Sustained use of marine subsidies promotes niche expansion in a wild felid

The use of marine subsidies by terrestrial predators can facilitate substantial transfer of nutrients between marine and terrestrial ecosystems. Marine resource subsidies may have profound effects on predator ecology, influencing population and niche dynamics. Expanding niches of top consumers can impact ecosystem resilience and interspecific interactions, affecting predator-prey dynamics and competition. We investigate the occurrence, importance, and impact of marine resources on trophic ecology and niche dynamics in a highly generalist predator, the caracal (Caracal caracal), on the Cape Peninsula, South Africa. Caracals have flexible diets, feeding across a wide range of terrestrial and aquatic prey. We use carbon and nitrogen stable isotope analysis of fur samples (n = 75) to understand trophic position and niche shifts in coastal and inland foragers, as well as the implications of a diet rich in marine resources. We found significant differences in isotope signatures between these groups, with higher δ13C (P < 0.05) and δ15N values (P < 0.01) in coastal foragers. Isotope mixing models reveal that these elevated signatures were due to non-terrestrial food subsidies, where approximately a third of coastal foraging caracal diet comprised marine prey. The addition of marine prey species to diet increased both the trophic level and isotope niche size of coastal foraging caracals, with potential impacts on prey populations and competition. Our results suggest that marine prey are an important dietary resource for coastal foraging caracals, where seabirds, including two endangered species, are a major component of their diet. However, there are likely risks associated with these resource benefits, as routine consumption of seabirds is linked with higher pollutant burdens, particularly metals. Increased encounters between this terrestrial predator and seabirds may be a result of increased mainland colonies due to changes in habitat availability and the highly opportunistic and generalist foraging behaviour of a native predator.

Thermal stress gradient causes increasingly negative effects towards the range limit of an invasive mussel

Environmental filtering (EF), the abiotic exclusion of species, can have first order, direct effects with cascading consequences for population dynamics, especially at range edges where abiotic conditions are suboptimal. Abiotic stress gradients associated with EF may also drive indirect second order effects, including exacerbating the effects of competitors, disease, and parasites on marginal populations because of suboptimal physiological performance. We predicted a cascade of first and second order EF-associated effects on marginal populations of the invasive mussel Mytilus galloprovincialis, plus a third order effect of EF of increased epibiont load due to second order shell degradation by endoliths. Mussel populations on rocky shores were surveyed across 850 km of the south-southeast coast of South Africa, from the species' warm-edge range limit to sites in the centre of their distribution, to quantify second order (endolithic shell degradation) and third order (number of barnacle epibionts) EF-associated effects as a function of along-shore distance from the range edge. Inshore temperature data were interpolated from the literature. Using in situ temperature logger data, we calculated the effective shore level for several sites by determining the duration of immersion and emersion. Summer and winter inshore water temperatures were linked to distance from the mussel's warm range edge (our proxy for an EF-associated stress gradient), suggesting that seasonality in temperature contributes to first order effects. The gradient in thermal stress clearly affected densities, but its influence on mussel size, shell degradation, and epibiosis was weaker. Relationships among mussel size, shell degradation, and epibiosis were more robust. Larger, older mussels had more degraded shells and more epibionts, with endolithic damage facilitating epibiosis. EF associated with a gradient in thermal stress directly limits the distribution, abundance, and size structure of mussel populations, with important indirect second and third order effects of parasitic disease and epibiont load, respectively.

Prey and predator density-dependent interactions under different water volumes

Predation is a critical ecological process that directly and indirectly mediates population stabilities, as well as ecosystem structure and function. The strength of interactions between predators and prey may be mediated by multiple density dependences concerning numbers of predators and prey. In temporary wetland ecosystems in particular, fluctuating water volumes may alter predation rates through differing search space and prey encounter rates. Using a functional response approach, we examined the influence of predator and prey densities on interaction strengths of the temporary pond specialist copepod Lovenula raynerae preying on cladoceran prey, Daphnia pulex, under contrasting water volumes. Further, using a population dynamic modeling approach, we quantified multiple predator effects across differences in prey density and water volume. Predators exhibited type II functional responses under both water volumes, with significant antagonistic multiple predator effects (i.e., antagonisms) exhibited overall. The strengths of antagonistic interactions were, however, enhanced under reduced water volumes and at intermediate prey densities. These findings indicate important biotic and abiotic contexts that mediate predator-prey dynamics, whereby multiple predator effects are contingent on both prey density and search area characteristics. In particular, reduced search areas (i.e., water volumes) under intermediate prey densities could enhance antagonisms by heightening predator-predator interference effects.

