Kr. Blanche et al., Rainfall-contingent detection of fire impacts: Responses of beetles to experimental fire regimes, ECOL APPL, 11(1), 2001, pp. 86-96
Fire is a valuable tool for maintaining biodiversity in many ecosystems thr
oughout the world, particularly tropical savannas. An understanding of the
impact of different fire management regimes is essential if desired outcome
s are to be achieved. Numerous studies have shown patchy and inconsistent r
esponses of insects to fire. This may be because detection of responses to
fire was contingent upon environmental conditions such as rainfall. Here we
examine how ground-active beetles respond to savanna fires and how detecti
on of these responses is contingent upon the amount of rain occurring prior
to sampling. We also test the extent to which family-level responses are "
taxonomically sufficient" to reveal responses detected at the species level
.
Our study was part of a landscape-scale, replicated field experiment at Kap
alga Research Station, Kakadu National Park, in the wet-dry tropics of nort
hern Australia. Over a 7-yr period (1988-1994), ground-active beetles were
monitored using pitfall traps. In 1988 and 1989 fires were excluded from al
l of the 15-20-km(2) experimental compartments. From 1990 to 1994, three ex
perimental fire regimes were applied: burning annually early in the dry sea
son (May/June), burning annually toward the end of the dry season (Septembe
r/October), and exclusion of fire. Each treatment was replicated three time
s. Pitfall trapping began at the start of the wet season in 1988 and contin
ued twice a year, in the middle of the dry season (July/August) and early i
n the wet season (November/December), until the conclusion of the study at
the end of 1994.
Beetle abundance in dry-season samples was too low and variable for any mea
ningful statistical analyses. For wet-season samples, repeated-measures ana
lysis of variance revealed that fires late in the dry season significantly
reduced the number of ground-active beetle individuals, families, and speci
es. However, detecting this decline was contingent upon substantial amounts
of rain falling prior to sampling. In addition, canonical correspondence a
nalysis (CCA) showed that fire intensity and amount of rainfall prior to sa
mpling interacted to change abundance of several common beetle families and
species. Fires late in the dry season appeared to disadvantage some common
wet-adapted taxa, while favoring other common groups that were dry-adapted
. A Mantel test, comparing dissimilarity matrices based on abundances of co
mmon families and species, confirmed that analysis at the family level was
taxonomically sufficient to reveal most responses found by analysis at the
species level.
The strong influence of rainfall on detection of beetle responses to fire p
oses a challenge to managers using such insects to monitor fire-prone ecosy
stems. Unless monitoring strategies include rainfall in the sampling design
, burning prescriptions may not appear to produce desired or consistent out
comes; fire impacts may be there, but go undetected.