Why is the world green? The interactions of top–down and bottom–up processes in terrestrial vegetation ecology

Abstract

A classic question in plant ecology is "why is the world green?" That is, if plants are food for animals why do not animals eat all the available food -changing a 'green world' into a 'brown world'. We first reviewed this question in 2009 and now revisit our arguments in the light of new data and new thinking. Here we argue that (1) the top-down bottom-up dichotomy is probably too simple for understanding a complex system -such as vegetation -rich in feedback processes. (2) Nevertheless it appears that bottom-up processes are generally more important for maintaining the presence of some sort of vegetation while top-down control process are generally more important in determining the type of vegetation at a site. (3) Although this review mainly takes a qualitative and experimental approach to the question, we also argue that simple well-known mathematical models from population ecology can be very informative in thinking about the types of explanations for the green world phenomenon, and demonstrating that it is rarely a simple choice between one form of control or another.

Spatial diversification of agroecosystems to enhance biological control and other regulating services: An agroecological perspective

“…considering tritrophic interactions as trophic levels are highly overlapping (Wilkinson and Sherratt, 2016), by spatially diversifying crop and non-crop habitats represents the first two phases proposed by Zehnder et al (2007) for managing arthropod pests without chemical pesticides in a context of organic farming and is the main component of agroecological crop protection described by Deguine et al (2016). Although they can be implemented at the farm level, they together induce a diversification at the landscape scale, influencing insects (both pests and natural enemies) that are highly mobile, easily crossing farm borders.…”

‘Remote’ behavioural ecology: do megaherbivores consume vegetation in proportion to its presence in the landscape?

“…One possible reason for this is that the current study focused on the southern portion of KNP, whereas the former included the northern region in which the tree communities are dominated by a single-species: mopane Colophospermum mopane. Given the high concentration of defensive secondary compounds in mopane (Styles & Skinner, 2000;Kohi et al, 2010), and the tendency of many (but certainly not all) animals to avoid high loads of such anti-feedants by diversifying their diets (Freeland & Janzen, 1974;Wiggins, McArthur & Davies, 2006;Torregrossa, Azzara & Dearing, 2011;Wilkinson & Sherratt, 2016), elephants might under-utilize mopane. Such a foraging behaviour would necessitate greater consumption of grasses in habitats such as the northern KNP, and consequently reduce any relationship between browse/graze consumption and availability for the park as a whole.…”

The Psyllids Triozoida Limbata Supplant the Defenses of Psidium Guajava Plants to Garentee Feeding and Protection

“…"Why is the world green?" is a classical question in plant ecology not yet fully responded by the scientists [1,2]. The core of this puzzle question relies on the fact that although many herbivores, mainly insects, can reduce the plant growth and reproduction and increase their mortality, in its majority plants cover the terrestrial world.…”

“…The core of this puzzle question relies on the fact that although many herbivores, mainly insects, can reduce the plant growth and reproduction and increase their mortality, in its majority plants cover the terrestrial world. One partial response for this question is that most herbivores species cannot obtain the quantity and or quality of food resources required to grow, survive, or reproduce at high rates [2,3] because plants synthesize secondary metabolites and proteins that are toxic, repellent and or that inhibit insect herbivores digestion. These chemical defenses have been evolving throughout the arms race between plants and insects and gave the plants the ability to recognize signals from damaged cells and activate their immune response against the attackers [4].…”