Research Article |
Corresponding author: Natasha Lombard ( n.lombard@sanbi.org.za ) Academic editor: Marcos A. Caraballo-Ortiz
© 2021 Natasha Lombard, Margaretha Marianne Le Roux, Ben-Erik van Wyk.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Lombard N, Le Roux MM, van Wyk B-E (2021) Electronic identification keys for species with cryptic morphological characters: a feasibility study using some Thesium species. PhytoKeys 172: 97-119. https://doi.org/10.3897/phytokeys.172.53484
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The popularity of electronic identification keys for species identification has increased with the rapid technological advancements of the 21st century. Although electronic identification keys have several advantages over conventional textual identification keys and work well for charismatic species with large and clear morphological characters, they appear to be less feasible and less effective for species with cryptic morphology (i.e. small, obscure, variable characters and/or complicated structures associated with terminology that is difficult to interpret). This is largely due to the difficulty in presenting and illustrating cryptic morphological characters unambiguously. When taking into account that enigmatic species with cryptic morphology are often taxonomically problematic and therefore likely exacerbate the taxonomic impediment, it is clear that species groups with cryptic morphology (and all the disciplines dependent on their correct identification) could greatly benefit from a user-friendly identification tool, which clearly illustrates cryptic characters. To this end, the aim of this study was to investigate and develop best practices for the unambiguous presentation of cryptic morphological characters using a pilot interactive photographic identification key for the taxonomically difficult plant genus Thesium (Santalaceae), as well as to determine its feasibility. The project consisted of three stages: (1) software platform selection, (2) key construction and (3) key evaluation. The proposed identification key was produced with Xper3 software and can be accessed at http://www.xper3.fr/xper3GeneratedFiles/publish/identification/1330098581747548637/mkey.html. Methodologies relating to amongst others, character selection and delineation, visual and textual descriptions, key construction, character coding and key evaluation are discussed in detail. Seventeen best practices identified during this study are subsequently suggested for future electronic key compilation of species with cryptic morphology. This study indicates that electronic identification keys can be feasible and effective aids for the identification of species with cryptic morphological characters when the suggested best practices are followed.
Best practice, interactive key, key construction, photographic key, Santalaceae, South African plants, taxonomic impediment, Xper3
Species identification underpins the majority of biological sciences (
Although electronic identification keys have several advantages over conventional identification keys [as detailed in
Enigmatic species with cryptic characters such as many plants, insects, bryophytes and microorganisms are common and are often surrounded by much taxonomic uncertainty (
Thesium is a hemi-parasitic plant genus of ± 350 species that has its centre of diversity in southern Africa, with ± 175 species (
The aim of this study was to investigate and develop best practices for the unambiguous presentation of cryptic morphological characters using a pilot interactive photographic identification key. The project was developed by (1) identifying practical, easy-to-use software with which to construct a photographic identification key, (2) producing a pilot identification key for 25 Thesium species found in the eastern part of South Africa and (3) evaluating the effectiveness of the identification key with a target group of users from different backgrounds. We subsequently propose a multi-access interactive photographic identification key produced with Xper3 software.
As the intent of this study was to investigate and demonstrate principles behind the unambiguous presentation of cryptic characters and not to produce a comprehensive field-ready identification key, a subset of 25 species (Table
The 25 Thesium species included in the pilot interactive photographic identification key, as well as the most recent taxonomic treatment for each species.
