The conservation and ecology of Ischnocarpus exilis and I. novae-zelandiae, two threatened New Zealand cresses

Abstract

Sixty-five percent of New Zealand's threatened plants are herbaceous, yet herbaceous species have received relatively little research attention. Very little experimental work is done to scientifically test hypotheses thought to be governing any threatened plant species in New Zealand. This study experimentally tested several hypotheses related to the restriction and decline of two of New Zealand's threatened herb species, Ischnocarpus exilis and I.novae-zelandiae, in order to make recommendations for their conservation management. This was done with a glasshouse experiment, a series of field experiments, demographic monitoring and seed bank analysis.

I.exilis is critically endangered and exists in only one small population I.novae-zelandiae has declined throughout its range and has disappeared from former habitat of tussock grassland. Both species are confined to marginal rock outcrop habitat, possibly due to competition from invasive weeds.

A glasshouse competition experiment showed that the presence of Hieracium pilosella (mouse-eared hawkweed) at two different densities had a negative effect on Ischnocarpus germination, seedling survival, growth rate, and fecundity. Since I.novae-zelandiae is found growing underneath tussock and shrub canopy in the northern-most portion of its range, I also tested for a facilitory effect of Festuca novae-zelandiae (hard tussock) on I.novae-zelandiae to determine if a loss of tussock cover could also be important in the decline of the species. Germination of I.novae-zelandiae was favoured by the presence of F.novae-zelandiae, indicating that shelter from tussock canopy may be important for I.novae-zelandiae establishment.

A field experiment confirmed that weed competition had a negative effect on I.exilis establishment with germination of experimentally sown seeds significantly higher in non-weedy habitat and higher in weeded plots in weedy habitat. This experiment, combined with seed bank analysis, also showed that the distribution of I.exilis is limited by low seed production and poor dispersal ability, and not by specific habitat requirements. Therefore, the management of weeds within the present habitat of I.exilis is essential, as is the enhancement (introduction of additional I.exilis plants) of the population to increase seed production, and reintroductions of populations in other locations.

In a second field experiment, seeds were sown into different habitat types, and in weeded and non-weeded treatments within those habitats, to examine the effects of habitat type and weed competition on I.novae-zelandiae establishment. Different factors were found to impact on I.novae-zelandiae establishment depending on habitat conditions. In dry conditions, little establishment occurred in open modified tussock grassland habitat compared to adjacent sheltered rock outcrop habitat, suggesting that shelter was a prerequisite for establishment in dry conditions. In the sheltered conditions of rock outcrop habitat however, weed competition significantly decreased germination rates. Thus I.novae-zelandiae may be limited by an inability to establish in open modified tussock grassland, and by weed competition within its current rock outcrop habitat. Demographic processes did not appear to limit natural I.novae-zelandiae populations, although when the habitat is highly weed-invaded, seed production may be lower. Regular demographic monitoring of populations of I.novae-zelandiae throughout its range, and monitoring and control of weeds within those populations is required.