This is because of the edge effects of their irregular shapes, their frequent disturbance, and their proximity to roads and houses. The pekarangans used for the Jambi study had unusually low levels of plant diversity, which may account for the results. Despite this, the gardens apparently still attract birds due to their food resources. A similar finding was repeated in a separate West Java study, indicating children shoot birds in the gardens and take their eggs while adults kill or chase them due to the perception of them as pests. Plant diversity in pekarangans arises from complex interactions between several factors that are not fully understood. These include environmental stability, the tropical climate that is favorable to plant growth, and their close proximity to the owners' domestic activities. Other natural factors are size, temperature decrease due to elevation, precipitation, and climatic events like El Niño. Anthropological factors include individual preferences and market proximity. The diversity of plants aids individual plants to adapt to a changing environment, helping them survive in the long term. The biodiversity in the multi-layered system also helps to optimize solar energy and carbon harvesting, cool the domestic climate, protect the soil from erosion, and accommodate habitats for wild plants and animals. The genetic diversity also gives protection from the effects of pests and diseases. As an example, the abundance of insectivorous birds in the gardens helps control pests, helping the garden remain productive. While on per individual basis pekarangans store only small amounts of carbon due to their size, on per area basis they hold an amount of carbon that is similar to primary or secondary forests, and greatly surpassing "Imperata" grasslands and fallow lands. Plant diversity in pekarangans tends to increase as their size increases. Diversity of crop species, however, might reach a plateau in very large gardens. Larger pekarangans have a lower density of crop species because of more constant cultivation patterns. A pekarangan smaller than is insufficient for plant diversity and crop production. Some plant types, such as trees higher than 10 meters (33 ft), spice plants, and industrial crops are almost not present in gardens of or less. Home gardens in Java tend to be smaller; the majority of them are smaller than , as suggested by a report from 2004. Meanwhile, similar gardens in other Indonesian islands tend to be larger. Their average size is estimated to be ; a few reach the size of . Pekarangans at high altitudes tend to have a smaller size, increased density of plants, and a smaller range of plant diversity.