Andrew D. Irving

Restoring Coastal Plants to Improve Global Carbon Storage: Reaping What We Sow

Long-term carbon capture and storage (CCS) is currently considered a viable strategy for mitigating rising levels of atmospheric CO2 and associated impacts of global climate change. Until recently, the significant below-ground CCS capacity of coastal vegetation such as seagrasses, salt marshes, and mangroves has largely gone unrecognized in models of global carbon transfer. However, this reservoir of natural, free, and sustainable carbon storage potential is increasingly jeopardized by alarming trends in coastal habitat loss, totalling 30–50% of global abundance over the last century alone. Human intervention to restore lost habitats is a potentially powerful solution to improve natural rates of global CCS, but data suggest this approach is unlikely to substantially improve long-term CCS unless current restoration efforts are increased to an industrial scale. Failure to do so raises the question of whether resources currently used for expensive and time-consuming restoration projects would be more wisely invested in arresting further habitat loss and encouraging natural recovery.

Interactive effects of sedimentation and microtopography on the abundance of subtidal turf-forming algae

Abstract We quantified the percentage cover of turf-forming algae among locations spanning > 1000 km of continuous South Australian coastline and found that they formed one of the most extensive subtidal habitats on reefs devoid of macroalgae. We then experimentally tested the hypothesis that sediment deposition and microtopography of substratum interact to control turf alga abundance (percentage cover and biomass). Both microtopography (topographically simple and complex) and sedimentation (natural and reduced) were manipulated orthogonally to test their independent and interactive effects on subtidal turf-forming algae. This field experiment demonstrated that reduced rates of sedimentation have positive effects on the percentage cover and biomass of turf-forming algae (Feldmannia spp.) and that a very small increase in topographic complexity (microtopography) can have large positive effects (> 36% increase) on the biomass of turf-forming algae under conditions of reduced sedimentation. These results support new models that suggest differences in microtopography may account for observed variation in the effects of sedimentation on the abundance of turf-forming algae. Importantly, heavy rates of sediment deposition can negate the positive effects of greater topographic complexity at fine spatial scales (i.e. microtopography). These results raise serious concerns about gaps in our knowledge of the effects of sedimentation on turf-forming algae and its consequences for current declines in the diversity and cover of algae in disturbed coastal habitats.

Switching from negative to positive density-dependence among populations of a cobble beach plant

Interactions among species occur across a continuum from negative to positive and can have a critical role in shaping population and community dynamics. Growing evidence suggests that inter- and intra-specific interactions can vary in strength or even switch direction (i.e., negative to positive) depending on environmental conditions, consumer pressure, and also among life-history stages. We tested the hypothesis that seedlings and adults of the intertidal annual forb Suaeda linearis growing on New England shores exhibit positive density-dependence under physically stressful conditions high on the shore (i.e., greater temperatures, evaporative stress), but negative density-dependence under physically milder conditions low on the shore. Among experimental treatments of plant density (dense versus sparse) at each shore height, plant biomass, length, and number of leaves/branches were greater in dense stands high on the shore (positive density-dependence), but greater in sparse stands low on the shore (negative density-dependence). Such responses were consistent among life-history stages and generally consistent between sites. As a more direct measure of fitness, per capita seed production was also positively density-dependent high on the shore, but negatively density-dependent low on the shore. These results support the current theory predicting an increase in the frequency of positive interactions with increasing environmental stress and further emphasize the previously understated role of positive interactions in shaping and maintaining populations and communities.

