SpeciEs coexistence

Community structure changes

Species distribution and community structures, i.e., the relationships between populations of different species, have fluctuated considerably in relation to the increasing pressures imposed on them by human activities (Boivin et al., 2016). In order to best protect biodiversity, it is therefore necessary to understand the fundamental ecological principles that govern it, such as how species coexist, how they share space and resources, etc.

Ecological niche theory

Since the beginning of the 20th century, the distribution of species has been thought in terms of ecological niches. Indeed, each species has a fundamental niche that can be defined as a hyper-volume with n dimensions, where each dimension of space represents a resource or environmental condition that defines the requirements of a species to maintain a viable population (Hutchinson 1957). The realized niche is the set of conditions actually used by given animal (pop, species), after interactions with other species (predation and especially competition) have been taken into account. A major application of the niche is to explain the structure of communities (number of species, composition, relationships between species).

The competitive exclusion principle

The principle of competitive exclusion stipulates that two species with identical resource use patterns cannot continue to coexist in a stable environment and share the same niche if food resources are limiting, with one species eliminating the other (Hardin 1960). Thus, if 2 ecologically similar species coexist, it is because they have necessarily achieved a niche differentiation on one or more of the dimensions of their niche (e.g. spatial, temporal or trophic dimensions).

Adaptive radiation

Adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic interactions or opens new environmental niches (Schluter 2000). Starting with a single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits. The prototypical example of adaptive radiation is finch speciation on the Galapagos ("Darwin's finches"), but examples are known from around the world.

Four features can be used to identify an adaptive radiation (Schluter 2000):

• A common ancestry of component species: specifically a recent ancestry.

• A phenotype-environment correlation: a significant association between environments and the morphological and physiological traits used to exploit those environments.

• Trait utility: the performance or fitness advantages of trait values in their corresponding environments.

• Rapid speciation: presence of one or more bursts in the emergence of new species around the time that ecological and phenotypic divergence is underway.

"The most commonly quoted example of adaptive radiation is Darwin’s finches, discovered during Darwin’s voyage to the Galápagos archipelago. Speciation is the development of one of multiple new species in the evolutionary process, where the original species produces mutated forms which successfully survive in other environments due to these mutations. In the case of Darwin’s finches, adaptations occurred relatively rapidly. Blown over to various islands with different flora and fauna, beak morphology might ensure either the survival or the death of a bird. For example, warbler finches and ground finches have evolved from a common ancestor. Warbler finches have long, thin beaks perfect for eating insects. Ground finches have thick, blunt beaks ideal for breaking over the husks of nuts and seeds. The fifteen species of finches found at the Galápagos archipelago make up a monophyletic group, or a group of organisms all descended from one ancestral species. The common ancestor is not known due to a lack of DNA, but fossils from two species of ground finches, Geospiza nebulosi and Geospiza magnirostris have the thick, blunt beaks of their descendants. This would indicate that warbler finches are the result of speciation through the process of adaptive radiation. Upon landing on an island with few nuts and seeds but many insects, those specimens with longer, thinner beaks (mutations) were more likely to survive and reproduce. Natural selection increased the survival rates of long-beaked birds on this island where they interbred, eventually leading to a phenotype common to this new species." (https://biologydictionary.net/adaptive-radiation/)

Parallel evolution is defined as the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different groups. Parallel evolution is sometimes difficult to distinguish from convergent evolution. Parallel evolution occurs when different species start with similar ancestral origins, then evolve similar traits over time. This kind of thing happens because the two different species, though they don't necessarily share a common ancestor, experience similar kinds of environmental pressures and survive only by undergoing similar adaptations. A classic example of parallel evolution is found among plants, in which several similar but distinct forms of leaf evolved in parallel and are evident today.

Convergent evolution is the process in which species that are not closely related to each other independently evolve similar kinds of traits. For example, dragonflies, hawks, and bats all have wings.

Before my PhD, I carried out studies on feeding ecology, species coexistence and niche partitioning in seabirds within the framework of ecological niche theory and competitive exclusion principle.

Sea duck project

During my Master1 internship at the Université du Québec à Rimouski in 1999-2000 under the supervision of Magella Guillemette, I studied resource partitioning and trophic segregation within the guild of 12 North American sea duck species during winter using stomach contents analysis data.

We investigated the influence of body mass on the diet composition of predators. Because the basal metabolic rate increases allometrically with the body mass of organisms, small animals have higher energy requirements relative to their body mass than large species. Therefore, the energy value of the diet should decrease with the body mass of the species. We showed that this hypothesis was verified within the North American sea duck guild, with large species feeding predominantly on low energy bivalves while small species fed predominantly on higher energy crustaceans (Ouellet et al. 2013 PLoS ONE). However, the two species of goldeneye, although having similar sizes and therefore similar energy requirements, differed in the proportion of crustaceans and bivalves included in their diet, thus allowing for niche sharing. We proposed a new hypothesis on the evolution of the diet of sea ducks based on metabolic, aerodynamic, digestive and temporal constraints, as well as on competition avoidance.

Penguin & seal project

During my Master2 internship at the CEBC-CNRS in 2003 under the supervision of Yves Cherel, I studied resource partitioning and trophic segregation within the guild of 5 sympatric diving predator species (penguins and fur seals) breeding in summer on the Crozet Archipelago (Cherel et al. 2007 J Anim Ecol). For this, I analyzed the stomach contents of the different species. I also used an innovative approach based on stable isotope analysis of δ15N and δ13C in the blood and nails of the different species in collaboration with Keith Hobson from the University of Saskatchewan in Canada.

Consumers are enriched in δ15N with respect to their food and as a result, δ15N provides information on the trophic level of the species. In contrast, δ13C varies very little along the food chain and is primarily used to determine the primary sources of food webs. In the marine environment, δ13C can also indicate the feeding zone of organisms along a latitudinal gradient, with low-latitude plankton being enriched in δ13C compared to high-latitude plankton.

We showed that although the 3 penguin species feed sometimes on similar preys, king penguins select larger individuals than rockhopper and macaroni penguins, thus allowing for niche partitioning. In addition, in summer, δ15N and δ13C defined 2 distinct trophic levels and 3 distinct feeding areas, respectively, thus characterizing 4 non-overlapping trophic niches within the diving predator guild of the Crozet Islands (Cherel et al. 2007 J Anim Ecol). Our study highlighted the promise of the stable isotope approach to study the structure, functioning and dynamics of food webs, a key step towards measuring and better predicting the impacts of global changes on ecosystem functioning.