Direct and indirect interactions of native and introduced species in coastal habitats

<p>Current research has emphasized the need to identify and quantify the effects of positive and negative interactions (both direct and indirect) between species, taking into account the influence of abiotic conditions and spatial scales. In this research it is particularly challenging to adequately assess and predict the impact of introduced species on native communities. This study examined interactions of introduced and native species on coastal sand dunes in New Zealand. Substantial areas of this habitat have become dominated by the highly competitive exotic sand-binder marram grass (Ammophila arenaria) and other exotic weeds, replacing native species, including the native eco-engineering spinifex (Spinifex sericeus). First, I examined direct interactions (competition and facilitation) between marram and spinifex along an abiotic stress gradient where experimental plots were subject to different restoration management techniques. In my large-scale dune experiment I planted 2475 spinifex seedlings in three different treatments (bare sand, live marram, dead marram) including an unplanted control, and monitored the plots for one year along exposed marram grass-dominated dune fields near Whanganui, West Coast, North Island/New Zealand. The stress gradient hypothesis predicts facilitation will be greatest where stress is most severe. I hypothesized facilitation of spinifex plantings and other self-colonizing plants in dead sprayed marram compared to live marram and a change of interaction between spinifex and marram grass along an abiotic stress gradient. Spinifex survival was not significantly different across treatments, but the interaction between treatment and location from the sea as well as pre-existing marram cover were significant predictors of plant growth in a linear mixed effect model. Exotic weeds such as Senecio elegans, Conyza canadensis, dandelions and legumes were facilitated by sprayed marram grass relative to abundances in live marram grass, while grasses other than marram grass and spinifex performed best in bare sand, in the absence of competitors. For S. elegans, abundances were higher closer to the sea. Spinifex growth was greatest in plots closest to the sea. Comparing spinifex growth in live marram grass and dead marram showed similar patterns at the fore, mid and back dune, but sprayed maram showed better facilitation of spinifex. For restoration plantings it is only recommended to plant into dead marram if the site is not weed prone as weeds were facilitated in the same way as spinifex and in some instances appeared to hinder spinifex growth. Crucial for a successful conversion from marram to spinifex is that the initial marram cover is not too high prior to spraying- lower densities allow for better spinifex growth. Second, I examined indirect competition with a survey of natural populations and a common garden experiment at a shingle beach. I was interested in determining the influence of plant density at different spatial scales as well as plant morphology on insect abundances. I studied indirect interactions by choosing native and introduced Senecio spp. as host plants for insect colonization and incorporated fine scale plant density (50cm radius circle, area = 0.8 m²) and coarse scale plant density (6m quadrat annulus, area = 32 m²) as predictors for insect colonization in addition to plant morphology. I surveyed Senecio spp. over a period of three growth seasons and conducted an experiment where I manipulated densities of pairs of species of either introduced Senecio elegans, S. skirrhodon and native S. lautus. My survey of natural populations and the manipulative field experiment show a negative impact of high conspecific and heterospecific plant density on the colonization of the seed head predator fly Sphenella fascigera. Along with plant density, plant size was a significant predictor of insect abundances. Higher densities of introduced S. elegans indirectly facilitated S. lautus at fine spatial scales by reducing the incidence of S. fascigera. This supports the resource dilution hypothesis which predicts higher insect herbivore numbers on isolated resource patches. Thus, my results provide empirical evidence for apparent facilitation of a native plant by an introduced plant via a shared herbivore. For future assessments of the impacts of invasive species it will be important to consider the net-outcome of direct and indirect competitive and facilitative interactions. In particular, for restoration purposes in stressful environments removal of invasive species may have to occur in a carefully controlled manner taking into account the abiotic conditions and spatial scales at which interactions occur.</p>

