Variation in thermal tolerances of native freshwater fishes in South Africa's Cape Fold Ecoregion: examining the east–west gradient in species' sensitivity to climate warming

The Cape Fold Ecoregion (CFE) is a biodiversity hotspot with high levels of endemism in its freshwater fish fauna. This study examined inter and intra-specific variation in critical thermal maxima (T_Cmax) for eight native species of freshwater fish from the CFE. Cape galaxias Galaxias zebratus, Breede River redfin Pseudobarbus burchelli, Berg River redfin Pseudobarbus burgi, Clanwilliam redfin Pseudobarbus calidus and fiery redfin Pseudobarbus phlegethon were the most thermally sensitive (T_Cmax = 29.8–32.8°C). Clanwilliam rock-catfish Austroglanis gilli, Eastern Cape redfin Pseudobarbus afer and Cape kurper Sandelia capensis were moderately sensitive (T_Cmax = 33.0–36.8°C). An increase in intra-specific thermal sensitivity of S. capensis was observed from east to west. The results were related to in situ water temperature, which influenced T_Cmax for all species, suggesting that thermal history is a major driver of variation in thermal tolerance amongst populations. These thermal tolerance data for freshwater fishes in the CFE demonstrate that resilience to climate warming follows a geographical cline and that the more sensitive western species and regions are conservation priorities.     

Swimming with the giant: coexistence patterns of a new redfin minnow Pseudobarbus skeltoni from a global biodiversity hot spot

Ecological niche theory predicts that coexistence is facilitated by resource partitioning mechanisms that are influenced by abiotic and biotic interactions. Alternative hypotheses suggest that under certain conditions, species may become phenotypically similar and functionally equivalent, which invokes the possibility of other mechanisms, such as habitat filtering processes. To test these hypotheses, we examined the coexistence of the giant redfin Pseudobarbus skeltoni, a newly described freshwater fish, together with its congener Pseudobabus burchelli and an anabantid Sandelia capensis by assessing their scenopoetic and bionomic patterns. We found high habitat and isotope niche overlaps between the two redfins, rendering niche partitioning a less plausible sole mechanism that drives their coexistence. By comparison, environment–trait relationships revealed differences in species–environment relationships, making habitat filtering and functional equivalence less likely alternatives. Based on P. skeltoni's high habitat niche overlap with other species, and its large isotope niche width, we inferred the likelihood of differential resource utilization at trophic level as an alternative mechanism that distinguished it from its congener. In comparison, its congener P. burchelli appeared to have a relatively small trophic niche, suggesting that its trophic niche was more conserved despite being the most abundant species. By contrast, S. capensis was distinguished by occupying a higher trophic position and by having a trophic niche that had a low probability of overlapping onto those of redfins. Therefore, trophic niche partitioning appeared to influence the coexistence between S. capensis and redfins. This study suggests that coexistence of these fishes appears to be promoted by their differences in niche adaptation mechanisms that are probably shaped by historic evolutionary and ecological processes.     

Are native cyprinids or introduced salmonids stronger regulators of benthic invertebrates in South African headwater streams?

The introduction of nonnative salmonids in the Southern Hemisphere generally leads to a reduction in invertebrate abundance and changes in assemblage composition. In the Cape Floristic Region of South Africa, introduced rainbow trout Oncorhynchus mykiss is the dominant predator in many headwater streams, where they have replaced small‐bodied native fishes such as Breede River redfin Pseudobarbus burchelli. To examine the consequences of this species replacement on food web structure, we used a month‐long field experiment to compare the top‐down effects of Breede River redfin and rainbow trout on benthic invertebrate assemblages (abundance and composition) and basal resources (periphyton and particulate organic matter) in 1 × 1.5 m of plastic cages. Benthic invertebrate abundance was more strongly depleted in the cages with redfin than in the cages with trout, and redfin and trout had distinct effects on invertebrate assemblage composition. On the other hand, neither redfin nor trout had a significant influence over standing stocks of periphyton or organic matter, implying that their differential effects on benthic invertebrates did not cascade down to the base of the stream food web in our experiment. Gut content analysis showed that aquatic invertebrates contributed more to the diet of redfin, while terrestrial invertebrates contributed more to the diet of trout, which may be responsible for the relatively weak effect of trout on aquatic invertebrates. This pattern contrasts with nonnative salmonid impacts elsewhere in the Southern Hemisphere. That trout can strongly alter the structure of benthic invertebrate assemblages, in addition to severely depleting native fish abundance, in Cape Floristic Region headwater streams should be weighed into management decisions, and our findings highlight the need for a detailed understanding of species‐specific top‐down effects where native predators are replaced by invasive predators.     

