Collapse and reorganization of a food web of Mwanza Gulf, Lake Victoria

Exploring multiple stressor effects with Ecopath, Ecosim, and Ecospace: Research designs, modeling techniques, and future directions

Understanding the cumulative effects of multiple stressors is a research priority in environmental science. Ecological models are a key component of tackling this challenge because they can simulate interactions between the components of an ecosystem. Here, we ask, how has the popular modeling platform Ecopath with Ecosim (EwE) been used to model human impacts related to climate change, land and sea use, pollution, and invasive species? We conducted a literature review encompassing 166 studies covering stressors other than fishing mostly in aquatic ecosystems. The most modeled stressors were physical climate change (60 studies), species introductions (22), habitat loss (21), and eutrophication (20), using a range of modeling techniques. Despite this comprehensive coverage, we identified four gaps that must be filled to harness the potential of EwE for studying multiple stressor effects. First, only 12% of studies investigated three or more stressors, with most studies focusing on single stressors. Furthermore, many studies modeled only one of many pathways through which each stressor is known to affect ecosystems. Second, various methods have been applied to define environmental response functions representing the effects of single stressors on species groups. These functions can have a large effect on the simulated ecological changes, but best practices for deriving them are yet to emerge. Third, human dimensions of environmental change - except for fisheries - were rarely considered. Fourth, only 3% of studies used statistical research designs that allow attribution of simulated ecosystem changes to stressors' direct effects and interactions, such as factorial (computational) experiments. None made full use of the statistical possibilities that arise when simulations can be repeated many times with controlled changes to the inputs. We argue that all four gaps are feasibly filled by integrating ecological modeling with advances in other subfields of environmental science and in computational statistics.

Accumulation characteristics and ecological implications of heavy metals in surface sediments of the Mwanza Gulf, Lake Victoria

The distribution of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn and their fraction characteristics (except Hg) were investigated in surface sediments of the Mwanza Gulf, Lake Victoria. The ecological risks, bioavailability, and mobility of the metals were also evaluated by using enrichment factor (EF), contamination factor (C_f), geo-accumulation index (I_geo), potential ecological risk index (RI), risk assessment code (RAC), individual contamination factor (ICF), and global contamination factor (GCF). Results showed that there were moderate accumulations of heavy metals in sediments from the southern part of the gulf. The mean C_f of heavy metals ranged from 1.19 (Ni) to 2.85 (Hg) suggesting moderate contamination of heavy metals in the sediments while I_geo results showed that the sediments are mainly contaminated by As, Cr, and Hg. The average potential ecological risk of heavy metals in sediments of the Mwanza Gulf is at moderate level (RI 205.49). Hg and Cd posed considerable or moderated risks with mean ecological risk of 114.18 and 44.16, which accounted for 51.08% and 21.54% of the total RI, respectively. High bioavailability and mobility of heavy metals were found in sediments near Mwanza city, particularly Zn and Cd, of which the bioavailability risks were at medium to high levels. Given the biological and environmental importance of the Mwanza Gulf and Lake Victoria, emission paths and bioaccumulation of heavy metals through food webs should be studied carefully to ensure the safety of food and the health and well-being of humans.

Adult Atlantic salmon have a new freshwater predator

The Atlantic salmon (Salmo salar) is one of the world's most emblematic freshwater fish. Despite conservation and rehabilitation plans, populations of this species are dramatically declining due to human impacts such as habitat fragmentation, overfishing and water pollution. Owing to their large body size, anadromous adults were historically invulnerable to fish predation during their spawning period migration. This invulnerability has disappeared in Western Europe with the introduction of a new freshwater predator, the European catfish (Silurus glanis). Here we report how adults of Atlantic salmon are predated in the fishway of a large river of SW France, where the delayed and narrow passage created by the structure increases the probability of predator-prey encounter. We assessed predation risk by monitoring salmon and catfish in one fishway of the River Garonne, using video fish-counting from 1993 to 2016. We analysed the predation strategy of catfish using observations made with acoustic camera and RFID telemetry in 2016. Our results demonstrate a high predation rate (35%-14/39 ind.) on salmon inside the fishway during the 2016 spawning period migration. Our results suggest that a few specialized catfish individuals adapted their hunting behaviour to such prey, including their presence synchronized with that of salmon (i.e, more occurrences by the end of the day). Such results suggest that the spread of European catfish will potentially impact migration of anadromous species through anthropized systems.

Food-web stability signals critical transitions in temperate shallow lakes

A principal aim of ecologists is to identify critical levels of environmental change beyond which ecosystems undergo radical shifts in their functioning. Both food-web theory and alternative stable states theory provide fundamental clues to mechanisms conferring stability to natural systems. Yet, it is unclear how the concept of food-web stability is associated with the resilience of ecosystems susceptible to regime change. Here, we use a combination of food web and ecosystem modelling to show that impending catastrophic shifts in shallow lakes are preceded by a destabilizing reorganization of interaction strengths in the aquatic food web. Analysis of the intricate web of trophic interactions reveals that only few key interactions, involving zooplankton, diatoms and detritus, dictate the deterioration of food-web stability. Our study exposes a tight link between food-web dynamics and the dynamics of the whole ecosystem, implying that trophic organization may serve as an empirical indicator of ecosystem resilience.

