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 edge-specialist 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/

ecotone.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 greatest (species specific)

http://www.class.uidaho.edu/italy2004/ecology_files/ecotone.jpg

Species specificity

• (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)

Turner et al. 1990

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 species, after microbial intervention, can change nutrient cycles

http://thegreencuttingboard.blogspot.com/Arundo-donax.jpg

• 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 system

• (d) multiple responses – common where other characters are also relevant, or where there is a complex evolutionary/historical relationship

Shugart, 1990

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 greater

Wiens, 1992

• 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 cost-benefit 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/

kokerboom_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

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