Climate, energy and diversity

Abstract

In recent years, a number of species-energy hypotheses have been developed to explain global patterns in plant and animal diversity. These hypotheses frequently fail to distinguish between fundamentally different forms of energy which influence diversity in dissimilar ways. Photosynthetically active radiation (PAR) can be utilized only by plants, though their abundance and growth rate is also greatly influenced by water. The Gibbs free energy (chemical energy) retained in the reduced organic compounds of tissue can be utilized by all heterotrophic organisms. Neither PAR nor chemical energy influences diversity directly. Both, however, influence biomass and/or abundance; diversity may then increase as a result of secondary population dynamic or evolutionary processes. Temperature is not a form of energy, though it is often used loosely by ecologists as a proxy for energy; it does, however, influence the rate of utilization of chemical energy by organisms. It may also influence diversity by allowing a greater range of energetic lifestyles at warmer temperatures (the metabolic niche hypothesis). We conclude that there is no single species/energy mechanism; fundamentally different processes link energy to abundance in plants and animals, and diversity is affected secondarily. If we are to make progress in elucidating these mechanisms, it is important to distinguish climatic effects on species' distribution and abundance from processes linking energy supply to plant and animal diversity.

Figure 1

(a) Mean (black) and annual (grey) range of sea-surface temperature (SST) along 170° W in the Pacific Ocean. Note how the largest seasonal variations are in the temperate latitudes. Data are averages for the period 1983–2003. The AVHRR Pathfinder v. 5.0 SST data were obtained from the Physical Oceanography Distributed Active Archive Centre (PO.DAAC) at the NASA Jet Propulsion Laboratory, Pasadena, California (http://podaac.jpl.nasa.gov). (b) Mean (black) and annual (grey) range of synoptic air temperature for terrestrial habitats. Mean annual temperature data (°C) for the period 1961–1990 at 10 min resolution interpolated from station means (New et al. 2002), and resampled to an equal-area grid using a Behrmann projection at a resolution of 96486.2 m at the standard parallels of 30° N and 30° S (provided by R. G. Davies). Data are for North and South America (available at http://www.cru.uea.ac.uk/cru/data/tmc.htm), and plotted after pooling into bins of 1° of latitude, with the range calculated from seasonal average maximum and minimum in each bin.

Figure 2

Temperature and metabolic niches. (a) Variation in resting metabolic rate in teleost fish (Clarke & Johnston 1999). Note the wider range of values at higher temperatures. (b) Diagram showing how absolute aerobic scope also increases with temperature. This arises because resting metabolic rate increases positively with temperature and relative aerobic scope (the ratio of active to resting metabolism) is temperature invariant. Although the model was based on data for teleost fish, it also applies to other ectotherms (Clarke 2003). (c) Energetic niches in mammals (black) and birds (grey). Data are for field metabolic rate, and have been corrected for body mass assuming a mass exponent of 0.75 (redrawn from Anderson & Jetz 2005). Note the wider range of values at higher temperatures.

Figure 3

A conceptual diagram showing the complexity of processes that link energy supply to organism diversity. Solid arrows illustrate the transfer of energy, with photosynthetically active radiation (PAR) shown as an open arrow and chemical energy shown as grey arrows. The influence of temperature is all-pervasive and complex; it influences the rate of utilization of energy (left-hand side of the diagram) and also the availability of water, and rate of mutation and population processes linking abundance to diversity (right-hand side of the diagram). This diagram emphasizes that there is no single species/energy hypothesis, that different processes underpin the evolution of plant diversity and animal diversity, and that temperature has a complex influence.