Top down versus bottom up

Where the Wild Things Were discusses examples of ecosystems from the point of view of trophic cascades. Since this topic is new to me, I decided to dedicate this blog to a short review about top down versus bottom up forces. Bottom up is where primary producers and nutrients dictate community structure and diversity. It seems that ecologists predominantly think that consumers (ex. top predators) impact producers (ex. plants) but are not the key components driving productivity in ecosystems. Thus, most ecologists appear to prefer bottom-up control of species productivity and biodiversity. However, this topic is highly

debated and recent papers show that top predators can alter entire systems and convert habitats ( ex. overgrazing by herbivores due to lack of predators converting savannas to deserts). In these instances, studies show that top predators can benefit plants by controlling densities of herbivores. Most likely different ecosystems respond differently to both top down and bottom up presurres. These papers give several interesting examples that highlight the importance of top predators in ecosystem diversity. One study looks at "islands" in the Venezuelan rainforest with and without top predators (see Figure 4). This study shows that top down control highly effects community diversity. Islands without predators showed high densities of herbivores and seed predators resulting in a decreased number of seed recruitment and saplings. Another example can be seen in Western Atlantic salt marshes (see Figure 5). This study shows that marshes can be over run in the absense of the top predator the blue crab.  In this case, the prey of the blue crab is a snail who lives on fungus growing on plant tissue. Large numbers of this snail have been shown to deplete marsh vegetation and expose mudflat patches in the absence of the blue crab. Based on these papers, it seems like there is a combination of both top down and bottom up pressures in ecosystems. In some ecosystems, it seems that humans play a huge role in altering trophic levels by removing predators or top down forces resulting in loss of diversity and habitat integrity. 

Silliman, B. R. & Angelini, C. (2012) Trophic Cascades Across Diverse Plant Ecosystems. Nature Education Knowledge 3(10):44