woman with camera

Cats must stay indoors to protect wildlife, says European law

Published: 27th November 2019 Last updated: 15th April 2020

Free-ranging domestic cats harm biodiversity by killing wild animals, disturbing them, transmitting diseases, and in other ways. Legal experts at Tilburg University analyzed the nature conservation legislation of the European Union in light of the growing scientific evidence on cats’ wildlife impacts. Their conclusion: EU member states must control stray and feral cats where these threaten protected species and sites, and must prohibit people from letting their pet cats roam outdoors.

It is common knowledge that domestic cats (Felis catus) hunt birds and other small animals. In the last fifteen years, however, scientific evidence regarding the magnitude and variety of cats’ wildlife impacts has greatly increased. It shows that in Europe and worldwide, domestic cats are amongst the most harmful invasive alien species. In light of this evidence, two Tilburg University lawyers analyzed European nature conservation law, in particular the 1979 Birds Directive and the 1992 Habitats Directive. Their results have just been published in the peer-reviewed Journal of Environmental Law .

Outdoor cats at odds with EU law

Regarding (unowned) stray and feral cats, the study shows that the EU Directives require the removal or control of such cats where they pose a threat to protected species or protected Natura 2000 sites. Regarding (owned) pet and farm cats, the law requires EU member state authorities to ensure that letting cats roam free is forbidden and effectively prevented.

“This conclusion may come across as extraordinary,” says lead author Arie Trouwborst, “but it really is the outcome of a standard interpretive analysis of legal requirements that have been around for decades, in light of current knowledge on cats’ biodiversity impacts.”

Protected wildlife

EU law requires member states to prohibit and prevent the “deliberate” killing, capturing and disturbing of a range of protected animals, including all native birds. The term “deliberate” has been consistently explained by the EU Court of Justice as covering not only cases where somebody intends to affect a protected animal, but also cases where somebody “accepts the possibility” of such consequences – as an unwanted but accepted side-effect. “Letting your cat wander outdoors fits that category,” says Trouwborst.

Widespread non-compliance

A cat-free landscape is, of course, not what we see, and domestic cat management continues to be a blind spot in the application of nature conservation law across Europe. The question why this might be so is also explored in the study. Candidate reasons for not upholding the law include practical feasibility, the interests of domestic cats, and those of their owners, but none of these survive scrutiny.

Political sensitivity

Despite the harm it causes, granting outdoor access to pet cats is an entrenched habit, and political inconvenience might be an underlying reason for inaction. “Cat management is a perennially controversial issue,” says co-author and professor of EU law Han Somsen, “but fear of political unpopularity is evidently not a valid justification for failing to comply with legal obligations.”

Low-hanging fruit

Moreover, the benefits of overcoming this political hesitation are great, especially against the backcloth of the worsening biodiversity crisis. The expected gains of removing cats from the landscape are considerable, and the costs are low when compared to the action needed to address climate change or to achieve ecologically sustainable agriculture.


Arie Trouwborst & Han Somsen (2019). Domestic cats (Felis catus) and European nature conservation law – applying the EU Birds and Habitats Directives to a significant but neglected threat to wildlifeJournal of Environmental Law, doi: 10.1093/jel/eqz035.

Note to editors

For more information please contact Arie Trouwborst of the Department of Public Law and Governance, Tilburg Law School / Tilburg University, The Netherlands, at a.trouwborst@tilburguniversity.edu; tel. +31 13 4668704.