It says so much about Edward II's turbulent reign that for centuries after William the Conqueror put the rebellious Waltheof to death in 1076, no English earls were executed, then in Edward's reign and just afterwards, the first three years and a few months of his son Edward III's reign - a period which to all extents and purposes belongs to Edward II's era - seven (seven!) earls were executed. Another two earls, meanwhile, died in battle: Gilbert de Clare, earl of Gloucester and Hertford, at Bannockburn in June 1314, and Humphrey de Bohun, earl of Hereford and Essex, at Boroughbridge in March 1322. Furthermore, rumours were current in the fourteenth century that Guy Beauchamp, earl of Warwick, was poisoned in August 1315 with the connivance of Edward II himself (in revenge for Guy's abduction of Piers Gaveston in June 1312). Quite honestly, to be an earl in Edward II's era and to die of natural causes was a real achievement. After Edward III took over the governance of his own kingdom from his mother in October 1330, no other English earl was executed for the remainder of his long reign.
Piers Gaveston, earl of Cornwall was stabbed and beheaded on the orders of several of Edward II's disgruntled barons, including the earls of Warwick, Lancaster, Hereford and Arundel, at Blacklow Hill in Warwickshire on 19 June 1312. There was and is some dispute as to whether he was really earl of Cornwall when he died, as the title had been taken from him the previous autumn when he was exiled for the third time, but certainly, Edward II thought he was, having granted him the title and lands again in January 1312. The next earl of Cornwall was Edward's younger son, 12-year-old John of Eltham, granted the earldom by his mother Isabella and brother Edward III in October 1328; it's probably revealing of Edward II's feelings about Piers Gaveston that he didn't give anyone else the earldom once held by his beloved, not even his own son.
Thomas of Lancaster, earl of Lancaster and Leicester, was beheaded outside his own Yorkshire castle of Pontefract on 22 March 1322, having lost the battle of Boroughbridge six days earlier. Numerous fourteenth-century chroniclers pointed out that Edward II executed his cousin in revenge for Piers Gaveston's death just under a decade earlier. Probably born in late 1277 or early 1278, Thomas was forty-four when he was executed, and because he was of royal birth - his father was Edward I's only brother and his mother was the niece of Louis IX of France and the widow of Enrique I of Navarre - his death shocked contemporaries. If we decide that Piers Gaveston doesn't really 'count' as an earl, and he was Gascon and not English anyway, Thomas was the first English earl executed for 246 years.
Andrew Harclay, earl of Carlisle, was, as sheriff of Cumberland, the man chiefly responsible for destroying the Contrariant army at the battle of Boroughbridge on 16 March 1322, and shortly afterwards was rewarded with the new earldom of Carlisle by a grateful Edward II. Andrew soon fell foul of the king, however, when in early 1323, sick of the king's inability to protect the north of England from Scottish raids, he came to an agreement with Robert Bruce without Edward II's knowledge or consent. Andrew suffered the traitor's death in Carlisle on 3 March 1323, probably in his early fifties, having been earl of Carlisle for less than a year. The next earl of Carlisle was James Hay, a whole 300 years later in 1622.
Hugh Despenser the Elder, earl of Winchester, was hanged in Bristol on 27 October 1326, aged 65. Edward II had left him in Bristol to hold the city against Queen Isabella and her invasion force, but it fell to her after a few days, and Hugh was given a show trial and immediately hanged, then beheaded after death. His body was suspended for several days on the gallows by the arms, and then, horribly, fed to dogs, and his head was taken to Winchester for public display. He had been earl since May 1322, a little under four and a half years, and there was no other earl of Winchester until the 1470s.
Edmund Fitzalan, earl of Arundel, aged 41, was beheaded in Shrewsbury on 17 November 1326, also on the orders of Queen Isabella and Arundel's cousin Roger Mortimer. It seems that he wasn't even allowed the semblance of a trial before his death, which was a horribly slow and painful one: it took the incompetent executioner at least seventeen and perhaps more than twenty strokes of the axe to sever his head. Arundel, present at the execution of Piers Gaveston in 1312, had become a staunch ally of Edward II and the Despensers, and his son and heir Richard, who restored the family fortunes and then some in later years, was married to Hugh Despenser the Younger's eldest daughter Isabella.
Edmund of Woodstock, earl of Kent, the youngest son of Edward I and half-brother of Edward II, and grandson of Philip III of France, was beheaded in Winchester on 19 March 1330, aged 28. He had been making attempts to free his supposedly dead half-brother the late king from captivity in Corfe Castle, Dorset, and was beheaded after being forced to wait around on the scaffold for many hours because the executioner had fled, unwilling to participate in the judicial murder of a king's son. His wife Margaret Wake was almost nine months pregnant with their son John at the time.
Roger Mortimer, earl of March, was hanged at Tyburn on 29 November 1330, aged 43 or 44, on the orders of Edward III. A list of fourteen charges was issued against him during parliament, which reveals the young king's fury that March's dominance of his government made him feel that he was "like a man living in custody". As with Hugh Despenser the Younger, lord of Glamorgan, almost exactly four years earlier, March was accused of usurping royal power which did not belong to him.
The earls who died of natural causes were:
Henry de Lacy, earl of Lincoln, died in 1311, aged about 50 or 52. Henry's heir was his daughter Alice (1281-1348).
Aymer de Valence, earl of Pembroke, son of Henry III's half-brother William de Valence. He died in France in June 1324 in his forties or fifties, on his way to Charles IV's court in Paris, somewhat in disgrace with Edward II. Possibly Pembroke's sudden and unexpected death - he collapsed and died in the arms of an attendant without being shriven - spared him from later execution either by Edward II or Queen Isabella.
Robert de Vere, earl of Oxford (1257-1331): a weirdly obscure nobleman who played no role whatsoever in events of Edward II's reign, and at least part of the fact that he survived both the reign and Queen Isabella's regime unscathed was because he wasn't important enough for anyone to bother with.
John of Brittany, earl of Richmond (c. 1266-1334): a younger son of Edward I's sister Beatrice, and thus a first cousin of Edward II and of Thomas and Henry of Lancaster. Oddly, John never married. He was loyal to Edward until 1322, and later joined Isabella in France. Although he mostly grew up in England, where his relatives affectionately called him Briton, he was always something of an outsider in English politics.
John de Bohun, earl of Hereford and Essex (1305-36): Edward II's nephew, and, with his younger brothers, a close ally of their cousin Edward III.
Thomas of Brotherton, earl of Norfolk (1300-38): Edward II's other half-brother; possibly, he survived Isabella and Roger Mortimer's regime because his son and heir was married to Mortimer's daughter Beatrice.
Henry of Lancaster, earl of Lancaster and Leicester (c. 1280-1345): brother and heir of Thomas, first cousin of Edward II, with whom he was somewhat in disgrace from 1322 to 1326. Henry played an important role in Edward's downfall in 1326/27, but two years later fell out badly with Isabella and Mortimer and rebelled, and again, was perhaps lucky not to be executed or judicially murdered.
John de Warenne, earl of Surrey and Sussex (1286-1347), nephew of the obscure Robert de Vere, earl of Oxford, and nephew-in-law of Edward II: switched sides from Edward to the queen at the right moment in 1326 and thus saved himself from the horrible execution of his brother-in-law the earl of Arundel, and died the day before his 61st birthday in June 1347.
And finally, for the sake of completeness, I should also mention Edward III, who was made earl of Chester in late 1312 when he was a few days old.