We investigated changes in species composition and diversity of nocturnal geometrid moth communities along an altitudi-nal transect covering intact closed forests below 3000 m, a mosaic
Trang 1of Geometrid Moths on
Mt Kilimanjaro
Jan C Axmacher, Ludger Scheuermann, Marion Schrumpf, Herbert V.M Lyaruu, Konrad Fiedler, and
Klaus Müller-Hohenstein
INTRODUCTION
Fires often occur along the upper margin of the montane rain forest on Mt Kilimanjaro (Beck
et al 1986; Blot 1999; Meyer 1890; Salehe 1997) This is especially true for exceptionally dry periods which regularly appear during the
El Niño southern oscillation (ENSO) phases
As a consequence, the forest boundary has been profoundly lowered, and the forest has been replaced by heathland dominated by Erica excelsa, E trimera, and E arborea (Hemp and Beck 2001), with some small isolated patches
of forest remaining above the current closed forest boundary
This change in the vegetation is accompa-nied by various changes in ecological condi-tions The temperature drops more rapidly at nightfall outside the forest, and the microcli-mate is drier within the open heath vegetation, leading to the disappearance of many ferns and epiphytic plants outside the forest
The objective of our study was to explore how these fire-induced changes affect herbivo-rous insects along the upper forest margins and
in forest remnants We investigated changes in species composition and diversity of nocturnal geometrid moth communities along an altitudi-nal transect covering intact closed forests below
3000 m, a mosaic of forest remnants and heath-land at 3100 m, and heathheath-land vegetation at 3300
m The sites at 3100 m, therefore, enable a direct comparison of geometrid moth communities in heathland and forest fragments, whereas the
lower and higher portions of the transect, albeit the altitudinal differences between the sites, pro-vide comparison of community structure and diversity of geometrid moths between large closed forest and heathland habitats Geometrids were selected for four reasons: (1) They are one
of the three most diverse families of Lepi-doptera (2) They are taxonomically rather well known (Scoble 1999) (3) They proved to be suitable indicators of environmental change in a number of studies in tropical forests (e.g Beck
et al 2002; Brehm 2002; Holloway 1984; Intachat et al 1999; Kitching et al 2000) (4) They regularly occur in quite large numbers
at high altitudes at which most other insects are not very common (Brehm 2002)
TEST SITES AND METHODS
S TUDY A REA
Mt Kilimanjaro, the highest mountain in Africa (5892 m), is situated 300 km south of the equa-tor in Tanzania It is a volcanic complex com-posed of three volcanic centers, covering an area of roughly 80 km × 40 km
Within the Mt Kilimanjaro area, the study sites are located in the southwest part of the mountain along the Machame tourist route below the Shira plateau, along an altitudinal gradient from 2300 to 3300 m The upper por-tions of this area were heavily affected by forest and heath fires during the last El Niño event (1996 to 1997), when large areas of the 3523_C005.fm Page 69 Wednesday, November 23, 2005 7:50 AM
Trang 2ericaceous belt on the Shira plateau burned
(Salehe 1997) These fires also affected the
heath sites included in this study, whereas the
forest remnants and forest sites remained intact
M ETHODS
Geometrid moths were investigated at 13 sites
Eight of these sites were located within the
intact forest belt in the ranges 2300 m, 2580 m,
2700 m, and 2900 m; two plots consisted of
isolated patches of remnant forest at 3100 m;
and three sites of heathland ranged from 3100
to 3300 m
Moths were attracted by an
accumulator-powered weak UV-containing light source
(Syl-vania blacklight-blue, F 15 W/BLB-TB) placed
within a white reflective gauze cylinder
(diam-eter 0.8 m, height 1.6 m) From the gauze,
insects were sampled manually using plastic
jars prepared with diethylether The samples
were taken from 7 to 10 P.M local time between
October 2001 and January 2002 Catches were
restricted to periods without strong moonlight,
avoiding the period from 5 d before to 5 d after
full moon For each site, two to four night
catches were pooled for analysis Subsequently,
specimens were spread, sorted to
morphospe-cies level, and taxonomically identified as far
as possible at the Zoologische Staatssammlung
in Munich
Data handling and statistical analysis were
carried out using Microsoft Access, SPSS,
Esti-mateS (Colwell 2000), and an Excel macro
pro-vided by Meßner (1996) for calculating the
nor-malized expected species shared (NESS) index
and Sørensen index of similarity
RESULTS
A total of 2158 individuals representing 92 morphospecies were sampled (Table 5.1);
41 (44%) of these morphospecies representing
1563 individuals (72%) could be identified to species level
P ROPORTIONS OF S UBFAMILIES WITHIN
G EOMETRIDAE
In the study area, three subfamilies of geometrid moths, Larentiinae, Ennominae, and Geometrinae, are largely represented and can
