The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts

Climate-driven changes in biotic interactions can profoundly alter ecological communities, particularly when they impact foundation species. In marine systems, changes in herbivory and the consequent loss of dominant habitat forming species can result in dramatic community phase shifts, such as from coral to macroalgal dominance when tropical fish herbivory decreases, and from algal forests to ‘barrens’ when temperate urchin grazing increases. Here, we propose a novel phase-shift away from macroalgal dominance caused by tropical herbivores extending their range into temperate regions. We argue that this phase shift is facilitated by poleward-flowing boundary currents that are creating ocean warming hotspots around the globe, enabling the range expansion of tropical species and increasing their grazing rates in temperate areas. Overgrazing of temperate macroalgae by tropical herbivorous fishes has already occurred in Japan and the Mediterranean. Emerging evidence suggests similar phenomena are occurring in other temperate regions, with increasing occurrence of tropical fishes on temperate reefs.     

Identifying the interacting roles of stressors in driving the global loss of canopy‐forming to mat‐forming algae in marine ecosystems

Identifying the type and strength of interactions between local anthropogenic and other stressors can help to set achievable management targets for degraded marine ecosystems and support their resilience by identifying local actions. We undertook a meta‐analysis, using data from 118 studies to test the hypothesis that ongoing global declines in the dominant habitat along temperate rocky coastlines, forests of canopy‐forming algae and/or their replacement by mat‐forming algae are driven by the nonadditive interactions between local anthropogenic stressors that can be addressed through management actions (fishing, heavy metal pollution, nutrient enrichment and high sediment loads) and other stressors (presence of competitors or grazers, removal of canopy algae, limiting or excessive light, low or high salinity, increasing temperature, high wave exposure and high UV or CO₂), not as easily amenable to management actions. In general, the cumulative effects of local anthropogenic and other stressors had negative effects on the growth and survival of canopy‐forming algae. Conversely, the growth or survival of mat‐forming algae was either unaffected or significantly enhanced by the same pairs of stressors. Contrary to our predictions, the majority of interactions between stressors were additive. There were however synergistic interactions between nutrient enrichment and heavy metals, the presence of competitors, low light and increasing temperature, leading to amplified negative effects on canopy‐forming algae. There were also synergistic interactions between nutrient enrichment and increasing CO₂ and temperature leading to amplified positive effects on mat‐forming algae. Our review of the current literature shows that management of nutrient levels, rather than fishing, heavy metal pollution or high sediment loads, would provide the greatest opportunity for preventing the shift from canopy to mat‐forming algae, particularly in enclosed bays or estuaries because of the higher prevalence of synergistic interactions between nutrient enrichment with other local and global stressors, and as such it should be prioritized.     

From marine deserts to algal beds: Treptacantha elegans revegetation to reverse stable degraded ecosystems inside and outside a No‐Take marine reserve

Canopy‐forming algae play a key role in temperate coastal ecosystems sustaining complex habitats that provide food and refuge for rich associated biotic communities. These macroalgae are in decline in many coastal areas, where overgrazing by herbivores can lead to the loss of these highly structured and diverse habitats toward less complex sea urchin barren grounds. Once established, low productive barren grounds are considered stable states maintained by several positive feedback mechanisms that prevent the recovery of marine forests. To revert this global decline, restoration efforts and measures are being encouraged by EU regulations and local actions. Here, we tested the success of active revegetation techniques as a tool to promote functional and productive Treptacantha elegans forests in sea urchin barren grounds under different restoration strategies (active, and combined active with passive strategies). Active revegetation was performed in 6 barren grounds, 3 located inside a Mediterranean No‐Take marine reserve (active and passive strategy) and 3 outside (active strategy alone), following a three‐step protocol: (1) sea urchin population eradication, (2) seeding with Treptacantha elegans, and (3) enhancement of T. elegans recruitment. Revegetation success was assessed 1 year later in the six barren grounds, but was only achieved after combining active with passive restoration strategies. Our results encourage revegetation of barren grounds to shift from less productive habitats to complex T. elegans forests, highlight the potential of the combined passive and active restoration strategies, as well as the important role of marine reserves not only in conservation but also in ecological restoration.     

