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SCALETOOL IntroductionDriversBiodiversityPolicies and managementConnectivity and protected areas
Differences between structural and functional connectivity How to assess connectivity - methods and tools From movement to dispersal to connectivity Connectivity and Natura 2000 Key messages for the connectivity of protected areas
 

From movement to dispersal to connectivity

Movement indicates every movement during all-day life of individuals (foraging, home-ranging, etc.). In contrast, dispersal describes a movement away from the place of birth towards another environment for reproduction. Thus, dispersal is an ecological key process and inevitable for connectivity managements.
Movement of individuals or propagules is an important phenomenon in many ecological systems. For animals, qualitatively different types of movement can be identified, including home-ranging, dispersal, migration and nomadism. While all these behaviours fulfil specific functions for the individuals in their corresponding environments, dispersal, especially, is considered a key process for many organisms if landscapes are fragmented, i.e. remaining resources are distributed in separated habitat patches. Dispersal is not only species-specific but can also be sex-biased.

Dispersal can be defined as: "movements away from the place of birth towards another location or social environment for reproduction." Thereby, dispersal leads to a redistribution of population density in the landscape and can facilitate the recolonisation of empty patches or the "rescue" of small populations by receiving immigrants. In this context, Taylor et al. (1993) coined the term "landscape connectivity" as "the degree to which the landscape facilitates or impedes movement among resource patches". One aspect of this concept is the arrangement of patches in the landscape (structural connectivity). In contrast, functional connectivity focuses on the ability of individuals to disperse within this spatial setting from a species-specific perspective. In many cases, connectivity is considered important for the survival of species in fragmented landscapes. Correspondingly, large efforts are undertaken regarding the conservation and restoration of connectivity both in ecological research and in applied conservation (Levins 1966, Hanski 1999, Fahrig 2003, Henle et al. 2004, EEA 2014).

Assessing the interaction of dispersal and connectivity using RangeShifter

Within the SCALES-project, the individual-based, spatially-explicit population model RangeShifter was developed, which can investigate how the landscape, dispersal, and demography jointly determine connectivity (Bocedi et al. 2014). Assessing the impact of dispersal on connectivity, RangeShifter was used to simulate a hypothetical species inhabiting a woodland network within a highly anthropogenic landscape (see Figure). Assumptions made regarding demographics, dispersal behaviour, and habitat-dependent movement mortality can substantially alter projected outcomes. The example simulates a patch-based, stage-structured population exhibiting density-dependent emigration and which was initialised in a single patch (highlighted in red in Figure a) over a simulation period of 100 years. The outcomes demonstrate the importance of considering multi-generation connectivity.

Connectivity depended upon how dynamics and dispersal were modelled. A sexual species, whose individuals had to find a mate in order to colonise new patches, had occupied only 17 ± 3.0% of suitable patches after 100 years (see Figure b). When females settled as soon as they found a patch of suitable habitat and males settled only in patches with female(s) present, the overall patch occupancy increased to 27 ± 7.5% (see Figure c). In a female-only model, where mate-finding no longer limited colonisation, patch occupancy increased to 64 ± 6.5% (see Figure d).

These results were obtained assuming that dispersing individuals responded to the landscape in terms of movement choices, but that per-step mortality was constant across different landscape types. Introducing potentially more realistic habitat-specific movement mortalities, specifically higher across roads, urban and arable (see Figure e), substantially reduced network connectivity, both in terms of overall patch occupancy (6 ± 2.4%) and waiting time to colonisation (see Figure b,e, right-hand panels).

These simulation experiments demonstrate that connectivity is a joint product of demographic and dispersal traits as well as landscape structure and composition. Consequently, tools like RangeShifter, with options to represent these different aspects, are a step forward towards more biological realism in connectivity research.

Results of a simulation experiment with RangeShifter. Colors indicate the proportion of suitable patches occupied after 100 years (left) or the waiting time to colonisation (right). For details please refer to the text.



References

Bocedi, G., S.C.F. Palmer, G. Pe'er, R.K. Heikkinen, Y.G. Matsinos, K. Watts, and J.M.J. Travis. 2014. RangeShifter: A Platform for Modelling Spatial Eco-Evolutionary Dynamics and Species' Responses to Environmental Changes. Methods in Ecology and Evolution 5:388-96.

EEA (European Environment Agency). 2014. Spatial Analysis of Green Infrastructure in Europe - European Environment Agency (EEA). Publication. http://www.eea.europa.eu/publications/spatial-analysis-of-green-infrastructure.

Fahrig, L. 2003. Effects of Habitat Fragmentation on Biodiversity. Annual Review of Ecology, Evolution, and Systematics 34:487-515.

Hanski, I. 1999. Habitat Connectivity, Habitat Continuity, and Metapopulations in Dynamic Landscapes. Oikos 87:209-19.

Henle, K., D.B. Lindenmayer, C.R. Margules, D.A. Saunders, and C. Wissel. 2004. Species Survival in Fragmented Landscapes: Where Are We Now? Biodiversity & Conservation 13:1-8.

Levins, R. 1966. The Strategy of Model Building in Population Biology. American Scientist 54.

Taylor, P.D., L. Fahrig, K. Henein, and G. Merriam. 1993. Connectivity Is a Vital Element of Landscape Structure. Oikos 68:571-73.

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