Benthic diatom-based indices and isotopic biomonitoring of nitrogen pollution in a warm temperate Austral river system

Rivers are impacted by pollutants from anthropogenic activities such as urbanisation and agricultural practices. Whilst point source pollution has been widely studied and in some cases remediated, non-point pollutant sources remain pervasive, particularly in developing countries that lack economic and human specialist capacity. Monitoring of pollution levels in many regions is additionally challenged by a lack of robust indicators for nitrogen inputs, however, diatom community indices and analysis of variation in microphytobenthos (MBP) stable isotope analysis variations have potential. The present study investigates variations and utilities in benthic diatom indices and MPB δ¹⁵N along different river sections (n = 31) of an austral river between two seasons (wet and dry), testing for relationships with key environmental variables (physical, water and sediment), in the context of N monitoring. One hundred and eighteen diatom taxa belonging to 36 genera were identified, with physical (water flow), water (nitrate, P and total dissolved solids) and sediment (B, Ca, Cr, Na, N, P, SOM, Pb and Zn) variables correlating to one or more of the 12 diatom indices presented. In particular, Biological Diatom Index, Biological Index of Water Quality, Central Economic Community, Index of Artois-Picardie Diatom (IDAP) and Sládeček's Index were strongly explained by sediment variables, whilst Descy's Pollution Index and Schiefele and Schreiner's Index were explained by water and physical variables. Whilst MPB δ¹⁵N were within the "no impact" level in the wet and dry seasons at reference (i.e. unpolluted) sites, all sites located in agricultural or urban areas, and downstream of sewage discharges, had a wider range that encompassed increasing organic impacts ("inorganic impacts" to high "organic impacts"). Temperature and turbidity (negative), as well as dissolved oxygen, waterway width and depth (positive), significantly affected MPB δ¹⁵N, whilst effects of chemistry variables were less apparent. Overall, we found that MPB δ¹⁵N signatures were significantly correlated with Trophic Diatom Index, the Specific Pollution Sensitivity Index and the Artois-Picardie Diatom Index, suggesting the utility of diatoms and MPB δ¹⁵N in assessments of aquatic pollution. In turn, MPB δ¹⁵N values are strong indicators of N pollution across spatial and seasonal gradients. Thus, the results showed the effects of sediment variables on diatoms to be strong, indicating that sediment rather than water characteristics more strongly structure diatom communities. Thus, sediment variables should be sampled when conducting bioassessment studies.

Inter-Population Similarities and Differences in Predation Efficiency of a Mosquito Natural Enemy

Predation is a critical factor that mediates population stability, community structure, and ecosystem function. Predatory natural enemies can contribute to the regulation of disease vector groups such as mosquitoes, particularly where they naturally co-occur across landscapes. However, we must understand inter-population variation in predatory efficiency if we are to enhance vector control. The present study thus employs a functional response (FR; resource use under different densities) approach to quantify and compare predatory interaction strengths among six populations of a predatory temporary pond specialist copepod, Lovenula raynerae, from the Eastern Cape of South Africa preying on second instar Culex pipiens complex mosquito larvae. All individuals from the sampled populations were predatory and drove significant mortality through per capita predation rates of 0.75-1.10 mosquitoes/h at maximum densities over a 5-h feeding time. Individuals from all copepod populations exhibited Type II FRs with no significant differences in attack rates. On the other hand, there were significant differences in handling times, and therefore also maximum feeding rates (maximum experimental prey density: 32), suggesting possible genetic differences among populations that influenced predation. Owing to a widespread distribution in arid landscapes, we propose that predatory calanoid copepods such as L. raynerae play a key regulatory role at the landscape scale in the control of disease vector mosquito populations. We propose that these ecosystems and their specialist biota should thus be conserved and enhanced (e.g., via selective breeding) owing to the ecosystem services they provide in the context of public health.