Isotopic variation in five species of stream fishes under the influence of different land uses

The aim of this study was to test if changes in land use alter the isotopic signature of fish species, promoting changes in the trophic position and food resource partitioning between these consumers. Three different systems were investigated: pasture streams (n = 3), streams in sugar cane plantations (n = 3) and reference streams (n = 3). Fish species Aspidoras fuscoguttatus, Astyanax altiparanae, Characidium zebra, Hisonotus piracanjuba and Knodus moenkhausii were selected, and their nitrogen and carbon isotopic compositions were estimated to assess changes in the trophic level and partitioning of food items consumed. The composition of δ¹³C (‰) only differed among the land use categories for A. altiparanae, H. piracanjuba and K. moenkhausii. Resource partitioning was different for all species, with changes in the sources or proportions they consumed in each land use category, but only A. altiparanae introduced new food sources in large quantity in altered land uses. It is important to note, however, that the results from the resource partitioning analysis are limited due to large overlapping of isotopic signatures between the analysed food resources. All fish species exhibited variation in δ¹⁵N (‰), with the highest values found in streams under sugar cane or pasture influence. Despite the variation in nitrogen isotopic values, only C. zebra and H. piracanjuba displayed changes in trophic level. Therefore, it is believed that the increase in the δ¹⁵N (‰) value of the individuals collected in streams under the influence of sugar cane or pasture was due to the greater influence of livestock dung and chemical and organic fertilizers. The results also highlight the importance of studying consumer species along with all forms of resources available at each location separately, because the signatures of these resources also vary within different land uses.     

Summer food of sympatric red fox and pine marten in the German Alps

Based on fecal analyses, we compared summer diet composition and trophic niche breadth for the sympatric red fox Vulpes vulpes (n=55 scats) and pine marten Martes martes (n=64) in the foothills of the German Alps. Mammals accounted for 41 and 51% of the consumed biomass by pine martens and red foxes, respectively, and no single mammal species exceeded 8% of the diet. The larger red fox consumed a wider range of prey sizes than the smaller pine marten, and both consumed large amounts of plants and also insects. Whereas the Levins index suggested that both predators have specialist feeding niches, the Shannon-Wiener index showed that both predators were relatively generalist. Despite its preliminary nature, our study suggests that a strict distinction between generalist and specialist trophic niches is not justified for medium-sized carnivores in the Alps, particularly as results greatly depend on the indices used.     

Trophic niche partitioning of cryptic species of long-eared bats in Switzerland: implications for conservation

Dietary niche partitioning is postulated to play a major role for the stable coexistence of species within a community, particularly among cryptic species. Molecular markers have recently revealed the existence of a new cryptic species of long-eared bat, Plecotus macrobullaris, in the European Alps. We studied trophic niches as well as seasonal and regional variations of diet in eight colonies of the three Plecotus species occurring in Switzerland. Faeces were collected monthly from individuals returning to roost after foraging. Twenty-one arthropod categories were recognized from the faeces. All three species fed predominantly on Lepidoptera, which made up 41%, 87% and 88% (means across colonies) of the diet composition of P. auritus, P. macrobullaris and P. austriacus, respectively. The occurrence of numerous fragments of both diurnal and flightless insects in the diet of P. auritus (but rarely in the diet of the other two species) indicates that this species mostly gleans prey from substrates. P. austriacus and P. macrobullaris are more typical aerial feeders. The latter two species have narrow trophic niches, whilst P. auritus has a much broader diet. Comparison of intraspecific and interspecific niche overlaps in P. auritus and P. macrobullaris in sympatry suggests dietary niche partitioning between these two species. In contrast, the high similarity of the trophic niches of P. austriacus and P. macrobullaris, associated with a typical parapatric distribution, indicates competitive exclusion. The best conservation measures are preservation and restoration of habitats offering a high abundance of moths, the major prey of the three Plecotus species.     

