Coastal systems like barrier islands are good systems to further test ecological community assembly frameworks presented for highly dynamic, physically stressed systems. Barrier islands are dynamic in nature and can change morphologically over short temporal scales depending what plant communities are present and the intensity of press and pulse disturbance events they experience. Physical processes of barrier islands tend to attract higher focus, but understanding interactions with plant communities are critical for predictions of how barrier island resilience will be affected by disturbances associated with changing climate and sea-level rise.
Chapter 1: Deterministic or Stochastic?
Components of each theory feed into mechanisms of community assembly in dynamic systems like barrier islands. For example, neutral theory acknowledges communities as non-equilibrial entities which justifies consideration of stochastic affects when studying dynamic systems. Niche theory, while assuming assemblage equilibrium, acknowledges differences in species traits determined by varying biotic and abiotic filters. A knowledge gap remains in understanding how components of each of these theories fits into community assembly in dynamic systems, how balance between the two theories changes with disturbance, and how they affect community resilience and function.
Chapter 2: Intraspecific trait variation
When using trait-based methods to understand resilience to disturbances, it is critical that functional trait redundancy and intraspecific trait variation are considered, as both could contribute to ecosystem resilience. Ecosystem state and function can be maintained through contingent trait redundancy, described as scenarios in which unique species are contingently similar in certain contexts. Unique species may function similarly during certain disturbances but structural differences preserve unique vulnerabilities to concurrent phenomena. This trait-based redundancy mechanism provides an ecological contingency plan that allows for ecosystem and community functions to be achieved through various trait combinations in unique species. When functions are achievable in many different ways, disturbances and stressors (i.e. storms, flooding, increased nutrient limitation etc.) are less likely to present a risk to all functionally related components at once. Understanding the divergence of traits within individual dominant species may allow for a more complete understanding of what drives community structure after disturbance impacts cease and community recovery begins. To date, these mechanisms of community response after disturbance has accrued little focus, but can be studied thoroughly in barrier island systems, which have few dominant species and rapid disturbance recovery.
Chapter 3: Plant community response to nutrient enrichment
One way in which researchers have recently addressed further understanding the effect of biotic interactions and abiotic conditions on community patterns is through manipulation of nutrient enrichment. Nutrient deposition is an abiotic condition that significantly contributes to biotic interactions, as well as community structure and function, and is increasing with climate change and anthropogenic influence. Seitzinger et al. (2002) suggest that rates of nutrient inputs are expected to significantly increase in coastal systems by 2050, making it critical to better understand the affects these additions have on community assembly, structure, and function on barrier islands. Nutrient manipulation plots and plant trait-based ecology can be used to investigate these complex questions that involve multiple scales and co-occurring phenomena. Manipulation studies like this will also help to inform how plant community assembly processes and future plant community trajectories are altered by relationships between changing abiotic conditions and biotic interactions.
Chapter 4: Trait-based relationships with island topography
The ecological dilemmas introduced above all feed into understanding large scale dynamics of ecosystems and landscapes. The theory of multiple causality of land forms has been adopted to explain the presence of barrier islands. There has been a significant amount of work contributing to topography-vegetation patterns using dominant barrier island species. For example, Virginia barrier islands with large dunes and high Ammophila breviligulata abundance is coupled with less disturbed swales dominated by Spartina patens. Conversely, islands with low elevation and active wash over areas are completely dominated by S. patens. To date, a knowledge gap remains in understanding associations between different barrier island topographies and the functional trait composition of communities. This is an important key if we are to understand how climatic effects will change topography-vegetation feedbacks - causing possible alterations in disturbance regime, and thus community assembly processes.