Sand Dune Restoration in New Zealand: Methods, Motives, and Monitoring

<p>Sand dunes are critically endangered ecosystems, supporting a wide variety of specialist native flora and fauna. They have declined significantly in the past century, due to coastal development, exotic invasions, and stabilization using marram grass (Ammophilia arenaria). An increasing number of restoration groups have carried out small scale rehabilitations of using native sand binding plants spinifex (Spinifex sericeus) and pingao (Desmoschoenus spiralis). However like many other restoration ventures, efforts are not formally monitored, despite the potential for conservation of species in decline. This thesis seeks to investigate the social and ecological aspects of sand dune restoration in New Zealand. Firstly, the status of restoration in New Zealand was examined using web based surveys of dune restoration groups, identifying motivations, methods, and the use of monitoring in the restoration process. Secondly, the ecology of restored and marram dominated sand dunes was assessed. Vegetation surveys were conducted using transects of the width and length of dunes, measuring community composition. Invertebrates were caught using pitfall traps and sweep netting, sorted to order, and spiders, beetles and ants identified down to Recognizable Taxonomic Units (RTUs) or species where possible. Lizards were caught in pitfall traps, and tracking tunnels tracked the presence of small mammals in the dunes. Analysis of each variable involved the comparison of biodiversity data between restored and marram dominated dunes, at six sites across the Wellington region. The survey of dune restoration practitioners confirmed that restoration was generally based on the motivation of erosion protection and foreshore stabilization, however an increasing number of groups were interested in the conservation of flora. Conservation of fauna was a priority for only one of the respondents. Informal monitoring of restoration attempts was carried out by the majority of groups, but specific biodiversity monitoring or monitoring using systematic scientific methods was carried by only a small proportion of groups. Re-vegetation of dunes commonly used a small suite of native sand binding species mostly pingao and spinifex. Species in decline such as sand tussock (Austrofestuca littoralis) and sand daphne (Pimelia arenaria) were only planted at a small proportion of sites. Restoration of dune ecosystems has the potential to not only enhance erosion protection and sand stabilization mechanisms, but to benefit native flora and fauna endemic to sand dunes. Identifying biological change and carrying out biodiversity monitoring may be beneficial in maximizing the ecological effectiveness of restoration attempts. Marram dunes contained higher foliage cover, vegetation height and vegetation species diversity than restored dunes. Abundance and diversity of beetle, spider, and ant families were higher in marram dominated dunes. Estimated population size of common skink (O. nigraplantare polychroma) and mouse population density was also higher in marram dunes. These results were positively correlated with the percentage of vegetation foliage cover and vegetation species diversity, suggesting that the habitat conditions created by marram grass were favored by fauna. These results suggest that for maximum biodiversity gains, future dune restoration attempts should increase vegetation cover, and include a wider range of plant species. Species in decline known to be important for fauna, such as pohuehue (Muehlenbeckia spp.), sand pimelia, and sand coprosma (Coprosma acerosa) should also be included for reciprocal benefits for conservation of flora and fauna. Marram grass could also be incorporated into restoration, as its mass removal may have considerable consequences for fauna using it as a refuge, and it appears to provide desirable habitat for fauna.</p>

Sand Dune Restoration in New Zealand: Methods, Motives, and Monitoring

<p>Sand dunes are critically endangered ecosystems, supporting a wide variety of specialist native flora and fauna. They have declined significantly in the past century, due to coastal development, exotic invasions, and stabilization using marram grass (Ammophilia arenaria). An increasing number of restoration groups have carried out small scale rehabilitations of using native sand binding plants spinifex (Spinifex sericeus) and pingao (Desmoschoenus spiralis). However like many other restoration ventures, efforts are not formally monitored, despite the potential for conservation of species in decline. This thesis seeks to investigate the social and ecological aspects of sand dune restoration in New Zealand. Firstly, the status of restoration in New Zealand was examined using web based surveys of dune restoration groups, identifying motivations, methods, and the use of monitoring in the restoration process. Secondly, the ecology of restored and marram dominated sand dunes was assessed. Vegetation surveys were conducted using transects of the width and length of dunes, measuring community composition. Invertebrates were caught using pitfall traps and sweep netting, sorted to order, and spiders, beetles and ants identified down to Recognizable Taxonomic Units (RTUs) or species where possible. Lizards were caught in pitfall traps, and tracking tunnels tracked the presence of small mammals in the dunes. Analysis of each variable involved the comparison of biodiversity data between restored and marram dominated dunes, at six sites across the Wellington region. The survey of dune restoration practitioners confirmed that restoration was generally based on the motivation of erosion protection and foreshore stabilization, however an increasing number of groups were interested in the conservation of flora. Conservation of fauna was a priority for only one of the respondents. Informal monitoring of restoration attempts was carried out by the majority of groups, but specific biodiversity monitoring or monitoring using systematic scientific methods was carried by only a small proportion of groups. Re-vegetation of dunes commonly used a small suite of native sand binding species mostly pingao and spinifex. Species in decline such as sand tussock (Austrofestuca littoralis) and sand daphne (Pimelia arenaria) were only planted at a small proportion of sites. Restoration of dune ecosystems has the potential to not only enhance erosion protection and sand stabilization mechanisms, but to benefit native flora and fauna endemic to sand dunes. Identifying biological change and carrying out biodiversity monitoring may be beneficial in maximizing the ecological effectiveness of restoration attempts. Marram dunes contained higher foliage cover, vegetation height and vegetation species diversity than restored dunes. Abundance and diversity of beetle, spider, and ant families were higher in marram dominated dunes. Estimated population size of common skink (O. nigraplantare polychroma) and mouse population density was also higher in marram dunes. These results were positively correlated with the percentage of vegetation foliage cover and vegetation species diversity, suggesting that the habitat conditions created by marram grass were favored by fauna. These results suggest that for maximum biodiversity gains, future dune restoration attempts should increase vegetation cover, and include a wider range of plant species. Species in decline known to be important for fauna, such as pohuehue (Muehlenbeckia spp.), sand pimelia, and sand coprosma (Coprosma acerosa) should also be included for reciprocal benefits for conservation of flora and fauna. Marram grass could also be incorporated into restoration, as its mass removal may have considerable consequences for fauna using it as a refuge, and it appears to provide desirable habitat for fauna.</p>

