Ecosystems are deceptively simple to set up. You don't need to audit energy, nitrogen, carbon, etc.
If there are cats in your forest, you need enough rodentia and avia for them to eat. To support the avian and rodent population you need insects, seeds, and fruit. To support the insects and the plants you need offal, death, and decay.
If there is an explosion in the rodents' food supply, as there is in India every 47-ish years, the rodent population will explode (into a plague in India's case). If this is the food source for your cats and birds then those populations will also increase rapidly -- followed by a die-off when they have eaten all the rodents who can no longer reproduce quickly enough to feed the cats because all their food is also gone.
So, if you have a particular apex predator that you want in your world then you can start top-down -- what does that predator eat, what does its prey eat, and so-on.
Is it a pack animal? Then it probably includes very large animals in its diet; prey which is too large for a single member of the pack to kill on its own -- that's why the species lives in a pack. Solitary predators tend to be very powerful but can only take down prey which are twice or thrice their mass.
If you have a useful omnivorous or herbivorous middle-cycle animal you want in your world then not only do you need a food source to support it, but you have to have a predator above it to maintain a balanced population, otherwise you will be overrun by this animal.
Rabbits, for example, can literally have a maximum of 12 litters of kits per year! One female can easily produce 5 dozen offspring every year. That feeds a lot of foxes, assuming that you have enough roughage to feed the rabbits.
Desert ecologies can be very interesting and challenging. Most deserts aren't large enough to be devoid of life, but the deeper you go into a desert the more sterile it gets. You won't find a wide variety of large animals in the desert, and when you do they will almost always be herbivorous specialists. If they're carnivores then they're almost always nocturnal.
I get pretty involved in some of my simulations: how many calories in plant nectar or in a particular nut, how many calories in this type of bug, how many calories does a particular predator animal need to thrive versus how many to just survive, what type of insect is needed to pollinate a particular type of plant, etc. I do that just because it's interesting and fun to watch, but you don't have to do that to create a believable ecology.
One of the biggest problems I had with many Dungeon Masters was the number of dragons they have in their worlds -- they were all over the damn place! Everyone had a flippin' dragon! One country had an entire dragon air force!
Impressive, but what do they eat?!? A dragon is a very high-energy animal -- it's difficult to keep a dragon without the surrounding countryside showing the effects of it! One of my players once asked me why dragons always live in some "waste" somewhere. I laughed and said, "It wasn't a waste before the dragon got there!" Which later led into a humorous interaction when the dragon finally introduced himself, but that's a story for Quora.
The point is that it doesn't take much to create a believable, workable, and even flexible ecology. And it's worth the effort: a believable ecology makes for a much more enjoyable world. Just remember that at its most basic level your ecology will probably be powered by death and decay. No death, no decay, no nutrients, no plants, no insects, no anything else.