations due to intraspecific competition was estimated at 40%. Principles of intraspecific competition in plants were tested for applicability to T. crocea populations. Juvenile mortality due to competitive stress was density dependent. Aggregated distributions of one year old clams changed to random or regular distribution of adults. Normal size-frequency distribution for juveniles became skewed for older groups. A bimodal size-frequency distribution of the population was related to selective mortality in 1–3 year old clams. Adult mortality due to crowding was less severe but significant. Growth rates were inhibited by competition. Deformations in morphology resulted from crowding. Intraspecific competition for space and light by adults inhibited recruitment of young. Animal adaptations to reduce mortality under crowded conditions were also important. Larvae aggregated on settling and oriented with posterior ends pointed away from nearest neighbors. Positional alignment within the substratum was selectively advantageous. Burrowing posteriorly was preferential, but anterior and sideways burrowing as well as twisting within the burrow were also observed. Movement within substratum served to reduce local damage to the shell. Proteinaceous deposits secreted through perforations in the shell reduced subsequent damage. T. crocea populations exhibited many animal adaptations that reduced mortality during the first years of life, but as cohorts matured, plant-like patterns of competitive interaction became more significant.     

A double test of the parasite manipulation hypothesis in a burrowing bivalve

The parasite manipulation hypothesis predicts that parasites should be selected to manipulate host behaviour to facilitate transmission to the next host. The bivalve Macoma balthica burrows less deep when parasitized by the trematode Parvatrema affinis. Shallow burrowing increases the likelihood of ingestion by birds, their final hosts, and therefore this has been interpreted as manipulation by the parasite. When unparasitized, M. balthica displays seasonal changes in burrowing depth, becoming less accessible to predators in winter. If shallow burrowing of parasitized individuals is due to direct manipulation by the parasite, the availability of parasitized individuals should be high throughout the year, or at least especially in the season when most birds are present and potential transmission rates are highest. We compared burrowing depths of parasitized and unparasitized individuals in a single population during seven consecutive years. Parasitized individuals showed reduced burrowing depths but, in contrast to the prediction, the effect of parasites on availability to predators was smallest, not largest, in the season with the highest bird numbers. The parasite P. affinis competes for energy with the host, and M. balthica with low energy stores are known to reduce depth of burrowing. When we included size-corrected somatic ash-free dry mass (as an estimate of the energy stores) in our statistical analysis, the effect of infection on burrowing depth disappeared. Thus the effect of infection on burrowing depth is likely to be an unavoidable, indirect effect of the channelling of energy towards the parasite, causing the starving individual to take greater risks in the acquisition of food. Since both the seasonal pattern and the magnitude of increased availability of parasitized individuals are inadequate, the increased exposure of parasitized M. balthica to the final host does not seem to represent an example of adaptive host manipulation by the parasite.     

Pattern-oriented modelling: a 'multi-scope' for predictive systems ecology

Modern ecology recognizes that modelling systems across scales and at multiple levels-especially to link population and ecosystem dynamics to individual adaptive behaviour-is essential for making the science predictive. 'Pattern-oriented modelling' (POM) is a strategy for doing just this. POM is the multi-criteria design, selection and calibration of models of complex systems. POM starts with identifying a set of patterns observed at multiple scales and levels that characterize a system with respect to the particular problem being modelled; a model from which the patterns emerge should contain the right mechanisms to address the problem. These patterns are then used to (i) determine what scales, entities, variables and processes the model needs, (ii) test and select submodels to represent key low-level processes such as adaptive behaviour, and (iii) find useful parameter values during calibration. Patterns are already often used in these ways, but a mini-review of applications of POM confirms that making the selection and use of patterns more explicit and rigorous can facilitate the development of models with the right level of complexity to understand ecological systems and predict their response to novel conditions.     