Biomorphogenic Feedbacks and the Spatial Organization of a Dominant Grass Steer Dune Development

Nature-based solutions to mitigate the impact of future climate change depend on restoring biological diversity and natural processes. Coastal foredunes represent the most important natural flood barriers along coastlines worldwide, but their area has been squeezed dramatically because of a continuing urbanization of coastlines, especially in Europe. Dune development is steered by the development of vegetation in interaction with sand fluxes from the beach. Marram grass (Calamagrostis arenaria, formerly Ammophila arenaria) is the main dune building species along most European coasts, but also in other continents where the species was introduced. Engineering of coastal dunes, for instance by building dunes in front of dikes, needs to be based on a solid understanding of the species’ interactions with the environment. Only quantitative approaches enable the further development of mechanistic models and coastal management strategies that encapsulate these biomorphogenic interactions. We here provide a quantitative review of the main biotic and physical interactions that affect marram grass performance, their interactions with sand fluxes and how they eventually shape dune development. Our review highlights that the species’ spatial organization is central to dune development. We further demonstrate this importance by means of remote sensing and a mechanistic model and provide an outlook for further research on the use of coastal dunes as a nature-based solution for coastal protection.

Biomorphogenic feedbacks and the spatial organisation of a dominant grass steer dune development

Nature-based solutions to mitigate the impact of future climate change depend on restoring biological diversity and natural processes. Coastal foredunes represent the most important natural flood barriers along coastlines worldwide, but their area has been squeezed dramatically because of a continuing urbanisation of coastlines, especially in Europe. Dune development is steered by the development of vegetation in interaction with sand fluxes from the beach. Marram grass (Calamagrostis arenaria, formerly Ammophila arenaria) is the main dune building species along most European coasts, but also in other continents where the species was introduced. Engineering of coastal dunes, for instance by building dunes in front of dikes, needs to be based on a solid understanding of the species' interactions with the environment. Only quantitative approaches enable the further development of mechanistic models and coastal management strategies that encapsulate these biomorphogenic interactions. We here provide a quantitative review of the main biotic and physical interactions that affect marram grass performance, their interactions with sand fluxes and how they eventually shape dune development. Our review highlights that the species spatial organisation is central to dune development. We further demonstrate this importance by means of remote sensing and a mechanistic model and provide an outlook for further research on the use of coastal dunes as a nature-based solution for coastal protection.

Accuracy of the UAV-Based DEM of Beach–Foredune Topography in Relation to Selected Morphometric Variables, Land Cover, and Multitemporal Sediment Budget

Recent developments in unmanned aerial vehicles (UAVs) have resulted in high-resolution digital elevation models (DEMs) of vulnerable coastal environments, including beach–foredune topography. If performed repetitively, they can offer an excellent tool to determine the spatial and temporal changes in the sediment budget, which may be required for proper land management. However, the quality of a UAV, slope parameters, and vegetation significantly influence DEM accuracy. The aim of this study is to compare precise GPS-RTK transects across a section of the South Baltic coast in Poland with those obtained from a DEM based on high-resolution and high-accuracy images obtained by a wind-resistant, high-quality fixed-wing UAV during beyond visual line of sight operation (BVLOS). Different land cover classes, slope inclination, and general curvature, as well as surface roughness, were taken into consideration as possible factors influencing the uncertainty. The study revealed that marram grass greatly affects the accuracy of the UAV-derived model and that the uncertainty of the UAV-derived DEM increases together with increasing slope inclination and, to a lesser degree, with increasing general slope curvature. We showed that sediment budget determinations with the use of a UAV-based DEM are correct only where grass cover is sparse, in our study, up to 20% of the area.

