TOKYO, June 18 (Reuters) - Japan's Aderans Holdings Co Ltd (8170.T) has decided to terminate its anti-takeover defence plan, after a U.S. activist fund successfully replaced the struggling wig maker's board last month.

More than a dozen Japanese companies have scrapped "poison pill" schemes this year, including Rohm Co Ltd (6963.OS), though some analysts said such moves were unlikely to spread quickly among other firms.

In a statement released on Thursday 18 June, Aderans cited changes in its business environment among the reasons for the decision, but Nomura Securities senior strategist Kengo Nishiyama said it was more likely because of its defeat to U.S. fund Steel Partners at its annual shareholders' meeting.

"(The measures) were against Steel Partners, and they have lost meaning now that the fund effectively got control," he said.

Confectionery maker Ezaki Glico Co Ltd (2206.T) also decided last month not to seek shareholder approval to continue its defence measures. The company denied the connection but the move came after Steel Partners sold all of its stake in the company late last year.

About 570 Japanese companies have introduced poison pill schemes since around 2005, according to Nomura.

But the number of firms adopting such plans has been on the decline as companies feel less pressure than a few years ago to protect themselves given the struggles of funds and other potential aggressors amid the recession.

Electronic device maker Rohm, which last month scrapped its defence plan, said the threat of being taken over has become relatively small in the current economic climate.

Aderans, Ezaki Glico and others also said there has been more legal protection to keep takeover attempts in check since they introduced their defence schemes.

"Defence measures have not necessarily been used in the best interest of shareholders," said Nomura's Nishiyama.

"But at the same time, companies do not see compelling reasons to end them right away. So, the number of those terminating them is likely to grow but not very quickly," he said. (Reporting by Taiga Uranaka; Editing by Chris Gallagher)