The relative importance of herbivory and abiotic conditions to demographic rates of a threatened plant in Florida, Opuntia stricta (Haw.) haw
The impact of insect herbivory on native plants may be manifested in different ways, depending on which part of the plant’s life history is affected and whether the herbivore is native or invasive. Our study system consists of the invasive South American cactus moth, Cactoblastis cactorum (Berg) and the specialist native bug, Chelinidea vittiger McAtee. Both attack populations of native prickly pear cacti (genus Opuntia) in Florida. Our objective in this study was to quantify the relative importance of herbivory (invasive and native) and abiotic conditions in explaining variation in demographic rates of O. stricta.
At three locations in the Florida Panhandle, we tagged 17-19 plants of O. stricta and resurveyed each at least twice a year for five years (2009-2014), recording plant size, fruit abundance, and insect presence. We calculated three weather variables from daily temperature and precipitation data from the NOAA National Climate Data Center: mean daily precipitation, percentage of days with rain, and average degree day. We used generalized linear mixed models in SAS 9.4 to quantify the relative ability of insect presence and weather variables to explain variation in three vital rates of O. stricta: relative growth rate, probability of fruit production, and fruit abundance.
Opuntia stricta relative growth rate between times t and t-1 was negatively associated with plant size[t-1], the percentage of days with rain within the time period, and the presence of the native bug[t-1] and invasive moth[t-1]. It was positively correlated with the average degree day within the time period. The probability of fruit production was only significantly negatively correlated with plant size[t-1]. In contrast, fruit abundance was positively correlated with plant size[t-1], average relative growth rate, the percentage of days with rain and average degree day. Fruit abundance was negatively correlated with mean daily precipitation and the presence of the native bug[t-1].
Our results show that insect herbivory and weather do not affect all demographic rates equally. In general, the association between O. stricta demographic rates and insect herbivory was negative, suggesting that plants may not be able to compensate for biomass lost to herbivores, at least in the short-term.
In future work we will quantify the relative importance of abiotic and biotic variables in explaining survival of O. stricta. Finally, we will build a population model to estimate the effect of the insect herbivory and weather on O. stricta population dynamics.