Download Drivers of Species diversity

Species diversity
What determines:
1) Species distributions?
2) Species diversity?
Patterns and processes
At least 120 different (overlapping) hypotheses
explaining species richness ...
We are going to think about general principles and the
most obvious hypotheses
Hierarchy ... and some terms
• Species
– species richness, species diversity
• Populations
• Communities
– ”community assembly”
• Ecosystems and biomes
• Environmental variation ... ”Habitat”
• What explains the distribution of plant species
(and thus the local species richness) in Skåne?
• Why are there no lions in Skåne?
Floran i Skåne
Tyler et al. (2007)
”Drivers” of diversity
■ What controls species distributions and
therefore levels of species diversity??
Species diversity
Ecology and/or chance?
Scale
• The same general processes operate on all
spatial and temporal scales
Adaptation?
Habitat and adaptation
”Adaptation”
Process of evolution by natural selection
• Basic physiological and morphological
characteristics of a species
– determine whether a species can grow in a particular
place
• Large scale patterns
– often determined by physiological constraints. e.g.
climate
• Fine scale patterns
– Determined by e.g. local variation in soil properties
Adaptation
• Natural selection
• Operates on individuals
• Better relative survival and reproductive
fitness
... depends on available genetic variation
... large scale adaptation within species
... speciation
Large scale patterns often determined by physiological
constraints (e.g. climate)
Large scale distribution
(European beech)
Modern natural distribution of Fagus sylvatica according to Atlas Florae
Europaeae (Jalas & Suominen, 1972–99), bold black line; and areas where
F. sylvatica would be the dominant tree under natural conditions (Bohn
et al., 2000), shaded areas.
Fine scale patterns; often determined by e.g. local variation in soil properties
Fine scale distribution (mine tailings with very high levels of lead and zink)
Why may there be problems with
predicting future distributions from
present distributions and climatic limits?
Why may there be problems with predicting future
distributions from present distributions and climatic
limits?
• Other factors are involved
– Purely spatial processes (”false correlations”)
– Historical processes (present distribution may not be in equilibrium
with present-day climate)
• The distributions and species diversity that we
observe at the present day are a complex
reflection of processes that operate/have
operated on different spatial and temporal
scales
• Adaptation and ecology
• Non-adaptive processes: history, chance
[The impacts of these two types of driver will be a
central thread in my lectures and in the exercise at
the end of the biodiversity theme]
History
• Immigration patterns
• Barriers
• Chance
[we have already discussed how ancient chance events may still
be evident in patterns of present day genetic variation within
species...]
(Dispersal)
Horse chestnut
Cameraria ohridella
Big seeds!
Dispersal
Species pool
• Regional species pool
• Species within a region (scale!)
– The species in the pool are a subset of the species
that could potentially exist under the particular
climatic and environmental conditions in that
particular region
– Filtered by historical processes: migration,
dispersal, previous regional processes
Habitat heterogeneity
Adaptation
Different environmental conditions within a
particular area (applies at different scales)
Land-cover heterogeneity
Habitat heterogeneity
Scale!
Agricultural intensification
Fig. 1. The multivariate and interacting nature of farming practices and some of the routes by which farming
practice impacts on farmland birds. Arrows indicate known routes by which farming practices indirectly or
directly affect farmland bird demography and therefore local population dynamics and finally total population
size.The goal of manipulating farming practice is to impact on population size. Rather than identifying key
routes through this web to change in a piece-wise fashion (e.g. insecticide usage), we suggest that
management designed to increase habitat heterogeneity is likely to benefit the organisms in such a way as to
meet the management goals
Benton, Vickery & Wilson (2003) Trends Ecol Evol
Competition
Competition
•
•
•
•
“Complete competitors cannot coexist"
Competition for light (shading): above ground
Competition for nutrients: below ground
Competition can lead to exclusion of most
species OR to the coexistence of many species
Competitive exclusion
Fertile (= high levels of nutrients) and moist conditions mean
that vegetation grows vigorously and there is high biomass
(productivity is high)
There is competition for light and no room for non-vigorous
species
”complete
competitors
cannot
coexist”
”Niche”
• Species’ ecological situation
• Abiotic variables (e.g. soil moisture or nutrients)
• Biotic interactions (e.g. competition or interactions with
predators, mycorrhizal fungi etc. etc.)
