Tree growth was investigated in Isle Royale National Park in Michigan to reveal the influence of herbivores and carnivores on plants in an intimately linked food chain. Plant growth rates were regulated by the amount of certain types of animals present (moose and wolves), and the plants only responded to annual changes in primary productivity when they were no longer under pressure from moose. The study revealed top-down control in the Isle Royale ecosystem.
The top-down model predicts that changes in density at one trophic level are caused by opposite changes in the next higher trophic level. In other words, if the number of carnivores decreases, the amount of herbivores should increase. Thus changes in primary productivity (the energy flow of plants), becomes noticeable when higher trophic levels are removed. When carnivores are released from a three-level system (carnivores, herbivores, and primary producers), control is passed to the herbivores. The bottom up theory predicts that changes in higher trophic levels does not affect density patterns in lower levels.
Ring growth analysis revealed cyclic intervals of ring growth suppression that accompanied elevated moose densities in the ecosystem. In the west end of the island there was higher evapotranspiration (which is related to primary productivity) than in the east end, where moose levels were higher because of a lag in the moose density cycle. In the early summer temperatures, AET (evapotranspiration) was supposed to be particularly high across the island. Figure 3 reveals that when wolves were scarce, moose numbers grew and balsam fir were depleted (as evidenced by decreased tree ring growth). This is why the wolf and tree ring cycles resemble each other and are the opposite of the moose cycle. Figure 3E reveals that a clear trend in AET can be seen when the moose population was low, but that there was no reasonable pattern when the moose population increased.
The opposite cycles of wolf/fir growth and moose, coupled with the fact that vegetation dynamics were more closely linked to wolf-moose interactions rather than to seasonal weather patterns suggests top-down control in the Isle Royale ecosystem. Furthermore, there is a more intelligible pattern in primary productivity linked to seasonal conditions when plants are released from herbivore predation.