Macroalgal forests and sea urchin barrens: Structural complexity loss,fisheries exploitation and catastrophic regime shifts

In contrast to ecosystems that change smoothly and continuously in response to various stressors, some transitions between states with radically different properties can occur abruptly. An example are the sea urchin barrens and canopy algae (e.g. kelp beds) which represent alternative stable states. More precisely, the variation in grazing intensity in coastal rocky system may drive switches between one complex state into the barren state, the former dominated by erect algae and the latter by encrusting coralline algae and bare rock. Identifying the causes that drive a complex system towards a phase-shift becomes crucial for implementing strategies for the successful conservation and/or recovery of marine forests. Mathematical models that aim to assess effects of fisheries and sea urchins-seaweeds interaction may contribute to understand mechanisms driving transitions between alternative states. Fisheries exploitation has been considered the main driver of urchin population density transitions, with consequent effects on canopy algae distribution. The major novelty of the model here presented is the incorporation of habitat structural complexity, which explains the effect of algal biomass loss on coastal fish assemblages and the strong irreversibility of the system. We have found that as some critical parameters change macroalgae are more resilient and this may give rise to new scenarios, such as the emergence of new stationary states.     

Near‐term impacts of coral restoration on target species,coral reef community structure,and ecological processes

The global decline of corals has created an urgent need for effective, science‐based methods to augment coral populations and restore important ecosystem functions. To meet this challenge, the field of coral restoration has rapidly evolved over the past decade. However, despite widespread efforts to outplant corals and monitor survivorship, there is a shortage of information on the effects of coral restoration on reef communities or important ecosystem functions. To fill this knowledge gap, we examined the effects of restoration on three major criteria: diversity, community structure, and ecological processes. We conducted surveys of four restored sites in the Florida Keys ranging in restoration effort (500–2,300 corals outplanted) paired with surveys of nearby, unmanipulated control sites. Coral restoration successfully enhanced coral populations, increasing coral cover 4‐fold, but manifested in limited differences in coral and fish communities. Some restored sites had higher abundance of herbivorous fish, rates of herbivory, or more juvenile‐sized corals, but these effects were limited to individual reefs. Damselfish were consistently more abundant at restored compared to control sites. Despite augmenting target coral populations, 3 years of coral restoration has not facilitated many of the positive feedbacks that help reinforce coral success. In a time of increasingly frequent disturbances, it is urgent we hasten the speed at which reefs recover important ecological processes, such as herbivory and nutrient cycling, that make reefs more resistant and resilient if we are to achieve long‐term restoration success.     

Exposure to herbivores increases seedling growth and survival of American chestnut (Castanea dentata) through decreased interspecific competition in canopy gaps

The successful development of early stages of blight‐resistant hybrid stock has increased hopes for restoration of American chestnut (Castanea dentata) to eastern North American forests. However, these forests have undergone substantial ecological change in the century since the functional extirpation of American chestnut, and it remains unknown to what extent American chestnut will be able to recolonize contemporary forests. In particular, high densities of white‐tailed deer (Odocoileus virginianus) and competition with mesophytic tree species such as maple (Acer) may impede chestnut regeneration, much as they affect oak (Quercus). We used a split‐plot analysis of variance (ANOVA) design to examine the effects of canopy gaps and herbivory on survival and growth of third generation backcrossed (BC₃) hybrid chestnut seedlings over two growing seasons in central Indiana, U.S.A. Only 4 of 588 (0.7%) seedlings in closed‐canopy plots survived to the end of the study, as opposed to 264 of 589 (45%) seedlings in gap plots. Within the gap treatment, fencing was associated with reduced chestnut survival as well as reduced herbivory and increased cover of non‐chestnut vegetation. Our results indicate that herbivory may indirectly benefit chestnut regeneration by suppressing competition. However, this beneficial effect is likely context‐dependent and additional work is needed to establish the conditions under which it occurs.     

Responses of algal communities to gradients in herbivore biomass and water quality in Marovo Lagoon, Solomon Islands

Settlement tiles were used to characterise and quantify coral reef associated algal communities along water quality and herbivory gradients from terrestrial influenced near shore sites to oceanic passage sites in Marovo Lagoon, the Solomon Islands. After 6 months, settlement tile communities from inshore reefs were dominated by high biomass algal turfs (filamentous algae and cyanobacteria) whereas tiles located on offshore reefs were characterised by a mixed low biomass community of calcareous crustose algae, fleshy crustose algae and bare tile. The exclusion of macrograzers, via caging of tiles, on the outer reef sites resulted in the development of an algal turf community similar to that observed on inshore reefs. Caging on the inshore reef tiles had a limited impact on community composition or biomass. Water quality and herbivorous fish biomass were quantified at each site to elucidate factors that might influence algal community structure across the lagoon. Herbivore biomass was the dominant driver of algal community structure. Algal biomass on the other hand was controlled by both herbivory and water quality (particularly dissolved nutrients). This study demonstrates that algal communities on settlement tiles are an indicator capable of integrating the impacts of water quality and herbivory over a small spatial scale (kilometres) and short temporal scale (months), where other environmental drivers (current, light, regional variability) are constant.     

