Valuing Common Species

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Science  08 Jan 2010:
Vol. 327, Issue 5962, pp. 154-155
DOI: 10.1126/science.1182818

Aldo Leopold's dictum that “To keep every cog and wheel is the first precaution of intelligent tinkering” (1) has been oft repeated in the context of environmental management. The argument is beguilingly simple. In the absence of a detailed understanding of what each species does in an ecosystem, it would be foolish to allow the loss of any one of them. It is the precautionary principle writ large and, given its enormous ramifications for the ways in which people interact with the natural world, ecologists have spent much intellectual energy, time, and resources in determining whether it has a strong empirical basis (2). Indeed, some of the best-known recent ecological experiments have examined the consequences of varying the numbers of species in a small area on ecosystem function. This focus assumes that the importance of retaining Leopold's cogs and wheels lies mostly in the differences between them. However, a growing body of work on common species underlines that having sufficient copies of some key pieces may be equally, and perhaps often more, important.

Common yet declining.

Some of the most familiar common species are in marked decline. For example, European populations of the house sparrow (Passer domesticus) and the starling (Sturnus vulgaris) have fallen dramatically. The trends shown were determined from annual national breeding bird surveys (17).


Arguably, the importance of naturally common species—those that are abundant and widespread—in shaping the world around us is so blatant that it is easily overlooked. Within any given taxonomically defined assemblage (such as vascular plants, bees, amphibians, or birds), such species are in the distinct minority—the state of being very common is actually very rare—but they contribute much of the structure, biomass, and energy turnover of the majority of terrestrial and marine systems (3, 4). They may exert a profound influence on the prevailing environmental conditions experienced by other species and thus those that can coexist. Even in tropical forest landscapes, some of the most species-rich ecosystems that exist, it is not the full diversity of trees that is most apparent but the smaller number of species that have come to dominate.

Few experiments have been conducted explicitly to determine the contribution of common species to ecosystem function. However, those that have been done confirm the importance of these species. In particular systems, common species can, for example, be responsible for the bulk of primary production, carbon storage and flows, and sediment mixing (58). Moreover, meta-analyses of the results of experiments that have manipulated species richness have shown that the standing stock (total abundance or biomass) and resource depletion found in the experimental treatments with most species tend to be similar to those of the most productive species in single-species treatments (9). This result might be explained by the sampling effect: Assemblages with larger numbers of species are more likely to contain those species that are most productive.

In consequence, common species underpin the provision of many ecosystem services, the benefits that ecosystems provide to people. This does not pertain solely to the more familiar supporting services (such as soil formation, primary production, nutrient cycling, and water cycling) and regulating services (such as air quality regulation, climate regulation, water regulation, and pollination). There is also growing evidence that physical and visual encounters with the natural world positively influence human health and well-being, most noticeably in urban areas where such access is limited but where most people live (1012). Inevitably, it is species that are common within cities (when native, they are often also common more widely) that are chiefly responsible for these interactions.

The null expectation is that the contribution of common species to ecosystem structure, function, and services will be strictly proportional to their abundance. However, the extent to which this expectation is met, or to which common species contribute supraproportionally (that is, they are keystone species) or subproportionally, remains poorly understood.

Ecologists and evolutionary biologists have found that common species also contribute to structuring the world around us in less obvious ways. For instance, they disproportionately influence spatial patterns of variation in species richness and species turnover (13, 14), provide temporal continuity to assemblage structure through greater species longevity (15), and exhibit some of the classical latitudinal and altitudinal patterns of variation in body size, coloration, and reproductive investment that can result in the same species looking and behaving differently in different parts of the world (16).

The importance of the role that common species play would perhaps largely be of interest only to biologists if they remained common. However, systematic declines are now frequent (see the figure), with common species lying at the heart of each of the major pressures on biodiversity.

First, common species are the main victims of habitat loss, fragmentation, and degradation. Indeed, to a first approximation, common species are habitat loss, yet our awareness of this fact is dulled by the presentation of statistics in terms of areal declines of forests, grasslands, coral reefs, and the like, rather than the numbers of individuals that have been removed.

Second, common species are the main objects of large-scale overexploitation. Although many species may be involved, the vast majority of the resources resulting from logging, fishing, and the bushmeat trade come from just a few. Sustainable use is principally a matter of how we deal with common species.

Third, common species are frequent casualties of the invasion of species, whether after accidental or intentional introduction, that are alien to an area. Indeed, some of the environmentally and economically most important alien species are those that have killed or replaced the natural dominants.

Finally, it is the impacts of habitat loss, overexploitation, and invasive species on common species that lead to the most pronounced resultant cascades of reductions and losses of other species, because common species shape their environments and are involved in large numbers of biotic interactions (such as herbivory, predation, and parasitism).

One might perhaps argue that the declines of previously common species are of limited concern, because others will increase in abundance and distribution to take their places. In some cases, it is doubtless true that other species will come to dominance, although these typically tend to be rather different in biological characteristics (for example, smaller sizes, shorter generation times, and greater propensity for boom-and-bust dynamics) and in the ecosystem services that they provide (a grassland is not a forest, and jellyfish-dominated oceans do not provide large quantities of fish for human consumption). Moreover, in an increasing number of cases, a troubling dynamic appears to be playing out, with declines in common species reflecting large net losses in the numbers of individuals of an assemblage (4). Many examples come from intensively used areas of agriculture and urbanization in the developed world, perhaps presaging such problems elsewhere. Previously common species, including the rocky mountain grasshopper and the passenger pigeon, have been driven extinct through anthropogenic activities. However, typically, the more substantive concern is the extinction of commonness, or the ecological and functional extinction of common species.

None of this is to say that rare species are unimportant: There is ample evidence to the contrary. Rather, we need to give weight both to retaining the different kinds of Leopold's cogs and wheels and to ensuring that we have sufficient of each.

References and Notes

  1. Data from European Bird Census Council/Royal Society for the Protection of Birds/BirdLife/Statistics Netherlands; see
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