Dynamics of ecologies within geophysical settings

Biological communities comprise intricate and complex networks of symbioses, food webs, and resource sharing and exhibit their own internal dynamics. They also arise and change within the dynamic geophysical landscape of the surface of the Earth, which includes plate tectonics, volcanism, orogeny, subsidence, weathering, ocean circulation, the growth and retreat of ice sheets, and changing patterns of precipitation and temperature. The differing temporal scale of these processes produces widely varying ecological and evolutionary impacts on community development and persistence. For example, as volcanic islands subside, carbonate reef communities will experience decreasing ambient light and temperature; near-shore nutrient levels will also change as the volcanic rocks are progressively weathered and nutrient inputs change. Where possible, reef ecosystems will compensate by migrating towards shore and perhaps by changing community composition. Forested catchments will also develop with tectonic uplift or subsidence as temperatures and precipitation change with altitude. Rapid changes in the intensity of thermohaline ocean circulation will affect communities that depend either on the upwelling of nutrient-rich deeper waters or the sinking of oxygen-rich waters. Soil communities will respond to many factors, including the geology of the substrate rock and the local hydrology. These geophysical constraints influence the establishment and persistence of ecological niches, and facilitate biotic niche construction as a result of interactions between species. Many of these changes will reflect geophysical events on an evolutionary time scale rather than biological adaptation within communities: The geophysical fragmentation and/or isolation of communities and biogeographic units (provinces) (e.g., by continental-breakup or orogeny) has been long considered to dissociate gene pools and promote speciation (Fig. 1). Although paleontologists are often unable to detect slow changes in populations over time, sensitive differential comparisons between populations using DNA-based methods may reveal underlying patterns in the response of communities to changes in the geophysical character and connectivity of habitats.

These patterns of dynamic biological change occur on time-scales longer than human records and their study requires the interpretation of geological, geochemical, climatological, and paleontological records. Critical intervals in Earth history (periods involving mass extinction events or dramatic changes in ecological structure) are a means of studying the dynamics of the coupled biosphere-geosphere system, just as one means of determining the dynamics of a mechanical system is to subject it to a sudden impulse and observe the dynamics of its response. These events include the well-known Cretaceous-Tertiary, Permian-Triassic, and end-Devonian events (which included mass extinctions) as well as the low-latitude glaciation events in the late Neoproterozoic, the Late Paleocene Thermal maximum, the Devonian radiation of vascular plants, and the spread of C4 plants during the late Cenozoic. Most previous investigations of these events have sought one or more causes for these catastrophes. The goals of future research should include the detailed, high-resolution reconstruction of the geologic record through these intervals and the testing of different models of ecosystem response using this record.

Beyond our own planet, will an increased understanding of the interplay between geophysical and ecological forces allow us to elucidate some of the general principles for the development of planetary biospheres? Or is the process so stochastic that the current organization of life on Earth is the product of multiple contingencies, with little or no predictive power for other, slightly dissimilar planets? For example, is it inevitable that the concentration of molecular oxygen will rise to appreciable levels in the atmosphere of an Earth-like planet, as the evidence suggests happened on our world 2-2.5 billion years ago? Or are there circumstances in which an anaerobic atmosphere can be maintained indefinitely, even if oxygenic photosynthesis has evolved? Will photosynthesis necessarily evolve, and is the evolution of vascular plants inevitable or does this depend on the exact geology of the planet, the composition of the atmosphere, or the impinging flux of ultraviolet radiation? Are the same feedbacks established between the biosphere and climate that may exist in the Earth system? Will biospheres be able to maintain a low but finite amount of CO2 in the atmosphere despite changes in the luminosity of the parent star, plate tectonics, and volcanism over time? These questions may not remain simple speculation for long: Astronomers are considering space-based observatories with unprecedented angular resolution and sensitivity capable of detecting Earth-sized planets around orbiting nearby stars. With the more ambitious designs we will be able to obtain spectra of their reflected or emitted light. If a planet has an atmosphere, certain gases may (e.g., oxygen, ozone, methane) produce absorption lines in the spectrum, and abundant levels of these gases could suggest the intervention of biology (in particular a photosynthetic surface biosphere) on a planet. It is possible that in the not-so-distant future we may have additional data points to map to our increased understanding of how biological and geological systems develop together.

Figure 1. The Galapagos islands, volcanic edifices on a mid-ocean ridge whose individual, distinct faunal communities partly inspired Charles Darwin's theories of evolution and speciation (courtesy the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE).