Susan S. Bell

Habitat structure: The evolution and diversification of a complex topic

Habitat structure, by definition, is a component of every ecological study. This book deals with a particular type of structure, that provided by the arrangement of objects in space. Even restricted in this way, habitat structure conjures up a multitude of images in the minds of ecologists, from concrete topographic features to near-abstractions like ‘patches’, ‘mosaics’, and ‘gradients’. The variety of types of physical habitat structure has, in turn, spawned a wealth of narrowly defined terms meant to convey subtle aspects of the relationship between organism and structure. While these terms may do exactly what was intended of them, we suggest that the gain in precision is offset by a loss in generality. The various subdisciplines of ecology adopt terminologies, and experimental techniques related to them, largely for the cognoscente. ‘Profile of vegetational density’ and ‘canopy surface structure’, for instance, may end up having explicit meaning for a particular ecologist, whereas ‘substrate heterogeneity’ and ‘enemy free space’ may not. Yet, all reside under the broad umbrella of habitat structure.

Seagrass landscapes: a terrestrial approach to the marine subtidal environment

Subtidal seagrass habitats are prime candidates for the application of principles derived from landscape ecology. Although seagrass systems are relatively simple compared to their terrestrial counterparts in terms of species diversity and structural complexity, seagrasses do display variation in spatial patterns over a variety of scales. The presence of a moving water layer and its influence on faunal dispersal may be a distinguishing feature impacting ecological processes in the subtidal zone. Studying seagrass-dominated landscapes may provide a novel approach to investigating questions regarding self-similarity of spatial patterns, and offers a new perspective for analysing habitat change in a variety of marine environments.

Faunal response to fragmentation in seagrass habitats: implications for seagrass conservation

Fragmentation in seagrass systems results in changes to landscape features that may have implications for fauna. We examine published studies to identify whether faunal abundance shows any relationship with patch size of seagrass beds, suggesting preferential use of the edge or interior by seagrass associated taxa. In a series of studies in Tampa Bay, FL, we also examined: (1) the relationship between abundance of both fish and amphipod fauna and seagrass patch size in 24 seagrass (Halodule wrightii) beds (5–93 m2) in 1994 and 1995; (2) whether abundance of the infaunal polychaete, Kinbergonuphis simoni, was significantly different within the 1 m edge versus interior of two seagrass (Halodule wrightii ) beds of similar size and age; and (3) compared the spatial distribution of the tube-building polychaete, Spirorbis spirillum, in Thalassia testudinum seagrass beds in two sites in Tampa Bay. Neither review of the published literature on fauna and seagrass patch size nor the data presented from our Tampa Bay studies suggest that habitat fragmentation has any consistent impact on fauna over the spatial scales that have been investigated. Likewise, little evidence exists that identifies any taxonomic group to be fragmentation sensitive in that they differentially utilized edge or core areas of seagrass patches. While we did detect a reduction of both infaunal and epibenthic polychaetes at the 1 m edges of seagrass beds relative to interior areas, the reduction was not similar along all edges. Events such as seagrass die off or a high incidence of boat propeller-damage over an extensive area may be required to detect effects of habitat fragmentation on fauna. Given that patch size alone does not appear to adequately account for variation in faunal abundance, we suggest that restoration efforts might best focus upon locating areas with similarity of landscape context or patch characteristics other than patch size.

An Interdisciplinary and Synthetic Approach to Ecological Boundaries

Abstract We introduce a collection of articles that proposes conceptual and methodological tools to advance the integrated study of ecological boundaries. A number of studies are germane to understanding the structure and function of boundaries over a wide array of ecological systems and scales. However, these studies have not been unified in a consistent theoretical framework. To integrate these seemingly disparate studies and to advance future research on boundaries, these articles present a common conceptual framework, a classification of the different types of boundaries and their potential functions, and statistical and modeling approaches that can be applied to a wide range of systems, processes, and scales. We summarize the themes that emerge from these articles and suggest questions to guide future research.

