Agricultural expansion has transformed and fragmented natural forest habitats at an alarming rate, and dramatic increases in agricultural intensification have since taken place in order to keep pace with human population growth and food demands. This simultaneously poses a considerable threat to biodiversity in agricultural landscapes, as production land is now one of the largest terrestrial biomes on the planet. Therefore, its contribution to biodiversity conservation is critical.
Links between the intensification of agricultural systems and ‘in situ’ declines of biodiversity on farmland have been well documented. However, despite growing recognition that system inputs such as fertiliser and livestock can move or ‘spillover’ into adjacent natural habitats, there has been no direct quantification of the extent of impacts in recipient ecosystems. These abiotic and biotic pathways can cause dramatic impacts on the diversity, composition, and functioning of remaining natural ecosystems, and on their ability to provide a variety of essential ecosystem services. Due to concerns regarding future food security, balancing trade-offs between agriculture and conservation has subsequently become a hot topic in ecological research. However, without any direct quantification of the off-site ecological impacts of agricultural intensification in mosaic landscapes, it is inherently difficult to fully evaluate strategies aimed at balancing production and conservation.
Using New Zealand farming systems as a case study, this thesis aims to address gaps in our current understanding of how increasing agricultural intensification impacts biodiversity in native forest remnants embedded within production landscapes. The first main chapter explores whether the magnitude of ecological impact in forest remnants (for a suite of 26 response measures) and severity of edge effects, scale with the degree of land-use intensity in surrounding agricultural pastures. This chapter also examines whether ecological responses differ in remnants ‘spared’ for conservation purposes (i.e. where livestock are excluded by fencing). The second chapter uses a model food-chain approach native to New Zealand, to test whether nutrient spillover from agricultural pastures influences plant-herbivore and herbivore-parasitoid interactions in forest remnants. This chapter also includes a large-scale common garden fertilisation experiment using the same tri-trophic system, which was established to examine bottom-up multi-trophic responses to the independent and combined effects of Nitrogen (N), Phosphorus (P) and cow dung and urine. The third main chapter uses a novel stable isotope approach for quantifying community-wide incorporation of resources into trophic structure. I test for the first time whether increasing intensity of farming systems drives greater nutrient spillover spatially into adjacent forest remnant soils and examine scaling effects of 15N (as a marker for anthropogenic N) through multiple trophic levels.
Beyond finding that agricultural land-use intensity generally has negative off-site effects on biodiversity, the key findings of this thesis were (i) spillover of nitrogen and phosphorus from agricultural systems into adjacent remnant soils is exacerbated by increases in land-use intensity, with (ii) percolating bottom-up effects on plant and insect community dynamics. (iii) The magnitude of ecological impacts in forest remnants scales linearly with increasing land-use intensity, as does (iv) direct anthropogenic N enrichment across multiple trophic levels, which has the potential to severely jeopardise the stability of ecologically important remnant habitats. (v) Although there were stark structural differences in remnants with and without livestock exclusion, impacts of land-use intensity on ecological response metrics were actually comparable across all sites. (vi) Livestock exclusion should be a priority first step towards conserving native forest remnants, however it should be recognised that fencing does not prevent abiotic channels of nutrient spillover (fertiliser drift, overland flow, leaching) in land characteristic of land spared for nature. (vii) Consequently, increasing land-use intensity compromises the effectiveness of the land-sparing trajectory for conserving native biodiversity, which is currently undertaken in New Zealand production systems.
Given the overall strength of these findings and the novel, ecosystem-wide and landscape-scale approaches taken to address fundamental questions, the work in this thesis greatly enhances our knowledge of the relationships between agricultural productivity and ecological impacts in spatially-coupled ecosystems. This is highly important, not only in New Zealand but worldwide, as it is anticipated that unstoppable human population growth and food security pressures will cause ecological impacts both on the farm and in adjacent natural ecosystems to become even more severe. Therefore, determining the relationship between land-use intensification and biodiversity loss represents the cornerstone of sustainable agricultural development in the future.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/8712 |
| Date | January 2013 |
| Creators | Deakin, Elizabeth Louise |
| Publisher | University of Canterbury. Biological Sciences |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Elizabeth Louise Deakin, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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