Animal Activity in Uncontrolled Terrestrial Communities as Determined by a Sand Transect Technique

1. What they did

Bider wanted a single, simple method that could record the activity of a whole community of ground-dwelling animals, from insects and worms to mammals and birds, in natural conditions. Every animal moving on the ground leaves a trail in fine sand, so he built strips of sand about 3 ft wide and several hundred feet long, and noted the location and direction of every track crossing them at regular intervals.

• The transects. Near Lac Carré, Quebec, he built three: an ecotone transect (1962) running 450 ft through an open field and 300 ft into the woods, a 783-ft field transect and a 450-ft woods transect (both 1963). Soil was cleared of roots and pebbles, compacted and topped with fine sand. After each reading the sand was swept smooth with a soft push broom.
• Reading the tracks. Tracks were read every 2 hours, day and night (at night by gas lantern), and recorded in 10-ft units on data sheets.
• 1963: four tracking periods. Late June, late July, late August to early September, and mid-October, with each transect divided into 150-ft sections to compare where animals were active.
• 1964: continuous tracking. A polyethylene canopy, 6 ft wide and suspended 6 ft high, kept rain off the ecotone transect so it could be tracked continuously for 165 days, alongside detailed weather records.

He did not try to count individual animals: the measure was the number of crossings, which reflects activity rather than population size.

2. What they found

The community

• The study produced about 182,000 crossings. The community comprised invertebrates and 58 vertebrate species, grouped into 38 categories where species were hard to tell apart.
• The ecotone transect recorded about twice as many species as the field or woods transects (51 against 32 and 30); with the canopy and continuous tracking, 59 species and 9 more were recorded.
• A 150-ft section recorded as many species as a whole 783-ft transect in similar habitat. Both lynx species and a mountain lion crossed, the latter a record for the region.

Use of space

• In the woods, most species eventually used the whole area at random (16 of 20 species used the deeper sections equally). Activity was often higher in the first 150 ft inside the forest edge and lower in the next 150 ft, an “edge and vacuum” effect.
• In the fields, activity was more patchy: many species used pockets of the field, and activity was lowest near the field edge.
• Deer mice moved along set paths from a homesite to feeding areas. Meadow voles and shrews crossed mostly at about 10 major runways (82% and 83% of their crossings), which took up less than 4% of the field transect.
• Snowshoe hares shifted during the summer from the deep woods toward the forest edge and into the fields, following the food supply. Garter snakes seemed to move down to the fields to give birth and back to the woods to hibernate.
• Red squirrels and chipmunks used separate areas when both were most active in late summer, the squirrels carrying cones from a spruce and a chipmunk carrying leaves to a hole.

Time of activity

• Peaks of activity of different species were most spread out on warm July nights, and squeezed into the first hours after dusk on cold nights. In autumn, arhythmic animals (voles, shrews, snakes) shifted their activity from night to day.
• Through the season, activity rose and fell with life events: short bursts during mating (chipmunks in a “frenzy” along the forest edge; five hares chasing in daylight), high steady activity while storing food, and sudden surges during migrations of frogs, salamanders, snakes and birds.
• Weather shaped activity within each season. Activity was lowest the first day after a front; diurnal animals peaked on the last clear day and nocturnal animals on warm cloudy nights. Rain suppressed insect activity, stimulated meadow voles, and did not change shrew activity. Toads were hardly active when dusk temperatures were not above 50°F.
• Ermine crossing many times in one day left mostly a crouched gait instead of their normal hopping gait.

3. What they concluded

• Activity in a natural community is controlled at four levels, each setting limits for the next: the year (population size), the season and its life events, each weather system, and individual climatic events.
• Simpler animals respond mainly to physical conditions such as temperature and rain; more advanced animals depend more on behaviour. Bider called this a “physio-ethological activity gradient”.
• Activity data from a single day are worthless. Analyses should compare similar days within one year, within one seasonal event and from similar points in weather systems.
• One full weather system is the minimum time needed to detect the basic species of a community in a given season.
• Bider considered the sand transect his greatest contribution: in his view, the most practical, simple, economic and least biased way to get copious data on animal activity.

4. How tracking was or was not used

Tracking was the method. Every track crossing the sand was identified and recorded by location and direction. Identification used Murie’s Field Guide to Animal Tracks; the author notes that Murie’s drawing for red-backed voles is wrong (the drawings were not Murie’s own). Red-backed and meadow voles were separated by print size and depth, and by gait (meadow voles walk; red-backed voles use a triangular hopping gait), with decisions on new runways postponed until the animal was seen; the author estimates this kept errors under 1%. Chipmunks, red squirrels, grey squirrels and woodchucks were separated by stride width, print weight and distance between jumps. Shrews and jumping mice could not be told apart reliably and were grouped by family. Questionable tracks were circled and timed in the sand for checking. A reference collection of real prints was made by spraying the sand with lacquer and setting it on plaster of Paris.

5. Tracker or observer certification

Not reported. The author says that identifying tracks “is a question of practice and experience”. Student assistants did much of the reading; no training, certification or test of their skill is described. The author notes that some squirrel tracks gave the assistants the most trouble.

6. Other technologies, skills or innovations

• A transect canopy of polyethylene film that allowed continuous tracking through rain and limited dew.
• A sand recipe and maintenance routine: fine sand (particles of about 0.1 mm) to record small details, raking to dry the track after rain, and herbicide to control weeds.
• Weather recording with a time-lapse movie camera (1962 to 1963), then a rain and snow gauge, dew gauges, a wind instrument and a pyrheliometer.
• Lacquer and plaster casts of sand prints for teaching and checking.

7. Implications for conservation (from the paper)

• The edge and vacuum pattern at the forest edge may matter in applied ecology, where strip cutting is used to produce a greater abundance of animals.
• The yearly number of crossings by transient animals during the dispersal phase might be a good index of a population’s success in rearing young.