Proceedings of the conference held 8-9 October 1994, Footscray, Melbourne
Biodiversity Series, Paper No. 8
Department of the Environment, Sport and Territories, 1996
15. The impact of fire on soil invertebrates in E. regnans forest at Powelltown, Victoria
Museum of Victoria
Fire is an important factor in the development and maintenance of many forests in Australia. The current study examined the abundance and diversity of soil invertebrates at two sites in Eucalyptus regnans forest at Powelltown, Victoria. Seasonal distribution of soil arthropods was examined and the effects of fire (Ash Wednesday wildfire of 14/2/1983) on general abundance and seasonal distribution at the study sites were recorded. Correlation of arthropod abundance with soil moisture, rainfall and temperature were also examined. The findings are compared with other relevant fire and arthropod research in Australia.
The invertebrate fauna of E. regnans forest soil was dominated (numerically) by Acarina (mites) and Collembola. In general, arthropods were more abundant during winter and spring and abundance of most arthropod taxa was correlated with rainfall and/or temperature.
Following the fire, total arthropod and nematode abundance increased. The increase in arthropod abundance was primarily due to increased abundance of Acarina, Collembola and insect larvae. The abundance of other arthropod taxa was either unaffected or decreased after fire. Most taxa recovered to pre-fire abundance within 11 months. The Crustacea, particularly Amphipoda, were the most adversely affected by fire. Amphipoda were absent from all post-fire samples.
Fire intensity was an important factor affecting the recovery of population levels. Conditions prevailing after a high intensity fire adversely affected more invertebrates, particularly during winter months. Increased exposure of the ground layer after a high intensity fire may have significantly altered invertebrate populations.
Species composition of Formicidae (ants) and Pseudoscorpionida was altered by fire.
Key words: fire impact, soil invertebrates, abundance, diversity, fire intensity, E. regnans forest, Victoria.
Wildfires have probably been common in southern Australia since the late Tertiary (Main 1976). The biota of forest and woodland communities in the region are expected to have highly evolved adaptations to fire (Whelan & Main 1979). As a result, periodical fires are unlikely to constitute major disturbances in fire-adapted communities. Furthermore, the distribution of some communities, including Eucalyptus regnans, is maintained by fire. Fire is also indicated in the development and maintenance of some faunal communities (Huhta 1971; Recher et al. 1974).
The impact of fire on the soil and litter invertebrate communities in Australian forests has seldom been studied. The structure of soil and litter invertebrate communities has traditionally been poorly studied because of the complexity of organisms, a lack of knowledge of their biology and the limitations of techniques for examining the system as a whole (Usher et al. 1978). The diversity of invertebrates presents some formidable problems in studying and understanding the role of fauna in the soil system, particularly in Australia where the difficulties are accentuated by a lack of taxonomic work (New 1984). Attempts to quantify the influence of fire in Australia on soil and/or litter invertebrates have been reviewed by Campbell and Tanton (1981), Leonard (1974) and Suckling and MacFarlane (1984).
Each year many areas of Australian forest and woodlands burn, either by prescribed burning or wildfire, and fire is increasingly utilised by forest managers to reduce fire hazard. It is important to understand the impact of fire on invertebrates as they are crucial to the stability, regulation and functioning of all forest ecosystems (Reichle 1977; Gill 1981; Flinn et al. 1983). Future use of fire in the management of Australian forests should be based on a sound knowledge of its potential impact upon components of the community.
Two sites were selected in Eucalyptus regnans forest near Powelltown, approximately 70 km east of Melbourne (Figure 15.1). Similarity in elevation, aspect, soil type, fire history and plant community were factors in site choice. A full description of each site, including rainfall, temperature, geology, geomorphology, soils, vegetation and fire history are given in Coy (1991).
The wildfire which burnt through the area in February 1983 varied in intensity as do all wildfires. As a result the two sampling sites were burnt at recognisably different intensities. Site 1 was subjected to a low intensity ground fire resulting in minimal mid-storey and canopy damage. By comparison, the high intensity ground fire at site 2 destroyed the canopy, mid-storey and under-storey vegetation, greatly increasing the exposure levels of the underlying soil.
At the two study sites, four 10 m by 10 m grids were established for soil core, litter and vegetation sampling. Four or eight soil cores (6.3 cm diameter x 10 cm depth) were collected each month for ten months prior to the "Ash Wednesday" fires on Feb 14 1983 and ten months after the fires at site 1. Monthly sampling at site 2 commenced in June 1982, seven months before, and continued until November 1983; eight months following the fire. The position of soil core samplings were determined by random co-ordinates on the grids and invertebrates were extracted from the cores using modified Berlese funnels. This method of extraction gives a measure of activity rather than absolute abundance. All invertebrates collected were sorted to ordinal level and the Pseudoscorpionida and Formicidae were further identified to species.
The moisture content of all samples was measured by oven drying extracted samples to a constant dry weight.
