Energy flows reveal declining ecosystem functions by animals across Africa – Nature

Report on Ecosystem Energetics: Quantifying Functional Decline and its Implications for the Sustainable Development Goals

1.0 Introduction: A Functional Approach to Biodiversity for Sustainable Development

Understanding the consequences of biodiversity loss is a central challenge for achieving the Sustainable Development Goals (SDGs), particularly SDG 15 (Life on Land). Traditional metrics that track species numbers are often insufficient for assessing changes in critical ecosystem functions that underpin human well-being and planetary health. These functions—such as pollination, seed dispersal, and nutrient cycling—are vital for food security (SDG 2: Zero Hunger), climate regulation (SDG 13: Climate Action), and the overall resilience of terrestrial ecosystems.

This report details an ecosystem energetics approach, a physically meaningful method for translating changes in animal populations into a suite of ecosystem functions. By quantifying the flow of energy through animal communities, this methodology provides a robust tool for tracking progress towards global sustainability targets and informing effective conservation and restoration strategies.

2.0 Methodology: Measuring Energy Flows Across Sub-Saharan Africa

The analysis quantifies historical changes in energy flows through mammal- and bird-mediated ecosystem functions across sub-Saharan Africa, a region critical for global biodiversity. The approach compares contemporary conditions with a pre-industrial baseline to assess the impact of human activity.

2.1 Analytical Framework

1. Baseline Estimation: Historical species abundances and distributions were established using population density models and habitat-adjusted range maps to represent a pre-industrial state.
2. Energy Calculation: The annual food energy consumed by each species was calculated using allometric equations based on body mass, diet, and metabolic rates. This provides a common currency (energy) to compare the functional contribution of diverse species.
3. Contemporary Assessment: Current energy flows were quantified by adjusting historical abundances based on the Biodiversity Intactness Index (BII), which estimates population changes in response to different land uses (e.g., croplands, settlements, protected areas).
4. Functional Grouping: Species were classified into functional groups (e.g., pollinators, seed dispersers, grazers, carnivores) to assess how specific ecosystem functions have been altered, linking biodiversity status directly to services relevant to the SDGs.

3.0 Key Findings: A Continental Decline in Ecosystem Function

The analysis reveals a significant degradation of animal-mediated ecosystem functions across sub-Saharan Africa, with profound implications for achieving SDG 15 and related goals.

3.1 Overall Decline in Trophic Energy Flow

• Total energy flow through wild bird and mammal populations has decreased by more than one-third, now standing at only 64% of historical levels.
• This decline represents a major loss in the capacity of ecosystems to perform essential functions, directly undermining targets within SDG 15 to halt biodiversity loss and restore degraded ecosystems.
• The most severe functional losses occur in intensively used landscapes:
  • Settlements: Energy flow is reduced to 27% of historical levels, impacting SDG 11 (Sustainable Cities and Communities) by diminishing local ecosystem services.
  • Croplands: Energy flow is reduced to 41% of historical levels, threatening functions like pollination and pest control that are critical for SDG 2 (Zero Hunger).

3.2 Disproportionate Collapse of Megafauna Functions

• Functions performed by megafauna (large herbivores and apex carnivores) have collapsed outside of protected areas.
• Energy flows through nutrient dispersers (large herbivores) and grazers are only 26% and 32% intact, respectively.
• This loss compromises large-scale ecosystem processes such as vegetation structuring and nutrient cycling, which are crucial for landscape resilience (SDG 15) and carbon storage (SDG 13).

3.3 Biome-Specific Functional Degradation

• Forests: The decline is primarily driven by the loss of arboreal birds and primates, impacting seed dispersal and forest regeneration. This degradation hinders efforts to combat deforestation under SDG 15.
• Grassy Systems: The loss of terrestrial herbivores is the dominant factor, altering vegetation structure and fire regimes.
• Arid Systems: The decline of burrowing mammals is a significant contributor, affecting soil health and water infiltration, which are important for combating desertification (Target 15.3).

4.0 Implications for the Sustainable Development Agenda

The ecosystem energetics approach provides a powerful tool for integrating biodiversity into the broader sustainable development framework, offering actionable insights for policy and practice.