Species | Taxonomic treatment used |
Thesium angulosum A.DC. |
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Thesium asterias A.W.Hill | Hilliard 2006 |
Thesium confine Sond. |
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Thesium costatum A.W.Hill |
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Thesium cupressoides A.W.Hill |
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Thesium davidsoniae Brenan | Brenan 1985 |
Thesium durum Hillard & B.L.Burtt |
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Thesium goetzeanum Engl. |
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Thesium gracilarioides A.W.Hill |
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Thesium gracile A.W.Hill |
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Thesium gypsophiloides A.W.Hill |
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Thesium impeditum A.W.Hill |
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Thesium magalismontanum Sond. |
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Thesium multiramulosum Pilg. | Hilliard 2006 |
Thesium natalense Sond. | Lombard et al. in prep. |
Thesium ovatifolium N.Lombard & M.M.le Roux |
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Thesium pallidum A.DC. |
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Thesium procerum N.E.Br. |
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Thesium racemosum Bernh. |
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Thesium resedoides A.W.Hill |
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Thesium scirpioides A.W.Hill | Lombard et al. in prep. |
Thesium transvaalense Schltr. |
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Thesium utile A.W.Hill |
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Thesium vahrmeijeri Brenan |
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Thesium zeyheri A.DC. |
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Xper3 was chosen as the platform for the present study as it is a free access self-controlled programme where no external data storage or servers are needed, and which includes all of the functionalities required by the authors (e.g., multi-access keys, visual and text descriptors and species profiles) (
Construction of the identification key was completed in four steps: 1) data collection, 2) taxonomic and character backbone construction, 3) character coding and 4) species profile compilation.
Characters and character states used
A total of 26 characters were used in the key (Table
Images
Live material was photographed in the field during the flowering seasons (September to February) of 2016, 2017 and 2018 using a Canon EOS 400D camera and Canon EF 100 mm/2.8 USM macro lens. Where live material could not be accessed for certain characters or species, photographs of herbarium material were used. One of the advantages of electronic identification keys is that they can continuously be updated and current images can be replaced with superior images as they become available. Flowers from herbarium material were rehydrated by placing them in Windowlene (cleaning agent) for 15 min before being photographed. Herbarium material were photographed with standard smartphone cameras (Huawei P9 lite, Samsung S7) by aiming the smartphone camera lens at the eyepiece of a light microscope (Nikon SMZ 745 T stereo microscope, Nikon Corporation) so that the enlarged image becomes visible through the eyepiece and then taking the photo. Photographs were later edited where necessary to enhance characters using Microsoft PowerPoint software v. 14.0.7229.5000 (Microsoft Corporation). All photographs included here and in the key were taken by the authors unless stated otherwise. Bract shape and placental column shape photographs were supplemented with illustrations to ensure unambiguity (
The 26 characters and their respective character states used to distinguish between selected Thesium species in a pilot interactive photographic identification key. Definitions of characters and character states are given in the identification key (http://www.xper3.fr/xper3GeneratedFiles/publish/identification/1330098581747548637/mkey.html).
Character | Character state | |||||||
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1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |
Distribution in South Africa (Province) | Eastern Cape | Free State | Gauteng | KwaZulu-Natal | Limpopo | Mpumalanga | Northwest | Northern Cape |
Habit 1 (shape) | Erect | Virgate | Decumbent or procumbent | |||||
Habit 2 (woodiness) | Woody | Herbaceous | ||||||
Habit 3 (branching position) | Unbranched | From the lower third | From the middle third | From the upper third | ||||
Root system | Branched | Underground stem | ||||||
Vegetative scales | Present | Absent | ||||||
Plant height | (Actual measurement in m) | |||||||
Stem cross-section | Smooth | Ribbed (sulcate) | Winged (alate) | |||||
Plant hairiness (indumentum) | Hairs absent (glabrous) | Hairs present (pubescent) | ||||||
Foliage type | Leaves | Scales | ||||||
Leaf orientation | Appressed | Spreading | Not applicable | |||||
Leaf attachment | Fused to stem (decurrent) | Not fused to stem (not decurrent) | ||||||
Inflorescence 1 - apex | Indeterminate | Determinate | ||||||
Inflorescence 2 - structure | Raceme-like | Cymes | Spike-like | Solitary | ||||
Inflorescence 3 - synflorescence flower arrangement combinations | Monochasium | Dichasium | Not applicable | |||||
Flower shape | Cup-shaped (stellate/ patelliform) | Bell-shaped (campanulate) | Tubular | |||||
Involucral bracts | Absent | Present | ||||||
Bract fusion to flower stalk (bract recaulescence) | Not fused | Partially fused | Fully fused | Not applicable | ||||
Bract shape | Lanceolate | Linear | Ovate | Deltoid | ||||
Corolla lobe shape | Triangular | Linear | ||||||
Flower disc | Present | Absent | ||||||
Corolla lobe margin hairiness (indumentum) | Dense hairs | Sparse hairs | Lacinulate | Papillose (ciliate or erose) | Smooth (glabrous) | |||
Style length | Sessile | Short | Long | |||||
Stigma position | Below the anthers | In line with the anthers | Above the anthers | |||||
Placental column shape | Straight | Twisted | ||||||
Fruit length | (Actual measurement in mm) |
Taxonomic backbone
The first data to be uploaded into Xper3 were the scientific names of the 25 Thesium species included in this study (Fig.
