New research is illuminating how the movement of plants and animals across different landscapes, known as dispersal networks, plays a crucial role in maintaining biodiversity.

Most ecological models work on the assumption that life follows unvaried - or homogenous - pathways through habitats. A recent study from Concordia researchers, published in the journal Trends in Ecology and Evolution, calls for greater value to be placed on the variations - or heterogeneity - in this movement.

Variety over volume

Conservation projects often focus on how much habitat animals and plants can access, but more is not always better.

"Understanding the heterogeneity of the dispersal networks of species in a conservation project is crucial because it can have a strong influence on diversity," explains lead author Paul Savary. The Horizon postdoctoral fellow is a junior associate at the Loyola Sustainability Research Centre.

"With insufficient connectivity, species may not have access to resources required to flourish; too much and they may face insurmountable competition."

A realistic perspective on species movement

"When we observe what is happening with real species, it's a bit like how humans move in a city," Savary explains. "We may use the metro, ride a bike, walk or take a car. The places we want to access are not distributed over a regular grid. Destinations are irregularly distributed across the city."

Two factors determine the heterogeneity of dispersal networks. The first factor is the landscape. Structurally, features of a habitat, such as water sources, steep cliffs and dense woods, are not evenly distributed.

The second factor is biological. Not all species move in the same way. A bear cannot fly like a bird, and a sparrow does not fly like an eagle. This variation in movement style is referred to as "dispersal abilities". Different species cannot cross the same types of landscape elements and so do not have the same dispersal abilities.

"We see this every day," Savary explains. "Urban infrastructure can never be a perfect grid because it is constrained by features such as topography and riverbanks.

"So, we select our mode of transportation, or dispersal ability, based on how far we need to go and our perception of the obstacles to getting there, such as distance, traffic or time constraints."

Savary adds that these variables lead us to have heterogeneous dispersal networks. Those who bike to work engage in a different network than those who drive, and people who live in highly walkable neighbourhoods have smaller networks than those living in suburban communities.

If our cities were built in a perfect grid, then a higher population would distribute the load evenly across pathways. But as many of us have observed, this is not the case. Certain nodes become overcrowded bottlenecks, such as at highway interchanges and some metro stations.

Instead of a grid, Savary says dispersal networks are more like a multicoloured tangle of yarn, where each colour represents a species, and the curves of the yarn show the complexity of its movements.

Untangling the processes that underlie the patterns

Early conservation efforts focused on creating large, protected habitats that tend to be isolated from each other. More recent scholarship has highlighted the value of creating several smaller habitat patches, which together cover the same land area but can be more interconnected.

Savary's new research is co-authored with Department of Biology professors Jean-Philippe Lessard and Pedro Peres-Neto. Their study provides ecologists with clear steps on how to incorporate analyses of heterogeneous dispersal networks into their research. The approach is intended to ensure a better understanding of how to protect specific species based on their real-world behaviour.

Conservation work is no small feat, but new technologies such as growing banks of genetic data, improved satellite mapping and increased GPS tagging projects offer valuable field data to researchers.

"Most of the time we can't observe the process, we just observe patterns. But what we are interested in is the process," Savary says.

"It's a detective quest. By looking deeper at how landscape structure and species characteristics moderate processes and shape patterns, we are better able to predict the impacts of conservation efforts. Species can continue interacting, moving and fulfilling their needs."

Read the full cited paper: "Heterogeneous dispersal networks to improve biodiversity science."

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