Biodiversity and Conservation (2006)
DOI 10.1007/s10531-005-5411-z
Springer 2006
-1
The short-term impact of forest fire on soil
invertebrates in the miombo
G. SILESHI* and P.L. MAFONGOYA
World Agroforestry Centre (ICRAF), Zambia-ICRAF Agroforestry Project, P.O. Box 510089,
Chipata, Zambia; *Author for correspondence: Address: P.O. Box 511118, Chipata, Zambia.
(e-mail: [email protected] or [email protected])
Received 29 November 2004; accepted in revised form 31 March 2005
Key words: Forest fire, Macrofauna, Miombo, Soil invertebrates
Abstract. A study was conducted in eastern Zambia to assess the impact of fire on soil invertebrate
communities between December 2003 and November 2004. Soil samples were collected 4 times
from secondary miombo forest patches that were burnt in July–September 2003 and 2004 and
patches not affected by fire. Macro-invertebrates were hand-sorted from soil samples and their
population densities computed. The total number of orders per sample and the population density
of Annelida, Chilopoda, Arachnida and some Hexapoda were lower under burnt forest patches
compared to unburnt ones throughout the study period. Although the difference between burnt and
unburnt patches in populations of other taxa such as Lepidoptera and Diptera remained below the
threshold of statistical significance, fire appeared to have reduced their densities. It is concluded
that fire can alter the structure of soil invertebrate communities through direct mortality or by its
effect on availability of food resources. Further studies are needed to establish the linkage between
fire, invertebrate community structure, ecosystem processes and floristic composition of the
miombo.
Introduction
The miombo is the largest continuous dry deciduous forest in the world
extending across much of central, eastern and southern Africa including
Angola, DR Congo, Malawi, Mozambique, Tanzania, Zambia and Zimbabwe
(Campbell et al. 1996). It is a woodland vegetation dominated by slow growing
mainly deciduous trees of the genus Brachystegia, Julbernardia and Isoberlinia
forming a 15–20 m high, single storey, light but closed canopy (Lawton 1978).
Recently it has been identified as one of the world’s biodiversity hotspots that
need a global conservation strategy (Mittermeier et al. 2003). Fire is the major
ecological factor, which regulates the dynamics of miombo ecosystem (Lawton
1978). Forest fires are almost invariably started by people (Frost 1996) deliberately for instance by livestock owners seeking to promote a green flush for
their animals, by rodent hunters (Ajayi and Kwesiga 2003), by people creating
firebreaks around their homesteads, people clearing land for cultivation,
smoking out beehives or making charcoal in the forest (Piearce 1986). It is
generally agreed that frequent uncontrolled fires are harmful both to vegetation and soil (White 1993).
Conservationists are concerned about large vertebrates such as mammals
and birds or forest plant diversity, while lesser emphasis is placed on invertebrates. Invertebrates are important ecological agents in miombo woodlands
(Frost 1996). Recent evidence shows that there is a tight link between soil
invertebrates and fundamental soil functions such as carbon mineralization,
the establishment of complex organic material, nitrogen fixation and denitrification, all important determinants of plant growth (Bignnell and Eggleton
2000; Lavelle et al. 2003). Some invertebrates such as termites and earthworms
are major determinants of soil structure, important conservators of fertility and
landscape architects (Jones et al. 1994; Bignnell and Eggleton 2000). Many soil
invertebrates are also important players in terrestrial food webs, where they
serve as food sources for many predaceous invertebrates and vertebrates.
However, there is a dearth of information on the effect of burning on the
biodiversity of invertebrates and the ecosystem functions they provide. The
objective of this preliminary study was to assess the impact of forest fire on soil
invertebrate communities.
Materials and methods
The study area
The study was conducted at Msekera (1339¢ S, 3234¢ E, altitude 1025 m) in
Eastern Province of Zambia. The climate of eastern Zambia is subtropical with
three distinct seasons: the warm rainy season (November–April), cool-dry
winter (May–August) and hot-dry season (September–October). The rainfall
pattern is unimodal (mean annual = 960 mm, range = 887–1014 mm) with
approximately 85% of the rains falling during December–March. The average
air temperatures vary between the means of 15 and 18 C during the coldest
months of June and July; and between 21 and 26 C during the hottest months
of September and October.
