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Testing Patch-Mosaic Burning in South Africa: the ant perspective
Kate Parr
Percy FitzPatrick Institute, University of Cape Town,
Rondebosch 7701, South Africa
Supervisors:
Prof W. 1. Bond
Dr. H. Robertson
Project submitted in partial fulfillment of the requirements for thedegree of Master of Science in
Conservation Biology, University of Cape Town.
February 1999
Format: Journal of Applied Ecology
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The copyright of this thesis rests with the University of Cape Town. No
quotation from it or information derived from it is to be published
without full acknowledgement of the source. The thesis is to be used
for private study or non-commercial research purposes only.
Testing patch-mosaic burning in South Africa: the ant perspective
KATE PARR
Perc>~ FitzPatrick Institute, University ofCape Town, Rondebosch 7701, South Africa
Summary
1.
There is much . debate regarding the most appropriate burning regime for the conservation of
biodiversity in savanna areas. A patch-mosaic system of burning is based on the premise that fire
pattern is a surrogate for diversity. To determine the effectiveness of a patch-mosaic system in
promoting biotic diversity, ants were studied on grassland patches of different post-bum age, and
fire frequency.
2.
A total of 66 ant species (22 genera) were recorded in pitfall traps set in 6 grassland sites,
representing three fire regimes (young and frequently burned, young and infrequently burned, and
old and infrequently burned).
3.
Species accumulation curves showed that species richness (for both observed and predicted)
declines with decreasing fire influence, being greatest on young and frequently burned plots and
lowest on old and infrequently burned plots. There was pronounced dominance by a few species
on young and infrequently burned plots, and greater equitability at other sites.
4.
A generalised linear model showed that 23 ant species (35 % total species) were affected by patch
age or fire frequency. With a regime of frequent fires nine species could be lost, and with a
regime ofinfrequent fires four species could be lost.
5.
Functional Group analysis revealed that the relative abundance of Cryptic Species and Specialist
Predators decreased with decreasing fire influence. Opportunists were more abundant on old and
infrequently burned plots.
6.
Litter cover and the percentage of total foliage biomass at ground level and 0-20 em, increased
with decreasing fire influence. Species richness was found to be inversely correlated to the
proportion of foliage biomass at ground level (r2 = 0.93; n = 6; p <0.001).
7.
Ant diversity is affected by frequency of burning, as well as post-bum age. Thus if frequency of
burning is standardised, and patch age is varied, not all species will be adequately conserved. A
patch-mosaic system by varies both fire frequency and age after fire, and thus produces patches
each with a unique fire history promoting the conservation of invertebrates such as ants.
Key-words: patch-mosaic burning, ants, heterogeneity, biodiversity conservation, frequency, post-bum age
1
Introduction
The maintenance of biotic diversity and ecosystem functioning is one of the main goals of
conservation (Groombridge 1992). Fire, as a major disturbance force in savanna systems, is one
way that this aim can be fulfilled. Fire and herbivory, regarded as crucial determinants of natural
savanna vegetation, modify broad patterns set primarily by rainfall and edaphic factors, and hence
play a role in determining the structure and function of these systems (Walker 1987~ Scholes &
Archer 1997). Fire management in protected areas is a contentious issue however, with
considerable debate as to the most appropriate burning regime.
Burning policies in protected areas have progressed through various phases ranging from
laissez jaire, to strictly controlled, prescribed fire regimes (Mentis & Bailey 1990; van Wilgen et
al. 1999) where frequency and seasonality of fires is standardised. Rigid prescribed fire
management which still persists in some protected areas (e.g. Etosha National Park, Namibia
(Stander, Nott & Mentis 1993; Du Plessis 1997» has been suggested to result in system
simplification and homogenisation (Mentis & Bailey 1990; Scholes & Walker 1993; van Wilgen et
al. 1999). The importance of inherent variability, a key characteristic of savanna systems, is
largely ignored with prescribed burning policies.
It is now recognised that spatial and temporal heterogeneity and complexity are crucial
elements in the functioning of ecosystems (Christensen 1997), and are important components of
biotic diversity (Braithwaite 1996). With the paradigm shift from homogeneity to patchiness in
ecology much attention has focused on patch theory (Weins 1997). Huston (1994) describes how
patchiness in pattern creates heterogeneity in resource availability, which in tum provides an array
of opportunities for colonisation and survival. It is this existence of opportunities that fosters
diversity, and it is this mixture of patches that forms the total resource base for biotic diversity.
2
Disturbances such as fire are important mechanisms for producing (and maintaining)
spatial heterogeneity (Schwilk, Keeley & Bond 1997). Fire, thus, plays a major role in structuring
ecological systems by producing a spatio-temporal mosaic of patches at different successional
stages (Turner et al. 1994; Moloney & Levin 1996). These diverse conditions may prevent
community domination by one or few species (Bond & van Wilgen 1996), and allow for the
persistence of fire-dependent species in a community subject to regular fires (Frost 1984).
Changes to natural disturbance regimes constitutes one of the major ways that humans
have altered ecosystems, and thus the biological diversity that occurs within them (Bond & van
Wilgen 1996; White & Harrod 1997). There is now growing concern that in some ecosystems
such regularity of burning could have adverse ecological effects. Hence management regimes that
result in the homogenisation of habitats should be avoided (Law & Dickman 1998).
It is suggested that a burning regime that will promote heterogeneity and patchiness in the
landscape is preferable (Saxon 1984; Russell-Smith 1997). In a patch-mosaic burning system, fire
parameters are varied to create a mosaic of patches representative of a range of fire histories
which generates heterogeneity within the landscape (Brockett, Biggs & van Wilgen 1999). Such a
burning system is based on the premise that patchiness is a major source of diversity (Pickett &
Rogers 1997). It is assumed that if fire pattern is regarded as a surrogate for biodiversity, then this
system should maintain biotic diversity.
The question remains though: is the theory behind the patch mosaic system borne out in
reality? Much of the research and quantitative evidence supporting the need to incorporate
variability and heterogeneity originates in Australia. Botanical studies have revealed that static fire
intervals may be detrimental (Keith & Bradstock 1994), and variable fire intervals should be
promoted where the aim is the conservation of biodiversity (Morrison et al. 1995; Gill &
McCarthy 1998). Faunal studies too indicate that at the landscape level, the ideal fire regime for
3
the conservation of a wide range of species is one where there are a variety of bums producing a
mosaic of patches (Braithwaite 1987; Woinarski 1990; Andersen 1991a; Andersen et al. 1998).
