Seed dispersal is a key process affecting the structure, composition and spatial dynamics of plant populations. Numerous plant species in the tropics rely upon animals to disperse their seeds. Humans have altered mammalian movements, which will likely affect seed dispersal distances (SDD). Altered SDD may have a range of consequences for plant communities including reduced seedling recruitment and plant biomass, seed trait homogenization, altered gene flow and a reduced capacity to respond to environmental changes. Therefore, modelling the consequences of altered animal behaviour on ecosystem processes is important for predicting how ecosystems will respond to human impacts. While previous research has focused on the link between animal species extirpation and SDD, it remains unclear how changes in mammalian movement will impact SDD. Here we implemented a mechanistic modelling approach to examine how mammalian movement reductions impact SDD in the tropics. We combined allometric theory with a mechanistic seed dispersal model to estimate SDD via the movement of 37 large frugivorous mammals (> 10 kg) in the tropics under different levels of human footprint, a global proxy of direct and indirect human disturbances. Our results suggest that assemblage-level SDD reductions are estimated to be up to 80% across the tropics in response to human disturbance. This is particularly the case in areas with high human impact such as agricultural landscapes and suburban areas. The region with the largest reductions in SDD was the Asia-Pacific with average reductions of 25%, followed by Central-South America (16%) and then Africa (15%). Our study provides insights into how human-induced changes in movement behaviour of large mammals could translate into altered ecosystem functioning.

This repository contains the code and data to run an IBM simulating mammalian seed dispersal in the tropics from Tucker, M.A,., Busana, M., Huijbregts, M.A.J., & Ford, A.T. (2021). Human-induced reduction in mammalian movements impacts seed dispersal in the tropics. Ecography. 10.1111/ecog.05210

You will need both .R files to run the simulations ("Tucker_SDD_Sims_Run.R","Tucker_SDD_Sims_Source.R").

Example data is provided to illustrate which information is required to run the simulations ("ExampleSimulationData.csv"). The HFI (Human Footprint Index) and NDVI (Normalised Difference Vegetation Index) data can be values set by the researcher, or values extracted from a raster. The step length (distance travelled per hour in metres) is calculated using the equations from the above manuscript.

The species-level data that was used to run the simulations is also included ("SpeciesData.zip"). The species-level data spans 37 terrestrial mammals (>10kg) across the tropics. The HFI data was extracted from 1 km global human footprint index (HFI for 2009; Venter et al. 2016). The HFI and NDVI data were extracted from each grid cell across the entire range of each species based on the IUCN range data (IUCN, 2019).

The NDVI data was extracted from Copernicus 1 km PROBA-V global normalized difference vegetation index (NDVI) data (1999–2017 average; Jacobs and Wolfs 2019). 

References:

IUCN (2019). The IUCN Red List of Threatened Species. Version 2019-2. https://www.iucnredlist.org. Downloaded on February 2019.

Jacobs, T. and Wolfs, D. 2019. Normalized difference vegetation index: long term statistics 1 km: GLOBE 1999–2017 (V2.2.1). – <https://land.copernicus.eu/global/products/ndvi>, accessed 17 April 2019.

Venter, O. et al. 2016. Sixteen years of change in the global terrestrial human footprint and implications for biodiversity conservation. – Nat. Comm. 7: 12558.