be found along the whole altitudinal gradient Two other subfamilies, Sterrhinae and Desmo-bathrinae, also occur but account for less than 3% of individuals as well as species at any site and disappear at sites above 2900 m Concern-ing the number of individuals caught, Larenti-inae was the dominant subfamily throughout the whole study area, always accounting for more than 50% of the total catch Within the closed forest, Ennominae proved to be the sec-ond most abundant subfamily, whereas they were much rarer in the heath, where the pro-portion of Geometrinae was very high In the forest fragments, Geometrinae and Ennominae were caught in roughly equal proportions (Fig-ure 5.1)
When considering the proportions of sub-families according to the occurrence of mor-phospecies on the plots, Larentiinae are domi-nant on all plots, followed by Ennominae Geometrinae accounted for a smaller share of species richness, and the proportions seen in heathland are not different from those obtained
at some lower forest sites (Figure 5.2)
TABLE 5.1
Number of catches performed and number of geometrid moth individuals and
morphospecies caught at each site
Site
2300 F1
2300 F2
2580 F1
2580 F2
2700 F1
2700 F2
2900 F1
2900 F2
3100 Ff1
3100 Ff2
3100 E
3300 E1
3300 E2
Trang 3Effects of Fire on the Diversity of Geometrid Moths on Mt Kilimanjaro 71
A LPHA -D IVERSITY
Fisher’s alpha proved to be a reliable measure
of the diversity of moth communities in a
num-ber of studies (e.g Brehm 2002; Chey et al 1997; Schulze 2000), as long as the samples fit a log-series distribution This is true for all the sites sampled in this study Once sample
FIGURE 5.1 Proportions of subfamilies, based on numbers of individuals Sites are sorted by altitude (Site codes: altitude in meters, F = forest, Ff = forest fragment, E = heath vegetation).
FIGURE 5.2 Proportions of subfamilies, based on numbers of morphospecies Sites are sorted by altitude (Site codes: altitude in meters, F = forest, Ff = forest fragment, E = heath vegetation).
Forest
Forest 100
75
50
25
0
2300 F1 2300 F2 2580 F1 2580 F2 2700 F1 2700 F2 2900 F1 2900 F2 3100 F
3100 E 3300 E1 3300 E2
Site
Forest
Forest 100
75
50
25
0
2300 F1 2300 F2 2580 F1 2580 F2 2700 F1 2700 F2 2900 F1 2900 F2 3100 F
3100 E 3300 E1 3300 E2
Site
3523_C005.fm Page 71 Wednesday, November 23, 2005 7:50 AM
Trang 4size exceeds 120 individuals per site in
spe-cies-poor communities as in the case of Mt
Kilimanjaro, Fisher’s alpha shows a greater
independence of sample size than most other
alpha diversity measures (Hayek and Buzas
1997) This number was exceeded in our study
at all but one site, namely, the forest fragment
3100 Ff 2
When comparing geometrid diversity
from different sites, two main results emerge
First, geometrid communities at Mt
Kiliman-jaro are not very diverse overall (values
rang-ing from 9 to 15 in closed forests above 2250
m) Second, the values from the forest below
the upper treeline below 3000 m and in the
forest fragments at 3100 m do not differ
sig-nificantly, whereas diversity in the heath both
at 3100 m and 3300 m is significantly reduced
(Figure 5.3) compared to all forested sites
B ETA -D IVERSITY
To study the beta-diversity of the moth
commu-nities in the study area, two similarity measures,
the Sørensen index and the NESS index, were chosen As an ordination method, nonlinear multidimensional scaling was applied (see Beck et al 2002 and Schulze and Fiedler, in press, for methods) Figure 5.4 shows the ordi-nations of the samples for the Sørensen index and the NESS index for a low sample size parameter m = 1 to emphasize the similarity of moth ensembles with regard to the most abun-dant species and a higher parameter m = 37 with stress on the occurrence of rare species (Grassle and Smith 1976)
In all three ordinations, the sites are primar-ily arranged according to their altitudinal posi-tion along the first axis Apart from this, the forest plots are always positioned closely together and separated from the remaining sites
by the first dimension In all three ordinations, communities from forest fragments are posi-tioned closer to the heathland than to the forest sites A clear distinction between heath and for-est fragments is only discernible with the NESS index, particularly with m = 1 According to the
FIGURE 5.3 Diversity of Geometridae: Fisher’s alpha The bars indicate the standard deviation, calculated using EstimateS (From Colwell, R.K [2000] Sites are sorted by altitude (Site codes: altitude in meters, F
= forest, Ff = forest fragment, E = heath vegetation.)
2300 F1 2300 F2 2580 F1 2580 F2 2700 F1 2700 F2 2900 F1 2900 F2 3100 F
3100 E 3300 E1 3300 E2
20 18 16 14 12 10 8 6 4
Site
Forest Heath
Forest fragment
Trang 5Effects of Fire on the Diversity of Geometrid Moths on Mt Kilimanjaro 73
FIGURE 5.4A Two-dimensional nonlinear scaling of samples of Geometridae, based on matrices calculated with (A) Sørensen and NESS similarity indices, (B) small sample-size parameter m = 1, and (C) large sample-size parameter m = 37 Numbers correspond to the altitude in meters; = forest, ● = forest fragment, X = heath Low stress values indicate an excellent goodness-of-fit of the ordinations to the initial similarity data.
