Census techniques and Pollard & Yates

[Michael Gochfeld]

In recent inquiries regarding butterfly censusing the following book was
mentioned.

Pollard E and TJ Yates. Monitoring Butterflies for Ecology and Conservation.
Chapman and Hall, now available in paperback.   The 1993 hardcover edition sold
out quickly (that may mean that the publisher was chintzy and didn’t print many
copies).

This is a very readable book.  I lament the fact that species’ English names
are not capitalized so for someone unfamiliar with the fauna, you can’t always
tell what is an adjective or what is part of the English name for example:
“small heath, ringlet and green-veined white.”

Nonetheless a lot of it reads like Ken’s butterflying accounts.  Habitat
descriptions and butterflies seen (and counted).

Pollard and Yates describe in detail the methods and results from their census
technique (hereafter referred to as “Pollard”).  The results and trends are
fascinating (and sad in some cases).
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In our New Jersey book we identified four published methods of censusing
butterflies along transects. They differ only slightly:

King method: all butterflies seen are counted and the distance at which each
individual is first spotted is recorded.  Butterflies beyond 5 m are not
counted.  This seems awkward if butterflies are numerous.

Sides: all individuals seen within 5 m on either side of the observer are
counted (pretty much the same results as King method, but doesn’t require
separate distance estimate for each countee).

Pollard: all individuals seen in front of the observer at a range of 5 m or
less are counted.
Pretty much the same, but no fair looking behind you (unless you are in Puma
habitat).

Douwes: all individuals seen to the right of the observer within a 5 m distance
are counted. Americans are allowed to use 16 ft as an approximation.

The latter method allows you to look to one side only, thereby reducing the
likelihood of overlooking less conspicuous individuals.  But what if something
really good flies by to your left.

Our transect (about 300 m) is covered by using the Douwes method on both the
inbound and outbound legs.  We therefore have a result that approximates the
Sides: method, the transect is divided up into segments, and for each segment
we use only the maximum count, thereby approximating the Pollard method
(confused yet).  Mainly we are interested in the consistency between the two
counts (within an hour period).  By covering the same area we can gain an
estimate of the possibility of overlooking certain species entirely as well as
an estimate of count precision (usually only one or two species are numerous
enough for this to be an issue).

Each segment thus becomes a mini census tract about 10 m wide and 30 m long.

In any case we do the same thing on each census (approximately weekly). We also
record the relative abundance of all flowers in each segment (about 110
identified species) and note which nectar sources are being used.

For each butterfly we note whether it was flying, sitting, courting, mating,
feeding when first encountered, and the nectar source (butterflies are never
too numerous for this, wish they were).  However, it is not always easy to tell
whether a butterfly is actually nectaring or merely sitting on a flower.

We once tried comparing the four methods by having four columns and indicating
for each individual encountered whether it would be “countable” under a
particular method. This required two people walking in tandem, one looking left
and one looking right. Followed, in some cases by an argument over whether an
individual was countable, and what to do if it crossed from the left front to
the right rear.

It was actually tricky (maybe more skillful mortals would find it easy), but
the results were (not surprisingly) pretty close.

Michael Gochfeld
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