Jon J. Sullivan

The impact of herbivory on plants in different resource conditions : A meta-analysis

Understanding how plant recovery from herbivory interacts with the resource environment is necessary to predict under what resource conditions plants are most affected by herbivory, and ultimately how herbivory impacts plant population dynamics. It has been commonly assumed that plants are generally best able to recover from herbivory when growing in high resource conditions, an assumption which is supported by some models (e.g., the continuum of responses model) but opposed by others (e.g., the growth rate model). The validity and generality of any effects of resources (light, nutrients, and water) on plant recovery from herbivory were tested with mixed-model, factorial meta-analyses using a log response ratio metric applied to plant growth and reproduction data from the ecological literature. In total, 81 records from 45 studies were included in the growth meta-analysis, and 24 records from 14 studies in the reproduction meta-analysis. High resource levels and the absence of herbivory both strongly increased plant growth and reproduction. There was no significant overall interaction between growth or reproduction after herbivory and re- source conditions, but the interaction terms were significant for each plant functional group in the growth meta-analysis. Basal meristem monocots grew significantly more after her- bivory in high resources, while both dicot herbs and woody plants grew significantly more after herbivory in low resources. A similar result was found in the 34.6% of growth records where exact- or overcompensation occurred. Overcompensation was more likely in high resources for monocots and in low resources for dicot herbs. The reproduction data set was too small to subdivide. These qualitative differences between monocot and dicot herbs and woody plants explain many of the contradictory results in the literature and show that no single current model can account for the responses of all plants to herbivory.

Quantifying the benefits of mast seeding on predator satiation and wind pollination in Chionochloa pallens (Poaceae)

Mast seeding could be favoured by a number of factors, including predator satiation and wind pollination. When both these factors could be involved, determining which is more important has been very difficult. In this paper we present 10 years of data on flowering intensity, seed predation and estimated pollination success for Chionochloa pallens (Poaceae), an alpine tussock grass, at Mt Hutt, South Island, New Zealand. C. pallens suffers from high levels of pre-dispersal seed predation (up to 94%) and is wind pollinated, so provides an ideal experimental system. C. pallens had very variable flowering intensity between years (coefficient of variation of floret production (CV)= 1.80). A significant predator satiation effect was found. Predation was not affected by the absolute level of flowering intensity in the current year, but did vary significantly with the change in flowering intensity between the current and previous years. Therefore, predator satiation seemed to involve a numerical response (starving the predator in low seed years). There was a significant wind pollination benefit from larger flowering efforts, which depended only on the size of the current flowering effort. Therefore, for benefits via pollination, C. pallens populations with high levels of resources can satisfactorily produce a large flowering effort every year; in contrast, to gain benefits via predator satiation, fluctuating flowering intensity is required regardless of mean levels. The relative benefits of masting versus constant reproduction were modelled; benefits from wind pollination increased very little with increased CVs, to give a benefit of 2.3% fewer florets with unexpanded ovaries at the field CV of 1.8 than at CV = 0. The overall mean flowering effort is apparently high enough in the study population for efficient pollination even if masting did not occur. Predator satiation gave large benefits from masting, from 8% of all florets at Cv = 1.25 increasing rapidly to 42% at CV = 1.8 and 62% at CV = 2.25. Therefore predator satiation is vastly more important than wind pollination in favouring mast seeding in C. pallens, providing 92-95% of the benefits at CVs>1.5. The rapid increase in benefits from predator satiation at CV > 1.5 may help explain why Chionochloa species have such high CVs.

People and Time Explain the Distribution of Naturalized Plants in New Zealand1

Abstract New Zealand has as many naturalized plant species as natives, and the invasion has yet to slow. The current composition and distribution of the naturalized flora are dominated by the time since species introduction and by human factors such as population density, at spatial scales from region to neighborhood. Additional index words: Eradication, introduction effort, invasive plant, propagule pressure, weed. Abbreviations: NZ, New Zealand; SE, standard error of the mean.

