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Selective pressures on cacti needle length


The Cylindropuntia bigelovii cactus, also called the teddy-bear cholla plant, is a species of cactus found in the water deplete Sonoran Desert. One form of physical variation between the cacti is needle length, which can vary from 1 cm to up to 5 cm in length. It is known that selective pressure from lack of water in the desert keeps the needles quite short to minimize surface area and therefore losses of water in the hot environment. 5000 cacti were examined to determine the average needle length amongst teddy-bear cholla. The results are summarized in Table 1.
Table 1. The distribution of Cylindropuntia Bigelovii Needle length.
1-2 (cm)2-3 (cm)3-4 (cm)4-5 (cm)
Number of cacti19125606311897
In a follow-up study, it was shown that the Neotoma Lepida or the Desert Woodrat, is the primary teddy-bear cholla predator. It was also shown that the rats are best warded off from eating cacti that have needles longer than 4.2 cm. It was also observed that Sand Flies also consume teddy-bear cholla; however they only seem to target infant cacti that have yet to grow needles longer than 0.5 cm.
Two years following the study, the population of Desert Woodrat in the Sonoran desert quadruples. How would you expect the data in Table 1 to shift following this change?
Choose 1 answer: