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155BIOTROPICA 34(1): 155±167 2002

Comparison and Origin of Forest and Grassland Ant Assemblages in the High Plateau of Madagascar (Hymenoptera: Formicidae) 1

Brian L. Fisher

2

Department of Entomology, California Academy of Sciences, Golden Gate Park, San Francisco, California,

94118, U.S.A.

and

Hamish G. Robertson

3 Life Sciences Division, South African Museum, P.O. Box 61, Cape Town 8000, South Africa

ABSTRACT

We assessed species richness and composition of ant assemblages in adjacent montane forest and secondary (anthro-

pogenic) grassland habitats in the central plateau of Madagascar. We used ®ve quantitative methods (leaf litter sifting,

two types of pitfall traps, beating low vegetation, and soil digging) and compared methods within and across habitats.

Sample-based and occurrence-based accumulation curves demonstrated that the ef®ciency of ant inventory methods

is habitat speci®c. Litter sifting, however, was the single most ef®cient method in both habitats. Overall, our analyses

of the relative ef®ciency of methods recommend the use of sifting and beating in the montane forest site, and sifting

alone in the grassland site. In four of ®ve methods, more species were collected in the grassland site (31 spp.) than

in the forest site (26 spp.). Occurrence-based accumulation curves based on all methods demonstrated that species

richness was similar in the two habitats, reaching a maximum difference of approximately one species. Only ®ve

species were shared between the grassland and forest sites. The presence of a high number of ant species restricted to

the grassland site (18 spp.) is the ®rst record of high endemism in this habitat in Madagascar and may have strong

implications for the reconstruction of the natural vegetation types at the time humans arrived. Their presence suggests

that a comparable open habitat, such as montane woodland, shrubland, or thicket, was present on Madagascar long

before humans developed the secondary grasslands less than 2000 years ago. These results are contrary to the ``classical

hypothesis'' that the central plateau was a continuous region of closed forest. These results support the hypothesis

that the montane regions, including the central plateau, once contained areas of habitat with an open structure and

that the endemic ants now found in the secondary grasslands were originally native to such a habitat.

REÂSUMEÂ

La richesse en espeÁces et la composition des fourmis ont eÂte inventorieÂes dans la foreÃt montagneuse du plateau central

de Madagascar et dans la prairie secondaire adjacente. Cinq meÂthodes quantitatives ont eÂte utiliseÂes (tamisage des litieÁres,

deux types de trous-pieÁges, battage des veÂgeÂtations basses et lavage de terre). Nous avons compare l'ef®cacite de ces

meÂthodes dans chacun et entre les deux habitats. Les courbes d'accumulation d'espeÁces baseÂes sur l'eÂchantillon et sur

l'occurrence ont montre que l'ef®cacite des meÂthodes d'inventaire de fourmis est habitat-deÂpendant. Cependant, le

tamisage des litieÁres se montrait la meÂthode la plus ef®cace dans les deux habitats. Nos analyses sur l'ef®cacite relative

des meÂthodes recommandent l'utilisation combineÂe du tamisage des litieÁres et du battage de veÂgeÂtation dans la foreÃt

montagneuse, et l'utilisation seule du tamisage des litieÁres dans la prairie secondaire. Dans quatre des cinq meÂthodes,

plus d'espeÁces ont eÂte collecteÂes dans la prairie (31 spp.) que dans la foreÃt (26 spp.). Les courbes d'accumulation

d'occurrence baseÂes sur toutes les meÂthodes ont deÂmontre que la richesse en espeÁces de ces deux habitats est similaire,

atteignant seulement une diffeÂrence maximale d'approximativement une espeÁce. Pourtant, seulement cinq espeÁces parta-

geaient les deux habitats. La preÂsence de plusieurs espeÁces de fourmis uniquement trouveÂes dans la prairie (18 spp.) est

la premieÁre observation d'une forte endeÂmicite de cet habitat et peut avoir une importante implication pour la restauration

