What exactly do year to year changes in species index values stand for?
Such changes stand for fluctuations in population numbers, which we should interpret with some caution. PECBMS aims to detect long term rather than short term trends in population indices. Year to year changes, even when large, have limited relevancy to conservation. For instance, some bird species drop in numbers in reaction on extreme whether events (e.g. hard winters), but their populations may recover very quickly. Some species fluctuate in numbers naturally because of fluctuating food availability.
Added to such natural yearly fluctuations comes statistical noise. Therefore one should pay attention to the standard error of an index or trend. Values with large standard errors are less precise and indices based on such values will fluctuate more strongly.