R. D. Pridmore

Scale‐Dependent Recolonization: The Role of Sediment Stability in a Dynamic Sandflat Habitat

An important ecological issue is developing an understanding of how patterns and processes vary with scale. We designed a field experiment to test how differences in the aerial extent of disturbance affected macrofaunal recolonization on a sandflat. Three different plot sizes (0.203 m 2 , 0.81 m 2 , and 3.24 m 2 ) were defaunated, and samples were collected to assess recovery over a 9-mo period. As the sandflat used for the experiment was prone to disturbance by wind-driven waves, we also measured changes in sediment bed height (an indicator of sediment stability) over the course of the experiment. Most common species revealed significant relationships between density and disturbance plot size. Scale-dependent recovery was also demonstrated by differences in species assemblage structure over the course of the experiment. Relative rates of colonization varied by 50% between large and small experimental plots. However, these differences were not related to specific species, particular functional groups, or potential modes of colonization. The results revealed an unusually slow rate of faunal recovery following defaunation. Both increasing numbers of colonists and density changes in ambient sediments made an important contribution to recovery. The relationship found between changes in sediment bed height and wind velocity indicated that wind-driven wave disturbance was an important factor influencing sediment instability. Sediment instability was higher in all experimental plots than in the ambient sediments, due to the initial removal of a dense spionid polychaete tube mat characteristically found at the study site. Sediment instability also increased with increasing plot size. Thus in this dynamic sandflat habitat, faunal emigration from recovering disturbed patches of sediment may significantly slow rates of recolonization. These results demonstrate that incorporating patch size, emigration, recovery time, and interactions between hydrodynamic conditions and habitat stability (particularly where colonists influence sediment stability) are crucial to generating a general understanding of recovery processes in soft-sediment habitats. While our results demonstrate the need for caution in scaling-up from small-scale studies, they do indicate that larger scale disturbances that destroy organisms with a role in maintaining habitat stability are likely to result in very slow recovery dynamics, particularly in wave-disturbed soft-sediment habitats.

Spatial structure of bivalves in a sandflat:: Scale and generating processes

A survey was conducted during the summer of 1994 within a fairly homogeneous 12.5 ha area of sandflat off Wiroa Island, in Manukau Harbour, New Zealand, to identify factors controlling the spatial distributions of the two dominant bivalves, Macomona liliana Iredale and Austrovenus stutchburyi (Gray), and to look for evidence of adult–juvenile interactions within and between species. Most of the large–scale spatial structure detected in the bivalve count variables (two species, several size classes of each) was explained by the physical and biological variables. The results of principal component analysis and spatial regression modelling suggest that different factors are controlling the spatial distributions of adults and juveniles. Larger size classes of both species displayed significant spatial structure, with physical variables explaining some but not all of this variation. Smaller organisms were less strongly spatially structured, with virtually all of the structure explained by physical variables. The physical variables important in the regression models differed among size classes of a species and between species. Extreme size classes (largest and smallest) were best explained by the models; physical variables explained from 10% to about 70% of the variation across the study site. Significant residual spatial variability was detected in the larger bivalves at the scale of the study site. The unexplained variability (20 to 90%) found in the models is likely to correspond to phenomena operating at smaller scales. Finally, we found no support for adult–juvenile interactions at the scale of our study site, given our sampling scale, after controlling for the effects of the available physical variables. This is in contrast to significant adult–juvenile interactions found in smaller–scale surveys and in field experiments. Our perception of adult–juvenile interactions thus depends on the scale of study.

Bedload and water-column transport and colonization processes by post-settlement benthic macrofauna: Does infaunal density matter?

