Ecological strategies in fern evolution: a neopteridological overview

Abstract

Drawing inferences about the past from the ecology of living organisms is one of several approaches to reconstructing palaeo-environments. Pteridophytes are a major component of fossil floras, but their use as environmental indicators is constrained as much by lack of ecological data on living species as by an understanding of the distribution of fossils. Taking a neobotanical perspective, this paper discusses some important ecological strategies of ferns and allied plants and their underlying selection pressures, based on an extensive survey of tropical and temperate species and on horticultural experience of the behaviour of wild species in experimental cultivation. Broadly parallel developments to similar selection pressures and environmental responses have been sought from amongst distantly related extant families, to derive broad concepts of weaknesses and strengths inherent in the biology of these plants. From this evidence, seven main limitations and twelve important advantages imposed on pteridophytes by aspects of their biology are identified as follows: Limitations: • The handicap of an independent gametophyte stage • Single growing-point limitations of sporophyte architecture • Slow plant growth rates • Intolerance of widely fluctuating conditions • Poorly controlled evaporative potential • Uncontrolled high reproductive commitment • Need to ‘return to the water to breed’ Advantages: • Low-light photosynthetic ability • Diverse phytochemical armament • High disease resistance under saturated humidity levels • High tolerance of acute nutrient disequilibrium substrates • High migrational ability of the airborne spore • Spore tolerance of adverse aerial environments • Flexibility of breeding systems to match varying ecological opportunity • Revivalist tendencies of certain gametophytes • Potential longevity of resultant sporophytes • Exploitation of mycotrophy • Exploitation of potentials of polyploidy • Biotic independence It is argued that collectively these weaknesses and strengths provide a broad framework, which, operating in varied combinations, limit or open opportunities for exploitation of a considerable array of ecological habitats by Pteridophyta. Based on these data, several general ecological principles are developed. It is proposed that, through time, such strategies are likely to have opened many pteridophyte habitat opportunities, though not all of these will necessarily have left directly identifiable signals in the fossil record. Modern fern ecological limitations and advantages are shown to occur across broad taxonomic spectra and many are innate abilities of the plants. It is, therefore, argued that a similar general framework of weaknesses and strengths is likely to have operated in the past, and thus have been of similar relevance in defining and promoting the ecological achievements of the fossil pteridophytes in relation to selection pressures and consequent adaptations. This opens up the potential to extrapolate from the modern ecology for interpretation of palaeo-ecology and palaeo-environments. Examples of this potential are given for each limitation and advantage, where possible incorporating evidence from the fossil record.