−1.5
−1
−1
−0.5
−0.5
0
0
0.5
0.5
1
1
3300
3100 3100
2900
2700
2300 Stress: 0.04
2580
(a)
−1
−0.5
−0.5
0
0
0.5
0.5
1
1
1.5
1.5
3300
3100
3100
2900
2700 2300 Stress: 0.04
2580
(b)
−1.5
−1
−1
−0.5
−0.5
0
0
0.5
0.5
1
1
3300
3100 3100
2900 2700
2300 Stress: 0.06
2580
(c)
3523_C005.fm Page 73 Wednesday, November 23, 2005 7:50 AM
Trang 6Sørensen index, forest fragments are
indistin-guishable from the heath plot at the same
alti-tude (3100 m) Therefore, differentiation
between heathland and forest fragments is more
a matter of altered abundance relationships,
whereas species composition in forest
frag-ments cannot be distinguished clearly from
heathlands at identical altitudes
DISCUSSION
Our data reveal strong effects of fire-induced
spread of heathland on geometrid moth
com-munities at the three levels of subfamily
rela-tionships, alpha-diversity, and species
compo-sition On the level of subfamilies, in addition
to the complementary altitudinal increase of
Larentiinae and decrease of Ennominae,
Geometrinae appear to benefit from fire, as
they are much more numerous at the heathland
as compared to forest fragments situated at
similar altitudes However, this applies only to
two species of the genus Comostolopsis
(War-ren), which has become very abundant in this
habitat The altitudinal decrease of Ennominae
is even more accentuated in open heathland,
probably as a consequence of the rather strong
link of most Ennominae to forested habitats
The overall predominance of Larentiinae has
generally been observed at both high altitudes
and latitudes that might reflect particular
ther-mal adaptations of this moth subfamily (Brehm
2002)
Within the intact montane rain forest
occurring in the lower parts of the study area
below 3000 m, there was no observed decrease
in overall geometrid moth diversity with
increasing altitude The highest forest sites at
2700 and 2900 m had even slightly more
diverse geometrid moth communities Overall
diversity in this tropical montane forest was
nowhere higher than in temperate zone
wood-lands and much lower than in other tropical
mountain regions (Schulze 2000; Brehm
2002) Yet, species composition within the
for-est belt revealed a continuous change from
2300 to 2900 m, regardless of the analytical
methods chosen
Above 3000 m, the marked drop in
geometrid alpha-diversity of the heath in
com-parison to forest fragments in the direct vicinity
(Plot 3100 Ff 1 and 2 in comparison to 3100 E) shows that the destruction of the forest along the upper forest line can trigger distinct effects
on the diversity of these insects Forest rem-nants do not show a similar impoverishment However, with regard to species composition, forest remnants are more similar to the sur-rounding heathland than to intact forest It is, therefore, not yet clear whether such forest rem-nants could serve as a permanent refuge for forest species, offering the possibility of recol-onization if the forest could encroach into the heathland again with time Alternatively, these forest fragments might actually be sinks rather than sources, if the forest moth species are only present due to permanent immigration from the intact forest
CONCLUSION
Fire-induced conversion of montane forest into heath scrubland severely reduces diversity of geometrid moths If ENSO-triggered drought periods and related forest fires increase in fre-quency and severity, as is suggested by recent climate models, our data suggest massive con-sequences for large proportions of insects that are specifically dependent on the more humid and thermally less extreme and less variable conditions prevailing in the high-altitude closed forest
SUMMARY
Due to reoccurring fires, the upper forest boundary at Mt Kilimanjaro has become dis-tinctly lowered and fragmented These large-scale and long-term vegetation changes are reflected in the changes to geometrid moth communities These changes were studied on
13 sampling sites located within the intact for-est below 3000 m, as well as in forfor-est fragments and in the heath vegetation replacing the forest above 3000 m The overall diversity of geometrid moths decreases when the forest is replaced by heath The species composition, especially the abundance pattern of the species,
is dramatically altered Forest fragments remaining within heathland areas might serve
as refuge areas for some of the moth species,
Trang 7Effects of Fire on the Diversity of Geometrid Moths on Mt Kilimanjaro 75
but only to a limited extent, as is reflected by
strong faunal dissimilarities between intact
for-est and forfor-est fragments
ACKNOWLEDGMENTS
We would like to thank the German Research
Foundation (DFG, Mu 364 14-1, 2 & 3),
For-estry and Beekeeping Division (FBD),
Kili-manjaro National Park (KINAPA), Tanzania
National Parks (TANAPA), Tanzania
Commis-sion for Science and Technology (COSTECH),
the CITES-Office, Dr Axel Hausmann at the
Zoologische Staatssammlung, Munich, and
numerous other helping hands and minds for
their kind help and cooperation
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