Simon F. Thrush

Bioturbators enhance ecosystem function through complex biogeochemical interactions

Predicting the consequences of species loss is critically important, given present threats to biological diversity such as habitat destruction, overharvesting and climate change. Several empirical studies have reported decreased ecosystem performance (for example, primary productivity) coincident with decreased biodiversity, although the relative influence of biotic effects and confounding abiotic factors has been vigorously debated. Whereas several investigations focused on single trophic levels (for example, grassland plants), studies of whole systems have revealed multiple layers of feedbacks, hidden drivers and emergent properties, making the consequences of species loss more difficult to predict. Here we report functionally important organisms and considerable biocomplexity in a sedimentary seafloor habitat, one of Earths most widespread ecosystems. Experimental field measurements demonstrate how the abundance of spatangoid urchins—infaunal (in seafloor sediment) grazers / deposit feeders—is positively related to primary production, as their activities change nutrient fluxes and improve conditions for production by microphytobenthos (sedimentatry microbes and unicellular algae). Declines of spatangoid urchins after trawling are well documented, and our research linking these bioturbators to important benthic–pelagic fluxes highlights potential ramifications for productivity in coastal oceans.

Spatial patterns in soft-bottom communities

The spatial patterns exhibited by soft-bottom macrobenthic organismshave become recognized for their potential to play an importantrole in determining both the ecology of these species and our ability to study it. Recent studies have shown that spatial scales of field sampling or experimentation are important influences on data interpretation. The presence of patches, density gradients and spatially autocorrelated variables may confound designs and affect the validity of inferential statistics. Future studies must integrate the intensity and form of patterns from various spatial and temporal scales if we are to understand the process responsible for generating pattern.

Forecasting the limits of resilience: integrating empirical research with theory

Despite the increasing evidence of drastic and profound changes in many ecosystems, often referred to as regime shifts, we have little ability to understand the processes that provide insurance against such change (resilience). Modelling studies have suggested that increased variance may foreshadow a regime shift, but this requires long-term data and knowledge of the functional links between key processes. Field-based research and ground-truthing is an essential part of the heuristic that marries theoretical and empirical research, but experimental studies of resilience are lagging behind theory, management and policy requirements. Empirically, ecological resilience must be understood in terms of community dynamics and the potential for small shifts in environmental forcing to break the feedbacks that support resilience. Here, we integrate recent theory and empirical data to identify ways we might define and understand potential thresholds in the resilience of nature, and thus the potential for regime shifts, by focusing on the roles of strong and weak interactions, linkages in meta-communities, and positive feedbacks between these and environmental drivers. The challenge to theoretical and field ecologists is to make the shift from hindsight to a more predictive science that is able to assist in the implementation of ecosystem-based management.

The Effect of Spatial and Temporal Heterogeneity on the Design and Analysis of Empirical Studies of Scale-Dependent Systems

Processes interacting across scales of space and time influence emergent patterns in ecological systems, but to obtain strong inference and empirical generalities, ecologists need to balance reality with the practicalities of design and analyses. This article discusses heterogeneity, scaling, and design analysis problems and offers potential solutions to improve empirically based research. In particular, we recommend bridging the dichotomy between correlative and manipulative studies by nesting manipulative studies within a correlative framework. We suggest that building on variation, by designing studies to detect variability, rather than fighting it often leads to an increase in generality. We also emphasize the importance of natural history information for determining likely scales of spatial and temporal heterogeneity and the probable occurrence of feedback loops, indirect effects, and interacting processes. Finally, we integrate these concepts and suggest planned iterations between multiscale studies to build up natural history information and test the strength of relationships across space and time. This offers a way forward in terms of heuristically developing models and determining ecological generalities.

Seabed drag coefficient over natural beds of horse mussels (Atrina zelandica)

Measurements of seabed drag coefficient, C 100 , were made under tidal currents at four sites in Mahurangi Harbour, New Zealand. At the first three sites the dominant roughness element was the pinnid bivalve, Atrina zelandica (horse mussel). At the fourth site, which was devoid of horse mussels but covered in cockle shells, patches of seaweed and crab burrows, C 100 was smallest (0.0055), but still twice as large as the value typically applied to abiotic, flat, cohesionless seabeds (0.0025). The mean drag coefficient plus-or-minus standard error at the three sites with horse mussels was: 0.0082 ± 0.0010 (site 1); 0.0096 ± 0.0009 (site 2); 0.0115 ± 0.0016 (site 3). There were no clear differences amongst sites 1, 2 and 3 in terms of the attributes of individual horse mussels (e.g. shell height, width or orientation), which could have been used to explain the ranking of the drag coefficients. There were, however, differences amongst the three sites in terms of spatial distribution of individual bivalves. The site with the highest density of horse mussels, site 1, had the lowest drag coefficient and an areal concentration (λ) of horse mussels higher than typical values cited for the critical concentration (λ c ) for the onset of skimming flow over various idealized, three-dimensional roughness elements. At sites 2 and 3, the drag coefficient was given by: Formula math. which was valid for λ < λ c , where K is von Karmans constant, k is the horse mussel height (i.e., protrusion above the seabed), m 100 and λ c 0.2. The stable eddies that are hypothesized to lodge between roughness elements at concentrations greater than λ c may influence benthic community dynamics.

