Ecosystem Builder
Build a sustainable food chain across a given terrain. Ecosystem is a legacy Solve scenario — if your invitation still includes it, here is how it works and how to practise for it.
What is the Ecosystem Builder?
The Ecosystem Builder is a simulation scenario in McKinsey’s Solve assessment. You act as an environmental researcher tasked with constructing a sustainable food chain across a specific terrain. Your goal is to select a set of species that can survive together — each predator must have enough prey to meet its calorie requirements, every predator-prey pair must produce a calorie surplus, and all species must be compatible with the terrain and climate conditions of the site.
The difficulty comes from cascading constraints: a predator consumes whichever prey provides the greatest number of calories first, so the order and composition of your food chain affects the entire system. A single miscalculation collapses the ecosystem. Candidates need to hold multiple rules in mind simultaneously, under time pressure, without making an arithmetic error.
The rules
When multiple prey species offer the same calorie value, the predator consumes them equally. The ecosystem is only valid when every pair in the chain has a calorie surplus and every species is terrain-compatible. Running these checks quickly and in the right order is a skill — and practising them until they are automatic is what makes the difference on test day.
Why candidates struggle
Practise the Ecosystem scenario.
Build your understanding from scratch.
Three full Ecosystem simulations built from real past PSG data, with detailed solutions for every question.
- 3 × full Ecosystem simulations
- Real past PSG data
- Detailed solutions for every question
- Instant access
Also included in the Ultimate PSG Bundle, alongside Redrock, Sea Wolf and SFL practice tests.
Ecosystem FAQs
What is the McKinsey Ecosystem scenario?
The Ecosystem Builder is a simulation scenario in the McKinsey Solve assessment. You act as an environmental researcher building a sustainable food chain across a terrain: each predator must meet its calorie needs, every predator-prey pair must produce a calorie surplus, and all species must be compatible with the site conditions. It tests systemic thinking and accuracy under pressure.
Is Ecosystem still part of McKinsey Solve in 2026?
Rarely. Ecosystem was largely replaced by Sea Wolf in 2024, and most 2026 candidates sit Redrock, Sea Wolf and the Sustainable Futures Lab. Some invitations still include it, so check yours before deciding what to practise.
Why do candidates find the Ecosystem scenario hard?
The difficulty comes from cascading constraints: one incorrect calorie calculation can invalidate the entire chain, terrain-compatibility mismatches are easy to overlook, and validating everything under time pressure takes longer than most candidates expect. Holding all the rules in mind at once is what makes it challenging.
How many Ecosystem practice tests does PSG Solver include?
PSG Solver includes three full Ecosystem practice tests built from real past PSG data, each replicating the scenario with detailed solutions for every question. They are available on their own or as part of the Ultimate PSG Bundle.