Biology of insects that feed in the inflorescences of Chionochloa (Poaceae) in New Zealand and their relevance to mast seeding

Abstract Inflorescences of New Zealand Chionochloa species are attacked by at least three insects, two flies and a moth. There has been disagreement about the identity of various life stages of these insects. We followed the seasonal pattern of occurrence of the two fly species that fed in Chionochloa pallens inflorescences in a population on Mt Hutt, Canterbury. Eggs and larvae of Diplotoxa similis (Diptera: Chloropidae) appeared in the inflorescences as soon as they emerged. The larvae are principally flower feeders, and most D. similis individuals had pupated by the end of the flowering period. Diplotoxa similis adults emerged from the puparia at the end of the season, and probably overwintered as adults. The second fly was an undescribed cecidomyiid (Diptera: Cecidomyiidae). Eggs of the cecidomyiid are laid into the C. pallens florets at the time of flowering, and hatch into mobile, translucent, early‐instar larvae. Late‐instar larvae were less mobile and opaque orange, and probably dropped from the inflorescences late in the season. The third species, Megacraspedus calamogonus (Lepidoptera: Gelechiidae) has large mobile caterpillars which appear early in the season, but there is doubt about its egg morphology and oviposition sites. On two dates there was a negative correlation between densities of the two fly species among plants. Since D. similis appears first, it may be able to usurp resources and reduce densities of the cecidomyiid. A review of known occurrences of the three insects suggests that, compared to D. similis and M. calamogonus, the cecidomyiid (1) has a greater geographic range and (2) occurs on more Chionochloa species; however, these trends might be due to poor sampling of D. similis and M. calamogonus early in the season. The cecidomyiid appears to be less easily satiated than D. similis by masting in Chionochloa.

Insect predation of seeds of native New Zealand woody plants in some central South Island localities

Abstract Samples of fruit collected from woody plants in central South Island lowland forests, showed that nine species were more or less heavily affected by insect seed predation (more than 10% of seeds and up to 63% eaten in some samples). One obligate seed-eater each occurred in seven of these; the rest had two or more. Two were also attacked by facultative seed-eaters. A further eight species of plants were affected to a lesser degree by obligate or facultative seed-eating insects. Moth larvae from nine families and ten genera are the main seed eaters, but weevils are important for some plant species.

Why is mast seeding in Chionochloa rubra (Poaceae) most extreme where seed predation is lowest

Abstract Mast seeding (highly variable population seed crops among years) is common in the New Zealand flora, and is especially evident in high elevation populations of Chionochloa (snow tussocks). However, mast seeding is less pronounced at low elevations in New Zealand plants, including C. rubra. Past studies of high elevation Chionochloa populations showed that the main benefit of mast seeding is the satiation of pre‐dispersal insect seed predators. We therefore predicted that, to be able to set seed regularly, either C. rubra should suffer less predation than other Chionochloa species, or low elevation populations of C. rubra should experience lower seed predation than higher elevation populations. To test these predictions we surveyed seed predation in C. rubra and neighbouring C. pallens and C. flavescens at 22 New Zealand sites spanning over 1000 m in altitude. Total seed predation was not significantly lower in C. rubra (48%) than in C. pallens (22%) and C. flavescens (54%). Seed predation significantly increased with decreasing altitude, in direct contradiction to our prediction. This reveals a reproductive paradox: why do low elevation populations of C. rubra not show more pronounced mast seeding? We discuss three possible answers: (1) C. rubra has not formed altitudinal ecotypes with different critical temperatures for heavy floral induction; (2) the seed predators at low elevation are harder to satiate; or (3) low elevation populations suffer stronger disadvantages from masting through high intraspecific competition and or density dependent seedling mortality. We suggest that this paradox may also occur in many other masting plants with wide altitudinal ranges, that predator satiation will often break down at the lower altitudinal limit of the plant, and that seed predation may limit the lower elevational range of some masting plant species.