Ecological strategies of (pl)ants: Towards a world-wide worker economic spectrum for ants

Abstract

Current global challenges call for a rigorously predictive ecology. Our understanding of ecological strategies, imputed through suites of measurable functional traits, comes from decades of work that largely focussed on plants. However, a key question is whether plant ecological strategies resemble those of other organisms.Among animals, ants have long been recognised to possess similarities with plants: as (largely) central place foragers. For example, individual ant workers play similar foraging roles to plant leaves and roots and are similarly expendable. Frameworks that aim to understand plant ecological strategies through key functional traits, such as the 'leaf economics spectrum', offer the potential for significant parallels with ant ecological strategies.Here, we explore these parallels across several proposed ecological strategy dimensions, including an 'economic spectrum', propagule size-number trade-offs, apparency-defence trade-offs, resource acquisition trade-offs and stress-tolerance trade-offs. We also highlight where ecological strategies may differ between plants and ants. Furthermore, we consider how these strategies play out among the different modules of eusocial organisms, where selective forces act on the worker and reproductive castes, as well as the colony.Finally, we suggest future directions for ecological strategy research, including highlighting the availability of data and traits that may be more difficult to measure, but should receive more attention in future to better understand the ecological strategies of ants. The unique biology of eusocial organisms provides an unrivalled opportunity to bridge the gap in our understanding of ecological strategies in plants and animals and we hope that this perspective will ignite further interest. Read the free Plain Language Summary for this article on the Journal blog.

Keywords: ants; ecological strategy; functional trait; leaf economic spectrum; plant traits; trade‐off; worker economic spectrum.

FIGURE 1

The environment drives the energetic balance of organisms, selecting for a range of ecological strategies. Examples for ants (a) and plants (b) are shown below. Energy assimilated from the environment is devoted to growth and maintenance or reproduction. Constraints of physics and biology limit the viability of ecological strategies, leading to trade‐offs. Trade‐offs (examples 1, 2 and 3 are detailed in the text) can occur at different scales within an organism, that is, at the colony or whole plant level, the organ level (e.g. ant workers or plant leaves) or the reproductive unit level (alate ants or plant seeds). The environment (both biotic and abiotic; and evolutionary history of a species) drives the ecological strategy of species, determining, for example, whether their investment in reproduction focuses on quantity or quality.

FIGURE 2

Cumulative publications using the terms ‘traits’, ‘ecological strategies’ and ‘trait space’ in the animal and plant literature from Web of Science between 1998 and 2020. Traits have been considered twice as often in publications relating to plants as animals. While ‘trait space’ is more common in the animal literature, the term ‘ecological strategies’ is used approximately five times more often in the plant literature. ‘Ecological strategies’ considers the coordinated trait variation resulting from ecological and evolutionary trade‐offs. ‘Trait space’ represents the multidimensional position of species based on a range of measurable traits. Trait space may be used to predict ecological strategies, but does not explicitly hypothesise any trade‐offs among traits. Search terms are provided in Appendix S1.

FIGURE B1

Parallels between ants and leaves, showing ants as the leaves of the colony.

FIGURE B2

Global ant morphological trait space. Heat colour represents the density of species in each plane (redder colours = more species). Grey contours represent the 50th (inner), 95th and 99th (outer) percentiles. Arrows represent correlations of traits with the PC axes. All morphological traits are relative to size except size itself. Position of the independent data (shaded in grey) represents their correlation with the PC axes. Arrows not drawn for the independent data for visual clarity. Correlations are scaled differently between each panel, and between the trait and independent ecological datasets for visual clarity. Percentages in the axis labels indicate the fraction of total worker ant morphological variation captured by each axis.