Developing an identification key for the economically important genus Inga

Inga is a genus of about 300 tree species in the Leguminosae (sub-family Mimosoideae), restricted to tropical America. The genus has a long history of utilisation dating back over 2,000 years and has been traditionally cultivated largely for its edible fruits. An understanding of the local economic importance of Inga is well established and developing work on taxonomic resources for the genus is underway at Kew, principally preparation of an interactive identification key to its species. 

Taxonomy and ecology 

The genus Inga is a common feature in lowland and montane rainforest throughout Central and South America, from Mexico to as far south as Uruguay. Its highest species diversity is in Central America, and in the foothills of the Andes in Colombia, Ecuador and Peru. Inga occupies a wide range of habitats and occurs at elevations from sea level up to 3,000 m. Species can be found in rainforest on non-flooded land, and in periodically or permanently flooded and disturbed land, often closely associated with rivers.  

Morphology and evolution 

Inga is notorious for the uniformity of its morphology. All species are small to large trees up to 40 m tall, with smooth bark, and all have paripinnate leaves (compound leaves which terminate in a pair of leaflets) with characteristic foliar nectaries found between each leaflet pair along the leaf rachis. These glands produce nectar day and night and attract mostly ants, which defend the plant against herbivores (Koptur, 1984). The leaf rachis in many species is winged to varying degrees (as in the photo of Inga vera shown below). 

Inga is thought to have diversified relatively recently in evolutionary history, with speciation occurring in the last 10 million years, and many species arising as recently as 2 million years ago (Richardson et al., 2001). 

Previous taxonomic work 

The earliest significant treatment of the genus Inga was by George Bentham at Kew, in 1875. His treatment included 140 species. 

In the 1980's it was realised by the botanical community that, in order to increase the scale of utilisation of Inga, a more up-to-date treatment of the genus, including species descriptions and illustrations, was required. This would allow local and cultivated species to be more easily identified.   

Terry Pennington, an Honorary Research Associate at Kew, and an expert in tropical ecology, published a monograph in 1997: The Genus Inga: Botany. This comprehensive tome describes 258 species, complete with distribution maps and illustrations produced by the prolific botanical artist Rosemary Wise. During the course of the five-year project, Terry spent two years in the field collecting Inga species from wild populations, and he examined nearly 10,000 herbarium specimens. Alongside his monograph, Terry also published books on the utilisation of Inga (1998), and the genus Inga in Ecuador (1997) and Peru (1997). 

Development of an interactive identification key to Inga to be implemented on Kew’s Neotropikey site 

As a sandwich student from the University of Reading working in the Americas team at Kew since August 2014, I have been helping to develop the web-based Neotropikey, Kew’s collection of identification resources for the plants of the Neotropics. 

I have worked with my supervisors, Bente Klitgaard and Anna Haigh, to source images of, and test genus-level interactive keys to the families Plantaginaceae and Lauraceae written by the Brazilian botanist Dr Vinicius C. Souza and the Missouri-based Dutch botanist Dr Henk van der Werff, respectively. Both these treatments are now available on the Neotropikey site. All the identification keys found on Neotropikey were created using the program Lucid 3.5, a software package designed to allow users to create matrix keys, in contrast to 'rule-based' dichotomous keys, which are commonly found in field guides.  

Earlier in 2015, having gained more experience using Lucid, I was presented with the challenging task of creating an identification key to all the species of Inga, using a combination of Terry Pennington’s Inga monograph, and the vast Inga collections held in Kew’s herbarium. This was necessary because the regional keys in Terry's monograph are further divided by taxonomic sections of Inga, but the monograph provides no key to sections, so that only those very well acquainted with the species-level taxonomy will know which sub-key to use. When completed, my new key aims to provide a relatively quick and reliable tool for identifying Inga species in the future. 

In discussion with Terry Pennington, a list of morphological characters important for species level identification was created, and I'm now well advanced with the task of inputting data for all species into Lucid. Once I have worked through the 258 species described in Terry’s monograph, the next step will be to gather data for about 50 species which have been described since the monograph was published. Later, images for each species and their taxonomically significant characters will be sourced; these will include herbarium specimen scans, field photographs and line drawings. The final key is to be ready for incorporation into Neotropikey by the end of my 12-month sandwich placement, in August 2015.