For comparing invertebrate abundance during the pre- and post-fire periods of the study, only data collected in comparable months from each period were included in analyses. Statistical analyses were selected from the SPSSX software package (Nie et al.1986). T-tests and one-way ANOVA were used to compare sample means from different months, sites and conditions (burnt versus unburnt). These tests are strong and robust (Ferguson 1971; Sokal & Rolfe 1969; Zar 1974). Pearson Correlation Coefficients were used to examine for correlation of invertebrate abundance with soil moisture content and Spearman Correlation Coefficients were calculated for invertebrate abundance and monthly rainfall, minimum and maximum temperatures.
The major limitation of the study was the lack of an ongoing control not affected by fire. This limits the ability to separate annual variation from the variation due to fire (treatment). Consequently the variation in abundance of invertebrates can only be discussed in terms of a comparison of the composite conditions prevailing before and after fire. Similarly, the brevity of pre-fire sampling restricts the analysis of annual variation without fire.
Replicated samplings are restricted to the same forest, in close proximity to one another and are therefore pseudo-replicates rather than true replicates from a series of forest blocks. Pseudo-replicates are not truly independent of one another.
15.4.1 Total invertebrate abundance
Total invertebrate abundance significantly increased at both sites following fire (Figure 2a). The increased abundance of invertebrates collected immediately after fire in the current study (February 1983) suggests that many soil invertebrates survive fire (Figures 2d, f, g, j, l, o, r). Individuals may have survived fire by moving down the soil profile, particularly the true soil fauna with suitable morphological attributes, such as size and shape, for movement through the inter-ped spaces. Rice (1932), Coults (1945) and Gillon (1970) all report invertebrates surviving fire either under charred forest debris on the ground or in the soil.
The concentration of invertebrate survivors in the soil and on its surface may be responsible for the increased number of invertebrates in samples immediately after fire. The three-dimensional above-ground habitat was reduced to an almost two-dimensional habitat (habitat compression), greatly decreasing the area available for invertebrates. Majer (1980), Whelan et al. (1980), O'Dowd and Gill (1984) agree that habitat simplification is important in the apparent increase in invertebrate abundance following fire. However, these authors used pitfall trapping techniques which are particularly susceptible to the effects of habitat compression.
Few studies on the effects of fire on soil and litter invertebrates have been carried out in Australia, and the results of the current study appear to conflict with those of most other Australian studies. Decreased abundance of invertebrates after fire has been recorded in most Australian research (eg. Springett 1976; Majer 1980; Bornemissza 1969; McNamara 1955; O'Dowd & Gill 1984; Abbott 1984). Only one study (Whelan et al. 1980) reported increased invertebrate abundance following fire, which was attributed to habitat compression.
In the current study, the increase in total invertebrate abundance was primarily due to an increase in micro-invertebrate groups, mites (Acarina) and springtails (Collembola); whereas abundance of most mesofaunal invertebrates decreased following fire. The response of the most numerically abundant taxa in collections significantly affect the pattern described by the total invertebrate numbers. The taxa present in collections and their respective abundance is a consequence of collection technique (Southwood 1978; Greenslade & Greenslade 1971; Huhta 1971; Majer 1980). In most other Australian studies the techniques used minimised the collection of the micro-invertebrates.
Some non-Australian studies are also at variance with the current study as authors report decreased micro-invertebrate abundance following fire (eg. Pearse 1943; Huhta et al. 1967; Buffington 1967). The comparability of these studies should be considered with the following points in mind:
- Australia has a long history of fire in the development and evolution of its flora and fauna;
- different sampling techniques were used resulting in different components of the invertebrate fauna being examined; and 3) different vegetation communities, soil types and fire regimes were studied.
Post-fire activation of dormant invertebrates and hatching of diapausing eggs may significantly contribute to increased invertebrate abundance after the fires. Many invertebrates survive unfavourable conditions by entering a dormant or resistant state (diapause or aestivation) in which development is arrested (Tauber & Tauber 1976; Highnam & Hill 1977). The activation of these states is controlled by external stimuli such as temperature, humidity and/or photoperiod which brings about an alteration in hormonal levels of the animal. Low moisture levels and high temperature prior to the Ash Wednesday fires may have increased the prevalence of dormant forms of invertebrates at the study sites.
Survival of invertebrates subsequent to fire is affected by a variety of biotic and edaphic factors. Important biotic components include food source (plant or prey), competition, predation (including parasites) and the relationship between species. Edaphic factors important to soil organisms include weather (precipitation, insolation, temperature and wind), microclimate (soil moisture, humidity and temperature), chemical (nutrients) and physical (soil texture and structure).
