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Abstract

Introduction Different climatic regions across the globe are associated with biomes that differ in their cover of woody plants, such as grasslands, savannas, and forests (Whitaker, 1962). For a long time, researchers have assumed that abiotic factors control the spatial distribution of woody plant-dominated ecosystems or biomes. According to this idea, rainfall and temperature determine the transitions from deserts to grasslands to savannas and eventually to forests (e.g., Prentice et al., 1992). However, we increasingly realize that biomes may be far less fixed entities than previously assumed. An alternative view for many regions might be that of “ecosystems uncertain,” which Whittaker (1975) defined as zones “in which either grassland or one of the types dominated by woody plants” may occur under the same climatic conditions. As Bond (2005) discusses, many of these “ecosystems uncertain” may be seen as “consumer-controlled ecosystems” where plant consumers, such as herbivores and fire, prevent a closed forest from developing and are a major determinant of the ecosystem state. Bond (2005) showed that such “ecosystems uncertain” may in fact cover a very large part of the world (Fig. 5.1). More recently, several global analyses confirmed that across a large part of the global land surface, tree cover is indeed bimodal (Staver et al., 2011) or even trimodal (Scheffer et al., 2012). This means that under the same climatic conditions, a system may be in a treeless, savanna, or forest state; this pattern has been described for (sub)tropical (Hirota et al., 2011; Staver et al., 2011), as well as boreal (Scheffer et al., 2012), parts of the world. From this, we can conclude that, rather than being purely controlled from the bottom up, ecosystems, and even biomes, are shaped by interacting bottom-up and top-down factors (Polis and Strong, 1996). There is general agreement that both bottom-up and top-down factors affect plant communities (Polis and Strong, 1996), but the question remains what the relative strengths of such top-down and bottom-up processes are and whether we can find general spatial and temporal patterns in their effects (Gripenberg and Roslin, 2007).

Published in

Title: Trophic Ecology : Bottom-Up and Top-Down Interactions across Aquatic and Terrestrial Systems
Publisher: Cambridge University Press

SLU Authors

UKÄ Subject classification

Ecology

Publication identifier

  • DOI: https://doi.org/10.1017/CBO9781139924856.006
  • ISBN: 9781107077324
  • eISBN: 9781107077324

Permanent link to this page (URI)

https://res.slu.se/id/publ/132032