How mechanisms underlying food-web stability may influence ecosystem regime shifts is not well understood. Combining food-web and ecosystem modelling, Kuiper et al. show that destabilizing reorganization of a small number of key trophic interactions precede catastrophic changes in shallow lake ecosystems.

Ecosystem-level effects of a globally spreading invertebrate invader are not moderated by a functionally similar native

Biological invasions are a key element of human-induced global environmental change. However, lack of knowledge of the indirect consequences of invasions, combined with poor understanding of how their ecological effects depend upon competitive attributes of the receiving community, hinders our ability to manage and predict the effects of invasive species on ecosystems. We established an experiment using a combination of both additive and substitutive experimental designs to explore the effects of the globally spreading mysid shrimp Hemimysis anomala on the biological structure of outdoor pond mesocosms in the absence and presence of a functionally similar native competitor, Mysis salemaai. The naturally smaller H. anomala had considerably stronger effects on primary producers, multiple aspects of consumer assemblages and overall biological structure of the ponds in comparison with the functionally similar native. Moreover, the magnitude of these effects was generally independent of the presence of M. salemaai and even total mysid density. Hemimysis anomala reduced both the abundance and diversity of zooplankton assemblages significantly, triggering a strong trophic cascade on phytoplankton and a simultaneous increase of benthic invertebrate biomass. These findings indicate that invasion by H. anomala may exacerbate the effects of nutrient enrichment on lakes. Our results demonstrate that introduced species can, irrespective of the presence of functionally similar natives, induce complex changes to ecosystems that reach beyond direct consumptive effects. Moreover, the cascading indirect effects of invasion can exacerbate the impacts of other stressors. Disregarding the complexity of indirect effects therefore risks underestimating significantly the global ecological footprint of biological invasions.

Levels and patterns of persistent organic pollutants (POPs) in tilapia (Oreochromis sp.) from four different lakes in Tanzania: geographical differences and implications for human health

In Tanzania fish is one of the most important protein sources for the rapidly increasing population. Wild fish is threatened by overfishing and pollution from agriculture, industries, mining, household effluents and vector control. To monitor possible implications for public health, the geographical differences of the occurrence and levels of persistent organic pollutants (POPs) in tilapia fish (Oreochromis sp.) from four different Tanzanian lakes were investigated in 2011. Concentrations of organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), polybrominated diphenyls (PBDEs) and hexabromocyclododecane (HBCDD) were determined in pooled samples of tilapia muscle from Lake (L) Victoria, L. Tanganyika, L. Nyasa (also called L. Malawi) and L. Babati in Tanzania in 2011. Levels of Σ-DDTs (274 ng/g lipid weight (lw)) and sum of 7 indicator PCBs (Σ-7PCBs) (17 ng/g lw) were significantly higher in tilapia from L. Tanganyika compared to the other lakes. The highest levels of Σ-endosulfan (94 ng/g lw) were detected in tilapia from L. Victoria. Toxaphenes were detected in low levels in fish from L. Tanganyika and L. Babati. Results revealed a geographic difference in the use of DDT and endosulfan between L. Victoria and L. Tanganyika. Low ratios of DDE/DDT in tilapia from L. Tanganyika indicated an on-going use of DDT in the area. Median levels of ΣBDEs, including BDE-209, were highest in L. Victoria (19.4 ng/g lw) and BDE-209 was present in 68% of the samples from this lake. The presence of BDE-209 indicates increasing influence of imported products from heavy industrialized countries. The measured POP levels in the studied tilapia were all below MRLs of EU or were lower than recommended levels, and thus the fish is considered as safe for human consumption. They may, however, pose a risk to the fish species and threaten biodiversity.

The resilience and resistance of an ecosystem to a collapse of diversity

Diversity is expected to increase the resilience of ecosystems. Nevertheless, highly diverse ecosystems have collapsed, as did Lake Victoria's ecosystem of cichlids or Caribbean coral reefs. We try to gain insight to this paradox, by analyzing a simple model of a diverse community where each competing species inflicts a small mortality pressure on an introduced predator. High diversity strengthens this feedback and prevents invasion of the introduced predator. After a gradual loss of native species, the introduced predator can escape control and the system collapses into a contrasting, invaded, low-diversity state. Importantly, we find that a diverse system that has high complementarity gains in resilience, whereas a diverse system with high functional redundancy gains in resistance. Loss of resilience can display early-warning signals of a collapse, but loss of resistance not. Our results emphasize the need for multiple approaches to studying the functioning of ecosystems, as managing an ecosystem requires understanding not only the threats it is vulnerable to but also pressures it appears resistant to.