MODELING SEAGRASS LANDSCAPE PATTERN AND ASSOCIATED ECOLOGICAL ATTRIBUTES

A predictive model for seagrass bed coverage (presence/absence at 1-m res- olution) and ecological attributes of the bed, such as biomass and shoot density, would be a valuable management tool. But forming such a predictive model is complicated by a number of factors that strongly influence seagrass bed structure and our interpretation of its ecological function. The factors include the effects of waves and water depth (hydro- dynamic setting) and the spatial and temporal scales of the sampling technique itself. In this study, we examined the coherence of predictions of seagrass cover and ecological attributes of temperate, mixed-species seagrass derived from two common sampling tech- niques, (video) line transect (commonly used by biologists) and grid-sampled surveys (often used in remote sensing). Mapping resolution was held constant at 1 m, and the two tech- niques applied across seagrass beds of varying coverage that reflected the effect of a hydrodynamic gradient ranging from patchy, high-energy beds to continuous cover, low- energy beds. We found that the prediction of seagrass coverage as a function of hydro- dynamic setting can be improved not only by increasing the spatial extent of sampling at a fixed resolution (1 m), but also by ensuring that data for both dependent (e.g., percent cover) and independent (e.g., wave exposure) variables are averaged over similar scales (spatial extent and resolution). Large-scale features of the landscape, such as patches several meters in width, appeared to be best quantified by sampling over a large spatial extent, as with the video transects. Therefore, contiguous sampling over a broad spatial extent, as opposed to our numerous, somewhat smaller sampling (grid-sampled, 50 3 50 m areas) is the more appropriate strategy for predicting the probability of seagrass bed cover. Con- versely, we found that ecological attributes of the seagrass bed (biomass, shoot density, and sediment composition) were best characterized by sampling over a shorter spatial extent (i.e., ,50 m), indicating that very localized conditions may have influenced patterns of seagrass community attributes. Generalizing information about seagrass bed ecological attributes obtained from high-resolution samples (fine scale) taken over a broad spatial extent (coarse or landscape scale), as may occur with resource surveys and impact assess- ments, has the potential to be highly misleading, especially in patchy environments. The influence of sampling scale and survey method on the prediction of coverage and ecological attributes of seagrass beds reveals the need to carefully choose sampling designs to evaluate seagrass distribution and their associated ecological characteristics in the Beaufort, North Carolina (USA) area, and perhaps in other like habitats.

DYNAMICS OF A SUBTIDAL SEAGRASS LANDSCAPE: SEASONAL AND ANNUAL CHANGE IN RELATION TO WATER DEPTH

The spatial heterogeneity of a subtidal marine landscape and the areal extent of both monospecific and mixed patches of seagrass species were studied in Tampa Bay, Florida, USA. Specifically, we examined the temporal dynamics of seagrass distribution and its relationship to water depth and the serial replacement of one species by another. The ∼5-ha landscape was mapped at 1-m intervals in the spring and fall of 1994 and 1995. The landscape consisted of monospecific and mixed patches of seagrass (47%) and bare sediment (53%). Halodule wrightii was the most abundant seagrass (∼74%), while Thalassia testudinum was second most common (20%), and mixed patches of H. wrightii and T. testudinum composed the remaining 6%. There was an overall increase in seagrass of 14% from spring 1994 to fall 1995. The majority of change occurred along the margins of existing seagrass patches (i.e., H. wrightii invading bare sediment). Typically, “new” patches were the result of the transition of one seagrass species to another ...