On our best behavior: optimality models in human behavioral ecology

This paper discusses problems associated with the use of optimality models in human behavioral ecology. Optimality models are used in both human and non-human animal behavioral ecology to test hypotheses about the conditions generating and maintaining behavioral strategies in populations via natural selection. The way optimality models are currently used in behavioral ecology faces significant problems, which are exacerbated by employing the so-called ‘phenotypic gambit’: that is, the bet that the psychological and inheritance mechanisms responsible for behavioral strategies will be straightforward. I argue that each of several different possible ways we might interpret how optimality models are being used for humans face similar and additional problems. I suggest some ways in which human behavioral ecologists might adjust how they employ optimality models; in particular, I urge the abandonment of the phenotypic gambit in the human case.     

Climate change,phenological shifts,eco-evolutionary responses and population viability: toward a unifying predictive approach

The debate on emission targets of greenhouse gasses designed to limit global climate change has to take into account the ecological consequences. One of the clearest ecological consequences is shifts in phenology. Linking these shifts to changes in population viability under various greenhouse gasses emission scenarios requires a unifying framework. We propose a box-in-a-box modeling approach that couples population models to phenological change. This approach unifies population modeling with both ecological responses to climate change as well as evolutionary processes. We advocate a mechanistic embedded correlative approach, where the link from genes to population is established using a periodic matrix population model. This periodic model has several major advantages: (1) it can include complex seasonal behaviors allowing an easy link with phenological shifts; (2) it provides the structure of the population at each phase, including the distribution of genotypes and phenotypes, allowing a link with evolutionary processes; and (3) it can incorporate the effect of climate at different time periods. We believe that the way climatologists have approached the problem, using atmosphere–ocean coupled circulation models in which components are gradually included and linked to each other, can provide a valuable example to ecologists. We hope that ecologists will take up this challenge and that our preliminary modeling framework will stimulate research toward a unifying predictive model of the ecological consequences of climate change.     

Time for a change: dynamic urban ecology

Contemporary cities are expanding rapidly in a spatially complex, non-linear manner. However, this form of expansion is rarely taken into account in the way that urbanization is classically assessed in ecological studies. An explicit consideration of the temporal dynamics, although frequently missing, is crucial in order to understand the effects of urbanization on biodiversity and ecosystem functioning in rapidly urbanizing landscapes. In particular, a temporal perspective highlights the importance of land-use legacies and transient dynamics in the response of biodiversity to environmental change. Here, we outline the essential elements of an emerging framework for urban ecology that incorporates the characteristics of contemporary urbanization and thus empowers ecologists to understand and intervene in the planning and management of cities.     

Host specificity and population dynamics of a sponge-endosymbiotic bivalve

We assessed the host-use pattern of the sponge-endosymbiotic bivalve Vulsella vulsella and its demographic consequences in an inland sea in Okinawa Island, Japan. Vulsella vulsella utilized only one massive globular sponge species Spongia sp. as a host, and no Spongia sp. without V. vulsella were found. Individual sponges contained 9-248 live bivalves and 0-222 dead bivalves. The densities of live and dead bivalves in individual sponges were approximately constant irrespective of sponge size, indicating that available space is very scarce inside each sponge. The size distribution of bivalves was skewed to small, young individuals less than 30 mm in shell height, although the estimated largest possible size was 106 mm. The bivalve population at each sampling date was composed of three yearly cohorts, and recruitment of juveniles occurred in the summer. The bivalves became sexually mature as males within one year after recruitment and changed sex from male to female as they grew. The size and sex distributions of the bivalve were largely similar among sponges regardless of sponge size, suggesting that the recruitment, growth, longevity, and sex change of the bivalve were strictly regulated, probably by the high water temperature and strong waves generated by typhoons in summer months.     

Bioinformatics tools in predictive ecology: applications to fisheries

There has been a huge effort in the advancement of analytical techniques for molecular biological data over the past decade. This has led to many novel algorithms that are specialized to deal with data associated with biological phenomena, such as gene expression and protein interactions. In contrast, ecological data analysis has remained focused to some degree on off-the-shelf statistical techniques though this is starting to change with the adoption of state-of-the-art methods, where few assumptions can be made about the data and a more explorative approach is required, for example, through the use of Bayesian networks. In this paper, some novel bioinformatics tools for microarray data are discussed along with their 'crossover potential' with an application to fisheries data. In particular, a focus is made on the development of models that identify functionally equivalent species in different fish communities with the aim of predicting functional collapse.     

