Animal dispersal
Dispersal refers to the movement of an organism from the place where it was born (its natal area) to somewhere else. Have you ever picked a dandelion seedhead and blown on it, making a wish as you watch the seeds float in the air? As the seed is carried by a bundle of feathery bristles and drifts on the breeze, you’re witnessing the process of seed dispersal.
Dandelion seeds
Dandelion seeds float away in the wind. To make sure at least some of the seeds land in a suitable growing place, the plant has to produce lots of seeds.
Just like the dandelion, animals also disperse, allowing them to seek a home in a new location where they can survive and reproduce.
It’s helpful to think of dispersal as like purchasing a one-way ticket. Imagine packing your suitcase, waving goodbye to your old bedroom and your family and flying off to a new destination, with no plan to return. In most animals, it’s the young or juvenile individuals that disperse. They leave the place they were born to find a new home of their own. This movement is usually permanent or done very irregularly.
Dispersal versus migration
Dispersal is different from another process of animal movement called migration. If dispersal is like buying a one-way ticket, migration is more like having a return ticket – you travel to a different place but come back again later. Migration is typically done by adults and happens on a regular, often seasonal basis. Animals often move between breeding and non-breeding sites, such as birds flying to warmer places for the winter and then returning to breed in the spring. The flight of the godwit is an excellent example of seasonal migration, where the birds undergo an astonishing 29,000 km round trip between Aotearoa New Zealand and Alaska each year.
Reasons for dispersal
Dispersal can be a risky behaviour. It can require a lot of energy to disperse, there’s a chance of being eaten by a predator during the journey and it’s possible that a suitable new area to live is never found. So why do animals disperse?
Like most animal behaviour, dispersal is a balance between the costs versus the benefits the animal gains from doing that behaviour. Living creatures are constantly making decisions that allow them to survive and reproduce. Dispersal allows animals to reduce competition with family members or other individual, and it can reduce inbreeding by helping animals avoid mating with close relatives and explore new habitats.
Spiderlings preparing to balloon
Square-ended crab spiderlings climb to the tip of a harakeke flower head in preparation to balloon.
Spiderlings use ballooning as a form of passive dispersal to find new habitats and avoid competition with siblings.
Passive and active dispersal
Dispersal can be passive or active. Spiderling ballooning is an example of passive dispersal. Animals that use passive dispersal do not choose where they end up. Instead, they rely on features in their environment like wind or ocean currents to disperse. Spiderling dispersal is an obvious example of passive dispersal because we can see evidence of this in the thousands of silk threads floating in the air during winter. However, there are many other animals that rely on this type of dispersal too. Small insects get carried around by the wind or hitch-hike on the backs of birds, and marine larvae such as crabs, mussels and coral drift with ocean currents.
In contrast, active dispersal describes animals that purposefully move their bodies to reach a new area. Think of actions like flying, running, swimming and slithering. Active dispersers are animals that are good at moving from place to place by themselves – described as being highly vagile. For example, hatchling turtles crawl across beaches to the water and then swim to their feeding grounds, and birds often fledge their nests and disperse to new areas away from their parents. Many large insects such as butterflies and beetles also actively disperse through flight or by crawling.
Life cycle of freshwater eels
The life cycle of freshwater eels is still somewhat of a mystery as some parts of the eel cycle have not yet been observed, such as spawning.
Artwork by: Bruce Mahalski
Some animals may use a mixture of active dispersal, passive dispersal and migration –depending on the life stage. In Aotearoa New Zealand, tuna (longfin eels) use all three types of movement. Adult tuna actively disperse by leaving Aotearoa to spawn. The larvae passively disperse by floating on ocean currents and then become active as elvers, swimming upstream to find a place to live.
Gold Coast eco bridge
Roads may disrupt animal dispersal, migration and other movements. Specially constructed overbridges and underpasses connect natural corridors. This eco bridge provides safe passage for wildlife crossing from one side of the M1 motorway to the other – located in New South Wales, Australia.
Human impacts on animal dispersal
Like many aspects of animal behaviour and ecology, humans have an impact on how animals move around. A major way that humans interrupt dispersal is through the building of roads and buildings that block an animal’s pathway. Major roads can act as barriers that animals are afraid to cross or can result in death if they do take the risk. For example, a study in the United States found that timber rattlesnakes are reluctant to cross roads, resulting in populations one on side of the road being genetically distinct to snakes on the other side of the road.
Streetlights, noisy cities and pollution can also all affect the way animals move to new places. Reflective surfaces like glass on the windows of homes and skyscrapers are often not noticed by flying animals, resulting in high rates of collisions and death. Similarly, artificial light at night is increasingly being recognised to interrupt numerous aspects of animal behaviour, including dispersal and migration. City lights can act like ecological traps, attracting animals that eventually become exhausted from circling lit areas.
Declining habitat quality and fragmentation can also affect dispersal. If humans cause habitats to decline and chop up suitable places for animals to live into smaller, isolated patches that are too far apart, animals may be more inclined to disperse but struggle to do so.
Habitat fragmentation can also affect dispersal through the act of chopping up suitable places for animals to live into smaller, isolated patches. If too far apart, animals may struggle to disperse in between these patches. In relation to this, if habitat quality declines animals may be more inclined to disperse – meaning in this case humans are causing higher rates of dispersal than may be normal.
Novel pottery fish ladders
Some freshwater fish species migrate for reproduction purposes. Culverts and other land use changes have made it difficult for these fish to move around. Conservationsists have developed a variety of methods to help. The late Coromandel potter, Barry Brickell, used plates and bowls to create novel fish ladders.
However, there are actions we can take to help mitigate some of the effects people have on animal movement. In many countries, people have built wildlife bridges, corridors or tunnels to help animals move between fragments of habitat. You can also help by volunteering at your local restoration project by planting trees, weeding or trapping predators.
Related content and activity ideas
Spiderling ballooning is a form of passive dispersion. Learn more about ballooning and animal dispersal with the activity Spider parachutes.
Citizen science projects Loss of the Night and Globe at Night look at the vexing issue of light pollution, which can affect animals and plants and disrupt ecosystems.
Hamilton Halo – bringing tūī back to Hamilton City is a success story. Kirikiriroa Hamilton is awash with tūī thanks to increased planting, especially along corridors like streams and gully systems.
Read about Ahi Pepe MothNet – a project that began when an Ōtepoti Dunedin scientist wondered whether artificial lighting had impacts on moth communities.
Meet the author of this article, Dr Chrissie Painting, in our All about insects webinar.
Chrissie was part of the team who developed these Buzz in the Garden activities:
Useful links
Find out more about Dr Chrissie Painting’s outreach Cultivating Connections: Lessons on Community Engagement from the Chatham Islands in this Royal Society Te Apārangi article.
Acknowledgement
This article was written by Dr Chrissie Painting, Senior Lecturer, Ecology, Biodiversity and Animal Behaviour at the University of Waikato Te Whare Wānanga o Waikato.
