Abstract
Many conceptual syntheses in ecology and evolution are undergirded by either a patch- or continuum-based model. Examples include gradualism and punctuated equilibrium in evolution, and edge effects and the theory of island biogeography in ecology. In this study, we sought to determine how patch- or continuum-based analyses could explain variation in concentrations of stream macronutrients and system metabolism, represented by measures of productivity and respiration rates, at the watershed scale across the Kanawha River Basin, USA. Using Strahler stream order (SSO; continuum) and functional process zone (FPZ; patch) as factors, we produced statistical models for each variable and compared model performance using likelihood ratio tests. Only one nutrient (i.e., ) responded better to patch-based analysis. Both models were significantly better than a null model for ecosystem respiration; however, neither outperformed the other. Importantly, in most cases, a combination model, including both SSO and FPZ, best described observed variation in the system. Our findings suggest that several patch- and continuum-based processes may simultaneously influence the concentration of macronutrients and system metabolism. Nutrient spiral- ing along a continuum and the patch mosaic of land cover may both alter macronutrients, for example. Similarly, increases in temperature and discharge associated with increasing SSO, as well as the differences in light availability and channel morphology associated with different FPZs, may influence system metabolism. For these reasons, we recommend a combination of patch- and continuum-based analyses when modeling, analyzing, and interpreting patterns in stream ecosystem parameters.
Highlights
Describing and quantifying how structure and processes vary over space and time is a fundamental issue in science, in general, and for ecology and evolution, in particular
We looked at each nutrient independently rather than using a multivariate approach because each may have a different source or uptake rate
Our results suggested little support for the single-factor patch- (FPZ) or continuum-based (SSO) models; in many cases, a combination was preferred
Summary
Describing and quantifying how structure and processes vary over space and time is a fundamental issue in science, in general, and for ecology and evolution, in particular. There have been two philosophies: gradualism and abrupt change. Debate between proponents of each of these philosophies has played out many times in the history of ecology and evolution. In macroevolution, Darwin (1859) and others emphasized that small, gradual changes over long periods of time could produce great diversity, whereas Eldredge and Gould (1972) argued that punctuated equilibria were key in forming the diversity of life on earth. Discrete patch-based models have been broadly tested and represent a paradigm that is fundamental for conservation biology and landscape ecology (Levins 1970, Wilson 1980, Pulliam 1988, Forman 1995). Landscape mosaics (Wiens 1992), the theory of island biogeography (MacArthur and Wilson 1967), and metapopulation dynamics (Hanski 1994) are some examples of patch-based theories. Other viewpoints include patch dynamics models (Wu and Loucks 1995, Dunn and Majer 2007), which are used to describe ideas of nonequilibrium and heterogeneity in nature
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