Leafwing Population Dynamics (Genus Anaea, Lepidoptera; Nymphalidae) and Viability of the Endangered Florida Leafwing

of a dissertation at the University of Miami. Dissertation supervised by Professor Carol C. Horvitz and Joyce Maschinski. No. of pages in text. (161) Butterfly populations are in decline in Florida. An example is the Florida leafwing, Anaea troglodyta floridalis (Nymphalidae), a species endemic to the pine rocklands in South Florida. I used viability analysis of population dynamics and extinction risk to evaluate the sensitivity of A. t. floridalis to disturbances such as fire. Using an annual population density index estimated from monthly count data over 15 years, I estimated a time series of relative population growth rates. To these relative growth rates I fit density independent and density dependent population dynamics models and estimated both intrinsic growth rate (r0) and carrying capacity (K) in response to fire. Projecting density independent and density dependent population models into the future by simulations indicated that the density dependent model was the more appropriate and also provided a more optimistic viability prediction. Sensitivity of relative growth rate to density increased in the absence of fire, but this sensitivity decreased when fires occurred due to post-fire increase in carrying capacity. While my analysis suggested that improving carrying capacity would reduce the extinction risk for this endangered endemic butterfly, I cannot say for sure how to improve carrying capacity. Carrying capacity in butterflies is generally a function of either predator or host plant abundance, and I addressed this issue using two common leafwings, A. aidea and A. andria, as proxies for the endangered leafwing. In an unusual application of mark-recapture methods, I repeatedly surveyed 510 A. aidea caterpillars in their natural environment for one month in the spring. I asked how the probability of surviving and growing varied across that month, and tested for the effect of temperature on growth and the effect density on survival. In mark-recapture, the demographic process is partially “hidden” due to imperfect detection, so I modeled caterpillar demography as a hidden Markov process using Bayesian procedures. With data on survival and development of individuals during each 3-day time step, I modeled the entire process of juvenile development as a column stochastic Markov process. The stage of each individual alive at time t identifies a particular column, which contains all possible fates (dead or alive and which stage they would be in) for individuals in that stage 3 days later. These columns together describe the entire juvenile demographic process (from egg to pupa) as a stage-structured matrix model that contains the probabilities of surviving and growing for each stage after a 3-day time step. Using two multinomial probability functions, I estimated the joint likelihood of being alive and observed in a particular stage at each time step given the demographic matrix, the probability of being observed, and the individual’s stage at the previous time step. During this survey, caterpillar densities increased to a maximum then declined as individuals either died or pupated. I observed both live and dead caterpillars with high probability, and was able to determine the cause of death for most corpses encountered. The abundance of caterpillars attracted multiple insect and arachnid predators, and survival declined as per capita predation rate increased over time. Survival of fifth instars improved at high density, with density scaled by the size of all caterpillars on a host plant, but this is arguably an artifact resulting from the time lag between predator and prey abundance. The most likely model showed that survival of fifth instars declined over the growing season while early instars (first to fourth) had constant high survival. Similarly, the most likely model supported time-invariant stage-specific growth rates, despite a compelling relationship between growth rates and temperatures. This stage-structured matrix model represented the recruitment process as a progression of individuals from egg to pupa through all juvenile stages, which are absent in most studies of butterfly demography. This work not only demonstrated that predator abundance affected Anaea population dynamics, but it also showed that predation may play a role in density dependence. Insect herbivores commonly feed on multiple plants throughout their geographic range, but feed locally on only one or a few plants. I asked how specialized were the diets of Anaea aidea (tropical) and Anaea andria (temperate). Rearing almost 300 caterpillars in the lab on controlled diets of one of five Croton host plants, I evaluated the performance of caterpillars in terms of survival to pupation, development time, pupa mass, and adult size. Both leafwings showed a moderate ability to feed on multiple Croton, but tended to perform best on their local host. Even though each leafwing was capable of surviving to pupation on non-local hosts and local non-hosts, either growth or development time was less than ideal compared with the local host. The tropical leafwing performed well on all but one Croton in this experiment and performed better on its local host compared with the temperate leafwing on its local host. The temperate leafwing’s local host, C. argyranthemus was a poor host for both leafwings, and C. monanthogynus (non-local host for the temperate leafwing and local non-host for the tropical leafwing) was a suitable host for both species. The results of this experiment and my experience with this system indicated to me that the endangered Florida leafwing might be restricted to a single host only because there are no suitable Croton nearby. I argue that the Florida leafwing could potentially feed successfully on other species of Croton, particularly those known as hosts of other Anaea in the Caribbean. Further experimental work would be needed to test this hypothesis and to determine if there would be management implications of finding alternative hosts. Surviving inhospitable periods or seasons may greatly affect fitness. Evidence of this exists in the prevalence of dormant stages in the life cycles of most insects. Here I focused on butterflies with distinct seasonal morphological types in which one morphological type, or form, delays reproduction until favorable conditions return, while the other form develops in an environment that favors direct reproduction. For two butterflies, Anaea aidea and A. andria, I tested the hypothesis that the development of each seasonal form involves a differential allocation of resources to survival at eclosion. I assayed differences in adult longevity among summer and winter forms in either a warm, active environment or a cool, calm environment. Winter form adults lived 40 times longer than summer form but only in calm, cool conditions. The magnitude of this difference provided compelling evidence that the winter form body plan and metabolic strategy favor long term survival. Neither adult feeding nor reserve size appeared to be the main cause, leaving resource conservatism as the primary explanation. This research suggests that winter form adults maintain lowered metabolic rate, a common feature of diapause, to conserve resources and delay senescence while overwintering. Seasonal climatic fluctuations affect the life history of most organisms. Leafwing butterflies have a complex life cycle that is characterized by there being multiple generations within a year, distinct seasonal forms of adults, and relatively long-lived adults who feed on nitrogen-rich sources. During a single year, the stage structure of the population changes seasonally as do the probabilities of survival and growth and the amount of reproduction. This section addresses the question: when the fates of individuals depend on both life stage and season, how do life stage transitions within each season contribute to population dynamics evaluated over the entire seasonal cycle? I combined demographic rates estimated for the tropical leafwing in the field and in the laboratory to construct a periodic stage-structured matrix model that encapsulates the progression of the population through each seasonal phase along with the demography within each phase. I used a megamatrix form of the periodic matrix model. This approach is distinct from compiling a series of annual matrices as the particular products of the single phase matrices, which is the more commonly utilized approach. I show that these two approaches to the problem are equivalent and that the analytical properties of the megamatrix provide the seasonal population dynamic properties of interest. My results show that stable stage structure, reproductive value, and elasticity all changed seasonally, highlighting the importance to annual dynamics of different life stages in different seasons. Using elasticity of the annual growth rate to demographic rates for each stage and season, I found that overwinter survival of adults has the largest overall effect on average annual dynamics. During the breeding season, from spring to fall, reproduction and growth have the largest effect. This research is the first application of a periodic megamatrix to model seasonal butterfly population dynamics. With this approach I demonstrated that the importance of each life stage depends on season, and my results suggest which selection pressures maintain two distinct seasonal forms and life histories in leafwing butterflies, Anaea.