Varley Gradwell 2006

Why do similar species have opposite responses to habitat edges?
Robert M. Ewers
Institute of Zoology and University of Cambridge

Summary of project

This project represented what I believe is the first experimental manipulation of
species to try and elucidate the mechanisms underlying species responses to habitat
edges. The study was carried out at the Hope River Forest Fragmentation Project in
tlre Southern Alps of New Zealand, where myself and Raphael Didham have an
extensive dataset detailing the response of ca.900 beetle species to forest-grassland
edges.

I focused my study on a pair of Mecodema species in the family Carabidae; M.
rugiceps and M. fulgidum. These species are relatively abundant, morphologically
very similar, but have contrasting response to habitat edges. M. rugiceps is restricted
to forest, and at the site I selected for experimental work, was restricted to deep forest
(never encountered closer tlian 250 m to the forest edge). By contrast, M. fulgidum is
a habitat generalist that is present in both the forest and in the surrounding grassland
matrix.

I was able to perform the translocations as planned and described in my proposal.
However, the results were not at all what I had expected. In large part, I believe this
is because New Zealand experienced one of the coldest summers on record. This
project was designed primarily to detect differential mortality rates between the two
species across a forest-grassland edge, with the expectation that the two species would
vary in their ability to survive the variation in microclimatic conditions across the
edge. The study was planned and executed in the austral summer (December 2006 -
April 2007) when conditions are hottest and standing microclimate gradients are at
their strongest. But the persistent cold weather during the summer resulted in low, or
even completely absent, standing microclimatic gradients (analogous to those
expected during winter months). To work around this problem I extended my initial
planned fieldwork period of one month. to four and a half months, waiting and hoping
for settled summer conditions to arrive, but was mostly disappointed.

The results from my planned experiment are not striking, but there are some
interesting patterns; there was significant mortality in the forest specialist M. rugiceps
but not for the habitat generalist M. fuligdum However, there was no significant
correlation between mortality rate and distance to edge for M. rugiceps. The
mortality occurred almost exclusively at the very end of the summer, when conditions
were at their hottest. To examine this trend in more detail, I was able to purchase
thirty years of climate data from a nearby weather station. Although not yet formally
analysed, the data indicate that mortality began to occur when soil moisture became
much dryer than average at the very end of my field season. I am investigating
whether this trend will be strong enough to justify publication. If nothing more, these
data serve as a strong indication that M. rutgiceps, which is weakly water-loving and
burrows underground during the day, may have its spatial distribution limited by
seasonal changes in soil moisture. Although the patterns I have observed are not what

I had originally set out to detect (temporal rather than spatial variation in mortality
rate), they do lead to the specific hypothesis that temporal variation in soil moisture is
one ecological mechanism that may underlie species responses to habitat edges.
Generating an explicit hypothesis such as this was one of the primary purposes of this experiment.