I've been monitoring population trends of a threatened bird species in a developing country. My protocol uses numbers of breeding adults (not juveniles) to assess long-term population trends. My local colleagues are asking me why juveniles are not included in the population assessments. I know that only counting adult breeders is common protocol for many wildlife population studies, mainly because the adult population is more stable and less susceptible to short-term population fluctuations due to high juvenile mortality rates. However, I can't seem to find good references that specifically address this. Can anyone help me out with some references? Thanks in advance!
Hello Andrea,
I can't help with bird or mammalian references, but 'wildlife' has a broader connotation. In freshwater crayfish, adults may predate on juveniles, leading to juveniles preferring different habitats, less usable by the larger adults. Demers A et al. 2003/2004, Bull. Fr. Peche Piscic. 370-371: 127-137, available on my RG page, contains references to this topic. Hope this helps. Julian
The following paper and its references may help you:
MG Brasher, RM Kaminski, B Lwjr, 2002. Evaluation of indicated breeding pair criteria to estimate mallard breeding populations. Journal of Wildlife Management, 66 (4) :985-992.
This study is about neonatal ungulate predation. They list some possible biase handling neonates. I don't know if it can help you but it may inspire you for birds, for exemple in neonates tracking and handling.
As suggested, juveniles often aren't included because they don't directly contribute to population growth until they reach adulthood and breed. An individual that doesn't breed doesn't affect the population's future.
It's also important to note that different methods are usually required to sample different life stages. Juveniles are often too small to be caught and marked, are less mobile and thus less likely to be caught, or aren't detectable by methods used to find adults such as call/song surveys for birds.
The importance of juvenile vs adult survival is a whole sub-field - check out this paper and the sources it cites, particularly Sandercock et al and Wilson and Martin, which address birds.
Article Variation in vital-rate sensitivity between populations of T...
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