Sampling invertebrates and data analysis
Three sampling sites about 2 km apart from each other were located in the
Msekera area where patches of the secondary miombo forest was burnt by
forest fires in July–September 2003 and 2004. All the fires in the study area
were started by people, either building fire-breaks around their fields or by
rodent hunters. These uncontrolled fires subsequently led to outbreaks of large
wildfires of variable intensity, and affected patches of the secondary forest. At
each site, soil samples were collected from forest patches that were burnt and
adjacent patches that were not affected by fire (hereafter referred to as unburnt)
4 times between December 2003 and November 2004 to coincide with contrasting periods in the climatic cycle of the study area; (1) in December – the
beginning of the rainy season, (2) in February – mid-rainy season, (3) in July –
mid-dry season, and (4) in November – end of the dry season. A total of 16, 26,
18 and 28 samples were collected from the same patches in December, February, July and November, respectively. Half of the samples were from the
burnt patches in the same general area and the other half from unburnt patches
in adjacent areas. A metallic monolith (25 cm · 25cm and 25 cm depth) (Lavelle
et al. 2003) was placed over a randomly selected spot and, it was driven into the
soil to the ground level using a metallic mallet. The soil was removed from the
monolith and macrofauna were hand-sorted from each sample and counted.
Here macrofauna is defined as an invertebrate group found within soil samples,
which has more than 90% of its specimens visible to the unaided eye (Lavelle
et al. 2003). The invertebrates collected were then separated into higher taxa
(family and order) and their densities per square meter were computed. Some
invertebrate taxa occurred erratically in both burnt and unburnt patches. The
populations of those taxa that occurred in sufficient numbers to be analyzed in
a statistically meaningful manner were compared using two-sample (burnt and
unburnt) t-test assuming unequal variance. Data were subjected to t-test using
PROC TTEST (SAS 2002–2003) after transforming densities into logarithms.
Results
Effect of fire on the number of invertebrate orders
From a total of 88 samples collected, 2892 individual macro-invertebrates
belonging to the phyla Annelida, Mollusca and Arthropoda were recovered. The
total number of orders per sample (Figure 1) recorded under the patches burnt in
July–September 2003 was significantly lower than that under unburnt patches in
December 2003 (t = 3.3, n = 14, p = 0.005), February 2004 (t = 3.1, n = 22,
p = 0.006) and July 2004 (t = 5.2, n = 14, p = 0.0001). Though the difference
was not significant at the prescribed probability level in November (t = 2.0,
n = 26, p = 0.056), the number of invertebrate orders was lower under patches
burnt in July–September 2004 compared to unburnt patches.
Effect of fire on population density of various invertebrate taxa
The population of total macrofauna (all macro-invertebrate taxa combined)
under the burnt patches was significantly lower than that under the unburnt
patches in December (t = 4.3, n = 14, p = 0.005). Even though the difference
between the two was not significant in February (t = 1.0, n = 22, p = 0.327)
and July (t = 1.2, n = 14, p = 0.253), total invertebrate densities were lower
under burnt patches (Figure 1). The densities under unburnt patches and
patches that were burnt in July–September 2004 did not differ in November
2004 (t = 1.2, n = 26, p = 0.238).
Figure 1. Number of orders per sample (a) and total macro-invertebrate densities (numbers of all
invertebrates per square meter) (b) under burnt and unburnt miombo in December 2003–
November 2004 at Msekera, estern Zambia. Vertical lines represent the standard errors of means.
