Berlin-based SEO platform provider Searchmetrics reports that space occupied by the Knowledge Graph and what Google calls "universal search elements" (AdWords ads, maps, photos, news) on the first page of results for travel-related searches have reduced the average number of organic results displayed from 10 to fewer than 9.
Google displays the Knowledge Graph in response when its system recognizes or suspects a "semantic entity" behind a search term, or in response to questions about:
- Famous people or their relatives
- Companies and employees
- Recipes or food
- Movies or novels
- Distances between things
- Conversions of measurements
In regard to SEO and marketing, Searchmetrics sees both pros and cons of Google's increased use of the Knowledge Graph. The quantity of information displayed in the Knowledge Graph will make clicking through to a website unnecessary for some users, resulting in decreased traffic and ad revenues. But being listed in the Knowledge Graph with a site link is a positive thing.
Searchmetrics recommends using markup language like that offered by schema.org on Web pages to increase the likelihood of getting into the Knowledge Graph.
Here is more information from Google about the Knowledge Graph.