The second way that non-native species can be beneficial is that they act as catalysts for restoration. This is because the presence of non-native species increases the heterogeneity and biodiversity in an ecosystem. This increase in heterogeneity can create microclimates in sparse and eroded ecosystems, which then promotes the growth and reestablishment of native species. In Kenya, guava has real potential as a tool in the restoration of tropical forest. Studies of isolated guava trees in farmland showed that they were extremely attractive to a wide range of fruit-eating birds. In the course of visiting them, birds dropped seeds beneath the guavas, many of them from trees in nearby fragments of rainforest, and many of these seeds germinated and grew into young trees. Surprisingly, distance to the nearest forest didn't seem to matter at all – trees up to 2 km away (the longest distance studied) were just as good as trees much nearer to forest fragments. Guavas establish easily on degraded land, and each tree is potentially the nucleus of a patch of regenerating rainforest. Of course, most seedlings that grow beneath guavas are just more guavas, but guava is an early-successional tree that soon dies out when overtopped by bigger trees, nor does it actively invade primary forest. Invasive alien trees can also be useful for restoring native forest. In Puerto Rico, native pioneer trees could cope with natural disturbances such as drought, hurricanes, floods and landslides, but are mostly unable to colonise land that has undergone deforestation, extended agricultural use and eventual abandonment. In these sites, low-diversity pioneer communities of invasive trees develop, but over time native trees invade. Alien pioneers may dominate for 30 to 40 years but the eventual outcome, after 60 to 80 years, is a diverse mixture of native and alien species, but with a majority of native species. In the absence of the initial alien colonists, abandoned agricultural land tends to become pasture and remain that way almost indefinitely.