Exploring Landscape Heterogeneity: Patterns, Impacts, and Functions

1. BCB 322:Landscape Ecology Lecture 6: Emerging patterns I: Heterogeneity

2. Heterogeneity • Patterns arise in any landscape as a result of the underlying processes • Disturbance and fragmentation are closely allied, and have significant impacts on the environment • Heterogeneity is the main pattern in any landscape, & is inherent at all scales • “The uneven, non-random distribution of objects” (Forman, 1995) • Since every pattern in a landscape results from, and produces new processes, heterogeneity is important for landscape function • The analysis of heterogeneity is fundamental to the understanding of landscape functions and spatial ecological processes non-random

3. Heterogeneity • Three types to be considered: • Spatial: variation in space, either horizontally (as under human disturbance regimes) or vertical (uneven vegetation distribution above ground – generally natural) • Temporal: similar to spatial, but implies variation for a http://www.class.uidaho.edu/italy2004/ecology_files/patchSteppingStones.jpg single point over time. Two areas may have the same spatial variation, but differ in time. • Functional: variation in distribution of communities/ populations. Linked to life history of organisms • Soil composition is a strong driver of heterogeneity, & it varies strongly from individual plots (1-10m) to the full landscape (Becher, 1995). It can also vary vertically in the soil profile • The effect of geological heterogeneity is unpredictable, and a probabilistic approach is used when modelling it

4. Heterogeneity • By creating borders and edge effects, additional processes are set up in the landscape, influencing the flux of materials • Since plant and animal species respond fairly rapidly to changes in mosaic heterogeneity, minor variations can often be observed with remote sensing techniques. • Effectively, heterogeneity can exaggerate biological-environment interactions (eg: skylarks avoid small prairie spaces, even though they are functionally similar to larger patches) (Farina, 1998) • Heterogeneous landscapes can show several different stable configurations, and can shift between them rapidly given sufficient incitement. • This is termed a polyclimactic state, and it is determined by variations in deterministic & stochastic factors (wind, climate extremes, climate change, edaphic conditions) and internal factors (disease, predation, invasion)

5. Disturbance • Hence disturbance can play a role in maintaining heterogeneity and preventing stable equilibria developing. • A moderate disturbance regime can increase heterogeneity, but it depends on the initial conditions • Homogeneous system: heterogeneity increases to the midpoint, then falls • Highly heterogeneous: after an initial gain in heterogeneity, there is a rapid dropoff (Kolasa & Rollo, 1991)

6. Disturbance • Obviously, some disturbance can be useful for maintaining heterogeneity. • Likewise, heterogeneity can impact on the disturbance regime: • fires spread less readily in a mixed woodland than in a pure coniferous matrix (reduced disturbance) • predators (eg: foxes) in woodlots & parklands in an agricultural environment can impact on livestock (increased disturbance) • Species distribution depend on community heterogeneity: large sample areas include more species, and thus should be more similar than smaller ones with a limited selection of species. http://pelotes.jea.com/AnimalFact/Mammal/Fox01.jpg

7. Structure • Three main types of heterogeneity structure (Addicot et al., 1987): • Divided homogeneous (suitable patches in an unsuitable matrix) • Undivided heterogeneous (patches of varying suitability) • Divided heterogeneous (varying suitability patches in an unsuitable matrix

8. Scale • Fire, grazing & the two in combination were tested in grasslands (Glenn et al, 1992) • Locally, burning seemed to have higher heterogeneity than grazing, whilst the corollary was true at a regional scale. • Overall, untreated local plots had the most heterogeneity, but regional responses varied to a large degree, depending on season of burning (spring burning then grazing increased heterogeneity, autumn burns reduced it) • This variation makes studies complicated – some processes depend on patchiness, but not all. • General, organismal responses are a good measure by which to assess relevant scale. (neighbourhood scale) • Neighbourhood for vagile species is obviously their territory or resource area • For sessile species it is more complex, but it can be estimated according to the areas from which food, predators and other foragers come.

9. http://www.rangemagazine.com/features/spring-05/pics/1HWANGE1badland.jpghttp://www.rangemagazine.com/features/spring-05/pics/1HWANGE1badland.jpg Animals • Spatial heterogeneity is one of the main factors determining bird species diversity in the landscape (MacArthur et al., 1962) • Bird diversity was also higher in shrubby areas because shrubs have higher heterogeneity than tree areas (even though trees have more variation in canopy structure) • Animals also drive heterogeneity: grazing animals at high densities can permanently alter vegetation cover by compacting soil and removing vegetation (eg: rural goat grazing in Zimbabwe) • Animals can even alter geomorphology, & all sizes can impact heterogeneity: • Insects (ants & termites shift soil, dung beetles enrich soil) • Small mammals (moles & rabbits digging soil, other rodents distributing seed) • Medium mammals (beaver dams, pigs rooting) • Large mammals (elephants/buffalo opening up cover) • All sizes of birds affect seed distribution & enrich soil with droppings

