To understand the effects of habitat fragmentation, people often look through the lens of species-area relationships and focus on individual patches. While this is a useful approach, it doesn’t necessarily help to understand how fragmentation might affect the landscape as a whole unit.
A species-habitat network is a tool to envision how habitat fragmentation might affect an entire landscape, rather than just an individual species or site.
Palmeirim, A.F., Emer, C., Benchimol, M., Storck-Tonon, D., Bueno, A.S. and Peres, C.A. 2022. Emergent properties of species-habitat networks in an insular forest landscape. Science Advances 8(34): eabm0397.
Within the network, each species in the landscape is a node that is linked to another set of nodes matching the habitat patches where that species occurs. By creating this network, the properties of the entire landscape can be quantified into metrics such as connectance or modularity. These can then give a better understanding of ecosystem properties such as function and resilience.
Species-habitat networks may be useful in theory, but they need to be tested with empirical data. A recent study does so by using an insular fragmented forest landscape in Brazil to examine the emerging properties of species-habitat networks.
The Balbina Hydroelectric Reservoir was created in the late 1980’s in Central Brazilian Amazonia by damming the Uatumã River and creating a landscape of over 3,500 forested islands surrounded by an open-water matrix. Thirty years later, researchers analyzed surveys from 22 of the forest islands, which supported over 600 species including medium-large mammals, nonvolant small mammals, lizards, understory birds, frogs, dung beetles, orchid bees, and trees. This data allowed them to create a species-area network, which quantified what was going on the landscape as a result of the fragmentation.
In this case, habitat fragmentation resulted in simpler species-habitat networks – generally not a good thing in terms of ecosystem stability and resilience. The emergent properties of the species-habitat network showed that there were widespread, nonrandom local species extinctions. This was seen from a combination of metrics: low connectance (the rate between actual and potential species occurrences), low modularity (the strength of module formations comprised by subgroups of patches occupied by the same species), and high nestedness (the degree to which the network is formed by sites and species with fewer interactions that are connected to species-richer sites and widely distributed species).
In addition, simulations that removed islands from the landscape by size in order from large to small showed low network robustness (tolerance to secondary extinctions following area-driven habitat-patch removal from the landscape). Species declined much faster when sites were removed from the largest to the smallest than from the smallest to the largest.
Species persistence varied a lot across taxa, emphasizing the still-complex nature of trying to quantify interactions at a landscape scale. As expected based on traditional methods, forest area and body size remained important drivers structuring the network. Despite the variation across so many taxa, the species-habitat network remains a useful tool for determining how large-scale fragmentation may disrupt ecosystem functioning at the landscape scale.
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