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Summary1. The habitat accommodation model for animal succession proposed that animal species enter a succession when changes in the vegetation succession reach a threshold of habitat suitable to that species. As the vegetation succession moves on, the habitat becomes less suited to that species and it is competitively excluded by species better suited to the habitat. 2. The main species in the mammalian succession following fire for wet heath in eastern Australia are rodents, with Pseudomys gracilicaudatus (eastern chestnut mouse) being followed by Rattus lutreolus (swamp rat) which becomes dominant with increasing time since fire. The abundance of both species has been shown to correlate with vegetation density, but in markedly different ways, and asymmetric interspecific competition has been demonstrated using controlled, replicated removal experiments in the field. 3. We used this system to examine if vegetation density is causal, manipulating the habitat by clipping to remove 60-70% of the vegetative cover from the 10 m × 10 m area surrounding each of six trapping stations on each grid. There were four experimental plots clipped, each with two abutting grids, one clipped and one unclipped, and a further four control plots each with two abutting grids that remained undisturbed. 4. We monitored the effects on each species with three censuses in January 1993 (summer) before clipping in early February, and on five further censuses, three in February (late summer) to assess immediate, short-term effects, one in August (winter) and one in December 1993 (early summer). 5. The abundance of Rattus lutreolus was significantly reduced by clipping the vegetation, while the abundance of Pseudomys gracilicaudatus remained relatively unchanged by the clipping. 6. Two species that are abundant on early succession stages in dry heath succession, but occurred at very low abundance on these wet heath habitats before clipping, P. novaehollandiae (New Holland mouse) and Mus domesticus (house mouse), showed marked increases in abundance on clipped and control plots soon after clipping. 7. An abundance index based on the standardized difference between clipped and control plots, was used to assess responses to habitat manipulation. Rattus lutreolus demonstrated significant negative index values, Pseudomys gracilicaudatus had index values close to zero until the winter census, P. novaehollandiae and Mus domesticus showed positive abundance indices soon after clipping. The order of significant responses to the habitat manipulation was revealed as: Mus domesticus → Pseudomys novaehollandiae → P. gracilicaudatus → Rattus lutreolus . 8. The impact of a habitat manipulation experiment on these four species of rodents produced a retrogression of the small-mammal succession. This demonstrated a causal role for vegetation density, which provided insight into the mechanisms that operate during the succession following fire, and supported the habitat accommodation model for animal succession.

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It is now well known that the pair interaction between two hydrocarbon molecules in water has distinctly different properties from the bulk hydrophobic interaction familiar to the biochemist, which is modeled by the transfer of a hydrocarbon from aqueous solutions to pure liquid hydrocarbon. We consider experimental data for pair interactions, which have been fitted by a simple empirical potential function, and point out some of their properties. (i) Surface free energy and cosphere overlap models, of the type considered until now, cannot reproduce correctly both the pair and bulk hydrophobic interactions. (ii) Pair interactions though still attractive are strikingly weaker in aqueous solution than in the gas phase, in contrast to the usual view of hydrophobic interactions. (iii) For pair interactions in water, the solvent-separated configuration is less important than the contact configuration if the hydrocarbon has more than two carbon atoms.Hydrophobic "bonding" involves the association of nonpolar groups in an aqueous environment. Kozak et al. (1) have drawn the distinction between bulk hydrophobic interactions-that is, "interactions involving large clusters of nonpolar groups as may, for instance, be found in the interior of a protein molecule"-and interactions between small numbers of nonpolar groups as may, for instance, be found at the surface of a protein molecule. These qualitatively different interactions are generally lumped together in the analysis of biochemical systems. For macromolecules, models of bulk hydrophobic interactions are appropriate for the buried molecular interior, while models for pair hydrophobic interactions are probably more appropriate at the molecular surface where there are many neighboring water molecules present. The distinction between these two kinds of interactions is important because they can be quite different in magnitude. As described below, the relative effect of water, in general, is to increase the bulk interaction and decrease the pair interaction for saturated hydrocarbons, although both are decreased for benzene. The macroscopic features of both bulk and pair hydrophobic interactions have been extensively studied and much is known about them (2-23). At the molecular scale a knowledge of aqueous hydrophobic interactions implies a knowledge of the potentials of average force between the hydrophobic species, and many questions about these potentials remain unanswered. For a complete understanding of these interactions in biochemical processes, we need a detailed knowledge of the forces involved. This paper presents some conclusions about the effects of water on bulk and pair interactions of hydrophobic molecules and suggests some tests of molecular scale models of these interactions. Pair Versus Bulk InteractionsThe relationships between pair interactions, bulk interactions, hydrophobic solvation, and the transfer of hydrophobic groups between water and hydrophobic medium is illustrated by the following thermodynamic cycle. nR(g, c@) AG,(g, Ce) IR (g co)H...