The Mammal Census Project

Some of you have asked if we could review the history, goals, and plan of the mammal census project on BCI. Even though it's recorded in some entries on our website, some of our schools back home don't have easy access to the web, so we summarize here:

Jackie and Greg are running a project to collect long-term (15 years) data on the numbers of individuals of different species of mammals on Barro Colorado Island, Panama. People used to think that because there are no sharp changes in temperature in the tropics, and no severe winter weather, that everything stays the same all year round and from one year to another. Yes, they knew it rained a lot for part of the year, and was dry the other part, but folks didn't figure that affected the mammals much. They certainly didn't think that mammals in the tropics had big changes in their numbers like the lemmings do in Alaska and Scandinavia or the voles do in New Jersey. The theory was that the tropics are extremely stable places to live and that life is fairly easy for mammals. So we checked some of that and found quite a lot of evidence that mammal numbers do change a lot and that some years they have trouble getting enough to eat. Early researchers in the 1930's reported that some years there was famine in the forest on BCI and many mammals died. More recent observations in the 70's showed that famines occur periodically. We found that the end of the rainy season and the early dry season (November-March) are difficult times every year because fruits tend to be scarce. We also thought there might be a somewhat irregular cycle of good years and bad years. Our long-term data data will be analyzed over the next year to look for such patterns.

And now we are trying to find out why things change in this forest. We have, for example, been measuring fruit production by some trees that are important to some mammal species. Fruit production, and therefore food supply, vary from year to year. Rainfall and sunshine are both important for trees to produce fruit, so these are logical factors to look at. Some of you have already sent us e-mail messages asking if the El Nino is causing the recent drought on BCI, and, indeed, the El Nino weather pattern might prove to be very important in understanding our data on mammals.

We have a specific routine for collecting data to determine the changes in numbers of mammals from one year to the next. We do our census in early dry season. We walk at a rate of one kilometer per hour. We look around, up and to the sides, etc. We record all mammals except bats (later I will tell you what the bat folks are doing). For every animal we see, we record identity of the species and an estimate of:

the distance between the animal and the observer,
between the animal and the trail, and
the height of the animal off the ground.

That sounds like a lot of stuff to write down, but if you practice estimating distances (in meters) and then develop a routine, it's not hard to do. These numbers will later be used to estimate:

the ease with which animals will allow observers to approach,
the width of the strip of land that is being censused (different in different parts of the island where vegetation may be very open or very dense), and
the levels of the forest being used by different species.

We also trap and mark squirrels to get a mark-and-resighting estimate of the size of the squirrel population. We put little ear-tags (like having pierced ears) and necklaces with colored beads on the squirrels in one 10-hectare area, and later look to see what proportion of the squirrels have markings. (This is similar to a mark-and-recapture study of goldfish crackers that can be done in the classroom to estimate the "population" of "goldfish"-- e-mail me when I go home if you want a copy of this lesson). These data on individual squirrels who have become friends over the years are interesting because we have learned that squirrels may live to be 10 years old, something biologists didn't suspect before, but that Greg and I guessed at a few years ago. This means that one very knowledgable mother squirrel might build up a whole inventory of information about a variety of food sources in her territory and raise lots of babies to succeed in her place. This is another reason why long-term studies are important to do.

Our basic data, however, are in the form of numbers of animals seen per kilometer. For example, in 1996, we did about 120 km of census and saw about 4 agoutis per kilometer, on the average. This was down from 5 agoutis per km in a previous season. We want to look at fruitfall data too and compare to see if there is an obvious reason why agouti numbers were down. We will later send some samples of our data and more explanation.

We have to census in the morning between 7 am when it's just light enough to see clearly in the forest,and noon, when many animals take a siesta for the rest of the day, coming out to feed again in the late afternoon. And we do about 20 km of night census too, which is more tiring and time-consuming for each animal we find and identify (it takes longer to be sure of an identification when working at night). We do day and night censuses because different species are active in the dark, while some species sleep. We take headlamps (like the ones that miners wear) and a 500,000 candlepower spotlight. It's harder to see animals at night, but the equipment gives us a special advantage. Most animals that are active in the night have a special part of their eye in the back of the eyeball (tapetum) that reflects light and so intensifies their ability to see in low light levels. The light of a spotlight is also reflected back to our eyes (you must hold the light at or just above your own eye level) and so we see bright "eyeshines". Different species also have characteristic eyeshines: bright yellow, brilliant green, dull red, etc. We record all the mammals we see, except bats, and where we saw them, as well as the time.

So, that's what we do! You probably thought of some more questions, so send us a message! Hasta luego!