des types de veÂgeÂtation naturelle initiale de l'õÃle. Leur preÂsence suggeÁre que des habitats ouverts similaires tels que les

formations arbustives de montagnes et les fourreÂs, eÂtaient preÂsents aÁ Madagascar longtemps avant la transformation de

ces habitats en prairie secondaire par les humains. Ces reÂsultats contredisent l'hypotheÁse classique qui avance que le

plateau central a eÂte uniquement compose d'une reÂgion de foreÃt dense. Ainsi, ces reÂsultats supportent l'hypotheÁse que la

reÂgion montagneuse contenant le plateau central, renfermait des habitats aÁ structure ouverte et que les fourmis endeÂmiques

actuellement trouveÂes dans la prairie secondaire sont, a l'origine natives de ces habitats.

Key words: collection methods; deforestation; Formicidae; grassland; inventory design; Madagascar; tropical montane forest.

1 Received 2 October 2000; revision accepted 27 March 2001. 2

E-mail: b®sher@calacademy.org

3

E-mail: hrobertson@iziko.org.za

156 Fisher and Robertson

SINCE HUMANS COLONIZEDMADAGASCARca1500±

2000 years ago (MacPheeet al.1985, Burney

1987a±c), it is estimated that more than 80 percent

of Madagascar's original habitat has been destroyed (Du Puy & Moat 1996, 1998). Most of the island (ca72% of the total land surface) is now composed of secondary (anthropogenic) grasslands, which are depauperate, dominated by pantropical species characteristic of disturbed settings, and burnt an- nually (Abrahamet al.1996, Lowryet al.1997).

The central plateau region of Madagascar, which

includes the Ankazomivady region studied here, is now dominated by this extremely impoverished¯o- ra.

How, when, why, and to what extent the cen-

tral plateau of Madagascar has been transformed from its primary vegetation since the arrival of hu- mans have been of great interest and debate (re- viewed in Gade 1996, Lowryet al.1997). The arguments differ in their interpretation of the orig- inal vegetation type, but all conclude that ®re was the dominant force in altering the vegetation. The central difference among the views is the extent to which grasslands were present when humans ®rst started altering the environment. The so-called ``classical hypothesis'' holds that prior to human ar- rival, the central plateau of Madagascar was covered by essentially continuous, dense, climax forest (Pe- rrier de la BaÃthie 1921, 1936; Humbert 1927,

1949; MacPheeet al.1985: 465; Burney 1987a:

130; Gade 1996). On the other hand, studies based

onpollen core samples taken across the island ar- gue that grasslands were present when humans ar- rived. One interpretation of the pollen core sam- ples holds that the grassland vegetation occurring today is primarily a degraded form of this original climax vegetation, which consisted of open grass- land areas devoid of any woody elements (MacPhee et al.1985; Burney 1987a, b). A second interpre- tation based on pollen cores and other evidence suggests that open grasslands were not dominant when humans arrivedbutthat grasses were present only as a minor component of the vegetation, which included forest and less densely wooded veg- etation such as shrubland and thicket (Lowryet al.

1997). In this latter case, the secondary grasslands

of today were once a mosaic of forest, thicket, and shrubland in which grasses were present.

Gade (1996) and Lowryet al.(1997) have

summarized several lines of evidence supporting the idea that the central highlands of Madagascar were once forested (Gade 1996) or composed of forest, shrubland, and thicket (Lowryet al.1997) and that the vast grasslands seen today resultedfrom anthropogenic burning. One of the argu- ments for this scenario is based on known high levels of plant and animal endemism in other nat- ural habitats in Madagascar, contrasted with low levels in grasslands. If the grasslands were ancient on Madagascar, Gade (1996) and Lowryet al. (1997) reasoned that a rich endemic herbaceous ¯ora would have evolved. To further support this claim, Gade (1996) noted that the highlands of