Increased abundance of some invertebrate orders during the post-fire autumn and spring of the current study may have been affected by the periodic litterfall of the forest (Figure 15.2a & b). Litterfall in E. regnans forests is characterised by periodic increase coinciding with barkfall in autumn and spring (Ashton 1975). Plant regrowth and accumulation of litter following fire represents an increase in resources with time after fire, and may differ from the pre-fire resource in terms of quality as well as quantity. Bornemissza (1969) reported the speed of reinvasion by soil invertebrates following fire was associated with the accumulation of leaf litter under trees and with the regeneration of herbs and shrubs in exposed areas.
Many fungal fruiting bodies were obvious at both study sites after the fires and increased growth of fungi has been recorded after fire. These may also be an important food resource to fungivorous invertebrates such as many Collembola.
After fire, particularly a high intensity fire, an initial increase in the abundance of some invertebrates is followed by a significant winter decline (eg. Figures 15.2b, l). Invertebrates which survive the fire, or which are recruited to the post-fire population, share a depleted resource (habitat compression). It is not uncommon for a species to utilise its entire available food source with a resulting sharp reduction in population due to starvation (Ross 1965). Rice (1932) reported reduction in the abundance of many species during the winter following a fire.
The abundance of some predators (Chilopoda, Hymenoptera, Opilionida and Araneae) decreased after fire (Figures 15.2b, m,p,q). Reduced predation would influence the recovery of prey populations, therefore, conditions prevailing after high intensity fire may favour reproductive success and survival of prey populations.
Note: Standard deviations for each abundance are given in Coy (1991).
Annual variation in climate and general climatic region are important regulators of abundance of soil invertebrates. Correlations between abundance of various orders and soil moisture, rainfall and temperature were detected during this study (Coy 1991). The soil reflects the general climate but moderates its extremes (Ross 1965). Most soil inhabitants are poorly adapted for surviving periods of low moisture and high temperatures, particularly in moist forest environments, and survive only within narrow ranges of climatic or microclimatic variation. Many have poor control of water loss-particularly the Crustacea and myriapods which lack an impermeable cuticle. When humidity levels in their environment fall appreciably, evaporation from the body surface and the respiratory organs increases (Humphreys 1975) and rapidly leads to dehydration. Increasing temperatures serve to increase evaporation in many invertebrates and thereby increase the rate of dehydration.
The removal of vegetation by fire, especially at site 2 of the current study, exposes the ground layer and habitats to greatly increased fluctuations in microclimate. Changes in microclimate affect the abundance and activity of soil invertebrates (Wallwork 1976). The diurnal and seasonal rhythm of temperature and humidity is greatest in exposed areas and progressively less in more shaded or protected areas (Ross 1965). Post-fire exposure of the ground layer, resulting from the different fire intensity at each site was far greater at site 2. The fall in invertebrate abundance during the post-fire winter at this site coincided with the onset of severe frost and low temperatures (Bureau of Meteorology, unpub. data), and may be related to increased exposure to climatic extremes.
An increase in soil pH following fire in the present study (Coy 1991) reflects changes in chemical properties such as the cation exchange capacity. The chemical composition of soil often determines the abundance and distribution of invertebrates (eg. Ross 1965). Furthermore, vegetation growth and therefore the food source of some invertebrates can be affected by the chemical composition of soil. Fire, particularly high-intensity fire, causes the mineralisation of much of the litter and vegetation; increases in a number of nutrients including nitrogen, phosphate, potassium, calcium, sodium and magnesium follow (O'Connell et al. 1979; Abbott et al. 1984).
[Table 1] Frequency of invertebrate Orders collected in soil samples (1= present in all months).
Order coding:- Is, Isopoda; Am, Amphipoda; Ch, Chilopoda; Di, Diplopoda; Pa, Pauropoda; Sy, Symphyla; Co, Collembola; Pr, Protura; Po, Psocoptera; He, Hemiptera; Cl, Coleoptera; Hy, Hymenoptera; La, Insect Larvae; Ac, Acarina; Op, Opilionida; Ar, Aranaea; Ps, Pseudoscorpionida; Pe, Onychophora; Si, Siphonaptera; To, Thysanoptera; Th, Thysanura; Or, Orthoptera; Ip, Isoptera; Sc, Scorpione; Ta, Tardigrada.
