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Description
Ecologists and paleontologists commonly use diversity measures to analyze how processes (e.g., competition during invasion) influence community structure. Based on generic diversity, it is hypothesized that if no incumbents are lost, and invaders succeed, then the pre-invasion community was not saturated and competition was not a driving force in structuring the community (e.g. Patzkowsky and Holland 2003, 2007). However, using diversity as a measure of competition does not take into account 1) morphological shifts by incumbents to avoid competition (character displacement), or 2) losses at lower taxonomic levels (species loss). By comparing the morphology of incumbents pre- and post-invasion the change or loss of a morphotype can be illustrated, whether this is character displacement or species loss. To test morphospace as a measure of competition, brachiopod morphology was used as a proxy for niche space occupation, to explore competition in the fossil record during the Richmondian invasion (Ordovician) in the Illinois Basin. The external shape of three pairs of potentially competing brachiopod genera were analyzed; each pair consisted of one incumbent and one invader. Using Baseline Shape co-ordinates (Bookstein 1991), morphology of incumbents was compared pre- and post-invasion. In each incumbent-invader pair it is possible to differentiate the incumbents pre- and post-invasion, the incumbent became more morphologically different from the invader, and landmarks driving the morphological differences of Platystrophia and Hebertella shift away from the invader in morphospace. Thus, competition was a driving force in the brachiopod community, and morphological changes could reflect either competition-induced character displacement, or species loss. Therefore, morphology may be a better measure of ecological patterns through time than diversity counts, as it can illustrate niche space occupation and reflect taxonomic loss.
