McKinsey Solve 2025: Ecosystem Deep Dive

This article is a deep-dive into the Ecosystem mini-game of the McKinsey Solve. All insights are updated for the 2025 version.

Please read the main article on the McKinsey Solve (2025 edition) if you haven’t done so.

Overview of McKinsey Solve: Ecosystem

1st game in McKinsey Solve

In the 2025 version of the McKinsey Solve test, Ecosystem is the 1st mini-game that the candidate has to take. It has remained in this position ever since the 2020 version of the test, and it is unlikely to be replaced in the near future.

Your job is to build an ecosystem

In the Ecosystem test, the objective of the candidate is two-fold. The first (and main) objective is to build an 8-species food chain from a collection of 39 species. The second objective is to place that food chain in a location that is suitable for every species.

The candidate is free to choose whichever objective first, but at MConsultingPrep, we recommend taking the first objective first because once this is done, the second objective can be completed in mere seconds.

Time limit is 35 minutes

The time limit allocated to the Ecosystem test is 35 minutes. The bulk of this time limit is spent on building the food chain.

As we shall see later, this limit is quite generous. Most candidates, with preparation, can complete the test in 15-20 minutes, even taking into account test-running food chains.

Key concepts in Ecosystem

Species (& their requirements)

The species are the most fundamental building blocks in the Ecosystem game.

There are 39 species per Ecosystem session: 30 animals and 9 producers. Animals are species that consume other species; producers, on the other hand, do not consume other species - they are often plants, fungi, corals, etc. depending on the scenario.

Each species comes with a set of requirements: calories provided, calories needed, species that they eat, species that eat them, as well as suitable conditions such as depth, elevation, temperature.

Species names are based on real-life species, such as Bluefin Tuna, Hammerhead Shark, etc., which are often suggestive of what they eat (for example, a Hammerhead Shark is almost always a predator), but that does not always apply.

All 39 given species are placed in the “Help Center” at the bottom-left corner of the screen, while selected species go to the “Species Select” panel.

Location (& its conditions)

In Ecosystem, the location of the food chain is displayed on the “map” in the center of the screen, and chosen through a pin (similar to the one you find in Google Maps). You can use the mouse to move the pin around until you find a suitable location.

Each location is defined by 7-8 conditions: depth/elevation, temperature, rainfall, windspeed, etc. With the exception of depth and elevation, every condition varies almost randomly between locations. These conditions correspond to the requirements of each species.

These are shown on a condition panel on the right side of the screen.

Scenarios 

There are three scenarios in the Ecosystem game: the Coral Reef, the Forest, and the Mountain. The game rules stay the same between the scenarios, which are only for cosmetic purposes.

• In Coral Reef, all species are marine species (i.e. fish, corals, etc.) and all conditions are changed into sea-related ones (depth, temperature, water pH, water current, etc.)

• In Mountain and Forest, all species are land-based, and all conditions are changed into land-based ones (elevation, temperature, soil pH, wind speed, etc.). The only difference between Mountain and Forest is the appearance of the “map”.

Eating Rules

Rules as stated by McKinsey

The species with the highest 'Calories Provided' eats first. It eats its 'Food Source' with the highest 'Calories Provided'. In case of a tie, it eats equally from both species.

When a 'Food Source' is eaten, its 'Calories Provided' decrease permanently by an amount equal to the eating species' 'Calories Needed'. If the eating species needs more calories, it eats another 'Food Source' based on current 'Calories Provided'.

Then the species with the next highest current 'Calories Provided' eats. Species who end with their 'Calories Needed' fully met and more than zero 'Calories Provided' survive.

These rules are displayed in-game on the Species Select panel.

A few key details

There are a few key details you should take note of here that is very important for understanding how food chains are resolved:

1. CALORIES PROVIDED decides which species gets to eat, not Calories Needed. 

It might be tempting to think that species who need more will eat first, but that’s not how it works here, and if you make this mistake the results will be “less than optimal”.