A pre‐adaptive approach for tropical forest restoration during climate change using naturally occurring genetic variation in response to water limitation

Effective reforestation of degraded tropical forests depends on selecting planting material suited to the stressful environments typical at restoration sites that can be exacerbated by increased duration and intensity of dry spells expected with climate change. While reforestation efforts in nontropical systems are incorporating drought‐adapted genotypes into restoration programs to cope with drier conditions, such approaches have not been tested or implemented in tropical forests. As the first effort to examine genetic variation in plant response to drought in a tropical wet forest, we established a watering experiment using five replicated maternal lines (i.e. seedlings from different maternal trees) of five dipterocarp species native to Borneo. Apart from the expected species level variation in growth and herbivory (3‐fold variation in both cases), we also found intraspecific variation so that growth in some cases varied 2‐fold, and herbivory 3‐fold, among genetically different maternal lines. In two species we found that among‐maternal line variation in growth rate was negatively correlated with tolerance to water limitation, that is, the maternal lines that performed the best in the high water treatment lost proportionally more of their growth during water limitation. We argue that selection for tolerance to future drier conditions is not only likely to impact population genetics of entire forests, but likely extends from forest trees to the communities of canopy arthropods associated with these trees. In tropical reforestation efforts where increased drought is predicted from climate change, including plant material resilient to drier conditions may improve restoration effectiveness.     

Herbivory facilitates growth of a key reef‐building Caribbean coral

The decline of reef‐building corals in conjunction with shifts to short‐lived opportunistic species has prompted concerns that Caribbean reef framework‐building capacity has substantially diminished. Restoring herbivore populations may be a potential driver of coral recovery; however, the impact of herbivores on coral calcification has been little studied. We performed an exclusion experiment to evaluate the impact of herbivory on Orbicella faveolata coral growth over 14 months. The experiment consisted of three treatments: full exclusion cages; half cage procedural controls; and uncaged control plates, each with small O. faveolata colonies. We found that herbivorous fish exclusion had a substantial impact on both macroalgal cover and coral growth. Fleshy macroalgae reached 50% cover within some exclusion cages, but were almost absent from uncaged control plates. Critically, O. faveolata calcification rates were suppressed by almost half within exclusion cages, with monthly coral growth negatively related to overgrowth by fleshy macroalgae. These findings highlight the importance of herbivorous fishes for coral growth and the detrimental impact of macroalgal proliferation in the Caribbean. Policy makers and local managers should consider measures to protect herbivorous fishes and reduce macroalgal proliferation to enable coral communities to continue to grow and function.     

Bird predation enhances tree seedling resistance to insect herbivores in contrasting forest habitats

According to the associational resistance hypothesis, neighbouring plants are expected to influence both the insect herbivore communities and their natural enemies. However, this has rarely been tested for the effects of canopy trees on herbivory of seedlings. One possible mechanism responsible for associational resistance is the indirect impact of natural enemies on insect herbivory, such as insectivorous birds. But it remains unclear to what extent such trophic cascades are influenced by the composition of plant associations (i.e. identity of ‘associated’ plants). Here, we compared the effect of bird exclusion on insect leaf damage for seedlings of three broadleaved tree species in three different forest habitats. Exclusion of insectivorous birds affected insect herbivory in a species-specific manner: leaf damage increased on Betula pendula seedlings whereas bird exclusion had no effect for two oaks (Quercus robur and Q. ilex). Forest habitat influenced both the extent of insect herbivory and the effect of bird exclusion. Broadleaved seedlings had lower overall leaf damage within pine plantations than within broadleaved stands, consistent with the resource concentration hypothesis. The indirect effect of bird exclusion on leaf damage was only significant in pine plantations, but not in exotic and native broadleaved woodlands. Our results support the enemies hypothesis, which predicts that the effects of insectivorous birds on insect herbivory on seedlings are greater beneath non-congeneric canopy trees. Although bird species richness and abundance were greater in broadleaved woodlands, birds were unable to regulate insect herbivory on seedlings in forests of more closely related tree species.     