A patch-mosaic system is relatively new to southern Africa, and the effectiveness of this
system in promoting biotic diversity has not been tested. In this study, I investigate the potential
of a patch-mosaic system for African savannas by testing whether a mosaic of different patch
types is effective in promoting biotic diversity. Ants were used in this study for a number of
reasons: ants are highly abundant and diverse in savanna systems, and their wide distribution
throughout the world in diverse habitats makes ants strong indicators of biological diversity
(Roth, Perfecto & Rathcke 1994; Folgarait 1998). As indicators of ecosystem condition, ant
assemblages often reflect the degree of habitat disturbance/ succession in a community (Majer
1983, 1985; Andersen 1990, 1997c; Vanderwoude, Andersen & House 1997a; Peck, McQuaid &
Campbell 1998). Ants are functionally important in savanna systems, playing an important role in
structuring communities (Folgarait 1998). Furthermore, they are sampled and sorted to
morpho species level with relative ease. In using ants as indicators, I am not suggesting that they
are a surrogate for other biodiversity, but rather that they may reflect patterns of loss of diversity
of other species.
Difference in patch age is likely to influence biotic diversity (e.g. Donnelly & Giliomee
1985; York 1994), but the extent to which frequency influences diversity is not as clear, and the
importance of varying burning regime is unknown. Different species may be favoured by
, particular fire histories, and different combinations of patch age and frequency: some species may
favour young and frequently burned areas, others infrequent fires, and others young and
infrequent bums. If this is the case, then variance in burning regimes is important, and should be
an inherent feature of any burning system that is implemented.
I sampled ant diversity on patches of different post-bum age and different fire frequencies
(fire return periods). I tested two hypotheses, firstly, patch post-bum age has no effect on ant
4
diversity and secondly, difference in fire frequency has no effect on ant diversity. If patch age
alone influences diversity, then a patch-mosaic burning system is no better than other burning
systems (e.g. block burning) in promoting diversity. If fire frequency influences diversity, uniform
frequencies imposed by block-burning systems may cause the loss of some species dependent on
frequencies different from those imposed by management.
Study Area
The study was carried out at Pilanesberg National Park (pNP) located in the North West
Province, South Africa (27 0 06' S, 25 0 15' E). The park, established in 1979, covers 50 000 ha,
and is roughly circular in shape. The park comprises an ancient alkaline volcanic crater, which
represents the second largest ring complex in the world (Lurie 1973). Rainfall is highly seasonal,
with most of the annual mean, 630mm, falling between October and March.
The vegetation is a moist savanna (Sour Bushveld; Acocks (1975) Veld Type No. 20),
consisting of tufted perennial grasses and trees which are broad-leaved and deciduous. The
perennial grasses give a continuous to patchy fine-fuel distribution of 500-7000 kg/ha depending
on rainfall in the previous growing season. Following a bum, sufficient grass-fuel to carry a fire
has usually accumulated after one or two growing seasons. Fires (both natural and prescribed)
usually occur in the dry season between April and December (Brockett, Biggs & van Wilgen
1999). Block burning was carried out in PNP between 1981 and 1988. From 1989 to present, a
patch-mosaic system of burning has been implemented.
Most of the naturally occurring fauna have been reintroduced to the park. This includes
species of antelope, zebra (Equus burchelli), giraffe (Giraffa camelopardalis) , elephant
(Loxodonta africana), white and black rhino (Ceratotherium simum and Diceros bicornis),
hippopotamus (Hippopotamus amphibius) and warthog (Phacochaerus aethiopicus).
5
Methods
Grassland sites were selected from a 1981 unpublished vegetation map of PNP which was
developed from an earlier map produced during the park's development phase (Farrel, van Riet &
Tinley 1978). This was to ensure that all plots started as grassland before burning was
implemented in the park, and thus any changes in vegetation could be related to the fire regime.
Since 1981 fires in PNP have been recorded onto 1:50 000 maps of the park. I consolidated fire
maps for the period of patch-mosaic burning (1989-1998) and period of block-burning (19811988) to produce a post-fire fuel age map. I calculated the fire return period (FRP) of each patch,
and patches of required age/frequency combinations were identified (Table 1). Young plots were
categorised as those where time since last fire (age of fuel) was 1 or 2 years (i. e. last burned 1997
or 1996). Old plots were those that last burned 4 to 6 years ago. Frequently burned plots were
classified as having a mean FRP of less than three years, and infrequently burned plots, a mean
FRP of more than three years. A three year threshold was chosen as this is the average FRP in
Pilanesberg (Brockett pers. comm.). Grassland patches with required fire regimes were located,
and variables such as slope, aspect and geology standardised where possible.
Table 1. Plot characteristics in terms of post-bum age and fire frequency
Plot
Fire regime
Age: post-burn
(years)
MeanFRP
(years)
Al
Young /
frequent
A2
Young/
frequent
BI
Young/
infrequent
B2
Young /
infrequent
CI
Old /
infrequent
C2
Old /
infrequent
1
1
1
2
4
6
3.2
3.8
2.1
(
2.7
3.2
3.2
) Decreasing influence of fire)
6
ANT SAMPLING
I studied ground foraging (epigaeic) ants in six 1 ha grassland plots representing two replicates of
each of three fire regimes: young and frequently burned (plots Al and A2), young and
infrequently burned (Plots B1 and B2) and, old and infrequently burned (plots C1 and C2) (Table
1). Each 1 ha plot was positioned at least 50 m away from adjacent patches with different fire
regimes to minimise edge effects.
Within each plot, I sampled ants using 20 pitfall traps (7 cm diameter plastic cups) located
in a 5 by 4 grid with 10m spacing. Traps contained 50 ml 50% ethylene-glycol solution as a
preservative, and operated for six days from 20 October to 26 October 1998. Throughout this .
sampling period the weather was hot and dry. The contents of one trap in Plot Al were
destroyed, and consequently excluded from analysis.
Samples were washed through a fine aquarium sieve, and stored in 80 % ethanol. Ants
were sorted to species-level, and either named to species or assigned species code numbers within
each genus (number code applying only to this study). Voucher specimens were mounted for each
plot, and are deposited in the South African Museum, Cape Town.