I interpret this rule as “which species who contribute more to the system will be rewarded first”, and ever since it feels much more intuitive to me.

2. It’s the CURRENT Calories Provided that matters, not the original. 

This has HUGE implications because it forces you to test-run a food chain to know the results - and test-running takes time. 

And then after test-running, you might also get nasty surprises, for example, one species being one-upped by another, getting their only food source stolen and ending up starving.

Consequently, you should build your food chain as linearly as possible - the less criss-crossing and overlapping, the better, because you can predict it more easily, and test-run it quicker too.

Explanation with example

So how does it all work out in practice?

We’ll run through a sample food chain just so we can see all the rules coming to life.

The Lion is the animal with the highest Calories Provided, so it starts eating.

It has two food sources: Tiger and Horse. Between these two, the Horse has the higher Calories Provided, so the Lion picks the Horse.

The Lion “takes” 3000 calories from the Horse, reducing its own Calories Needed down to 0 and Calories Provided of the Horse to 2000.

Then the Elephant has the next-highest current Calories Provided, so it eats. 

Now, like the Lion, the Elephant has both the Horse and Zebra as Food Sources. Originally the Horse provided more calories, but now, the Horse’s Calories Provided had gone down to 2,000, lower than the Zebra at 3,000, and so the Elephant picks the Zebra instead

The Elephant takes all of the Calories Provided from the Zebra, leaving it with 0.

However, the Elephant still needs 1000 calories more, so it turns to the Horse and eats 1000 calories. The Horse has 1000 Calories Provided left and the Elephant is fully satisfied.

After the Elephant, the Horse has the next-highest current Calories Provided, so it eats.

Now, both Fruits and Nuts - the Food Sources listed under the Horse - provide the same amount of calories so the Horse eats from both of them equally, taking 1000 calories from each.

The Horse now has 0 Calories Needed, while Fruits and Nuts each has 3,000 Calories Provided left.

The final species to eat is the Tiger.

It goes for its sole Food Source, which is Nuts.

It takes all the 3000 Calories Provided from Nuts, but because it now runs out of food sources, it has to stop eating.

Now that all the species have eaten, we can run the survival check.

5 out of 8 species survive. So this food chain needs to be fixed, and to do so we usually need to remove the “dead” species and replace them with more sustainable ones.

Recommended approach

Divide species into three groups

Straight-up picking 8 species from the 39 species is going to be very difficult. However, McKinsey gives us a clue as to how they want us to approach the problem: if we look at the condition requirements of all the species, we can see that they can be divided into equal, 13-species groups.

Within each group, every species will require the same conditions. Species of a group can only eat, and be eaten by, other species of the same group.

So, instead of randomly going around between all 39 species, we must first decide which of the 13-species groups we should work with first.

Choose the group with the best producers

How to choose the best group? The answer is simple: go for the group whose producers have the highest calories provided. 

When you start the game, simply go to the Help Center > Species > Producers tab, open all the producers and note down their Calories Provided on a sheet of paper, along with their Depth / Elevation to differentiate between each group. 

Then sum up all the calories of same-group producers, and choose the group with the highest sum. This does not guarantee the group will yield a food chain, but it does give you a better chance.

Note: if you see a producer with an empty “Eaten By” field, avoid that species group. Producers that are not eaten by any other species cannot contribute their calories, so the entire species group has a very low chance of yielding a working, 8/8 food chain. 

Limit testing time to 10 minutes

When you test-build and test-run your food chains, try to allocate only 10 minutes to each species group before moving on.

There is a not-insignificant chance that you have to test all three groups before finding a food chain, so if you spend more than 10 minutes on each group, you risk running out of time before the food chain is completed.

Tips to build food chain faster

Use all the producers & apex predators

When you add a species group, always add all three producers. These producers have no calories requirement, so they are effectively free points.

The same goes with apex predators. These are species that are not eaten by any other species, and they tend to consume only a small amount of calories, so they survive easily. They tend to carry the same name as real-life apex predators too (so if you see a Great White Shark it’s almost definitely an apex predator). There can be more than one apex predator per group.

