Latitudinal Gradients

Climate may also play an important role in moderating the strength of herbivore cycles. Some herbivore cycles are more pronounced at higher latitudes than at lower ones, although the strength of the relationship depends on the particular taxa under scrutiny. Many extrinsic factors vary with latitude, most importantly seasonality, strength and body size (Bergmann's rule). The strongest patterns in cycling along latitudinal gradients are found in Fennoscandian (Finland, Norway, Sweden, and the Kola Peninsula in Russia) voles and lemmings. Population cycles are very strong at high latitudes and lessen in strength as latitude decreases. In these systems, it is hypothesized, and supported with small-scale manipulative experiments, that the type of predators associated with latitude may help explain the dampening of cycles at lower latitudes. In particular, specialized predators are more abundant at higher latitudes than at lower latitudes, and generalist predators are more abundant at lower latitudes. Specialized predators that depend on voles to survive will decrease in population density, following the dynamics of the vole increase and decrease. Generalist predators, on the other hand, have the ability to switch from voles as the herbivores begin their population decline, thus moderating the decline itself. In other words, specialists destabilize vole populations and gener-alists stabilize them. Thus, latitude itself does not cause the cycles, but influences the predators that are probably drivers of cycles.

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