The Effects of Habitat Geometry on Territorial Defense Costs: Intruder Pressure in Bounded Habitats

Computer simulation models were used to explore the effect ofhabitat geometry on intruder pressure for territories in differentlocations within a patch of uniformly high quality habitat,for territories in patches of different shapes and sizes, andfor patches surrounded by different types of suboptimal habitats. In models in which the edges of the habitat patch are impermeable(hard-edged), intruders do not leave the central territorialhabitat, H, and intruder pressure is lower for territories onthe edge of H than for more centrally located territories. Averageintruder pressure for any given loop of territories (, for loop = i) is positively relatedto both the proportion of territories on the edge of the patch(ESR) and the average distance moved by intruders. In models in which the edges of H are permeable (soft-edged),intruders are able to move between H and the surrounding habitats,which were of two types: sinks (no intruders generated there)and reserves (a source of intruders). The presence of sinksdramatically reduces both and average intruder pressure over H () as compared to hard-edged habitats, and both and are negatively related to the proportion of territories on theedge of the patch (ESR). Conversely, and are positively related to ESR when the surrounding habitat acts as an intruder-reserve. Data from empirical studies of territorial species agree withmany of the direct and indirect qualitative predictions of thesemodels. The effects of habitat geometry on defense costs maybe important in many territorial species, and should be takeninto account in future studies.     

The Influence of Light and Temperature on Isoprene Emission Rates from Live Oak

There is a growing awareness of the role of vegetation as a source of reactive hydrocarbons that may serve as photochemical oxidant precursors. A study was designed to assess independently the influence of variable light and temperature on isoprene emissions from live oak (Quercus virginiana Mill.). Plants were conditioned in a growth chamber and then transferred to an environmentally controlled gas-exchange chamber. Samples of the chamber atmosphere were collected; isoprene was concentrated cryogenically and measured by gas chromatography. A logistic function was used to model isoprene emission rates. Under regimes of low temperature (20°C) or darkness, isoprene emissions were lowest. With increasing temperature or light intensity, the rate of isoprene emission increased, reaching maxima at 800 μE m^-2 s^-1 and 40–44°C, respectively. Higher temperatures caused a large decrease in emissions. Since the emissions of isoprene were light-saturated at moderate intensities, temperature appeared to be the main factor controlling emissions during most of the day. Carbon lost through isoprene emissions accounted for 0.1 to 2% of the carbon fixed during photosynthesis depending on light intensity and temperature.