No abstract

A power analysis allows estimation of the probability of detecting upward or downward trends in abundance using linear regression, given number of samples and estimates of sample variability and rate of change. Alternatively, the minimum number or precision of samples required to detect trends with a given degree of confidence can be computed. The results are applicable to an experimental situation in which samples are taken at regular intervals in time or space. The effects of linear and exponential change and of having sample variability be a function of abundance are investigated. Results are summarized graphically and, as an example, applied to the monitoring of the California sea otter population with aerial surveys.

This paper considers three concepts of stability as they relate to the dynamics of distinctive patch types of algal canopy guilds in southern and central California kelp communities: (I) persistence of a patch through more than one generation of the dominant species, which was evaluated by using life tables and observations of patch borders; (2) inertia or the resistance of different patches to invasion or disturbance, which was evaluated by artificially enhancinggametophytes by transplanting sporogenic material, by removing canopy, and by evaluating some important disturbance processes; and (3) resilience or recoverability of a patch following a perturbation sufficient to allow invasion of different species, which was studied by defining some of the mechanisms of successful invasion or succession. By working in distinct habitats in southern (Pt. Lorna and Santa Catalina Island) and central (Pt. Piedras Blancas) California, we could evaluate different types of physical stresses as they related to these stability concepts.Taller perennial canopy guilds were dominant competitors for light, but were more susceptible to physical wave stress. Dominance hierarchies in the competition for light appeared to be reversed in areas exposed to increasing wave stress. The main causes of mortality at Pt. Lorna were entanglement with storm-dislodged Macrocystis plants and, in some areas, sea urchin grazing. Mortality in central California was due to winter storms. In most cases, distinct patches resisted invasion for> 10 yr. The mechanisms of resistance involved (1) competition for light and, possibly, nutrients, and (2) limits to spore dispersal. When succession occurred, it was often mediated by many factors, including seasonality of spore production, which coincided with winter storm-related mortalities; mechanisms of kelp dispersal, which were most effective via drifting plants and fragments of fertile material held against the substrate by invertebrates; and survivorship of gametophytes and small sporophytes, which was influenced by local scour and grazing. Appropriate spatial scales, stability, and succession studies in these kelp communities were determined by the size of the disturbed area, which varied from the free space resulting from detachment of single plants to the free space resulting from catastrophies such as overgrazing or unusual storms. Temporal scales were influenced by seasonality of disturbance and algal reproductive condition and aperiodic episodes of cool, nutrient-rich water advected into the patch.There appeared to be conflicting morphological adaptations of the canopy guilds: exploitation of light was enhanced at higher canopy levels, whereas the lower canopy levels were better adapted to tolerate stress from wave surge. The adaptations of the algae appeared to form four distinct groups of tactics: (I) ruderals or plants, such as Nereocystis and Desmarestia, with opportunistic life histories;(2) kelps, such as Macrocystis, adapted to exploitative competition for light and nutrients; (3) kelps (Eiseni...

The consequences of accepting a false null hypothesis can be acute in conservation biology because endangered populations leave little margin for recovery from incorrect management decisions. The concept of statistical power provides a method of estimating the probability of accepting a false null hypothesis. We illustrate how to calculate and interpret statistical power in a conservation context with two examples based on the vaquita (Phocoena sinus), an endangered porpoise, and the Northern Spotted Owl (Strix occidentalis caurina). The vaquita example shows how to estimate power to detect negative trends in abundance. Power to detect a decline in abundance decreases as populations become smaller, and, for the vaquita, is unacceptably low witin the range of estimated population sizes. Consequently, detection of a decline should not be a necessary criterion for enacting conservation measures for rare species. For the Northern Spotted Owl, estimates of power allow a reinterpretation of results of a previous demographic analysis that concluded the population was stable. We find that even if the owl population had been declining at 4% per year, the probability of detecting the decline was at most 0.64, and probably closer to 0.13; hence, concluding that the population was stable was not justified. Finally, we show how calculations of power can be used to compare different methods of monitoring changes in the size of small populations. The optimal method of monitoring Northern Spotted Owl populations may depend both on the size of the study area in relation to the effort expended and on the density of animals. At low densities, a demographic approach can be more powerful than direct estimation of population size through surveys. At higher densities the demographic approach may be more powerful for small populations, but surveys are more powerful for populations larger than about 100 owls. The tradeoff point depends on density but apparently not on rate of decline. Power decreases at low population sizes for both methods because of demographic stochasticity.

We assessed scientists' ability to detect declines of marine mammal stocks based on recent levels of survey effort, when the actual decline is precipitous. We defined a precipitous decline as a 50% decrease in abundance in 15 yr, at which point a stock could be legally classified as "depleted" under the U.S. Marine Mammal Protection Act. We assessed stocks for three categories of cetaceans: large whales (n = 23, most of which are listed as endangered), beaked whales (n = 11, potentially vulnerable to anthropogenic noise), and small whales/dolphins/porpoises (n = 69, bycatch in fisheries and important abundant predators), for two categories of pinnipeds with substantially different survey precision: counted on land (n = 13) and surveyed on ice (n = 5), and for a category containing polar bear and sea otter stocks (n = 6). The percentage of precipitous declines that would not be detected as declines was 72% for large whales, 90% for beaked whales, and 78% for dolphins/porpoises, 5% for pinnipeds on land, 100% for pinnipeds on ice, and 55% for polar bears/sea otters (based on a one-tailed t-test, ␣ = 0.05), given the frequency and precision of recent monitoring effort. We recommend alternatives to improve performance.Key words: decision analysis, marine mammals, monitoring, trends, trends in abundance, statistical power.In 1994 the U.S. Marine Mammal Protection Act (MMPA) was amended to implement a new management approach designed to identify excessive human-caused mortality in U.S. waters (often referred to as the PBR scheme, for Potential Biological Removal) . Scientists designing this approach recognized 157 158 MARINE MAMMAL SCIENCE, VOL. 23, NO. 1, 2007 that trends in abundance would not be effective to identify at-risk stocks because the statistical power to resolve trends in abundance was very low with the data series that were available at that time. The new management approach was therefore based on estimates of direct, human-caused mortalities as well as information on stock abundance and structure. The implicit assumption behind the new management approach was that direct mortality, such as bycatch in fisheries, was the main threat to marine mammal populations.However, several marine mammal stocks experienced severe declines that were not, so far as we know, a result of direct, human-caused kills. The noteworthy examples are the western stock of Steller sea lion , Merrick et al. 1992, Sease et al. 2001, several stocks of harbor seals in Alaska (Pitcher 1990, Ver Hoef and Frost 2003), the southwestern stock of sea otter in western Alaska (Doroff et al. 2003), and Hawaiian monk seals (Antonelis et al. 2006). Because the PBR system was not designed to identify stocks declining due to factors other than direct kill, we wondered whether this new approach, involving new and more-or-less regular population surveys instituted since 1994, has improved scientists' ability to detect trends in marine mammal stocks, particularly those resulting from causes such as depletion of prey base, ecosystem changes...