Using stable isotopes to test for trophic niche partitioning: a case study with stream salamanders and fish

1. Stream salamanders and fish often co‐occur even though fish prey on and outcompete salamanders. However, the mechanisms that allow palatable salamanders to coexist with fish are unknown. 2. We tested mechanisms in the field that promote coexistence between Idaho giant salamanders (Dicamptodon aterrimus) and stream salmonid fishes in headwater streams. Previous research in this system indicated that salamander dispersal did not promote coexistence with fish. We tested the hypothesis that D. aterrimus shift their diet when they occur with fish, facilitating coexistence through local niche partitioning. 3. We used nitrogen and carbon stable isotopes to describe the trophic niche of D. aterrimus and fish in three co‐occurring populations of salamanders and fish and three populations of salamanders without fish. We used two approaches to quantify trophic niche partitioning with stable isotopes: 95% kernel density estimators and isotopic mixing models. 4. We found that salamanders and fish were generalists that consumed aquatic invertebrates primarily, but both species were also cannibalistic and predatory on one another. We also found no support for trophic niche partitioning as a coexistence mechanism because there were no differences in the trophic niche metrics among salamander populations with and without fish. 5. Although we did not identify mechanisms that facilitate salamander and fish coexistence, our empirical data and use of novel approaches to describe the trophic niche did yield important insights on the role of predator–prey interactions and cannibalism as alternative coexistence mechanisms. In addition, we found that 95% kernel estimators are a simple and robust method to describe population‐level measure of trophic structure.     

Seasonality and interspecific competition shape individual niche variation in co‐occurring tetra fish in Neotropical streams

The drivers of intraspecific niche variation and its effects on species interactions are still unclear, especially in species‐rich Neotropical environments. Here, we investigated how ecological opportunity and interspecific competition affect the degree of individual trophic specialization and the population niche breadth in tetra fish. We studied the four ecologically similar species (Psalidodon aff. gymnodontus, P. aff. paranae, P. bifasciatus, and Bryconamericus ikaa) in subtropical headwater streams (three sites with two co‐occurring species and three sites with only one species). We sampled fish in two contrasting seasons (winter/dry and summer/wet) and quantified their trophic niches using gut content analysis. Psalidodon bifasciatus was the only species distributed over all the sampled streams. We observed seasonal differences in population trophic niche breadth of P. bifasciatus just when this species co‐occurred with P. aff. gymnodontus. These findings confirm the complex nature of the effects of interspecific competition, depending, for instance, on the identity of the competitor. The degree of individual specialization of P. bifasciatus was higher in the winter, and it was not influenced by the presence of another species. Conversely, the other two Psalidodon species studied presented greater individual specialization in the summer, when fish consumed a higher proportion of allochthonous items (terrestrial insects and seeds), and there were no effects only for B. ikaa. Herein, our results suggest that seasonality in food‐resource availability is a major driver of niche variation and it has the potential to play an important role in how these similar tetra species interact and coexist. Abstract in Portuguese is available with online material.     

Trophic positions of omnivores are not always flexible: Evidence from four species of freshwater crayfish

Omnivores are generally believed to be flexible in their diet and trophic position: seasonal, ontogenetic and site‐based differences in trophic position have been observed. We compared consumed and assimilated diet among four species within a group of omnivorous freshwater crayfish, to determine whether species that occur together at a site occupy different trophic positions. Diets of Geocharax falcata, Gramastacus insolitus, Cherax destructor and Euastacus bispinosus (Decapoda: Parastacidae) were compared using stable isotopes (δ¹³C and δ¹⁵N) and gut content analysis across nine sites that varied in their species composition. Gramastacus insolitus consumed mainly plant material across all sites. Geocharax falcata consumed either plants or animals or both at different sites. Its trophic level was consistently similar to G. insolitus, despite differences in gut contents and source for dietary carbon. Cherax destructor consumed animals and had a relatively stable trophic position among sites. Relative trophic position of these three species was consistent across sites and regardless of food consumed, they were positioned as omnivores at a lower trophic level than predators but higher than primary producers and herbivores. Euastacus bispinosus occupied a higher trophic level than other invertebrate species but δ¹³C levels did not differ among sites. Cherax destructor and G. falcata may show flexibility in food sources and in the assimilation of food that determines their trophic position relative to other crayfish species. In contrast, G. insolitus and E. bispinosus are likely to show both a more fixed diet and less flexible trophic position. Therefore, not all omnivores show the flexible diet and trophic position generally reported in the literature. Some species of omnivorous crayfish may maintain a relatively constant trophic position across sites, seasons or changes in food availability regardless of whether their consumed diet alters or not.     

