Broadly speaking, herbivore populations can be regulated by density-dependent mechanisms or affected by density-independent mechanisms. These factors are key in understanding how herbivore populations cycle. Density-independent factors affect population densities of the animal in question, but are not affected by changes in densities themselves so that there is no feedback in the system. These factors may include climatic variables, sunspots, or some types of human landscape-alteration. Any particular factor may act as a density-independent or density dependent mechanism given a certain set of circumstances. For example, a disease may be density dependent if higher host population densities increase its transmission rate, but density independent if it is not contagious or otherwise is unaffected by the host population.
Density-independent factors are rarely the sole determinants in causing cycles, but they are important in providing the arena in which density-dependent mechanisms play out. For example, density-independent factors may influence the amplitude of the cycles. Researchers at one time believed that density-independent factors were relatively unimportant in either predicting or influencing the increases and decreases of cycles. We now know that these factors can be vital in exaggerating, dampening, or stabilizing the effects of density-dependent factors.
One density-independent factor that was invoked to explain some aspects of the hare-lynx cycles is the activity of sunspots. Since sunspot activity affects the entire planet, the effects may be responsible for the synchronization of hare population cycles across vast areas of high-latitude boreal forest. The sun goes through a 10-year cycle of solar activity with distinct and longer periods of high-amplitude cycles every 80-90 years, or what is known as the Gleissberg period. Although the relationship between sunspot activity and hare density drifts out of synchrony over many hundreds of years, they are forced back into synchrony at the onset of a new Gleissberg period of high sunspot activity.
Thus, it appears that sunspot activity by itself is not sufficient to cause or maintain the cycles we observe in herbivores, but may be necessary to force the population synchrony over large geographic areas. Several hypotheses have been put forth to explain the mechanism behind this forcing, but the researchers believe that sunspot activity seems to affect large-scale climatic variables such as precipitation, which can affect plant growth. Increased solar radiation can also affect the defensive compound concentrations in certain plants. Since the biochemical pathways for protection against UV radiation and antiherbivore chemical defense often use the same precursor or intermediate molecules, increased resource allocation to UV-protection during periods of high sun-spot activity may cause concomitant decreases in antiherbivore protection. This would presumably allow hares to eat more food, reproduce more, and thus increase population density.
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