Abstract:
The yucca-yucca moth interaction is a classic case of obligate mutualism. Female moths pollinate and oviposit in the gynoecium of the flower; however, maturing larvae eat a fraction of the developing seeds. We studied within-fruit distributions of four seed types (fertile, infertile, eaten and uneaten seeds) in order to evaluate costs and benefits in a Yucca schottii population in southeastern Arizona. We focused on how the spatial arrangement of seeds affected larval behaviour and, hence, the costs of the mutualism to the yucca. Infertile seeds were distributed throughout both infested and uninfested locules. Additionally, moth larvae feeding in a single locule preferred fertile seeds and even avoided infertile seeds and left the fruit significantly more often when they encountered infertile seeds. We suggest that, regardless of the cause of infertile seeds, they function as blocking units within seed locules and therefore reduce seed predation by moth larvae. We also suggest that, together with certain other fruit traits, the presence of infertile seeds promotes the evolutionary stability of this pollination mutualism. © 1996 Chapman & Hall.
Abstract:
In North American deserts, many species of cactus attract ants to their extrafloral nectaries; the ants actively defend the food source, and hence the plant, against herbivores. In thermally extreme environments, however, networks of positive and negative interactions like these are likely to be sensitive to the thermal limitations of each of the interacting species. We compared the thermal tolerance of a common phytophagous cactus bug, Narnia pallidicornis (Hemiptera: Coreidae), to that of the ants that defend the cactus Ferocactus wislizeni in the Sonoran Desert, USA. We used flow-through respirometry to experimentally determine the thermal limit of the herbivore and compared this to the thermal limits of the ant defenders, determined previously. In the field, we recorded herbivore frequency (proportion of plants with N. pallidicornis) and abundance (the number of N. pallidicornis per plant) in relation to ambient temperature, ant species presence and identity, and fruit production. We show that N. pallidicornis has a higher thermal tolerance than the four most common ant mutualists, and in the laboratory can survive very high temperatures, up to 43°C. Herbivore frequency and abundance in the field were not related to the daily high temperatures observed. Plants that were not defended by ants were occupied by more N. pallidicornis, although they showed no reduction in fruit set. Therefore, herbivory is likely to continue on fishhook barrel cacti even at high temperatures, especially those temperatures beyond the thermal tolerance of the ant defenders. The consequences of increased herbivory, however, remain unclear. Mutualisms are essential for ecosystem functioning; it is important to understand the thermal sensitivity of these interactions, especially in light of expected increases in global temperature regimes.
Abstract:
Pollinators feed on patchily distributed plants, forcing them to decide when to move between patches and when to switch plant species. The "plant-pollinator landscape' is shaped by an interacting set of plant and pollinator attributes: flowering phenology sets the distance that pollinators need to travel between patches of a given species to obtain sufficient food, while the pollinators' search capacities and dietary specificity determines the likelihood that they can and will make that journey. These sets of traits do not vary independently, allowing us to identify several characteristic landscapes and the types of organisms that occupy them. This chapter describes five such landscapes, attempting in each case to determine whether phenological variation within plant species helps to explain the ability of pollinators to persist. The first two landscapes are occupied by highly specialized pollinators and (respectively) synchronously flowering and asynchronously flowering plants. Intraspecific phenological variation is likely to have critical consequences for pollinator persistence in these two landscapes. The third and fourth landscapes are occupied by relatively generalized pollinators and (respectively) synchronously flowering and asynchronously flowering plants. The former is probably the most common landscape, encompassing most temperate plant-pollinator interactions. Pollinators in these landscapes are less likely to be strongly influenced by phenological variation in any one of their resource plants, due to their ability to switch foods easily while remaining local. The fifth landscape is occupied by generalist pollinators that may migrate. Little is known about resource use of individual migrants, but it is possible that at least some of them rely on geographical gradients in flowering time of certain preferred species. -from Author
