One of the most significant developments in macrophysiology has been the debate about the mechanistic underpinnings of the scaling of life-history and physiological attributes and its implications for understanding the diversity of life across a range of hierarchical levels (Brown et al. 2004
; Makarieva et al. 2005
). This issue has revitalized an interest in how and why physiological and life-history traits are related to body size and temperature. It is these strong relationships, especially within higher taxa, which reveal profound, but neglected, macrophysiological impacts of overexploitation: the erosion of large areas of physiological phenotypic space.
At both the inter- and intraspecific levels, overexploitation has resulted in a reduction in the size of the targeted organisms. Perhaps the most widely recognized of humankind's effects on diversity has been the removal of large-bodied species from the terrestrial landscape: as a consequence of overexploitation most continents no longer house the very large grazers and predators they once did (Jablonski 2007
). Where very large animal species still occur, such as in Africa, their abundance is much reduced (Skead 2007
), and they are frequently restricted to relatively small managed areas. Moreover, in human-dominated systems, even comparatively small, large-bodied species are no longer present. In marine systems, large whale, fish and turtle species are in decline as ever lower levels in the marine food web are targeted to satisfy human resource demands. The resulting environmental problems are often not recognized as originating from the removal of large-bodied species (Jackson et al. 2001
At the intraspecific level, similar targeting of large individuals has been common. In many commercially exploited fish species, mean body size has declined dramatically over the past several decades (Sibert et al. 2006
; Olden et al. 2007
). Similar declines in the mean body size of mammal species have been recorded, and especially of the horn-bearing males that are of most interest to trophy hunters (Coltman et al. 2003
). Large trees have also been targeted, and it is common now to find particular individuals known and protected simply for their large size and antiquity (The Tree Register 2006, http://www.treeregister.org/index.html
By removing large species and individuals, and by reducing their abundances, overexploitation has substantially reduced the range of physiological phenotype space once covered and has selected for a radically different suite of life histories (Kuparinen & Merilä 2007
). In consequence, unique physiologies typical of larger species and individuals are threatened. These include regional heterothermy as found in fish species such as tuna (although apparently not all tuna populations are overexploited; Sibert et al. 2006
), the capacity for the use of low-quality plant material such as in large herbivorous mammals and the resilience to overcome prolonged periods of resource deprivation that is a characteristic of the larger individuals of many species (because storage capacity rises more steeply with mass than maintenance metabolism). The latter change makes clear that the erosion of physiological phenotypic space is not only intrinsically worrying, but will also have profound consequences for the ways in which plants and animals respond to other forms of environmental change over the short and long term.
Increases in the variability of climatic conditions are forecast under most global climate change scenarios (Easterling et al. 2000
). In many taxa, strong relationships exist between the tolerance of stressful conditions and body size. Likewise, large active dispersing organisms tend to have greater dispersal capacities than do smaller ones, whereas for passive dispersing organisms no clear relationship with mass exists (Jenkins et al. 2007
). Dispersal is not only a key factor determining survival of habitat fragmentation, but also plays a significant role in influencing adaptation to new environments and the likelihood and extent of plasticity (Chown & Terblanche 2007
). In consequence, the loss of large-bodied individuals and species will reduce tolerance to variable environments and result in a loss in the capacity of systems to recover from or resist perturbation via either fragmentation or increases in temporal variability of abiotic conditions. Large and small individuals also play different functional roles in food webs (Cohen et al. 2005
), suggesting that the removal of the large-bodied will lead to trophic restructuring, even though the species may not have been removed from the system. It is by understanding the interactions between body size, life history and functioning over large spatial and temporal scales that macrophysiology can help assess the impacts of overexploitation and complement other investigations of the same issue, as well as draw attention to its significance even where extinction is not an outcome.