Adult infauna as facilitators of colonization on intertidal sandflats

Adult macrofauna can be present during the initial stages of recolonization as a result of their resistance to disturbance or migration. They may have an important effect on recolonization of disturbed patches because of their greater size and higher rates of feeding and bioturbation, relative to invading larvae and juveniles. An experiment was conducted to assess the influence of the polychaetes Heteromastus filiformis (Claparede) and Aonides oxycephala (Sars) and the bivalve Tellina liliana Iredale on the structure of assemblages colonizing small defaunated patches. Both T. liliana and A. oxycephala, when added separately to defaunated sediments, facilitated the colonization of other species and/or conspecifics. The addition of H. filiformis to defaunated patches had little influence on the population density of common taxa but did increase the variety of colonizing taxa, especially polychaetes. These results emphasise the importance of facilitatory interactions in macrofaunal recolonization of intertidal sandflats. The patterns of species association apparent from this experiment are discussed in relation to the disturbances created by feeding rays in this habitat.

Macrobenthic community composition of six intertidal sandflats in Manukau Harbour, New Zealand

Abstract Macrobenthic community structure was assessed on intertidal sandflats, a predominant feature of Manukau Harbour. Thirty‐six replicate core samples were collected from each of six 9000 m2 sites, during October 1987. The distribution of individuals amongst taxa was similar at the six sites, but the numerical dominance of the most common taxa changed appreciably from site to site. A shift from polychaete‐ to bivalve‐dominated communities was apparent between sites. Examination of a hypothesised relationship between sediment grain size and trophic structure of the macrobenthos highlights the difficulties in using this relationship to monitor environmental change. Identification of core taxa by DECORANA ordination and TWINSPAN classification is considered a useful mechanism in defining taxa likely to play major roles in influencing community structure and function.

Real world biodiversity–ecosystem functioning: a seafloor perspective

The effective application of biodiversity-ecosystem function (BEF) research to societal needs amid the Anthropocene represents the next grand challenge for ecology. Biodiversity knowledge that is most meaningful to society must reconcile insights derived from theory with detailed experiments and broad-scale trends. This perspective requires science that addresses high species richness, redundancy, and natural variability, which simplified model systems cannot mimic. Here, we illustrate solutions of biodiversity knowledge to management and societal problems that combine BEF with scaling experiments, analysis of BEF along environmental gradients, and mapping technologies. We primarily draw examples from biophysical interactions in seafloor environments, which cover 70% of the Earth and add significantly to global ecosystem functions and services.

Improving ecosystem service frameworks to address wicked problems

Complex problems often result from the multiple interactions between human activities and ecosystems. The interconnected nature of ecological and social systems should be considered if these “wicked problems” are to be addressed. Ecosystem service approaches provide an opportunity to link ecosystem function with social values, but in practice the essential role that social dynamics play in the delivery of outcomes remains largely unexplored. Social factors such as management regimes, power relationships, skills, and values, can dramatically affect the definition and delivery of ecosystem services. Input from a diverse group of stakeholders improves the capacity of ecosystem service approaches to address wicked problems by acknowledging diverse sets of values and accounting for conflicting world views. Participatory modeling can incorporate both social and ecological dynamics into decision making that involves stakeholders, but is itself a complex social undertaking that may not yield precise or predictable outcomes. We explore the efficacy of different types of participatory modeling in relation to the integration of social values into ecosystem services frameworks and the generation of four important elements of social capital needed to address wicked problems: enhancing social learning and capacity building; increasing transparency; mediating power; and building trust. Our findings indicate that mediated modeling, group mapping, and mental/conceptual modeling are likely to generate elements of social capital that can improve ecosystem service frameworks. Participatory simulation, system dynamic modeling, and Bayesian belief networks, if utilized in isolation, were found to have a low likelihood of generating the social capital needed to improve ecosystem services frameworks. Scenario planning, companion modeling, group model building, and participatory mapping all generate a moderate to high level of social capital elements that improve the capacity of ecosystem service frameworks to address wicked problems.

Structure of the internal boundary layer over a patch of pinnid bivalves ( Atrina zelandica ) in an estuary

Measurements of tidal-current boundary-layer flow over an experimental 2-m by 2-m patch of pinnid bivalves (Atrina zelandica) in a northern New Zealand estuary are presented. Previous work demonstrated a link between mesoscale (order 100 m) patchiness of the benthic biota and time-averaged boundary-layer dynamics. The aim in this new experiment was to describe the three-dimensional structure of turbulence at the patch scale (order 1 m). Flow over three densities of Atrina was investigated: 340 individuals per 4 m 2 , 50 individuals per 4 m 2 and zero individuals. An internal boundary layer (IBL) grows downstream from the leading edge of the patch at the base of the ambient boundary layer. One meter in from the leading edge, the top of the IBL was ∼12 cm above the bed for the high-density patch and ∼6 cm for the low-density patch. Flow in the IBL was three-dimensional in that vertical and transverse mean velocities were nonzero, secondary Reynolds stresses were nonzero and comparable with the primary stress, and velocity spectra deviated from scaling relationships for two-dimensional flow. Thus, the observed IBL was still in its infancy, i.e., it consisted of a roughness sublayer only as the distance from the leading edge of the patch was not enough for development of a second, overlying logarithmic layer. In summary, the IBL that envelops the Atrina patch is a region of lower mean longitudinal velocities but more energetic turbulence relative to the ambient boundary layer. The former translates into shelter, which some organisms might take advantage of, and the latter translates into increased vertical exchange across the top of the IBL, which might enhance fluxes of nutrients, colonists and suspended sediments, and might have implications for deposition and resuspension of organically rich biodeposits. The results extend our knowledge of turbulence over patches of suspension feeders at the 1-m scale and therefore provide information needed to improve depiction of flow in models of suspension-feeder-flow interactions.