• [gaps for establishment ... ”regeneration niche”]
Niche separation
•
•
•
•
“Complete competitors cannot coexist"
Low-nutrient and dry conditions are stressful
Competetion for nutrients leads to ...
Co-existence of functionally different species through
niche separation: different ways of acquiring the scarce
nutrients (functional diversity)
• Highly diverse systems are often nutrient-poor
[In the short term, sorting of existing species and, in the long term ”character
displacement” and speciation]
[Fundamental and realized niches]
Functional diversity
What kind of things do organisms do within a community
or ecosystem?
”Traits” (functional or otherwise) = particular
characteristics of an organism (influence a species’
fundamental niche)
§ Rooting depth, ways of acquiring nutrients, leaf area, plant height, leaf
thickness, how seeds are dispersed etc. etc.
■ adaptation to different environments
■ competition ■ dispersal
Communities can be characterized in terms of the diversity of
functional traits (”functional diversity”)
Functional differentiation
Functional diversity
Intermediate disturbance hypothesis
Highest diversity at ”intermediate” levels of disturbance
”Level” can refer to the intensity, frequency or size of the
disturbance ... or the time since disturbance
Mixture of different processes (dispersal and competition)
e.g. Gentle (”intermediate”) disturbance keeps fine scale habitat
heterogeneity (microhabitat diversity)
e.g. Open ground. Initial colonization by a few species  more
species arrive and the first ones hang on  the environment
changes as succession proceeds (soil changes that may be
influenced by the plants themselves).  the vegetation
becomes closed and there are less gaps for establishment
e.g. Productivity: (disturbed) low productivity  intermediate 
high productivity (competitive exclusion)
Connell 1978: Science
• Intuitively sensible hypothesis, but can be explained, or
viewed in a range of different ways
• Has been a lot of recent controversy about this
hypothesis
• Different ways of looking at ecology: ”empirical” (based
on data and observations) and ”logical” (based on
theories about how nature should work)
• Interplay between productivity and disturbance
Huston 2014 Ecology
Species pool
• If we have a set of species within a region or local area,
what determines whether they will occur in a particular
patch of apparently suitable habitat?
• Dispersal gives access to the habitat
• History of loss plus fragmentation coupled with a poor
dispersal ability that stops recolonization...
• Real habitat specialists ... Habitat may not actually be
suitable
• Local extinction ... lose species from the regional or local
pool
Regional Species Pool
Zobel 1997 Trends Ecol. Evol.
Landscape and habitat fragmentation
• ”Landscape modification and habitat
fragmentation are key drivers of global species
loss” (Fischer & Lindenmayer 2007: Global Ecol. Biogeogr.)
• Connectivity, patch size, edge effects, habitat
heterogeneity, local extinction
Fragmentation
Patches of habitat are large and close to
each other: they support large
populations
Patches become smaller and more
sharply separated: they support smaller
populations
The distance between fragments
increases: the smallest patches can no
longer support populations: dispersal
and gene flow are limited
Edge effects
• Edge effects: fertilizer and nutrients spread in
from adjacent habitats
• Unexpected effects of fragmentation on
species diversity?
Edge effects
• Edge effects: fertilizer and nutrients spread in
from adjacent habitats
• Unexpected effects of fragmentation ...
• High diversity in small fragments as a result of
edge effects
• What ”kind” of diversity are we interested in?
