Rhodolith structural loss decreases abundance, diversity, and stability of benthic communities Reduction and homogenization of structurally complex habitats is occurring throughout the world. Species that increase structural complexity often have a disproportionate influence on the structure and function of an ecosystem. Rhodoliths are carbonate-forming species that increase structural complexity of the benthos over an otherwise soft bottom habitat. The goal of this study was to characterize the associated community inhabiting rhodolith beds and to make a comparison to adjacent crushed rhodolith habitat to elucidate the ecological role of rhodoliths and allude to the impact of rhodolith loss. Community composition within rhodolith beds and crushed rhodolith habitats was created from surveys of four functional groups including macroalgae, infaunal and epifaunal invertebrates, and fishes. Observed differences in community composition were due to both habitat and season. Community differences by habitat were driven by greater abundance of fleshy attached algae, infauna, and epifauna, and fishes within rhodolith beds. These surveys suggest the loss of rhodolith structure results in reduced abundance of macroalgal, infaunal, and epifaunal taxa. Rhodolith size and complexity appears relatively low compared to other rhodolith beds worldwide, suggesting a potential cause of reduced faunal abundance and diversity. Factors reducing rhodolith structural complexity, such as mooring chains, are predicted to diminish community biodiversity and abundance.
Gabara SS, Hamilton SL, Edwards MS, Steller DL (2018) Rhodolith structural loss decreases abundance, diversity, and stability of benthic communities at Santa Catalina Island, CA. Mar Ecol Prog Ser 595:71-88. https://doi.org/10.3354/meps12528
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Rhodolith food webs are detrital based and rhodoliths trap detritus contributing to their role as a foundation species In many coastal marine systems with low productivity, cross-habitat exchange of macroalgal material has been shown to have significant bottom-up effects. California rhodolith beds are slow-growing carbonate red algae that support algal and invertebrate communities and are often adjacent to highly productive kelp beds. To determine trophic structure and sources of productivity supporting a rhodolith bed off Catalina Island, samples of water, flora, and fauna were run for carbon and nitrogen stable isotope analysis. Using cluster analysis on carbon and nitrogen isotope values of 12 invertebrate consumer taxa, four trophic groups were identified: planktivore, detritivore, herbivore, and predator. The isotope value of organic matter within rhodoliths was similar to the values of both fresh and drift Macrocystis pyrifera, suggesting kelp may contribute to the organic matter within rhodoliths, however benthic biofilms or algae such as diatoms may also contribute. Detritivores appeared to consume a mixture of sources suggesting sediment within rhodoliths is an amalgam of suspended particulate organic matter from the water column and benthic algae including giant kelp M. pyrifera particulates derived from adjacent kelp beds. Dominant macroalgae within rhodolith beds was epiphytic red macroalgae (~10% cover) that propagated on rhodoliths and appeared to contribute little to the food web relative to suspended particulate organic matter and M. pyrifera subsidies. As detritivores and predators were greatly influenced by kelp, temporal fluctuations of drift subsidies from adjacent kelp beds and water motion likely has dramatic effects on kelp retention and therefore secondary production and community structure of adjacent rhodolith beds.
Gabara, S.S. Trophic structure and potential carbon and nitrogen flow of a rhodolith bed at Santa Catalina Island inferred from stable isotopes. Mar Biol 167, 30 (2020). https://doi.org/10.1007/s00227-019-3635-9
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Kelp Forests
Kelp forest deforestation leads to lower gross primary production, respiration, and the range between these metrics
Trophic interactions can result in changes to the abundance and distribution of habitat-forming species that dramatically reduce ecosystem functioning. In the coastal zone of the Aleutian Archipelago, overgrazing by herbivorous sea urchins that began in the 1990s resulted in widespread deforestation of the region’s kelp forests, which led to lower macroalgal abundances and higher benthic irradiances. We examined how this deforestation impacted ecosystem function by comparing patterns of net ecosystem production (NEP), gross primary production (GPP), ecosystem respiration (Re), and the range between GPP and Re in remnant kelp forests, urchin barrens, and habitats that were in transition between the two habitat types at nine islands that spanned more than 1000 kilometers of the archipelago. Our results show that deforestation, on average, resulted in a 24% reduction in GPP, a 26% reduction in Re, and a 24% reduction in the range between GPP and Re. Further, the transition habitats were intermediate to the kelp forests and urchin barrens for these metrics. These opposing metabolic processes remained in balance; however, which resulted in little-to-no changes to NEP. These effects of deforestation on ecosystem productivity, however, were highly variable between years and among the study islands. In light of the worldwide declines in kelp forests observed in recent decades, our findings suggest that marine deforestation profoundly affects how coastal ecosystems function.
