Chapter 2 the Historyofassociations Between Plants and Animals
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Chapter 2 The historyofassociations between plants and animals Conrad C.Labandeira^ in estimating the duration of associations, and limitations 2.1 Introduction on the use of modern ecological analogues for correctly Ever since the greening of a strip of equatorial coastline interpreting fossil material. In this context I have that fringed tropical oceans approximately 425 Ma, chosen three ecomorphological units•functional feed- plants and animals have been extending their ecological ing groups, dietary guilds and mouthpart classes • as reach on land. Today their terrestrial representatives con- yardsticks to anchor much of the discussion of the stitute the overwhelming bulk of macroscopic diversity fossil record. Emphasis will also be placed on results on the planet, and occur in profusion in virtually all from varied phylogenetic studies for inferring the history terrestrial habitats. The earth's land surface today is of associations between plant hosts and their animal fundamentally an abode for vascular plants and five herbivores. A glossary of technical terms is shown in major groups of macroscopic animals: nematodes, gas- Box 2.3 at the end of the chapter (p. 73). tropods, onychophorans, arthropods and vertebrates. Two subgroups of this menagerie • seed plants and in- 2.2 Why study plant-animal associations of sects• contribute the preponderance of this diversity, the past? and overwhelmingly outnumber in species all other groups combined. Virtually all terrestrial life is affected in The evolutionary history of biodiversity is essentially a some way by their myriad associations. chronicle of species interactions; species do not occur in The origin and evolution of associations between splendid isolation. This precept suggests that an under- vascular plants and animals has a rich but undervalued standing of the history of life and the generation of biodi- fossil history. This history has been integrated into bio- versity must include an examination of when, how and logical and palaeobiological perspectives, each of which why organisms have associated during geological time. By has explored the deep historical dimension of vascular contrast, the overwhelming effort in palaeobiology and plant and animal associations. The empirical data under- biology has concentrated on examinations of isolated lying both of these approaches will be interpreted in this species in time and space. As indicated by Price in Chapter chapter within a context of such theoretical questions 1, there has been a growing realization that comprehend- as the initial launching of herbivory on land; the origin ing associations locked within a biota is considerably and early expansion of major ecological associations; the more than the sum of constituent species lists, and alter- establishment of modern associations and the survival natively requires placing associations in an evolutionary of ancient relationships; and the roles of taxonomic ex- context (Price 1997). Early efforts at unravelling these tinction, ecological convergence and escalation in long- associations focused on gross correlations between the term patterns of herbivory. Both palaeobiological and taxonomies of plant hosts and their insect herbivores biological approaches have unique strengths and liabili- (Ehrlich & Raven 1964), followed by more refined hy- ties that include the varied uses of uniformitarianism, the potheses (e.g. Funketal. 1995), demonstrating that cycles quality of fossil evidence for inferring past associations, of adaptations of plant defence and reciprocal counter- the role of combining phylogenetic and ecological data adaptations of herbivore attack resulted in the long-term *7^¿/í is contribution 69 of the Evolution of Terrestrial Ecosystems Consortium at the National Museum of Natural History. 26 Associations between plants and animals 27 improvement of individuals towards coping with decreas- or predictable way. Associations range from antagonistic ing odds of survival. Curiously, this research did not influ- interactions such as prédation or parasitism, to neutral ence palaeobiology. relationships such as commensalism, to the collectively By contrast, the escalation hypothesis of Vermeij beneficial arrangements of various mutualisms. Feeding (1987) was proposed from an evaluation of the fossil or feeding-related processes constitute the bulk of known record of marine invertebrates. Vermeij's formulation relationships between plants and animals, and thus an as- describes escalation as the increasing adaptiveness of sociation is conceived as an available food resource niche' lineages through time resulting from successively higher that is variably occupied by different hosts and consumers levels