Comparisons of densities of (numbers m 2) of the various taxa under burnt
and unburnt patches during the wet season and dry season are presented in
Tables 1 and 2, respectively. Out of the 65 valid comparisons presented in the
tables, 79% showed a decrease in densities in the burnt patches, while only
17% showed an increase in burnt patches. During the wet season, invertebrate
densities were significantly (p<0.05) lower in the burnt patches in 10 out of the
33 valid comparisons (Table 1). During the dry season, densities were significantly lower in the burnt patches in seven out of the 33 valid comparisons
(Table 2). The populations of Annelida (earthworms), Chilopoda (millipedes),
Arachnida (spiders and mites) and some Hexapoda especially Hymenoptera
(mainly ants), Hemiptera (bugs) and Coleoptera (beetles) under burnt patches
were consistently lower compared to those under unburnt patches. Among the
Coleoptera taxa affected by fire were Staphylinidae, Carabidae, Curculionidae
and Scarabaidae whose populations remained lower under burnt miombo
patches throughout the study period (Tables 1 and 2). Diplopoda (centipedes)
and Isoptera (termites) showed variable response to burning. Centipede populations were significantly lower in the burnt patches in December 2003, while
in February 2004 their population was higher in the burnt patches. Termite
populations though lower in burnt patches in December 2003 (Table 1),
showed an increase afterwards in the same patches compared with unburnt
patches (Table 2).
Discussion
Overall, fire appeared to have a negative effect on soil invertebrates. However,
some invertebrates were significantly more affected than the others.
Haplotaxida
Neogastropoda
Geophilomorpha
Juliformia/Isopoda
Araneae, Acari, Solifugae
Isoptera
Blatodea
Hemiptera
Neuroptera
Coleoptera
Annelida (Oligochaeta)
Gastropoda
Diplopoda
Chilopoda
Arachnida
Hexapoda (Insecta)
Not identified
Not identified
Various
Various
Various
Various
Blatellidae
Cydinidae
Myrmeleontidae
Carabidae
Staphylinidae
Scarabaiedae
Tenebrionidae
Curculionidae
Total Coleoptera
Various
Various
Formicidae
Family
48.0
2.0
24.0
72.0
18.0
396.0
2.0
8.0
2.0
22.0
18.0
42.0
16.0
14.0
112.0
32.0
10.0
246.0
14.0
2.0
2.0
46.0
6.0
78.0
2.0
6.0
0
4.0
2.0
16.0
6.0
0.0
28.0
4.0
0
36.0
0.212
*
0.023
0.584
0.265
0.046
*
0.536
0.351
0.075
0.034
0.113
0.338
0.062
0.004
0.251
0.078
0.096
41.8
16.0
33.2
65.2
7.4
615.4
1.2
4.9
0
9.8
11.1
60.3
12.3
7.4
100.9
45.5
2.5
166.2
3.7
0
51.7
8.6
8.6
376.6
0
0
1.2
2.5
1.2
29.5
16.9
4.9
51.7
8.6
0
88.6
Burnt
Unburnt
Probability
Unburnt
Burnt
February 2004
December 2003
*Density of taxon erratic and statistically meaningful comparison was not possible.
Diptera
Lepidoptera
Hymenoptera
Order
Class
0.0002
0.022
0.022
0.001
0.967
0.664
*
0.039
0.337
0.138
0.004
0.267
0.885
0.511
0.051
0.481
0.337
0.650
Probability
Table 1. Densities (numbers m 2) of major soil macro-invertebrate taxa in burnt and unburnt patches of secondary miombo and significance of difference
(probability of p>t) in densities during the rainy season at Msekera, eastern Zambia.
Haplotaxida
Neogastropoda
Geophilomorpha
Juliformia/Isopoda
Araneae, Acari, Solifugae
Isoptera
Blatodea
Hemiptera
Neuroptera
Coleoptera
Annelida (Oligochaeta)
Gastropoda
Diplopoda
Chilopoda
Arachnida
Hexapoda (Insecta)
Not identified
Not identified
Various
Various
Various
Various
Blatellidae
Cydinidae
Myrmeleontidae
Carabidae
Staphylinidae
Scarabaiedae
Tenebrionidae
Curculionidae
Total Coleoptera
Various
Various
Formicidae
Family
1.6
1.6
8.0
25.6
4.8
38.4
0
11.2
3.2
1.6
1.6
33.6
11.2
14.4
64.0
1.6
3.2
248.0
0
0
0
8.0
2.0
208.0
4.0
2.0
0
2.0
0
2.0
8.0
6.0
20.0
0
2.0
10.0
0.343
0.343
0.087
0.041
0.566
0.196
0.184
0.071
0.168
0.878
0.343
0.009
0.599
0.229
0.028
0.343
0.687
0.008
5.7
1.1
2.3
25.1
11.4
42.3
*
11.4
*
2.3
6.9
12.6
8.0
3.4
33.1
6.9
*
37.7
0
6.9
1.1
6.9
2.3
74.3
*
3.4
*
2.3
0
13.7
5.7
0
22.9
1.1
*
40.0
Burnt
Unburnt
Probability
Unburnt
Burnt
November 2004
July 2004
*Density of taxon erratic and statistically meaningful comparison was not possible.