Resilience and restoration of tropical and subtropical grasslands,savannas, and grassy woodlands

Despite growing recognition of the conservation values of grassy biomes, our understanding of how to maintain and restore biodiverse tropical grasslands (including savannas and open‐canopy grassy woodlands) remains limited. To incorporate grasslands into large‐scale restoration efforts, we synthesised existing ecological knowledge of tropical grassland resilience and approaches to plant community restoration. Tropical grassland plant communities are resilient to, and often dependent on, the endogenous disturbances with which they evolved – frequent fires and native megafaunal herbivory. In stark contrast, tropical grasslands are extremely vulnerable to human‐caused exogenous disturbances, particularly those that alter soils and destroy belowground biomass (e.g. tillage agriculture, surface mining); tropical grassland restoration after severe soil disturbances is expensive and rarely achieves management targets. Where grasslands have been degraded by altered disturbance regimes (e.g. fire exclusion), exotic plant invasions, or afforestation, restoration efforts can recreate vegetation structure (i.e. historical tree density and herbaceous ground cover), but species‐diverse plant communities, including endemic species, are slow to recover. Complicating plant‐community restoration efforts, many tropical grassland species, particularly those that invest in underground storage organs, are difficult to propagate and re‐establish. To guide restoration decisions, we draw on the old‐growth grassland concept, the novel ecosystem concept, and theory regarding tree cover along resource gradients in savannas to propose a conceptual framework that classifies tropical grasslands into three broad ecosystem states. These states are: (1) old‐growth grasslands (i.e. ancient, biodiverse grassy ecosystems), where management should focus on the maintenance of disturbance regimes; (2) hybrid grasslands, where restoration should emphasise a return towards the old‐growth state; and (3) novel ecosystems, where the magnitude of environmental change (i.e. a shift to an alternative ecosystem state) or the socioecological context preclude a return to historical conditions.     

Disturbance and reef topography maintain high local diversity in Ecklonia radiata kelp forests

Disturbance of competitive‐dominant plant and algae canopies often lead to increased diversity of the assemblage. Kelp forests, particularly those of temperate Western Australia, are habitats with high alpha diversity. This study investigated the roles of broad‐scale canopy loss and local scale reef topography on structuring the kelp‐dominated macroalgal forests in Western Australia. Eighteen 314 m² circular areas were cleared of their Ecklonia radiata canopy and eighteen controls were established across three locations. The patterns of macroalgal recolonisation in replicate clearances were observed over a 34 month period. Macroalgal species richness initially increased after canopy removal with a turf of filamentous and foliose macroalgae dominating cleared areas for up to seven months. A dense Sargassum canopy dominated cleared areas from 11 to 22 months. By 34 months, partial recovery of the kelp canopy into cleared areas had occurred. Some cleared areas did not follow this trajectory but remained dominated by turfing, foliose and filamentous algae. As kelp canopies developed, the initial high species diversity declined but still remained elevated relative to undisturbed controls, even after 34 months. More complex reef topography was associated with greater variability in the algal assemblage between replicate quadrats suggesting colonising algae had a greater choice of microhabitats available to them on topographically complex reefs. Shading by canopies of either Sargassum spp. and E. radiata are proposed to highly influence the abundance of algae through competitive exclusion that is relaxed by disturbance of the canopy. Disturbance of the canopy in E. radiata kelp forests created a mosaic of different patch types (turf, Sargassum‐dominated, kelp‐dominated). These patch types were both transient and stable over the 34 months of this study, and are a potential contemporary process that maintains high species diversity in temperate kelp‐dominated reefs.     

Herbivory in canopy gaps created by a typhoon varies by understory plant leaf phenology

1. Availabilities of light and soil nitrogen for understory plants vary by extent of canopy gap formation through typhoon disturbance. We predicted that variation in resource availability and herbivore abundance in canopy gaps would affect herbivory through variation in leaf traits among plant species. We studied six understory species that expand their leaves before or after canopy closure in deciduous forests. We measured the availabilities of light, soil nitrogen, soil water content, and herbivore abundance in 20 canopy gaps (28.3–607.6 m²) formed by a typhoon and in four undisturbed stands. We also measured leaf traits and herbivory on understory plants. 2. The availabilities of light and soil nitrogen increased with increasing gap size. However, soil water content did not. The abundance of herbivorous insects (such as Lepidoptera and Orthoptera) increased with increasing gap size. 3. Concentrations of condensed tannins, total phenolics, and nitrogen in leaves and the leaf mass per area increased in late leaf expansion species with increasing gap size, whereas none of the leaf traits varied by gap size in early leaf expansion species. 4. Herbivory increased on early leaf expansion species with increasing gap size, but decreased on late leaf expansion species. In these late leaf expansion species, total phenolics and C : N ratio had negative relationships with herbivory. 5. These results suggested that after typhoon disturbance, increased herbivory on early leaf expansion species can be explained by increased herbivore abundance, whereas decreased herbivory on late leaf expansion species can be explained by variation in leaf traits.     

