Are tree plantations promoting homogenization of mammal assemblages between regions with contrasting environments?

Aim

The expansion of agriculture is promoting the loss of natural environments and their biotic homogenization. We aimed at understanding whether the replacement of forests and grasslands by tree plantations leads to biotic homogenization of mammal assemblages of two contrasting Neotropical ecoregions or if dispersal or environmental limitations keep their original assemblages clearly differentiated.

Location

Argentina, Upper Paraná Atlantic Forest, Southern Cone Mesopotamian Savannas and Iberá marshes

Taxon

Mammals

Methods

We conducted two camera-trap surveys, deploying 184 camera-trap stations in continuous forest, fragmented forest, and pine plantations in the Atlantic Forest, and 234 in grassland, fragmented forest, and pine plantations in the Mesopotamian Savannas. We evaluated the similarity of the assemblages among the environments and regions, generating all possible pairwise comparisons using three similarity indices: Sørensen (species identity), Horn (common species), and Morisita-Horn (dominant species). Using variation partitioning diagrams and redundancy analysis, we evaluated the spatial structure of mammal assemblages and the influence of environmental variables.

Results

There was a greater similarity in species identity between different environments within each region than between similar environments in different regions. Common and dominant mammal assemblages of tree plantations tended to be similar between regions and were different from assemblages of the natural environments within the same region. Fragmented forest assemblages were very similar among regions. Assemblages were spatially structured but most of the variation between regions was explained by the environmental variables.  

Main conclusions

Each region has a distinct pool of species, which is partially explained by environmental factors, such as the differential representation of native environments in the landscape. However, an expansion of tree plantations and forest fragmentation in the Atlantic Forest could lead to biotic homogenization between regions due to an increase in the abundance of generalist species.

Methods
We conducted camera-trap (Reconyx HC500) surveys in two contrasting ecoregions ("Region"): in the Upper Paraná Atlantic Forest ecoregion ("Atlantic Forest" in the dataset), and in the Southern Cone Mesopotamian Savannas and the Iberá marshes ("Mesopotamian Savannas" in the dataset).  

In the Atlantic Forest, we randomly distributed 184 camera-trap stations (with a minimum distance of 2 km between stations) in three main environments: continuous forest, fragmented forest, and tree plantations (Iezzi et al., 2018). Continuous forest stations (53 camera-trap stations) were located in the largest continuous native forest block (~370,000 ha), surrounded in a 2-km radius by more than 75% of forest cover. Fragmented forest stations (69 stations) were in forest remnants with varying degrees of anthropogenic disturbance, outside the continuous forest (Iezzi et al., 2019). Stations in tree plantations (62 stations) were located in 4- to 14-year-old pine plantation stands. The cameras were active between May 2013 and December 2014 for an average of 49.8 days (range: 12 - 123 days), totaling 9,171.8 camera days of effort.

In the Mesopotamian Savannas, we deployed 234 camera-trap stations, at least 2 km apart, in three environments: native grassland (89 camera-trap stations), fragmented forest (54 stations), and tree plantations (91 stations; Iezzi et al., 2020). Stations in tree plantations were immersed in 1- to 30-year-old pine plantation stands. Approximately half of the stations within each environment were located in fields with cattle (N=107; Di Bitetti et al., 2020). Cameras were active between May 2016 and March 2017, with a  mean effort of 44.9 days (range: 21 - 67 days) per station, totaling 10,493.9 camera-trap days.

Stations were not baited and were located off-road, attached to a tree trunk or stake at 25-50 cm above ground level. More than 1 h had to pass for successive pictures of a species to be considered independent records. Records of small (<200 g) sigmodontine rodents were categorized as “sigmodontines” since it was difficult to identify them at species level. We excluded the records of domestic exotic mammals and 328 records that were impossible to identify at species level. Those sites with no records (9 stations) were excluded from the dataset.

For each species, we report the relative frequency of records at each station (# of independent records / # of camera-trap days).