|Change following fire||D||D||-||-||I||-||I||D||-||I||-||-||-||I||-||-||-|
|Change following fire||D||D||D||-||D||-||-||-||I||-||-||D||I||I||D||D||-|
Notes: D, decrease; I, increase. (Orders codes as for Table 1). *** = P <0.001, ** = P<0.005, * = P<0.01
|Difference between sites||Is||Am||Ch||Di||Pa||Sy||Co||Pr||Po||He||Cl||Hy||La||Ac||Op||Ar||Ps|
|Before fire, Year 1||NS||NS||NS||NS||NS||NS||NS||NS||NS||**||NS||*||NS||NS||NS||NS||NS|
|After fire, Year 2 fire||NS||NS||*||NS||***||NS||*||NS||NS||NS||NS||*||NS||***||NS||NS||NS|
Order codes as for Table 1
15.4.2 The diversity and abundance of invertebrate orders
Variation in the total invertebrate abundance is of little value when attempting to evaluate ecologically significant changes in community structure. Accordingly, specimens were identified to ordinal level. In general, the number of orders detected increased marginally following the fires but fewer orders were represented in each monthly collection (Table 1). If orders which represented less than 0.01 per cent of the individuals collected were excluded from the comparison, the number of orders represented decreased following fire, due to the absence of Amphipoda. Plowman (1979), Bornemissza (1969) and Springett (1976) also report a decrease in the number of taxa represented after fire. Campbell and Tanton (1981) suggest that the reduction in numbers observed by some authors was due to inadequate techniques. It follows from this that the reduction in species richness may be an effect of low densities rather than localised extinctions. Only Whelan et al. (1980) reported an increase in the number of taxa represented after fire.
The response of each order to the conditions prevailing after fire, as compared to their respective abundance before fire, varied (Table 2). At site 1, three orders decreased in abundance following fire, and four orders increased. Following a higher intensity fire at site 2, seven orders decreased in abundance while three orders increased. The variation in responses of the orders as compared to the response of the invertebrates as a whole, accentuates the importance of studying lower taxonomic groupings of animals.
Abundance of invertebrates collected from each site was compared to investigate similarities or differences arising from site differences, and from the difference in fire intensity experienced at each site. Similarity of invertebrate abundance at the two sites before the fires, coupled with a significant difference between them after the fires, implicates fire intensity as affecting the response to post-fire conditions (Table 3). Before the fires only the Hemiptera and Hymenoptera abundance was significantly different at the two sample sites of the current study. Both of these orders were poorly represented in samples. Following the fires the abundance of five orders were significantly different at the two sites, including four of the most abundant orders, Chilopoda, Pauropoda, Collembola and Acarina.
Abundance of both orders of the Crustacea represented in the study, Isopoda (slaters) and Amphipoda (land hoppers), was significantly reduced following fire (Table 2). The absence of crustaceans immediately following fire suggests that much of the existing population was destroyed. Amphipods were absent from all samples collected during the post-fire period of the study (Figure 2b). The Isopoda, although present in four samples collected after the fires, was also greatly reduced in frequency and abundance (Figure 2c).
The Crustacea are predominantly marine organisms (Schram 1986) and terrestrial forms are usually poorly adapted to their terrestrial conditions. Their general strategy is to avoid typical terrestrial conditions and they are often restricted to cool, moist habitats (Edney 1960). Water economy, particularly the control of water loss, is a crucial factor affecting the survival and distribution of terrestrial crustaceans. Unlike many other terrestrial invertebrates, the Crustacea lack a waterproof integument, therefore water loss in terrestrial crustaceans is greater than in insects (Edney 1957a; 1957b). Consequently the Crustacea are much less successful on land than insects or arachnids.
The control of water loss is essential for effective gaseous exchange in respiratory organs, the maintenance of body fluids and breeding (Edney 1957a; Waloff 1941). Adaptations to terrestrial conditions within the Crustacea are most advanced in the isopods (slaters) and include specialised respiratory organs and corresponding changes in the circulatory system (Silen 1954). The amphipods (land hoppers) retain a more archaic respiratory system which requires moist air to function (Waterman & Chase 1960). Water uptake mechanisms also limits the distribution of most terrestrial crustaceans to moist habitats with free water and a water vapour saturated atmosphere. Temperature acts in conjunction with moisture levels to restrict the survival of crustaceans on land (Schram 1986; Edney 1951). Some correlation of crustacean abundance with rainfall and temperature was evident during the current study (Coy 1991).
Given the lack of adaptations to terrestrial conditions, it is not surprising that the Crustacea are most seriously affected by the conditions prevailing during and after fire in the current study. The group least adapted to terrestrial conditions, the amphipods, appeared to suffer the greatest demise. Other Australian studies (O'Dowd & Gill,1984; Springett 1976) also reported a decrease in the abundance of Crustacea after fire. Conversely Majer (1980) reported a slight increase in isopod abundance following fire, but the numbers recorded were low.
The four classes of Myriapoda – Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda and Symphyla – represented an average of 4.14 per cent of the total specimens collected. Myriapods are primarily found in cool, damp habitats including the soil and leaf litter of forests and woodlands. Water loss presents a serious problem as an impervious, wax-like epicuticle is not normally present in this group of organisms (Blower 1974). Both moisture and temperature are regarded as important factors regulating distribution and activity in myriapods (Lewis 1974). Temperature is particularly important as it affects the strength of responses to moisture or relative humidity (Cloudsley-Thompson 1968). The abundance of Chilopoda, Diplopoda and Pauropoda in E. regnans forest soils was correlated with minimum and maximum temperature.