Niche and area of distribution modeling: a population ecology perspective

Statistical modeling of areas of distribution of species by correlative analysis of the environmental features of known presences has become widespread. However, to a large degree, the logic and the functioning of many of these applications remain obscure, not only due to the fact that some of the modeling methods are intrinsically complex (neural networks, genetic algorithms, generalized additive models, for example), but mainly because the role of other ecological processes affecting the species distributions sometimes is not explicitly stated. Resorting to fundamental principles of population ecology, a scheme of analysis based on separation of three factors affecting species distributions (environment, biotic interactions and movements) is used to clarify some results of niche modeling exercises. The area of distribution of a virtual species which was generated by both environmental and biotic factors serves to illustrate the possibility that, at coarse resolutions, the distribution can be approximately recovered using only information about the environmental factors and ignoring the biotic interactions. Finally, information on the distribution of a butterfly species, Baronia brevicornis , is used to illustrate the importance of interpreting the results of niche models by including hypothesis about one class of movements. The results clarify the roles of the three factors in interpreting the results of using correlative approaches to modeling species distributions or their niches.     

Integrating individual behavior and population ecology: the potential for habitat-dependent population regulation in a reef fish

We used the predictions of the ideal free and ideal despoticdistributions (IFD and IDD, respectively) as a basis to evaluatethe link between spatial heterogeneity, behavior, and populationdynamics in a Caribbean coral reef fish. Juvenile three-spotdamselfish (Stegastes planifrons) were more closely aggregatedin patch reef habitat than on continuous back reef. Agonisticinteractions were more frequent but feeding rates were lowerin the patch versus the continuous reef habitat. Growth rateswere lower in patch reef habitat than on the continuous reef,but mortality rates did not differ. A separate experiment usingstandard habitat units demonstrated that the patterns observedin natural habitat were the result of the spatial distributionof the habitat patches rather than resource differences between habitats. Our results do not follow the predictions of simpleIFD or IDD models. This deviation from IFD and IDD predictionsmay be the result of a number of factors, including lack ofperfect information about habitat patches, high movement costs,and higher encounter rates of dispersed patches. Our resultsdemonstrate that behavioral interactions are an integral partof population dynamics and that it is necessary to considerthe spatial organization of the habitat in both behavioraland ecological investigations.     

Assembly and response rules: two goals for predictive community ecology

Abstract. Assembly rules provide one possible unifying framework for community ecology. Given a species pool, and measured traits for each species, the objective is to specify which traits (and therefore which subset of species) will occur in a particular environment. Because the problem primarily involves traits and environments, answers should be generalizable to systems with very different taxonomic composition. In this context, the environment functions like a filter (or sieve) removing all species lacking specified combinations of traits. In this way, assembly rules are a community level analogue of natural selection. Response rules follow a similar process except that they transform a vector of species abundances to a new vector using the same information. Examples already exist from a range of habitats, scales, and kinds of organisms.     

The evolutionary ecology of biotic association in a megadiverse bivalve superfamily: sponsorship required for permanent residency in sediment

Background

Marine lineage diversification is shaped by the interaction of biotic and abiotic factors but our understanding of their relative roles is underdeveloped. The megadiverse bivalve superfamily Galeommatoidea represents a promising study system to address this issue. It is composed of small-bodied clams that are either free-living or have commensal associations with invertebrate hosts. To test if the evolution of this lifestyle dichotomy is correlated with specific ecologies, we have performed a statistical analysis on the lifestyle and habitat preference of 121 species based on 90 source documents.