Alien Plant Invasions,Introduced Ungulates,and Alternative States in a Mesic Forest in Hawaii

Restoration by natural successional processes after removal of perturbations may not be feasible for many degraded ecosystems. Controlling major ecological threats such as non‐native ungulates is often a critical first step toward restoring native communities but past degradation, interactions with alien species and abiotic features may create conditions requiring additional intervention to ensure effective conservation. We monitored a series of fenced plots within diverse mesic forest on western Kauai in the Hawaiian Islands from 1998 to 2005 to determine the effects of ungulate removal on native and alien plant species. Relative to unfenced control plots, germination of seedlings and frequency of understory species of both native and alien species increased in the fenced plots. Density of both native and alien canopy and understory species declined more in unfenced than fenced plots, but density of native species declined more than alien species density in both fenced and unfenced plots. In fenced plots, the frequency of larger alien woody species and cover of an alien, mat‐forming fern species increased over time, indicating that fencing may encourage alien species that could interfere with regeneration of native species. Our study suggests that effective conservation of this and other remnant native Hawaiian forests will require both ungulate exclusion, removal of alien plant species with especially detrimental effects on native species, and proactive restoration programs for native species without natural sources of propagules. As the effects of invasive species continue to escalate, continental ecosystems lacking high endemism may also require similar interventions to preserve their biodiversity.     

Algal resistance to herbivory on a Caribbean barrier reef

Field and laboratory research at Carrie Bow Cay, Belize showed that macroalgae, grouped in functional-form units resisted fish and urchin herbivory in the following order (from high to low resistance): Crustose-Group, Jointed Calcareous-Group, Thick Leathery-Group, Coarsely Branched-Group, Filamentous-Group and Sheet-Group; thereby supporting the hypothesis that crustose, calcareous and thick algae have evolved antipredator defenses and should show the greatest resistance to herbivory with a gradation of increasing palatability towards filaments and sheets. Of the 21 species examined, several (e.g.,Dictyota cervicornis on grids,Laurencia obtusa andStypopodium zonale) had exceptionally low losses to fish grazing, probably due to chemical defences. The sea urchin,Diadema antillarum, was more inclined to feed on algae with known toxic secondary metabolites than were herbivorous fishes; hypothetically related to the differences in mobility and concomitant modes of feeding. Tough leathery forms such asSargassum polyceratium andTurbinaria turbinata resisted grazing by bottom feeding parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) but were susceptible when suspended midway in the water column, possibly due to the presence of rudderfishes (Kyphosidae) which readily consume drift Sargassaceae. The overall tendencies support our predicted relationship between grazer-resistance and algal morphology. In conjunction with our previously reported findings concerning primary productivity, toughness and calorimetry for many of the same species, these results lend credence to generalizations relating form with function in marine macroalgae.     

Global patterns of insect herbivory in gap and understorey environments,and their implications for woody plant carbon storage

Insect herbivory is thought to favour carbon allocation to storage in juveniles of shade‐tolerant trees. This argument assumes that insect herbivory in the understorey is sufficiently intense as to select for storage; however, understoreys might be less attractive to insect herbivores than canopy gaps, because of low resource availability and – at temperate latitudes – low temperatures. Although empirical studies show that shade‐tolerant species in tropical forests do allocate more photosynthate to storage than their light‐demanding associates, the same pattern has not been consistently observed in temperate forests. Does this reflect a latitudinal trend in the relative activity of insect herbivory in gap versus understorey environments? To date there has been no global review of the effect of light environment on insect herbivory in forests. We postulated that if temperature is the primary factor limiting insect herbivory, the effect of gaps on rates of insect herbivory should be more evident in temperate than in tropical forests; due to low growing season temperatures in the oceanic temperate forests of the Southern Hemisphere, the effect of gaps on insect herbivory rates should in turn be stronger there than in the more continental temperate climates of the Northern Hemisphere. We examined global patterns of insect herbivory in gaps versus understories through meta‐analysis of 87 conspecific comparisons of leaf damage in contrasting light environments. Overall, insect herbivory in gaps was significantly higher than in the understorey; insect herbivory was 50% higher in gaps than in understoreys of tropical forests but did not differ significantly between gaps and understories in temperate forests of either hemisphere. Results are consistent with the idea that low resource availability – and not temperature – limits insect herbivore activity in forest understoreys, especially in the tropics, and suggest the selective influence of insect herbivory on late‐successional tree species may have been over‐estimated.     

