Top–down impacts of avian predators are often overlooked in marine environments despite evidence from other systems that birds significantly impact animal distribution and behavior; instead, birds ...are typically recognized for the impacts of their nutrient rich guano. This is especially true in shallow seagrass meadows where restoration methods utilize bird perches or stakes to attract birds as a passive fertilizer delivery system that promotes the regrowth of damaged seagrasses. However, this method also increases the local density of avian piscivores that may have multiple unexplored non-consumptive effects on fish behavior and indirect impacts to seagrass communities. We utilized laboratory and field experiments to investigate whether visual cues of avian predators impacted the behavior of the dominant demersal fish in seagrass habitats, the pinfish Lagodon rhomboides, and promoted cascading interactions on seagrass-associated fauna and epiphytes. In laboratory mesocosms, pinfish displayed species specific responses to models of avian predators, with herons inducing the greatest avoidance behaviors. Avoidance patterns were confirmed in field seagrass meadows where heron models significantly reduced the number of fish caught in traps. In a long term field experiment, we investigated whether avian predators caused indirect non-consumptive effects on seagrass communities by monitoring fish abundances, invertebrate epiphyte grazers, and the seagrass epiphytes in response to heron models, bird exclusions, and bird stakes. On average, more fish were recovered under bird exclusions and fewer fish under heron models. However, we found no evidence of cascading effects on invertebrate grazers or epiphytes. Bird stake treatments only displayed a simple nutrient effect where higher bird abundances resulted in higher epiphyte biomass. Our results indicate that although birds and their visual cues can affect fish and epiphyte abundance through non-consumptive effects and nutrient enrichment, these impacts do not propagate beyond one trophic level, most likely because of dampening by omnivory and larger scale processes.
The influence of dietary elemental contents on consumer stoichiometry was investigated in selected and combined soft tissues (as a proxy of the whole individual) of the omnivorous sea urchin, ...Lytechinus variegatus. We raised urchins for 4 months in controlled seawater tanks using three different diets with different nutritional contents (from lower to higher: seagrass, red macroalgae, and a formulated diet). Individuals fed the different diets varied an average of 19.7, 19.4, and 38 % in C:N, C:P, and N:P ratios, respectively, with stronger temporal variability for C:P and N:P ratios across tissues and whole individuals. This resulted in homeostasis parameters (1/H) of −0.45, 0.09, and 0.38, respectively, for C:N, C:P, and N:P, indicative of homeostatic to weakly homeostatic organisms, at least for C:P and N:P ratios. Individuals fed the nutrient-rich formulated diet had higher growth rates (14 ± 0.83 g WW month⁻¹) than those fed macroalgae or seagrass (9.3 ± 0.57 and 3.4 ± 0.33 g WW month⁻¹, respectively). However, rapid body increments in more nutritional diets caused both a decrease in the %N and an increase in the %P of soft tissues, which resulted in significant but opposite effects of diet stoichiometry and growth in sea urchin C:N (R = −0.74 and R = 0.93, for diet and growth effects, respectively) and N:P ratios (R = 0.60 and R = −0.63, also, respectively, for diet and growth effects). Among potential compensatory mechanisms helping to preserve certain levels of homeostasis, ingestion rates (g WW diet per g WW of urchin) were higher for seagrass and macroalgae diets than for the nutrient-rich formulated diet. In contrast, absorption and growth efficiencies displayed significant negative associations with nutrient contents in diets and did not exhibit nutritional compensation. Overall, our results suggest that resource stoichiometry strongly determines the growth rate of individuals (R = 0.88, P < 0.01), and moderate variability in C:N:P ratios of sea urchins possibly arise from differences in the allocation of proteins and RNA to body components, similarly to what has been proposed by the growth rate hypothesis.
Four decades of research findings on the ecology of seagrasses and their animal and plant associates are described here, along with some of the major changes and advances that have taken place in our ...understanding of the inner workings of these amazingly productive and diverse ecosystems. Of primary importance are shifts in the recognition of: 1) the importance of the nursery role of seagrasses and how it can be quantified; 2) the importance of direct herbivory in the trophic ecology of seagrass-dominated ecosystems; and 3) the primacy of consumer effects in determining the abundance of algal epiphytes that colonize leaves and other aboveground seagrass tissues. Exciting new areas of investigation include elucidating rhizosphere interactions, including developing a deeper understanding of the importance of nutrient foraging by seagrass roots, and of the microbes in the sediments that fix nitrogen and reduce toxic hydrogen sulfide concentrations. Increased understanding of the roles of genetic diversity and the size and integration of seagrass clones, as well as additional and better evaluations of the relative value of different seagrass meadows as finfish nurseries are also needed. Of great importance for seagrass researchers of the future is spending large amounts of time in the field developing a thorough understanding of the natural history of the many and varied inhabitants of seagrass meadows. The knowledge gained by this investment of time will be essential for the development of meaningful, new hypotheses and the means for testing them.