of selection pressure by predators on prey. It differs in time and space. An instructive example are marat- from various coevolutionary hypotheses by ( 1 ) a focus on tialean ferns and their insect herbivores, both ofwhich un- the adaptation of evolving species to a changing environ- derwent a complete taxonomic turnover during the 300 ment rather than through interbiotic associations, and (2) million years since the Late Pennsylvanian (see p. xii for a the primacy of antagonistic associations such as prédation time scale). During the Late Pennsylvanian almost all the over less antagonistic associations such as mutualism. The fundamental ways that insects consume land plants were escalation hypothesis was a logical extension of the Red established • chewing, piercing and sucking, galling, bor- Queen hypothesis proposed earlier (Van Valen 1973), in ing, and consumption of spores and prepoUen (Laban- which an adaptation in one species affected the relative deira 1998a). Only leaf-mining was apparently absent. non-adaptedness of other species, resulting either in the These basic feeding styles, or functional feeding groups, simultaneous improvement of interacting species or their also occur in the modern descendants of marattialean marginalization through extinction or relegation to safer ferns, albeit by herbivores from lineages that have evolved marginal environments. more recently than the late Palaeozoic. In a similar con- Until recently, palaeobiology never developed a text, Lawton and colleagues (1993) provide an example methodology to extensively test trends in the fossil record from the modern herbivores of the cosmopolitan bracken resulting from patterns seen in modern plant•insect asso- fern, in which continental geography rather than geologi- ciations. Rather than investigation of mutualisms or other cal time is examined. They conclude that the current modes of accommodation, overt prédation has been the insect herbivores of bracken are highly variable globally, centre of study. Much of this reluctance is attributable to often with phylogenetically disparate insect taxa occupy- criticisms regarding a poor fossil record, the inability of ing the same functional feeding group on different conti- fossils, if found, to illuminate relevant questions of phy- nents, and in some regions particular functional feeding logeny, or perceived limitations in the interpretation of groups may be occupied by multiple taxa or even remain evidence from plant•insect associations (Farrell & Mitter vacant. The salient point is that while plant hosts and their 1993; Shear & Kukalová-Peck 1990). Nevertheless there varied insect herbivores evolve and are constantly replaced is an informative and interpretable record that has been in time and space, their associations nonetheless remain documented and used productively. This is particularly constant. A Palaeozoic palaeodictyopterid insect imbib- true for well-preserved deposits that provide unique win- ing vascular tissue sap from a marattialean tree fern is dows into the past (Crepet 1996; Labandeira 1998a). In functionally playing the same role as an aphid today feed- particular, examination of fossil plant•insect associations ing on the same tissues in an angiosperm (Labandeira & is essential for testing hypotheses regarding the macroevo- Phillips 1996). An appropriate analogy is a soccer team in lutionary dynamics of plants and their insect associates. which the positions remain the same but players occupy- These data are the source for patterns, especially temporal ing these positions are periodically rotated, retired and trends, that can support or refute hypotheses originating recruited. from modern ecological studies (Coley 1999). 2.2.2 Theoretical issues about the origin and 2.2.1 Associations and time evolution of associations An association is an ecological relationship that occurs Four basic questions have been identified that are of inter- indirectly or directly among two or more participants est to palaeobiologists and biologists. All are united by that use a resource in a repeatable or otherwise stereotyped explicit reference to geological time, which provides the 28 Chapter 2 chronometer by which long-term trends and processes are (a) Ecological saturation hypothesis cahbrated and ultimately interpreted. loOn 2.2.2.1 How were plant•animal associations launched during the Palaeozoic? 50. Three hypotheses have been proposed for assessing how ecological units, such as functional feeding groups, di- etary guilds and mouthpart classes, expand in macro- evolutionary time (Strong et al. 1984). The first lb) Expanding resources hypothesis hypothesis, the ecological saturation hypothesis (ESH), 100 n advocated by palaeobiologists, maintains that the total number of ecological positions, or roles, has remained ap- t H proximately