Australian National University researcher Brett Howland has found big mobs of kangaroos destroy the grassland habitats of reptiles.

"When there are too many kangaroos, they over-graze grasslands until they are like a lawn, which leaves lizards with no shelter," said Mr Howland, from the Fenner School of Environment and Society.

"Just because kangaroos are native doesn't mean they don't do damage. We have to regulate their numbers if we want to retain a variety of reptiles," he said.

Mr Howland studied lizard populations, which provide a good yardstick for the health of grasslands. Lizards depend on grass cover for food and shelter, they are an important part of food webs, they are food for birds and small mammals, and provide pest control by eating insects.

In his studies over five years on properties mostly in the ACT, and two in Victoria and two in NSW, Mr Howland found grass over 20 centimetres tall housed the greatest number of reptiles.

Some of the properties had low numbers of kangaroos, others had medium or high numbers. He noted how reptiles differed in each setting.

"The current number of kangaroos in some of Canberra's parklands, over 300 per square kilometre, removes all tall grass. We should be controlling kangaroo numbers to at most 100 kangaroos per square kilometre in grasslands on average, and even less in treed areas."

Mr Howland said his area of expertise was not kangaroo culling. "My data indicates what the grass should look like, how you achieve that is more of a policy issue."

ACT government ecologist Dr Don Fletcher said the number of kangaroos culled in grasslands in the ACT was consistent with Mr Howland's research. In other bushland communities culling was scaled in proportion to canopy cover.

"What that means is .9 kangaroos in open woodland, .5 in woodland and .1 kangaroos per hectare in forest and open forest," Dr Fletcher said.

Mr Howland's study found in areas where the grass was higher than 20 centimetres more than twice as many reptiles, and nearly three times as many species of reptile, than when grass was short.

"Many reptiles are under threat – species such as the striped legless lizard are on the vulnerable list. They face possible extinction in the near future," Mr Howland said.

"However, there are millions of eastern grey kangaroos in Australia, making them one of the most populous large mammals in the world."

Without threats from indigenous hunters and native predators such as the dingo, kangaroo numbers had skyrocketed, Mr Howland said.

"We are still coming to grips with managing biodiversity," he said. He hopes his research will cause people to appreciate the need for land management.

"You can't lock a reserve and throw away the key, that doesn't work," Mr Howland said.

ACT Government ecology Dr Don Fletcher, in grassland it is 100 per kilometre, in other communities it will be scaled in proportion to canopy cover,, that means .9 in open woodland, .5 in woodland and .1 kangaroos per hectares in forest and open forest.

In typical reserve which have a mix of communities, you end up one kangaroo per hectare in grassland, but most have forest and woodland and ends up being less than one kangaroo per hectare.

We are leaving behind somewhat fewer than what he is saying.

We have differences in reserves because reserves differ in proportion of vegetation types.

In future we will review numbers and formulas and say should be have different formula. At moment.

In 2009 less 500 kangaroos shot, and has gradually worked up in numbers. More research has been coming out, Mr Howland's was the last adding more and more information to the conservation picture.