Madagascar today have none of the ecological com-

plexity of East African savannas whereplants and mammals show remarkable adaptations to ®re. All living mammalian species on Madagascar are forest dwelling, and no amphibians or reptiles are known to be endemic to the secondary grassland areas (Gade 1996, Raxworthy & Nussbaum 1996). Fur- thermore, the absence of endemic amphibians and reptiles in the high plateau grasslands was used by

Raxworthy and Nussbaum (1996) as strong evi-

dence that the current grasslands are recent and arti®cial, and that the original vegetation and en- demic herpetofauna have been lost. As a result, the secondary grassland habitat has been viewed as a bleak, sterile landscape with few endemic taxa. Since ants are successful invaders of disturbed hab- itats in Madagascar and throughout the world (Williams 1994, Fisheret al.1998), the ant assem- blages in these grassland habitats would be expect- ed to show a similar pattern of low diversity and few endemics. We provide the ®rst detailed account of the ant assemblages in grasslands of the high plateau and present evidence ofhigh levels of en- demism.

Our ®rst objective addressed the need to de-

velop ef®cient inventory methods for these habitat types. We used ®ve quantitative methods (leaf litter sifting, two types of pitfall traps, beating low veg- etation, and soil digging) and compared their rel- ative ef®ciency within and across habitats. Our sec- ond objective wastocompare the species richness and composition of ant assemblages in adjacent forest and grassland habitats in the central plateau of Madagascar. Speci®cally, we wanted to test the hypothesis that the ant fauna in the grassland would contain no or few endemics and be domi- nated by invasive exotics.

METHODS

STUDY SITES.Ants were intensively surveyed in a

montane forest block (Ankazomivady) and the ad- jacent exotic grassland between 6 and 15 January

1998. The two sites were located in the central

high plateau of Madagascar in the province of Fia-

Forest and Grassland Ant Assemblages 157

narantsoa. The forest block was recently (8±12 years ago) cut off from the extensive forest block to the east, which extends to the east and south toward Vohipara, Vohiparara, and Ranomafana (Ifandiana; Goodmanet al.1998). The inventories were conducted at the following sites: (1) Anka- zomivady Forest site located 29 km SSW of Am- bositra, 4.6 km SW of Ambalamanakana, 20

846.69S, 47

809.9

9E at 1700 m elevation. The

forest had been selectively logged for large trees and showed numerous signs of use by zebu (Bos indi- cus), bamboo harvesting, and wood cutting. Good- manet al.(1998) provides a botanical description of the forest; (2) Grassland site located 28 km SSW of Ambositra, 4.1 km SW of Ambalamanakana, 20

846.59S, 47810.19E at 1670 m elevation. The age

of the grassland is dif®cult to determine but dis- cussions with the elders in Ambalamanakana, a neighboring village, revealed that the grassland had been present for longer than 50 years and subjected to burning every few years. S

URVEY METHODS.Five quantitative methods were

used at the forest and grassland sites. Each method consisted of taking samples along a 250 m transect. Separate transect lines were used for each method, except for beating and sifting, which followed the same transect line. Transects were parallel andca

10 m apart. The grassland transects ran perpendic-

ular to the forest edge and beganca100mfrom the forest edge. The forest transects were located over 500 m from the forest edge. (1) Pitfall traps (test tubes). Fifty pitfall traps were spaced at 5 m intervals along the transect. Pitfall traps consisted of test tubes (18 mm internal diam.

3150 mm length) partly ®lled to a depth

ofca50 mm with soapy water and 5 percent eth- ylene glycol solution, which were inserted into PVC sleeves and buried with the rim ¯ush to the soil surface. Traps were left in place for four days. (2) Pitfall traps (cups). Fifty pitfall traps were spaced at 5 m intervals along the transect. Pitfall traps consisted of plastic cups (65 mm internal diam.

