Leaf breakdown and colonisation by invertebrates in a headwater stream: comparisons of native and introduced tree species

Leaf breakdown, colonisation by invertebrates, food choice by a facultative shredder, and composition and activity of the microflora of leaves of three native and three introduced trees were investigated in a small headwater stream, South Island, New Zealand. Leaves of each species were immersed and collected after 1, 32, 60, and 95 days to measure mass loss and invertebrate colonisation. Additional leaves collected after 60 days were used for food choice and respiration experiments, and others taken after 95 days (some species) were examined with scanning electron microscopy. Leaf breakdown rates followed the sequence: elm > red beech > willow > mahoe > oak > mountain beech. Elm (introduced) and mahoe (native) supported the highest invertebrate densities, but shredders were most abundant on willow (introduced) and red beech (native) leaves. In laboratory choice trials the facultative shredder Olinga jeanae showed a preference for elm and red beech leaves which were fastest to break down and had high respiration rates. We found that leaves of introduced trees can be preferred by shredders and hence, no strong associations were apparent between shredders and these native trees.