Ecological Hangover

Large herbivores have a large impact on the vegetation they rely on. They affect the structure, composition and dynamics of plant communities. 50,000 years ago, many more herbivore giants existed, and there is considerable evidence to suggest that their absence has affected the vegetation communities they left behind. 

Johnson (2009) argues that extinct megafauna once kept the landscape more open, creating gaps in the vegetation. This creates a mosaic landscape: a mix of open, undisturbed and somewhere-in-the-middle plant communities. A greater variety of habitats equals a greater variety of species. More sunlight in cleared areas allows different species of plant to grow, and a mosaic landscape where plant communities are moving through various successional stages would at any one time would increase the diversity of this landscape. They also enabled accelerated nutrient cycling via urine and faces and altered fire regimes by reducing accumulation of dry plant matter (Johnson, 2009). 

So when these giants went extinct, and the habitats became more uniform for lack of megafauna-made gaps, plant community diversity may have decreased along with them. The mosaic would have become more like a wheat field. 

Plant communities that once defended against browsing mega-herbivores with spines and toxins are now inedible by modern stand-ins. The plant's seed dispersal mechanisms too, designed for the missing megafauna, are now obsolete. These plants may be in decline for this reason, meaning that plant communities may be in a process of relaxing from the selective pressures the megafauna imposed. Contemporary plant communities may therefore be far from stable and established, but in a transitional period (Johnson, 2009).    

On its last legs? The Javan Rhinoceros
From: It's nature

There is side-note to this debate: ecological extinction. The Javan Rhino suffers from this condition, and it's likely that towards the end of their time on Earth mammoths, mastodons and other giant herbivores suffered the same fate. Ecological extinction occurs when the populations of a species are so low in number that they no longer fulfill their ecological role, such as tree-felling, seed dispersal etc. 

Speaking of Rhinos, it's the ZSL "Responding to the Rhino Poaching Crisis" meeting next week, so I'll be posting about these endangered megafauna soon!