Character backbone
After the taxonomic backbone was completed the character backbone was compiled. This was done by adding each character and its corresponding character states into Xper3. Each character and character state was listed in the key using descriptive terms such as flower shape, style length etc. (e.g., Fig.
In addition to terminology and textual descriptions, each character and character state was also visually represented with a figure plate containing representative photographs. For example, the character vegetative scales, was illustrated with three photographs; two plants with vegetative scales and one without vegetative scales (Fig.
An example of the visual and textual presentation of a character, vegetative scales, in the user interface of the interactive photographic identification key. A representative images of each character state (present and absent) of the character, with the relevant structures further emphasized using circles and arrows B a textual description of the character.
An example of the visual and textual presentation of character states in the user interface of the interactive photographic identification key. For the character, root system, each character state (branched and underground stem) is A illustrated with multiple photographs to show variation, as well as B a textual description.
After constructing the taxonomic- and character backbones of the key, character states were manually coded for each species in Xper3, for example, the style length of T. angulosum is long (Fig.
The final step in key construction was to create a profile for each species (Fig.
Target group testing
A target group testing was done at a Plant Specialist Group meeting at Buffelskloof Nature Reserve, South Africa. The 22 participants included amateur botanists, conservationists, ecologists, environmental consultants, horticulturists and taxonomists. Participants were divided into six groups of three or four and provided with access to the Xper3 key on their smartphones and laptops, a printed dichotomous key (automated by the Xper3 platform), a microscope and a flower dissecting kit (a razor blade, tweezers and dissecting needle). Each group was given ample fresh material of three Thesium species (T. confine Sond., T. procerum N.E.Br. and T. utile A.W.Hill) and was asked to identify them accurately at their own pace. During the exercise participants provided feedback which allowed characters, character states, terminology and photographs that caused confusion and/or uncertainty to be identified. Participants also gave general feedback on the usability of the key and all of these suggestions were incorporated into an improved key (discussed below). The authors aim to continuously improve the key by trial and revision and also expand the key by systematically adding more Thesium species.
Checkbase
In addition to the target group testing, the identification key was also evaluated using Checkbase, a build-in tool provided by Xper3. Checkbase provides information on discrimination between (1) items (species), (2) descriptions (characters) and (3) character states, as well as (4) missing character states.
In the current age of information and digital technology more emphasis is being placed on the development of electronic resources to advance the identification of species, which is vital for all practices related to or dependent on biological studies (
While Xper3 software provided a pragmatic platform for key construction in this study, the general principles and best practices discussed hereafter, can be applied to any software with the relevant functionalities (e.g., DELTA, http://www.delta-intkey.com/; Lucid, http://www.lucidcentral.org/).
