Body Size Overlap, Habitat Partitioning and Living Space Requirements of Terrestrial Vertebrate Predators

Farlow, J.O. and Pianka., E.R. (2003) Body size overlap, habitat partitioning, and living-space requirements of terrestrial vertebrate predators: implications for large-theropod paleoecology. Historical Biology 16: 21-40.

Since we've already discussed this week's chapter in Macroecology, I will limit this post to discussing the article. Considering our discussion on emergent traits and macroevolution, I feel this article is well suited to our current topic.
Farlow and Pianka set out to relate variations in morphology, feeding strategy and habitat partitioning in modern predators to their dinosaurian analogues. They begin by setting some basic definitions of "hunting set" and "guild". A hunting set refers to "a group of species with similar body build and hunting strategy" ie. felids with conical teeth; whereas a guild refers to "a group of species that exploit the same class of environmental resource in a similar way", ie. canids and felids. Guilds are by definition more of a blanket term and could potentially include one or more hunting sets.
The main focus of the paper is on the coexistance of multiple predators that share the same hunting set. As such, the authors utilize the Hutchinsonian ratio of 1.3 (large species mouthpart size/small species mouthpart size) to delineate the potential of coexistance, with the largest species generally having a much larger ratio than smaller species.
From here, the authors plunge into a detailed comparison of the coexistance of multiple species of lizards, crocodylians, raptors and mammals. In general, species of similar size (ratio below 1.3) are found to 1) hunt different prey, utilize different hunting methods or hunt in different areas, as seen in veranid lizards; 2) segregate the shared habitat, as seen in crocodylians and mammals; 3) separate themselves temporally, as in raptors; or 4) have different morphological adaptations for hunting different prey, as seen in mammals.
These results are then correlated to predatory therapods from the Dinosaur Park Formation, which include two large species (tyrannosaurids), two medium species (dromaeosaurids) and one unclassified small species. Interestingly, most of the Hutchinsonian ratios for the therapods were below 1.3, indicating that they should not coexist without some secondary adaptation or separation. Within the dromaeosaurids, which are separated from the tyrannosaurids by both size and hunting strategy, the two species are separated enough by size to enable coexistance.
The tyrannosaurids present something of a problem for the authors, as they do not vary widely in any preservable way. They are of similar size, morphology and hunting strategy. This leaves only the possibility of a segregation in the habitat or prey choice. This is conceivable due to additional evidence from the Hell Creek Formation, which preserves both species of tyrannosaurs, but in different ratios. It's possible the two were separated along a north-south gradient, which would have also separated the two species in prey choice.
This presents another interesting question. Just how large would a large therapod's range be? The authors examine this by applying body size : habitat size ratios of modern endotherms and ectotherms to therapods. The results indicate that were therapods endotherms, their ranges would be so immense as to disallow the type of spatial coexistance we see in the fossil record. This leaves two options: either dinosaurs were ectotherms, which is widely disputed, or like modern mammals, they were able to finely partition their shared habitats through structural and behavioral adaptations.