If the text you read does not have a DOI, please paste a complete reference of your source. We suggest you use Google Scholar to find the reference, click on cite, and copy and paste here the reference on APA format. For example the classic paper by Folke et al on regime shifts would be:
Folke, C., Carpenter, S., Walker, B., Scheffer, M., Elmqvist, T., Gunderson, L., & Holling, C. S. (2004). Regime shifts, resilience, and biodiversity in ecosystem management. Annu. Rev. Ecol. Evol. Syst., 35, 557-581.
If you have more than one reference, please use IDs as follows to separate them "(#1) ...... (#2) ......". Those IDs must be the same between the reference text and DOI.