Habitat associations of imperilled fishes after conservation intervention in the Cape Fold Ecoregion,South Africa

The aim of this study was to examine the habitat associations of a native cyprinid community of the recovering Rondegat River in the Cape Fold Ecoregion of South Africa as part of a long-term native fish abundance monitoring project. Relative abundance data were extracted from underwater video camera footage across the longitudinal gradient of the river in three sampling instances. Using multivariate methods the authors assessed community composition with respect to habitat, its overlap with a protected area and species-specific abiotic predictors of relative abundance. Distance from the uppermost site in the river was the most significant predictor of species abundance, indicating spatial segregation and varying overlap between species. The protected status of sites in the upper reaches, vegetated substrates and the size of individual sites were the most impactful on the relative abundance of the endangered fiery redfin Pseudobarbus phlegethon. The results of this study indicate that underwater video monitoring is an effective and low-cost approach that can inform conservation recommendations. Reducing agricultural runoff and sedimentation in the lower reaches may be useful further interventions to maintain key habitats of submerged vegetation.     

Characterizing the trophic niches of stocked and resident cyprinid fishes: consistency in partitioning over time,space and body sizes

Hatchery‐reared fish are commonly stocked into freshwaters to enhance recreational angling. As these fishes are often of high trophic position and attain relatively large sizes, they potentially interact with functionally similar resident fishes and modify food‐web structure. Hatchery‐reared barbel Barbus barbus are frequently stocked to enhance riverine cyprinid fish communities in Europe; these fish can survive for over 20 years and exceed 8 kg. Here, their trophic consequences for resident fish communities were tested using cohabitation studies, mainly involving chub Squalius cephalus, a similarly large‐bodied, omnivorous and long‐lived species. These studies were completed over three spatial scales: pond mesocosms, two streams and three lowland rivers, and used stable isotope analysis. Experiments in mesocosms over 100 days revealed rapid formation of dietary specializations and discrete trophic niches in juvenile B. barbus and S. cephalus. This niche partitioning between the species was also apparent in the streams over 2 years. In the lowland rivers, where fish were mature individuals within established populations, this pattern was also generally apparent in fishes of much larger body sizes. Thus, the stocking of these hatchery‐reared fish only incurred minor consequences for the trophic ecology of resident fish, with strong patterns of trophic niche partitioning and diet specialization. Application of these results to decision‐making frameworks should enable managers to make objective decisions on whether cyprinid fish should be stocked into lowland rivers according to ecological risk.     

Diets and resource partitioning between larvae of three anisopteran species

Analysis of resource partitioning between larvae of three Anisopteran species showed that Aeshna cyanea and Anax imperator (both Aeshnidae) tended to occupy similar ecological niches which were not shared by Libellula depressa (Libellulidae).The diets of these predators and comparisons between trophic availability and diets indicated that prey species eaten varied according to season and predator species, and that some selection of prey species occurred.     

THE DIET AND IMPLICATIONS OF DIETARY CHANGE OF CAPE GANNETS ON BIRD ISLAND,ALGOA BAY

Batchelor, A. L. & Ross, G. J. B. 1984. The diet and implications of dietary change of Cape Gannets on Bird Island, Algoa Bay. Ostrich 55:45-63.

The diet of the Cape Gannet Sula capensis breeding on Bird Island, Algoa Bay, South Africa, was investigated. Approximately 2000 regurgitations representing 13300 prey items were examined. Twenty-three teleost and two cephalopod species were recorded in the diet. More than 90 % of the numerical composition consisted of the three species Pilchard Sardinops ocellata, Anchovy Engraulis capensis and Saury Scomberesox saurus. The proportions of these three species in the diet changed during the sampling periods. These changes were not related to the implied migration patterns of these species. Chick growth rates were investigated in conjunction with the dietary study. These results indicated that Cape Gannet chicks on a diet of Pilchard and/or Anchovy were more likely to fledge successfully than those on a diet of Stockfish Merluccius capensis or Saury. These observations were based on differences in growth rate, and peak and fledgling mass, The data suggest that Pilchard is a preferred prey species and its presence or absence in regurgitationsMayreflect a similar situation on the feeding grounds. The observed increase in the Bird Island gannet populationMaybe related to the availability of high-quality food and consequent reduced pre-breeding mortality. In the absence of historical dietary data, further detailed population dynamics studies are required to support this supposition.     

