def split_train_test(df_features, test=['2008', '2009', '2010', '2011', '2012', '2013']):
    """
    Split the feature DataFrame in test and train set. Use the last three
    years by default as test set.
    """
    df_features = df_features.reset_index()
    idx = df_features['year'].isin(test)
    train = df_features[~idx]
    test = df_features[idx]
    
    train.set_index(['site_id', 'species', 'year'], inplace=True)
    test.set_index(['site_id', 'species', 'year'], inplace=True)
    
    return(train, test)