In addition to pitfall trapping, observations of ants at tuna fish baits were used to quantify
the relative diurnal behavioural dominance of species. Behavioural dominance is based on
competitive interactions and competitive exclusion. Seven days after the pitfall traps were emptied
(3rd November 1998), tuna fish baits were set at each site every 2 m along a 20 m transect
running through the sampling plot. All species at the baits were recorded after 5, 15, 25, 35, 45
and 60 minutes. The abundance of species at the bait was scored according to a six-point scale: 1,
1 ant; 2, 2-5 ants; 3, 6-10 ants; 4, 11-20 ants; 5, 21-50 ants; 6, >50 ants. This scaling takes into
account distortions which
m~y
occur due to placement of baits near nest entrances or foraging
7
trails (Andersen 1997a). Specimens of all species were collected with an aspirator for laboratory
identification.
Pitfall trap species abundances were also scored according to the six-point scale described
for tuna-baits.
VEGETATION SAMPLING
Microhabitat vegetation was assessed for each plot. Ten, 3 m long transects were placed through
the centre of the middle ten pitfall traps in each plot, running in the same direction. Vegetation
(species present), litter and bare ground were recorded every 10 cm along each transect.
To provide an indication of vegetation structure and biomass, I constructed foliage
profiles from the amount of vegetation (measured as area of plant matter per unit volume of
space) at different heights (MacArthur & MacArthur 1961). They are thus an approximation of
the vertical distribution of biomass. Poles (1.5 m high divided into height classes of 20 em) were
used to determine foliage density profiles. Vegetation structure was surveyed every 5 m along
two 40 m-transects, in both directions perpendicular to the main transect.
DATA ANALYSIS
Diversity indices
To compare a diversity between plots, abundances (excluding tuna bait data) were used to
calculate species richness (i.e. total number of species) (S), Shannon index of diversity (H '),
Shannon evenness index (E), Ricklefs diversity index of the effective number of species (eH ')
(Ricklefs 1979), Simpson's dominance index (D) and Berger-Parker's dominance index (d)
(Magurran 1988). Simpson's index is heavily weighted towards the most abundant species in the
sample, and is less sensitive to species richness, and Berger-Parker's index simply expresses the
proportional importance of the most abundant species (Magurran 1988).
8
Estimation of Species Richness
Local species richness can be estimated by extrapolating species accumulation curves and
using non-parametric techniques based on the distribution of individuals among species, or of
species among samples (Colwell & Coddington 1994). Species accumulation curves (observed:
number of species observed in the pooled samples (SOBS)) for the 20 pitfalls per plot, as well as
abundance-based coverage estimator (ACE) of the total number of species in the local community
from which the samples were taken, were computed using EstimateS (Version 5, Colwell 1997),
and plotted for each succeeding station sample. The program randomised sample order 100 times,
and averaged randomisations to produce smooth species accumulation curves. ACE is a nonparametric method to improve the estimate of species richness, and is based on those species with
10 or fewer individuals in the sample (Chao, Ma & Yang 1993).
Influence of patch age and frequency on species
Regression analysis was performed with generalised linear models (McCullagh & Neider
1983, GENSTAT, Release 4.1 for Windows) using both abundance (number of ants per pitfall;
Poisson distribution) and incidence of occurrence data (presence/absence of species in pitfalls;
binomial distribution) to determine whether species were significantly influenced by patch age and
frequency. Regression analysis produced positive and negative values for the constants 'age'
(negative indicated preference for young patches, and positive a preference for old patches) and
'frequency' (negative indicated preference for frequently burned areas, and positive a preference
for infrequently burned patches).
Pseudoreplication might be viewed as problematic here (Hurlbert 1984), because although
each pitfall sample per plot is independent from every other, they all come from the same plot.
Due to the nature of the sampling, this kind of problem is inevitable, and does not detract from
the results, or implications for fire management and biodiversity conservation.
9
Functional groups
The great diversity of ants collected, and the absence of information on their specific
biology mean it is difficult in many 'instances to interpret their responses to fire at a species level.
Assigning species to functional groups according to their habitat distributions and relative
behavioural dominance provided an alternative way of categorising ant species using ecological
criteria. This functional group approach was first used in studies of the Australian arid zone
(Greenslade 1978), but has since been applied elsewhere (see Greenslade 1985; Andersen 1986,
1991a, 1997a).
Patterns of functional group composition were examined for each patch type, and burn
characteristic. Functional groups used in this study are as follows (based on Andersen 1991a):
1. Subordinate ·Camponotini (e.g. Camponotus), which co-occur with, and are behaviourally
submissive to more abundant, highly active and aggressive species.
2. Hot Climate Specialists (e.g. Ocymyrmex), taxa adapted to hot arid environments with
morphological, physiological or behavioural specialisation's which reduce interactions with
more dominant species.
3. Cold Climate Specialists (e.g. Anoplolepis, Lepisiota), have distributions centred mainly on
temperate regions.
4. Tropical Climate Specialists (e.g. Aenictus), similar to above, but distribution is centred on the
humid tropics.
5. Cryptic Species (e.g. Oligomyrmex), which nest and forage predominantly within the soil and
litter, and have little interaction with epigaeic ants.
6. Opportunists (e.g. Tetramorium, Technomyrmex), are unspecialised 'weedy' species
characteristic of habitats supporting low ant diversity.
10
7. Generalised Myrmicinae
(e.g. Monomorium,
Pheidole,
Crematogaster), which
are
unspecialised, but highly competitive taxa.
8. Specialist Predators (e.g. Pachycondyla, Cerapachys), have little interaction with other ants
due to large body size, low foraging densities and specialised diet.
Andersen's Dominant Dolichoderinae category is not used here as they are not considered
dominant (i.e. abundant, highly aggressive species) in southern Africa (Wheeler 1922).
Results
SPECIES RICHNESS AND COMPOSITION OF ANTS
A total of 66 ant species comprising 22 genera was recorded in pitfall traps, with the richest
genera being Tetramorium (14 species), Monomorium (10), Camponotus (7), Pheidole (7) and
Lepisiota (6) (Table 2). The most abundant species were Pheidole sp. 5, representing 17.5% of all
ants found in traps, Pheidole sp. 7 (16.9%), Pheidole sp. 2 (13.5%), Monomorium albopilosum
(Emery) (8%) and Crematogaster sp. 1 (4.7%).
