Activity for servicing assets and the related changes in fair value for the three months ended March 31, 2020 and 2019 was as follows:

	Three Months Ended March 31,
	2020			2019
Beginning balance	$	19,471		$	19,693
Additions, net		1,393			1,102
Changes in fair value		(3,064	)		(1,261	)
Ending balance	$	17,800		$	19,534

Loans serviced for others are not included in the Consolidated Statements of Financial Condition. The unpaid principal balances of these loans serviced for others as of March 31, 2020 and December 31, 2019 were as follows:

	March 31,		December 31,
	2020		2019
Loan portfolios serviced for:
SBA guaranteed loans	$	1,240,319	$	1,231,959
USDA guaranteed loans		124,396		119,047
Total	$	1,364,715	$	1,351,006

Loan servicing revenue totaled $2.8 million and $2.5 million for the three months ended March 31, 2020 and 2019, respectively. Loan servicing asset revaluation, which represents the changes in fair value of servicing assets, resulted in downward valuations of $3.1 million and $1.3 million for the three months ended March 31, 2020 and 2019, respectively.

The fair value of servicing rights is highly sensitive to changes in underlying assumptions. Changes in secondary market premiums contribute to the change in fair value of servicing rights while prepayment speed assumptions have the most significant impact on the fair value of servicing rights.

Generally, as interest rates rise on variable rate loans, loan prepayments increase due to an increase in refinance activity, which may result in a decrease in the fair value of servicing assets. Measurement of fair value is limited to the conditions existing and the assumptions used as of a particular point in time, and those assumptions may change over time. Refer to Note 16—Fair Value Measurement for further details.