Diversity and endemism of woody plant species in the Equatorial Pacific seasonally dry forests

In the first comprehensive review of the floristics of neotropical SDF Alwyn Gentry (1995) noted that SDF ecosystems were less species rich and contained only a subset of the plant diversity found in the more humid forests. The lower diversity in the Equatorial Pacific SDFs is clearly due to the low levels of diversity within families and genera. A notable exception is Leguminosae. This family showed high levels of diversity at the generic (34 genera, 19% of the total), specific (70 species, 22% of the total) and endemic species level (15 endemics, 21% of the total). This is not surprising since several studies have shown that this family is among the most, if not the most, prominent members of SDF in the Neotropics (Gentry 1995; Pennington et al. 2006). Malvaceae, on the contrary, are not necessarily regarded as important constituents of tropical dry forest communities (Pennington et al. 2006). Our data indicated that it is by far the second most important family contributing to the number of genera (15 genera, 8% of the total), species (19 species, 6% of the total) and endemic species (6 species, 9% of all endemics), although our results were based on an expanded Malvaceae concept (including 14 species from the former Sterculiaceae, Tilliaceae and Bombacaceae). Especially interesting was the subfamily Bombacoideae, contributing with several taxa (9 species, 6 genera). Gentry (1993), referring to the northern Peruvian SDFs already stated, “Fabaceae is the most speciose and dominant family of trees. Bombacaceae, though less speciose, are represented by five different genera of large trees and are probably more dominant here than elsewhere on earth”, a statement that we can certainly extend to the SDFs in the Equatorial Pacific region. A narrow concept of Malvaceae would place Boraginaceae, Cactaceae and Moraceae in second place, all with 12 species.

In contrast to the low generic and specific diversity (as compared to humid rainforests), levels of endemism seem to be among the highest in the continent. We found 67 endemic species, which represent 21% of the total of woody SDF species reported in the Equatorial Pacific region. This percentage is similar to what Dodson and Gentry (1991) reported for the flora of a SDF in Ecuador and similar to their total estimate for the entire dry forest region in western lowland Ecuador. Considering only SDFs, they estimated that 19% of the species should be endemic (approximately 190 species). The whole flora of the region, including other vegetation types below 900 m.a.s.l., was estimated at some 6,300 vascular plant species, of which 20% or 1,260 species would be strictly endemic to the region (Dodson and Gentry 1991). Kvist et al. (2004) found similar levels of endemism for the Gesneriaceae in Ecuador (23 of 107 species). These endemism levels are very similar to what Gentry (1982) estimated for the Chocó flora, one of the worlds most publicised regions in terms of plant diversity and endemism. It was recently that the Equatorial Pacific SDFs and the Chocó were jointly considered as one of the hotspots of biodiversity in the world, (Mittermeier et al. 2005), with an estimated endemism level of 25%. This estimation seems to hold true, at least for the woody component of the Equatorial Pacific SDFs.

There is little comparable information about levels of endemism in other SDF regions in the Neotropics as most data are from local checklists and inventories (e.g., Lott and Atkinson 2006 for SDF floristic checklists in Mexico and Central America). Available data suggest that the Equatorial Pacific SDFs are intermediate in levels of endemism as compared to other SDF regions. The Chiquitano SDFs in eastern lowland Bolivia seems to have the lowest endemism level of all neotropical SDF regions with only three endemic woody species out of 155 reported trees, a fact probably explained by the recent geological past of the area into which the extant flora arrived from more northerly latitudes after the last glacial maximum (Killeen et al. 2006). Intermediate levels of endemism have been reported for the dry Andean valleys in Bolivia, where 18% of the total native flora is considered endemic (López 2003). A study of three plant families (Labiatae, Asclepiadaceae, Acanthaceae) in the same region showed higher levels of endemism (33%), although care has to be taken to extrapolate these figures as there is ample variation in the level of endemism between different families (Wood 2006). The highest levels of endemism in neotropical SDFs have been found in the Brazilian Caatinga and in Mexico. In the former, 41% of the 932 known plants are endemic (Silva et al. 2003), whereas 52% of the species of Leguminosae, the most important and dominant SDF family in the Neotropics, are restricted to this biome (Queiroz 2006). Finally, Mexican SDFs are estimated to have 60% of endemic species (Rzedowski 1991).

Both countries have also variants of inter-Andean SDF, which are best represented in the long and deep valleys of Peru. The most important of these dry valleys, the Rio Marañon valley, is located east of the northwestern Peruvian coastal SDF and connected to them by the lowest mountain pass of the whole Andean chain, the Porculla Pass (2,165 m.a.s.l.). It has been suggested, that this pass has favoured the immigration and exchange of SDF biota, which evolved either in the Marañon valley or the coastal SDF (woody plants: Linares-Palomino et al. 2003; birds: BirdLife International 2003, herpetofauna: Venegas 2005). Thus, the strong presence of a group of woody species in the data, which are preferentially found at altitudes above 1,000 m.a.s.l., could be composed of species that are also found in the Marañon valley. Indeed, several species show distributions extending into this valley (e.g., Eriotheca discolor, Erythroxylum novogranatense, Loxopterygium huasango, Trichilia tomentosa, Clavija euerganea, Mauria heterophylla, Inga oerstediana).