Species richness of ants was greatest in recently burnt plots, and declined with decreasing
fire frequency (Table 3). Patch type species richness was 51, 42 and 34, on the young and
frequently burned, young and infrequently burned, and old and infrequently burned patches
respectively. The number of unique species (species only occurring on one plot) was highest on
young and frequently burned plots (13 species), followed by old and infrequentlyburned plots (7
species). The total number of unique species on young and infrequently burned plots was only
five.
All diversity indices other than the Berger-Parker index, tended to decrease with
decreasing fire influence (Table 3). The Berger-Parker index indicates that the proportional
11
Table 2. Composition of ant species collected in pitfall traps. Data are numbers of ants per species for
each plot, and species' Functional Group classification (SC= Subordinate Camponotini, CCS= Cold
Climate Specialist, HCS= Hot Climate Specialist, CS= Cryptic Species, OPP= Opportunist, GM=
Generalised Myrmicinae, SP= Specialist Predator). Incidence of occurrence data per species for each
.
plot is in brackets. Plot types: YIF = young and frequently burned, YII = young and infrequently burned,
0/1 = old and infrequently burned.
Species
Plot
Functional
Group
Al
YIF
Aenictinae
Aenictus eugenii Emery
Cerapachyinae
Cerapachys wroughtoni Forel
Cerapachys sp. 1
Dolichoderinae
Tapinoma sp. 1
Technomyrmex albipes (F.Smith)
Formicinae
Anoplolepis custodiens (F.Smith)
Anoplolepis sp. 1
Camponotus aequitas Santschi
Camponotus cinctellus (Gerstacker)
Camponotus debellator Santschi
Camponotus mayri Forel
Camponotus nasutus Emery
Camponotus.petersii Emery
Camponotus vestitus (F.Smith)
Lepisiota arnoldi (Forel)
Lepisiota capensis (Mayr)
Lepisiota crinita (Mayr)
Lepisiota spinosior (Forel)
Lepisiota sp. 5
Lepisiota sp. 6
Plagiolepis sp. 1
Myrmicinae
Cardiocondyla emeryi Forel
Crematogaster sp. 1
Crematogaster sp. 2
Leptothorax simoni (Emery)
Meranoplus glaber Arnold
Monomorium albopilosum Emery
Monomorium katir Bolton
Monomorium mictilis Forel
Monomorium oscaris Forel
Monomorium pulchrum Santschi
Monomorium setuliferum Forel
Monomorium springvalense Forel.
Monomorium sp. 1 (near taedium
Bolton)
A2
YIF
YII
B2
Y/I
CI
Oil
C2
Oil
7 (1)
TCS
7
2 (1)
SP
SP
OPP
OPP
Bl
Total
no. of
ants
2
1
1 (1)
11 (5)
1 (1)
19 (4)
15 (6)
19 (10)
2 (2)
11 (5)
1 (1)
12 (7)
4 (1)
9 (7)
6 (2)
10 (6)
9 (4)
16 (2)
3 (1)
7 (5)
1 (1)
25 (8)
3 (2)
1(1)
1 (1)
1 (1)
CCS
CCS
SC
SC
SC
SC
SC
SC
SC
CCS
CCS
CCS
CCS
CCS
CCS
CS
9 (6)
14 (6)
1 (1)
20 (11)
2 (2)
OPP
OM
OM
CCS
HCS
OM
OM
OM
OM
OM
OM
OM
OM
2 (1)
42 (9) 147 (12)
3 (1)
15 (2)
3 (3)
2 (2)
62 (15) 7 (4) 81 (17)
5 (2)
3 (3)
1 (1)
12 (3)
17 (8)
10 (5)
22 (8)
15 (12)
1 (1)
3 (3)
3 (3)
1 (1)
6 (4)
2 (2)
2 (2)
4 (3)
1 (1)
1 (1)
2 (1)
10 (7)
2 (1)
1 (1)
30 (7)
1 (1)
18 (5)
23 (6)
12
10 (6)
1 (1)
1 (1)
16 (6)
25 (10)
54(13) 175(10)
23 (7)
9 (5)
2(2)
3 (3)
59 (12)
25 (9)
1 (1)
15 (8)
2 (2)
7 (5)
1 (1)
6 (2)
3 (3)
50
70
9
43
7
3
9
1
1
3
11
1 (1)
1 (1)
1
77
67 (17)
86 (17)
17 (10)
5 (4)
1 (1)
2
1
2
1
65
7
189
102
4
2
318
5
18
73
1
32
250
52
Table 2.-cont.
Species
Monomorium sp. 5 (near mavide
Bolton)
Monomorium sp. 8 (salomonisgroup)
Ocymyrmex foreli Arnold
Oligomyrmex sp. 1
Pheidole sp. 1
Pheidole sp. 2
Pheidole sp. 3
Pheidole sp. 4
Pheidole sp. 5
Pheidole sp. 6
Pheidole sp. 7
Tetramorium ?baufra Bolton
Tetramorium constanciae Arnold
Tetramorium ericae Arnold
Tetramorium frigidum Arnold
Tetramorium khyarum Bolton
Tetramorium laevithorax Emery
Tetramorium mossamedense Forel
Tetramorium notiale Bolton
Tetramorium repentinum Arnold
Tetramorium sericeiventre Emery
Tetramorium setigerum Mayr
Tetramorium setuliferum Emery
Tetramoriumsimillimum (F.Smith)
Tetramorium umtaliense Arnold
Ponerinae
Anochetus levaillant Emery
Leptogenys schwabi Forel
Odontomachus troglodytes Sanschi
Pachycondyla caffraria (F.Smith)
Pachycondyla sennaarensis (Mayr)
Pachycondyla strigulosa (Emery)
Platythyrea lamellosa (Roger)
Total no. of ants
Plot
Functional
Group
Total
no. of
ants
Al
Y/F"
A2
B1
B2
C1
C2
Y/F
YII
3 (2)
21 (6)
Oil
1 (1)
Oil
GM
Y/I
2 (2)
GM
2 (2)
16 (5)
7 (3)
3 (2)
5 (2)
RCS
CS
GM
GM
GM
GM
GM
GM
GM
Opp
OPP
Opp
Opp
Opp
OPP
OPP
Opp
OPP
OPP
OPP
OPP
OPP
OPP
22 (13)
26 (6)
33 (15)
6 (6)
24 (13)
1 (1)
SP
SP
OPP
SP
SP
SP
SP
1 (1)
1 (1) 108 (12) 4 (2)
99 (5)
42 (1)
73 (12)
35 (8)
2 (1)
4 (2)
31 (8)
9 (2)
2 (2)
3 (3)
671 (19) 19 (4)
1 (1)
6 (1) 669 (18)
2 (2)
60 (6)
2 (1)
12 (4)
1 (1)
3 (1)
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
7 (4)
13 (6)
7 (5)
14 (9)
1 (1)
2 (1)
1 (1)
1 (1)
9 (4)
4 (3)
3 (3)
11 (7)
2 (1)
2 (1)
1 (1)
2 (2)
2 (2)
15 (8)
8 (4)
3 (1)
1 (1)
1 (1)
1 (1)
1 (1)
5 (4)
2 (2)
3 (3)
4 (3)
2 (2)
560
1187
851
27
33
111
1
12 (1)
125
326 (14) 540
37
46
17 (4) " 710
1
675
62
2
19 (5)
35
1
1 (1)
4
1
20 (12)
61
3
1
6 (6)
23
2 (2)
2
1 (1)
37
2
5
2 (1)
1 (1)
5
1
3
8
2
2
10
567
3995
2 (2)
2 (2)
531
13
299
importance of the most abundant species was greatest on young and infrequently burned plots.
