Ecological Rehabilitation, an approach to assisting ecosystems modified by invasive plants : Applied and Validated via Kaitorete Spit, a case study

Abstract

Invasive plants modify ecosystem structure and function, which can result in an alternative novel state - jeopardising the ecosystems ecological value. The impact of invasive plants on communities, ecosystems or landscapes can be a catalyst for undertaking ecological restoration, with the primary goal of returning the ecosystem to the state prior to the invasive plant driven modification. This thesis asks whether the attainment of a prior state and the eradication of an invasive plant(s) is a realistic objective and whether or not a revision of the current practice might determine an alternative approach to assisting ecosystems modified by an invasive plant(s).

These questions are explored through the literature; the concerns with the current practice of ecological restoration are discussed using relevant examples. A new approach, adopting the term ‘Ecological Rehabilitation’ is proposed, which is then applied and tested through a case study; Lupinus arboreus invasion of the Kaitorete Spit dune ecosystem, Canterbury, New Zealand.

Ecological rehabilitation differs from ecological restoration as it,

1) Determines and takes into consideration how an invasive plant has impacted upon ecosystem structure and function, which aids in mitigating the plants impact,

2) Gathers relevant information from a range of spatial and temporal scales - from the individual species to the landscape and from the daily to the annual, information which is then used to identify how the invasive plant has modified the ecosystem and develop a successional model for the pre-invasion ecosystem,

3) Constructs the reference system from a range of temporally and spatially varied avenues of inquiry rather than selecting an extant ecosystem as the reference system,

4) Derives the rehabilitation goals from the reference system, the goals represent the rehabilitation target rather than identifying a static target,

5) Acknowledges that the invasive plant may be a component of the rehabilitated ecosystem, but success is achieved by conserving the ecosystem value rather than attaining a specific biological assemblage.

The application and testing of the ecological rehabilitation approach required field-based inquiry, laboratory and computer-based analysis and biographic/archival searches to determine, at varying scales, the impact of L. arboreus on the Kaitorete Spit ecosystem. A cyclical succession model for the dune ecosystem without the presence of the invasive plant is proposed based on the development of a theoretical chronosequence. The impact of the invasive plant on an endemic widow spider is investigated and discussed in view of focal species used in ecological restoration. The invasion of L. arboreus across the dune ecosystem is mapped from aerial photographs and landscape ecology concepts of invasion are applied to assist in developing a management strategy. The impact of additional plant available nitrogen on several native and introduced species is examined through a greenhouse experiment and the impact of eradicating L. arboreus from the dunes on the soil nutrient concentration and the vegetation assemblage is examined.

The reference system for Kaitorete Spit is presented based on the results of the research and the concept of environmental thresholds is discussed in light of the research findings, with the suggestion that they a) are barriers to autogenic recovery and b) are crossed in the opposite order to that proposed by Richard Hobbs. The concept of ecological rehabilitation is discussed in light of the Kaitorete Spit case study. Ecological Rehabilitation is an alternative approach to assisting ecosystems modified by invasive plants – or any modified ecosystem – so long as a reference system can be constructed.