Tucker at al. 2018

Moving in the Anthropocene: Global reductions in terrestrial mammalian movements

Blog author: Devra Hock

Author:

Marlee A Tucker: Senckenberg Biodiversity and Climate Research Centre, Senckenberg Gesellschaft für Naturforschung, 60325 Frankfurt (Main), Germany

            Post-doctoral researcher in movement ecology at the Senckenberg Biodiversity and Climate Research Centre and Goethe University, Frankfurt, Germany

            PhD at Evolution and Ecology Research Centre at the University of New South Wales, Australia

            

            Research Interests: Aim to examine animal movement patterns and behaviors from a macroecological perspective. More broadly, interested in large scale patterns in ecology, biogeography and evolution that can aid our understanding of species vulnerability to changing environments that can be utilized for conservation. This includes global patterns in species richness, species extinction risk, energetics and allometric scaling.

Summary/Main Points:

1. Main Question:

            Background— Earth’s surface has been modified 50-70% by human activities, causing changes in biodiversity and ecosystem functions. Human footprint is impacting loss of habitat and biodiversity and how animals move through fragmented and disturbed habitats. Previous research on the extent of animal movements and how they are affected by anthropogenic impacts on landscapes has been done in local geographic regions or within single species. These studies display a decrease in animal movements as a result of habitat fragmentation, barrier effects, or resource changes. Only a few studies reporting animal movements increasing. 

            Main Questions— Conducted a global comparative study of how the human footprint affects movements of terrestrial nonvolant mammals. For each mammalian individual, locations were annotated with the Human Footprint Index. Also included other covariates that are known to influence mammalian movements: environments with lower productivity, body size, dietary guild. 

2. Methods: 

            Human Footprint Index—an index with a global extent that combines multiple proxies of human influence: the extent of built environments, crop land, pasture land, human population density, nighttime lights, railways, roads, and navigable waterways. HFI ranges from 0 (natural environments) to 50 (high-density built environments)

            Normalized Difference Vegetation—well-established, satellite-derived measure of resource abundance for both herbivores and carnivores

            Calculated displacements as the distance between subsequent GPS locations of each individual at nine time scales ranging from 1 hour to 10 days. For each individual at each time scale, calculated the 0.5 and 0.95 quantile of displacement. Examine the effect of human footprint on both the median and long-distance movements for within-day movements (1 hr time scale) up to longer time displacements of more than 1 week (10 day). 

            Used linear mixed-effects models that accounted all covariates, taxonomy, and spatial autocorrelation

3. Results:

            Found strong negative effects of the human footprint on median and long-distance displacements of terrestrial mammals.

            Displacements of individuals across species living in areas of high footprint were shorter than displacements of individuals in low footprint by as much as a factor of 3.

            Median displacements for carnivores over 10 days were 3.3km in areas of high footprint versus 6.9 km in areas of low footprint

            Maximum displacements for carnivores at 10 days averaged 6.6 km in areas of high footprint versus 21.5 km in areas of low footprint. Effect significant on all temporal scales with 8 hrs or more between locations.

            Effect not significant at shorter time scales, suggesting human footprint affects ranging behavior and area over longer time scales, rather than altering individual travel speeds

Human footprint index was separated into two components: the individual behavioral effect represented by individual variability of HFI relative to the species mean, and the species occurrence effect as the mean HFI for each species. 

Results indicate behavioral as well as species effects. Significant behavioral effect on median displacements and on long-distance displacements at most time scales were observed. Species occurrence effect was only significant over longer time scales. 

Body mass, dietary guild, and resource availability were also related to movement distances. Larger species traveled farther than smaller species. Also, a negative relationship between resource availability and displacement, such that movements were on average shorter in environments with higher resources. Carnivores traveled on average farther per unit time than herbivores and omnivores. For all variables, effects were significant across time scales longer than 8 hrs for both median and long-distance displacements.

4. Discussion/Conclusion:

            Reduction of mammalian movements in areas of high HFI stems from two nonexclusive mechanisms: 1) movement barriers such as habitat change and fragmentation, and 2) reduced movement requirements attributable to enhanced resources. Both mechanisms vary responses across populations or species, acting together on single individuals or populations. 

            Consequences of reduced vagility affect ecosystems regardless of underlying mechanisms and go beyond the focal individual themselves. Animal movements are essential for ecosystem functioning because they act as mobile links and mediate key processes such as seed dispersal, food web dynamics, and metapopulation and disease dynamics. 

            The global nature of reduced vagility across mammalian species that is demonstrated in the paper suggests consequences for ecosystem functioning worldwide. 

Questions/Comments:

             I thought this research was presented very clearly. The graphs are also very illustrative of the patterns in the data, especially the two groups visible in the Human Footprint Index and the Normalized Difference Vegetation Index. 

            One thing I would have liked to see discussed is the relationship between Human Footprint Index and the Normalized Difference Vegetation Index. HFI measures the level of human impact on a variety of factors, while the NDVI looks at level of productivity/resource abundance. Very similar patterns are seen with animal displacement and I wonder what everyone thinks about the relationship between these two indices.