Garden Size, Householder Knowledge, and Socio-Economic Status Influence Plant and Bird Diversity at the Scale of Individual Gardens

Domestic gardens collectively cover substantial areas within cities and play an important role in supporting urban biodiversity and ecosystem functioning. The extent of their contribution to urban biodiversity is ultimately determined by the attitudes, motivations, and practices of their owners. We identified variables characterizing individual householders and their properties that explained variation in perennial plant and avian species richness and plant diversity among 55 gardens in New Zealand. The size of the vegetated area was most important in explaining bird and plant diversity, independent of property size. Also important and positively associated with plant richness were socio-economic status and ability to discriminate between native and exotic species. Median housing age and neighborhood green space were less important. Contrary to expectations, better educated householders who demonstrated pro-environmental orientation (NEP) did not necessarily have gardens with greater plant diversity, however, people with higher NEP scores tended to have gardens with more structurally complex vegetation. Similar variables were important in explaining native and exotic species richness, but higher exotic plant diversity was associated with older people with smaller properties of lower value. Avian species richness increased primarily with vegetated area, but also the areas of beds and hedges. We demonstrate that although householder knowledge is an important determinant of garden biodiversity, vegetated area is most important. Promoting urban garden biodiversity requires that larger vegetated properties be supported and encouraged, and that planners should consider biodiversity when formulating policies concerning garden size, property size, and consents that may result in a progressive increase in the proportion of built over/paved areas.