Escalating herbivory and resulting adaptive trends in calcareous algal crusts

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Abstract
Evolutionary changes in herbivore abundance, diversity, and ability to excavate calcareous substrata occurred independently in three major herbivore groups: mollusks, urchins, and fish. These changes in herbivory have escalated dramatically since the mid-Mesozoic Era to the extent that herbivory today is more intense than at any time in the past.Today, calcareous red algal crusts (i.e., corallines) are a conspicuous and frequently dominant element of shallow marine communities. Corallines and their (assumed) parent taxon, solenopores, probably always required grazing to remain free of epiphytes. The large scale evolutionary trend toward increased grazing intensity may have given the corallines a distinct advantage over the more herbivore-susceptible solenopores. Four anatomical characters unique to corallines may have facilitated their rapid expansion into shallow reef environments and subsequently (more than 100 million years later) allowed them to withstand the more intense levels of grazing that evolved in those environments. Today corallines are the only algal form to thrive under, and often even require, intensive herbivory. The extinction of solenopores and the adaptive radiation of corallines parallel a gradient of escalating herbivory over time.Herbivores capable of excavating calcareous substrate are also frequently capable of denuding, and preventing the establishment of, stands of large leathery (kelp-like) macrophytes. It is possible that large-scale increases in herbivory have caused large-scale reduction of algal biomass in benthic communities. The presence of algal forests especially in the Paleozoic and early Mesozoic Eras may be a new variable to consider in reconstructing paleoenvironments.