**3. Using Plant DNA Barcodes to Understand Ecological Patterns and Evolutionary Processes**

With the advancement of DNA barcodes as a reliable source for genetic species identification, biologists increasingly use this tool to track species interactions. Such studies are now being conducted across the globe in both temperate and tropical environments. The review by Gostel and Kress [5] in this Special Issue outlines the recent progress that has been made in these investigations as a result of novel computational and sequencing capacities, high-throughput barcoding methodologies, and the expansion of the global DNA barcode database.

Three additional papers in the Issue highlight how DNA barcodes help to uncover previously obscure interactions, e.g., mate location in highly complex tropical forests; the abundance and diversity of large mammalian herbivores and their woody plant food sources; and in plant-insect pollinator communities. In an attempt to uncover how Orthopterans (crickets, katydids, and grasshoppers) use acoustic cues to find mates, Palmer and colleagues [6] turned to plant DNA barcodes to test the specificity of food plants that could facilitate mate location in katydids on Barro Colorado Island in Panama. Their results showed that most katydids are generalist herbivores and food choice would most likely not facilitate mate location. In a semi-arid African savanna Freeman et al. [7] demonstrated the important role of megaherbivores in shaping vegetation across landscapes. Using data from plant DNA barcodes, they were able to ascertain that some habitats, which deter large mammalian herbivores, serve as refuges for plant species that otherwise are quite palatable to these animals. Finally, a comparison between plant metabarcoding and non-molecular methods of tracking plant-pollinator interactions demonstrated the advantages of a DNA barcode approach in determining the complexity of these communities [8].

### **4. Plant DNA Barcodes and Human Interactions**

It should not be forgotten that one of the most important plant-animal interactions on the planet is between plants and humans. Three final papers in the Special Issue address the application of DNA barcodes to tracking medicinal plants, invasive species, and habitat conservation. Jamdade et al. [9] demonstrate that as the DNA barcode library is built for the flora of the United Arab Emirates the current plant DNA barcode regions provide sufficient markers for the safe usage, prevention of adulteration, and the regulation of medicinal plant trading. DNA barcode sequence data were employed by Yessoufou and Ambini [10] to build a molecular phylogeny of the 210 known naturalized alien woody plants in South Africa. Based on this phylogeny they demonstrated that the benefits humans obtain from an alien species had significant evolutionary signal, but that non-invasive species exhibited more benefits to humans than their introduced, invasive counterparts. Such phylogenetic metrics can also contribute to plant conservation. Pearl et al. [11] generated DNA barcodes

for 366 species of plants in the heathland ecosystems in Queensland, Australia. The resulting measures of phylogenetic diversity found in these communities combined with other patterns of diversity suggested contrasting conservation and managemen<sup>t</sup> implications for these historical "refugial environments".

This Special Issue of *Diversity* on "Plant DNA Barcodes, Community Ecology, and Species Interactions" provides a taste of the current variety of investigations and publications that are a result of the expansion of DNA barcodes in the biological sciences. It is hoped that the papers contained herein will inspire and encourage future applications of DNA barcoding to the exploration of ecological and evolutionary systems across the globe.

**Author Contributions:** W.J.K. and M.R.G. contributed equally to this publication. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** W.J.K. and M.R.G. are grateful to the authors who have contributed their work to this Special Issue.

**Conflicts of Interest:** The authors declare no conflict of interest.
