Because of its remoteness and strict protection, the Selvagens Islands may represent one of the last remaining intact marine ecosystems in the eastern Atlantic. With this in mind, we set out to examine the value of Selvagens as a baseline for ecosystem health that can be used to assess ecosystem state not only within the region, but also globally.
Our main objective was to compare the benthic communities and fish assemblages around a densely populated and highly developed island Madeira with the Selvagens Islands Nature Reserve, a strict marine protected area with almost no direct human impacts. An additional goal of our research was to examine the efficacy of marine protected areas around Madeira, comparing them to the Selvagens Islands Nature Reserve, and fished areas around Madeira.
Data were collected by all authors in a collaborative effort. Non-invasive research was conducted, which included photographs and visual estimates described in the methods. No vertebrate sampling was conducted and therefore no approval was required by the Institutional Animal Care and Use Committee. Our data are available at Data Dryad: doi: We conducted in situ surveys of fishes, algae, and benthic macro-invertebrates within two depth strata 10 and 20 m at 29 locations at Selvagens in September and 36 locations around Madeira in July Fig 1. Oceanographic conditions e.
Previous studies in the region have not shown seasonal differences in reef fish abundance [ 16 , 23 ], and we therefore feel that our sampling effort in not confounded by potential temporal variability in these reef fish assemblages.
Sampling locations were restricted to rocky boulder habitat at both locations to reduce the influence of habitat on fish assemblage structure, which is known to exist around Madeira [ 24 ]. Weather conditions precluded sampling along parts of the north coast of Madeira. At Madeira Island, the Garajau Marine Protected Area MPA prohibits the take of marine life except bait fish for the tuna fishery, which consists of small coastal pelagic species [e.
The Rocha do Navio MPA permits extraction of nearshore resources using spearfishing and line fishing, as well as collecting of invertebrates and tuna bait fishing. The Garajau MPA accounted for only 4. All surveys were conducted on open-circuit scuba between the hours of and to reduce the influence of crepuscular variation in the fish assemblages.
Dive duration ranged from 60—90 min depending on habitat and environmental conditions. Characterization of the benthos was conducted along one 50 m-long transect run parallel to the shoreline at each of the two depth strata. For algae and sessile invertebrates, we used a line-point intercept methodology along each transect, recording the species or taxa found every 20 cm on a fiberglass measuring tape. For mobile invertebrates, we counted individuals in twenty-five 50 x 50 cm quadrats randomly placed along each of the m transects.
At each depth stratum within a site, divers counted and estimated lengths of all fishes encountered within fixed-length m belt transects whose widths differed depending on the direction of swim. Swimming duration per transect varied from 10—15 min, depending on habitat complexity and fish abundance. Three replicate transects were performed per site at each depth stratum.
Fishes were identified to species level in all cases. Fish total length TL was estimated to the nearest cm. Fishes were tallied by length and individual-specific lengths were converted to body weights. Numerical density abundance was expressed as number of individuals per m 2 and biomass density was expressed as g per m 2.
Length-weight fitting parameters were obtained from FishBase [ 25 ]. The sum of all individual weights and numerical densities was used to estimate biomass and numerical density by species. Fish species diversity were calculated from the Shannon-Weaver diversity index: , where p i is the proportion of all individuals counted that were of taxa i. Fishes were categorized into four trophic groups top predators, herbivores, secondary consumers, and planktivores based on information from FishBase [ 25 ].
Resource commercially targeted species were designated based on expert opinion of dive operators, fishers, and resource managers.
Principal Coordinate Analysis PCO was used to compare sessile benthic functional groups between islands. Data were arcsine square root transformed prior to analysis. Similarity percentages analysis SIMPER was used to examine differences in sessile benthic functional group cover between islands and depths. Fish assemblage characteristics and trophic biomass between islands were compared using linear mixed models LMMs with island group, depth strata, and their interaction treated as fixed factors.
Stations were treated as a random effect to account for spatial autocorrelation in these data.
Fish trophic group biomass was tested for differences between islands and depths using multivariate analysis of variance MANOVA. Canonical discriminate analysis was used to identify and display the nature of the significant differences among islands and depths found by the MANOVA. Trends in the trophic groups were represented as vectors given by correlations of these variables with the canonical variates. The strength of each variable in discriminating among groups was displayed graphically as the length of these vectors.
Although our main focus was on comparisons between Selvagens and Madeira, the few small MPAs around Madeira provided insights into the potential benefits of protection within this highly developed island. Overall community structure between islands and depths were compared using Principal Components Analysis on dominant sessile benthic cover, mobile invertebrates, and fish biomass by trophic group with supplemental variables island, depth strata projected onto the unconstrained ordination using the ordination program CANOCO version 5. Functional groups were centered subtraction of the average of the column values and standardized division by the standard deviation of the column values , resulting in each column having zero mean and unit variance.
