Abstract
This study is an interdisciplinary ecological study addressing the fine-scale relationships between plants, invertebrates and the environment in an alpine ecosystem. Alpine environments are marked by steep environmental gradients and complex habitat mosaics at various spatial scales. Regular forming periglacial patterned ground landforms on the Old Man Range, Central Otago, South Island, New Zealand present an ideal medium for studying plant-invertebrate-environment relationships due to their partitioning of the landscape into discrete units of contrasting environmental conditions, and the existence of some baseline knowledge of the soil, microclimate, vegetation and flora.
The study was conducted in three types of patterned ground (hummocks, stripes and solifluction terraces) on the Old Man Range. Each component of the study was sampled at the same spatial scale for comparison. Temperature was recorded in the soil and ground surface from April 2001 to March 2004 in microtopographic subunits (microsites) of each patterned ground landform. Plant species cover was sampled within each microsite; invertebrates were sampled from soil cores taken from the same locations as plant samples in April 2001 and September 2001. The two sampling occasions coincided with autumn before the soil freezes, and winter when maximum freezing was expected.
Fine-scale changes in the topographic relief of the patterned ground led to notable differences in the timing and duration of snow. The steepest environmental gradients existed during periods of uneven snow distribution. The soil in exposed or south-facing microsites froze first, beginning in May, and typically froze to more than 40cm depth. Least exposed microsites rarely froze. Within the microtopography, patterns of freezing at specific locations were consistent between years with only minor differences in the timing or depths of freezing; however, notable variation in freezing existed between similar microsites.
Within the microtopography, different assemblages of organisms were associated with different microsites. In total, 84 plant and lichen species were recorded, grouping into six community types. Species composition was best explained by growing degree-days, freeze-thaw cycles, time frozen and snow-free days; species diversity and richness increased with increasing environmental stress as indicated by freeze-thaw cycles, time frozen and exposure.
In total 20,494 invertebrates, representing four Phyla, 12 Classes, 23 Orders and 295 morpho-taxa were collected from 0.17m³ of soil. Acari, Collembola and Pseudococcidae were the most abundant invertebrates. Over 95% of the invertebrates were found in the plant material and first 10cm depth of soil. Few significant relationships were found between diversity, richness or abundance of invertebrate taxa and the microsites; however, multivariate analyses identified distinct invertebrate assemblages based on abundance. Invertebrate composition was best explained by recent low temperature and moisture, particularly in winter; however, plant composition also explained invertebrate composition, but more so in autumn.
This research has shown that organisms in the alpine environment of the Old Man Range are sensitive to fine-scale changes in their environment. These results have implications as to how historical changes to the ecosystem may have had long-lasting influences on the biota, as well as how a currently changing climate may have further impacts on the composition and distribution of organisms.