Case study 1: terrestrial

Hypothetical Case Study: Housing Development Project

At the end of this case study, you should be able to:

Apply the SGBA to a development project impacting terrestrial and freshwater ecosystems primarily and secondarily, respectively.
Use the SGBA to calculate Habitat Biodiversity Units (HBUs) for baseline and post-development habitats as functions of habitat area, distinctiveness, condition, creation and enhancement multipliers.
Identify habitat trading rules for enhancement across habitats and habitat biodiversity compensation between terrestrial and freshwater ecosystems.
Describe how overall biodiversity net gain may be achieved from biodiversity net loss and gain from terrestrial and freshwater ecosystems, respectively.
Suggest mitigation strategies to enhance, maintain biodiversity or minimise its loss.

Project Description & Objectives

The Housing & Development Board (HDB) plans a new residential development project on a 10-hectare site in Singapore. This site is located at the confluence of two major roads, adjacent to existing managed green spaces. The proposed development includes the construction of residential blocks of flats, an internal road network, and several recreational areas. Our objective is to utilize the SGBA Metric to evaluate the biodiversity impacts of this project, establishing a baseline and assessing the potential post-development biodiversity units, and to provide recommendations that align with sustainable development goals.

Current Land Use

The development site is currently characterized by a mix of managed vegetation, some naturally regenerated areas, and existing drainage infrastructure. Specifically, the land use at the site is primarily:

Exotic-dominated secondary forest: Representing areas with mature trees dominated by non-native species, often found in disturbed habitats.
Turf: Areas of managed grass.
Concrete drain/ canal: Existing man-made drainage channels.

Biodiversity Accounting Assessment

The assessment follows the Singapore Biodiversity Accounting Metric (SGBA), which quantifies the biodiversity value of habitat parcels based on their area, distinctiveness, and condition. For created or enhanced habitats, a "difficulty of Creation/ Enhancement" risk multiplier is also applied to account for the inherent challenges in establishing new habitats.

Selected Habitat Types

For this case study, we have selected the following habitat types, all of which are permissible under SGBA trading rules and are not designated as "Irreplaceable" or "Avoid at all costs":

Exotic-dominated secondary forest: A common habitat type in Singapore, characterized by exotic plant species.
Turf: A very common managed vegetation type with low biodiversity value.
Concrete drain/ canal: Man-made channels typically supporting limited aquatic life, given its low water quality in some areas.

For post-development, we consider the creation of:

Urban park/ garden: Managed green spaces within urban areas.
Native-dominated secondary forest: A higher-value forest type, if restoration is possible.
Naturalised stream: An enhanced concrete drain transformed into a more natural, vegetated watercourse.

Tabulation of Parameters for Terrestrial and Freshwater Ecosystems

Detailed SGBA calculations for what follows may be found herein: Case study 1 XL vs GS- SG Biodiversity Accounting Metric 1.1

The tables below present the baseline biodiversity units and the projected post-development units, including creation and enhancement. The "Total Units" for baseline are calculated as Area x Distinctiveness Score x Condition Score.

Table 1: Baseline Habitat Units (Pre-development) of Terrestrial and Freshwater Ecosystems.

For created/enhanced units, the "Difficulty of Creation Multiplier" is also applied: Area x Distinctiveness Score x Target Condition Score x Difficulty of Creation Multiplier.

Table 2: Post-Development Habitat Units (Creation and Enhancement) of Terrestrial and Freshwater Ecosystems.

a: Despite being considered a terrestrial habitat creation, this represents the inevitable loss of previous habitats for development. SGBA accounts for this loss.

b: Replacing parts of exotic-dominated forest and turf (see accompanying SGBA file). While a lower distinctiveness than the lost forest, it contributes to overall terrestrial units and is an acceptable outcome for urban greening. Time to Target Condition: 10 years.

c: This represents an enhancement from the original exotic-dominated secondary forest (Moderate distinctiveness to High distinctiveness), adhering to the SGBA rule of "same broad habitat or higher distinctiveness habitat required" for the 'Secondary rainforest' broad type. The multiplier accounts for high difficulty in creating this habitat type. Time to Target Condition: 25 years.

d: This represents an enhancement from a concrete drain (Very Low distinctiveness to Low distinctiveness), adhering to the SGBA rule of "same broad habitat or higher distinctiveness habitat required" for the 'Rivers and streams' broad type. The multiplier indicates low difficulty in naturalizing a drain. Time to Target Condition: 1 year.

Note: While Time to Target Condition (i.e., b - d) does not count towards HBU calculation, it provides a guardrail for developmental planning and monitoring.

SGBA Application: Net Biodiversity Change Summary

By comparing the total HBUs before and after development, we can determine the net change in biodiversity for the development project.

Overall Biodiversity Unit Change:

Net Change in Terrestrial Units: Post-development Terrestrial Units (4 + 4.98) - Baseline Terrestrial Units (72.0 + 1.75) = 9.98 - 73.75 = -64.27 units (Net Loss)
Net Change in Freshwater Units: Post-development Freshwater Units (1.0) - Baseline Freshwater Units (0.5) = + 0.5 units (Net Gain)

Combined Net Change in HBUs = Post-Development HBUs - Baseline HBUs

Combined Net Change in HBUs = (9.98) - (74.25) = -64.27 HBUs

Combined Net Percentage Change = (Net Change in HBUs / Total Baseline HBUs) × 100% Combined Net Percentage Change = (-64.27/ 74.25) × 100% = -86.56%

Consultant's Insights and Recommendations

This case study for the development project highlights several important aspects of using the SGBA Metric:

Quantifying Impacts: The SGBA provides a clear, numerical value of the biodiversity units associated with a site before and after development. In this hypothetical scenario, while there is a significant net loss in terrestrial biodiversity units (-64.27 units), there is a net gain in freshwater biodiversity units (+ 0.5 unit) due to the enhancement of the concrete drain into a naturalized stream. The SGBA is designed to capture these changes across different ecosystems.
Informing Design Decisions: The metric encourages developers to consider biodiversity at the earliest stages of design. The calculated net loss in terrestrial units indicates a need for more ambitious mitigation strategies. While the development includes urban park/ garden and some native-dominated secondary forest, these are not sufficient to offset the loss of the larger area of exotic-dominated secondary forest.
Adherence to Trading Rules: The proposed enhancements adhere to SGBA trading rules, for example, improving a concrete drain (Very Low distinctiveness) to a naturalized stream (Low distinctiveness) is a trade-up. Similarly, enhancing a portion of the existing forest to native-dominated secondary forest (High distinctiveness) is a positive step. However, the overall scale of these positive changes is currently not enough to achieve net gain for terrestrial habitats.
Guiding Mitigation Strategies: The results indicate where efforts should be concentrated. To improve the terrestrial net change, further strategies could include:
- Increased Green Infrastructure: Integrating more skyrise greenery (e.g., living roofs and walls) throughout the residential blocks, which can provide habitat and urban biodiversity benefits.
- Habitat Enhancement: Exploring localized widening of green corridors or enhancing other retained areas within the study area to create larger patches of native-dominated secondary forest or even freshwater swamp forest where feasible, to increase distinctiveness and condition.
- Prioritizing Lower Value Habitats for Development: If alternative site layouts are possible, prioritizing development on areas with Extremely Low or No Value habitats (like turf or developed land) would help minimize losses from higher distinctiveness habitats.

The SGBA is an invaluable tool for transparently assessing biodiversity impacts and guiding decisions towards minimizing loss and achieving net gains in Singapore's unique urbanized landscape. This hypothetical case study demonstrates its utility in identifying key impacts and informing subsequent ecological recommendations.

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