Biotic and abiotic effects to vital rates do not necessarily translate to equivalent effects on population growth. It is important to consider these effects when predicting how population growth rates are affected by threats such as invasive species. We explored the joint effects of native and invasive insect herbivory and weather on plant population growth using a set of closely interacting species: two species of native cacti (Opuntia) in Florida and two cactus moth species (the native Melitara prodenialis and the invasive South American Cactoblastis cactorum).
Between December 2012 and August 2015, we surveyed all cacti (2908 plants) in 287 1-m2 plots across seven habitat patches (five islands and two barrier dune habitat patches) within Guana Tolomato Matanzas National Estuarine Research Reserve (near St. Augustine, Florida along the Atlantic Coast). During each summer and for each individual cactus, we recorded the number of segments, flowers, and fruit. Each summer and winter, we noted cactus moth presence/damage in addition to cactus survival and recruitment of new cactus individuals.
We modeled relative growth rate and fruit production using generalized linear mixed models and performed contingency table analyses on our survival data.
Results/Conclusions
While 63% of marked O. stricta plants were observed to have moth damage during the course of the study, the invasive and native moths were only observed on 6.3% and 1.8% of plants, respectively. Insect prevalence was even lower on O. pusilla: both moth species were observed on less than 1% of plants and only 13% of plants showed moth damage.
A small fraction of plants produced fruit: 0.2% and 4% of O. pusilla and O. stricta, respectively. In contrast, at least 11% of each species produced clonal offspring through segment loss.
Of the 2908 plants surveyed during the study, approximately 50% recruited into plots after the initial survey, a majority of which were clones (660 and 335 O. pusilla and O. stricta individuals, respectively). We only observed seedling recruitment for O. stricta (n = 109).
Moth damage was negatively associated with survival for both cactus species. Opuntia stricta infested by the invasive moth also had lowered survival. Mortality was highest for clonal plants, compared to seedlings and plants of unknown reproductive mode.
We are working to incorporate abiotic variables (temperature and precipitation) into our vital rate analyses and will synthesize our results with a size-structured population model.