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. In this study, we document dispersal and colonization by post-settlement benthic macrofauna on a large (250×500 m) area of intertidal sandflat at Wiroa Island in the Manukau Harbour, New Zealand, over a 2-week period in February 1994. We examine the effects of variation in the natural density of large (>15 mm) Macomona liliana Iredale (a tellinid bivalve) on these processes. Post-settlement transport was measured using cylindrical bedload and water-column traps positioned at 22 experimental sites within the study site. Macrofaunal colonization was documented using small pans of defaunated sediment deployed at each of the experimental sites. Greatest macrofaunal colonization of pans occurred in areas of greatest sediment reworking and deposition, suggesting transport and deposition of post-settlement stages may be passive processes. For some species [e.g. Austrovenus stutchburyi (Wood)] there was a significant positive relationship between their abundance and the weight of sediment in bedload traps, as well as wind conditions over the study period. This is consistent with passive transport processes, but does not exclude the possibility of active dispersal processes also being important. Differences in the hydrodynamic conditions (e.g. increased wind generated wave activity) over the 2-week study may have contributed to changes in abundance on the 2 sampling dates. For other species (e.g. Macomona liliana) the absence of significant positive relationships between abundance in either bedload or water-column traps and the weight of sediment retained in traps, as well as between ambient densities and the net flux of individuals across sites, is consistent with active dispersal and colonization. There was a negative relationship between background density of large Macomona liliana and numbers of smaller conspecifics (≤4 mm) and other bivalves [Austrovenus stutchburyi, Cyclomactra ovata (Gray) and Nucula hartvigiana Pfeiffer] colonizing pans of defaunated sediment. The results suggest that 10 to 100s m scale variation in the natural densities of an abundant siphonal surface deposit feeder may have a significant influence on dispersal and colonization and, thus, the spatial patterns of macrofauna on the tidal flat.

Scaling-up from experiments to complex ecological systems: Where to next?

a , b c d e * S.F. Thrush , D.C. Schneider , P. Legendre , R.B. Whitlatch , P.K. Dayton , a f a g h J.E. Hewitt , A.H. Hines , V.J. Cummings , S.M. Lawrie , J. Grant , a a i R.D. Pridmore , S.J. Turner , B.H. McArdle National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand Ocean Sciences Centre, Memorial University of Newfoundland, St. John’s, Canada ALC5S7 c ́ ́ ́ Departement de Sciences Biologiques, Universite de Montreal, C.P. 6128, succursale Centre-ville, ́ ́ Montreal, Quebec H3C 3J7, Canada Dept. Marine Sciences, University of Connecticut, Groton, CT 06340-6097, USA Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0201, USA Smithsonian Environmental Research Center, P.O. Box 28, Edgewater, MD 21037, USA Culterty Field Station, University of Aberdeen, Newburgh, AB40AA, Scotland Dept. of Oceanography, Dalhousie University, Halifax, Canada B3H 4JI Biostatistics Unit, School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand

Matching the outcome of small-scale density manipulation experiments with larger scale patterns: an example of bivalve adult/juvenile interactions

Generalising or scaling up from small-scale experiments to larger areas is an important challenge for both ecology and conservation biology. This study describes a technique that attempts to meet this challenge by combining spatial mapping with small-scale process experiments. Specifically, we evaluate the density effects of large individuals (>15 mm shell length) of a tellinid bivalve (Macomona liliana Iredale) on macrofauna in 0.25 m2 experimental plots within the natural density variation of large Macomona over a 12.5 ha site. By mapping the spatial distribution of large Macomona before conducting the experiment, we were able to identify homogeneous areas with different background densities of large Macomona and embed 22 experimental locations within the natural density-scape. Within each location, four experimental densities were added to plots from which all large macrofauna (>4 mm) had been previously removed. Macrofauna were sampled 22 days after the start of the experiment and significant negative treatment effects of high densities of large Macomona were identified by ANOVA for juvenile bivalves Macomona (<4 mm), Austrovenus stutchburyi (Gray) (<4 mm), the isopod Exosphaeroma falcatum Tattersall and the total number of individuals. Generalised linear models were then used to include the effect of background density variation of large Macomona in the analysis. Only Austrovenus (<4 mm) demonstrated a significant interaction between the background and experimental densities of large Macomona. This resulted from background densities of large Macomona having a significant effect on Austrovenus (<4 mm) in the two lowest density treatments only. Significant effects were detected only because we had planned the study to cover the various background densities of Macomona. The effect of experimental and background density variation of large Macomona on Macomona (<4 mm), Exospheroma, nemerteans and the total number of individuals were similar in direction and strength. Except for nemerteans, all relationships were negative, with low densities of macrofauna associated with high experimental and background densities of large Macomona. This implies that large-scale extrinsic factors (e.g., elevation, exposure to wave disturbance) are not the only features influencing the distribution of Macomona at the scale of the study site; intrinsic processes operating on smaller scales are also important. This scale-dependent response would not have been uncovered, had we not conducted a larger-scale survey in concert with the smaller-scale experiment.

