"The Snail and the Eagle," by Marcus Gheeraerts the Elder, Flemish, c.1520-1590. Drawn for an edition of Aesop's Fables, 1567.
“No one method seems essential in the formation of oceanic species complexes. The problem is extraordinarily complex. The combination of factors involved in the evolution of land snails with their sedentary habits may not be the same as those influencing flying insects.” p302 (Zimmerman, 1942)
When Louis Pasteur did his famous experiment with the swan-necked flask, he was essentially showing the potential importance of dispersal limitation to microbial community assembly.
Dispersal limitation is also a huge driver of biogeographic patterns. That there are marsupials in in New Guinea and Australia, but not in nearby islands in Indonesia is the legacy of dispersal limitation. Dispersal limitation is also the reason there are no endemic lizards or rodents (or many other groups) in Hawaii. But here we come to some interesting patterns, because over relatively short distances and periods of time, you might expect that many lizards and rodents would be able to out-disperse a land snail, but land snails have independently arrived in Hawaii at least 9 or 10 times. Some of the research I’m working on is focused in Palau, which is much closer to mainland source biotal areas, and here ants have probably independently colonized more frequently than land snails. In this archipelago landscape, dispersal and dispersal limitation at various scales in space and time play important roles in generating the diversity patterns that I’m interested in.
When researchers study dispersal and its evolutionary and ecological ramifications, they might study the behavior of individual animals, or estimate the probability distribution of seeds landing at a given distance from the parent tree. They might make complex models of ocean currents, or infer ancient dispersal from molecular phylogenetics or fossil data. Dispersal ability involves a variety of different traits, is often context-dependent, and can evolve. It’s honestly pretty tough to write up a succinct lil blog post about dispersal, but I’m going to try.
In Vellend (2010, 2016)’s framework, dispersal is analogous to gene flow in population genetics: it’s a way new species (rather than alleles) can enter a system over time. In “Evolutionary Community Ecology,” McPeek refers to “sink species” as one of four types of species present in communities (other authors have used more or less this term before, but McPeek is the first to list them as a fundamental species type along with coexisting species, neutral species, and walking dead species). To be a sink species in a particular place, a taxon is likely abundant in a nearby area, where it is a coexisting (or, more rarely, neutral?) species, and only exists as a sink species where it does because of a continual rain of propagules where conditions are more favorable. If one could magically turn off the tap of dispersal, the sink species would become a walking dead species, and drift to extinction (or, more rarely, they might adapt to their sink habitat when gene flow from the more favorable habitat ceases).
In another sweeping book published around the time Vellend (2016) and McPeek (2017) came out, Leibold & Chase (2018)’s book “Metacommunity Ecology,” describes different types of metacommunities. Just like the population genetics concept of a metapopulation implies that there are some local populations within which gene flow is likely, and among which gene flow is less likely but still substantially non-zero, metacommunities are patchy assemblages among which there is appreciable, but not continuous dispersal and interaction. One of the four main types of metacommunities is the mass effects (ME) archetype, which involves species that McPeek (2017) would characterize as source-sink. Another metacommunity framework they describe is species sorting (SS). If the habitat parameters of the patch determine species composition, then species sorting is involved. When we look at different communities, species sorting and dispersal limitation are useful hypotheses for why certain species are not found in some patches. In some areas of the world, serpentine soils create a dramatic mosaic of habitats that are unsuitable for many plant species. Many obligate vernal pool species can disperse widely on the wind or bird feed (or digestive tracts), but cannot survive in bodies of water that are not ephemeral because they get eaten by fish. Some stream invertebrates can effectively avoid fish predation, but require really high dissolved oxygen, so are primarily found in certain parts of fast-flowing streams. The freshwater habitats of Massachusetts could be thought of as a metacommunity with strong species sorting, but also likely some source-sink dynamics, and perhaps neutral dynamics and patch dynamics as well.
Implicit in discussions of organismal dispersal abilities is the configuration of landscapes (or the connectivity of bodies of freshwater, or the currents of the ocean, etc etc). Depending on the researcher, there may be more focus on landscape configuration (e.g., connectivity, number of and distance between islands, habitat heterogeneity), or more focus on organismal traits. In animals (and sometimes other types of organisms) there might be a focus on behavior; but in animals with undirected dispersal stages and most other types of organisms, the prevailing winds, currents might be much more important.
Most organisms have at least some stage of their life cycle that is capable of dispersal: many sessile marine invertebrates have planktonic larvae, for example, and most plants have seeds or spores that can be carried by the wind, water, or animals. It is sometimes useful to distinguish among active (e.g., flying, swimming, walking) vs. passive dispersal. There are a number of terms associated with passive dispersal, many of which end in -chory: anemochory (wind dispersal), hydrochory (dispersal by flowing water or currents), zoochory (dispersal by animals). Zoochory can be further divided up depending on which organism is doing it: you could refer to anthropochory for human-mediated dispersal, chiropterochory for bat-mediated dispersal, or myrmecochory for ant-mediated dispersal. Some researchers also distinguish between seeds that attach to the outside of animals (epizoochory) vs. seeds that pass through the digestive tract (endozoochory). Some plants and fungi have ballistically driven seed or spore dispersal, which some researchers refer to as “autochory.” Zoologists refer to small animals that disperse by attaching to other animals as “phoretic.”
Finally, a big chunk of invasion ecology is tightly tied to dispersal ecology. Introduced species are species that exist outside of their native range because of some human action (homogocene, etc.), so invasion ecology is principally concerned with species that have undergone an anthropogenically-mediated range expansion. Which species are prone to “anthropochory”? Whether or not species brought by humans will become established, and whether, further, they will competitively displace or prey upon native species to a problematic extent is within the Vellend’s “selection” component.
