Biodiversity and Conservation 8: 1205–1218, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Genetic diversity of Couratari multiflora and Couratari guianensis (Lecythidaceae): consequences of two types of rarity in central Amazonia NADJA LEPSCH-CUNHA1,∗ , PAULO Y. KAGEYAMA2 and ROLAND VENCOVSKY3 1 Projeto Dinâmica Biológica dos Fragmentos Florestais, Instituto Nacional de Pesquisas da Amazônia (PDBFF/INPA), Ecologia, CP 478, 69011-970, Manaus, AM; 2 Escola Superior de Agricultura ‘Luiz de
Queiroz’, Universidade de São Paulo (ESALQ/USP), CP 09, Depto de Ciências Florestais, 13418-970, Piracicaba, SP; 3 Escola Superior de Agricultura ‘Luiz de Queiroz’, Universidade de São Paulo (ESALQ/USP), C.P. 83, Depto de Genética, 13418-970, Piracicaba, SP, Brazil; ∗ Author for correspondence (fax: 092 642 2050; e-mail:
[email protected]) Received: 25 August 1998; accepted in revised form 7 January 1999
Abstract. We quantified the within-population genetic variation of Couratari multiflora and C. guianensis, two tree species found in ‘terra firme’ forests of central Amazonia. Both species have some ecological features in common, but they differ in population abundance across their geographic ranges. While C. multiflora has been found only in low-density populations in all sites studied to date, C. guianensis is relatively common in some sites and very scarce in others. In a 400-ha plot, we found 41 and 29 adults of C. multiflora and C. guianensis, respectively. Twenty-two saplings of C. guianensis and 103 seedlings of C. multiflora were also examined. The mean expected heterozygosities (Hem ) of seedlings and adults of C. multiflora were 0.431 and 0.436, and the mean fixation indices (Fm ), 0.114 and 0.176, respectively. For C. guianensis, saplings and adults presented Hem equal to 0.425 and 0.429, and the Fm were 0.393 and 0.527, respectively. These low-density populations of two congeneric species did not differ in terms of genetic diversity, but rather they differed in terms of mean observed heterozygosity (Hom ), and therefore Fm . The species with variable population density had lower Hom and greater Fm relative to the species that is always found in low-density. Key words: Amazonia, conservation, Couratari spp., genetic diversity, rare tropical tree species
Introduction A ‘rare’ species may be a species with narrow geographical range (i.e., endemic), a species with narrow habitat specialization, a species with low local population density, or any combination of these (Rabinowitz 1981). In the first two types, the species could be locally abundant, and in the last two, they could have wide geographical range. For tropical forests, a high percentage of plant species are locally represented by one individual or less per hectare (Hubbell and Foster 1986; Rankin-de-Merona and Ackerly 1987; Gentry 1990). For the majority of these locally uncommon tropical species, it is not known what type of rarity they exhibit, i.e., if they have higher
1206 abundance in other parts of their geographic ranges. Even considering that abundance variation is a common feature of tropical species (Schoener 1987; Brown et al. 1995), natural reserves in the Amazon will probably not represent the supposed highdensity populations of many species. However, the need to understand the genetic and demographic consequences of local rarity is an urgent matter. Although previous studies suggested that in general rare tropical tree species appear to have much lower genetic diversity (Hamrick and Murawski 1991) and lower outcrossing rates (Muraswki and Hamrick 1991) than trees at higher densities, more recent studies have shown contrasting results. The genetic studies done with low-density-tree species, with few exceptions, revealed outcrossing rate estimates higher than 80%, large effective pollen movement, and for some species, high gene diversity (O’Malley and Bawa 1987; O’Malley et al. 1988; Hall et al. 1996; Stacy et al. 1996; Nason and Hamrick 1997). Therefore, the early expectations that the scattered-tropical trees were self-fertilizing (Baker 1959), with relatively little genetic diversity and high levels of endogamy (Corner 1954), do not apply for many of these species studied to date. Four types of rarity were recognized for the tropics: (1) the species that are uniformly rare almost throughout their range; (2) the species that are common in some places but are rare in others; (3) the endemic species, generally abundant in a specific area; and (4) the clustered species, which occur in high densities but the total population in a given site is low (Bawa and Ashton 1991). These demographic features are probably reflected in the genetic structures of populations, and broad conservation generalizations will not apply equally to all types of rarity. We are concerned with contrasting levels of genetic diversity between the first two types of tropical-rare species cited above, through the study of two congenerictree species with different population abundance patterns. Couratari multiflora (J.M. Smith) Eyma is probably uniformly rare throughout its range. Its populations have been found at low densities in all sites where it was examined. Couratari guianensis Aublet has a distinct demographic pattern, with low-density populations in some sites and with relatively high density in others (Prance et al. 1976; Mori and Collaborators 1987; Mitchell and Mori 1987; E. Vidal, IMAZON, personal communication; ISA reporter). Here we report the results of an isozyme study in which we estimated withinpopulation genetic variation and fixation indices for these two tree species. We determined these estimates for different size/age classes within each species, comparing seedlings, saplings and adults. We addressed the following question: will species with different demographic structures (fluctuating and uniform densities) have different levels of inbreeding and within-population genetic variation?
1207 Methods Species descriptions C. multiflora occurs from Venezuela throughout the Guianas and the Brazilian Amazon. C. guianensis has wider distribution, occurring from Costa Rica to the whole Amazonian region (Mori and Prance 1990). They are emergent trees reaching 40–50 m in height, leafless when flowering and fruiting, and possess wind-dispersed seeds. Their flowers are characterized by a strongly asymmetric androecium which produces a nectar reward for few-specialized pollinators such as large-euglossine bees (Nelson et al. 1985; Mori and Boeke 1987; Mori and Collaborators 1987). Study site and field collection Field-work was conducted at a 1,000 ha study area or area within a continuous forest (Reserve 1501) administered by the Biological Dynamics of Forest Fragments Project (BDFFP). The BDFFP is a joint project of the Smithsonian Institution (SI, Washington, DC, USA) and the Instituto Nacional de Pesquisas da Amazônia (INPA, Manaus, Amazonas, Brazil). The Reserve is located about 90 km from Manaus (2◦ 30 S, 60◦ W), in the Distrito Agropecuário, an area of 500,000 ha of non-flooded (‘terra firme’), relatively undisturbed forest at 80–110 m above sea level. The average temperature of the area is 26 ◦ C with a maximum of 35–39 ◦ C and a minimum of 19–21 ◦C. The annual rainfall ranges from 1,900–2,300 mm, with a rainy season from December to April and a dry season from May to November (Lovejoy and Bierregaard 1990). Our study was initiated based on a previous work of the Lecythidaceae family realized in a 100-ha plot within the Reserve (Mori and Lepsch-Cunha 1995). This plot includes 18 and 6 individuals considered adults (≥20.0 cm DBH, diameter at breast height), of C. multiflora and C. guianensis, respectively. In order to survey more individuals of these two species, the plot size was increased to 400-ha, and all individuals ≥20 cm DBH were inventoried, yielding 41 and 29 adults of C. multiflora and C. guianensis, respectively. In addition to the adults, 22 young individuals of C. guianensis were sampled among 1.0 to