Assessing sediment particle-size effects on benthic algal colonisation and total carbohydrate production

Increased sedimentation and siltation associated with anthropogenic environmental change may alter microbial biofilms and the carbohydrates they produce, with potential bottom-up effects in these ecosystems. The present study aimed to examine to what extent carbohydrate (associated with biofilm exopolymer) concentration and benthic algal biomass vary among different sediment types (size-structure categories) using a microcosm experiment conducted over a period of 28 days. Substrate treatment and time had a significant effect on the total chlorophyll-a concentrations, whilst a significant interaction was present in the case of total sediment carbohydrates. Total sediment carbohydrates did not relate significantly to chlorophyll-a concentrations overall, nor for any substrate treatments owing to a non-significant 'chlorophyll-a × substrate' interaction term. The diatom community characteristics across sediment sizes were unique for each treatment in our study, with unique dominant diatom taxa compositions within each sediment size class. The finest sediment particle-size (<63 μm) may be the least stable, most likely due to lower binding. We anticipate that the current study findings will lead to a better understanding of how different sediment types due to sedimentation and siltation will impact on primary productivity and the composition of diatom communities in aquatic systems.

Macroinvertebrate functional feeding group alterations in response to habitat degradation of headwater Austral streams

Protecting the structural and functional integrity of lotic ecosystems is becoming increasingly important as many ecological systems face escalating pressures from human population growth and environmental impacts. Knowledge on the functional composition of macroinvertebrates in austral temperate streams is generally lacking hindering the design and implementation of water management and restoration goals. Therefore, this study examined the effects of urban land-use activities on the benthic macroinvertebrate functional feeding guild structure among different stream orders in an austral river system (Bloukrans River) in the Eastern Cape Province of South Africa. Water quality and macroinvertebrate community data were collected across two seasons from 18 sites in two different stream order categories (i.e. 1, 2 + 3), following standard methods. We separated macroinvertebrates into functional feeding groups (FFGs), which we then used to assess the effects of riparian condition on FFG organization. Linear mixed effects model (LMM) results demonstrated that total dissolved solids (TDS), canopy cover, phosphate and channel width were the key variables that described the major sources of variation in macroinvertebrate FFGs. Based on FFG proportions, collector-gatherers were the most abundant in the Bloukrans River and represented 71.3% of the macroinvertebrate assemblages. The FFG ratios indicated that all the eighteen sites were strongly heterotrophic (i.e. streams received additional sources of energy from leaf litter and other organic matter), showed below expected linkage with riparian input and stable substrates were limited. The FFG ratios offered some insights into the overall functioning of Bloukrans River system. Our results highlight the importance of including macroinvertebrate functional diversity as a complementary approach to assess the ecological integrity in management and restoration plans of river systems.

River nutrient water and sediment measurements inform on nutrient retention, with implications for eutrophication

The consideration of nutrients in pollution dynamics is important for environmental management and conservation. Developing countries are yet to appreciate the aquatic ecosystem pollution impacts on their economies and as such, information on water pollution dynamics is limited. This study assessed the spatio-temporal dynamics of nutrient loading and retention in stream water and sediments in the Bloukrans River system, Eastern Cape province, South Africa over the course of the wet and dry season. Sediment and water samples were analysed for total phosphorus (TP) and nitrogen (TN) concentrations, and were used in combination with river flow discharge, to determine nutrient loads. The study results highlight that river discharge plays a significant role in temporal differences in sediment and water column nutrient concentrations. The mean sediment nutrient concentration was high for the dry season, with high values being observed for the urban river system. Nutrient loads were high above the sewage treatment works outflow (i.e. urban sites), as such, a decreasing trend was observed with increasing distance from the urban environment. Nutrient loads were generally high for the dry season in comparison to the wet season indicating organic matter retention (i.e. accumulation from burst sewage pipes) most likely due to low flows. While it was evident that the ageing wastewater infrastructure contributed to the observed state of the Bloukrans River, the high natural nutrient retention capacity seemed to mitigate eutrophication of downstream aquatic ecosystems. As such, the nutrient retention capacity and management of the system is central to the entire Bloukrans River catchment management practices. Therefore, the study contributes to our understanding of water and sediment nutrient pollution dynamics in an arid temperate river landscape where vast spatio-temporal differences in base flow characterise the riverscape.

