In my opinion, I think conserve 15% of every biome would be enough to ensure the long term persistence of global biodiversity. 

Based on my research, large region of Earth which consists of certain climate and types of living things is called biome. Biome is also known as the large community of vegetation and wildlife adapted to a specific climate. 

I have read about something related to this where it says that tundra biome covers about 20% of the Earth. I wonder if we consume more than 15% of the biome, would it be enough to ensure the long-term persistence of global biodiversity. 

habitat destruction & degradation,  fragmentation and  overexploitation of the resources available in our earth are also the causes to such to happen.

Can anyone tell what trans boundary protected area is… 

It is a Geographical space where it is managed and legally protected to achieve the long term conservation of nature, Where it also allows higher control of pest species and even alien invasive species 

for your info the main considerations for developing a trans boundary protected area also involves  ecologically  where a  protected region is likely to be more efficient if it follows natural ecosystem boundaries rather than political boundaries. 

  first is Human  where  Social considerations are highly regarded in establishing trans boundary protected areas, placing particular importance on the needs of traditional communities, many of which cross political boundaries or live migratory lifestyles, or to foster peace and collaboration between neighbouring countries (sometimes following armed conflicts) and another is  Legally where as tran sboundary protected areas may often occur in areas where more than one sovereign legal regime applies, they require special legal agreements between the authorities involved.