Relationships between the effects of insect herbivory and sheep grazing on seasonal changes in an early successional plant community.

The effects of spring grazing by sheep and of natural levels of insect herbivory were studied in 1985 on a limestone field abandoned from arable land for four years. A split-plot design was adopted in which paddocks, arranged in Latin squares, were either left ungrazed or heavily grazed by sheep for ten days in April. Within each paddock plots were either sprayed regularly with Malathion-60 or untreated.Natural levels of insect herbivory, compared to the reduced levels in insecticide-treated plots, had effects of similar magnitude to those from the short burst of spring grazing. Many attributes of the grazed/insecticide-treated sward were either increased or decreased by a factor of two within a season. Both types of herbivore caused changes in the direction of plant succession as well as in its rate. Effects on early successional species were large and similar when caused by either type of herbivore. Effects on later successional species were often smaller, but also showed differences in the action of the two herbivore types, as did effects on sward height, species richness and total cover. The effects of sheep and insect herbivory were not always additive or in the same direction.The results suggest that manipulations of both mammal and insect herbivores may be powerful tools for directing changes in plant community composition.

Population processes in a large herbivorous reptile: the giant tortoise of Aldabra atoll.

Physical barriers divide the population of giant tortoises (Geochelone gigantea Schweigger) on Aldabra into several sub-populations of different density, which nevertheless are similar genetically. We measured individual growth rates in each sub-population. Mortality was estimated using data from Bourn and Coe (1979). Reproduction and recruitment were studied using data from previous work (Swingland and Coe 1979) and our own estimates of clutch size, egg weight, and laying frequency from 1975 to 1981.Individual growth rates were strongly dependent only on individual size and sub-population density and not on age or sex. Within a sub-population, the relationship between specific growth rate and size (linear measure) was best fitted by a Gompertz model, except for very young tortoises which grew faster in volume, though not in weight, than expected. Animals at high densities grow slowly to a small size whereas those at low densities grow fast to a large size. At very high density many juveniles remain at a small size without growing or maturing.Mortality of larger (> ca. 5 years old) animals was independent of density, but did depend on size in the highest-density sub-population, as predicted by the Gompertz growth model.Reproduction and recruitment were negatively density-dependent over the whole density range (5 to 35 animals ha-1) studied. Clutch size and laying frequency were strongly influenced by sub-population density, but egg weight was not. Laying frequency varied within sub-populations according to rainfall (presumably via annual food supply).All except one sub-population are seen as stages in the development of the same interactive system. Competition between individuals is nearly, but not purely, of scramble type. The remaining sub-population is either a distinct interactive system in which food supply for very young animals is important, or it is a non-interactive system controlled by the effect of natural enemies on very young animals. This suggests that the equilibrium density and/or dynamics of giant tortoise populations are highly sensitive to mortality factors affecting very young animals.In low density sub-populations the animals are large, have many young, low relative reproductive effort, and a short generation time. In high density sub-populations they are small, have few young, high relative reproductive effort, and a long generation time. This variation is largely phenotypic. It is anomalous with respect to r-K life history theory but is a logical consequence of indeterminate growth combined with size-determined risk and benefit functions and may have contributed to the giant tortoises' success as island colonisers.

Feeding ecology and seasonal movements of giant tortoises on Aldabra atoll.

We investigated seasonal changes in diet and distribution of giant tortoises (Geochelone gigantea (Schweigger)) on Aldabra atoll in the Indian Ocean. Animals were counted and their activity and feeding behaviour recorded on transects where vegetation composition and primary production had been studied (Gibson and Phillipson in press a, b).There were striking seasonal shifts in tortoise distribution, and male, female, and juvenile tortoises were found in different proportions in different vegetation types.Tortoises are selective grazers, feeding on a wide range of foodstuffs of which the most important (61% of feeding observations) was tortoise turf. Diet varied seasonally, with shrub leaves (mostly litter) overtaking tortoise turf in importance in the late dry season; diet broadened as the dry season progressed. Male and female diets were not significantly different but juveniles fed on herbs and mosaic rock vegetation more often than adults.Seasonal shifts in distribution are due to movements in response to changes in food availability, measured by the foods' cover abundances and production phenologies. Tortoises concentrate on preferred foods when available, but become less selective as production falls. Some differences in size and sex class distribution between habitats can also be explained by food availability.In the late dry season density peaks on the coastal Sporobolus virginicus (L.) Kunth sward. A detailed study showed that, while at least 20% of the population uses the sward each year, visit times are short and turnover of tortoises high, as would be expected on a non-preferred food.The giant tortoise interacts with its food supply similarly to other large herbivores, except that the low maintenance needs of this large poikilotherm allow it to develop unusually high population densities.

Niche use patterns among some Stenodemini (Heteroptera: Miridae) of limestone grassland, and an investigation of the possibility of interspecific competition between Notostira elongata Geoffroy and Megaloceraea recticornis Geoffroy.

It was suggested in a previous paper that mortality patterns in two species of Stenodemini (M. recticornis and N. elongata) could be explained by interspecific competition (Gibson 1976a)Such competition would have been generated by seasonal changes in the foodplants used by the two species resulting in both species using virtually the same range of plants at one time of year.As with a number of phytophagous insects (McNeill and Southwood 1978) many changes in foodplant use could be explained by differential seasonal changes in nitrogen content of the plant part (leaves) that the bugs were using in different plant species. Each bug had an ideal food nitrogen range which it used whenever possible. Lack of fit to this mechanism could often be explained by defensive chemistry and physical properties of some grass species.Although the total bug population was taking under 1% of the area's primary production, a potential refuge foodplant for one bug species was very heavily eaten where it was rare, and behavioural interference between bug species suggested that space to feed in was a limiting resource.Laboratory and semi-field culture experiments showed that the presence of one bug species adversely affected the growth and/or survival of the other, although the outcome of competition was different to that in the field, probably due to the different relative and absolute densities used.Although one cannot be completely certain without field manipulation experiments, it is extremely likely that interspecific competition was occurring and could be stabilised by the losing species (N. elongata) having a 'refuge generation'.Competition between stenodemine species on the study area could usually be avoided by separation along a number of resource axes e.g. foodplant species, plant part, nitrogen level of food or emergence timing. In the present circumstance, a particular combination of bug and plant species in the area has forced one species pair into overlap. It should be possible to predict the guild structure and competition relations in other areas from this.

The importance of foodplants for the distribution and abundance of some stenodemini (Heteroptera: Miridae) of limestone grassland.

The distributions and abundances of five species of Stenodemini (Heteroptera) and one of Rhopalidae (Heteroptera) were monitored over a 2-year period on a 1 ha square of limestone grassland near Oxford.At the same time, the distribution of all grass species in the area was mapped.By comparing the distributions of insects with their potential foodplants, and by carrying out feeding trials on selected insects, an outline of the foodplants selected by different species of insect was built up.Each insect fed on several foodplants at once, but this spectrum often changed seasonally and with the age of the insect. Most grass species were used by at least one insect species.There were considerable overlaps between the feeding spectra of different bugs. However, ecological separation was usually achieved by emergence timing and/or feeding on different parts of the same grass species.One species pair remained with an almost complete overlap in the resources that were measured (Notostira elongata and Megaloceraea recticornis). Since these bugs reached very high combined densities, and one of them suffered very high mortality at a time of overlap, it is possible that these two species were competitors.Sward height did not influence insect distributions within the range present on the study site. Nymphal parasites were extremely rare and unlikely to have affected their hosts' distributions.