Trait-dependent modification of facilitation on cobble beaches

Fundamental gaps remain in our knowledge of how positive species interactions, such as facilitation and mutualism, structure and maintain populations and communities. Foundation species create extensive biogenic habitats, but we know little of how their traits, such as density, age, and patch size, modify their ability to facilitate other species. We tested the role of facilitator traits in cobble beach plant communities in New England, USA. In this system, intertidal beds of the cordgrass Spartina alterniflora facilitate populations of halophytic forbs at higher shore elevations by buffering wave action, stabilizing cobbles, and limiting physical disturbance. Using descriptive and experimental techniques, we tested the hypotheses that (1) the density and height of cordgrass shoots modify the strength of the cordgrass-forb facilitation, and (2) cordgrass peat alone contributes to the facilitation of forbs. Increased shoot density, as well as the combination of cordgrass peat and shoots, positively affected two life history stages (seedling and adult) of the abundant forb Suaeda linearis, demonstrating that cordgrass traits modify the strength of facilitation in this system. Since the expression of traits varies within and among patches of any given foundation species, traits can and should be used to predict the strength of facilitation, to guide the development of conservation strategies, and to develop more accurate models of species interactions.

Identifying knowledge gaps in seagrass research and management: an Australian perspective

Seagrass species form important marine and estuarine habitats providing valuable ecosystem services and functions. Coastal zones that are increasingly impacted by anthropogenic development have experienced substantial declines in seagrass abundance around the world. Australia, which has some of the worlds largest seagrass meadows and is home to over half of the known species, is not immune to these losses. In 1999 a review of seagrass ecosystems knowledge was conducted in Australia and strategic research priorities were developed to provide research direction for future studies and management. Subsequent rapid evolution of seagrass research and scientific methods has led to more than 70% of peer reviewed seagrass literature being produced since that time. A workshop was held as part of the Australian Marine Sciences Association conference in July 2015 in Geelong, Victoria, to update and redefine strategic priorities in seagrass research. Participants identified 40 research questions from 10 research fields (taxonomy and systematics, physiology, population biology, sediment biogeochemistry and microbiology, ecosystem function, faunal habitats, threats, rehabilitation and restoration, mapping and monitoring, management tools) as priorities for future research on Australian seagrasses. Progress in research will rely on advances in areas such as remote sensing, genomic tools, microsensors, computer modeling, and statistical analyses. A more interdisciplinary approach will be needed to facilitate greater understanding of the complex interactions among seagrasses and their environment.

Testing for thresholds of ecosystem collapse in seagrass meadows

Although the public desire for healthy environments is clear-cut, the science and management of ecosystem health has not been as simple. Ecological systems can be dynamic and can shift abruptly from one ecosystem state to another. Such unpredictable shifts result when ecological thresholds are crossed; that is, small cumulative increases in an environmental stressor drive a much greater change than could be predicted from linear effects, suggesting an unforeseen tipping point is crossed. In coastal waters, broad-scale seagrass loss often occurs as a sudden event associated with human-driven nutrient enrichment (eutrophication). We tested whether the response of seagrass ecosystems to coastal nutrient enrichment is subject to a threshold effect. We exposed seagrass plots to different levels of nutrient enrichment (dissolved inorganic nitrogen) for 10 months and measured net production. Seagrass response exhibited a threshold pattern when nutrient enrichment exceeded moderate levels: there was an abrupt and large shift from positive to negative net leaf production (from approximately 0.04 leaf production to 0.02 leaf loss per day). Epiphyte load also increased as nutrient enrichment increased, which may have driven the shift in leaf production. Inadvertently crossing such thresholds, as can occur through ineffective management of land-derived inputs such as wastewater and stormwater runoff along urbanized coasts, may account for the widely observed sudden loss of seagrass meadows. Identification of tipping points may improve not only adaptive-management monitoring that seeks to avoid threshold effects, but also restoration approaches in systems that have crossed them.

Contemporary reliance on bicarbonate acquisition predicts increased growth of seagrass Amphibolis antarctica in a high-CO2 world

We find energetically costly bicarbonate pathways exist in three temperate seagrasses and then provide evidence that indicates greater growth and photosynthetic efficiency for bicarbonate users in a high CO2 world. Greater growth might enhance the future prosperity and rehabilitation of these important habitat forming plants, which have experienced declines of global significance.

