By Jackie Willis
April 4, 2004
The relationships between predators and their prey are fundamental to understanding food webs, yet these connections are very difficult to study in detail. Even when large quantities of data are available, scientists may arrive at incorrect conclusions because they have not looked at all sides of the story, or all factors in the "equation" of an ecosystem.
For example, there is a Canadian cat species, the lynx, which feeds mainly on snowshoe hares. The hares are relatives of rabbits, but have wide feet that act as "snowshoes," allowing the hares to run swiftly across the surface of deep snow instead of sinking under and getting stuck. Their predators, the lynxes, also have big, broad paws for running on snow, the better to catch hares. Hares are swift and alert, so lynxes may catch only a small percentage of the hares they pursue. However, lynxes try often and do eat lots of hares.
When Europeans settled in Canada, they trapped many species of mammals for their fur, including the lovely spotted lynx, and the soft, warmly furred snowshoe hares. The fur trade became a big business, and detailed records were kept of the numbers of hares, lynxes, and other species that were killed for their fur and shipped to Europe to make hats and coats. The numbers of hares and lynxes recorded in the 18th and 19th centuries were often in the tens of thousands each year, but varied drastically in some years. These changes in numbers were in cycles of 10 or 11 years, up and down. People assumed that the lynxes ate the hares and reproduced too much, so that there were too many lynxes eating too many hares, and so the hare numbers declined. Thus people concluded that the predator controls the prey population.
Scientists thought they were so smart. They had loads of data to show the lynx-hare cycles and they could explain how it all happens. However, hares are not isolated in this ecosystem. Hares have to eat something, and they are herbivores that eat young shoots of shrubs and small trees. Botanists studying the food of the hares found that certain willow species favored by the hares can produce toxic chemicals that interfere with reproduction by the hares. When the willows are not chewed up very much, the new shoots contain little toxins, and are tasty and nutritious. But if there are lots of hares chewing up the new shoots, then the plants strike back by producing bitter-tasting toxins. The hares reproduce less on this diet, and so fewer hares leads to a food shortage for the lynxes as well. Pretty soon both hares and lynxes are thin and hungry and many die and fail to leave babies. When the willows are no longer being eaten, they go back to producing new shoots that have no toxins, and then the hares can increase and so can the lynxes. This all takes about 10 years to repeat itself.
So, maybe it's the plants controlling the herbivores from the "bottom" of the food pyramid, rather than the predators controlling from the "top." Or maybe there are pressures from both the predators and the plants that interact in more complex ways. Now biologists are more cautious about conclusions and try to look broadly at many interacting components in ecosystems before drawing conclusions. On BCI, for example, there were scientists who felt that the ecosystem is not "normal" because they did not see many signs of predators. Now we know that the predators are there, but are just hard to detect. Our remote-sensing camera system has helped in this regard.
Also, scientists have thought that the snowshoe hare and lynx story of changing numbers is typical of species in the cold, wintry north, but not in the tropics where the climate is relatively stable. However, the results of the mammal census have shown us that the numbers of only some species stay the same, while many others do show sharp changes. Agoutis on BCI in the last 6 years showed declines in the numbers of sightings on the average of approximately 10% each year. This has coincided with a time when the number of ocelots on the island has remained high at 26 individuals. So did the ocelots eat lots of these agoutis?
How many agoutis do ocelots actually eat? We just don't know. Even with all 30 of our cameras working, we rarely get photos showing ocelots with prey. In hundreds of ocelot still photos we have just two cases of predation-- one ocelot catching an opossum, and one carrying a spiny rat. The opossum photo is interesting because it's part of a story. We have a photo first of a small "common Opossum" of the type called "Didelphis." He is staring into the camera and the time stamp on the photo shows when the photo was taken The next photo was taken 3 minutes later, because the camera system was set to wait three minutes before taking the next photo. This prevents the camera from using up an entire roll of film on one animal that stands in front of the camera. The next photo shows the male ocelot "Colmillo" (which means"Tooth") grasping something dark and furry in his toothy jaws and using his paw to hold it, in the exact same location as where the opossum stood. Then Colmillo is gone and hours later that night a big opossum stands in the same spot, with his fur sticking up and his eyes wide open in alarm. I went to the location after I developed the film, and found tufts of opossum fur at the exact spot where the small opossum stood. I imagine that the big opossum was very frightened by the evidence of violence on that spot and the bright flash from the camera.
People on BCI rarely see ocelots, and hardly ever observe predators in action. Yet, two remarkable observations occurred in the last few months. On Feb. 4, 2004, Ruffino Gonzalez, a worker who gathers plant data regularly on BCI, saw an ocelot catch a howler monkey. The cat climbed into a low tree, following the monkey upwards. The cat caught the monkey by the throat, and they tumbled to the ground, where the ocelot killed the monkey with a neck bite, and dragged it away into the forest. The other howler monkeys kept screaming for a long time.
On another morning in March, Guillermo Sanchez, our field assistant who tends the camera-traps, saw an ocelot carrying a heavy load-- a dead adult coatimundi. These are very special sightings. We also know from the agouti radio-tracking work of Enzo Aliaga-Rosell that agoutis are often eaten by predators. We know from Ricardo Moreno's study of the fur and bones found in nearly 100 scats of ocelots on BCI that agoutis may make up 20% of the diet of BCI ocelots. Our video camera-trap allowed us to capture one more case of predation by ocelots, this time on an agouti. You can view our video clip that shows a mother ocelot bringing her large kitten at night to share an agouti that she killed earlier in the day. The tape was recorded in infrared light, so the cats would not be disturbed.
Likewise, we have been watching for changes in sloth numbers of BCI, because Ricardo's scat data show that sloths make up about 30% of ocelot diet. We found declines of sloths and frequent encounters of dead sloths on BCI in the past few years. However, t's so difficult to count sloths accurately, and they never appear in our camera-trap photos, so it's hard to say what's happening. We don't even know if the ocelots climb trees to catch sloths or just grab them whenever they journey down low in the trees. No one has observed an ocelot catch a sloth on BCI, and ocelots don't eat many sloths in other countries.
What does this all mean for the food web? Well, it means that agoutis and ocelots are connected in an important predator-prey relationship. In this case on BCI, it may be that the decline of agoutis was caused by the ocelots that eat them. But we don't know for sure that the plants that are eaten by the agoutis are not the primary cause for the change in agouti numbers. We know that fruit crops vary substantially over the years, even though the climate is tropical. Tropical climates may not include cold weather, but they are not necessarily the same every year (think of El Nino changes). Rainfall changes a great deal, and rainfall affects the fruit crops of forest trees and shrubs. We are working on fruit production data from several sources to look for connections at the base of the ecological food pyramid. What we find so far is that the story is very complex and defies explanation. I am leaning toward an explanation that involves both predators and plants as major players in the control of the mammalian herbivores. This is an area that new young scientists will want to explore for many years to come, because these secrets will not yield easily to explanation.