Keryn B. Gedan

How will warming affect the salt marsh foundation species Spartina patens and its ecological role

Foundation species structure environments and create refuge from environmental stress. In New England high salt marsh, the grass Spartina patens is a foundation species that reduces salinity, anoxia, desiccation, and thermal stresses through canopy shading and root proliferation. In a factorial S. patens-removal and warming field experiment, foundation species removal strongly impacted every aspect of the community, reiterating the important role of the foundation species S. patens in the high marsh. Given this central role, we hypothesized that facilitation by the foundation species would be even more important under warmer conditions by ameliorating more severe thermal stress. However, the ecological role of S. patens was unaffected by experimental warming, and, independent of the presence of the foundation species, warming had only weak effects on the salt marsh ecological community. Only the foundation species itself responded strongly to warming, by significantly increasing aboveground production in warmed plots. Apparently, amelioration of thermal stress is not as important for salt marsh ecosystem function as S. patens’ moderation of salinity and desiccation stresses. From these experimental results, we anticipate that climate change-associated thermal stress will not greatly affect S. patens-dominated high marsh communities. In contrast, foundation species loss, an emergent conservation issue in Atlantic salt marshes, represents a critical threat to salt marsh ecosystem function.

Substrate size mediates thermal stress in the rocky intertidal

Variation in physical factors, such as slope, orientation, and wind exposure, shapes thermal conditions. Variation in substrate size is common in many habitats, but its thermal consequences for organisms are not well characterized. Larger substrates should remain more thermally stable and act as thermal refuges for associated organisms during short, thermally stressful periods such as midday temperature peaks or tidal exposure. In observations and a transplant and thermal integration experiment, we found that larger rock substrates stayed cooler and facilitated greater survival of the barnacle Semibalanus balanoides in the high intertidal relative to small substrates during the hot summer months in southern New England, USA. However, in thermally benign northern New England, rock substrate size had no effect on barnacle distributions, indicating that the thermal effects of substrate size are mediated by regional climate.

Small-mammal herbivore control of secondary succession in New England tidal marshes

Secondary succession is impacted by both biotic and abiotic forces, but their relative importance varies due to environmental drivers. Across estuarine salinity gradients, physical stress increases with salinity, and biotic stresses are greater at lower salinities. In southern New England tidal marshes spanning a landscape-scale salinity gradient, we experimentally examined the effects of physical stress and consumer pressure by mammalian herbivores on secondary succession in artificially created bare patches. Recovery was slower in marshes exposed to full-strength seawater, where physical stress is high. Compared to full-strength salt marshes, recovery in low-salinity marshes was much faster and was influenced by small-mammal consumers. At lower salinities, small mammals selectively ate and prevented the establishment of several native and two invasive, nuisance species (Typha angustifolia and Phragmites australis) but were unable to control the expansion of established P. australis stands. By controlling the establishment of competitively dominant species and the trajectory of secondary succession in low-salinity marshes, small mammals may play a cryptic keystone role in estuarine plant communities and are a critical, overlooked consideration in the conservation and management of estuarine marshes.