An integrative method for the evaluation, monitoring, and comparison of seagrass habitat structure

Assessing environmental condition is essential for the management of coasts and their resources, but better management decisions occur when large databases are simplified into more manageable units of information. Here we present the habitat structure index (HSI), which enables rapid assessment and direct comparison of seagrass habitat structure using scores of 0 (poor) to 100 (excellent) based on integrating five habitat variables: area, continuity, proximity, percentage cover, and species identity. Acquiring data to calculate the HSI can be done in situ or from video recordings, and requires relatively simple methodology of belt transects, estimating percentage cover, and basic taxonomy. Spatiotemporal comparisons can usefully identify locations and periods of seagrass habitat change, potentially providing an early warning indicator of habitat damage and decline in environmental quality. Overall, the integrative approach of the HSI represents a step toward simplifying the exchange of environmental information among researchers, coastal managers, and governing bodies.

Species coexistence and the superior ability of an invasive species to exploit a facilitation cascade habitat

Facilitation cascades generated by co-occurring foundation species can enhance the abundance and diversity of associated organisms. However, it remains poorly understood how differences among native and invasive species in their ability to exploit these positive interactions contribute to emergent patterns of community structure and biotic acceptance. On intertidal shorelines in New England, we examined the patterns of coexistence between the native mud crabs and the invasive Asian shore crab in and out of a facilitation cascade habitat generated by mid intertidal cordgrass and ribbed mussels. These crab species co-occurred in low intertidal cobbles adjacent to the cordgrass–mussel beds, despite experimental findings that the dominant mud crabs can kill and displace Asian shore crabs and thereby limit their successful recruitment to their shared habitat. A difference between the native and invasive species in their utilization of the facilitation cascade likely contributes to this pattern. Only the Asian shore crabs inhabit the cordgrass–mussel beds, despite experimental evidence that both species can similarly benefit from stress amelioration in the beds. Moreover, only Asian shore crabs settle in the beds, which function as a nursery habitat free of lethal mud crabs, and where their recruitment rates are particularly high (nearly an order of magnitude higher than outside beds). Persistence of invasive adult Asian shore crabs among the dominant native mud crabs in the low cobble zone is likely enhanced by a spillover effect of the facilitation cascade in which recruitment-limited Asian shore crabs settle in the mid intertidal cordgrass–mussel beds and subsidize their vulnerable populations in the adjacent low cobble zone. This would explain why the abundances of Asian shore crabs in cobbles are doubled when adjacent to facilitation cascade habitats. The propensity for this exotic species to utilize habitats created by facilitation cascades, despite the lack of a shared evolutionary history, contributes to species coexistence and the acceptance of invasives into a diverse community.

Decline and Restoration Ecology of Australian Seagrasses

Since the first version of this book almost 30 years ago, significant losses of seagrass meadows have continued to be reported from around Australia as a result of natural and human induced perturbations. Conservative estimates indicate losses over the past two decades have more than doubled that estimated in the late 1990s. Conservation and mitigation of disturbance regimes have typically been the first line of defence, but ecological restoration or intervention is becoming increasingly necessary in a rapidly changing environment, and is potentially a more effective management strategy where seagrass habitat is already lost or heavily degraded. Accordingly, there has been an increase in the number of restoration studies and projects feeding our knowledge-base of restoration practice across Australia. Yet despite this increase, successful restoration has been rare, often uncoordinated, and almost always at a scale that is orders of magnitude lower than the scale of loss. Clearly, our understanding of the ecological mechanisms underlying successful and unsuccessful seagrass restoration is not keeping pace with the rates of loss and societal needs for restoration. Indeed, many orders of magnitude more restoration effort, in terms of science and practice and their interactions, will be required to prevent further seagrass loss. The science of seagrass restoration or restoration ecology is still a young science, but has strong foundations built from several decades of ecological research addressing many aspects of ecological interactions in seagrasses. While restoration has strong scientific underpinnings from ecological theory, it is clear that restoration ecology can also contribute to ecological theory by providing new and novel opportunities to advance our understanding of the mechanisms that promote functional ecosystems. In this chapter, we provide examples of this understanding across the levels of biological hierarchy, from genes to landscapes, and where possible include future strategic research directions.