The altitudinal distribution of woody species and endemics showed two interesting relationships. In terms of absolute species numbers and endemics, the much more extensive coastal lowlands reported higher values than the sub-montane and mountainous areas. Nevertheless, once the effect of area had been taken into account by using the density of species per 1,000 km², instead of absolute species numbers, an opposite pattern emerged, showing that species richness and endemics per unit area were highest in the mountains, and decreased substantially towards the lowlands. Similar results, although for greater elevational gradients (sea level to tree-line and above) and across several major vegetation types, were obtained by Borchsenius (1997) and van der Werff and Consiglio (2004) for the vascular floras of Ecuador and Peru, respectively. Both studies found that the density of endemic and restricted-range species was greater in the Andes than in the lowland areas on either side of these mountains. Furthermore, Borchsenius’ study suggested that the southern Andes, part of which is included in our study area, appeared to be particularly rich in endemic species.

The geographical analysis by political units showed some interesting results. Loja, Cajamarca and Esmeraldas are the units where most vascular plants have been reported (with total vascular plant endemics highest in Cajamarca and Loja, Bracko and Zarucchi 1993; Jørgensen and León-Yánez 1999). In terms of woody SDF species, it seems that apart from Tumbes, Loja, El Oro and Cajamarca, the SDFs in the other regions appear to have been little collected. In addition, the high ratios of total vascular plants to woody SDF plants and of woody SDF endemics to total vascular plant endemics in Tumbes make this region probably the best representative of SDF vegetation in the study area.

The geographical distribution analysis showed that a substantial amount of the species, non-endemics (27.5%) and especially endemics (52.9–87.5%), have been reported in less than two provinces or departments. In some cases, this might be the result of little collecting (see below), but in the case of the endemic species, these are by definition restricted to a certain area and sometimes, within this area, they are rare and local. In the SDFs of the region, we face the severe problem of habitat destruction and some estimations consider that less than 5% of the area remains forested (BirdLife International 2003). The rarity of some species and habitat reduction potentially threatens the SDF. This could enhance so-called “Centinelan extinction” events, i.e., the disappearance of rare and restricted species due to forest clearance (after the disappearance of several endemic species in Cerro Centinela, Ecuador, Dodson and Gentry 1991; Wilson 1992). In contrast to this country-level definition of endemism, endemic species to the Tumbesian region have much wider geographical distributions (e.g., Aeschynomene tumbezensis, Carica parviflora, Tabebuia bilbergii, Eriotheca ruizzi and Pithecellobium excelsum). All five are characteristic (and in some cases dominant) trees and shrubs of the SDF in Ecuador and Peru, but not found outside this region.

Collection intensity of woody plants in the Equatorial Pacific region at altitudes below 1,100 m.a.s.l. has been unequal. This is a result of the efforts of individual botanists or institutions concentrating on specific areas in the region (cf. Borchsenius 1997). The SDFs in Guayas and Tumbes have benefited from thorough work from botanists from the Missouri Botanical Garden (D. Neill in Guayas, C. Díaz in Tumbes, respectively). The Manabí SDFs have good collections due to intensive collecting from Ecuadorean botanists (e.g., Hernández and Josse 1997). Esmeraldas has recently seen intensive collection efforts as part of a Smithsonian Institution project to inventory the flora of the Mache-Chindul Mountains (Clark et al. 2006). The other SDF areas are relatively little surveyed, as can be seen from the density of collections. It is rather surprising that otherwise well-botanised regions like Cajamarca (e.g., Sagástegui 1995) and especially Loja (Aguirre et al. 2002) lag so much behind other regions in our analyses. This shows that even though the Andean flora from these regions has been comparatively well collected, efforts need still to be made to increase the knowledge of other vegetation types occurring in them.

Dry lowland or Andean vegetation formations usually lack representation in protected area systems (e.g., Borchsenius 1997; López and Zambrana-Torrelio 2006). This is especially true in the SDF of Ecuador and Peru. There are 16 protected areas in the Equatorial Pacific region covering some 5,200 km², and some of these are not completely covered by SDF (e.g., the Santuario Nacional Manglares de Tumbes and Reserva Ecológica Manglares-Churute are mainly mangroves; PN Cerros de Amotape includes an extensive area which covers a more humid variant of seasonal forests, as does the Mache-Chindul Ecological Reserve). Thus, the true extension of protected SDF in the region is probably around 2,500 km², which represents approximately 5% of the estimated 55,000 km² of remaining SDF in the region. This is, however, an optimistic estimate since the vegetation these areas protect is not necessarily intact forest. It may sound contradictory, but several of them are composed of secondary highly disturbed regenerating vegetation (e.g., Josse 1997). Only forests on very steep slopes and areas of very difficult access remain close to undisturbed vegetation. Unfortunately, these are few. Due to the constant growing population in the region and the consequent demand for new arable and habitable land, the establishment of new protected areas in near-pristine vegetation is difficult. The development of initiatives such as the Northwestern Biosphere Reserve in Peru (which includes the PN Cerros de Amotape, RN Tumbes and CC El Angolo) should be an opportunity, especially since they conserve important areas of the Tumbes and Piura department (including an elevational gradient from sea level to 1,600 m.a.s.l.), which, as has been shown above, concentrate some of the most characteristic SDFs of the region. An extension of it into adjacent protected areas of Ecuador as a transboundary biosphere reserve, a conservation figure specifically encouraged by the ‘Seville + 5’ UNESCO-MAB meeting (UNESCO-MAB 2002), should be given highest priority. This step might not only enhance the conservation value of the region, but also provide a much more extensive corridor for the movement of organisms and better coordination of conservation tasks between both countries.