Total global reinsurance capital stood at $565bn (£399bn) at 31 December 2015, down by 2% from 2014, as estimated by Aon Benfield.
The reinsurance intermediary said the figures comprised capital from both traditional and alternative markets.
Based on its Aon Benfield Aggregate (ABA) report, which analyses the 2015 financial results of 27 major reinsurers, traditional capital decreased by 4% to $393bn (£278bn), driven by the strengthening of the US dollar and the impact of rising interests rates on bond valuations.
In contrast, alternative capital, sourced from third-party investors such as hedge funds, sovereign wealth funds, pensions and mutual funds, rose by 12% to $72bn (£51bn).
"This is reflected in robust levels of catastrophe bond issuance, further expansion of fully collateralized placements and growing utilisation of 'sidecar' vehicles," said the report.
Covering 10 years of data, the document showed alternative capital had been increasing its market share. It added reinsurers continued to incorporate alternative capital to lower their cost of underwriting.
Total premiums written by the ABA companies stood at $282bn (£199bn) in 2015, of which $206bn (£146bn) related to property and casualty (P&C) business.
The report found overall profitability remained relatively stable, but earnings were becoming increasingly reliant on benign catastrophe experience and material reserve releases. Growing price competition and low interest rates continue to pressure underlying returns on equity.
In terms of underwriting performance, P&C underwriting profit fell by 9% to $15.1bn (£10.7bn), of which $8.4bn (55% of the total) was derived from favourable prior year loss reserve development.
Mike Van Slooten, co-head of Aon Benfield's market analysis team, said: "The growing pressure on underlying earnings should be viewed against this backdrop, but in reality is likely to drive further M&A activity in the short to medium term."