4.1 Enhancing Monitoring for SDG 15 (Life on Land)

Traditional biodiversity metrics like the BII are poor predictors of specific functional declines, particularly for megafauna. An energetics approach offers a more nuanced and accurate way to:

• Track progress towards targets for halting biodiversity loss (Target 15.5) and restoring degraded ecosystems (Target 15.1).
• Guide restoration efforts by identifying the most energetically depleted functional groups, ensuring that restoration activities rebuild trophic complexity and ecosystem resilience.
• Inform the Planetary Boundaries framework by providing a clear, mechanistic link between biodiversity integrity and Earth system stability.

4.2 Supporting Climate Action and Food Security

• SDG 13 (Climate Action): By quantifying the decline of megafauna that influence vegetation structure (e.g., elephants), the approach helps to model the impacts of defaunation on carbon sequestration and storage in terrestrial ecosystems.
• SDG 2 (Zero Hunger): The analysis highlights the severe degradation of pollination (25% intact in croplands) and seed dispersal (14% intact in croplands), functions essential for the productivity and resilience of agricultural systems.

4.3 A Tool for Global Partnerships (SDG 17)

As a standardized, scalable metric, ecosystem energetics can be integrated into global biodiversity assessments like those by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES). It provides a common currency for governments, corporations, and conservation organizations to set, monitor, and achieve nature-positive targets in line with the UN Decade on Ecosystem Restoration.

5.0 Conclusion and Recommendations

The decline in energy flow through Africa’s animal communities signifies a critical loss of ecosystem function that jeopardizes the achievement of the Sustainable Development Goals. An energetics framework moves beyond simply counting species to measuring what they do, providing a vital link between biodiversity conservation and global sustainability.

5.1 Recommendations

1. Integrate Energetics into Policy: National and international bodies should adopt energy flow metrics to supplement existing biodiversity indicators, providing a more complete picture of ecosystem health for reporting on SDG 15.
2. Prioritize Functional Restoration: Conservation and restoration projects should set quantitative, function-based targets aimed at rebuilding energetically depleted guilds, such as large herbivores and arboreal frugivores.
3. Expand the Approach: Further research should expand this analysis globally and incorporate other critical taxa, such as invertebrates, to create a comprehensive understanding of animal-mediated functions in the Earth system.

1. Which SDGs are addressed or connected to the issues highlighted in the article?

The article primarily addresses issues related to biodiversity loss, ecosystem functionality, and land use change, which directly connect to several Sustainable Development Goals (SDGs). The most relevant SDGs are:

• SDG 15: Life on Land

  This is the most central SDG to the article. The entire study focuses on understanding how biodiversity loss in terrestrial ecosystems (specifically in sub-Saharan Africa) impacts their structure and function. It explicitly discusses the decline of animal populations (mammals and birds), the degradation of different biomes (forests, grassy systems, arid systems), the importance of protected areas, and the need for ecosystem restoration. The article states, “A key challenge for ecological science is to understand how biodiversity loss is changing ecosystem structure and function,” which is the core of SDG 15.

• SDG 2: Zero Hunger

  The article connects to SDG 2 through its discussion of land use change for agriculture. It mentions that “agricultural conversion depletes populations of large or frugivorous animals” and that energy flows have fallen to “41% (30–53%) in croplands.” This highlights the trade-offs between food production and biodiversity conservation. Furthermore, it touches upon ecosystem disservices where certain resilient species harm agricultural production, such as “avian granivory, which reached 108% (84–135%) of historical levels in croplands, where birds can take advantage of seed-rich crops.” This relates to the need for sustainable agricultural practices that maintain ecosystem health.

• SDG 13: Climate Action

  The article links biodiversity and ecosystem function to climate-related processes. It notes that animal-mediated functions are crucial for services like “storing carbon.” The decline of megafauna and seed dispersers is shown to have potential impacts on vegetation structure and, consequently, carbon sequestration. The text mentions that “defaunation affects carbon storage in tropical forests” and that elephants have the potential to “affect continental-scale carbon sequestration.” The proposed energetics approach aims to integrate these animal-driven functions into Earth system models, which is relevant for understanding and mitigating climate change.

2. What specific targets under those SDGs can be identified based on the article’s content?

Based on the issues discussed, several specific SDG targets can be identified:

1. Targets under SDG 15 (Life on Land)

   • Target 15.1: By 2020, ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services.
     