Diptera
Lepidoptera
Hymenoptera
Order
Class
0.032
0.454
0.559
0.049
0.073
0.238
*
0.175
*
0.999
0.041
0.735
0.630
0.082
0.162
0.139
*
0.703
Probability
Table 2. Densities (numbers m 2) of major soil invertebrate macrofauna taxa in burnt and unburnt miombo and significance of difference (probability of
p>t) in densities during the dry season at Msekera, eastern Zambia.
Earthworms, millipedes, ants and beetles appeared to be more sensitive to
forest fire. In the study area, fire occurs when many beetles are over-wintering
(Sileshi 2000; Sileshi and Kenis 2001), and are probably unable to escape the
flames. Although the difference between burnt and unburnt patches in densities
of other taxa (e.g. Lepidoptera, Diptera) remained below the threshold of
statistical significance, it appears that fire had considerable impact on their
populations. Lepidoptera larvae and pupae were absent from the burnt patches
during the rainy season unlike in unburnt patches. This is in agreement with
the observation that forest fires are partly responsible for the decline of edible
insects such as Lepidoptera caterpillars in parts of the miombo (Holden 1991;
Mbata et al. 2002). Other invertebrates such as snails (Gastropoda), centipedes
and termites showed variable response. During a fire, soil temperatures
decrease with increasing depth and soil organisms such as termites that live
deep in the soil may survive surface fires (Gillon 1983). Termites which are
capable of storing food in their nests also remain remarkably unaffected by
forest fire (Athias et al. 1975).
As can be seen in Tables 1 and 2, fire that occurred in July–September 2003
had negative impacts on populations of certain taxa throughout the rest of the
year. This may be due to an indirect effect of fire on food resources rather than
direct mortality. During a fire, nutrients are abruptly mineralized, and the
decrease in food sources may affect soil invertebrates. The effects of fire may
include large flux of nutrients leaving the ecosystem through volatilization and
rapid mineralization of nutrients, losses of nutrients through accelerated erosion
and leaching, adverse changes in hydrology, degradation of soil physical properties, and losses in microbial populations and associated processes (Neary et al.
1999). Thus the effects of repeated fires may be both severe and cumulative.
Direct mortality of fire-prone taxa, decrease in food resources and the other
adverse changes in soil processes may then alter the structure of invertebrate
communities as well as the flora. The review by Bignnell and Eggleton (2000)
indicated that removal of certain invertebrates such as termites could adversely
affect the hydrology of soils ultimately changing floristic compositions.
Frequent late season fires in addition to changing species composition also
affect vegetation structure transforming woodlands to open, tall grass savanna
with only isolated, fire tolerant canopy trees and scattered under-storey trees
(Frost 1996; Gambiza et al. 2000). A severe late fire may destroy the overstorey entirely and the resultant ‘fire hole’ may be rapidly colonized by
undesirable thicket shrubs and scramblers (Piearce 1986). This could render
parts of the miombo prone to invasion by alien plant species (e.g. Lantana
camara), which can in turn increase the danger of fire. Therefore, further
studies are needed to establish the linkage between fire, soil invertebrate
community structure, ecosystem processes and floristic composition of the
miombo.
Complete protection of the miombo from fire may be an impracticable ideal.
In Zambia, early burning has been promoted as a means to reduce fire damage
since 1978. However, there are practical problems to carry out early burning
successfully (Piearce 1986), and people have so many good reasons to use fire
(Holden 1991). It seems that forest fires will continue to threaten biodiversity in
the miombo unless concerted efforts are made to put in place appropriate
control measures.
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