390 mm length) partly ®lled to a depth of

ca40 mm with soapy water and 5 percent ethylene glycol solution, which were buried with the rim ¯ush to the soil surface. Traps were left in place for four days. (3) Leaf litter sifting. Invertebrates were extract- ed from samples of leaf litter (leaf mold, rotten wood) using a modi®ed form of the Winkler ex- tractor (Fig. 2 in Fisher 1998). The leaf litter sam- ples involved establishing 50 plots, each 1 m 2,at

5 m intervals along the transect line. The leaf litterinside each plot was collected and sifted through a

wire sieve of 1 cm grid size. Before sifting, the leaf litter material was chopped with a machete to dis- turb ant nests in small twigs and decayed logs. Ants and other invertebrates were extracted from the sifted litter during a 48-hour period in mini-Wink- ler sacks (for a detailed discussion of the mini-

Winkler method, see Fisher 1998).

(4) Soil digging. Subterranean ants were col- lected from 25 soil samples, spaced 10 m apart along the transect line. Soil sampling methodology consisted of digging a 30 330

330 cm hole and

searching the extracted soil for ants and ant brood. Small ants are dif®cult to discern in soil, but their white brood contrast well with the soil and facili- tate location of nests. (5) Beating low vegetation. Along the leaf litter transect, 25 beating stations were established 10 m apart. Ants on low vegetation and arboreal ants were sampled by holding a stretched 60

360 cm

white canvas platform below the undergrowth and beating the trunk of a tree (forest site) or clump vegetation (grassland site) three times with a stick. The dislodged ants that fell onto the canvas plat- form were placed in ethanol. This process was re- peated six times for each of the 25 beating samples. Therefore, each beating sample consisted of six dif- ferent plant subsamples, each beaten three times with a stick. The six beating subsamples were taken within a 5 m radius of the beating station along the transect.

Ants were also surveyed through general col-

lecting, de®ned as any collection that was separate from the ®ve quantitative transect methods, in- cluding searching in rotten logs and stumps, in dead and live branches, in bamboo, on low vege- tation, under canopy moss and epiphytes, and un- der stones. D

ATA ANALYSIS.Our analyses are divided into two

parts: (1) evaluation of inventory design: ef®ciency, cost (effort), and completeness of inventory meth- ods; and (2) comparison of ant species richness and composition in grassland and forest sites. Only rec- ords of ant workers were used in data analysis since the presence of queens or males in samples does not necessarily signify the establishment of a colony of that species within the transect habitat type. Our analyses of inventory design (ef®ciency, ef- fort, and completeness) and number of species col- lected are based on visual inspection of the shape and magnitude of species accumulation curves.

Comparison of measured species richness without

reference to accumulation curves is not meaningful

158 Fisher and Robertson

even when a standardized sampling effort is em- ployed (Colwell & Coddington 1994, Gotelli &

Colwell 2001). We used two types of accumulation

curves to address the two types of objectives. To analyze inventory design, species accumulation was plotted as a function of both the number of sam- ples taken and the number of species occurrences (see below). For comparing relative community species richness in the two habitats, species accu- mulation was plotted as a function of the number of species occurrences. These two types of accu- mulation curves evaluate two different aspects of the inventory process.

Species accumulation curves plotted as a func-

tion of the number of samples is a measure of spe- cies density, the number of species per unit of sam- ple effort, and may not re¯ect species richness (Go- telli & Colwell 2001). If habitats differ in density of individuals, or if two habitats differ in structure in a way that in¯uences the effectiveness of a par- ticular sampling method, then species accumula- tion curves plotted as a function of number of sam- ples may not re¯ect the actual species richness of the community. For example, high density habitats will need less sampling effort than low density hab- itats to reach equal levels of completeness. Al- though not always appropriate for comparing rel- ative species richness, we used sample-based accu- mulation curves as a tool to evaluate sampling de- sign and effort. Effort in this case was the time and cost to collect a unit sample in the ®eld. This isquotesdbs_dbs5.pdfusesText_10