Plot C1 tended to be an exception to these trends.
Table 3. Diversity indices for ant diversity at each site. Plot burning regimes: Y/F = young and frequently
burned, YII = young and infrequently burned, Oil = old and infrequently burned.
Plot
Burning regime
Species richness (S)
Predicted species richness (ACE)
Number of unique species
Shannon's diversity index (R')
Ricklefs effective species no. (eH')
Shannon's evenness index (E)
Simpson's dominance index (D)
Berger-Parker dominance index (d)
Al
YIF
38
46
5
2.94
18.9
0.81
13.11
0.19
A2
Y/F
32
38
8
2.47
11.8
0.71
7.46
0.26
BI
Yll
30
37
2
1.74
5.7
0.51
1.59
0.79
B2
Yll
29
35
3
1.06
2.9
0.31
2.87
0.56
CI
Oil
27
32
4
2.49
12.0
0.76
7.69
0.58
C2
Oil
21
32
3
1.61
5.0
0.53
2.75
0.22
Log species rank-abundance curves (May 1975) for the first 15 specres indicated
pronounced numerical dominance by a few species, on young and infrequently burnt plots, and
Plot C2 (Oil). There was greater equitability at the other sites (e.g. young and frequently burned
plots) (Fig. 1).
Species accumulation curves for observed species do not level off, but appear to be still
increasing (Fig. 2). The predicted number (according to ACE species-accumulation curves) of
species in the community at each plot shows higher species richness in young and frequently
burned plots, than young and infrequent plots, and higher species richness in these plots than old
and infrequent plots. Predicted plot species richness (as with observed richness) was found to
decline with decreasing fire influence (Table 3 and Fig. 2)
EFFECT OF PATCHAGE AND FIRE FREQUENCY ON ANT SPECIES
Results of regression with the generalised linear model indicate that some species of ant are
affected by patch age and frequency, and thus favour particular patch types (Table 4 and Fig. 3).
14
A
1000
~
100
o
e
as
"'0
c:
.......
::l
..0
<
10
Al
. -.
A2
1
-I
I
i
3
I
i
5
i
I
7
I
9
11
I
13
15
B
1000
~
1
100
<)
c:
as
~
c:
::l
..0
<
10
. --- - B2
Bl
3
5
7
9
11
13
15
c
1000
100
10
CI
+--r----r---,---,---.-..,.--,r---r-.---r---r--....---r---,.-'.=--,
3
5
7
9
11
13
C2
15
Speciesrank
Fig. 1. Species rank-abundance (log) curves for each patch type, using data from pitfall traps. Onlythe
most abundant fifteen species at each plot are considered. A = young and frequently burned plots, B =
young and infrequently burned plots, C = old and infrequently burned plots.
15
::I
A
T
30
I
25
~
20
+
15
T
10 -T,
5
0+--+----1f---+--+--l---+--+-l---+--+--t---+--+-+---:--+-+---j-----!
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20
SampIenumber
B
50
45
[)
"E::s
c:
40
35
(JJ
30
0-
25
v
'0
v
(JJ
v
.:::
.s::s
S
::s
U
20
T
t
t
t
I
t
A1
I-o-A2
I
15
1
i
I..-.-Bl
I-Q-
B2
!-tr-
C2
I
10
I-.-
5 T
o
I
I
I
I
I
I
I
I
I
1
2
3
4
5
6
7
8
9
I
,
I
I
I
I
I
I
10 11 12 13 14 15 16 17 18 19 20
SampIenumber
Fig. 2. Species accumulation curves. A : Observed number of species per plot (SOBS), B :
Predicted number of species per plot using abundance coverage estimator (ACE).
16
1
C1
!
1
Abundance data
Just over one third (35 %) of the total number of ant species in this study (23 species) are
significantly affected by age or frequency of burning in a patch. Of these, eight species were
significantly affected by patch age (12.1 %), all favouring young patches. Five species were
significantly affected by patch frequency (7.6 %); four favouring frequently burned areas, and one,
infrequently burned. Ten species (15.2 %) were significantly affected by both patch age and
frequency.
Incidence data
Eighteen species of ant are significantly influenced by patch age or frequency. Of these,
eight species (12.1 %) were affected by patch age alone, seven species (10.6 %) by patch
frequency alone, and three species affected by both age and frequency.
Nine species favour infrequently burned patches that could potentially be lost with a
regime of frequent fires. With infrequent fires, four species could be lost as they favour frequently
burned patches. A mosaic of only young patches in the landscape could result in the loss of three
species dependent on old patches.
Given that many ant species recorded in this study occurred at very low abundances and
incidence (many unique species), it is impossible to determine whether they are affected by patch
age and burning frequency. It is thus probable that the number of ant species affected by age or
fire frequency could be greater than stated here.
Graphical representations of selected species influenced by age and fire frequency (for
incidence of occurrence data) are shown in Figs 4 and 5. The number of species favouring
particular patch types was determined for both abundance and incidence of occurrence data (Fig.
