Ecology of Australia: the effects of nutrient‐poor soils and intense fires

Abstract

Australia, the flattest, driest, and geologically oldest vegetated continent, has a uniquely high proportion of nutrient‐poor soils. We develop a “Nutrient‐Poverty/Intense‐Fire Theory,” which postulates that most anomalous features of organisms and ecosystems of Australia are the evolutionary consequences of adaptations to nutrient poverty, compounded by intense fire that tends to occur as a result of nutrient poverty. The fundamental tenet of the theory is that plants growing in environments with plentiful light and periodic adequate moisture, but on soils poor in phosphorus, zinc, and other indispensible nutrients, can synthesize carbohydrates in excess of the amount that can be combined with, or catalyzed by, these nutrients for metabolism and production of nutrient‐rich foliage and reproductive tissues. They use this “expendable energy” to produce well‐defended foliage, large quantities of lignified tissues, and readily digestible exudates. Rapid accumulation of nutrient‐poor biomass, a result of low rates of herbivory, provides fuel for intense fire. Intense fire exacerbates nutrient poverty by volatilizing certain micronutrients critical for animals. Anomalous features of organisms of Australia that can be explained by this theory, rather than by climate or phylogenetic history alone, include the following: most woody plants have long‐lived, durable foliage; plants defend their tissues primarily with carbon‐rich but nutrient‐poor compounds; an unusually high proportion of plants protects seeds from fire and granivores in sturdy, woody capsules or follicles; plants allocate unusually large amounts of expendable energy to production of carbon‐based exudates, such as nectar and gums; an unusually high proportion of plant species is pollinated by vertebrates that average larger size than pollinators on other continents; herbivores are small and have slow metabolism; there are no ruminants, mammals that eat mainly subterranean plant matter, or fungus‐culturing termites and ants; vegetation dominated by leaf‐spinescent plants is more extensive than vegetation dominated by stem‐spinescent plants; nitrogen‐fixing plants are major components of most vegetation types; there is a higher proportion of myrmecochorous plant species than on any other continent; there are hardly any stem‐succulent and few leaf‐succulent, perennial, non‐halophytic plant species; and an unusually high proportion of bird species breeds cooperatively. Although the Nutrient‐Poverty/Intense‐Fire Theory can provide plausible explanations for these anomalous features, some puzzles remain, among them the great success of introduced herbivores, the lack of grazers on extensive grasslands on cracking clays, the apparently low productivity of ants, and the prominence of the parasitic plants of Australia. By examining the ratios of available energy to nutrients, particularly scarce nutrients, ecologists may identify processes not previously recognized as important for life forms or biotic adaptation on other continents.