Average dissimilarity in sessile benthic functional group cover between islands was Average dissimilarity of sessile benthic cover between depths was Similarity of Percentages SIMPER for sessile benthic cover by functional group most responsible for the percent dissimilarities between islands using Bray-Curtis similarity analysis of hierarchical agglomerative group average clustering. SIMPER for sessile benthic cover by functional group most responsible for the percent dissimilarities between depths.
Abundance values are means and one standard deviation of the mean in parentheses. Analysis of sessile benthic functional assemblage structure showed clear separation in ordination space, with sites at Selvagens showing higher concordance relative to Madeira Fig 2. Percent cover data were arcsine square root transformed prior to analysis. The sea urchin Diadema africanum was the most common mobile invertebrate observed at both islands Table 2. The average density at Madeira 4.
The hermit crab, Calcinus tubularis , was the second most abundant mobile invertebrate at Madeira, while at Selvagens the purple sea urchin, Paracentrotus lividus was most abundant. Densities of D. In contrast, densities of the black Arbacia lixula and purple P. SIMPER for mobile invertebrates most responsible for the percent dissimilarities between islands using Bray-Curtis similarity analysis of hierarchical agglomerative group average clustering.
Analysis of mobile invertebrate densities showed clear separation in ordination space between islands, with Selvagens showing higher concordance among sites Fig 3. PCO1 explained PCO2 explained an additional Data square root transformed prior to analysis. Overall species richness per transect was similar between islands, but there was a significant interaction between depth and island, with the 20 m sites at Madeira having significantly higher species richness compared with the other three depth x island combinations Table 3 , Fig 4.
Density number of individuals m -2 was significantly greater at Selvagens compared with Madeira, and significantly higher in the 10 vs. Total fish biomass g m -2 was 3. Biomass of resource species followed a similar pattern. Diversity was higher at Madeira compared with Selvagens and higher in 20 m compared with 10 m. Box plots showing median black line , mean red dashed line , upper and lower quartiles, and 5th and 95th percentiles.
Species richness transect, B. Number of individuals m -2 , C. Grams m -2 , and D. Shannon-Weiner Diversity. Results of Linear Mixed Models with station as a random effect, and Tukey-Kramer HSD honestly significant difference pairwise comparisons among fixed factors. Fish assemblage structure based on species biomass showed clear separation in ordination space between Selvagens and Madeira based on Principal Coordinates Analysis Fig 5. This separation was driven by Diplodus vulgaris towards Madeira and Kyphosus sectatrix , Serranus atricauda , Thalassoma pavo , Sparisoma cretense , and Bodianus scrofa towards Selvagens.
Boops boops was orthogonal to this primary axis. Average dissimilarity between islands was Average dissimilarity between depth strata was Square root transformation. Bray Curtis Similarity matrix.
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SIMPER for fish species by biomass g m -2 most responsible for the percent dissimilarities between islands using Bray-Curtis similarity analysis of hierarchical agglomerative group average clustering. There were large differences in the biomass of important resource fish species between Selvagens and Madeira Table 4. Chubs K. Another amberjack, S. Barred hogfish B.
Another predator of small sea urchins, S. A total of fifteen Dusky Grouper Epinephelus marginatus , which is listed as endangered by IUCN, were observed around Selvagens, ranging in size from 40 to cm. Only four dusky groupers were observed around Madeira and these were all exclusively found within the Garajau MPA. The m stratum at Selvagens was distinct from the other three island x depth combinations, with herbivores most responsible for this separation Fig 6.
The Selvagens 20 m stratum was distinct from the 20 m Madeira stratum but overlapped with the Madeira 10 m stratum. Top predators accounted for this separation and was orthogonal to herbivores. Herbivore biomass was seven times higher at Selvagens compared with Madeira, but there was a significant interaction with depth.
Secondary consumers had significantly higher biomass at Selvagens, as well as in the deep depth stratum. Planktivores were not significantly different between the two island groups or between depth strata. Box plots showing median black line , mean red line , upper and lower quartiles, and 5 th and 95 th percentiles. Top predators, B. Values are in g m Biomass in the Garajau MPA was 2. Values are means and standard error of the mean. There was clear separation between Madeira and Selvagens based on dominant sessile benthic cover, mobile invertebrates, and fish biomass by trophic group Fig 9.