Maize‐field complexity and farming system influence insectivorous birds’ contribution to arthropod herbivore regulation

The contribution of insectivorous birds to reducing crop damage through suppression of herbivory remains underappreciated, despite their role as cropland arthropod predators. We examined the roles of farming system, crop cover pattern, and structural configuration in influencing assemblage composition of insectivorous birds and their herbivorous arthropod prey across maize fields, and determined how bird exclusion affects crop herbivory levels. To achieve these objectives, we collected data across a sample of organic and conventional small‐scale non‐Bt maize farms in western Kenya. Assessments of abundance, diversity, and richness of insectivorous birds and abundance of their arthropod prey were compared between organic and conventional small‐scale non‐Bt maize on monocultured and inter‐cropped farms. We also employed bird exclusion experiments to assess impacts of bird predation on herbivorous arthropod abundance. Results showed that higher structural heterogeneity supported higher insectivorous bird richness, particularly under organic systems, dense trees, large woodlots, and thick hedgerows. Bird abundance further increased with crop diversity but not in relation to cropping method, hedgerow type, or percent maize cover per se. Conversely, herbivorous arthropod abundance and richness increased on conventional farms and those with higher percent maize cover, but were unaffected by cropping methods, tree, or hedgerow characteristics. Birds’ arthropod prey was more abundant under completely closed experimental plots compared with open or semi‐closed plots, confirming a significant linkage between birds and herbivorous arthropod suppression. In this study, we demonstrate importance of structural heterogeneity in agricultural landscapes, including diverse croplands and on‐farm trees to maximize insectivorous birds’ contribution to reducing crop arthropod herbivory. Abstract in Swahili is available with online material.     

Spatial dynamics of benthic competition on coral reefs

The community structure of sedentary organisms is largely controlled by the outcome of direct competition for space. Understanding factors defining competitive outcomes among neighbors is thus critical for predicting large-scale changes, such as transitions to alternate states within coral reefs. Using a spatially explicit model, we explored the importance of variation in two spatial properties in benthic dynamics on coral reefs: (1) patterns of herbivory are spatially distinct between fishes and sea urchins and (2) there is wide variation in the areal extent into which different coral species can expand. We reveal that the size-specific, competitive asymmetry of corals versus fleshy algae highlights the significance of spatial patterning of herbivory and of coral growth. Spatial dynamics that alter the demographic importance of coral recruitment and maturation have profound effects on the emergent structure of the reef benthic community. Spatially constrained herbivory (as by sea urchins) is more effective than spatially unconstrained herbivory (as by many fish) at opening space for the time needed for corals to settle and to recruit to the adult population. Further, spatially unconstrained coral growth (as by many branching coral species) reduces the number of recruitment events needed to fill a habitat with coral relative to more spatially constrained growth (as by many massive species). Our model predicts that widespread mortality of branching corals (e.g., Acropora spp) and herbivorous sea urchins (particularly Diadema antillarum) in the Caribbean has greatly reduced the potential for restoration across the region.     

Alien marine fishes deplete algal biomass in the Eastern Mediterranean

One of the most degraded states of the Mediterranean rocky infralittoral ecosystem is a barren composed solely of bare rock and patches of crustose coralline algae. Barrens are typically created by the grazing action of large sea urchin populations. In 2008 we observed extensive areas almost devoid of erect algae, where sea urchins were rare, on the Mediterranean coast of Turkey. To determine the origin of those urchin-less 'barrens', we conducted a fish exclusion experiment. We found that, in the absence of fish grazing, a well-developed algal assemblage grew within three months. Underwater fish censuses and observations suggest that two alien herbivorous fish from the Red Sea (Siganus luridus and S. rivulatus) are responsible for the creation and maintenance of these benthic communities with extremely low biomass. The shift from well-developed native algal assemblages to 'barrens' implies a dramatic decline in biogenic habitat complexity, biodiversity and biomass. A targeted Siganus fishery could help restore the macroalgal beds of the rocky infralittoral on the Turkish coast.