Assessment of monitoring power for highly mobile vertebrates

Abstract

Monitoring of population trends is a critical component of conservation management, and development of practical methods remains a priority, particularly for species that challenge more standard approaches. We used field-parameterized simulation models to examine the effects of different errors on monitoring power and compared alternative methods used with two species of threatened pteropodids (flying-foxes), Pteropus conspicillatus and P. poliocephalus, whose mobility violates assumptions of closure on short and long timescales. The influence of three errors on time to 80% statistical power was assessed using a Monte Carlo approach. The errors were: (1) failure to count all animals at a roost, (2) errors associated with enumeration, and (3) variability in the proportion of the population counted due to the movement of individuals between roosts. Even with perfect accuracy and precision for these errors only marginal improvements in power accrued (-1%), with one exception. Improving certainty in the proportion of the population being counted reduced time to detection of a decline by over 6 yr (43%) for fly-out counts and almost 10 yr (71%) for walk-through counts. This error derives from the movement of animals between known and unknown roost sites, violating assumptions of population closure, and because it applies to the entire population, it dominates all other sources of error. Similar errors will accrue in monitoring of a wide variety of highly mobile species and will also result from population redistribution under climate change. The greatest improvements in monitoring performance of highly mobile species accrue through an improved understanding of the proportion of the population being counted, and consequently monitoring of such species must be done at the scale of the species or population range, not at the local level.

Global meta-analysis reveals low consistency of biodiversity congruence relationships

“…However, these trends are likely to be the result of methodological difficulties. For example, many vertebrates routinely move distances ranging from metres to kilometres to forage, making it unclear whether assessing the 'distribution' of vertebrates at finer spatial scales than these routine movements is particularly meaningful 55 . Conversely, taxonomic completeness is low for many arthropod taxa as a result of their enormous diversity 56 , perhaps accounting for their lower use in global-scale biodiversity assessments.…”

Tick Paralysis in Spectacled Flying-Foxes (Pteropus conspicillatus) in North Queensland, Australia: Impact of a Ground-Dwelling Ectoparasite Finding an Arboreal Host

“…However, both methods result in an under-count of roughly the same magnitude, though stemming from different error sources [25], [51].…”

“…It is also very difficult to establish reliable population estimates for flying foxes because of the propensity for animals to change camps [20], [25], [66] and for large camps to disappear within days [50], [58], [68], [69], [70]. In the present analysis we used the results of two methods of establishing population data for flying foxes, fly-out counts and walk through day counts.…”

“…In the present analysis we used the results of two methods of establishing population data for flying foxes, fly-out counts and walk through day counts. Day counts avoid the errors of fly-out counts, such as missing a stream of flying out animals, but they are in general assumed to be less accurate due to reduced visibility of animals hanging closely together high up in trees and disturbance caused by the observer [25]. On the other hand, day counts have been found to have a higher level of precision ([71] cited in [25]).…”

“…The SFF was included in the Australian Environment Protection and Biodiversity Conservation Act 1999 list of vulnerable species in May 2002 [24]. While the population trends of the species are unknown [25], SFF populations are thought to be threatened by a range of factors, including loss of habitat, electrocution on power lines, entrapment in fruit netting, shooting in orchards, entanglement on barbed wire, and most recently by tick paralysis [13], [20].…”