Depth strata within islands were close to one another in ordination space, with the two depth strata at Selvagens having higher concordance compared to Madeira. The first two axes of the PCA biplot explained The major drivers of this separation for Selvagens were fish biomass of top predators and canopy algae. The separation for Madeira was driven by the hermit crab C. Erect algae, turf, and herbivore biomass were orthogonal to the primary vectors of island separation towards Selvagens, while barrens, invertebrates, and D. All data were centered and standardized.
Statistical results are shown in Table 6. The Selvagens and Madeira islands present a sharp contrast between a highly urbanized and developed island Madeira and a remote and virtually uninhabited marine reserve. The effects of fishing and land-based activities around Madeira have been well documented [ 31 — 34 ], and while we did not directly evaluate the anthropogenic stressors between Madeira and Selvagens, the extensive development and human activity at the former and the remoteness and near absence of human habitation at the latter clearly highlights the disparities in human pressure experienced between these two locations.
As a result of these differences, the Selvagens Islands may represent one of the last remaining intact marine ecosystems in the eastern Atlantic. Remote locations with limited fishing pressure and few land-based stressors are some of the few remaining examples of marine ecosystems without major anthropogenic influences [ 35 — 36 ]. Although the North Atlantic has been overexploited for centuries [ 37 — 39 ], the establishment of the Selvagens Islands Reserve in has resulted in the maintenance of a healthy ecosystem in a region where the surrounding seas are intensely fished and largely degraded.
While illegal fishing has been reported around Selvagens, the presence of park rangers and the recent addition of a Portuguese maritime police force seems to have limited the impact of these activities as shown by the high abundance of large fisheries species, including the dusky groupers—one of the main targets of recreational and commercial fisheries in the nearby islands and in the Mediterranean. The intertidal community around Selvagens was noteworthy for its abundance of large sun limpets Patella candei , particularly on Selvagem Pequena Fig This species has become rare throughout much of its range due to overfishing and is listed as in danger of extinction in the Canary Islands [ 40 ].
In addition, top-shell snails Phorcus atratus and other limpets, mainly Patella aspera and Siphonaria pectinata , were also common. These extremely high densities of intertidal grazers appear to limit macroalgal growth, which was restricted to small patches of turf algae, mainly Jania cf. The intertidal around Selvagens likely represents one of the few remaining intact ecosystems of its kind in the region. Turf algae and erect algae accounted for the majority of the sessile benthic cover around Selvagens, while Madeira was dominated by CCA, turf algae, sessile invertebrates, and sea urchin barrens.
Our results from Madeira are consistent with previous studies of the benthos around this island [ 31 — 33 ]. Erect macroalgae provides the main biological substrate for many organisms in the eastern Atlantic [ 17 , 41 — 42 ]. In the nearby Canary Islands, upright seaweeds are the principal engineering organisms on shallow rocky bottoms, providing complex habitats that support highly diverse communities [ 17 , 43 ].
The sea urchin D. The densities of D. In the eastern Atlantic, removal of top predators because of overfishing has been linked to hyperabundances of sea urchins predominantly Diadema , with the subsequent elimination of erect vegetative frameworks and the creation of barrens as an alternate stable state [ 17 , 20 , 43 — 47 ]. These upright seaweed beds exist due to the balance between seaweeds, herbivores, and predators [ 19 , 48 ], but human-derived stressors e.
Previous studies around Madeira have shown similar negative relationships between sea urchin densities and macroalgal cover [ 32 — 33 ]. The identity of predatory fishes on the sea urchin D. These authors also showed that the depletion of sea urchin predators in fished areas compared with MPAs resulted in increased sea urchin populations and cascading effects that reduces benthic diversity in areas open to fishing.
Species known to feed on and control sea urchins e. Total fish biomass was more than three times higher at Selvagens compared to Madeira, and biomass of top predators was an order of magnitude greater. Several commercial species e. Previous studies of the fish assemblages around Madeira dating back to the mids documented few large predatory fishes or other species with high resource value [ 24 , 50 — 51 ], suggesting that these species may have been subjected to overfishing for many years.
The prevalence and large sizes of this species at Selvagens is striking compared to Madeira, where it was only found within the highly restricted Garajau MPA. A similar pattern has been observed in other Macaronesian regions such as in the Canary Islands, where groupers are much more abundant around islands where fishing activities are restricted by MPAs and human population is low [ 16 ].
The Garajau MPA was established in and was the first exclusively marine protected area in Portugal. Overall biomass and that of top predators and secondary consumers was higher within this MPA when compared with areas open to fishing around Madeira and was comparable to Selvagens; however, its small size 3. The effects of fishing and other anthropogenic influences associated with urbanization are evident around Madeira [ 15 , 34 , 54 — 55 ].
