Geoscience Reference
In-Depth Information
These three scenarios are based on resource abundance,
gradient length and niche characteristics. They prompt the
question: what environmental and ecological factors
determine gradients and niches? These factors are the
basis for many of the explanations which have been
suggested in the debate on the global diversity gradient.
Table 22.4
summarizes the likely controlling factors, and
notes the supporting and contrary evidence for each
explanation.
We can conclude from
Table 22.4
that no single factor
explains the latitudinal gradient of biodiversity. Important
influences are history, climate, low seasonal variability,
complex plant structures with niche specialization,
and prey-predator effects. It is difficult not to agree with
MacDonald (2003) that: 'the high biodiversity of the
tropics has defied one single simple explanation and is
more realistically the product of many historical and
equilibrium factors.'
Biodiversity factor
Supporting evidence
Contrary evidence
Habitat diversity
1 Topography: changes in soils,
1 Local and regional rather than global
climates, reproductive isolation
effects
2 Complex vegetation structure gives a
2 Biodiversity in tropical deciduous
variety of habitats, which gives a variety of
forest and grassland higher than in similar
species, especially insects and birds
structures in mid and high latitudes
Large land area in tropics
Large populations run lower risk of
Large boreal forest biome has lower
extinction
biodiversity than smaller biomes, e.g.
temperate deciduous and Mediterranean
Environmental stability
1 Stable diurnal and seasonal climates
1 Why do species not evolve adaptations
in tropics
to climatic variability?
2 Larger ranges of species in high latitudes
2 Stable environments in deep oceans
have fewer species than unstable shallows
Disturbance: the
1
Support from field studies in the
1 Disturbance rates unknown for much of
'intermediate
boreal forest and the Great Barrier
the globe, but rates for tropical rain
disturbance hypothesis' of
Reef in Australia
forest may not be much less than
Connell (1978) states that
2 Support from computer modelling
for temperate forests
absenceof disturbance
leads to the extinction of
some species, and frequent
disturbance causes the
extinction of sensitive
species. Intermediatelevels
of disturbance give varied
habitats without extinctions
of sensitive species
Competition
1 Species more efficient within
1 No relevance to tropical tree diversity
narrow environmental microhabitats
2 Specialized niches and narrow food
2 Why not applicable to higher latitudes?
preferences decrease competition
Predation: high
Hypothesis of Janzen (1970) that
1 Why are there more predators
predation pressure
intense seed predation around
in the tropics?
keeps prey populations
tropical trees makes it difficult for
2 Field and laboratory studies show
low, decreasing
trees to establish close to another of
that large numbers of predators can
competitive exclusion,
the same species
lead to either more or less prey species
allowing more prey
species to evolve.
Productivity: more NPP
Correlation on the global scale between
1 Very productive ecosystems (estuaries,
can support more
NPP and the number of plant and animal
wetlands) have few species
species at higher
species
2
Regions like South Africa and Australia
trophic levels.
with highest diversities in regions of
intermediate productivity
