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 relevance to conservation. For instance, some bird species drop in numbers in reaction to extreme weather events (e.g. hard winters), but their populations may recover 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 fluctuate more strongly.