Biological monitoring in southern Africa: A review of the current status, challenges and future prospects

Biological monitoring programmes have gained popularity around the world particularly in southern Africa as they are fast, integrative and cost-effective approaches for assessing the effects of environmental stressors on aquatic ecosystems. This article reviews current efforts that have been made to use bioindicators (i.e. macroinvertebrates, diatoms and fish) in monitoring water resources and to summarise the challenges in employing these biological monitoring tools in southern Africa. In South Africa, macroinvertebrate (South African Scoring System (SASS)) and diatom based indices (e.g. South African diatom index (SADI)) have demonstrated their utility in identifying sources of impairment and determining the extent of impacts thus giving natural resource managers a scientifically defensible rationale for developing guidelines for conservation and management. Despite this advancement in South Africa, however, developing regionally appropriate quantitative tools for diagnosing ecosystem health is a pressing need for several other southern African countries. Together with sound scientific research, it is imperative for southern African countries to develop specific legislation and have mandated agencies, with proper training and funding to implement biomonitoring and bioassessments. We recommend for the advancement and adoption of biological criteria as an integrated approach to assessing the impact of human activities in riverine ecosystems of the southern African region.

Variation partitioning of benthic diatom community matrices: Effects of multiple variables on benthic diatom communities in an Austral temperate river system

This study explores diatom community dynamics in a highly modified semi-arid temperate region river system characterised by inconsistent river flow. Various water and sediment environmental variables were assessed using a multi-faceted analysis approach to determine the spatio-temporal drivers of benthic diatom communities in the river system. Overall, the diatom community was generally dominated by pollution tolerant species, reflecting the anthropogenic intensity and activities on the river system. Diatom community composition was found to be largely determined by water column chemistry variables particularly nutrient concentrations in comparison to sediment chemistry and physical variables. Strong seasonal diatom species composition was also observed and this was driven by strong seasonal variations in nutrient loads and metal concentrations, a result of the variable water flow across the two seasons. However, the greater temporal variation in communities was observed in the smaller systems with the mainstream river system being more homogenous over time. In addition, diatom community composition and environmental variables were found to be different and more pronounced between streams and mainstream sites, than between canals and streams. The study highlights the complex interaction between water column, sediment and physical variables in determining the diatom species composition in small river systems. It also highlights the importance of river flow inconsistency as an indirect variable that alters primary drivers such as nutrient concentrations in the water column and heavy metal levels in the sediment.

A quantitative analysis of microplastic pollution along the south-eastern coastline of South Africa

The extent of microplastic pollution (<5mm) in the southern hemisphere, particularly southern Africa, is largely unknown. This study aimed to evaluate microplastic pollution along the south-eastern coastline of South Africa, looking at whether bays are characterised by higher microplastic densities than open stretches of coastline in both beach sediment and surf-zone water. Microplastic (mean ± standard error) densities in the beach sediment ranged between 688.9 ± 348.2 and 3308 ± 1449 particles · m(-2), while those in the water column varied between 257.9 ± 53.36 and 1215 ± 276.7 particles · m(-3). With few exceptions there were no significant spatial patterns in either the sediment or water column microplastic densities; with little differences in density between bays and the open coast (P>0.05). These data indicate that the presence of microplastics were not associated with proximity to land-based sources or population density, but rather is governed by water circulation.