Centipedes and millipedes appear to survive the initial effects of fire with little alteration to the active populations. Conversely the two groups less well adapted to xeric conditions, the pauropodans and symphylans, were considerably reduced immediately after fire but increase rapidly to pre-fire levels. Two groups, the chilopods and the pauropods, exhibited significant changes in abundance after fire (Table 2) and between the sites after fire (Table 3); this indicates that response is affected by conditions prevailing after fire as well as fire intensity. Similar post-fire abundance patterns in each myriapod group were reported by Majer (1980), Springett (1976), O'Dowd and Gill (1984).
Chilopods are large, active and fast invertebrates which can move rapidly through the litter and soil to avoid decimation during adverse conditions. Habitat simplification immediately after fire would effect an increase in density and therefore an increased catch rate in samples. Subsequent decreases in chilopod numbers may have been due to limited resources, such as prey (Rice 1932), in burnt areas; alternatively declines may be due to a decreasing prey density as litter accumulated and increased the structural complexity of the habitat. Ahlgren (1974) reviewed several non-Australian studies and reported that chilopod populations were generally reduced after fire.
Diplopod abundance was very low throughout the study period. Many species survive dry periods as diapausing larvae in moulting chambers in the soil, or remain dormant deep in the soil (Gillon & Gillon 1976; Lewis 1974; Shaw 1968; O'Neill 1968; Toye 1967; Lawrence 1966). Dormancy is reported broken by rain (Lewis 1974) and may account for the appearance of diplopods with the onset of rain in April and May of the post-fire year in the present study (Figure 2e). Complete destruction of the litter layer and the organic rich upper soil layer would affect the ability of these saprophytic litter dwellers to survive post-fire conditions.
The taxonomy and biology of Australian pauropods is virtually unknown, yet they were the most abundant of the myriapods collected in soil samples during the present study (Figures 2d-g). At site 1, abundance of pauropods increased significantly following fire. These small animals are able to move deep into the soil thereby avoiding the immediate impact of fire. Although numbers collected decreased immediately after fire, a rapid recovery suggested their return to the upper soil layers and post-fire conditions favourable to pauropod survival. Starling (1944) suggests that fungal mycelia comprise a high proportion of their diet and post-fire conditions favour fungal growth (Ahlgren & Ahlgren 1965). These invertebrates lack trachea as well as a waterproof integument and are therefore liable to lose moisture through respiration and body surfaces. Conditions prevailing during winter after a high-intensity fire affected a decline in the number of pauropods collected. Increased exposure of the soil leads to decreased temperature and moisture vapour levels, in turn leading to decreased activity or survival of pauropods in the upper soil layers. Starling (1944) found that moisture and temperature were two main factors affecting pauropod distribution. Some correlation with temperature was detected in the present study.
The number of symphylans collected after fire in the current study was not significantly different from that collected prior to fire although an initial decrease was evident. These invertebrates, like other members of the Myriapoda, migrate deep into the soil during unfavourable conditions (Michelbaker 1938). Cloudsley-Thompson (1968) reports that the first instar of Symphyla never migrate to the soil surface and older Symphyla which do visit the soil surface retreat rapidly into the soil if disturbed. The rapid recovery in abundance suggests that suitable conditions prevailed to maintain the population. The proliferation of fungi and bacteria after fire (Ahlgren & Ahlgren 1965), a major component of the symphylan diet, assured an adequate food supply; in addition, a short development period (50-70 days) ensures the ability to rapidly increase when favourable conditions prevail.
15.4.5 Insecta and associated taxa
A total of 15 orders of insects were recorded during the study, but only six orders and the combined larvae of Hymenoptera, Coleoptera, Lepidoptera and Diptera were analysed. The abundance of other orders in samples was too low to allow for any meaningful trends to be discerned. The monthly abundance of Protura, Psocoptera, Hemiptera and Hymenoptera (Figure 2i, j, k, m respectively) were not significantly different throughout the study period. Changes in the abundance of these groups is therefore viewed with caution - they are more likely to be affected by statistical aberration caused by low numbers and associated data distributional problems. The trends in abundance of these groups are recorded in Table 2.
Insects are the most successful and adaptive group of invertebrates on land. All but the most primitive allies (entognathous hexapods) have developed efficient respiratory and circulatory systems, and an impermeable cuticle. Nevertheless, moisture conservation remains an important problem for all terrestrial insects. Response to the post-fire conditions varied within the orders of the Insecta.
The primitive insect allies represented in the current study are the Collembola (springtails) and the Protura. They rely upon moist habitats for terrestrial existence and are particularly adapted to these conditions (Cloudsley-Thompson 1988). Both groups were low in abundance immediately prior to and after fire (Figures 2h, i), suggesting that a large proportion of Collembola and Protura had succumbed to the summer conditions, retreated deep into the soil or entered dormant conditions before the fire. Many insects are able to withstand unfavourable conditions, including extremes of temperature, low humidity and scarce food supply, by entering diapause or aestivation and remaining dormant until conditions improve. Cryptobiosis and anahydrobiosis are extreme adaptations to dry conditions and have been recorded in Collembola (Poinsot-Balagner 1976; Wallace & Mackerras 1970). A summer fire may therefore be less detrimental than fires in other seasons because much of the population is in a form resistant to unfavourable (xeric) conditions.
