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River deltas represent globally important population centres and areas for food production, embedded on a nexus of land-water production. Yet pressures at the sea-land interface in these coastal areas are increasing from population growth, economic expansion and climate change. Over time, delta human populations have shifted from sea to land-based food production, the former more associated with local communities’ subsistence and the latter with exports of food products elsewhere leading to potentially irreversible land use-land cover changes (LULCC). While livelihood diversification by switching between or including both sea and land food production systems, could promote resilience and increase food security, food production based on sea and land-based resources has seldom been addressed in tandem, and potential unintended spill-over effects and LULCC may emerge which may jeopardise both. Here we examine the extent to which deltas have food production systems based on sea and land-based food sources (biomass, calories and protein) and which factors could drive these relationships, as transitions between sea and land food production in deltas may have local and global food security implications.We use global datasets across 235 large deltas, which include information on food biomass, calories and proteins from sea (fishing and aquaculture) and land (crops and livestock) production systems. Based on these indicators of food production, we calculated a sea:land ratio to examine to what extent food production of each delta is weighted towards the sea or land. We find clear geographical patterns in the sea:land food production ratio in global deltas: the majority of the deltas exhibit a dominance of sea-based food production (particularly those at higher latitudes) while the deltas in most of Europe, Central and Southern America, show a dominance of land-based food production. We found similar geographical patterns for biomass, calories and protein, with the relationship being stronger for protein than biomass or calories. We then examined how the sea:land ratio changes along gradients of human population density, and condition of the local ecosystems. Surprisingly, we found no relationship between population density and the sea:land ratio, indicating other factors may be at play (e.g., local context). Indeed, we found that as food production is mostly due to land contributions (i.e., the lower the sea:land ratio) the stronger the negative relationships with ecosystem conditions such as biodiversity intactness, soil and water quality. These results suggest that transitions between sea and land food production and subsequent LULCC can therefore be both complex and problematic in deltas globally, as these may represent switches in not just biomass but also nutritional quality, and may have severe implications for local ecosystem health and functioning. Increasing our understanding of what drives these transitions and associated LULCC at the nexus between land-water, and their effects, will allow more sustainable management of coupled food production systems and associated earth system processes.

River and sea ecosystem services contribute to the rapid and intensive development of delta social-ecological systems. This development, alongside other global change drivers, threatens the biodiversity of these deltas and in turn the ecosystem services that this biodiversity supports. However, biodiversity can itself mediate anthropogenic impacts by increasing ecosystem resilience. Linkages between biodiversity and ecosystem services are increasingly established, but we lack understanding of whether the mediating effects of biodiversity are global and ubiquitous, and whether they mediate global change drivers in deltas.Here, we examine the potential for biodiversity to mediate the relationships between five anthropogenic indicators and global change drivers (population, infrastructure, land use change, climate change in temperature and precipitation) and 19 ecosystem properties and services. We assess these relationships across a global dataset of 235 large deltas. We find that in 89% of cases, greater biodiversity (species richness and the intactness of biodiversity) is connected to a weakened or reversed association between anthropogenic drivers and ecosystem services. Such weaker or reversed associations were found across different ecosystem services (e.g. food production, carbon sequestration, soil regulation), most commonly with climate change and population.We then investigated the contribution of biodiversity and abiotic and anthropogenic drivers to delta ecosystem service supply. Ecosystem service supply was most strongly and consistently associated with abiotic drivers (mostly climatic), but biodiversity and anthropogenic drivers were also important to individual services (productivity and crop-related services respectively). Deltas showed fewer than expected associations between biotic, abiotic and anthropogenic indicators and ecosystem services, yet weakened or reversed associations were more frequent than in other social-ecological systems. Our results empirically show how biodiversity can both act as a resource and mediate social-ecological relationships, but that both of these roles could be compromised as deltas become more modified.

As planetary boundaries loom, there is an urgent need to develop sustainable equilibriums between societies and the resources they consume, thereby avoiding regime shifts to undesired states. Transient system trajectories to a stable state may differ substantially, posing significant challenges to distinguishing sustainable from unsustainable trajectories. We use stylized models to show how feedbacks between anthropogenic harvest regimes and resource availability drive transient dynamics. We show how substantial time lags may occur between interventions and social-ecological outcomes, and that sudden system collapses need not be linked to recent environmental changes. Historical reconstructions of island state populations show a variety of transient dynamics that closely corresponds to model expectations based on island differences in productivity and harvesting regime. We conclude that vulnerable social-ecological systems may persist when the population:resource ratio remains within a viable range of intermediate (rather than small) values, which implies that averting environmental crises may require counter-intuitive measures. Transient states influence the sustainable or unsustainable trajectories of social-ecological systems and lead to time lags between rapid regime shifts and the change or intervention which caused them, according to theoretical models of island states.

