More on Co-evolution and Interdependence

Just a couple of days ago I was talking about some of the ways that completely different species have a dependence upon the other. While many are obvious such as food and prey, most prey species need the predator to control their numbers; otherwise they outstrip their food supply. That scenario, as witnessed in many species when the predators are reduced or removed, has the effect of reducing the prey species more than would ever be lost via predation. That is because while some will die from starvation, the greater effect is reduced breeding rates and increased infant mortality. Thus the importance of a full range of species within any habitat is now well understood.

However, currant research is still illuminating some very interesting and subtle examples of interdependence. I still can’t get the image out of my head of scientists sniffing ants. See the previous post on this subject.

However, research is highlighting many surprising examples of the symbiosis of different species and the importance of large animals. As I have mentioned before, in Yellowstone National Park, they discovered that having a healthy and sizable population of Wolfs changed the behaviour of the Elk. By adjusting the way the Elk feed, by keeping them out in the open and away from the Forest and trees, willow and aspen regenerated. This in turn provided food and shelter for beavers, who by creating their pools and this in turn helped the dippers, amphibians and reptiles. The web of benefits extends out to include about one hundred species where a direct link can be shown to the reintroduction to the wolf more than a decade previously.

Research just published in Science Magazine, that journal for the AAAS, carried out by Florida University, has made some remarkable discoveries about the importance of the need for the large herbivores in protecting the acacia trees in the African Savannah.

While this may appear counter intuitive, the acacia trees need the large herbivores like Giraffes, Elephants and Antelope to nibble the trees to remain healthy. As what happens is when the trees are grazed the tree reacts by producing a sweet sap. This may in part be to help heal the wounds, but the important part is this sap attracts ants, specifically, the biting ant Crematogaster who act as body guards for the trees. When the herbivores come back to the Acacias, the ants fight off the grazers; this prevents the tree from being damaged beyond loosing some growth.

The researchers from Stamford University and Florida University isolated the trees, mimicking the loss of (extinctions) the herbivores by fencing the trees off. The effect was not what anyone expected to see as instead of the trees thriving the in fact suffered and became less vibrant and grew more stunted than the ones that were grazed. Part of the effect was that the tree by getting nibbled stimulated extra growth. But the process was more complex than that. Once grazing did not injure the trees they stopped producing the sap that attracted the ants. The loss of the ants allowed other insects to invade and bore into the trees weakening the trees. Therefore the trees needed to large mammals to browse them, to gain the protective alliance of the ants.

It shows that even in the natural world we all need friends.




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