Species coexistence in a changing world

Abstract

The consequences of global change for the maintenance of species diversity will depend on the sum of each species responses to the environment and on the interactions among them. A wide ecological literature supports that these species-specific responses can arise from factors related to life strategies, evolutionary history and intraspecific variation, and also from environmental variation in space and time. In the light of recent advances from coexistence theory combined with mechanistic explanations of diversity maintenance, we discuss how global change drivers can influence species coexistence. We revise the importance of both competition and facilitation for understanding coexistence in different ecosystems, address the influence of phylogenetic relatedness, functional traits, phenotypic plasticity and intraspecific variability, and discuss lessons learnt from invasion ecology. While most previous studies have focused their efforts on disentangling the mechanisms that maintain the biological diversity in species-rich ecosystems such as tropical forests, grasslands and coral reefs, we argue that much can be learnt from pauci-specific communities where functional variability within each species, together with demographic and stochastic processes becomes key to understand species interactions and eventually community responses to global change.

Keywords: climate change; competition; facilitation; functional traits; global change; heterogeneity; intraspecific variability.

FIGURE 1

Influence of intraspecific variability in the filtering of potential species integrating a community. (A) classical community assembly theory without taking into account intraspecific variability and (B) community assembly theory incorporating intraspecific variability. Species with mean trait values matching the abiotic requirements and being either ecologically different or capable of tolerating competition will contribute to the eventual community. By incorporating intraspecific variability, more species will pass biotic and abiotic filters because they are able to adjust by phenotypic plasticity or simply because they are genetically variable so more species could join the community in (B) than in (A). Each shape represents a species and each color represents a given trait value within a species. Dashed lines represent abiotic and biotic filters.

FIGURE 2

A theoretical scheme of coexistence and competitive exclusion between two species. If niche differences between competitors are greater than their fitness asymmetries then both species will show stable coexistence (blue region). In contrast, if fitness differences are greater than niche differences, then the species with higher fitness will exclude the other (red region). Fitness differences also determine which species dominates under stable coexistence. Figure adapted from MacDougall et al. (2009).

FIGURE 3

Representation of direct and indirect pathways relating abiotic and biotic factors with diversity. We show examples of five study systems, corresponding to (A) temperate forests, modified from Paquette and Messier (2011), (B) tropical forests, modified from Yasuhiro et al. (2004), (C) grasslands, modified from Gazol et al. (2012), (D) drylands, modified from Soliveres et al. (2014), and (E) alpine ecosystems, modified from Cavieres et al. (2014). Single arrows represent causal paths, where thickness is proportional to the path coefficient (solid: positive, broken: negative, dotted: non-significant). Interlinked influences of landscape conditions and local environmental factors are explaining species richness in contrasted biomes such as subtropical forests and temperate grasslands. However, diversity and coexistence are usually dependent on distinct factors in each biome (i.e., competitive exclusion is more relevant in temperate forests, whereas facilitation mediated by woody cover or cushion effects are more important in drylands and alpine ecosystems, respectively).

FIGURE 4

Global change drivers affect coexistence mechanisms in a number of ways, at various levels of biological organization (from individuals to species) and at various spatial and temporal scales. Individual fast responses to environmental change co-occur with alterations in species interactions, resource use and many other changes that interactively affect species coexistence. Changes observed at the community level are thus resulting from the direct effect of global change drivers on both coexistence mechanisms and individual species responses to changes in these drivers.