This research was published more than 5 years ago, some information may be out of date.

A new first-ever survey of conservation impact investing reveals a market of approximately $23 billion across just the last five years, and finds that investments in this space are expected to more than triple over the next five years (2014-2018). However, the report also finds that a substantial amount of potential private capital has not been deployed, demonstrating a need for a significant increase in the number of risk-adjusted investment opportunities. 

From the report:
"The report finds evidence of rapid growth and increasing interest in the market, which implies that much of the future of conservation lies with impact capital. Through an investor survey, we documented $23.4 billion in global conservation impact investments from 2009 through 2013. Investments by development finance institutions (DFIs) such as the International Finance Corporation totaled $21.5 billion; private investments accounted for $1.9 billion. While private investment accounted for a small share of the total market, we found that it grew at an average rate of 26% annually from 2009 through 2013. Further, from 2014 through 2018, private investors expect to deploy $1.5 billion of already-raised capital and to raise and invest an additional $4.1 billion."