Niche partitioning and niche filtering jointly mediate the coexistence of three closely related spider species (Araneae,Philodromidae)

1. Several mechanisms can mediate the coexistence of species, such as the neutral dynamic, niche filtering and niche partitioning. The present study investigated which of these mechanisms mediate the coexistence of closely related spider species at the scale of one locality. 2. The coexistence of three spider species of the genus Philodromus (Philodromidae) (Philodromus albidus, Philodromus aureolus and Philodromus cespitum) was studied. The study area comprised three habitat types, including a deciduous forest, a scrub and a plum tree stand. The spatial niche properties of the Philodromus species were explored by comparing their micro/macrohabitat preferences. The natural diet of these species was analysed. Laboratory experiments involving prey acceptance were conducted to investigate the trophic niche properties. The species' phenologies were studied to compare their temporal niches. 3. The philodromids had differentiated their trophic and habitat niches. The coexistence mechanisms were therefore assessed by studying the relationships between niche overlaps and the pairwise product of relative abundances. A negative relationship was observed between functional niche overlaps and the product of relative abundances, whereas a positive relationship was observed between spatial niche overlaps and the product of relative abundances. 4. The present results suggest that different mechanisms influence different niche dimensions. Niche partitioning influenced functional niches, whereas niche filtering influenced spatial niches. The results also suggest that the filtering process in one dimension was facilitated by niche differentiation in another dimension.     

Trophic niche similarities of sympatric Turdus thrushes determined by fecal contents,stable isotopes,and bipartite network approaches

An ecological niche has been defined as an n‐dimensional hypervolume formed by conditions and resources that species need to survive, grow, and reproduce. In practice, such niche dimensions are measurable and describe how species share resources, which has been thought to be a crucial mechanism for coexistence and a major driver of broad biodiversity patterns. Here, we investigate resource partitioning and trophic interactions of three sympatric, phylogenetically related and morphologically similar species of thrushes (Turdus spp.). Based on one year of data collected in southern Brazil, we investigated niche partitioning using three approaches: diet and trophic niche assessed by fecal analysis, diet and niche estimated by stable isotopes in blood and mixing models, and bipartite network analysis derived from direct diet and mixing model outputs. Approaches revealed that the three sympatric thrushes are generalists that feed on similar diets, demonstrating high niche overlap. Fruits from C3 plants were one of the most important food items in their networks, with wide links connecting the three thrush species. Turdus amaurochalinus and T. albicollis had the greatest trophic and isotopic niche overlap, with 90% and 20% overlap, respectively. There was partitioning of key resources between these two species, with a shared preference for fig tree fruits—Ficus cestrifolia (T. amaurochalinus PSIRI% = 11.3 and T. albicollis = 11.5), which was not present in the diet of T. rufiventris. Results added a new approach to the network analysis based on values from the stable isotope mixing models, allowing comparisons between traditional dietary analysis and diet inferred by isotopic mixing models, which reflects food items effectively assimilated in consumer tissues. Both are visualized in bipartite networks and show food‐consumers link strengths. This approach could be useful to other studies using stable isotopes coupled to network analysis, particularly useful in sympatric species with similar niches.     

The comparative reproductive styles of two closely related African minnows (Pseadobarbus afer andP. asper) inhabiting two different sections of the Gamtoos River system

Synopsis Two species of a monophyletic lineage of flexible-rayed redfin minnows,Pseudobarbus afer andP. asper, were studied to establish if there were any significant differences in their reproductive styles. They are sister species with few morphological or meristic differences.P. afer andP. asper are open substrate benthic spawners on coarse bottoms (rocks) and non-guarders of non-adhesive eggs. Their young are photophobic as free embryos. Riverine spawning sites indicated a conservative tendency and represented a phylogenetic constraint as compared to the more variable attributes, such as egg size, which were under environmental control within the limits expressed by the genotype. The combination of life-history attributes, gonadosomatic index, fecundity, egg size, investment per clutch, number of clutches per season and reproductive lifespan was found to be different forP. afer andP. asper. P. asper is derived and atypical of otherPseudobarbus species studied to date. Differences betweenP. afer andP. asper are directly related to the two distinct environments inhabited by these species-coastal Cape Fold Belt mountain streams and the inland Karoo streams of the Gamtoos River system.P. asper may have reverted, by juvenilization, to a more altricial form to survive the turbid, intermittent Karoo stream.     

