单选题 .  ①The occurrence of dense aggregations of the fruit-bearing tree Brosimum alicastrum on or near Maya ruin complexes in Central America was initially interpreted as evidence of ancient Maya silviculture, i.e., the stands were relics of orchards planted by the Maya. ②Later work, however, suggested that the stands resulted from the Artibeus bats that roost among the ruins. ③The bats fly to the forest, collect the fruit, bring it back to their roost, eat the fruit, and then discard the undamaged seed. ④The continual input of bat-dispersed seed has maintained the clumps of B. alicastrum around ruins for many hundreds of years.
    ①This observation, however, does not negate the possibility that B. alicastrum was used and managed by the Maya. ②In fact, if we examine the behavior, rather than the density or location, of these tree populations, we are presented with strong evidence of deliberate genetic improvement. ③This is especially notable in the B. alicastrum trees that cluster around the Maya ruins at Tikal in Guatemala: these trees' productivity is almost twice that of trees in Veracruz, Mexico, under almost identical environmental conditions.13.  "The continual input of bat-dispersed seed" is mentioned in the passage as part of an explanation for ______