Download BCB322: Landscape Ecology - University of Western Cape

BCB 322:
Landscape Ecology
Lecture 7:
Emerging patterns II
Ecotones
Ecotones
• First described in 1905 by Clements as “tension
zones” where principal species from adjacent
communities meet their limits (Farina, 1998)
• Importance further stressed by Odum (1959) as
transition zones between two communities.
• Situated where there is a change in the nature of
ecological transfers compared with patch interiors
(Farina, 1998)
• Basically, ecotones have species in common with
both adjacent interior communities, as well as edgespecialist species
• Tends to have higher diversity than surrounding
areas, and consequently may be interpreted as a
habitat in itself (frontier habitat) (Ricklefs, 1973)
Importance
• Provide landscape functions:
seeds & animals move
across them. & they act as
indicators of climatic shift
• High biological diversity, and
can act as refugia for
species under changing
conditions (climate/human
impacts)
http://www.geography4kids.com/misc/photos/ecot
one.jpg
• High rate of primary & secondary production, &
contribute to system integrity
• Riparian, coastal & lacustrine ecotones transmit
nutrients & water to aquatic ecosystems & back
• Understudied, but provide a good arena for study of
natural communities.
• Importance realised, & preserved in modern
conservation plans where possible
Basic concepts
• Areas where energy & material exchange are at a
maximum in a heterogeneous landscape
• In terrestrial systems, they correspond with changes
in soil/vegetation, & in heterogeneous systems are
situated at patch edges (structural)
• May also be functional, & may separate areas of
different maturity
• Found at all scales, both temporal & spatial
• Can be thought of as acting as “cell membranes” or
barriers (Forman & Moore, 1992), ensuring active &
passive transport between patches.
• Previous ecological measures considered archetypal
terrain (interiors), but understanding of landscape
ecology shows importance of boundaries between
regions
Difficulty of study
• Since they are temporary, and because they scale so
broadly, they can be hard to study
• Often structure & function not even related to physical
patterns, particularly as a result of human activity
• Obvious characteristics visible from the air are lost on
closer examination
• Because it is an edge, it
is hard to define the
boundaries of an
ecotone: some can be
fairly short, and others
broad even whilst
examining at the same
spatial scale
• Thus edges are
generally considered to
occur where contrast
between patches is
http://www.class.uidaho.edu/italy2004/ecology_files/ecotone.jpg
greatest (species
specific)
Species specificity
Turner et
al. 1990
• (a) ants, (b) mammals, (c) lizards (d) birds in a
grassland (2700m transect sampled at 30m intervals)
• Peaks correspond to discontinuities in resources/
habitats (pairwise sampling)
Heirarchical structure
• Spatial:
– Biome (blending of patches of different shapes & sizes from
adjacent biomes)
– Environmental mosaic (within biome: the shape & size of
internal patches)
– Patch (transition between patches)
– Population (for species with patchy distribution/ territory)
– Individual (eg: tree with localised water conditions or
allellopathy)
• Temporal:
–
–
–
–
–
104 years: climatic shift changes species distributions
103 years: historical movement of human civilizations
102 years: coastal/river dynamics
10 years: flooding regimes
Seasonal: snowmelt & water availability
Controlling factors
• Controls may be external or internal
• External include edaphic, microclimate,
human disturbance
• Intrinsic can be species-specific or
more broad-based
• Allelopathy: pines/eucalypts secrete
phenolic inhibitors into the soil, &
mosses lower pH (to 3!) of water in
contact with them to limit invasion of
other species
• Reproductive aggressiveness may limit
seeding of other plants (Arundo donax,
Phragmites, communis)
• Sediment capture from wind (dune
areas) or water (riparian) by certain
http://thegreencuttingboard.bl
ogspot.com/Arundo-donax.jpg
species, after microbial intervention,
can change nutrient cycles
• Animal activity (grazing, digging, seed predation, forest clearance)
play a role (eg: tree destruction by elephants creates shrubby
ecotones)
• Most internal factors still driven by external (moss may change pH,
but external conditions must still favour its growth initially)
Character of ecotone
• (a) gradual - common in
nature, such as transition
from grassland to woodland
• (b) discontinuous –
common where a species
reaches tolerance limit
(such as with salinity,
temperature, soil pollutants,
water levels) or biological
competition
• (c) hysteretic – occurs
where there is some
degree of latency in the
Shugart, 1990
system
• (d) multiple responses – common where other
characters are also relevant, or where there is a
complex evolutionary/historical relationship
Permeability
• Ecotones are important for
movement of both animal species
& materials/energy (such as
resources moved by ants from one
patch to another
• Permeability: a measure of the
capacity of an ecotone to deflect
movement of a vector (Wiens et al,
1985)
• Differs with vector strength (wind,
water), and material will be
deposited where kinetic drop is
Wiens, 1992
greater
• Topography affects dimension of the energy/material fluxes through
an ecotone due to kinetic energy & surface:volume ratio
• Eg: beaver dams have a high s:v ratio, increasing changes at
borders. Also, reduced kinetic energy results in deposition of
sediment
• High contrast between patches causes true impermeable barriers
(common in human-induced areas, rare in nature)
Animal movement
• Movement across ecotones is non-random & complex
• Depends on:
– Passive diffusion: depends on patch character, diffusion rate &
heterogeneity
– Active diffusion: active movement, dependent on movement rate
& type, organism density in patch & habitat preferences
– Probability of edge encounter: Animals may not encounter
ecotones – depends on arrangement, shape & size
– Decision to cross: can
be modelled as a costbenefit analysis given
species-specific
predation & resource
constraints
• Permeability tends to
increase with
increasing animal body
size.