Delta systems are growingly at risk from ongoing global changes, sediment dynamics, and face many pressures from both upstream land use change and downstream sea level rise. Historically, deltas have been key locations for human occupation, and currently hold over 340 million inhabitants globally. This continuing pressure and risks have led deltas to become locked-in, likely losing their ability to provide ecosystem services and resilience to global change. To assess whether global deltas ranging from living to locked-in have lost their resilience to changes, we (i) used historical HYDE data to reconstruct the development of population and land use in 48 major deltas over the last 310 years, (ii) determine whether deltas are in locked in states, and (iii) assess whether locked-in deltas are more at risk due to relative sea level rise (RSLR), hazards, anthropogenic condition, investment deficit, and provision of nature contributions to people. 46% of the analyzed deltas are living deltas (22 out of 48), i.e. yet to be locked-in. Of the locked-in deltas (26 out of 48), 21% (n=10) emerge due to engineered natural infrastructure as the development of cropland and irrigation and 33% (n=16) are locked-in due to institutional infrastructure, such as no development of population or agricultural land uses. We find that average risk index is higher for locked-in deltas due to natural infrastructure changes (higher hazard and anthropogenic risks for cropland development and higher investment deficit risk for irrigation development). Surprisingly, the most at risk deltas to future RSLR are the ones locked-in due to institutional changes and living deltas. Many locked-in deltas may be even more at risk when considering their current ability to supply regulating and material ecosystem services/nature contributions to people. Our results suggest that locked-in deltas might be more at risk from current pressures, due to a reduction in the functioning of the natural processes that govern deltas, while deltas locked-in due to social and institutional infrastructure will be more at risk in the future. Further work is necessary to understand whether these trends can be reversed.

Human modification of natural systems has typically enhanced provisioning ecosystem services (ES), such as agriculture, at the expense of biodiversity and other types of services. Long-term sustainability requires a balance of ES flows to both maintain human wellbeing, while preserving biodiversity. In river deltas this balance is critical – fertile flat land, well stocked fisheries, and water for use and navigation have promoted rapid population growth and development. Yet the same development degrades the key ES protecting these deltas from hazards and pollution. Many deltas face a critical juncture, at risk or already ‘locked-in’ to the need for engineered solutions to global change problems, unsustainable globally and in the long-term.We created a global dataset of 237 deltas and collected indicators of the extent each was modified from its natural state, and its ES supply. Several types of modification to the most important aspects of delta systems were considered – overall human impact (human footprint), pressure and demand on ES (population density), modification of water systems (flow disruption) and modification of natural productivity (human appropriation of net primary productivity); grouping deltas by modification state. The impacts of this human modification on over 50 robust biodiversity and ES indicators were then analysed.Firstly, we attempted to create bundles of commonly associated ES in delta areas. Hierarchical clustering highlighted several logical clusters related to crops, fisheries, water, species richness, biodiversity intactness and NPP. Secondly, we examined synergies and trade-offs between different ES. Provisioning services all showed clear correlations with one another, but clear trade-offs with supporting services or biodiversity. There were weaker synergies within and between regulating and supporting ES. Finally, the relationship between each type of modification and the ES was classified using six typologies fitted by applying a decision tree to their LOESS regression curves. Crop indicators typically had an inverted-U relationship, increasing in moderately modified deltas, but decreasing in the most modified, presumably pushed out by other land uses. While many other ES declined with modification, interestingly, species richness and intactness both began to increase again in the most modified deltas. In summary, this global analysis is the first to illustrate how ES vary along a gradient of development in deltas, and highlights the need to balance further modification against these critical services.

The spread of invasive alien species (IAS) is recognized as the most severe threat to biodiversity outside of climate change and anthropogenic habitat destruction. IAS negatively impact ecosystems, local economies, and residents. They are especially problematic because once established, they give rise to positive feedbacks, increasing the likelihood of further invasions and spread. The integration of remote sensing (RS) to the study of invasion, in addition to contributing to our understanding of invasion processes and impacts to biodiversity, has enabled managers to monitor invasions and predict the spread of IAS, thus supporting biodiversity conservation and management action. This chapter focuses on RS capabilities to detect and monitor invasive plant species across terrestrial, riparian, aquatic, and human-modified ecosystems. All of these environments have unique species assemblages and their own optimal methodology for effective detection and mapping, which we discuss in detail.