The tide is high, but it’s holding on: response of the grey bush-cricket, Platycleis albopunctata, to a storm surge

Coastal insects may be highly susceptible to population loss due to catastrophic inundation events. At two locations on the east coast of the UK (nature reserve and naturists’ beach), the response of the Nationally Scarce grey bush-cricket, Platycleis albopunctata, to the December 2013 storm surge was determined from long-term transect monitoring of stridulating males. In the post-surge seasons, males were more frequent on the back dunes, which would have been largely unsubmerged during the tidal event. Lower numbers of P. albopunctata were recorded on the fore dunes after 2013, probably due to submergence during the surge tide and extensive shingle deposition on its marram-grass, Ammophila arenaria, and open ground habitats smothering overwintering eggs. The heterogeneity of the dune habitat with slacks and ridges may render this species resilient to storm surges.

Accommodation space indicates dune development potential along an urbanized and frequently nourished coastline

With densely populated areas well below mean sea level, the Netherlands relies heavily on its dunes to ensure coastal safety. About half of the sandy coastline, however, is subject to structural marine erosion and requires frequent sand nourishment as a counteractive measure. A key component of present-day coastal safety policy is creating favorable conditions for natural dune development. These conditions essentially involve (1) a steady supply of wind-blown sand towards (2) a wide accommodation space where sand can accumulate and dunes are sheltered from frequent storm surge impacts. This paper examines to what extent an experimental mega-scale beach nourishment (termed Zandmotor in Dutch) has contributed to creating accommodation space favorable for dune development. Using publicly available airborne lidar data and Sentinel-2 satellite imagery, favorable accommodation space is identified by comparing recent changes in coastal morphology against dune vegetation-cover dynamics. With a focus on European marram grass (Ammophila arenaria) as the most prominent dune-building species, this paper demonstrates that the Zandmotor supports an especially high potential for incipient (embryo) dunes to develop as most of its favorable accommodation space is located on the beach. However, considering the conditions required for successful marram grass establishment as well as persistent anthropogenic disturbances arising from recreation and nature management practices, it is not likely that dune development along this urbanized coastline reaches its full potential.

Burial, erosion, and transformation of archaeological landscapes

A high proportion of archaeological sites are located on the world’s shorelines and recent research has documented the vulnerability of these sites to coastal processes and climate change. However, archaeological landscapes on many temperate coasts have already been degraded as a result of changes in dune dynamics related to changes in dune vegetation. These changes have produced marked spatial and temporal variations in patterns of burial and erosion in transgressive dune systems. This paper examines the modification and conservation of archaeological landscapes from a biogeomorphic perspective, using the example of marram grass ( Ammophila arenaria) invasion of dune systems in southern New Zealand. The impact of marram grass on dune system dynamics and the underlying archaeological landscape are complex. Full invasion may result in the general burial and protection of these landscapes, but the risk of degradation of sites is high during the invasion process. In southern New Zealand, marram invasion has resulted in the formation of stable foredunes, often associated with coastal progradation. Archaeological sites located close to the shoreline can be subject to either burial or erosion, or both, as marram grass establishes in the foredune zone. The spatial relationship between cultural sites and the shoreline may be lost as the coast progrades. The impact of marram invasion can extend throughout the hinterland dune system as a result of (i) dune mobility triggered by marram grass invasion and (ii) the development of a negative sand budget, which prevents or reduces beach-foredune-dune system sand exchange. The risk of degradation of the archaeological landscape can be significantly heighted by marram invasion, which can have profound implications for the preservation and interpretation of archaeological sites and materials. Paradoxically, dune system restoration may lead to the re-exposure of these sites, but the principal outcome of dune system restoration is expected to be a decline erosion (manifest as in deflation surfaces) and reburial of the archaeological landscape.