What concept implies that a firm's marginal revenue product curve for labor will slope downward in the short run?

Law of Diminishing Marginal Returns (LDMR). As in Economic theory, there will be fixed and variable factors of production in the short run. This would imply that beyond a certain level of production, the next unit of variable factor added to the production would result in a lower output as compared to the previous unit of variable input that was added to the production. This is ultimately due to the over usage of the fixed factors of production (such as machinery and infrastructure) and resulting in a less "efficient" amount of output due to the physical operating limits of fixed factors of production. As such in the short run, MR will slope downward if the firm is producing beyond its most efficient point of production to ensure more products can be produced given a limited amount of time.