Abstracts of Published papers in peer-reviewed journals
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- Lord, J.M. and Norton, D.A. 1990. Scale and the spatial concept of fragmentation. Conservation Biology 4: 197-202
- Norton, D.A. 1991(a). Trilepidea adamsii: an obituary for a species. Conservation Biology 5: 52-57
- Given, D.R. and Norton, D.A. 1993. A multivariate approach to assessing threat and for priority setting in threatened species conservation. Biological Conservation 64: 57-66
- Norton, D.A., Hobbs, R.J. and Atkins, L. 1995. Fragmentation, disturbance, and plant distribution: mistletoes in woodland remnants in the Western Australian wheatbelt. Conservation Biology 9: 426-438
- Awimbo, J.A., Norton, D.A. and Overmars, F.B. 1996. An evaluation of representativeness for nature conservation, Hokitika Ecological District, New Zealand
- Hobbs, R.J. and Norton, D.A. 1996. Towards a conceptual framework for restoration ecology. Restoration Ecology 4: 93-110
- Norton, D.A. and Carpenter, M.A. 1998. Mistletoes as parasites: host specificity and speciation. Trends in Ecology and Evolution 13: 101-105
- Norton, 1998(a). Indigenous biodiversity conservation and plantation forestry: options for the future. New Zealand Forestry 43(2): 34-39
1. Lord, J.M. and Norton, D.A. 1990. Scale and the spatial concept of fragmentation. Conservation Biology 4: 197-202
In a recent issue of Conservation Biology (Vol. 2, No 4), a special section was devoted to edge effects and fragmented landscapes. These papers and others (e.g. Lovejoy et al. 1986; Wilcove et al. 1986) typify the current view of fragmentation as a spatial phenomenon at the landscape scale. However, it needs to be recognized that fragmentation is not restricted to any particular scale, nor to the spatial domain as opposed to any other domain (e.g., temporal or functional). Fragmentation is simply the disruption of continuity. When defined in this manner, the concept of fragmentation can be applied to any domain in which continuity is important to the functioning of ecosystems. Because ecosystems function across a wide range of scales, fragmentation is not scale-limited. Here we discuss the effect of scale as applied to fragmentation in the spatial domain. In particular, we explore the conservation implications of scale in spatial fragmentation, since fragmentation is a major conservation issue.
2. Norton, D.A. 1991(a). Trilepidea adamsii: an obituary for a species. Conservation Biology 5: 52-57
The factors leading to the presumed extinction of Trilepidea adamsii (Cheesem.) Tiegh., an endemic New Zealand mistletoe, are discussed. Although several factors are involved, including habitat loss, overcollecting, reduced seed dispersal, and browsing by the brushtailed possum (Trichosurus vulpecula Kerr.), interactions among these factors appear to have been particularly important in this extinction. Possums, introduced from Australia last century, probably expanded too late to have had a major impact on Trilepeidea adamsii except perhaps at the very end, but they do seriously jeopardize the long term survival of several other plant species in New Zealand. Trilepidea adamsii illustrates the range of factors, and their interactions, that can lead to the extinction of a species.
3. Given, D.R. and Norton, D.A. 1993. A multivariate approach to assessing threat and for priority setting in threatened species conservation. Biological Conservation 64: 57-66
The use of multivariate techniques for assessing the threats facing species and for determining priority groupings in threatened species conservation is evaluated. Scores based on different criteria provide species profiles that allow species to be placed in multidimensional space. Species that group together are likely to be threatened for similar reasons. Multivariate techniques identify the main factors threatening species and highlight problems with linear ranking schemes, especially where species can have the same total score for very different reasons. Multivariate techniques also allow identification of redundant criteria used in deriving species scores, and are free of problems associated with interdependence between criteria. Multivariate techniques provide a powerful additional tool for determining management priorities and for assessing threat and can also be used for assessing changes in species status with time and for modelling potential management actions for improving the security of a species. However, in using these techniques it is essential to make a clear distinction between threat and priority.
4. Norton, D.A., Hobbs, R.J. and Atkins, L. 1995. Fragmentation, disturbance, and plant distribution: mistletoes in woodland remnants in the Western Australian wheatbelt. Conservation Biology 9: 426-438
Spatial heterogeneity and patchy distributions of species in intact landscapes are likely to lead to complex and unpredictable distribution patterns in remnants following fragmentation. We examined this proposition in relation to the mistletoe Amyema miquelii, which exhibits a clumped distribution in Eucalyptus salmonophloia woodlands in the Western Australian wheatbelt. We sampled mistletoe distribution and abundance in 14 woodland fragments ranging from 2.4 to 60.5 ha and in 14 sections of roadside corridors. These sites represent all known fragments and corridors containing E. salmonophloia in a 1680 square kilometre study area. We found that large fragments were more likely to have mistletoes than small fragments, but that small fragments either contained many or few to no mistletoes, reflecting the way fragmentation "samples" the pre-existing distribution. Superimposed on this sampling effect is the influence of disturbance. Fragments subjected to stock grazing contained no mistletoes. This indicates that grazing modified the habitat either for the mistletoe itself, through changed water relations, or for the frugivorous birds which may disperse mistletoe fruit, through removal of the shrubby understory. Only one A. miquelii plant was found on 26.3 km of roadside corridor, despite tree densities in corridors being similar to those in fragments. Roadside areas are generally considered good habitat for mistletoes, and their absence suggested that fruit-dispersing birds either did not use the corridors or did not stay in them long enough to deposit mistletoe seeds. These results indicate that, in order to predict biotic responses to fragmentation, information on distribution patterns and scales of patchiness in the prefragmentation landscape is required and the effects of fragmentation per se are likely to be confounded by other factors such as disturbance. Furthermore, quantifying fragmentation effects is difficult because of the small sample sizes typical of highly fragmented landscapes.
