Fecal Frenzy: How Much Scat is Enough?

The cruise yielded a veritable treasure trove of samples, but begged the question: how much scat is enough? Too few samples produce imprecise results, but excessive sampling can waste valuable financial resources. Fortunately, a timely paper by UBC’s Andrew Trites and Ruth Joy, recently published in the Journal of Mammalogy, offers a compass to guide researchers through the pitfalls of scatological excess.

Dietary snapshot
Typically, researchers identify and count prey in scat by examining several unique structures that resist digestion, such as the ear bones (otoliths) and jawbones of fish. More than 35 prey species have been described in pinniped scat, but only about 3-15 species turn up with any regularity: these findings were gleaned from studies of 10 to as many as 1,000 scat samples.

Seeking middle ground, Trites and Joy sought to answer two key questions: how does sample size affect statistical reliability, and how many samples are required to identify all prey species present?

Using computer models, they calculated the number of scats needed to accurately compare the diets of two populations. Naturally, the optimal sample size will vary with each study, depending on the number and distribution of prey species being tracked. But as a general rule of thumb, the authors suggest collecting approximately 60 samples in order to detect key prey species—that is, those occurring more than 5% of total collected samples.

Safety in Numbers
However, a different “golden number” is needed if the primary goal is to detect and track dietary differences across time or between areas. It was here that the study identified a curious inverse trend: 179 scat samples are needed to distinguish three prey types, but only 94 samples are required when six or more prey types are present.

The authors acknowledge that it may seem counter-intuitive to require more samples in order to distinguish fewer prey types: one might expect more samples would identify more prey types. But the study revealed that differences between populations with fewer types of prey are more difficult to detect than differences between populations that consumed a greater diversity of prey.

From a statistical point of view, fewer samples are needed to validate the existence of a difference when two populations show large differences. But with two closely matched populations, more samples are needed in order to confirm that the similarity is real and not just a result of sampling error. In essence, Trites and Joy advocate safety in numbers.

While the present study focuses on the numbers of scats needed to describe and compare diets, the authors warn that consideration must also be given to the geographic area, the time of year the samples are collected, and the sex and age classes of the individuals sampled. Thus, they write, “a thoughtfully collected, smaller sample of scats may be more representative of a population’s diet than a larger sample that has been haphazardly collected.”

The numbers of scats that researchers should collect to accurately describe the diet of Steller sea lions is undoubtedly much higher than many would have predicted. And while this discovery that biologists should be collecting at least 60 scats per site may not put a smile on every trowel-bearing researcher, it is an important rule of thumb that can be broadly applied to dietary studies of other mammal species living on land and at sea.

26 September 2005

Publication:
Dietary analysis from fecal samples: how many scats are enough? Trites, A.W. and Joy, R. 2005. Journal of Mammalogy 86(4):704-712. abstract Diets of mammals are increasingly being inferred from identification of hard parts from prey eaten and recovered in fecal remains (scats). Frequencies with which particular prey species occur among collections of scats are easily compiled to describe the average diet, and can be used to compare diets between and within geographic regions, and across years and seasons. Important to these analyses is the question of statistical power. In other words, how many scats should be collected to compare the diet among and between species? We addressed this problem using Monte Carlo simulations to analytically determine the consequence of sample size on the dietary analysis of scats using frequency of occurrence methods. We considered two questions: 1) how is the statistical power affected by sample size; and 2) what is the likelihood of not identifying a prey species? We randomly sampled predetermined numbers of scats (n=10–200) from computer-generated populations of scats containing prey of known species and frequencies of occurrences. We also randomly sampled a large database of field-collected scats from Steller sea lions (Eumetopias jubatus). We then used standard contingency table tests such as chi-square and Fisher’s exact test to determine whether differences between our samples and populations were statistically significant. We found a minimum size of 59 scats is necessary to identify principal prey remains occurring in >5% of scats. However, 94 samples are required when comparing diets to distinguish moderate effect sizes over time or between areas. These findings have significant implications for the interpretation of published dietary data, as well as for the design of future scat-based dietary studies for pinnipeds and other species.