Many ministry offices and functions overlap and a pastor is in fact an elder, as a bishop is also a pastor.
The word 'elder' in Greek is 'presbuteros', which evolved into 'prester' and eventually became 'priest', and refers to a mature, ordained believer who holds administrative office in the church. This differs from 'hieros' which is also translated 'priest', but refers to the more general 'priesthood of all believers' (Revelation 1:6) in which every believer has direct access to God.
A pastor is a 'shepherd' - or if you prefer, 'under-shepherd' to Christ our Great Shepherd.
In my hub 'The role of the prophet in the Church', I explain more fully the overlapping aspects of the so-called 'five fold ministry'.
Each office is in fact an equipping function of ministry, gifted through the Holy Spirit to his Church and are together analogous to the fingers of God's right hand - which work best and accomplish most when working together.
BISHOP evolved from the Greek EPISKOPOS. First the '-OS' inflection was dropped, before the EPISKOP became BISKOP, and finally BISHOP, but the original merely means Overseer.
So, who your episkopos happens to be, rather depends on what administrative level you occupy. Paul was an Apostle and ordained both Timothy and Titus as bishops, whilst Paul himself was their episkopos/bishop.
In this general sense, even a pastor may be considered the overseer/episkopos of his flock, so the qualifications of a pastor are really no different from those of a bishop, because a bishop has responsibilities for pastoral care, whilst a pastor has ministerial oversight over those in his care.
Contrary to what many suppose, the organisational model of Church ministry is firmly rooted in the rabbinical model of Judaism, and the terms we tend to consider as Christian would have been quite familiar to the early Church as Jewish.