Preliminary evidence for the organisation of a bacterial community by zooplanktivores at the top of an estuarine planktonic food web

As part of a larger investigation, the effect of apex predation on estuarine bacterial community structure, through trophic cascading, was investigated using experimental in situ mesocosms. Through either the removal (filtration) or addition of specific size classes of planktonic groups, four different trophic scenarios were established using estuarine water and its associated plankton. One such treatment represented a "natural" scenario in which stable apex predatory pressure was qualified. Water samples were collected over time from each of the treatments for bacterial community evaluation. These samples were assessed through pyrosequencing of the variable regions 4 and 5 of the bacterial 16S rRNA gene and analysed at the species operational taxonomic unit (OTU) level using a community procedure. The blue-green group dominated the samples, followed by Proteobacteria and Bacteroidetes. Samples were the most similar among treatments at the commencement of the experiment. While the bacterial communities sampled within each treatment changed over time, the deviation from initial appeared to be linked to the treatment trophic scenarios. The least temporal deviation-from-initial in bacterial community was found within the stable apex predatory pressure treatment. These findings are consistent with trophic cascade theory, whereby predators mediate interactions at multiple lower trophic levels with consequent repercussions for diversity.

Critical indirect effects of climate change on sub-Antarctic ecosystem functioning

Sub-Antarctic islands represent critical breeding habitats for land-based top predators that dominate Southern Ocean food webs. Reproduction and molting incur high energetic demands that are sustained at the sub-Antarctic Prince Edward Islands (PEIs) by both inshore (phytoplankton blooms; “island mass effect”; autochthonous) and offshore (allochthonous) productivity. As the relative contributions of these sustenance pathways are, in turn, affected by oceanographic conditions around the PEIs, we address the consequences of climatically driven changes in the physical environment on this island ecosystem. We show that there has been a measurable long-term shift in the carbon isotope signatures of the benthos inhabiting the shallow shelf region of the PEIs, most likely reflecting a long-term decline in enhanced phytoplankton productivity at the islands in response to a climate-driven shift in the position of the sub-Antarctic Front. Our results indicate that regional climate change has affected the balance between allochthonous and autochthonous productivity at the PEIs. Over the last three decades, inshore-feeding top predators at the islands have shown a marked decrease in their population sizes. Conversely, population sizes of offshore-feeding predators that forage over great distances from the islands have remained stable or increased, with one exception. Population decline of predators that rely heavily on organisms inhabiting the inshore region strongly suggest changes in prey availability, which are likely driven by factors such as fisheries impacts on some prey populations and shifts in competitive interactions among predators. In addition to these local factors, our analysis indicates that changes in prey availability may also result indirectly through regional climate change effects on the islands' marine ecosystem. Most importantly, our results indicate that a fundamental shift in the balance between allochthonous and autochthonous trophic pathways within this island ecosystem may be detected throughout the food web, demonstrating that the most powerful effects of climate change on marine systems may be indirect.

Trophic level stability-inducing effects of predaceous early juvenile fish in an estuarine mesocosm study

Background

Classically, estuarine planktonic research has focussed largely on the physico-chemical drivers of community assemblages leaving a paucity of information on important biological interactions.

Methodology/Principal Findings

Within the context of trophic cascades, various treatments using in situ mesocosms were established in a closed estuary to highlight the importance of predation in stabilizing estuarine plankton abundances. Through either the removal (filtration) or addition of certain planktonic groups, five different trophic systems were established. These treatments contained varied numbers of trophic levels and thus different “predators” at the top of the food chain. The abundances of zooplankton (copepod and polychaete), ciliate, micro-flagellate, nano-flagellate and bacteria were investigated in each treatment, over time. The reference treatment containing apex zooplanktivores (early juvenile mullet) and plankton at natural densities mimicked a natural, stable state of an estuary. Proportional variability (PV) and coefficient of variation (CV) of temporal abundances were calculated for each taxon and showed that apex predators in this experimental ecosystem, when compared to the other systems, induced stability. The presence of these predators therefore had consequences for multiple trophic levels, consistent with trophic cascade theory.

Conclusions/Significance

PV and CV proved useful indices for comparing stability. Apex predators exerted a stabilizing pressure through feeding on copepods and polychaetes which cascaded through the ciliates, micro-flagellates, nano-flagellates and bacteria. When compared with treatments without apex predators, the role of predation in structuring planktonic communities in closed estuaries was highlighted.