Abundance - the relative amount of a species in a particular ecosystem - is an important concept in ecology. Measures of abundance, which are approximated by counting the number of individuals in a sample area, are used to indicate population health and well-being. Thus, abundance has a central role in many ecological theories, including the limitation of species ranges and geographical patterns of speciation.
However, many of these ideas are based on a basic hypothesis about how abundance varies across a species' range. Specifically, it is generally assumed that abundance is greatest at the center of a species range and declines towards the edge of the range (the 'abundant center' can look much like a normal distribution). This simple assumption is often justified by the idea that abundance is tied to environmental gradients; at some point, the environmental conditions become less favorable and population numbers decline and, eventually, populations are no longer sustained because conditions become too extreme. In other words, local abundance is a reflection of how well a particular site meets the needs of a species. It approximates the niche. However, the 'abundant center' hypothesis assumes that niche conditions are spatially correlated and gradually decline toward range edges. This assumption is used extensively in theoretical and applied ecological research. It might also be inappropriate most of the time (Sagarin et al. 2006).
Why wouldn't abundance exhibit a uniform pattern across a range?
Essentially, abundance is controlled by many factors other than gradual environmental gradients. Canham and Thomas (2010) showed that trees in North America showed very little variation in local abundance across the species' ranges. Similarly, Sagarin and Gaines (2002) found that only 39% of the (145 diverse) cases that they examined even closely resembled a pattern of peak abundance in the center of the range. This is probably because, once an organism has colonized a site, factors like competition with other organisms and heterogeneity in local environmental factors like soil type, soil moisture, or topography are what modulate local abundance (Clark et al. 2011).
Is abundance still a useful indicator for ecology?
Given this issue, it is likely that abundance is not as useful as we thought for predicting the climate sensitivity of a species. This is an issue given that many models that predict species responses to climate ('climate envelope models') assume that abundance is a direct indicator of climate response.
Abundance is still valuable. In its basic definition it is still relevant as an indicator of population health. However, it is unlikely that most species will meet the assumption of an abundant center with gradual declines towards the edges. Consequently, we need to re-test many of our hypotheses about how species range dynamics function (Sagarin and Gaines 2002). To make abundance useful for predicting species responses to climate, it will be important to include other variables like physiological condition or population growth rates to get the whole picture (Clark et al. 2011). However, our realization that real abundance distributions are much more complex and varied provide an opportunity to re-evaluate theoretical concepts and develop better hypotheses to explain species dynamics (Sagarin et al. 2006). The possibilities are nearly endless!
- Canham CD, Thomas RQ. 2010. Frequency, not relative abundance, of temperate tree species varies along climate gradients in eastern North America. Ecology 91:3343-3440.
- Clark JS, et al. 2011. Climate change vulnerability of forest biodiversity: climate and competition tracking of demographic rates. Global Change Biology 17:1834-1849.
- Sagarin RD, Gaines SD. 2002. The 'abundant centre' distribution: to what extent is it a biogeographical rule? Ecology Letters 5:137-147.
- Sagarin RD, Gaines SD, Gaylord B. 2006. Moving beyond assumptions to understand abundance distributions across the ranges of species. Trends in Ecology and Evolution 21:524-530.