Lecture 13: Human-environment interactions - examples from Madagascar

Question 1

Human - environment interactions

Answer 1

• Environmental Determinism: Environment influences people (e.g., early agriculture in the Fertile Crescent, survival strategies in cold/icy or mountainous environments).
• Human Impact: People change the environment (e.g., heathlands by grazing, urban waste, deforestation).
• Interaction Model: Feedback loop between social system and natural ecosystem, mediated by ecosystem services.
• Are landscapes degraded or naturally unforested? (e.g., afforestation risks for savannas)

Question 2

Context Madagascar

Answer 2

• Madagascar is a micro-continent with diverse landscapes and climates.
• Originally thought to be heavily deforested, but evidence shows many areas were naturally open (e.g., woodland-grassland mosaics).
• Settlement history: Austronesian and Bantu migrations between 3000 and 1000 years ago.
• High erosion visible in satellite imagery, but lack of reliable data on natural vs. human-induced erosion.

Question 3

Lavaka Characteristics

Answer 3

• Deep, steep-sided gullies mainly in the central highlands.
• Pre-conditions: thick regolith, convex slopes, monsoon climate, seismicity, poor drainage.
• Found to exist before human arrival, but unclear to what extent humans influence their formation today.

Question 4

What are natural erosion rates and their drivers?

Answer 4

• Cosmogenic 10Be used to estimate erosion over 1000s-1 million years.
• Higher erosion = lower 10Be concentration.
• Higher in eastern escarpment & plains; lower in central highlands & grabens.
• Influencing factors: topography, river slope, lithology, seismicity, bioclimate, lavaka density.
• Pre-human erosion rates are low.
• Seismicity, lavaka, and river shape explain spatial variability.

Question 5

How are convex hillslopes shaped long-term?

Answer 5

• Hypothesis: protective laterite cap erodes at convexities, creating weaknesses.
• Laterite thickness decreases from hilltop to valley.
• Erosion increases downslope (from hilltop to valley).
• Lower chemical weathering at grassland convexities.
• Forest-to-grassland transition (1000–2000 years ago) caused up to 100x increase in erosion.

Question 6

Why do lavaka start at convexities? Role of runoff vs. groundwater?

Answer 6

• Hypothesis of Q2 was wrong!
• Less dense vegetation at the convexity might create a weakness
• Groundwater table too deep to initiate lavaka
• Groundwater is not starting the lavaka processes, likely initiated by concentrated surface runoff

Question 7

Is lavaka erosion increasing? Anthropogenic link?

Answer 7

• More lavaka appearing than stabilizing.
• Lavaka birth rate increasing since ~870 BP; mean lavaka age ~400 years.
• Matches floodplain data: erosion increase since 1000 BP, sharp rise since 400 BP.
• Population growth, cattle introduction, land use change are key drivers.

Question 8

How much sediment is mobilized by lavaka?

Answer 8

• Used UAV-SfM (high-res) and satellite DEMs (TanDEM-X 12m, Copernicus 30m).
• Calculated gully volumes and area-volume relationship.
• Copernicus underestimates volume.
• TanDEM-X accurate after volume breakpoint correction.
• Lavaka sediment rates: 10–130 t/ha/year = 500–9000 mm/kyr.

Question 9

Lavakas in Madagascar - Synthesis

Answer 9

• Erosion before humans: 6–19 mm/kyr.
• Now: 500–9000 mm/kyr in lavaka regions.
• Convexities are hotspots due to less vegetation.
• Anthropogenic pressure (land use, cattle, population) = clear impact.

Question 10

Natural capital accounting for eco-villages in Eastern Madagascar

Answer 10

1. Natural Capital Accounting: Framework
   * NCA complements GDP by measuring the condition and service flow of natural assets.
   * Aims to account for ecosystem degradation and benefits in both physical and monetary terms.
   * SEEA (System of Environmental Economic Accounting) is the UN’s standard framework.
   * SEEA-EA (Ecosystem Accounting) uses spatial units for mapping extent, condition, and service flows of ecosystems (provisioning, regulating, cultural).
2. PECOV Project
   * Conducted in Eastern Madagascar, supporting sustainable eco-village development.
   * Field data, satellite mapping, & stakeholder input.
   * SEEA-EA in tropical contexts with high biodiversity and pressure on natural resources.
3. Biophysical Accounts
   * Map ecosystem extent
   * Assess ecosystem condition
   * Measure ecosystem service flow in physical terms: Carbon storage and sequestration, Flood regulation and erosion control, Crop and fuelwood provisioning
4. Monetary Accounts
   * Assign economic value to services
   * Dependence of local livelihoods on ecosystem health.
   * Highlight economic loss due to degradation.
   * Support cost-benefit analysis of conservation policies.
5. Scenario Analysis
   * Simulate impacts of different land use and policy choices on ecosystem services.
   * Compare current trends vs. sustainable management vs. degradation scenarios.
   * trade-offs between development, conservation, and resilience.
   * Help inform eco-village planning and policy formulation.
6. Conclusions
   * Ecosystems underpin livelihoods & development.
   * Integrating ecosystem values in planning helps balance growth and conservation.
   * Provides evidence for sustainable resource use and poverty reduction.