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Understanding the extent to which ecosystem engineers modify the physical environment is challenging, particularly in dynamic and heterogeneous systems like estuaries, where both organism distributions and abiotic conditions vary spatially and temporally. While bioturbating shrimp can influence sediment properties through reworking, disentangling their pathways of effects on foundational species such as seagrass and bivalves remains complex. Shrimp density-driven changes in sediment composition and penetrability may not fully explain their negative impacts on structure forming species, highlighting the difficulty of linking biotic and abiotic interactions in variable environments. Furthermore, the emerging issue of antagonistic ecosystem engineering—where one engineer's activity prevents another from persisting at functionally relevant densities—compounds this uncertainty. This insight is crucial for predicting ecosystem resilience, guiding restoration efforts, and managing species interactions in dynamic coastal environments.
In Chapter 1, I explore the effects of bioturbating shrimp on seagrass. Because ecosystem engineers shape environmental conditions, interactions between ecosystem engineers can depend not only on the external environment but on “which species arrives when” within habitats. Yet, while endpoint outcomes for adults at high density have often been investigated, few studies have examined how these interactions change across density and life history stages. We tested for antagonistic engineering effects of the burrowing shrimp Neotrypaea californiensis (Dana, 1852) at a range of densities on eelgrass Zostera marina L., 1753, including seedlings as well as vegetative shoots. In an observational study, abrupt borders of eelgrass beds were not mirrored by shrimp, and shrimp were never excluded across the full range of observed eelgrass densities, patterns that are inconsistent with alternative stable states. However, eelgrass density declined with increasing shrimp density, and no eelgrass occurred at >336 shrimp m-2. Survival of eelgrass transplants also declined with increasing shrimp density, and in a manipulative experiment, seedlings declined more rapidly than vegetative shoots within a shrimp bed. Thus, shrimp have strong antagonistic engineering effects on eelgrass that increase with shrimp density and can preclude successful seedling establishment and persistence of vegetative shoots.
In Chapter 2, I explore the effects of bioturbating shrimp on oysters. Epibenthic organisms on intertidal flats can be affected by underlying sediments and by the activities of bioturbating species that live there. Therefore, bioturbating shrimp have two potential pathways to affect small clusters of juvenile oysters (seeded cultch): directly by moving sediment to the surface, or indirectly by affecting sediment properties (grain size, organic content, penetrability). We examined how oyster (Magallana gigas) survival and size responded to a) shrimp (Neotrypaea californiensis) density, b) mud content and penetrability of sediment, and c) shrimp density due to their effects on sediment properties (indirect pathway). Seeded cultch were deployed from spring through summer at 31 intertidal sites varying in both shrimp density and sediment properties within Willapa Bay, Washington (USA). Shrimp density was negatively associated with mud and organic content but positively with sediment penetrability, as expected from known ecosystem engineering effects of shrimp. However, neither mud content nor penetrability contributed statistically to the negative impact of shrimp density on oyster survival and size. No oysters survived the summer above 50 – 100 shrimp m-2, and remaining oysters were smaller with increasing shrimp density. Overall, negative effects of shrimp on benthic oysters likely occur through the deposition of sediment (28.9 ml burrow-1 day-1) rather than alteration of sediment properties. Our study highlights how the antagonistic ecosystem engineering effect of shrimp on oysters occurs independently of sediment responses to bioturbation and deposit-feeding and quantifies the conditions ensuring the persistence of ecologically- and commercially important foundation species.
In Chapter 3, I explore how transplants of seagrass into an area where it was previously excluded by burrowing shrimp can inform seagrass restoration strategies. Seagrass restoration has shown mixed results, even in environments that appear suitable, indicating on-going needs for improved restoration techniques. This study tracked eelgrass (Zostera marina) dynamics at two donor sites and one transplant site over multiple years, using resilience at donor sites and transplant establishment and expansion as key success measures. Despite finer sediments, higher shoot densities, and lower flowering frequencies at lower elevations, eelgrass morphology was similar at both high- and low-elevation donor sites. Recovery times increased with collection intensity, taking up to two years when large plots were completely cleared. Collected shoots were transplanted into plots of four sizes (0.0625–4 m²) and three densities (25–125 m²). Although larger, denser plots were expected to aid establishment in bioturbated areas, the highest initial establishment occurred in small, sparse plots. Over time, sparse and medium-density plots filled in, eventually converging with denser plots within a year. After two years, proportional shoot count changes were inversely related to plot size and density: small, sparse plots saw an 83-fold increase in shoot counts, while large, dense plots saw only a two-fold increase. Large, dense plots initially accumulated fine sediment and organic matter but lagged behind unvegetated areas after one year, likely due to bioturbator loss. Neither donor nor transplant sites showed facilitative effects, potentially due to intraspecific competition. Hydrodynamics around eelgrass shoots may have temporarily delayed sediment accumulation, a key ecosystem function. These findings contribute to improved eelgrass restoration strategies for fringe and upper margin intertidal areas.
Overall, I have addressed key interactions that shape the structure and function of intertidal soft sediments, showing how the outcomes among engineering species can become more predictable through incorporating factors such as density and per capita impacts. Borders between engineered habitats provide insight into species interactions, revealing how antagonistic ecosystem engineering shapes community structure. Our findings demonstrate that the effects of these interactions can vary across life history stages, with seedlings often more vulnerable than established individuals. For oysters, burrowing shrimp density serves as a strong predictor of performance, with sediment burial acting as the primary mechanism driving declines in survival and size. In eelgrass restoration, lower shoot collection intensity promotes faster donor site recovery, particularly in fringe and upper boundary habitats, ensuring resilience of natural beds. However, little evidence of self-facilitation in transplanted eelgrass suggests that intraspecific competition, rather than effects of positive density-dependence, constrains expansion, underscoring the complexity of habitat formation in bioturbated environments. Together, this research advances the understanding of ecosystem engineer interactions by demonstrating how antagonistic engineering can disrupt habitat formation and species persistence, emphasizing the importance of considering engineer density, life stage vulnerability, and sediment dynamics in conservation and restoration efforts.