The capacity of restored urban forests to support native birds: Ecological or social restoration?

Abstract

Urbanisation, which drastically and often irreversibly alters original habitat, is recognized as one of the leading causes of species extinctions. Biological invasions are viewed as the second most important cause of global biodiversity loss, and invasive species have been a major cause of island fauna extinctions. New Zealand cities are characterised by low levels of remnant indigenous vegetation and a unique, understudied assemblage of introduced mammals, resulting in a depauperate native forest bird community. The loss of native flora and fauna from urban areas is thought to lead to the ‘extinction of experience’, or loss of opportunities for city residents to interact with nature on a daily basis, which would allow them to develop a personal, emotional bond with the natural environment. This situation is believed to result in a vicious cycle of ever decreasing levels of native biodiversity and the continued disintegration of the human relationship with the natural world. Habitat restoration in cities could disrupt this vicious cycle by recreating native ecosystems where the majority of the world’s population now lives. To date, urban restoration research both internationally and in New Zealand has focused on plants. It is often assumed that restoring native vegetation will result in the recovery of native fauna and the ecosystem services they provide, such as seed dispersal and pollination. This assumption, however, has rarely been tested empirically. Restoration success should be measured on the scale of human experience, connection to nature and wellbeing, for it is on this level that urban restoration efforts have the most to contribute to global conservation progress. People create cities and are the source of both the challenges and solutions presented by urban ecosystems. Yet, little is known about the relationship between city residents and urban biodiversity. Recognizing that cities represent complex, interconnected networks of human actions and their effect on ecosystems and organisms, this thesis integrates the social and natural sciences in a multidisciplinary framework, with the overarching aim of investigating the capacity of urban forest restoration to break the vicious cycle of extinction of experience by providing habitat for native forest birds and reconnecting urban residents with nature in New Zealand cities. I used a phenomenological methodology to explore the experiences of people living near urban greenspaces. Using qualitative interviews, I explored Hamilton City residents’ perceptions and experiences of urban nature in frequently used parks and gardens. This research identifies a gap between what residents claim to value and their behaviour and actions affecting native biodiversity. Given this value-action gap, my interviews suggest we cannot currently rely on private gardens to provide adequate resources for native birds. This research highlights, however, the unique contribution that native bush parks make to human wellbeing and sense of place. Ecological restoration in cities therefore has great potential to enrich human lives. To evaluate restoration success by determining how species richness, abundance, diversity and community composition of native forest birds vary along a restoration chronosequence, I monitored birds and mammals at 43 sites in two cities in New Zealand’s North Island – Hamilton and New Plymouth. Sites represented three types of urban forest: unrestored (n = 6), restored (n = 25) remnant (n = 6), and rural forest remnants nearest to each city (n = 6). Restored sites formed an age gradient of 1 to 72 years since initial planting. We used NMDS for a visual representation, and PERMANOVA to test for differences in bird community composition across restored and reference sites. Structural equation modelling was used to investigate the relative importance of vegetation characteristics, landscape pattern and introduced mammals to native forest bird communities. Results demonstrate that urban forest restoration increases native bird species richness and diversity over time, and transforms the bird community from being dominated by introduced granivores and generalists, to one dominated by native forest birds. This research indicates, moreover, that lack of habitat is an important factor limiting native forest birds in New Zealand’s North Island cities. Current restoration efforts were found to benefit widespread, common native forest birds, but greater investments in predator control or eradication will be needed to re-create forests capable of supporting bird species of conservation concern. These same sites were used to investigate how introduced mammal communities change over time since initial restoration planting and what consequences this has for restoration success. I again used structural equation modelling to determine the relative importance of vegetation structure, landscape composition and interspecific interactions on introduced mammal community structure, as well as the impact of mammals on native forest regeneration. This research demonstrated that urban forest restoration provides good habitat for rats, possums and mice within 9 years of initial restoration planting. Rats were also found to decrease native sapling species richness and density, thereby changing the trajectory of forest recovery. This thesis advances theoretical and practical knowledge of the capacity of urban forest restoration to address the interconnected problem of native biodiversity loss in cities and the disintegration of human relationships with the natural world. Furthermore, it highlights the necessity of adopting a multidisciplinary approach in order to find ways of reconciling the needs of both people and native species in cities. Given that the health of people and the environment are inextricably linked, crossing the disciplinary boundary between the social and natural sciences provides one avenue for restoring human connections to the natural world.