Methodology/Principal Findings

Galeommatoidea has significant diversity in the two primary benthic habitats: hard- and soft-bottoms. Hard-bottom dwellers are overwhelmingly free-living, typically hidden within crevices of rocks/coral heads/encrusting epifauna. In contrast, species in soft-bottom habitats are almost exclusively infaunal commensals. These infaunal biotic associations may involve direct attachment to a host, or clustering around its tube/burrow, but all commensals locate within the oxygenated sediment envelope produced by the host’s bioturbation.

Conclusions/Significance

The formation of commensal associations by galeommatoidean clams is robustly correlated with an abiotic environmental setting: living in sediments (). Sediment-dwelling bivalves are exposed to intense predation pressure that drops markedly with depth of burial. Commensal galeommatoideans routinely attain depth refuges many times their body lengths, independent of siphonal investment, by virtue of their host’s burrowing and bioturbation. In effect, they use their much larger hosts as giant auto-irrigating siphon substitutes. The evolution of biotic associations with infaunal bioturbating hosts may have been a prerequisite for the diversification of Galeommatoidea in sediments and has likely been a key factor in the success of this exceptionally diverse bivalve superfamily.     

Seasonal respiratory responses to temperature and hypoxia in relation to burrowing depth in three intertidal bivalves

Respiratory responses to temperature and hypoxia in relation to burrowing depth were determined for winter- (W-C) and summer-conditioned (S-C) individuals of , , and . These bivalves occur sympatrically on sand-flats but display different sediment burrowing depths (. , <2 cm; . , 3–8 cm; . , 5–20 cm). A range depths over which daily temperature variation and O₂ concentration decline rapidly from surface values. Species thermal tolerance limits were found to decrease and to be more greatly temperature compensated with burrowing depth. Oxygen consumption rate (VO₂ increased with temperature to 25°C in . , but was, thereafter, regulated (Q10 1.0) up to 40°C while VO₂ increased with temperature in . and . until thermally streased at 25° to 30°C. The deposit feeding . does not acclimate VO₂ to temperature while . and . , both suspension feeders, show “reverse” acclimation [VO₂ (W-C) < VO₂ (S-C)] that conserves overwintering energy stores. The shallow burrowing . and deposit feeding . rarely experience hypoxia and are poor to non-regulators of VO₂ in reduced O₂ concentrations. In contrast, when winter-conditioned, . is a moderate regulator of VO₂, the degree of regulation increasing in S-C individuals which are exposed to higher levels of hypoxia.     

Reconciling classical and individual-based approaches in theoretical population ecology: a protocol for extracting population parameters from individual-based models

The two main approaches in theoretical population ecology-the classical approach using differential equations and the approach using individual-based modeling-seem to be incompatible. Linked to these two approaches are two different timescales: population dynamics and behavior or physiology. Thus, the question of the relationship between classical and individual-based approaches is related to the question of the mutual relationship between processes on the population and the behavioral timescales. We present a simple protocol that allows the two different approaches to be reconciled by making explicit use of the fact that processes operating on two different timescales can be treated separately. Using an individual-based model of nomadic birds as an example, we extract the population growth rate by deactivating all demographic processes-in other words, the individuals behave but do not age, die, or reproduce. The growth rate closely matches the logistic growth rate for a wide range of parameters. The implications of this result and the conditions for applying the protocol to other individual-based models are discussed. Since in physics the technique of separating timescales is linked to some concepts of self-organization, we believe that the protocol will also help to develop concepts of self-organization in ecology.     

On sampling procedures in population and community ecology

In this paper we emphasize that sampling decisions in population and community ecology are context dependent. Thus, the selection of an appropriate sampling procedure should follow directly from considerations of the objectives of an investigation. We recognize eight sampling alternatives, which arise as a result of three basic dichotomies: parameter estimation versus pattern detection, univariate versus multivariate, and a discrete versus continuous sampling universe. These eight alternative sampling procedures are discussed as they relate to decisions regarding the required empirical sample size, the selection or arrangement of sampling units, and plot size and shape. Our results indicate that the decision-making process in sampling must be viewed as a flexible exercise, dictated not by generalized recommendations but by specific objectives: there is no panacea in ecological sampling. We also point to a number of unresolved sampling problems in ecology.