Opuntia humifusa (eastern prickly pear cactus) is an endangered species in Canada, found principally in only one remaining location nationally, at Point Pelee National Park (PPNP). This study quantifies fruit and flower production in the population, the pad yellowing phenomenon that has been observed, overlying coverage and shade, and a variety of plant size metrics in order to assess the relationships between these factors. A variety of parametric (ANOVA, Pearson product-moment correlation, t tests) and nonparametric (Spearman's rank correlation, Kruskal-Wallis test) statistical analyses were conducted. Pad yellowing, a presumed sign of ill health, was not significantly related to any aspect of plant size or reproduction, and was statistically related only to shade (less yellowing with greater shade). Shade also affects plant height, with taller plants associated with greater shade. Reproductive effort is lower in shadier sites, and shade also appears to delay reproduction to later in the season. Published research generally suggests that increases in sexual reproduction are a sign of stress, suggesting an overall decrease in stress in the cactus population in the Park. However, I observed that reproduction decreased with shade, and increasing shade is known to cause population decline, making the effects of light stress at PPNP difficult to determine.
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