Software platforms with a multi-access approach, where a user can choose any of the available characters throughout the key, circumvent multiple problems associated with the identification of species, especially those with cryptic characters (
Multi-access keys furthermore cater for the unambiguous presentation of cryptic characters and character states by allowing authors to utilize numerous character divisions. For example, the inflorescence structure of Thesium species is an important distinguishing character, but often varies considerably and is notoriously difficult to interpret. Its incorporation into traditional textual keys (
Software allowing for updates and changes to be made to identification keys after publication is pertinent for species groups with cryptic morphology as these groups are likely to be taxonomically problematic and subject to ongoing taxonomic study. For example, subsequent to the construction of the identification key presented here, a Thesium species new to science was described (
Species profiles with supplementary information and media on each species form part of many software platforms and contribute considerably to accurate identifications (
Character and character state delineation
In this study, maximizing the number of valuable characters while minimizing the number of associated character states proved most pragmatic. Contrary to species groups with clearly defined morphological characters (e.g.,
The electronic key further improves identification efficiency by subdividing particularly confusing and cryptic characters into more digestible units (
Character and character state presentation
One of the main advantages of electronic identification keys when identifying species with cryptic characters is the illustration of characters using multiple aids, which greatly reduces ambiguity (
It is also true that images may contain only partial information (
Checkbase
The Xper3 evaluation tool Checkbase showed that five species pairs were only partially discriminated: (1) T. racemosum Bernh. and T. costatum A.W.Hill, (2) T. gracilarioides A.W.Hill and T. multiramulosum Pilg., (3) T. gracilarioides A.W.Hill and T. resedoides A.W.Hill, (4) T. gracile A.W.Hill and T. utile A.W.Hill, and (5) T. asterias A.W.Hill and T. ovatifolium N.Lombard & M.M.le Roux. These species pairs are morphologically similar and the coding of multiple character states to account for variation resulted in partial, but not full, overlap in some characters. While this result highlights the challenge of successfully separating species with cryptic morphology using electronic keys (as well as traditional keys), these species can nonetheless be successfully identified using their respective species profiles as discussed before. All of the characters and character states included in the key provided full discrimination between species (as opposed to only partial discrimination or no discrimination). One exception was the Western Cape Province character state under the geographical distribution character, as none of the species included in the key occur in the province. It was, however, retained along with the other eight provinces of South Africa for completeness and to allow for future expansion in the scope of the key.
Target group evaluation
The target group evaluation indicated that the proposed key could be useful for identifying species with cryptic morphological characters and provided valuable suggestions for improvement that were subsequently incorporated. Differences in user interpretation of character states had to be addressed and subsequently, following
During the evaluation, it was clear that some characters were problematic. Participants had very subjective interpretations of the degree of woodiness of plants (originally divided into herbs, suffrutices and shrubs) and consequently had trouble identifying the correct character state. To address this unambiguity, the number of character states was reduced to two: plants that were obviously herbaceous (including suffrutices) and robust woody plants. Corresponding textual descriptions were also revised and expanded, and clearer photographs were used to illustrate each character state. Similarly, the difficult-to-interpret inflorescence structure was simplified from six complex character states (e.g., monotelic racemose inflorescence with a terminal dichasial cyme, and simple or compound dichasial and monochasial cymes) to four, more general types (raceme-like, spike-like, cymes and solitary). The majority of participants were not able to utilize the placental column shape (generally < 2 mm) as they could not successfully dissect flowers to access this structure. Although there is little that can be done to improve this hurdle, the character was retained in the key as it is valuable for specialist use, and it is not crucial for species identification so that non-specialist users can simply forgo it.
The last method employed to improve the accuracy of the identification key was the coding of multiple character states (multiple correct answers) where necessary. This step is crucial as it accounts for intra-specific variation in characters, characters with continuous character states and also for user subjectivity (
Based on the pilot electronic identification key presented here, the following best practices are suggested for the unambiguous presentation of cryptic morphological characters and their character states in electronic identification keys: (1) maximization of the number of valuable characters; (2) minimization of the number of character states associated with each character; (3) division of difficult/complex characters into multiple simpler characters; (4) illustration of characters and character states using multiple aids such as visual and text descriptions; (5) illustration of character states using multiple photographs to show the entire range of variation (if applicable); (6) use of photographs of live material (as opposed to preserved material) and plants in situ where possible; (7) addition of labels and accents such as arrows or circles to photographs to highlight relevant characters; (8) tailoring text descriptions to the target audience(s) (generalist or specialist terminology, or both); and (9) coding for multiple character states (multiple correct answers) where intra-specific variation is present or if a species falls on or close to the border between two character states (to ensure that the discriminatory power of characters is not lost).
Other general best practices include: (10) ensuring sound taxonomy and clearly defined species concepts prior to key construction; (11) using software that allows for updates and improvements (as necessitated by user feedback and ongoing research), including the replacement of images with superior ones as they become available; (12) utilizing a multi-access key approach [as opposed to a single-access approach (dichotomous or polychotomous)]; (13) using species profiles with representative photographs and supplementary information including (14) photographs of diagnostic features and the general impression, size and shape (GISS); (15) detailed distribution maps (if species are geographically separated) and (16) diagnostic notes separating morphologically similar species; and (17) evaluation of proposed identification keys by participants from the target audience and the subsequent incorporation of feedback prior to publication.
Electronic identification keys are valuable resources for species identification, which underpins all biological sciences. This study contributes to the rather limited body of knowledge on the successful identification of enigmatic species with cryptic morphologies using contemporary identification aids. It has shown that well-constructed electronic identification keys are feasible and offer the possibility of accurate identifications, in particular for species with cryptic characters, despite apparent contradictory reports in the literature. We have gained valuable insights into not only the problems and challenges associated with the successful identification of Thesium species (as a practical example of species groups with cryptic morphology) but also possible solutions and circumventions for difficulties in electronic key construction.
Ultimately a sound knowledge of the taxonomy and diagnostic characters of the taxa will determine the quality and efficacy of the identification key, regardless of the technology used in its construction and presentation. High attention to the presentation of the characters and their respective states are critical. There is no substitute for careful field studies of live organisms in their natural environment to overcome the typical limitations imposed by preserved specimens. This means a much greater effort in data collection but also a much greater reward in achieving a high level of discriminatory power in the identification key. Such electronic identification keys maximise the benefits that can be derived from the use of digital images and undoubtedly increase the accuracy of identification and reduce ambiguities that lead to a more user-friendly product for both specialist and generalist users. This might be especially valuable in economically important species groups such as grasses, which are characterised by cryptic morphological characters, by expanding the suit of potential users to farmers, conservationists, ecologists and so forth. The gap between research and users can also be minimised by adding the latest information on subjects such as synonyms, ecology and potential uses to species profiles.
To our knowledge, the best practices suggested here (although a combination of novel and previously known guidelines) are the first guidelines on electronic identification key construction tailored to species with cryptic morphology. While these guidelines work well for Thesium, similar studies of other species groups with cryptic morphologies will test these best practices, and likely reveal additional challenges and guidelines. This study therefore serves as a starting point for similar studies.
The following persons and institutions are thanked: the Botanical Education Trust, Foundational Biodiversity Information Programme (Small Grant number 104931), University of Johannesburg, South African National Biodiversity Institute (SANBI) and National Research Foundation of South Africa (Grant number 84442) for funding; the National Herbarium under the SANBI for hosting the study and access to their collections; Andrew Hankey, Barbara Turpin, Bronwynn Egan, Daniel Nickrent, Delia Oosthuizen, Kate and Graham Grieve, John Burrows, Kagiso Mashego and Sedzani Simali for assistance in the field; persons from the Plant Specialist Group and Buffelskloof Nature Reserve for hosting and participating in the evaluation of the identification key; Sylvain Bouquin for technical assistance with the Xper3 platform; Daniel Nickrent from Southern Illinois University for his contribution to an initial version of the identification key, and advice on morphological terminology, characters and character states; two reviewers, Lynn Bohs and Muthama Muasya, for suggesting improvements to the identification key and manuscript.