You are all thinking so *small*. NESLife is for the creation of fascinating alternate biomes, not uninspired copies of Earth niches. At least, that's what it is for me.
And if you think this is exciting, just wait and see where I'm going with it next.
Organism: Altiku
Ancestor: Venku
Selective Pressure: Hyper-reduction in ammonia budget from Horod die-off and increased predation from High Needlers and others
Mutation: Evolution of a streamlined, cloud-soaring body, with a very low body mass. Pheromonic pathways to chase ammonia clouds and coordinate night-time drops to earth to absorb mineral nutrients. [Loss of parasitic ability.]
The physiological jump from Venku to Altiku is so drastic that some have hypothesized some sort of mutagen was at play, particularly the ionizing radiation of the Scouring Era. But this is unlikely due to the short period of high radiation exposure; rather the main reason for the speciation of the Altiku was a massive die-off in Venku species predicated by the destruction of their Horod ammonia sources en masse, and inability to compete with the Tenkus with their lower energy budget in the face of widespread Needler predation.
Only those Venku that had the most efficient nitrogen fixation pathways to the least cost - namely the lighter ones, were selected for, as were those that were more mobile; their increased ability to chase down increasingly scarce and volatile sources of ammonia rain made their sensory suite sensitive to even minute changes in humidity. (Interestingly enough, the sensory-pheromone complex located near the top of the plant acquired additional complexity during this period. The ability to secrete a touch-sensitive 'alarm' pheromone causing the night-browsing plants to release their roots and soar high into the sky was one of the main anti-predatory enhancements of the new genus.)
Lightness and mobility - combined with many scavengers and primary consumers' voracious appetite for low-hanging floaters now that other plant life was gone - naturally prioritized expansion into the untapped biome of the clouds. As such, the Altiku operates on an extremely tight nitrogen sugar budget. The gas sac comprises 90% of the mass of the creature, with the outer layer performing photosynthesis, the central layer nitrogen fixation, and the inner layer buoyant gas production.
But the finest accomplishment of the Altiku was invisible - A hyper-efficient biochemical pathway for nitrogen fixation, limiting the need for Altikus to land at all, with the exception of the acquisition of a few essential minerals. This caused the Altikus to become much more ambitious cloud chasers, as they now had to intake large quantities of ammonia directly rather than taking the easier route of leeching it from other plants. Increasingly Altiku packs are relying on mountain peaks to accomplish their mineral acquisition while avoiding predation, landing, leeching, and then soaring back into the sky for weeks at a time.
[Tenkus, with a much greater access to mineral nutrients and no serious plant competition, continued to be the mass-producing low-floater generalists, reproducing via spores rather than the occasional budding of the Altiku. But with high offspring comes the assumption of target status for land primary consumers. Avoiding this is the main reason for the Altiku's existence.]