Fragmented habitats may need to be better connected than previously thought

A model study reveals that the seeds of plants in small, fragmented habitats can abruptly evolve in such a way that they can no longer travel long distances. As a consequence, plants are less successful in reaching other habitats. This insight shows that fragmented natural areas may need to be better connected than previously assumed. The research, conducted by biodiversity researchers Monique de Jager and Merel Soons, was published yesterday in the scientific journal Ecology and Evolution.

Habitat fragmentation is a major cause of global biodiversity loss. Populations of plants and animals residing in small habitats are more vulnerable and face an increased risk of extinction. Moreover, fragmentation makes plants and animals less successful in returning to places from where they have disappeared. Additionally, fragmentation hampers the exchange of genes between populations, potentially limiting the capacity of groups of plants and animals to adapt to changing environmental conditions.

Abrupt Tipping Point

Efforts to restore nature frequently centre around establishing connections between isolated habitats. However, these measures often prove to be insufficiently effective. The new study conducted by De Jager and Soons now sheds light on a potential underlying cause: plants adapt to their confined habitats by developing seeds with limited dispersal capabilities.

De Jager explains how that works: "Initially, when a habitat becomes fragmented, it remains advantageous for plants to possess seeds capable of long-distance travel. This enables them to reach other suitable areas. Our mathematical models indeed confirm that as a large habitat fragments into smaller, increasingly dispersed habitats, seeds capable of traveling greater distances emerge. However, our models also reveal an abrupt tipping point: once the fragmentation reaches a certain threshold, the habitats become too distant from each other. The seeds are then simply unable to bridge the gap. The model suggests that seeds with limited dispersal capacities arise at this point, so that the majority of seeds end up within the suitable habitat in which they originated."

While this study is purely based on modelling, it highlights a plausible scenario that we should consider proactively.

Restoration

The researchers also employed the models to investigate the outcomes of reducing distances between habitats, such as through the restoration of intermediate areas. De Jager: "According to the model, a significant reduction in the distance between habitats is necessary to effectively reverse the effects of fragmentation. We calculated the distance at which the tipping point takes place where plants once again produce seeds capable of long-distance dispersal. It turns out that this distance between habitats is actually shorter than the distance between habitats at which the tipping point occurs when fragmentation actually increases. Hence, in order to counteract the effects of habitat fragmentation, nature must exhibit a lower degree of fragmentation than the level at which those effects initially occurred."

Mathematical models

The present study employed mathematical models to simulate the dispersal of plant seeds through water. Looking ahead, De Jager intends to explore whether such tipping points indeed manifest in nature. De Jager: "While this study is purely based on modelling, it highlights a plausible scenario that we should consider proactively."

Moreover, the researcher points out that a comparable mechanism likely operates in animals too. De Jager: "Animals have the capacity to adapt their behavior and to learn from one another. Once they have grasped that venturing beyond their habitat offers no advantage, they will stop doing it."

By offering more suitable habitats now, we can avert the adaptation of plants to confined and isolated environments.

Concrete evidence

Incidentally, there is an increasing body of evidence suggesting that habitat fragmentation can lead to a reduced long distance dispersal of plant seeds. For example, De Jager came up with the idea for the current model study when she read a scientific paper on the herb Crepis sancta. These plants generate two types of seeds: those that fall to the ground immediately and those that are transported by the wind. Interestingly, it turns out that Crepis sancta plants inhabiting small patches in urban areas exhibit a higher proportion of falling seeds compared to their counterparts outside the city. Urban Crepis sancta plants therefore appear to have been adapted relatively quickly to a fragmented environment, where wind-dispersed seeds often land on unsuitable surfaces like asphalt or stones. Seeds that fall on the ground immediately, on the other hand, settle within the same patch of fertile soil where their parent plant resides.

Prevention

According to De Jager, the model study underscores the importance of prevention over remediation. De Jager: "By offering more suitable habitats now, we can avert the adaptation of plants to confined and isolated environments. This undoubtedly represents the most favourable course of action, as once plants have reached a stage where their dispersal capacity is limited, reversing the consequences necessitates a more extensive set of measures."