http://home.earthlink.net/~gastropod/bvrdm.JPG
Role in landscape
• Modify type, extent & direction of movements between
landscape components
• (eg: riverine woodland increases field stability for
agriculture, whilst at the same time providing habitat for
the insects that feed on crops)
• Reduce negative impacts of wind & some disturbances
on patches, & modify soil & temperature character
• Edge effects generally increase biodiversity (not for all
species), and so may act as refugia for species.
• (eg: amphibians tend to live in ecotones, spend their
lives in terrestrial habitats, but move to ponds to breed
• Likewise, birds may roost in forest but feed in ecotones.
• Often possible to predict overall biodiversity by
examining ecotone density.
• Greatest biodiversity occurs with optimal blend of
patches & ecotones
Ecotone density
Naiman et al., 1988
Climate change
• Since ecotones may be edges of
species tolerances, range shifts
can be observed here first
• Ecotones used to study vegetation
shift (Delcourt & Delcourt, 1987)
• Ecotones have differing fragility:
mountain ecotones may show
change first
• Increasing temperatures will dry
soils. Although many species can
adapt their ecophysiology, broad
scale changes will result
• Biome-level ecotones will
experience reduced biodiversity
http://www.aphasiahelp.org/ourpages/brett/images/ko
kerboom_quivertree.jpg
• The quiver tree (Aloe dichotoma) is dying on the northern
extremes of its range, and advancing on the southern
ecotone (Midgely et al., 2005)
Economics
• Historically settlement is associated with ecotones
(lakes, river deltas, sea coasts) (Desaigues, 1990)
• Productivity of a system is assured by ecotones
• Removing ecotones was useful (lagoons, marshes,
woodland matrices) because it made land available, but
has long term effects in breaking nutrient/water cycles
• Eg: removal of riparian forest increases water
eutrophication as well as the chance of floods
• Hedges in Britain acted as windbreaks and changed
microclimate of soil, but agriculture intensification has
caused their removal
• Large ecotones (marshes & river meanders) are too
expensive to recover
• Hence it is important to preserve these systems to act
as functional entities & biodiversity refugia
Summary
• Transition zones between adjacent systems, with
characters defined by scale & interactions between
these zones
• Situated where rate & dimension of ecological transfers
changes
• Found at all spatial (biome to individual) & temporal
(millennia – daily) scales
• Hard to study because patterns vanish under close
examination
• Selectively permeable to organisms and abiotic factors
• Play a vital role in human-disturbed landscapes for
biodiversity maintenance
• Display changes in climatic conditions because species
exist at the edge of their ranges
• Zones in which human activity developed are
economically useful structures
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References
Clements, F.C. (1905) Research methods in ecology. University Publishing Co. Lincoln,
Nebraska, USA
Desaigues, B. (1990) The socio-economic value of ecotones. In: Naiman, R.J. & Decamps,
H. (eds). The ecology and management of aquatic-terrestrial ecotones. MAB, UNESCO,
Paris
Farina, A. (1998) Principles and Methods in Landscape Ecology. Chapman & Hall, London,
UK
Forman, R.T.T. & Moore, P.N. (1992) Theoretical foundations for understanding boundaries
in landscape mosaics. In: Hansen, A.J. a& di Castri, F. (eds). Landscape boundaries.
Consequences for biotic diversity and ecological flows. Springer-Verlag, New York
Midgley, G.M., Hughes, G., Thuiller, W., Drew, G. & Foden, W. (2005) Assessment of
potential climate change impacts on Namibia’s floristic diversity, ecosystem structure and
function. SANBI, Cape Town, UK
Naiman, R.J., Holland, M.M., Decamps, H. & Risser, P.G. (1988) A new UNESCO program:
research and management of land:inland water ecotones. Biology International, Special
Issue 17: 107-136
Odum, E.P. (1959) Fundamentals of ecology, 2nd editions. W.B.Saunders Company,
Philadelphia
Ricklefs, R.E. (1973) Ecology. Chiron Press
Shugart, H.H. (1990) Ecological models and the ecotones. In: Naiman, R.J. & Decamps, H.
(eds). The ecology and management of aquatic-terrestrial ecotones. MAB, UNESCO, Paris,
Turner, S.R., O’Neill, R.V., Conley, W., Conley, M., & Humphries, H. (1990 ) Pattern and
scale: statistics for landscape ecology. In: Turner, M.G. and Garnder, R.H. (eds).
Quantitative methods in landscape ecology: the analysis and interpretation of landscape
heterogeneity. Springer-Verlag, New York
Wiens, J.A., Crawford, C.S. & Gosz, R. (1985) Boundary dynamics: a conceptual
framework for studying landscape ecosystems. Oikkos 45: 421-427