3). More species favour young rather than old patches, and more species, infrequent rather than
frequent patches.
17
Table 4. Generalised linear model results showing ant species significantly affected by postburn age and fire frequency, p<O.OOl(***), p<O.Ol(**), p<O.05(*). A
= abundance data (no.
of ants/ pitfall trap), I = incidence of occurrence data (presence/absence in pitfalls).
Species
Post-burn Age
Young
Old
A
I
A
I
Aenictus eugenii
Camponotus cinctellus
Crematogaster sp. 1
Crematogaster sp. 2
Monomorium albopilosum
M mictilis
M oscaris
M setuliferum
M springvalense
Monomorium sp.l (near taedium)
Monomorium sp. 5 (near mavide)
Monomorium sp. 8
Ocymyrmex foreli
Pheidole sp. l
Pheidole sp. 2
Pheidole sp. 3
Pheidole sp. 4
Pheidole sp. 5
Pheidole sp. 7
Plagiolepis sp. 1
Tetramorium ericae
T. mossamedense
T. setuliferum
*
**
Frequency
Frequent
Infrequent
A
I
A
I
*
***
***
***
***
***
*
*
***
***
***
*
**
**
**
***
***
***
***
**
*
***
***
***
***
***
**
***
***
**
***
**
***
***
**
*
***
***
**
***
***
*
**
***
**
***
***
*
**
lD Abundance
E3 Incidence of
occurrence
Young
Old
Frequent
Infrequent
Patch variable
Fig. 3. Results of the generalised linear model showing the number of species significantly
(p~O.05)
**
**
favouring young or old patches and frequently or infrequently burned patches based
on abundance and incidence of occurrence data.
18
a.)
15
T
b.)
12
(\,)
6
(\,)
(,)
e
(,)
(\,)
s
9
fj
6
"':l
'u t:
~S
4
(,)
~
0
{:
0
-,-
2
3
0
0
Al
Bl
A2
B2
Cl
C2
8
c.)
o
d.)
(\,)
(,)
c: c:
"':l (\,)
'u t:
.S (,)fj
(\,)
_
<'$
Al
A2
Bl
B2
Cl
C2
Al
A2
Bl
B2
Cl
C2
15
12
6
(\,)
8
9
4
6
0
2
~'o
3
0
0
Al
A2
B2
Bl
Cl
C2
Plot
Plot
Fig. 4. Species shown with a generalised linear model to be significantly
(p~O.05)
influenced
by post-burn age of patch. All data is for the total incidence of occurrence in pitfall traps per
plot. Species favouring young areas: a.) Monomorium springvalense, b.) Plagiolepis sp. 1, c.)
Pheidole sp. 4. Species favouring old areas, d.) Pheidole sp. 2. (also see Fig. 5, Tetramorium
mossamedense).
a.)
(\,)
o
(\,)
c: oc:
(\,)
t:
(\,)
"':l
~~
-
~
~
(,)
g
{: '0
12
10
8
6
8
6
4'
4
2
0
2
0
Al
A2
Bl
B2
Cl
C2
20
c.)
(\,)
(,)
(\,)
12
10
b.)
A2
Bl
B2
Cl
C2
Al
A2
Bl
B2
Cl
C2
20
d.)
15
15
(,)
c:
e(\,)
(\,)
'u ~t: 10
.5 (,)
- g 5
;:'-
"':l
{:
Al
10
5
0
0
0
Al
A2
Bl
B2
Cl
C2
Plot
Plot
Fig. 5. Species shown with a generalised linear model to be significantly (p.s;O.05) influenced
by fire frequency. All data is for the total incidence of occurrence in pitfall traps per plot.
Species favouring frequently burned areas: a.) Crematogaster sp. 1. Species favouring
infrequently burned areas: b.) Tetramorium mossamedense, c.) Monomorium albopilosum, d.)
Pheidole sp. 5.
19
HERAVIOURAL DOMINANCE
Far fewer species (15 species) were recorded at the baits than in pitfall traps (66 species). This
could be an artefact of differential attractiveness to tuna baits, sampling only in the day, and only
on one occasion. No studies in South Africa have previously considered behavioural dominance,
and thus although the results do not enable a comparison between plots, it is interesting to note
that the numerical domination by Generalised Myrmicines is suggestive of poor representation by
Australian and North American behaviourally dominant groups (particularly dolichoderines).
Overall there appears to be low behavioural dominance, although .A. custodiens, M
albopilosum and Pheidole species are locally dominant. Local dominance of A. custodiens was
also noted in the Orange River Valley, South Africa (Dean & Bond 1990).
FUNCTIONAL GROUPS
The most abundant functional groups were Generalised Myrmicinae (particularly species of
Pheidole; 81 % of all individual ants) and Opportunists (8 %). These groups together accounted
for 89 % of all ants, despite contributing only 55 % of ant. species. Functional group composition
in pitfall traps varied across plots for both abundance and incidence of occurrence data.
There was no significant difference (X2 = 7.32, d.f= 6; X2 = 5.01, d.f = 6; X2 = 11.53, d.f
=
6; P >0.05) between scaled abundance data for each pair of plots within a patch type, and thus
data were pooled (Fig. 6). The relative abundance of Generalised Myrmicinae was 58.1 % on
young and frequently burned plots, 66.5 % on young and infrequent plots and 58.4 % on old and
infrequently burned plots. This very high abundance on young and infrequent sites is due to the
presence of two highly abundant Pheidole species (sp. 5 and 7) in these plots. The relative
abundance of Hot Climate Specialists, Cryptic Species and Specialist Predators all declined with
20
100%
90%
80%
70%
_opp
8c: 60%
~
[J
50%
]
;:l
CCS
~SP
.D
-:x: 40%
30%
DCS
20%
lIDSC
10%
E:lHCS
OGM
......
0%
B
Plot type
A
C
Fig. 6. Functional Group composition in pitfall traps for scaled abundance data, for combined
plots (A
= young
and frequently burned, B
= young and infrequently burned,
C
= old and
infrequently burned). The fimctional groups are: GM, Generalised Myrmicinae; HCS, Hot
Climate Specialist; SC, Subordinate Camponotini; CS, Cryptic Species; SP, Specialist
Predator; CCS, Cold Climate Specialist; OPP, Opportunist.
