Costs of passive dispersal in fragmented landscapes

In this post, Brittany Teller (@brittzinator) describes her recent paper with co- authors Adam Miller & Katriona Shea “Conservation of passively dispersed organisms in the context of habitat degradation and destruction

Leaving the natal location (hereafter, “dispersal”) can be a critical part of many species’ life cycles. If dispersing individuals help establish new populations, this colonization can help keep connected metapopulations viable in the face of disturbances like climate change. Traditionally, it has been hypothesized that highly dispersive species are generally resilient to changes in a landscape, because they may be able escape locally harsh conditions.

However, dispersal can also be risky if lots of offspring arrive in unsuitable habitat. In low-quality habitat, individuals are unlikely to establish—and even if they do, they are not likely to produce many offspring themselves. These circumstances can lead to population decline. In anthropogenically modified landscapes, the proportion of habitable area can decline rapidly—often too quickly for species to adapt.

The effects of habitat loss may be especially strong for passively dispersed species, like many plants or ballooning spiders, because abiotic forces like the wind primarily determine the areas in which propagules settle. When habitat is degraded or destroyed, passively dispersed species may continue to disperse a large number of offspring into newly degraded areas, and thereby become vulnerable to population decline.

Photo by L. Russo
Photo by L. Russo

Take for example, a species like butterfly milkweed Asclepias tuberosa. This plant species is valued for its unique interactions with insects (specifically, the Monarch butterfly), and also produces wind-dispersed seeds that have very little control over where they settle. Thus, this species has the potential to suffer reduced fecundity if too many seeds disperse into degraded areas (such as turf) or destroyed areas (such as parking lots). Even though a species like A. tuberosa currently appears to be unthreatened globally, it is listed as vulnerable or threatened in parts of the northeastern USA. Repeated loss of seeds to unsuitable habitat could limit the ability of the population to persist in the long-term, and could lead to regional extirpation.

In this paper, we use a general mathematical model to examine how dispersal propensity interacts with the circumstances of habitat degradation and destruction to determine passively dispersed species’ vulnerability to landscape alterations. In our model, we assume that populations are small, do not experience much density dependence, and do not evolve quickly. We find that in cases like these, passively dispersed species that are also highly dispersive may be vulnerable to population decline in the context of habitat degradation and destruction.

Importantly, we also compare highly dispersive species and more retentive species with similar life histories. Our results counter the prevailing ecological intuition that highly dispersive species are generally more resilient to landscape degradation than species that retain offspring locally. Instead, we find that in some cases, highly dispersive species can be more vulnerable than retentive species in degraded landscapes. Thus, we believe that conservation and management plans should carefully consider the effects of dispersal for both highly dispersive and poorly dispersing species.

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