5. Awimbo, J.A., Norton, D.A. and Overmars, F.B. 1996. An evaluation of representativeness for nature conservation, Hokitika Ecological District, New Zealand
Representativeness in the modern Hokitika Ecological District landscape is assessed by comparison with the distribution of landform-vegetation classes present in c. 1860 (prior to extensive European settlement). While upland landform-vegetation classes are well represented, lowland ones are not, especially on alluvial and coastal landforms. In these two classes, extant remnants account for 0.3% and 8.2% of their 1860 extent respectively. Options for increasing the representativeness of these landform-vegetation classes must rely largely on ecological restoration, as there are few remaining remnants not already protected. This under-representation may at first seem surprising, as Hokitika Ecological District is located in an area of New Zealand (west coast South Island) with a very high percentage of protected land (c. 75% protected), and emphasises the importance of considering spatial scale when looking at representativeness as a basis for the evaluation and design of protected natural areas. Any targets for land protection (e.g. 10% of the land area) should be stratified across a range of spatial scales from local (ecological districts in this study) to national.
6. Hobbs, R.J. and Norton, D.A. 1996. Towards a conceptual framework for restoration ecology. Restoration Ecology 4: 93-110
Heightening human impacts on the Earth result in widespread losses of production and conservation values and make large-scale ecosystem restoration increasingly urgent. Tackling this problem requires the development of general guiding principles for restoration so that we can move away from the ad hoc, site-and situation-specific approach that now prevails. A continuum of restoration efforts can be recognized, ranging from restoration of localized highly degraded sites to restoration of entire landscapes for production and/or conservation reasons. We emphasize the importance of developing restoration methodologies that are applicable at the landscape scale. Key processes in restoration include identifying and dealing with the processes leading to degradation in the first place, determining realistic goals and measures of success, developing methods for implementing the goals and incorporating them into land-management and planning strategies, and monitoring the restoration and assessing its success. Few of these procedures are currently incorporated in many restoration projects. The concept that many ecosystems are likely to exist in alternative stable states, depending on their history, is relevant to the setting of restoration goals. A range of measures, such as those being developed to measure ecosystem health, could be used to develop "scorecards" for restoration efforts. Generalizable guidelines for restoration on individual sites could be based on the concepts of designed disturbance, controlled colonization, and controlled species performance. Fewer explicit guidelines are available at the landscape scale, beyond nonquantitative generalities about size and connectivity. Development of these guidelines is an important priority so that urgent large-scale restoration can be planned and implemented effectively.
7. Norton, D.A. and Carpenter, M.A. 1998. Mistletoes as parasites: host specificity and speciation. Trends in Ecology and Evolution 13: 101-105
It has been estimated that c. 1% of all angiosperm species are parasitic and that about 40% of plant parasites are shoot parasites, parasitizing the above-ground parts of their host plants, while the other 60% are root parasites. Although much of the parasite literature focuses on animal parasites, plant parasites are ecologically and economically significant and share many features in common with animal parasites. Parasites can be defined as organisms that complete a whole stage of their life associated with a single host individual in a relationship that is beneficial to the parasite but not to the host. This definition includes all plant parasites, viruses, some phytophagous insects, parasitoids, and ecto- and endoparasites of animals such as lice and liver flukes.
Mistletoes, the predominant group of angiosperm shoot parasites, are a fascinating and diverse group of plants found in a wide range of ecosystems including boreal forests, tropical rainforests and arid woodlands. While seed of many plant parasites germinates only in response to chemical signals from host plants, mistletoe seeds germinate readily in almost all situations. However, the key limiting step in a mistletoe's life cycle is establishment, which is dependent on an appropriate disperser, deposition on a suitable sized branch, and mistletoe-host compatibility. In having such tight establishment requirements, mistletoes have much in common with many animal parasites. Like other plant and animal parasites, mistletoes also live in an intimate association with their hosts and derive nutrition from the host, and, of course, share a life-long association with a single host individual.
Mistletoes, as with other plant parasites have recently been described as both agricultural pests and as threatened species in different parts of the world. If we are to manage these species appropriately, it is important that we understand such basic aspects of their biology as patterns of host specificity and the ways in which they speciate. These aspects of mistletoe biology are, however, only beginning to be understood. Recent advances in our understanding of host specificity and speciation patterns in a variety of animal parasites provide valuable insights into similar patterns in plant parasites and especially in mistletoes, but also highlight key areas where our knowledge is still limited for these groups.
8. Norton, 1998(a). Indigenous biodiversity conservation and plantation forestry: options for the future. New Zealand Forestry 43(2): 34-39
Our goals in plantation forests should be to integrate production and protection in the same landscape (as advocated by the Resource Management Act 1991) rather than replacing one with the other. A review of indigenous biodiversity in New Zealand's plantation forests shows that many indigenous plants and animals occur in exotic plantations, with the number of species being dependent on plantation age, proximity to indigenous remnants and a variety of site factors (slope, aspect, etc).
Plantation forests contribute to the conservation of indigenous biodiversity through
- providing habitat for indigenous species;
- buffering indigenous forest remnants; and
- improving connectivity between remnants.
Options for enhancing indigenous biodiversity conservation in plantation forests include
- retention of indigenous forest;
- establishing a greater diversity of planted species;
- planting a diversity of tree species along streams and roads to provide additional habitat for indigenous animals; and
- modifying silvicultural practices within plantations.
It is suggested that through the use of spatial modelling, optimisation of the arrangement of different aged compartments, and different plantation species, will maximise both timber production and indigenous biodiversity within a forest thus allowing full integration of these two activities without the loss of production values.