Patterns in the spatial arrangements of polychaetes and bivalves in intertidal sandflats

Spatial autocorrelation correlograms based on Morans coefficient were constructed for common polychaetes (Goniada emerita, Heteromastus filiformis, Macroclymenella stewartensis, Boccardia spp. and Magelona? dakini)_and bivalves (Nucula hartvigiana, Soletellina siliqua and Tellina liliana) collected from intertidal sandflats of Manukau Harbour (New Zealand) during October, 1987. Patterns of heterogeneity on a scale smaller than inter-sample distance, homogenous density patches (5 to 30 m radius) and gradients in abundance running through sample sites (9 000 m2) were identified. Patterns could be defined even for species with distributions which, based on the variance: mean ratio test, were not significantly different from random. The possible influence on two of the study sites of sediment disturbances generated by feeding rays is discussed. Identification of spatial patterns is considered an important aspect of the design of surveys and manipulative field experiments.

Post-settlement movement by intertidal benthic macroinvertebrates: Do common New Zealand species drift in the water column?

Abstract To determine whether post‐settlement juveniles of New Zealand soft‐sediment macro‐fauna drift in the water column, samples were collected, day and night, above an intertidal sandflat in Manukau Harbour, New Zealand. Forty benthic taxa were collected in the net samples. Most taxa were Polychaeta (18 taxa), Bivalvia (9 taxa), or Amphipoda (4 taxa). All bivalves and gastropods collected were post‐settlement juveniles, 3 mm or less in size. Most of the polychaetes captured were either metamorphosing planktonic larvae or young benthic stages. In contrast, most of the captured arthropods were adults. Generally, benthic species found in the water column were also common in the benthos. Definite patterns in migratory activity were exhibited by some taxa over each 24 h sampling period. Taxa collected in the water column varied with sampling date, time of day, state of the tide, and depth of sampling. Only one of the benthic species collected in the net samples showed some ability to partition itself verti...

Scale-dependent benthic recolonization dynamics: life stage-based dispersal and demographic consequences

Soft-sediment recruitment dynamics are dependent upon two sources of colonists; larvae transported from the water column and post-settlement movement of juvenile and/or adult life-stages across the seabed. Differences in the relative dispersal ability of the different life-stages into disturbed patches of habitat should vary predictably with the spatial scale of the disturbance. Smaller patches with a greater edge:surface area ratio should be more influenced by the post-settlement colonist pool than larger patches possessing a smaller edge:surface area ratio. A life stage-based recolonization model, using a Polydora cornuta life table, has been developed to describe how differences in the immigration rates of larvae, juveniles and adults can influence within-patch recovery times. Model results indicate that immigration of adult stages into disturbed patches has the least influence on patch recovery time. In contrast, post-settlement juvenile stages generally has a pronounced effect on patch population dynamics. Experimental evidence of scale-dependent migration of different life-stages to disturbed patches is also presented for the spionid polychaete Boccardia syrtis. Future research on scale-dependent recolonization dynamics in soft-sediment habitats should focus on acquisition of within-patch demographic data in order to more fully understand the importance of post-settlement life-stage movement in regulating population and 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.