10. Foraging • Species-level heterogeneity requires individuals to modify their foraging habits – in larger patches they can afford to spend less time foraging • Time spent within a patch varies as the square of the linear dimension of the patch, whilst travel time between them varies linearly (ie): large patches are preferable and used in a more specialised way than small patches. (MacArthur & Pianka, 1966) • For large species, it has been found that they respond to landscape-level heterogeneity, but not locally (bison, Wallace et al., 1995) • Thus, they move in a determined manner (non-randomly) between patches, but within patches move randomly, minimising energy use (optimal foraging) • Similar observations of sheep grazing showed that they generally moved directly to the nearest plant, whilst occasionally moving between plants • It is possible that heterogeneity therefore plays a role in determining optimal foraging strategies for species, and may even enhance the efficiency

11. Animal movements • Certainly, movement within a matrix is determined by suitable/unsuitable patches, preventing animals from moving in straight lines (Johnson et al., 1992) • It may also play a role in the recollection of routes in a home range (homing ability) • Bumble bees’ foraging routes are generally longer in uniform stands, whilst in a varied stand they tend to backtrack more often • Certainly digger wasps (small range) use landmarks to find their way about, and are prepared to fly further in a varied landscape

12. Animal movements • Bees exhibit a similar behaviour • Homing ability drops off fairly quickly in flat or uniform landscapes (Plowright & Galen, 1985) • In comparison, in mountainous areas they can return from as far off as 9km, due to increased landmarks

13. Metrics of heterogeneity • We can measure the heterogeneity of a landscape in terms of any resource (soil structure, plant diversity, biomass, thicket structure, animal distributions…) • The variation in structure hence reflects changing functions and processes in the landscape • In order to assess this, we use several different metrics, each of which considers different aspects of the structure. • Fractal dimension (measures the complexity of edges) • Contagion (the extent of aggregation of patches) • Evenness (measures number of different patch types & their proportions in the landscape) • Patchiness (contrasts neighbouring patches in a matrix) • Li & Reynolds (1994) measured these different metrics against four components of heterogeneity

14. (Li & Reynolds, 1994)

15. Summary • Heterogeneity is the principal characteristic of any landscape • It varies due to underlying processes, and affects these processes in turn, by initiating or amplifying biological interactions • Can be considered in terms of temporal, spatial or functional components • Affects disturbance, although in both a positive & a negative manner • Heterogeneity affects animal processes (eg, grazing efficiency of ungulates) and is likewise affected by the interactions of animals with landscape functional components • Measured using different indices, including contagion, fractal dimension, evenness & patchiness

16. References • Addicot, J.F., Aho, J.M. & Antolin, M.F. (1987) Ecological neighbourhoods: scaling environmental patterns. Oikos49: 340-346 • Becher, H.H. (1995) On the importance of soil homogeneity when evaluating field trials. Journal of Agronomy & Crop Science74: 33-40 • Farina, A. (1998) Principles and Methods in Landscape Ecology. Chapman and Hall, London, UK • Forman, R.T.T. (1995) Land Mosaics. The ecology of landscapes and regions. Cambridge University Press, Cambridge. • Glenn, S.M., Collins, S.L. & Gibson, D.J. (1992) Disturbance in tallgrass prairies: local and regional effects on community heterogeneity. Landscape Ecology. 7: 243-251. • Johnson, A.R., Wiens, J.A., Milne, B.T. & Crist, T.O. (1992)Animal movements and population dynamics in heterogeneous landscapes. Landscape Ecology 7: 47-58 • Kolasa, J. & Rollo, C.D. (1991) Introduction: the heterogeneity of heterogeneity: a glossary. In: Kolasa, J. and Pickett, S.T.A. Ecological heterogeneity. Springer-Verlag, New York, pp 1-23 • Li, H. & Reynolds, J.F. (1994) A simualtion experiment to quantify spatial heterogeneity in categorical maps. Ecology75:2446-2455. • MacArthur, R.H., MacArthur, J.W. & Preer, J. (1962) On bird species diversity. II. Prediction of bird census from habitat measurements, American Naturalist 96: 167-174 • MacArthur, R.H. & Pianka, E.R. (1966) On optimal use of patchy environment. American Naturalist100: 603-610 • Plowright, R.C. & Galen, C/ (1985) Landmarks or obstacles: the effect of spatial heterogeneity on bumblebee foraging behaviour. Oikos44: 459-464