Factors affecting detection probability in plant distribution studies

P> Plant ecologists have been rather slow to appreciate the existence and the effects of imperfect detection probability in plants. Sources of heterogeneous detectability include differences in morphology or life-form, patch size, observers and survey effort. Understanding the relationship between such factors and detectability is crucial for the efficient design of new plant distribution studies and for the interpretation of existing ones. We have studied the factors affecting detectability in a large permanent plot (24 ha) in East China where the true distribution of six shrub and tree species was known from a detailed earlier inventory. Two observers independently resurveyed and recorded detection and non-detection of each species in each 20 x 20 m sampling quadrat. A total of 288 quadrats were resurveyed (218 by observer A, 211 by observer B and 141 by both). We used generalized linear mixed modelling to study the relationships between detection and species, observer, survey effort and patch size. Detectability of an occupied quadrat was remarkably low and ranged from 0.09 to 0.34 on average for the six shrub and tree species. Differences of detection among species were mainly as a result of distinctive morphology rather than life-form. There was no significant difference of overall detection probability between the two observers. Detectability increased to 0.95 as the survey path approached 20% area of the sampling quadrat and as a plant patch covered c. 19% of the area of the sampling quadrat. Synthesis. Our results suggest that imperfect detection is much more widespread than currently acknowledged by most plant ecologists. We identify several sources of heterogeneity in detectability (species, survey effort and patch size) that ought to be considered when studying and modelling the distribution of plant species. Detectability should be accounted for in plant distribution studies to avoid spurious inferences.