Table 1.

Overview of approaches to study viral adaptations to conditions that make them unsuccessful. We refer to viruses as unsuccessful when (i) there are low numbers of viruses in the environment; (ii) they have low or no viral infectivity and/or (iii) they have low or no viral replication ability and subsequent low or no release of viral particles into the environment. All approaches can be carried out at temporal and spatial scales.

Approach	Method	Examples *
Comparative studies
	Sampling of communities over time and correlation with changes in extrinsic factors	[95]
	Resurrection ecology, correlation of abundances with changes in the environment	[119]
Experimental studies
	Measurement of virus life cycle traits under different conditions	[83,85,100,101]
	Resurrection ecology, isolation of living viruses and measurement of life cycle traits under different conditions
Experimental evolution studies
	Virus evolution to different conditions (requires constant host)
	Host-virus coevolution under different conditions	[87,120]
	Virus (co)evolution in communities under different conditions	[65]
Modeling
	Virus population events across different conditions	[87]
Genomics
	Viromics to check for absence/presence of viruses across different conditions	[18]
	Genomics and phylogenetic trees to decipher evolution and past population events (bottlenecks, extinctions, migrations, etc.)	[115,121,122]

* Examples of algal viruses are provided but approaches can be applied to all other viruses. No examples given indicates that we are not aware of studies using those approaches.