     Explanation: The article directly addresses this by quantifying the degradation of ecosystem functions across Africa and proposing its energetics metric as a tool to “guide conservation and restoration” and help practitioners “conserve and restore functionally diverse, energetically intact ecosystems.” It aligns with the goals of the “United Nations Decade on Ecosystem Restoration (2021–2030).”
   • Target 15.3: By 2030, combat desertification, restore degraded land and soil, including land affected by desertification, drought and floods, and strive to achieve a land degradation-neutral world.
     
     Explanation: The study analyzes changes in various biomes, including “arid systems,” where functions performed by “burrowing mammals” have been significantly impacted. The goal of restoring energetically depleted guilds is a direct contribution to restoring degraded land.
   • Target 15.5: Take urgent and significant action to reduce the degradation of natural habitats, halt the loss of biodiversity and, by 2020, protect and prevent the extinction of threatened species.
     
     Explanation: The article’s core finding is the massive decline in animal-mediated functions due to biodiversity loss. It highlights the “collapse of large herbivores” and megafauna, noting that energy flows through these groups are “notably less intact (from 26–32%) than other ecosystem function groups.” This directly measures the consequences of habitat degradation and biodiversity loss that this target aims to halt.

2. Target under SDG 2 (Zero Hunger)

   • Target 2.4: By 2030, ensure sustainable food production systems and implement resilient agricultural practices that increase productivity and production, that help maintain ecosystems, that strengthen capacity for adaptation to climate change…
     
     Explanation: The article analyzes the impact of “croplands” on biodiversity and ecosystem function, showing a significant decline in energy flows in these areas. This underscores the challenge of making agriculture sustainable. The discussion of avian granivores in croplands also points to the need for agricultural systems that are resilient and in balance with local ecosystems.

3. Target under SDG 13 (Climate Action)

   • Target 13.3: Improve education, awareness-raising and human and institutional capacity on climate change mitigation, adaptation, impact reduction and early warning.
     
     Explanation: The article proposes a new scientific framework—ecosystem energetics—to “advance efforts to integrate animal-driven functions into biosphere and earth system models.” By providing a quantitative method to link biodiversity to large-scale processes like carbon cycling, it directly contributes to improving the institutional and scientific capacity to understand and model the impacts of ecosystem degradation on the climate system.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

Yes, the article explicitly uses, critiques, and proposes several indicators that can measure progress towards these targets.

• Biodiversity Intactness Index (BII)

  The BII is a central indicator used throughout the article to measure the impact of human activity on biodiversity. It is defined as a metric that “estimate[s] how human activity has changed the richness or abundance of species relative to remaining highly intact landscapes.” The study uses BII values to calculate the decline in species populations under different land uses. While the article critiques its inability to capture ecosystem function, the BII itself serves as a direct indicator for monitoring biodiversity loss, relevant to Target 15.5.

• Energetic Intactness / Trophic Energy Flows

  This is the novel indicator proposed and developed by the study. It is defined as “the percentage of historical energy flows through a group of species remaining in an ecosystem.” The article argues this is a more functionally relevant metric than BII. It provides specific measurements, such as the finding that “trophic energy flows have decreased by more than one-third” across sub-Saharan Africa. This indicator can be used to track the functional restoration of ecosystems (Target 15.1) and the specific decline in functions like nutrient cycling and seed dispersal.

• Species Population Densities and Abundance

  The study relies on datasets of “species population densities” as a foundational input for its models. Changes in the population density and abundance of key species (like large herbivores and apex predators) are an implicit indicator of ecosystem health and progress towards halting biodiversity loss (Target 15.5).

• Proportion of Land Under Different Land Uses

  The analysis is structured around different land use categories, including “strict protected areas,” “croplands,” “rangelands,” and “settlements.” The proportion of land in each category and the intensity of use are key drivers of the observed declines. The finding that energy flows fell to “27% (18–35%) of historical levels in settlements” and “41% (30–53%) in croplands” demonstrates how land use classification serves as an indicator of pressure on biodiversity, relevant to Targets 15.1, 15.3, and 2.4.

4. Create a table with three columns titled ‘SDGs, Targets and Indicators” to present the findings from analyzing the article. In this table, list the Sustainable Development Goals (SDGs), their corresponding targets, and the specific indicators identified in the article.

Source: nature.com