Download The effects of fire on invertebrate food web structure

PROGRAM B3.1: Impacts of Fire on Ecological Processes and Biodiversity
© BUSHFIRE CRC LTD 2008
The effects of fire on invertebrate food web structure in
the buttongrass moorlands of Tasmania
Fiona Christie1, Karl Brennan2 and Alan York1
1 School of Forest & Ecosystem Science, The University of Melbourne, Creswick 3363, VIC
2 Department of Environment and Conservation, Kalgoorlie 6430, WA.
This research aims to investigate the cycling of carbon and nitrogen through the decomposer and herbivore food webs using
stable isotope analysis (δ13C and δ15N) and to determine how fire affects food web structure and ecosystem functioning
Background
Insectivorous birds
Low intensity fires are used extensively in the management of Tasmania’s Buttongrass
Moorlands, despite being little known about the long term effects of repeated burning on
Invertebrate
predators & parasitoids
ecosystem functioning. Understanding patterns of species loss and the impacts on ecological
functioning are fundamental to predicting the long term effects of fire. In the Buttongrass,
Vertebrate herbivores
Invertebrate herbivores
invertebrates play key roles in the decomposition and mineralisation of organic matter and the
mobilisation of nutrients contributing directly to peat formation. Stable isotope “signatures”
can enable us to determine taxon relationships within the food web and make inferences about
Fire
Plants
potential flow on effects from fire induced changes in structure (Fig. 1).
Buttongrass moorlands – structural dynamics/community composition/
reproductive success/nutrient cycling
Methods
Invertebrate Decomposers
• Plants, and invertebrates were collected from Buttongrass Moorlands representing five
Aboveground
Belowground
Invertebrate Herbivores
Soil Fauna
different age classes (time since fire).
Figure 1: Simplified concept diagram outlining potential top-down (solid
arrows) and bottom-up (dashed arrows) impacts on plant and animal
assemblages. Changes in composition, abundance and richness at any level
may cause changes in the abundance and species composition at other
trophic levels with subsequent effects on rates of herbivory and
decomposition.
• Samples represented a wide range of functional groups including decomposers, primary
producers, herbivores and predators and identified to species/morphospecies.
• Specimens were oven dried and ground to a fine powder for analysis through the Isotope Ratio
Represents components of the food web which were sampled
Mass Spectrometer (IRMS) to determine natural abundances of δ13C & δ15N.
100
9.0
*
Recently Burnt
*
90
8.0
Long Unburnt
80
7.0
70
60
6.0
d 15N
*
50
40
5.0
4.0
30
*
3.0
20
2.0
10
1.0
Orthoptera
Opilionida
Isopoda
Hymenoptera
Hemiptera
Diptera
Diplopoda
Collembola
Coleoptera
Arachnida
Amphipoda
Acarina
0
Fig 2: Mean abundance of invert. orders from recently burnt and long unburnt
sites. Error bars represent SE of the mean (n = 5). * significant 0.05.
0.0
-24.0
-24.5
-25.0
-25.5
-26.0
-26.5
-27.0
d 13C
Fig 3: Dual isotope plot for δ13C & δ15N values of invert predators (spiders),
herbivores (Hemiptera: Ontiscus barbaris) and Buttongrass (Gymnoschoenus
sphaerocephalus) at recently burnt ( ) and long unburnt sites ( ).
Key findings to date
• Buttongrass moorlands support a diverse invertebrate fauna with almost 6000 individuals from 24 orders collected.
• Greater than 25% were spiders (Fig. 2).
• Recently burnt sites supported significantly more Arachnids, Coleopterans and Hymenopterans than long unburnt sites.
• Preliminary results to date show a decrease in δ13C but no change in δ15N with time since fire for many taxa (Fig. 3).
Implications and future direction
Further analyses of δ13C and δ15N from representative invertebrate and plant taxa will be used to trace the flow of nutrients through the
ecosystem and determine the impact of fire on food web structure and predict the stability of ecosystem processes and nutrient cycling.
Acknowledgements
Michael Driessen & Jayne Balmer (DPIW) , Lake St Clair Parks & Wildlife Staff, Alastair Richardson, Peter Davies & Todd Chaudry (UTas), Bron Appleby, Josie Lawrence, Amanda Ashton & Kath Whittaker