Too many mouths..
...with too much disposable income. In a world with not enough sustainable agricultural technology or "communism" to maintain a healthy population, balancing it's own needs with the needs of other species (aka biodiversity).
Sharks fin soup is not the start I suspect (See Cod populations), and Russia's near miss with the wheat crop of 2010 as yet another warning shot across the bows for the (increasingly fat and/or populous) human race.
There are simply too many of us to keep eating the way we do. We need to change and strictly manage what we eat, and/or how many of us are doing the eating, or improve the technology to give better yields with less environmental impact. If we do it now - while it's only a relatively small problem - we won't have to resort to some Soylent Green style future.
Depressing picture, but hey - it's Monday.
Roll on the denialist downvotes, while I pop out to the pub for a nice, juicy New Zealand steak and chips with shark fin soup for starters, rounded off with a 'massive-food-miles' 4-corners-of-the-world fruit salad covered in genetically-modified cream.