Ecosystem Physical Structure: Part 1

Ecosystem Physical Structure

Megafauna are capable of significantly shaping and altering landscape structure and functioning of ecosystems, consuming vegetation at higher rates than it can be replenished (as was alluded to as a mechanism for indirect megafaunal effects on ecosystem trophic structure in the past few posts – a la Elk in Yellowstone) and/or damaging/destroying it. Their impact on ecosystem physical structure is so significant that loss of megafauna is thought to have been responsible for past regime shifts in the world’s biomes – further loss could have equally drastic consequences in the future.

Woody plant abundance:

Megafauna can have substantial impacts on woody plant abundance, dependent on body size (greater body size ~ greater ability to consume destroy vegetation), feeding mode (browsers (eg Elk in YNP) directly inhibit abundance via consumption v grazers (eg Elephants) only occasionally feed on woody plants but suppress species abundance through trampling and breakage) and environmental factors controlling the abundance and ability of megafauna to access the plants.

A striking example of megafaunal dominion over woody plant abundance is outlined in a recent study from Bakker et al 2015, comparing Late-Quaternary megafaunal extinctions to analogues in modern exclosure experiments (where megafauna are artificially kept out of areas to simulate their removal from environments). African elephants’ (great in stature and strength ~ great in effect) propensity to pull out / push over shrubs and trees was found to account for >80% woody plant loss – exclusion of elephants resulted in an area with 42% more trees (as strikingly outlined in the following figure – infrared colour indicating higher concentrations of woody vegetation inside the exclosure, intensity of red indicating higher primary productivity)

Species composition effects:

Of course, megafaunal effects on plant abundance are much more nuanced than just equal destruction of all plant species in a given area. Whilst overall trends of more browsing-tolerant shrubs (more palatable plants being eaten in their stead) and more light-demanding woody species (vegetation openness created) are generally found, more complex diverse mosaics of spatially heterogeneous landscapes result from various interactive factors.

Woody plants can persist by defending themselves from browsing/grazing or associating with defended species – this often leads to a cyclic succession: woody plants survive by associating with poisonous / thorny shrubs, which they eventually outgrow and outcompete -> herbivory then cuts them back -> spatial and temporal distributions of this cycle results in a variety of grasslands, shrubs, and different size clumps of trees.

Plants persist by growing in inaccessible areas / areas with high predator activity, resulting in heterogeneously distributed ‘landscapes of fear’ avoided by megaherbivores, thus creating spatial variability in herbivory pressure and allowing local increases in woody plant abundance (eg back to the wolves’ effects on Elk in YNP)

Heterogeneity of landscapes also results from variation of behaviour within megafauna. For example, adult megaherbivores of sufficient stature become functionally immune to predation pressure and consequently frequent woody areas for greater forage while younger individuals, not yet large enough to avoid predation pressure, stay in grasslands where ambush is less likely.

Brief Conclusions

The result of these and other interacting factors are heterogeneous, diverse and robust ecosystems, with positive implications for the environment and for environmental services. 

Stay tuned next week for some interesting case studies in Africa, Australia and North America outlining more detail and nuance!