• Habitat specialists
Spatial structure in a fragmented
landscape
• Fragment size
– Habitat heterogeneity
– Probability of colonization
– Reduced edge effects (habitat quality)
• Connectivity
– Dispersal
– Realized connectivity
• Surrounding habitat matrix
– Local species pool
Fragmentation and stochastic
processes
• As the area of habitat become smaller and
fragments of habitat become more and
isolated
• Edge effects may reduce the suitability of the
habitat
• Species will be lost (increasingly randomly)
from small habitat patches
Extinction
• Local extinction (loss of a species from a habitat
fragment)
• Then regional extinction
• Gradual process: not just all individuals that die
suddenly
• Fragmentated habitats: what influences the
probability of local extinction?
– dispersal, connectivity, landscape context
• Loss of genetic variation contributes to the
problem of small populations. Genetic diversity is
important!
Extinction debt
• Habitat fragmentation ... fragments were once
part of a larger, more connected area of
habitat
• ”Inertia” .. the properties of the communities
in the fragments still reflect earlier times
• What will happen to species richness if there
is no further habitat fragmentation in the
future?
Grasslands
•
•
•
•
•
”Truncated succession”
Grazing
Low levels of nutrients
Intermediate disturbance
Fine-scale habitat heterogeneity ... gaps (”regeneration
niche”)
• Fine scale niche-differentiation (competition for nutrients)
[habitat fragmentation and history]
• Grassland specialists and habitat generalists
Dark diversity
• Regional species pool contains many species
that are ecologically suited to a particular
habitat ...
• But samples from plant communities typically
innclude only a proportion of the species that
could be there: the missing species = the
”Dark diversity”
– Functional traits that explain the dark diversity?
Neutral model
• Everything depends on neutral (not influenced
by ecological processes) processes such as
dispersal and species’ physiological tolerances.
Competition unimportant
• The model depends on the assumption that
dispersal is neutral and that dispersal traits
are non-adaptive (i.e. not subject to natural
selection)
Dispersal
But neutral (i.e. non-adaptive and random) processes
are contributing to patterns of distribution and
diversity on all spatial and temporal scales.
Examples
Large scale: immigration history and chance
Very fine scale: the processes involved in fine-scale
establishment in grassland. Small gaps and the
probability of a seed from a particular species finding
a gap.
The carousel model
5-year study, grasslands on Öland
“Species mobility on the scale of the small [0.001 m2 and 0.25
m2]quadrats used implies both appearance and disappearance
[of species] from these quadrats.”
"We postulate that in homogeneous, grazed, nutrient- and
water-deficient environments many species can reach virtually
all microsites within the plot, which we express through
the idea of the ‘carousel model’. We also question the
usefulness of the niche concept and re-interpret it by stating
that all species of this plant community have the same habitat
niche, while most of them are short-lived and have the same
regeneration niche. The essential variation amongst the
species is their individual ability to establish or re-establish by
making use of favourable conditions appearing in microsites in
an unknown, complex spatio-temporal pattern.
Van der Maarel, & Sykes 1993. Small-scale plant species turnover in a limestone grassland: the carousel model and some comments
on the niche concept. Journal of Vegetation Science 4: 179-188.
Characterizing species diversity
• Alpha and Beta diversity
• Within and between ”samples” or habitats
• Alpha diversity = ”Species diversity”
• Beta diversity = ”Species turnover”
[”Quadrats”
= samples]
■ Range expansion; immigration history (a lot of chance
events (non-adaptive processes))
■ Basic ecological amplitude: can the species grow in a
region? Abiotic environmental factors (adaptive
variation)
■ ■ Get the species into the regional species pool (chance +
general ecology and availability of suitable habitats)
■ Don’t lose species as a result of habitat fragmentation
(stochastic processes)
■ Habitat heterogeneity
■ Biotic interactions (competition)
■ All this goes on on different spatiotemporal scales!
Agricultural intensification & Habitat heterogeneity
Farmland biodiversity:
is habitat heterogeneity the key?
Benton, Vickery & Wilson 2003
TRENDS in Ecology and Evolution: 18
”Dark Diversity”
Dark diversity: shedding light on
absent species
Pärtel, Szava-Kovats & Zobel 2011
TRENDS in Ecology and Evolution: 26
General text book
Land Mosaics
R.T.T. Forman (in the library)