The abundance of Collembola significantly increased after fire at site 1, but did not alter at site 2 (Table 2). Immediately after fire, numbers remained low, but the Collembola quickly recovered to exceed pre-fire abundance. Survivors of the fire, migrating back into the upper layers or becoming active, would form the basis of a new population. Similar conclusions were reached by Majer (1980) and other Australian research reported similar patterns of collembolan abundance following fire (Hutson & Kirkby 1984; Whelan et al. 1980). However, some studies of non-eucalypt forests recorded decreased densities following fire (Springett 1971; Huhta et al. 1967). The rapid increase in numbers of Collembola collected after the fire in the current study indicates that favourable conditions prevailed, including ample food and low predator populations. Site differences in year 2 suggest that the lack of change at site 2 after the fire was affected by a post-fire winter decline resulting from increased exposure levels after a high intensity fire. The recovery of Protura was more prolonged but by November 1983 had reached pre-fire levels (Figure 2i).
The apparent increase in Collembola following fire in the current study could not be conclusively attributed to the effects of fire, but may have been entirely affected by climatic or other environmental conditions independent of fire. Below average rainfall in 1982 followed by more favourable conditions in 1983 may have resulted in the same abundance pattern. In particular, the absence of a post-fire winter decline at site 2 suggests that conditions prevailing after fire has little effect on the group as a whole. Campbell and Tanton (1985) similarly concluded that environmental conditions preceding fire may affect subsequent survival and recovery patterns in soil invertebrate fauna.
Psocopteran abundance significantly increased after fire at both sites (Figure 2j) but it is likely that these individuals were contaminants from the canopy. After fire there is reduced or no mid- and lower-storey vegetation to intercept these insects should they enter the area. This illustrates the importance in understanding the possible methods by which populations may increase in samples following fire.
Most of the Hemiptera (true bugs) collected belonged to the Reduvidae, which are predacious bugs, and these decreased in abundance after fire (Figure 2k). Habitat simplification immediately after fire may have masked the extent of abundance decline in this group. In general the results suggest that fire or post-fire conditions are deleterious to predatory insect populations at least in the short term. Research in Australia and other countries has reported decreased abundance of Hemiptera after fire (Ahlgren 1974; Majer 1980; O'Dowd & Gill 1984).
Hymenoptera (wasps, bees and ants) also decreased in abundance following fire. The majority of Hymenoptera collected during the current study were ants and the proportion of ants increased from 70.5 per cent before the fire to 80-90 per cent following fire. In contrast to the results of the current study, most other authors have recorded an increased abundance and species richness of ants following fire (O'Dowd & Gill 1984; Andersen & Yen 1985; Majer & Koch 1982; Majer 1980; Hurst 1970; Heywood & Tissot 1936; Rice 1932). Collection techniques could account for this difference; with habitat simplification leading to increased capture rates in pitfall traps as compared to soil cores. Ahlgren (1974) concluded that ants, except for those actively foraging outside the nest, are the least affected of all insects by fire because of their adaptability and preference for xeric conditions. By comparison, the ants collected during the current study are more suited to the cool, moist habitat of the E. regnans forest. The ant species collected during the current study are consistent with Andersen's (1986) description of mesic patterns of community organisation, with cool-climate specialists and cryptic species accounting for 63.5 per cent of total ant abundance.
A total of 14 species (or species groups) were collected during the study period (Table 4). Seven of the species can be regarded as epigaeic and seven as cryptic (Andersen, A. 1974, pers. comm.). Amblyopone sp., Discothyrea ?bidens Clark, and Prolasius niger Clark group were only recorded before the fires. P. niger was restricted to site 1, D. ?bidens to site 2, and Amblyopone sp. was recorded at both sites. Amblyopone sp. and D.?bidens are regarded as cryptic species whilst P. niger is epigaeic (Andersen, A. 1974, pers.comm.). Cerapachys (Lioponera) sp., Solenopsis sp. and Camponotus claripes Mayr group were only recorded after the fires. Solenopsis sp. is regarded as cryptic while the other two species are described as epigaeic (Andersen, A. 1994, pers.comm.). These data suggest that there was an increase in epigaeic forms following fire and a corresponding decrease in cryptic forms.
The number of species collected did not alter after fire, although species composition did change. Fire intensity may affect species richness which increased after a low-intensity fire but decreased after the high-intensity fire during the current study. This highlights the necessity for species level analysis of community changes.