100%
80%
u
o
5
g
60%
-Opp
(,)
(,)
0
~
[] ees
0
u
~
:su
-
40%
2!ii SP
~
DeS
lID se
20%
s ncs
DGM
0%
Al
A2
B2
BI
CI
C2
Plot
Fig. 7. Functional Group composition in pitfall traps based on incidence of occurrence data.
The plots are: Al and A2
= young
and frequently burned, BI and B2
=
young and
. infrequently burned, CI and C2 = old and infrequently burned. The fimctional groups are:
GM, Generalised Myrmicinae; HCS, Hot Climate Specialist; SC, Subordinate Camponotini;
CS, Cryptic Species; SP,' Specialist Predator; CCS, Cold Climate Specialist; OPP,
Opportunist.
21
decreasing fire influence. Opportunists appear to be influenced by patch age as they represented
21.9 % of all ants on old plots, compared to 14.2 and 11.8 % on young plots.
Functional group composition based on incidence of occurrence in pitfall traps (summed
over all pitfalls for each species) confirms that Generalised Myrmicinae, Cold Climate Specialists,
Specialist Predators and Cryptic Species all decline with increasing age. Conversely, Opportunists
appear to increase with increasing patch age (Fig. 7).
VEGETATION AND ABIOTIC CHARACTERISTICS
The percentage of total foliage biomass at ground level and 0-20 em generally increased with
decreasing fire influence (Fig. 8). Litter cover was also found to increase with decreasing fire
influence (Fig. 9). The relationship between proportion of foliage at ground level.and ant species
richness was highly significant: species richness inversely related to proportion of biomass/ foliage
at ground level (Spearman's r 2 = 0.93; n = 6; P <0.001) (Fig. 10). Bare ground cover was greatest
on young plots (especially young and frequently burned, 15.5%), and least on old and infrequently
burned plots (4.35 %).
Floral composition on the patches was variable. Loudetia simplex was found to occur on
young and frequently burned sites, Eragrostis rigidior only occurred on young plots (AI, A2 and
B 1), while Elionurus muticus (highly unpalatable grass) only occurred on infrequently burned
sites.
22
90
85
80
75
en
'3 en
s = 70
e- 8
o .$2
..J:j
65
OJ)
E .~ 60
8 .a 55
c,
50
45
•
•
•
•
•
0
0
Q)
Q)
Q)
-
0
0
0
Q)
•
2m
le<o.
l
oOm
0
Al
A2
CI
B2
BI
C2
Plot
Fig. 8. Percentage of foliage biomass at ground level (0 m) and 0-0.2 m for each plot. Al and A2 = young
and frequently burned, B I and B2 = young and infrequently burned, C I and C2 = old and infrequently
burned.
50
45
~ 40
0
(,,)
Q)
OJ)
E
Q)
8Q)
~
•
35
30
•
•
•
•
•
-25
20
Al
A2
BI
B2
C2
CI
Plot
Fig. 9. Percentage cover by litter for each plot. A I and A2 = young and frequently burned, B I and B2 =
young and infrequently burned, C I and C2 = old and infrequently burned.
40
en
Q)
'Q
35
Q)
C.
en
e0
30
"'"
Q)
..J:j
8::s
Z
25
•
20
45
50
55
65
60
70
Percentage foliage biomass at ground level
Fig. 10. Spearman's Rank Correlation between species richness and percentage foliage biomass at ground
level.
23
Discussion
FIRE AND ANT DIVERSITY
Both post-fire fuel age and fire frequency affected ant diversity in this study. The major trend
indicated that with reduced fire disturbance (i.e. older and less frequently burned plots), there was
a lowered richness and evenness, and an increased dominance of fewer ant species. This has been
shown to be the case for floristic diversity too. In a study in south-eastern Australia, frequently
burnt sites were species rich, and sites very infrequently disturbed by fire species were poor
(Prober & Thiele 1995; Morgan 1998; Morgan & Lunt 1999).
Species richness (observed (SOBS) and predicted (ACE)) decreased with increasing postburn age, and decreasing fire frequency. Similar trends have been reported elsewhere: for ants in
fynbos (Donnelly & Giliomee 1985), Eucalyptus forests (York 1994; Vanderwoude, Andersen &
House 1997b), and for insects in general (Force 1981). It follows that if biodiversity is measured
as species richness, frequent fires would thus be expected to maintain ant diversity. Species
richness however, takes no account of species composition; thus would frequent fires eliminate
ant species that prefer old vegetation, or rare fires eliminate young grassland specialists ?
This study shows that a single burning regime may compromise the survival of some
species. With frequent fires, nine species could be potentially lost (13.6 % of ants), and with
infrequent fires four species could be lost (6 % of ants). Eight species were affected by post-burn
age with all favouring young patches.
Different ant species thus show a range of responses to any particular fire (Andersen et al.
1998): some ant species have been shown to have flexible habitat requirements, while others are
more specific. York (1994) showed that there exists a suite of fire sensitive ant species that may
be lost where a burning regime is intensive, and simplification of habitats results. This may explain
24
the relatively high unique species count (7) on old and infrequently burnt plots. Decline in floristic
diversity in the absence of fire (prober & Thiele 1995), may affect some species of ant dependent
on early successional plants for food and shelter, and hence explain the potential loss of ant
species from old and infrequently burned grassland plots in PNP.
It is thus apparent that post-fire age, and FRP may have distinctly different effects on ant
community composition. For a given community assuming a disturbance gradient from very
frequent fires at one end, to an absence of fire at the other, it is likely that both extremes will
result in a loss of species. However, for different communities of ants, the interval between
burning which is optimal for sustaining diversity could differ (Gill 1996).
EFFECTS OF FIRE ON VEGETATION AND ANTS
Fire elicits a diverse array of responses by arthropods (Warren, Scifres & Teel 1987), affecting
patterns of population density and distribution, both directly through mortality and dispersal, and
indirectly through induced changes in host plant communities (Chambers & Samways 1998). Fire
often h~s little direct impact on communities of epigaeic ants due to their cryptic habits, tolerance
of dry soil which enables survival immediately post-fire, and social habits that are conducive to
rapid recolonization (Ahlgren 1974; Andersen 1991a). More importantly for ants, the effects of
fire are indirect, modifying habitat (composition and structure), altering food supplies and
influencing competitive interactions (Levieux 1983; Andersen 1983; Andersen & Yen 1985;
Greenslade 1997; Chambers & Samways 1998).