Gap Dynamics in a Seagrass Landscape

ABSTRACT We investigated gap dynamics within a shallow subtidal landscape characterized by seagrass vegetation and examined the relationship between gap formation and selected physical factors. The study was conducted over 2 y by using a biannual mapping of seagrass and water depth across an 48,800-m2 area in Tampa Bay, Florida. In addition, monthly sediment deposition or erosion was recorded at 96 locations within the landscape. Gaps represented from 2.4% to 5.7% of the seagrass landscape, and all were within monospecific stands of Halodule wrightii. Gaps ranged in size from 10 to 305 m2 and most frequently decreased in size over time. Most gaps were small and short lived (less than 6-mo duration), but the second age group most frequently recorded was at least 1.5 y old. No new species of seagrass invaded the gaps with Halodule replacing itself 100% of the time. Gaps were recorded over the entire range of water depths within the landscape. Neither gap area nor persistence of gaps was related to water depth. However gap area was associated positively with the number of extreme sedimentation events. Gaps originated not only from removal of interior vegetation (similar to classic gaps) but also from differential growth of the seagrass margin (similar to edaphic gaps). Distinct seasonal components to the mode of formation were detected with interior-produced gaps originating primarily in the winter and margin gaps most commonly during summer. These results combine to illustrate the importance of large-scale studies with fine-scale resolution for deciphering unique features of seagrass landscape dynamics. Our historical information suggests that a static enumeration of gaps may not provide an accurate assessment of disturbance intensity in this system, and the seagrass mosaic probably is explained best by a combination of disturbance regimes and edaphic factors, such as sediment stability. Moreover, we suggest that even in areas characterized by monospecific stands of vegetation and over short or moderate time periods, gaps indirectly may influence community structure and ecosystem function via modification of habitat arrangement.

Identifying Biotic Boundaries Along Environmental Gradients

We outline a new procedure for placing boundaries of faunal and floral distributions along environmental gradients. Four data sets on species distributions from the marine literature, each of which had been scrutinized previously for boundaries, were analyzed with the procedure. Placement of boundaries proved straightforward in all cases. Our procedure possesses several attributes that make it superior to other methods of boundary location: (1) it is graphical and amenable to exploratory data analysis, (2) it is based upon taxonomic similarity between locations that is judged in a probabilistic manner, (3) it uses all locations simultaneously, (4) it provides a level of confidence in any boundary, and (5) it is precise: two workers employing it will arrive at the same conclusion.

Meiofauna from Seagrass Habitats: A Review and Prospectus for Future Research

Although many studies exist which document abundances of epibenthic and sediment-dwelling macrofauna from seagrass habitats, little descriptive or experimental information is available on meiofauna from these systems. Much of this discreapancy is a result of sampling techniques or sample processing. Herein we critically review the literature on meiofauna from temperate and tropical seagrass systems and present data on meiofauna from three subcommunities within a Tampa Bay, Florida seagrass bed—seagrass blades, sediments surrounding individual culms and the water column. Four areas for future research are identified: 1) comparisons of macrofauna and meiofauna from seagrass sites and a description of their trophic interactions; 2) elucidation of relationships between meiofauna and algal epiphytes; 3) monitoring of vertical migration of meiofauna from sediments into the water column; and 4) biogeographic comparisons of 1–3 above.

Toward a landscape approach in seagrass beds: using macroalgal accumulation to address questions of scale

An experimental investigation of drift macroalgal accumulation in seagrass beds was conducted to determine if the relationship between passively dispersed plant structure and the spatial arrangement of rooted macrophytes differed when examined across two spatial scales. Experiments were performed from December 1992 to April 1993 at four different sites in Tampa Bay, Florida, utilizing artificial seagrass units (ASUs) of uniform shoot length and density but with different areal dimensions [1 m2 (S) versus 4 m2 (L)]. Drift macroalgae were also collected from 1 m×1 m plots of natural seagrass at each of the experimental sites from November 1990 to May 1992 to determine the relationship between macroalgal abundance and structural characteristics of natural seagrass. Disproportionately higher amounts of macroalgae were captured in L compared to S plots suggesting that macroalgal accumulation does not scale up directly with the areal dimensions of ASU patches. Higher amounts of algae recovered in L plots is in accordance with patterns expected if algae accumulate in zones of attenuated water flow. Neither seagrass shoot density nor blade length could adequately describe the patterns of algal accumulation. These combined results suggest that explanations for trapping/retention of passively dispersed particles should extend beyond traditional measures of vegetation complexity.