Image credit: Charles J. Sharp, CC BY-SA 4.0

When ecology fails: how reproductive interactions promote species coexistence (2021), Gómez-Llano et al., Trends in Ecology and Evolution. https://doi.org/10.1016/j.tree.2021.03.003

The Crux

Scientific literature, like many different aspects of society and culture, goes through periods where a given subject/topic is more prominent in the public conscience than others. Lately, the question of coexistence has been at the forefront of the minds of many community ecologists. Coexistence is the state in which two or more species can each maintain a population in the same habitat as each other, provided that the environmental conditions and species interactions that they experience remain stable. Many studies of coexistence have investigated how differences among coexisting species allow them to maintain their coexistence, which makes sense, as it’s hard to coexist with another species if they require the exact same food or habitat as you do.

Yet there are a lot of examples of coexisting species that seem to be almost identical. Some researchers have suggested that these networks of similar species are unstable and should break down over time. But are these groups of species truly doomed? Or are there other processes maintaining this seemingly unlikely coexistence?

Today’s authors suggest that reproductive interactions among species are what may allow such similar species to continue coexisting. While much of the work in this area is theoretical rather than empirical (see Did You Know?), the authors reviewed what empirical evidence they could. Today’s paper is a review (a paper that summarizes lots of previously published papers with the goal of synthesizing knowledge), so I will briefly touch on the main points as put forward by the authors.

Did You Know: Theoretical vs. Empirical Research

As the name implies, a theoretical study is one that focusses more on the theory of a given topic. This could be predation, coexistence, parasitism, etc. Theoretical studies do not involve experimentation, though they may cite previous experimental studies that support their arguments. Empirical studies, on the other hand, are those involving one or more experiments, and these experiments then generate new data which feed empirical results.

Background Theory

The authors highlight four different ways that males and females of two different species can interact with one another, then explain how each one of those mechanisms can promote coexistence. I briefly touch on each one, summarizing the relevant theory or empirical evidence.

Mechanism 1: Interactions between males and females of the same species

This mechanism has been studied in two main contexts: female mate choice and sexual conflict. Female mate choice being how/why a female chooses to mate with specific males, and sexual conflict being when the interests of males and females differ (males try to mate as much possible, females try to limit their matings and only mate a limited number of times with high quality males). Theory suggests that when population numbers are high, encounters between males and females increase, which decreases female fitness and reduces how many offspring they can produce. This reduces their population growth rate, which then allows the more rare species (i.e., less common/at a lower population number) to increase in number, which promotes species coexistence. While a number of studies have investigated this mechanism theoretically, no study has empirically addressed it.

Mechanism 2: Interactions between males and females of different species

Matings between different species are costly, because they usually produce offspring that are sterile (if they produce offspring at all!). To avoid these high costs, females may tend to aggregate in areas where there are more members of their own species, limiting costly matings between species. This then produces regional coexistence, whereby many patches are inhabited by only one of each species, but these populations exist side by side.

Empirical evidence suggests that costly effects of mating with different species vary with population density, meaning that costs will either increase or decrease as population density increases. A study with butterflies, for example, showed that females of species A often fail to mate as with males of species A when there were males of species B around, so they would rather not mate at all instead of risking mating with the wrong species. This failure to mate only increased when there were more males of species B around.

Mechanism 3: Interactions between males or females of the same species

All theoretical studies of mechanisms 3 have revolved around male-male competition, which is competition between males of the same species for access to females. For example, one hypothesis states that competition for territory among males is stronger within than between species, meaning that males compete more with members of their own species than members of another. If this competition then results in males of the same species establishing territories that are far from one another, this can allow males of other species to set up a territory between males of the other species (think a checkerboard pattern, with the black and white tiles being the territories of species A and B, respectively).

This can then result in competition among males increasing as population numbers increase, which will limit population growth rates at high density, promoting coexistence. This checkerboard territory example can actually be seen in studies of cichlid fish. Males of species A will chase other A males far from their territory, yet will only chase males of species B out of their immediate territory. This means that males of species B can have their territory next to that of an A male territory.

Mechanism 4: Interactions between males of females of two different species

Unlike the other three mechanisms, mechanism 4 has not been theoretically studied, yet empirical studies exist. In particular, studies have investigated competition between males of closely related species, where there may be little (or even no) visual differences between the species. For the males, accidentally mating with a female of the wrong species is less costly than not mating at all, so they will compete for every mating opportunity they get (with males of either their own species or males of another). While these studies have not fully explored how these interactions can influence species coexistence, such competition among males of different species can reduce sexual conflict for females of one species, which would increase population growth rate.

So What?

Reviews and syntheses are incredibly valuable for the scientific literature. Because they collate and summarize a large number of previous papers/studies, they allow other researchers to easily review and understand a given aspect of ecology. Today’s review in particular not only did that, but highlighted several gaps in our knowledge of mechanisms that may promote species coexistence. Doing so will inspire future studies and experiments, further advancing knowledge in the field.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who flies to Israel TOMORROW. 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.