Suppose you add all 3 producers, and 1 apex predator, that’s 4 out of 8 required species already selected. At that point selecting the remaining 4 species (all animals) will no longer be difficult.

Avoid clearly-red-flag animals

There are two ways an animal can be a “red-flag”: either it has very high Calories Needed, or it has very low Calories Provided. 

For the former case: each group there will be 1-2 animals with Calories Needed above 5000. Since the highest-Calories-Provided species tend to be the producers at around 4000-5000, an animal with a Calories Needed at 5000 or more will almost never survive.

For the latter case: in each group, there will be 1-2 animals with Calories Provided around 1500 or less. Since most other animals will require 2000-3000 Calories Needed, these species will likely be exhausted if you include them in your ecosystem.

Suppose you have a food chain with 4 species already selected (3 producers, 1 apex), and then you rule out 2 red-flag animals (1 for each type): at this point you will only need to select 4 more species out of a remaining pool of 7 species. 

It’s hard to get easier than this.

Build your food chain bottom-up

Theoretically speaking, you can build your food chain either top-down, or bottom-up. However, most people will find it easier doing so bottom-up (i.e. starting with producers).

We haven’t been able to pinpoint why this is the case, but for many years we’ve been showing our customers how to do things both ways, and when we survey our customers, most of those customers tell us that it’s 20-30% faster to do it bottom-up.

Use pen and paper

For the Ecosystem game, using good-old pen and paper is faster than any kind of “automated” Excel sheet. Especially if you know how to take notes efficiently.

Here’s how you do it:

• When you have chosen a species group, note down the producers at the bottom of your scratch paper, and the apex predators at the top, leaving enough space between them for two more rows of animals.

• For each species: note their Calories Provided above its name, and Calories Needed below the name. For the name: you can use a shortened version (e.g.: Great White Shark -> Gr White).

• Between the species: use arrows to display eating relationships; if A eats B, the arrow should point towards B, not the other way around.

Once this is done, you can start test-running the food chain. As calories change between eating turns, write the new numbers besides the old one.

Once all species have eaten, you can check which species survive and which ones do not.

Tips for choosing location

Ignore noise conditions

Generally speaking, if you see four “conditions” on each species card, two of them will be noise. In fact, I might dare say that only Depth / Elevation and Temperature matter.

A tell-tale sign of noise condition is that they are the same for every species - not just within each group, but within all 39 given species. The range of these conditions are also wide enough that no matter where you place the pin, you will get a suitable location.

Depth / elevation first, temperature second

Between depth/elevation and temperature, the former is derived from the location of the pin on the map, while the latter is randomly generated.

For example, in the Mountain scenario, if you move your pin around the top of the mountain, the elevation will always be high, but the temperature (and all the other conditions) vary considerably.

As a result, to choose location faster, you should look for an area on the map with the suitable depth/elevation first, then hover the mouse around until you get the suitable temperature.

A secret, under-10-minute strategy

Most prep sites would tell you that the fastest way to complete the Ecosystem test is to use an Excel-based or web-based solver, which is a tool where you can type in the names and calories of species and it will spew out a result.

But here’s the problem:

1. A solver would likely be considered cheating by McKinsey. While you can get away with it, you will likely be blacklisted if you are ever found out.

2. If you use a solver, it’s likely that your behavior will not look like a normal consultant doing the test, which will likely result in a rejection.

3. Most solvers are… pretty time-consuming, despite what they claim, simply due to the fact that you will need to type in all the species. About 15 minutes, in fact.

What we advocate at MConsultingPrep, is to stick to a technique we call “highest-to-highest”. This technique is so powerful that you can solve the Ecosystem test in 10 minutes, just using pen and paper. No calculator or excel sheet needed.

What’s more, this technique is totally legit, since it is derived directly from the Eating Rules.

If you want to learn more about this technique, you can find it as part of the McKinsey Solve Simulation package, developed by us at MConsultingPrep, used by thousands of candidates across five years, covering all the newest mini-games!