Climate-driven range shifts can result in altered mixtures of foundation species that can affect ecosystem structure and function. Higher diversity mixed assemblages may moderate disturbance impacts, ...yet this prediction has rarely been tested. The
Deepwater Horizon
oil spill provided an opportunity to examine the effects of salt marsh foundation species identity and composition on the response of both plant and faunal communities to oil disturbance. In the northern Gulf of Mexico, salt marshes are typically dominated by the foundation species smooth cordgrass,
Spartina alterniflora
. However, the sub-tropical black mangrove,
Avicennia germinans
, is expanding northward and increasingly growing with, and even replacing,
Spartina
. Based on a series of field surveys in the Chandeleur Islands, Louisiana, we found that oil exposure had few impacts on the plant community 5 yr post-oiling, though oil disturbance shifted the interaction among foundation species, increasing
Avicennia
’s competitive advantage and likely enhancing mangrove expansion. In contrast, abundance of the epibenthic animal community was reduced by ~30% and community composition was altered at oiled sites. Mixed foundation species assemblages provided few reductions in oil impacts within either trophic level. Strong habitat associations of epibenthic taxa, coupled with oil effects on individual foundation species, likely suppressed any advantage of mixed vegetation on the associated fauna. Our survey highlights that plant and epibenthic animal responses to disturbance can be decoupled, emphasizing the importance of examining both simultaneously to better understand and predict long-term responses.
The complex structure of seagrass beds provides increased protection for prey compared to unvegetated substrates. Additionally, most studies support the notion that predation intensity is negatively ...correlated with seagrass density. However, virtually every study to previously test this idea has used the same number of predators and prey across a gradient of seagrass density treatments. This design ignores another well-established fact: invertebrate and fish abundance increase with increasing seagrass density. Therefore, it makes sense to carry out predation experiments that increase the number of both predators and prey as seagrass density increases. Outdoor tank experiments (7 density treatments: 0 to 3000 leaves m–2) were carried out using artificial seagrass units modeled after turtlegrassThalassia testudinumand field mesocosm experiments (4 density treatments: 0 to 3000 leaves m–2) in livingT. testudinumin Big Lagoon (Florida, USA). In these experiments the absolute number of predators (the pinfishLagodon rhomboides) and prey (grass shrimpPalaemonetes pugio) were increased with increasing seagrass density. Both experiments yielded similar results: the only significant difference in predation rate was between the unvegetated treatment and the seagrass treatments as a group. These results suggest that dense seagrass does not necessarily provide increased protection for prey organisms.
We investigated the interaction between epiphyte-grazer abundance and eutrophication to assess the relative importance of top-down and bottom-up effects in subtropical seagrass meadows. In field ...experiments using a cageless technique to control amphipod abundance, we measured the effects of grazing and nutrient supply on the growth and productivity of shoalgrass Halodule wrightii and its epiphytes at both protected and wave exposed sites. Amphipod removal at the protected site resulted in 70% greater epiphyte loads on shoalgrass leaves and a 36% reduction in leaf biomass after 10 wk. At the wave-exposed site, where amphipod abundance was consistently low, we found no significant effects of grazer presence or nutrients on epiphyte load or leaf biomass. Average leaf length, however, was significantly reduced in nitrogen-enriched plots. Our results indicate that natural densities of amphipods can reduce seagrass loss by controlling epiphytes.
The importance of habitat factors in designing marine reserves and evaluating their performance over time has been regularly documented. Over three biennial sampling periods, we examined the effects ...of vegetated coverage and habitat diversity (i.e., patchiness) on fish density, community composition, and species-specific patterns along a gradient of protection from harvest in the shallow Spanish southern Mediterranean, including portions of the Tabarca marine reserve. With the exception of two herbivores (Sarpa salpa and Symphodus tinca), vegetated cover did not significantly affect fish densities, while habitat diversity was an influential factor across all three sampling periods. Overall, fish density was more positively associated with more continuous vegetated or unvegetated habitats, and was greatest in areas of highest protection (Tabarca II – Isla Nao site). These patterns were usually observed for four abundant fish species (Boops boops, Chromis chromis, Oblada melanura, and S. salpa). Fish community composition was distinct in the most protected portion of the Tabarca reserve, where it was also most stable. Our findings align with previous investigations of the Tabarca reserve and its surrounding areas, and demonstrate its continued effectiveness in conserving fish biomass and habitat. Together with effective management, marine reserves can facilitate greater species abundance, more stable biological communities, and resilient ecosystems.
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