Greenslade & Greenslade (1977) propose that vegetation has a threefold influence on ant
communities through its effects on microclimate, carrying capacity, and structural complexity of
habitat. Plants provide materials and nesting sites, and are a source of food. Microclimate
modifications by plant and litter (e.g. changes in moisture, and insolation levels which affect soil
25
surface temperatures) have been shown to have a significant impact on insect diversity (Lynch,
Johnson & Balinsky 1988), and more specifically on ant activity (Brian & Brian 1951; Levings
1983; Burbidge et al. 1992; Zimmer & Parmenter 1998). Thus as vegetation exerts a powerful
influence on ant communities, any changes in plant composition and structure are likely to be
reflected by changes in ant species and community composition (Andersen 1983). Frequent fires
may result in a more open plant community with higher day temperatures and low moisture levels
at the surface (Greenslade 1997), which will influence ant species composition since some ant
species are particularly sensitive to moisture levels and soil temperatures.
Herbaceous productivity (C4 grasses) generally increases in the post-fire period (Bailey
1988). With rapid grass regeneration and growth, food supplies may increase, favouring higher
species richness and diversity as more species are able to co-exist. Thus more recently burned
plots with higher productivity levels, had a greater diversity of ant species.
Results of the vegetation foliage densities illustrate that vegetation structure differs
according to burning regime. Fire acts to modify vegetation structure, litter cover and phytomass
(Chambers & Samways 1998) which influences foraging routes and ant behaviour (Zimmer &
Parmenter 1998). As fire influence decreases (age increases and frequency decreases), foliage
density at ground level and 0-20 cm increases, as does litter cover (Figs. 8 and 9). There is a clear
relationship between habitat structure and species richness (Fig. 10): differences in vegetation
structure significantly affect ants. It is thus the effect of habitat changes, especially structure,
rather than fire directly, that may account for the differences observed in this study.
In terms of functional group composition, the young and old patches produced two
distinctive communities. Figs 6 and 7 illustrate that the proportion of Opportunists, which favour
habitats supporting low species diversity, is greatest on old and infrequent plots, while
proportions of Cryptic Species, Specialist Predators and Generalised Myrmicinae are all higher on
26
young plots. The results of this study are similar to those of Vanderwoude, Andersen & House
(1997a, 1997b) who found the. proportion of Opportunists was greatest on unburned sites. The
decline in Cryptic Species with age 1S contrary to expected results, as these species usually favour
old, infrequently burned sites with higher litter loads, and greater cover. This discrepancy may be
due to the sampling method (Andersen 1991b) as ant species are differentially susceptible to
capture in traps (Marsh 1984). Habitat simplification on young patches may have substantially
increased capture efficiency of Cryptic Species that normally forage within dense ground layers of
litter and live grass biomass (Andersen & Yen 1985).
MAINTENANCE OF BIOTIC DIVERSITY FOR PROTECTED AREAS
If maintaining species diversity is one of the objectives of conservation, then it follows that a fire
regime should aim to maintain diversity of inter-fire intervals (frequency) in addition to the current
practise of maintaining a range of time-since fire (age) (Cary & Morrison, 1995; Morrison et al.
1995), thus promoting a range of fire types within a landscape (Braithwaite 1995).
Andersen (1991a) describes how in Australia, fire-free intervals of only a few years can
have important ecological consequences, and hence the maintenance of areas with such fire
frequencies should be an objective of savanna management. This study has demonstrated that both
time since fire and frequency of burning affect ant community composition. Thus any conservation
plan to conserve biodiversity should allow for a range of frequencies, in addition to a range of
post-fire fuel ages, or risk the loss of elements of diversity.
Short fire intervals maintain high ant species richness at small scales (individual patches).
However when managing for biodiversity at larger scales, between habitat diversity (beta) must be
considered. In order to maintain biotic diversity at both alpha and beta scales, it is clear that a
mosaic of all successional ages, and variety in fire frequencies is required. This mosaic may be
27
developed through implementation of a patch-mosaic burning system, where the aim is to
maintain landscape heterogeneity, both spatially and temporally. In PNP, the spatial heterogeneity
of the fire patterns increased over the last few years of implementation of the patch-mosaic
system. This resulted in higher spatial heterogeneity scores for the latter four years (1993-1996),
than the first four years (1989-1992) (Brockett, Biggs & van Wilgen 1999). This increased spatial
heterogeneity is produced by applying fires in a varied manner over successive fire seasons.
My study has shown that the resulting patches with different, and sometimes unique fire
histories (in terms of FRP and post-fire fuel age), support different suites of ant species. Spatial
heterogeneity in the landscape is important, and the patch-mosaic burning system is effective in
maintaining ant diversity. The temporal heterogeneity of the system still needs to be established,
and an index combining spatial and temporal heterogeneity developed.
Post-fire fuel age and FRP are only two variables, and it is important to consider that there
are many other factors not tested in this study (such as intensity and seasonality of fire, patch size,
shape and adjacent patch characteristics) that could contribute in some way to the maintenance of
diversity. It isimportant too to remember that the range of fire frequencies and post-burn ages
was limited in this study. Further studies are needed to investigate the implications and effects of
these variables on the conservation of biotic diversity. In addition, the communities in this study,
as in most ecological studies, are defined by the sampling methods used (Perfecto & Snelling
1995), and thus conclusions drawn apply mainly to epigaeicant species.
The patch-mosaic system enables the conservation of a range of species through the
patchy nature of the burning system resulting in a patchwork of mosaics each with unique fire
histories. A burning system that is less variable may result in homogeneity and the loss of biotic
diversity. The loss of particular elements of diversity could be especially problematic where
mutualisms are involved, and cascade effects probable.
28
Acknowledgements
I would like to thank the North-West Parks Board, and especially Bruce Brockett for logistical
support and the foresight to implement the Patch Mosaic System. I am particularly grateful to
William Bond for project funding and support, and Hamish Robertson for equipment and ant
identification. Thanks to both for valuable discussion and draft comments. My thanks also go to
Harry Biggs for his input into the planning stages of this project, Alan Andersen for his help with
Functional Groups, and I gratefully acknowledge the field assistance offered by Ida Mathe and
Gustav Mutjila.
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29
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