This is in stark contrast to Selvagens, which harbors rich benthic communities with diverse algal assemblages and high fish biomass, along with an abundance of large resource species.
The clear differences between these two islands highlights the importance of protecting the Selvagens, which harbors one of the last intact marine ecosystems in the North Atlantic, and the need to increase management and protection around Madeira if the ecosystem is to recover and provide the ecosystem services essential to the island community. No-take areas have been identified as an effective tool to restore erect macroalgal beds in other Macaronesian islands [ 56 ], and are known to provide overall ecosystem benefits both within and beyond their borders [ 57 ].
Benthic and fish communities at Selvagens resemble those of some MPAs around other areas of Macaronesia. Densities of the most common mobile invertebrate D. Similarly, fish biomass at Selvagens were similar to those in the La Graciosa no-take reserve, but lower than those in the Mar de Las Calmas [ 58 ]. These differences may be driven by inherent characteristics of both Selvagens and the Canary Islands, with the former showing a higher proportion of warm-temperate species and the latest having more complex and variable assemblages, likely due to the larger sizes of the islands and more heterogeneous habitats [ 59 ].
Threats to the Selvagens include illegal fishing within the reserve and unregulated or weakly monitored fishing for tuna and other target species surrounding the reserve. The current m depth limit for the reserve means that fishing can occur very close to the islands, with potential impacts to nearshore species.
Expansion of the reserve would provide protection for coastal and pelagic species, as well as reducing by-catch of sea birds, marine mammals, and large pelagic fishes e. In addition, the Selvagens may be an important biogeographical link between Madeira and the Canary Islands [ 60 ]. The results of this study reinforce patterns observed at smaller spatial scales within the Canary Islands [ 16 — 17 , 45 — 46 ], and are consistent with observations contrasting remote vs. The Selvagens harbor one of the last intact marine ecosystems in the North Atlantic, and maintains high coastal fish species diversity within a relatively small area [ 60 ].
The Selvagens Nature Reserve serves as a global model for what can be achieved elsewhere if species are protected and allowed to recover within their borders. Increased protection for this unique area is a precautionary bulwark against the degradation and decline of marine ecosystems throughout the region. Our results also identify the need for better fisheries and coastal zone management, as well as the need for larger and more effective marine protected areas around Madeira if the ecosystem is to recover and provide the ecosystem services essential to the island community. The Hydrographic Institute of Portugal graciously provided bathymetric information for Selvagens.
Browse Subject Areas? Click through the PLOS taxonomy to find articles in your field. Abstract The islands of Madeira and Selvagens are less than km apart but offer a clear contrast between a densely populated and highly developed island Madeira , and a largely uninhabited and remote archipelago Selvagens within Macaronesia in the eastern Atlantic. Introduction Macaronesia is a collection of four archipelagos Madeira, Selvagens, Azores, and Canaries located in the North Atlantic Ocean off the coasts of Europe and Africa [ 1 — 3 ], although Cape Verde is also included when considering terrestrial ecosystems [ 4 ].
Download: PPT. Fig 1. Sampling locations around Madeira and Selvagens in the eastern Atlantic Ocean. Materials and methods Ethics statement Data were collected by all authors in a collaborative effort. Sampling design We conducted in situ surveys of fishes, algae, and benthic macro-invertebrates within two depth strata 10 and 20 m at 29 locations at Selvagens in September and 36 locations around Madeira in July Fig 1.
Benthos Characterization of the benthos was conducted along one 50 m-long transect run parallel to the shoreline at each of the two depth strata. Fishes At each depth stratum within a site, divers counted and estimated lengths of all fishes encountered within fixed-length m belt transects whose widths differed depending on the direction of swim. This was a comprehensive survey of this remote archipelago underwater ecosystems, from shallow to deep waters.
There is little knowledge about the ecosystem of the Selvagens Islands, especially in the open waters and deep sea surrounding them. The main recommendations were centered on increasing the effectiveness of the surveillance in the islands and expanding the marine reserve. There is an opportunity to increase the percentage covered by MPAs in Portuguese waters due to the remoteness of this archipelago and the unique conditions of the coastal habitats around the islands.
The open waters are important corridors for marine megafauna moving through the eastern Atlantic. The deep sea contains important nursery habitats and deep water corals. Extending the current marine reserve to enclose these ecosystems will provide significant protection to many species and habitats. Ilhas Selvagens — Madeira Scientific expedition. Our Goal. Initiative status Stage 4 : Finished.