def convertStateToIndex(population, fire):
    """Convert state parameters to transition probability matrix index.
    
    Parameters
    ----------
    population : int
        The population abundance class of the threatened species.
    fire : int
        The time in years since last fire.
    
    Returns
    -------
    index : int
        The index into the transition probability matrix that corresponds to
        the state parameters.
    
    """
    assert 0 <= population < POPULATION_CLASSES, "'population' must be in " \
        "(0, 1...%s)" % str(POPULATION_CLASSES - 1)
    assert 0 <= fire < FIRE_CLASSES, "'fire' must be in " \
        "(0, 1...%s) " % str(FIRE_CLASSES - 1)
    return(population * FIRE_CLASSES + fire)