Using Facilitation Theory to Enhance Mangrove Restoration

Despite advances in positive interactions and facilitation theory in ecological research (1, 2), the concepts have failed to make a big impact on mangrove restoration ecology. Restoration applications of facilitation include reaping the community benefits of foundation species, positive density dependence, and facilitation cascades (3). In their June 2008 article, ‘‘Growth Performance of Planted Mangroves in the Philippines: Revisiting Forest Management Strategies,’’ Samson and Rollon discuss failed mangrove restoration projects in the Philippines and suggest that the poor growth and mortality of Rhizophora spp. seedlings were due to inappropriate placement of seedlings in low intertidal mudflat and seagrass habitats, instead of being planted in existing or deforested mangrove areas (many of which are now privately controlled). Planting mangrove seedlings at appropriate elevations to limit abiotic stress on seedlings is a major tenet of recent mangrove restoration work (4), and Samson and Rallon expand on this to say it must be the primary focus of future efforts if restoration is to succeed. This conclusion is overly narrow, whereas incorporating positive interactions into restoration practices will be complementary to ongoing approaches and result in mangrove forest restoration efforts having greater success under a wider range of environmental conditions. Although positive interactions, such as the planting of foundation species to initiate ecosystem development, are a key component of restorations, other positive interactions are rarely accounted for in marine restoration designs (3). The ability of mangroves to tolerate extremely stressful abiotic conditions is due in part to key adaptations (e.g., salt-excreting glands, aerial or prop roots, and pneumatophores) and to intraspecific positive feedbacks: neighboring trees’ leaky roots oxygenate soils and locally ameliorate high sulfide concentrations (5). Yet most mangrove restorations around the world, including the Philippines example, plant mangroves as single seedlings, evenly spaced, in rows. This configuration is based on the assumption that competition among seedlings needs to be minimized to foster establishment and growth. Thus seedlings need to be spaced well away from each other to maximize light availability and minimize competition between neighbors. However, whereas light availability can be a limiting factor at later stages in mangrove forest development, the limiting growth factors at the initial stages of mangrove establishment are edaphic stressors, such as low redox potential and high soil salinity, as recognized by the Samson and Rollon. Because coastal wetland plants engineer the substrate to ameliorate these harmful conditions, an effect that increases with wetland plant density (6), seedlings are likely to exhibit positive, not negative, density dependence because of the facilitative effects of neighbors on ameliorating anoxic soil conditions. Ecological theory and wetland experiments both predict that mangrove seedlings have a far better chance of survival if they are planted in clusters of several seedlings rather than plantation style. Planting seedlings in clusters will likely allow the necessary positive feedbacks to take root in the absence of adult plant roots or pneumatophores. For example, a black mangrove restoration in Mexico that planted five-seedling clusters resulted in notably high survival of planted seedlings (74%) after 4 years, despite being planted in a mudflat environment (7). Mangrove seedlings frequently suffer high rates of mortality, and clustered or redundant plantings allow surviving seedlings to compensate for lost neighbors. Nurse plants, which can serve the same purpose in a restoration as seedling clusters, promoting the facilitative species interactions that ameliorate abiotic stress, have also been found to improve mangrove restoration success (8). Higher plant densities have also been found to reduce herbivory on susceptible, young plants in other saline wetland environments (9). Coastal populations depend on mangrove ecosystems for economic products, such as shrimp ponds, fish farms, and timber products, and for the ecosystem services they provide, such as coastal stabilization, wave attenuation, and nursery habitat for fish. These services, from both natural and converted mangrove areas, cannot be minimized, and, as Samson and Rollon suggest, it may not be feasible or prudent in all cases to restore former mangrove areas to mangrove forest. In many cases, the best decision will include a mix of exploitation, conservation, and restoration (10). Whether mangrove restorations proceed in mangrove or nonmangrove habitats, restorations are much more likely to be successful if they assume positive, rather than negative, density dependence during initial ecosystem development and thereby capitalize on advances in facilitation theory. The old paradigm of applying terrestrial forestry nursery theory (minimize competition) to wetland restorations needs to be updated to current ecological theory, showing that positive interactions among plants in more physically harsh wetland systems are integral to plant establishment, survival, and success.

Livestock as a potential biological control agent for an invasive wetland plant

Invasive species threaten biodiversity and incur costs exceeding billions of US

Nature-Based Coastal Defenses: Can Biodiversity Help?

. Eradication efforts, however, are nearly always unsuccessful. Throughout much of North America, land managers have used expensive, and ultimately ineffective, techniques to combat invasive Phragmites australis in marshes. Here, we reveal that Phragmites may potentially be controlled by employing an affordable measure from its native European range: livestock grazing. Experimental field tests demonstrate that rotational goat grazing (where goats have no choice but to graze Phragmites) can reduce Phragmites cover from 100 to 20% and that cows and horses also readily consume this plant. These results, combined with the fact that Europeans have suppressed Phragmites through seasonal livestock grazing for 6,000 years, suggest Phragmites management can shift to include more economical and effective top-down control strategies. More generally, these findings support an emerging paradigm shift in conservation from high-cost eradication to economically sustainable control of dominant invasive species.

The present and future role of coastal wetland vegetation in protecting shorelines: answering recent challenges to the paradigm

Currently, coastal protection potential of ecosystems is estimated primarily as a function of its spatial extent and type. The degree to which coastal protection depends on aspects of biodiversity within these ecosystems is, however, less explored. Here, we provide a short summary of classical coastal protection strategies and the current state of knowledge of nature-based shoreline protection, and then discuss relevant biodiversity theory and the few studies that have investigated how species identity affects shoreline protection. This chapter provides the first attempt to identify the aspects of biodiversity that are likely to be important in enhancing coastal protection efforts.