Where The Wild Things Were: Re-wilding and the Sixth Extinction - Biodiversity Conservation and Environmental Change - page 35

ecosystem engineers shaped competition, facilitation and influence plant evolution, driving
the development of seed dispersal mechanism and plant chemical and structural defences
(Bond et al. 2004, Hansen and Galetti 2009, Johnson 2009, Corlett 2013, Galetti and Dirzo
2013, Seddon et al. 2014).
The role of megafauna in African savannas is clear, and it seems likely that the loss of
such creatures from ecosystems in Australia, Eurasia, and the Americas altered tree recruit-
ment, increased biomass accumulation, influenced habitat heterogeneity, and changed
fire regimes. Some plants would have lost the means of seed dispersal, and plants with
energetically costly defences to herbivory, such as thorns, spines, and unpalatable leaves,
would have lost their competitive advantage (Bond and Keeley 2005, Johnson 2009). Fur-
thermore, the loss of top carnivores would have had cascading effects on herbivores and
vegetation (Terborgh et al. 2006, Estes et al. 2011). In summary, we might expect the loss of
megafauna to reduce environmental heterogeneity, release competitors and prey (includ-
ing plants), change fire regimes, and reduce dispersal of some seeds (Corlett 2013, Galetti
and Dirzo 2013).
The palaeoecological record attests to the ecological effects of megafaunal extinction. Spo-
romiella are spores that are associated with herbivore dung, and alongside paleontological
evidence, radiocarbon dating, fossil pollen, and charcoal analysis, their presence can be used
to track the timing of megafaunal attrition and its effects on vegetation and fire. In North
America, there is evidence of increased fire following megafaunal extinction, a consequence
of reduced herbivory and associated increasing biomass. For example, Gill et al. (2009) used
Sporomiella , fossil pollen, and charcoal abundance to study the effects of megafaunal decline
at several sites in Indiana and New York. They found that combinations of plants with no
modern analogues developed at around 13,700 years ago, coinciding with the decline of meg-
aherbivores (Figure 3.2a). At this time, temperate broadleaved trees, particularly Fraxinus
(ash), Ostrya/Carpinus (hophornbeam/ironwood), and Ulmus (elm), coexisted with boreal
conifers such as Picea (spruce) and Larix (larch). Later, these assemblages were replaced by
pine ( Pinus ) and Oak ( Quercus ) forests (Gill et al. 2009). Similarly, fossil pollen, Sporomiella
and charcoal from four sites in New York State showed declining megaherbviore abundance
at the same time as increasing spruce/pine forest habitat about 14,500 years ago, preceding
the formation of so-called 'black mats'—dense, organic carbon- and charcoal-rich deposits,
thought to have been formed when wildfires swept the area, burning dead leaf litter and
unconsumed plant biomass (Robinson et al. 2005, Johnson 2009).
As a result of the loss of megafauna, fine-grained, biodiverse vegetation mosaics, main-
tained by a range of herbivores, were replaced by a coarser-grained, more dense vegetation
community, with raised water tables and reduced recycling of nutrients. Climate change
would have exacerbated these changes and following the accumulation of plant biomass, fire
replaced herbivores as the dominant consumer of plant matter (Robinson et al. 2005). In New
England, Faison et al. (2006) found evidence of open forest canopies associated with drier
climate and local disturbance by people, fire, and herbivores in the early Holocene. No simi-
lar vegetation communities were found until European settlement once more opened up the
forest canopy (Faison et al. 2006).