Species ecology can bias population estimates

Population indices are summary statistics computed from population monitoring data and routinely used in population management and ecological studies. Changes in a population index are assumed to reflect similar changes in population size, but the exact form of this relationship is often untested. We show that non-linear population index-size relationships lead to biased population change estimates. We then analyse the reliability of eight common population indices for a wide array of different types of species, showing that both systematic bias and considerable amount of error can arise from the interaction of ecological processes and survey methods. We describe the types of scenarios (e.g. types of species and how they are monitored) in which bias occurs, and what is the likely direction of the bias in each of them. Many typical population indices are biased; for example, those measuring abundance can often overestimate change, unless species individuals tend to establish completely non-overlapping territories, and those measuring the area utilized or species presence more likely underestimate changes in population size, especially when species' individuals or population processes lead to repulsion between individuals, or when there's limited amount of suitable resources to share. By pinpointing such scenarios our results help interpretation and design of population monitoring studies and therefore can improve the reliability and efficiency of conservation and management tasks.