144 PALAEOECOLOGY Figure Key graphs for the interpretation of fossil populations (A) A plot of the logistic equation, showing the rate of population increase slowing as the population size approaches the carrying capacity of the local environment (B) Survivorship curves for populations with a high juvenile mortality (concave curve), an equal chance of mortality at any age (straight line), and a high gerontic mortality (convex curve) the survivorship curve will be concave In environments in which there is an equal risk of mortality at any age, the survivorship curve will be straight The classic example of this type of curve is that of garden birds Palaeontological examples are found, especially in extreme environments in which mortality is generated by frequent, unpredictable, and unselective events In more stable environments, mortality rate is low initially but increases with age after a certain point, producing a convex survivorship curve In these cases, organisms can be fit enough to survive the variation seen in their environment, but this characteristic is reduced later in life through illness or gerontic effects Investigating Interactions between Organisms Organisms may interact in a number of different ways Most important among these are competition for resources (e.g., for nutrients, space, light, or mates), either intra- or interspecifically; predator– prey interactions; and symbiosis, which runs through a spectrum, from being beneficial to both partners (commensalism) to parasitism, in which one partner’s advantage is deleterious to the other Ecologists have found that these interactions play an important role in controlling the fitness of modern organisms, and competition and predation in particular are thought to be extremely important driving forces in evolution At first sight it might seem that the fossil record is likely to be virtually mute on the interactions between organisms; however, this is not the case Clearly, individual cases must be carefully assessed, but it is possible to generate quantitative data on interactions between organisms from the fossil record Competition The competitive exclusion principle suggests that two species that have exactly the same requirements cannot exist together within a habitat Where two species appear to have the same requirements, one will be prevented from maintaining a viable population by the success of the other, or one will be forced to adapt in order to use slightly different resources The latter strategy may involve character displacement (e.g., modification of filter-feeding apparatus to cope with particles of a different size) Competition has often been invoked as a cause of macroevolutionary change, although it is difficult to demonstrate For example, some have suggested that the post-Palaeozoic decline in articulated brachiopods was at least in part influenced by the superior competitive advantages of bivalves that may occupy the same niche, and a similar argument has been made to link the success of dinosaurs with the decline of synapsids towards the end of the Triassic One situation in which it has been possible to collect high-quality data concerns interactions between borers and skeletonized encrusters (e.g., bryozoans, oyster-like bivalves, and serpulid worms) These organisms have the advantage of having been preserved in situ and thus the fossils maintain their spatial relationships to one another and to the substratum It must be remembered that soft-bodied encrusters (e.g., anemones, sponges) that may also have co-existed will be unrepresented, although evidence of their existence may be revealed by bioimmuration (preservation by the overgrowth of a skeletonized organism) In these cases, it is also possible to examine competitive overgrowth It is known that space on hard substrates is at a premium in modern marine habitats, from which it may be inferred that taxa that are able rapidly to colonize space will be at an advantage over those that so less aggressively Much of this type of research has been centred on two clades of extant bryozoans, the cyclostomes and cheilostomes (see Fossil Invertebrates: Bryozoans) Both evolved independently from soft-bodied ctenostome ancestors, the cyclostomes in