Officials say cyanide used in gold mining was spread by poachers over the flat "salt pans". They also say lions, hyenas and vultures have died from feeding on contaminated carcasses or drinking nearby.
Nine suspected poachers have been arrested this month after the biggest, most brutal poaching spree on record. Three men were sentenced to up to 16 years in jail.
Mr Manvell joined Warblington School in 1988 and it is believed that he retired in 1998.
Headteacher Julia Vincent said: "We were saddened to hear of this tragic accident and our thoughts and prayers go out to his friends and family at this sad time."
The 67-year-old is understood to have also worked at the Avenue Tennis and Squash Club in the town.
Paula Fuge, personnel manager for Portsmouth Tennis Academy, who has been involved with Mr Manvell in the local tennis scene for more than 20 years, said: "I didn't believe it when I heard what had happened, you never expect that to happen."
Local guides travelling with the British man who was reportedly trampled to death in Tamil Nadu, India, attempted to get help from the nearby resort the man was staying at.
The man, reported to be 67-years-old, was ferried to the local Masinagudi hospital before being transferred to Gudalur Government Hospital, where doctors pronounced him dead, The Times of India reported.
There have been conflicting reports about the moments leading to the death of the British man killed inside a national wildlife park in Tamil Nadu, India.
Witnesses are reported to have said he failed to hear tourists warning him of the approaching elephants, whilst other accounts claimed he was engrossed in his photography and did not hear two elephants charge at him from behind.