Does it make sense to view much of contemporary biodiversity change in terms of adaptive ecological responses to changing environments?

There have been few topics more controversial in ecology over the past decade than the nature of recent biodiversity change. While there is some disagreement regarding precisely how biodiversity is changing, few would disagree that ecological communities generally are experiencing rapid human-induced changes in which species are present and abundant (i.e., change in species composition). However, a considerable range of attitudes towards such change seems to exist amongst scientists and the public. Some view human induced compositional change as a universally negative phenomenon, advocating for the restoration and preservation of historical baseline states; others view compositional change more favourably, as a necessary response to rapidly changing environments. I suspect most of us fall somewhere between these extremes, perhaps moving from one end of the spectrum to the other depending on the circumstances.

In a recent article in Global Change Biology, my colleagues and I introduced the framework of adaptive community dynamics. Among other things, we hope this framework might help people articulate under what conditions they consider compositional change to be desirable (or not), given particular sets of goals and values. At the core of our article is the idea that most of the turnover we see in ecological communities can usefully be viewed as ‘adaptive’, in the sense of improving the match between organisms’ traits and the environment. We explore potential implications of such a view for the functioning and management of ecosystems, concluding that there may often be a need to rethink default management advice centred on restoring historical baseline states, instead placing more emphasis on understanding how ecological communities adaptively respond to the continuing march of human-induced environmental change.

Inherent in a view of biodiversity change framed around adaptive dynamics, is the importance of distinguishing between environmental drivers (which cause adaptive compositional change) and ecological responses (the resulting compositional change itself). This is because the extent to which an ecological community, or any biological entity, is adapted or maladapted is entirely context dependent. We describe a community as being ‘well adapted’ to a particular environment if the organisms making up that community are well-equipped to survive and reproduce there (ie due to the traits they possess). This means that a change in the environment will alter the sets of species (and traits) that are likely to perform well under the new conditions (see figure). If the community was already reasonably well adapted, environmental change is likely to reduce the ‘fit’ between community members and the altered environment. However, the resulting compositional change should, in turn, act to ‘reinstate’ the fit between species’ traits and the environment, as selection favours species with traits that enhance survival and reproduction under new conditions. We refer to such compositional change as adaptive community dynamics.

It's important to note that when we use the term ‘adaptive’, we are not suggesting that compositional change is necessarily ‘good’ or ‘desirable’ in some sense. Rather, we argue that change in the identities and abundances of species within ecological communities is often underlain by processes that are very closely analogous (if not identical) to processes underlying adaptive evolution in biological populations* (ie via fitness differences and resulting selection).

As organisms within a (single-species) population encounter particular sets of environmental conditions, the fitness of an individual organism differs from the fitness of other organisms within the population which possess different genotypes. Similarly, within a (multi-species) ecological community, the fitness of an organism differs from (same-species) organisms with different genotypes, as well as from organisms of other species occupying that same community (which, of course, also have different genotypes). In both cases, processes of selection alter the relative numbers of different types of organisms inhabiting different or changing environments (ie via compositional change in genotypes and/or species). Any specific instance of adaptive community change of this kind might be considered desirable or undesirable, depending on the nature of environmental or compositional change, and on the values and priorities of particular observers.

As an example, most people, though not all, would agree that if the environment changes to the detriment of some critically endangered species – say the river it lives in dries out – this is bad (maybe the species doesn’t even have to be critically endangered if it’s particularly cute and cuddly…). This value assessment (change is bad) would not change if we were to describe the community dynamics underlying the creature’s decline as ‘adaptive’; if for example, other, perhaps less endangered species now inhabit the dried-out river. On the other hand, most people would likely also agree that if the environment changes to become more well-suited to the endangered species – perhaps the river was drying out but now is becoming wetter again – this is good. In both situations, compositional change is equally adaptive. Whether the change is considered good or bad depends on the values of the observer, explicit or implicit.

One way we expect adaptive dynamics to produce outcomes that will often be considered desirable is through the maintenance of ecosystem function. Well-functioning ecosystems emerge as the result of a plethora of interactions between species and environment (and between species and other species), which collectively regulate the circulation of energy and materials from the environment to ecological communities (and back again).

The execution of these interactions is contingent on species possessing traits suited to perform under the specific environmental conditions in which they find themselves. If conditions change, it follows that ecosystem function will only be maintained if the community ‘keeps up’ with the environment via increasing relative abundance and/or immigration of species with suitably adapted traits (ie via adaptive community dynamics).

Given the magnitude and ubiquity of human-induced environmental change already perpetuated throughout the Anthropocene, as well as the inevitability of changes yet to come (eg continued climate warming), facilitation or acceptance of adaptive community dynamics will often be a necessary response to human induced environmental change, particularly if the maintenance of ecosystem function is a stated goal. Prioritising other important goals, however, will require human responses that involve resisting adaptive dynamics, either by reversing or halting environmental drivers of change, or by directly intervening to reverse or halt compositional change itself. Such cases include a wide range of applications, from resisting environmental change to protect populations of endangered species, to boosting agricultural yields by reducing abundances of pest species that are well adapted to feed on crops (in order to feed our own populations).

However, despite the utility of resisting adaptive compositional change in many specific circumstances, it’s not clear that resistance should necessarily be the default course of action, as is often implicitly accepted. Maintaining maladaptive community states within radically altered environments will often not be feasible, and nor will completely halting the course of many environmental changes we’ve already set in place. Thinking in terms of adaptive community dynamics should help us examine underlying goals, values, and priorities in particular instances of ecological and environmental change and develop appropriate management strategies accordingly.

* As such, our framework is a natural extension of Mark Vellend’s “Theory of Ecological Communities”, and, unsurprisingly, it was Mark who came up with the idea of adaptive community dynamics.

References:

1. T. Carroll, F. Cardou, M. Dornelas, C. D. Thomas, and M. Vellend, ‘Biodiversity change under adaptive community dynamics’, Glob. Change Biol., vol. 00, pp. 1–14, Apr. 2023, doi: 10.1111/gcb.16680.
2. R. B. Primack et al., ‘Biodiversity gains? The debate on changes in local- vs global-scale species richness’, Biol. Conserv., vol. 219, pp. A1–A3, Mar. 2018, doi: 10.1016/j.biocon.2017.12.023.
3. C. D. Thomas, J. K. Hill, C. Ward, and J. H. Hatfield, ‘FAR‐sighted conservation’, Ecol Sol and Evidence, vol. 3, no. 4, Oct. 2022, doi: 10.1002/2688-8319.12188.
4. M. Vellend, The theory of ecological communities. in Monographs in population biology, no. 57. Princeton Oxford: Princeton University Press, 2016.