Intensive development of New Zealand’s indigenous grasslands: Rates of change, assessments of vulnerability and priorities for protection

Research was undertaken in the indigenous tussock grasslands of South Island of New Zealand in order to quantify past rates of conversion to agricultural land use and to develop vulnerability models to predict future conversion spatially and temporally. The study area was delineated using the median spectral reflectance of indigenous grasslands and included the largest extent of unprotected contiguous grasslands concentrated in the central South Island. Conversion from indigenous grasslands to a non-indigenous cover was quantified by comparative mapping over three intervals (1840-1990, 1990-2001, and 2001-2008). The basic premise in using satellite imagery to detect changes in land-use/cover is that these are revealed by changes in spectral signature. However, New Zealand’s indigenous and non-indigenous grasslands have overlapping spectral trajectories and high inter-annual variability, therefore contextual information was needed in order accurately map conversion from indigenous grassland cover to exotic pasture. Within the study area around the time of European settlement (1840) there were approximately 3.3 million hectares of indigenous grasslands. Between 1840 and 1990 around 1 million hectares of indigenous grasslands were converted to a non-indigenous cover. The extent of conversion during the preceding time period (1990-2008) was approximately 71,261 ha, of which 72% was converted to pasture and cropland and the remaining 28% to mining, urban settlements and exotic forestry. Although the overall rate of grassland conversion decreased relative to the period of European settlement and 1990, the proportion of remaining indigenous grasslands converted each year increased. Almost two-thirds of post-1990 conversion has occurred in environments with less than 30% indigenous cover remaining, and much is in land classified as non-arable with moderate to extreme limitations to crop, pasture and forestry growth. To assess the relative vulnerability of remaining areas of indigenous grassland to intensive land use (mainly intensive pasture production but also exotic conifer plantations, urban use and mining), spatial predictions using Generalized Additive Models (GAMs) were used to establish relationships between two different types of dependent (response) variables (presence or absence of conversion) and potential environmental and proxy socio-economic explanatory variables. The chosen predictors for the final model were used to map conversion probabilities in geographic space. The selected GAMs showed the mean probability of conversion in remaining indigenous grasslands was 0.15 and the mean area of conversion was 116 ha. Habitat that was most vulnerable to conversion was at moderate elevations and on medium slopes, and had previously been classified as being of low suitability for production. To interpret the regression models, plots of the partial response curves resulting from the model, and overall contributions of variables to the model, were used. The most important explanatory variables for predicting the probability of conversion in order of ‘alone contribution (the potential for each variable alone to explain conversion) was slope, rainfall, land tenure, distance to roads, proximity to existing agricultural, regional council, and mean annual temperature. Interpretation of the GAMs showed that conversion was negatively related to: slope, rainfall and distance roads; positively related to mean annual temperature; higher in the Otago and Canterbury regions and on privately owned or recently privatized lands, and peaked at intermediate proximity to roads. The prediction of the probability of conversion model was cross-validated both spatially and temporally. Temporal cross-validation compared predicted probabilities of conversion against reference maps of observed ‘current’ conversion. Spatial cross-validation evaluated model discrimination between ‘converted’ and ‘not converted’. Temporal and spatial performance was measured using the Receiver Operating Characteristic (ROC), a graphical plot of the true positive rate (sensitivity) as a function of the false positive (1-specificity) for different probability thresholds. For temporal cross-validation there was high correlation between ‘predicted’ and ‘observed’ (ROC = 0.913), and for spatial validation the relationship between the fitted and observed was also high (ROC= 0.921), indicating there was good discrimination between ‘converted’ and ‘not converted’. Integrating validated estimates of the probability of conversion (vulnerability) into conservation planning tools is an important component of conservation planning. Comparison of conservation prioritisation outputs with validated estimates of vulnerability of New Zealand’s indigenous grasslands showed variable effectiveness of vulnerability surrogates; one surrogate performed most poorly where vulnerability of grasslands to conversion was greatest and realized probability of protection was lowest. Furthermore, estimates of vulnerability using surrogates underestimated vulnerability on flat land that was closer to roads and overestimated areas on steeper land that was topographically invulnerable to conversion. There is an increased disparity between patterns of protection and patterns of conversion indicating that existing conservation planning tools are not effectively targeting the most vulnerable areas of remaining indigenous grasslands. An up-to-date validated vulnerability assessment offered a practical and a responsive technical bridge for the gap between science and implementation. This approach can be applied more widely to provide national models of vulnerability from representative samples of conversion

Patterns of past and recent conversion of indigenous grasslands in the South Island, New Zealand

We used recent satellite imagery to quantify the extent, type, and rate of conversion of remaining indigenous grasslands in the inland eastern South Island of New Zealand in recent years. We describe the pattern of conversion in relation to national classifications of land use capability and land environments, and ecological and administrative districts and regions. We show that although large areas of indigenous grasslands remain, grassland loss has been ongoing. Indigenous grassland was reduced in the study area by 3% (70 200 ha) between 1990 and 2008. Almost two-thirds of post-1990 conversion occurred in threatened environments with less than 30% of indigenous cover remaining, primarily in the Waitaki, Mackenzie and Central Otago administrative districts. This conversion occurred primarily on non-arable land. In the Mackenzie and Waitaki districts the rate of conversion in 2001-2008 was approximately twice that in 1990-2001. Opportunities to protect more of the full range of indigenous grasslands lie with the continuing tenure review process in these districts

The value of validated vulnerability data for conservation planning in rapidly changing landscapes

Data needed for informed conservation prioritization are generally greater than the data available, and surrogates are often used. Although the need to anticipate threats is recognized, the effectiveness of surrogates for predicting habitat loss (or vulnerability) to land-use change is seldom tested. Here, we compared properties of two different vulnerability surrogates to validated vulnerability—validated prediction of habitat conversion based on a recent assessment of land-use change. We found that neither surrogate was a particularly effective predictor of vulnerability. Importantly, both surrogates performed poorly in places most imminently threatened with habitat conversion. We also show that the majority of areas protected over the last two decades have low vulnerability to the most active threatening process in this biome (habitat conversion). The contrary patterns of vulnerability and protection suggest that use of validated vulnerability would help to clarify protection needs, which might lead to the improvement of conservation decisions. Our study suggests the integration of validated vulnerability into conservation planning tools may be an important requirement for effective conservation planning in rapidly changing landscapes. We apply our results to discuss the practical considerations and potential value of incorporating validated vulnerability into conservation planning tools both generally and in the context of New Zealand’s indigenous grasslands