What happens when the top man leaves?

Steve Jobs's departure on medical grounds from Apple shook the investment world last week.

Although his illness with cancer has been well known, Jobs's leaving still caused shares in one of the world's most successful consumer technology firms to fall 6 per cent, wiping billions off the company's value. Despite Apple having armies of executives and developers, it seems that the cult of the person at the top goes on.

Jobs is a prime example of how significant a single individual can be. This level of dependence forces investors to question whether a company with a key person can continue without them.

"There are risks involved if you're backing a one-man band," says David Kuo, the director of financial website Fool.co.uk. Steve Jobs is a classic example: a visionary chief executive who reinvigorated Apple following his return to the company.

Twitchy investors can rest easy if Jobs's previous absence in 2009 is any indication. Then he left for six months for a liver transplant to replace an organ damaged by pancreatic cancer. Any damage to the share price was short term. If this leave is more permanent, however, the question for investors is whether Jobs really is irreplaceable.

Many fund managers hold similarly important positions within their organisations and therefore potentially pose the same risk. Examples of this danger include Neil Pegrum quitting Insight Investment fund in 2004; the company closed the fund rather than bring in someone new. More recently, Gartmore's clients were quick to pull their money out after its star fund manager, Roger Guy, retired in November after 17 years. The company had little choice but to sell itself to rival Henderson.

Such stars will always be able to bolster value and also render that company or fund vulnerable should they up sticks. So which people in the fund management and company universes do investors need to be aware of?

Neil Woodford

Invesco Perpetual

Neil Woodford, 50, a fund manager at Invesco Perpetual, has been at it for over 20 years and has control of assets worth £18bn with his two main funds, the Invesco Perpetual High Income fund and the Invesco Perpetual Income fund.

"Invesco probably has the biggest problem of any fund group in the UK. If Woodford leaves, a large amount of money will probably walk out the door at the same time," says Ben Yearsley, an investment manager at Hargreaves Lansdown.

Despite well-documented periods of underperformance, Woodford is known for his emphasis on long-term returns and his funds have an outstanding track record. Even with a strong team of capable managers, Yearsley warns, losing Woodford would undoubtedly have an impact.

"The problem for them is he has been so good over the past two decades with investors buying the fund for him; whoever follows will definitely have a poisoned chalice."

Anthony Bolton

Fidelity China Special Situations

Rarely has the term "legendary fund manager" been more applicable than to Anthony Bolton, 60, who spent 28 years in charge of his flagship fund Fidelity Special Situations. Bolton will long be remembered for giving investors an average annual return of 19.5 per cent – compared with 13.5 per cent from the FTSE All-Share Index – before retiring from the fund at the end of 2007 which led to Fidelity splitting the fund to halve its size.

Bolton has since returned to head the £570m China fund launched last year, and it was his name that drew in record amounts of cash. However, experts contend that Fidelity may well have learnt from its experience.

"With the China fund, the situation is slightly different. For a start, Bolton has only committed to managing the fund until April 2013. Secondly, Fidelity has an excellent Asian team in Hong Kong where he is based – so any handover, when it happens, should be smooth," says Yearsley.

Hugh Young

Aberdeen Asia Pacific

Aberdeen Global Asia Pacific Equity Fund is yet another fund that could be exposed without its manager. With more than 20 years' experience, Hugh Young has established a standing as an Asia expert. Young lives in Singapore and his £2.4bn fund has seen a return of 121 per cent over the past five years.

While there is every indication that Young's departure would be disappointing for investors and Aberdeen is likely to see money leave, experts say that the fund should be able to weather the storm and investors will stick with it in the long term. "They have already appointed a younger member of their team to head up their emerging markets fund without any issues, and I am sure in time they will do the same on Asia," says Yearsley.

Mick Davis

Xstrata

Xstrata boss Mick Davis, 52, has built Xstrata into one of the biggest natural resources groups in the world.

"He has been instrumental in the growth of the mining group following a stream of well-timed acquisitions," says Jonathan Jackson, the head of equities at stockbroker Killik & Co.

Davis's biggest deal, on which he has built his reputation, was pre-Xstrata, when he worked at rival mining group Billiton and had a hand in merging the company with Australia's BHP in 2001. He left to take control of Xstrata, which was then a small Swiss steel alloys business. Once there, Davis made a series of high-profile acquisitions and now heads a company valued at £44bn.

"If he was to leave, that would be disappointing for shareholders and at least in the short term there would be a wobble," says Jackson.

Mike Lynch

Autonomy

As co-founder and chief executive of software company Autonomy, Mike Lynch has seen his tiny start-up blossom into the largest technology firm in Britain. It's no surprise that his name and Autonomy are synonymous when you hear that the software is based on his widely respected doctoral thesis.

With Dr Lynch as the brains as well as the face behind the company, his departure would be a significant blow. And, in a relatively small company worth £3.5bn, losing your top man is all the more likely to cause problems.

"He is seen as the key driver in the group's growth to become market leader in the development of meaning-based computing software. As a smaller company, Autonomy could be much more vulnerable," says Jackson.

Sir Martin Sorrell

WPP

When Sir Martin Sorrell, 65, founded advertising and marketing company WPP in the 1980s, it started life as a £1m business making wire baskets. Today the group is worth £10bn and Sorrell has been instrumental to this success.

WPP has recently added Blue State Digital to its long list of acquisitions – the digital agency behind Barack Obama's 2008 online fundraising campaign – which should make WPP an even more formidable presence.

"Sorrell is a classic example, taking the business from nothing to the world's greatest advertising company. He has been with WPP through thick and thin and is undoubtedly the driving force behind the company," says Kuo.