Host availability drives the spatiotemporal dynamics of interaction metapopulations across a fragmented landscape (2020) Opedal et al. 2020, Ecology. https://doi.org/10.1002/ecy.3186

Image Credit: Ferran Turmo Gort, CC BY-NC-SA 2.0, Image Cropped

The Crux

Ecology is all about understanding how biotic and abiotic factors interact within environments. Biotic factors are those that involve living organisms such as prey availability/resource abundance (i.e., the availability of food and resources?), competitor density, or predator density. Abiotic factors, however, are those that involve non-living aspects of the environment, such as rainfall or temperature. Studying how these various factors interact with one another allows researchers to better understand how and why ecological dynamics vary across a changing landscape.

One really cool thing about ecological dynamics is that they can play out across trophic levels, meaning something happening at the level of the resource (such as grass) can then result in changes at a higher trophic level, such as that of the consumer (deer) or predator (wolf). While there has been an enormous amount of work dedicated to understanding how these species interactions affect the species involved, much less is known about how these dynamics play out across a natural landscape. Today’s authors used a well-known model system (see Did You Know?) to study just that.

Did You Know: Model Systems

Model systems are specific biological systems that have been used by so many researchers to answer so many questions that they have become a staple of many different fields of research. Popular examples include flies (Drosophila), freshwater crustaceans (Daphnia), and even the insects that I worked with for my PhD (Enallagma damselflies) can be considered a model system for ecological research. In fact, the common English-language term “lab rat” comes from a model system, lab rats! Rats have been used so often and in so many ways that even non-scientists know them and use the term.

What They Did

The authors took advantage of the Åland (pronounced “oh-land”) Islands model system, an archipelago in the Baltic Sea off of the southwestern coast of Finland. This groups of islands contains a fragmented collection of ~4500 individual populations of the ribwort plantain (Plantago lanceolata), which is a host to the parasitic powdery mildew fungus (Podosphaera plantaginis) and the Glanville fritillary butterfly (Melitaea cinxia). On top of this, the butterfly is the host for a parasitoid wasp (Cotesia meltaearum). Researchers have conducted annual censuses of these plant, fungus, and insect populations since the year 2000, noting which populations have remained occupied and when/if new ones have popped up/gone extinct.

Using this data, today’s authors ran a series of models to understand if these species interactions, the abitoic environment, and the distance between the various populations can explain meta-population dynamics. A meta-population is a group of smaller populations that interact with one another. Think of Europe as a meta-population, with each individual country as a smaller population within it.

In other words, they wanted to know whether the presence of one or more of the species could be used to predict the presence (or extinction) of the other species in a given population? While many of these populations have the ribwort plantain, not all of them contain the mildew or parasitoid wasp, and we might expect the presence of the butterfly or mildew to predict the presence/absence of the other species. Additionally, is a population that is further away from another more likely to go extinct than one that is better connected?

What They Found

Not surprisingly, there has been variation in patch occupancy (meaning which populations were occupied) over the 19 years that the data was collected. In general, the butterfly, mildew, and parasitoid wasp were all more likely to colonize (and less likely to go extinct in) patches that had greater coverage of the ribwort plantain. This means that if a given population of the ribwort plantain covered 80% of the available space the other species would be less likely to go extinct than in a population where the plantain only covered only 40% of the available space. Additionally, better-connected populations served to increase occupancy rates and lower extinction probabilities.

Interestingly though, the authors found no effect of the various species interactions on the meta-population dynamics. This means that despite the variation seen in patch occupancy by the plantain, butterfly, mildew, and wasp, it cannot be attributed to the interactions that occur between these species. Instead, environmental drivers (e.g., temperature and rainfall) and host-plant availability had the strongest effects on meta-population dynamics.

Problems?

Obviously an ENORMOUS amount of data went into this study, yet some of that data was collected on a relatively coarse scale, meaning that data on individual populations was compared to climatic data on the regional scale. The authors note that strong environmental effects may have obscured the ability detect relatively weak effects of the species interactions on the meta-population dynamics. Finer-scale climate data may allow future researchers to better control for the strong effects of the environment such that we can better understand the effects of these focal species interactions on the meta-population as a whole.

So What?

Though they did not find evidence for an effect of species interactions on meta-population dynamics, the results of today’s study highlight how one key species within an ecological network can have cascading effects on every other species within it. In addition, these results show how species interactions may have different effects when considering their impact at different scales, which has implications for how we think about evolution in these meta-populations.

Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and has very recently (and successfully defended) his PhD. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.