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This chapter focuses on the distinction between ecosystem function and ecosystem dynamics. Ecosystem function refers to the manner in which the ecosystem of interest works, and interactions among its component parts and fluxes, including biotic and abiotic compartments. Meanwhile, ecosystem dynamics refers to variation in ecosystem function through time in response to perturbations that are continuous or stochastic in nature, or in relation to changes in ecosystem components. Therefore, the study of ecosystem dynamics derives from an understanding of ecosystem function, and this, in turn, depends critically on successful identification of the important drivers within the ecosystem. Inevitably, a discussion of ecosystem function and dynamics boils down to the factors that influence and contribute to variation in net ecosystem production—the result of net primary productivity and ecosystem respiration.

This chapter takes a look at the energetically based food web processes that can explain the observed relationship between net primary productivity and species diversity. It looks first at independent models of trophic dynamics, and next presents both theory and empirical evidence that the number of consumer species increases as the number of food chains within a community increases. This brings forth the conclusion that species diversity is influenced by both the rate and diversity of production. Through the study of productivity, the energetic efficiencies of consumers, habitat heterogeneity, and the frequency of disturbance, certain hypotheses have been derived to explain patterns in trophic structure and species richness at local, regional, and global scales. The chapter argues, then, that this richness of species is a function of how the rate of production and the diversity of production affect trophic structure, community organization, and patterns of energy utilization in species.

Net primary productivity (NPP) is the amount of C or biomass that accumulates over time and is photosynthesis—autotroph respiration. Annual NPP is estimated by summing positive biomass increments across time periods during the growing season, including offtake to herbivores, which can be high in grasslands. Remote sensing techniques that are used to assess NPP are discussed by the author. Belowground productivity can be high in grasslands, and it is important to carbon storage. Across grasslands on a geographic scale, NPP, N mineralization, and soil organic C all increase with annual precipitation. Within regions, NPP can be strongly affected by the proportion of C₄ plant species and animal species composition and diversity. Humans are adding more N to the environment than all the natural forms of addition (fixation and lightning) combined. Animals, especially herbivores, can have strong effects on how plants respond to changes in changes in resource availability.

Most of this book focuses on the biogeography and ecology of plants and various animal taxa on islands in the Sea of Cortés. These chapters highlight the historical and biogeographical factors that contributed to the patterns of species distribution and co-occurrence among islands. However, these patterns also reflect the action of ecological processes because the species present interact, directly or indirectly, within the food web that occurs on any given island. Island food webs may also be unique from other communities in the degree to which their structure and dynamics are also strongly influenced by the surrounding ocean. We believe that a deeper appreciation of the trophic connections between the sea and the land, and the resulting effects on the structure and dynamics of island food webs, is key to understanding the biogeography of species on islands. Many factors that operate through the food web can enhance or depress populations in a way that affects their local distribution and persistence, and, as a consequence, affects patterns of diversity on a biogeographical scale. Of these, we recognize three as being particularly important: the availability and quality of resources, competition, and consumption (i.e., by herbivores, predators, parasites). Bottom-up factors (nutrients, primary productivity, and food availability to consumers) set limits on island productivity and hence on the potential abundance of a particular group. Within a given community, secondary productivity and population density are subsequently constrained by top-down (i.e., consumption) and competitive effects. One of our goals in this chapter is to show how processes that influence productivity of gulf islands determine patterns of abundance of organisms on islands and affect interactions among species and trophic levels in these systems. Our second goal is to demonstrate the importance of spatial and temporal variability in productivity in determining the structure and dynamics of island food webs. Using our long-term studies of plants and consumers on islands in the northern gulf, we show that productivity varies greatly, both among years and islands, as a result of both local conditions and global climatic factors. Such variable productivity markedly affects food web dynamics and ultimately the abundance of species on the islands in the Sea of Cortés.