Edwards M, Konar B, Kim J-H, Gabara S, Sullaway G, McHugh T, et al. (2020) Marine deforestation leads to widespread loss of ecosystem function. PLoS ONE 15(3): e0226173. https://doi.org/10.1371/journal.pone.0226173
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How does kelp forest biodiversity affect food webs?
With increasing biodiversity loss occurring worldwide, there is a need to understand how these losses will affect ecosystem structure and function. Biodiversity loss leads to changes in species interactions, specifically the trophic complexity of food webs. How overall biodiversity loss affects the complexity of food webs can be described by changes to the diversity of food resources and the diversity of trophic levels. To understand how biodiversity affects trophic complexity of food webs we used ten islands across the Aleutian Archipelago to compare the alternate state communities found in kelp forest ecosystems (kelp forest and urchin barren communities) and then compared these to natural reference communities without local benthic production (their associated offshore communities). We constructed food webs for each community across the Aleutian Archipelago using primary producer and consumer carbon (δ13C, a proxy for food sources to a consumer) and nitrogen (δ15N, a proxy for consumer trophic level) stable isotope values. Our findings suggest that biodiversity loss (i.e. phase change from kelp forest to urchin barren) leads to reductions in trophic complexity, which was similar to naturally occurring communities with low local resource diversity. This was expressed by lower consumer isotopic dietary niche areas, especially omnivores and herbivores, and lower omnivore and carnivore trophic levels within the urchin barren communities. We clarify how biodiversity promotes food resources and increases trophic levels and complexity through critical trophic conduits.
Gabara SS, Edwards MS, Konar BH (2021) Biodiversity loss leads to reductions in community-wide trophic complexity. Ecosphere 12(2): e03361. https://doi.org/10.1002/ecs2.3361
Kelp Forests
Urchin Barrens
Offshore Habitats
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How reductions in sea otters, increases in urchin grazing, and resulting transitions from kelp forests to urchin barrens decreases growth and increases defense of a benthic perennial macroalga
A primary goal in the study of producer-herbivore interactions is to characterize the tradeoffs between primary producer growth and defense. Across the Aleutian Island Archipelago, the widespread decline in sea otters has resulted in reduced predation on sea urchins, which has led to increases in urchin populations, the formation of urchin barrens, and ultimately to overgrazing of much of the region’s kelp forests. The occurrence of both kelp forests and urchin barrens on islands, along with among island variation in the time period that urchin barrens have formed, presents a unique opportunity to characterize the extent to which exposure to intense herbivory and increased light may alter marine macroalgae growth and defense tradeoffs. To address this, we used a field caging experiment with Codium ritteri, a common perennial green macroalga in the Aleutian Archipelago, to test whether urchin barren macroalgae exhibit increased defenses and reduced growth relative to kelp forest individuals. Our results suggest that urchin barren C. ritteri had higher defense and lower growth relative to kelp forest individuals. In the laboratory we found little evidence for urchin barren C. ritteri growth under low light or altered defenses at high light. Grazing rates on C. ritteri were correlated with urchin biomass in the field suggesting higher herbivory intensity may shift primary producer energy allocation from growth to defense. Together, our data suggests that macroalgae occurring within kelp forests grow faster but are more palatable than macroalgae occurring in urchin barrens, which may increase urchin deforestation potential.
Gabara SS, Weitzman BP, Konar BH, Edwards MS (2020) Macroalgal defense phenotype correlates with herbivore abundance. Marine Biology 167, 179. https://doi.org/10.1007/s00227-020-03787-7