A preliminary assessment of the impact of alien rainbow trout (Oncorhynchus mykiss) on indigenous fishes of the upper Berg River,Western Cape Province,South Africa

Impacts of alien rainbow trout (Oncorhynchus mykiss) on critically endangered Berg River redfin (Pseudobarbus burgi), Cape kurper (Sandelia capensis) and Cape galaxias (Galaxias zebratus) in the upper Berg River were investigated in terms of predation and spatial interactions. Trout stomach contents revealed that invertebrates dominate trout diet within the study area, whilst only six fish were recovered from 45 stomachs. The apparent low fish predation success of O. mykiss within the stream suggests a smaller impact compared to that of other alien piscivores such as bass (Micropterus spp.). Galaxias zebratus was the only fish species identified as prey, and its conservation status in the river requires further investigation. Snorkelling surveys revealed that rainbow trout co-exist with S. capensis and adult P. burgi within pools on this river. Galaxias zebratus was absent from the pools, while P. burgi juveniles were segregated from rainbow trout along a depth gradient, possibly indicating avoidance behaviour. Sandelia capensis juveniles may avoid predation by hiding under rocks. Rainbow trout probably compete with indigenous fish for food and space in the pools, though this could not be quantified. The impacts of O. mykiss on all indigenous fauna within the river are likely to be density-dependent.     

Subterranean Sympatry: An Investigation into Diet Using Stable Isotope Analysis

In the Western Cape three species of mole-rat occur in sympatry, however, little is known about differences in their dietary preferences. Dietary composition of the three species; the common mole-rat (Cryptomys hottentotus hottentotus), the Cape mole-rat (Georychus capensis) and the Cape dune mole-rat (Bathyergus suillus) were examined using stable isotope analysis. Blood, fur and claw samples were collected from 70 mole-rats, in addition to several potential food items, to assess food selection of the three species under natural conditions. Overall there was a significant difference in the isotopic composition (δ¹³C and δ¹⁵N) between all three species and significant differences in their diet composition. There were also significant differences between tissues in all three species suggesting temporal variation in diet. The small size and colonial lifestyle of C. h. hottentotus allows it to feed almost 100% on bulbs, while the solitary and larger species G. capensis and B. suillus fed to a greater extent on other resources such as grasses and clover. B. suillus, the largest of the species, had the most generalized diet. However, overall all species relied most heavily upon geophytes and consumed the same species suggesting competition for resources could exist. We also showed a high level of individual variation in diet choices. This was most pronounced in B. suillus and G. capensis and less so in C. h. hottentotus. We demonstrate that stable isotope analysis can successfully be applied to examine dietary patterns in subterranean mammals and provide insights into foraging patterns and dietary variation at both the inter and intra population level.     

Food partitioning and spatial subsidy in shelter‐limited fishes inhabiting patchy reefs of Patagonia

The diets of the most conspicuous reef‐fish species from northern Patagonia, the carnivorous species Pseudopercis semifasciata, Acanthistius patachonicus, Pinguipes brasilianus and Sebastes oculatus were studied. Pinguipes brasilianus had the narrowest diet and most specialized feeding strategy, preying mostly on reef‐dwelling organisms such as sea urchins, limpets, bivalves, crabs and polychaetes. The diet of A. patachonicus was characterized by the presence of reef and soft‐bottom benthic organisms, mainly polychaetes, crabs and fishes. Pseudopercis semifasciata showed the broadest spectrum of prey items, preying upon reef, soft‐bottom and transient organism (mainly fishes, cephalopods and crabs). All S. oculatus guts were empty, but stable‐isotope analyses suggested that this species consumed small fishes and crabs. In general, P. brasilianus depended on local prey populations and ate different reef‐dwelling prey than the other species. Pseudopercis semifasciata, A. patachonicus and probably S. oculatus, however, had overlapping trophic niches and consumed resources from adjacent environments. The latter probably reduces the importance of food as a limiting resource for these reef‐fish populations, facilitating their coexistence in spite of their high trophic overlap.     

Gill Memorial Medal Address Holding the mirror up to nature

Since the early 1980s, there have been large decreases in numbers breeding in South Africa of three seabirds that compete with fisheries for food: African Penguin Spheniscus demersus, Cape Cormorant Phalacrocorax capensis and Bank Cormorant P. neglectus. By contrast, two other species that compete with fisheries but which have a larger foraging range when breeding or are nomadic between breeding localities, Cape Gannet Morus capensis and the nominate race of Swift Tern Thalasseus bergii bergii, have increased. Five seabirds that do not compete with fisheries for prey have maintained stable populations or increased. The decreases in the three species have led to deterioration in the overall conservation status of South Africa's seabirds, of which 10 of 15 species (67%) are presently classified as Threatened or Near-threatened, including six of the seven species that are endemic to the Benguela ecosystem off western southern Africa.