Fire and fire intensity seemed to have little influence on the abundance patterns of Coleoptera (beetles) (Figure 2l). Some species of beetles are attracted by smoke (Evans 1971) and this may have been responsible for the increased abundance during February 1983. The simplified habitat would have then caused increased beetle density in samples however, further investigation of the species present before and after fire is required to support these suggestions. Majer (1980) reported increased abundance of beetles after fire, while O'Dowd and Gill (1984) reported a decrease in beetle population following fire.
Because of the great diversity of structure of many insect larvae and the lack of taxonomic keys, larvae of Coleoptera, Lepidoptera, Hymenoptera and Diptera were considered as a single group in the current study. Abundance immediately after fire was lower than that recorded prior to the fire but a dramatic increase in the post-fire late winter and spring periods of the study, coinciding with hatching of soil borne eggs, resulted in an overall increase in abundance after fire (Figure 2n).
|Species||Site 1||Site 2|
|Cerapachys (Lioponera) sp.*||–||+||–||+|
|Discothyrea ?bidens Clark||–||+||–||+|
|Monomorium (Chelaner) sculpturatum Clark*||+||+||+||+|
|Strumigenys perplexa F. Smith||–||+||–||+|
|Iridomyrmex sp. A. (cf. rugoniger (Lowne) group)*||+||+||+||+|
|Iridomyrmex sp. B. (queens only - possibly a workerless parasite)*||+||+||+||+|
|Camponotus claripes Mayr group*||–||+||–||+|
|Prolasius pallidus Clark group*||+||+||+||+|
|Prolasius niger Clark group*||+||–||–||–|
* = epigaeic habits
In general arachnids are well adapted to terrestrial conditions although the soil mites lack special respiratory organs. Excessive heat is a constant problem for all arachnids because it increases the rate of water loss and increases the volume of body fluid contained within their hard exoskeleton causing increased internal pressure (Savory 1977). Arachnids generally utilise the damp conditions in soil to reduce water loss, but must sometimes seek out drier conditions to increase evaporative cooling.
The Acarina (mites) were the most numerous order in almost all samples, representing an average of 67.23 per cent of the total specimens collected. Opilionida (harvestmen), Araneae (spiders) and Pseudoscorpionida (pseudoscorpions) were far less numerous, but trends in abundance of these groups are reported (Table 2).
A significant increase in mite abundance after fire was recorded at both sites however, the increase was greater at site 1 (Figure 2o). Chandler et al. (1983), in a review of the effects of fire on soil and litter invertebrates, states 'all investigators agree that acarian populations are reduced by burning'. The results of the current study suggest that Chandler's statement does not necessarily apply to all fires, although the necessity of a concurrent control or unburnt sampling site is highlighted. The apparent increase in mite abundance after fire in the current study may have been even greater during the same period had the sites not been burnt. This suggestion is supported by the site comparison. No difference in abundance of this group was recorded between sites during the pre-fire period but a significantly lower abundance was recorded at site 2 after the fire. Again fire intensity is signified as important in post-fire response by affecting site conditions during the post-fire winter.
The majority of other studies have observed decreased Acarina abundance following fire (O'Dowd & Gill 1985; Curry et al. 1985; Whelan et al. 1980; Huhta et al., 1967; Bornemissza 1969; Metz & Farrier 1971). Techniques which did not target the collection of micro-invertebrates were used in all the above studies and are the likely cause of the disparate results. Only Majer (1980) reported an increased abundance of mites after fire.
The increased or unchanged abundance of mites immediately after fire would depend upon the survival of a large proportion of the existing population in refugia and by retreating down the soil profile (Campbell & Tanton 1981). During the current study increased proportions of juvenile mites and Astigmata were observed in post-fire samples. Astigmatid mites may have an advantage in surviving fire as these small mites normally inhabit the lower soil strata. Their small size enhances movement through the soil profile and many are tolerant of dry conditions (Spain & Hutson 1985; Plowman 1979; Wallwork 1976). Litter habitat compression after fire would lead to a greater density of mites in the upper soil layers as individuals migrated back to the soil surface. In addition, many mites have the ability to reproduce rapidly (Krantz 1978) and the increased proportion of mite larvae in post-fire samples indicated that reproductive success was involved in the rapid increase of mite numbers following fire.
No one environmental factor appears to be responsible for the distribution of mites. Increased rainfall and low temperatures were correlated with increased abundance of mites during the current study, suggesting both may be important in regulating density and activity of these animals. Other studies (Greenslade & Greenslade 1983; Hutson & Veitch 1987; Springett 1979; Holt 1985; Huhta et al. 1967) reported increased mite abundance during periods of high rainfall or high soil moisture content, regardless of temperature. Conversely Koch and Majer (1980) and Postle (1984) reported maximum abundance during the hotter dry months of their studies. Plowman (1979) recorded that maximum abundance of mites coincided with low soil moisture content. The effect of moisture and temperature on this group of animals is therefore unclear. Furthermore, other environmental factors such as litter fall, nutrient availability and humus development, have been implicated in affecting mite abundance (Plowman 1979; Bornemissza 1969).
