What are the main hurdles that terrestrial organisms (plants and animals) face?

Cal King gave you a good answer earlier. Let's first look at it:

"Gravity without buoyancy." Yup. Definitely true. The biggest plants use lignin and cellulose for support. The biggest terrestrial animals have strong bones for support, and use these in locomotion. Think about that next time you walk on your hind legs.

"availability of water." Yup. Definitely true. Us terrestrial dwellers are definitely not immersed in water, and yet we need water. Think about that next time you drink a SOLO(R) cup of water.

o Most plants have a "vascular system" which assists in moving water from the roots to the leaves.

o Most plants have a root system for getting water from the ground.

o Most plants have "bark" and/or "waxy cuticle" and/or "stomates that can be open or closed" to reduce evaporative water loss.

o Most terrestrial animals reduce evaporative water loss via having a relatively impermeable outer layer, and an enclosed tracheal, book lung, or lung system.

o Most terrestrial animals reduce water loss in waste removal by excreting uric acid or urea, which can be excreted with less water than ammonia could.

o You're equipped to drink liquid water. So are plenty of other terrestrial animals.

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A couple things that Cal King did not mention:

1. Temperature variation over the course of a day.

Yup. Water has nice thermal mass and a lot of energy is involved in changing its temperature, so a body of water isn't going to change temperature much over the course of a day.

Most plants don't do much about this. Some plants have chemistry that allow them to keep stomates closed during the day to reduce water loss when it's hot out.

Animals are typically active in a limited temperature range, and you'll see coping mechanisms like hibernation, estivation, being diurnal, being nocturnal, being underground, hanging out in the shade, basking in sunlight, ...

2. Sunlight is converted to heat and has deadly UV.

Ouch. Things that live beneath a meter of water don't have these problems.

Okay. We've discussed the temperature thing above.

UV can damage DNA. Terrestrial organisms can guard their cells with opaque layers (e.g. bark), outer pigmented layers, ancillary pigments, being nocturnal, hanging out in the shade, fur, parasols, umbrellas, ...

man