The other arachnids collected during the current study (Opilionida, Araneae and Pseudoscorpionida) were only present in low numbers throughout the study; however a significant decrease in abundance of Opilionida and Araneae was recorded after the fires at site 2 (Figures 2p, q). The post-fire decrease in opilionid abundance may relate to the groups reliance on water and moist conditions (Bishop 1949); throughout the current study opilionid abundance was correlated with soil moisture content.
The collection of Araneae was not enhanced by the techniques employed during the current study. Studies using pitfall trapping recorded far higher numbers, however, most studies do report a decreased abundance following fire (Springett 1976; Koch & Majer 1980; O'Dowd & Gill 1984; Ahlgren 1974; Huhta 1971). Other research has also reported that species richness and species composition were affected by fire; different guilds of spiders respond according to their biologies and behavioural patterns (Koch & Majer 1980; Huhta 1971; Curry et al. 1985).
In contrast no change in abundance of pseudoscorpions was detected during the study period. These carnivorous arachnids inhabit a wide variety of environments but are usually found in soil, litter, moss, under bark and in caves, (Gilbert 1951; Harvey 1983). Although some species can tolerate extremely xeric conditions (Savory 1977), the majority are susceptible to desiccation. An increased abundance immediately after the fire suggests that many pseudoscorpions survived the low-intensity fire. Vertical migration from refuges under bark and in the upper canopy of site 1, coupled with the effects of habitat compression (simplification), may account for the increase in abundance during February 1983 (Figure 2r). A species analysis of this group revealed only two species were present in samples, Austrochthonius australis and Pseudotyrannochthonius sp. nov.. Both species were collected before the fire although P. sp. nov. was restricted to site 2. Only A. australis was present in post-fire samples. Low density, rather than localised extinction, more probably accounts for the disappearance of P. sp. nov..
Total invertebrate abundance significantly increased following fire during the current study, but diversity at the ordinal level decreased. The increased catch of invertebrates during the post-fire period of the study may be related to the factors listed below: 1) rainfall for the year preceding the fire was significantly below average figures (Bureau of Meteorology, unpub. data); 2) the prevailing conditions may have been particularly unfavourable for soil invertebrate survival and activity; 3) increased catch rate due to habitat simplification, concentrating invertebrates into a decreased volume of habitat; and 4) an increase in the abundance of invertebrates.
Different taxa of soil invertebrates would contribute to post-fire populations according to their respective abilities to survive fire and the prevailing conditions. Following a high-intensity ground fire three orders (Isopoda, Amphipoda and Protura) decreased in abundance while four orders (Pauropoda, Collembola, Hemiptera and Acarina) increased. After the high-intensity crown fire at site 2 seven orders (Isopoda, Amphipoda, Chilopoda, Pauropoda, Hymenoptera, Opilionacea and Aranea) decreased in abundance while three groups (Psocoptera, Acarina and insect larvae) increased.
The general change in numbers of invertebrates collected after the fire must be interpreted with caution. Data from successive years before fire, or from comparable unburnt sites, is required to separate fire impact from annual variation without fire. Annual variation data is also required to determine if alterations in invertebrate populations following fire exceed those experienced in years without fire and thus determine the extent of the fire impact. Favourable climatic conditions during 1983, regardless of the fire event, may have led to increased abundance of active invertebrates. However, the response of some groups, particularly the Crustacea, suggest fire has an impact exceeding annual variation.
Fire intensity appeared to be important in affecting post-fire patterns of invertebrate abundance. In general, the invertebrate numbers after fire at site 1 exceeded the numbers at site 2. A lack of significant difference between invertebrate abundance at the two study sites before fire, followed by a significant difference after the fires, supports the conclusion that varying fire intensity affects populations and their micro-environments differently; even in the absence of annual variation data.
Throughout the current study, time and taxonomic difficulties prevented classification of most groups beyond the ordinal level. Fluctuations in abundance of an order represents the combined fluctuations of the species composing that order. Studies of species level changes following environmental alterations (eg. Huhta 1971; Andersen & Yen 1985) have described species replacement which would be masked if the studies had only been concerned with higher taxa. Further studies of the species present, and developmental stage of individuals, would give far greater information about the responses occurring within the forest soil community. The examination of species changes following fire is particularly important for understanding the extent and duration of community alterations. Studies involving higher taxonomic levels can, however provide clear indications of sensitive or robust taxa for inclusion in future studies. Long-term studies of the species composition of communities, and their response to fire, are urgently required; management decisions can then be based on a sound understanding of the ecological implications of fire.
The author wishes to thank Dr A. Andersen for identification of Hymenoptera: Formicidae, Dr M. Harvey for identifying the Pseudoscorpionida, and Forests Commission of Victoria (now DENR) for initial financial support.
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