Beyond species: How will global change affect species interactions?
Species responses to rapid global change have been summarized, somewhat caustically, as “adapt, move, or die.” But each species is embedded in a web of interactions with those around it. Species depend on some interactions (plants on pollinators for reproduction, predators on prey for food, screech owls on woodpeckers for nesting cavities) and harmed by others (competition, parasitism, being someone’s meal). Whenever species adapt, move, or die out, the effects ripple through ecosystems via these interactions.
Alpine paintbrush (CasIlleja species) add splash of colour to mountain meadows. They also provide rich nectar for pollinators and steal nutrients from the roots of neighbouring plants. Photo: Anna Hargreaves
It is hard enough to predict the effects of global change on single species; it might seem impossible to foretell the future of species interactions. Nevertheless, a few big-picture predictions can be made:
Prediction 1: Effects on species interactions will often dwarf direct effects
For example, warmer winters have facilitated population explosions of mountain pine beetles in Western Canada.[1] But it is not the beetles themselves we notice, but their devastating effect on forests, now estimated at two million square kilometres. On the flip side, restoring interactions can have equally large, positive effects. A famous example is the reintroduction of wolves to national parks (by humans to Yellowstone; by wolves themselves to Banff), which forced elk away from easily-hunted lowlands. Few park visitors will see wolves, but many will see the lush streambank vegetation and higher bird diversity their hunting has enabled.[2,3]
Prediction 2: Alpine species and interactions are most at risk from climate warming
Caterpillars are significant herbivores for mountain plants, and can kill many seedlings each spring. Photo: Anna Hargreaves
As the climate changes, species are moving to track their preferred climate. When biologists compare modern and historical field surveys on mountains around the world, they find species have moved upslope to cooler elevations far more than expected by chance, and have moved farther where local climate has warmed the most.[3]
The effect of moving up depends on where species start from. Lowland species tend to gain mountain territory, but mountain-top species get squeezed as they eventually run out of mountain to climb.[4] Biologists have long feared this “escalator to extinction” will push high-elevation specialists off mountain peaks, and last year, the first such local extinctions were documented.[5] Eight previously common birds have been lost from a 4000-metre peak in Peru since 1985. Some of the missing high-elevation birds ate insects, others dispersed seeds; we don’t yet know whether these interactions have been replaced by the mid-elevation birds that moved in.
Prediction 3: Forced migrations will reshuffle interactions
While mountain species are moving upward overall, different species are moving at very different rates.4 When communities do not move in synch, existing interactions break up, and novel interactions emerge. New interactions may simply replace old ones: for example, if a newly arrived insect pollinates local plants. Or novel interactions might reshape communities. An innovative experiment from the Alps simulated climate warming scenarios where plant communities either tracked climate together or pulled apart.[6] When communities were shuffled and new interactions created, plants from lower elevations consistently outcompeted those from higher elevations. Thus, species that stay put may be outcompeted by new arrivals, even if they can stand the heat.
Prediction 4: Disease and pests will add insult to climate injury
Climate change can tip the balance of interactions, and may often tip in favour of small organisms, including pests. Why? First, climate stress makes plants and animals more susceptible. An excellent example is the modern die-offs of endangered Whitebark pines. While many trees died from blister rust pathogen or bark beetles, recent work using tree rings to reconstruct growth rates suggests trees were more susceptible due to the effects of climate change, including drought, warming, and reduced snowpack.[7] Second, small organisms tend to have bigger populations and faster reproduction, making them evolutionarily adaptable and thus better poised to cope with rapid change. Recent genetic work suggests that evolution helped mountain pine beetles take advantage of warmer winters to cross the Rocky Mountains.[8] Finally, as insects are “cold blooded,” their activity and populations increase in warmer temperatures, as anyone caught in a warm day on the tundra knows. Global experiments suggest that insects currently play relatively small roles as predators[9] and herbivores[10] at high elevations and latitudes, but we should expect their role to increase in warmer times.
Prediction 5: Some interactions will prove resilient
Waterton Wildflowers, Canadian Rockies. Photo Celestine Aerden
Rapid change will disrupt the mountain ecosystems we know, but many interactions will prove resilient. Interactions are often generalized (most pollinators visit many flower species, for example), such that species on the move may functionally replace each other. Mountains naturally have highly variable climates due to their complex topography and weather patterns. Mountain species may therefore be adapted to cope with considerable upheaval, resilience that could buffer their interactions. Such resilience was proposed to explain why the longest running experiment of its kind found that glacier lily pollination stayed constant over almost three decades in Colorado’s Rockies, despite measurable climate change during that time.[11] It’s worth noting that this example comes from protected private land, where lilies have only one stress – climate change – to deal with at a time.
Conserving interactions
Species interactions are the glue that holds ecosystems together, and robust webs of interactions are more resilient to the insults of global change. The same conservation actions that will preserve the mountain wilderness so many Canadians love will help protect the interactions that wilderness depends on. Reducing the pace and extent of climate warming, preserving large tracts of intact natural land to maintain large populations, and connecting those lands to each other will help slow the rate of change and give species room to adapt and move.
Anna Hargreaves is a professor in the Department of Biology at McGill University. Her research covers ecology, evolution and conservation, with long term field sites in the Rocky Mountains of Alberta and ongoing collaborations in mountains from Alaska to Argentina.
References
1. K. R. Sambaraju, A. L. Carroll, B. H. Aukema, Multiyear weather anomalies associated with range shifts by the mountain pine beetle preceding large epidemics. Forest ecology and management 438, 86-95 (2019).
2. R. L. Beschta, W. J. Ripple, Riparian vegetation recovery in Yellowstone: the first two decades after wolf reintroduction. Biological Conservation 198, 93-103 (2016); M. Hebblewhite et al., Human activity mediates a trophic cascade caused by wolves. Ecology 86, 2135-2144 (2005).
3. I. C. Chen, J. K. Hill, R. Ohlemuller, D. B. Roy, C. D. Thomas, Rapid range shifts of species associated with high levels of climate warming. Science 333, 1024-1026 (2011).
4. B. G. Freeman, J. A. Lee-Yaw, J. M. Sunday, A. L. Hargreaves, Expanding, shifting and shrinking: The impact of global warming on species’ elevational distributions. Glob. Ecol. Biogeogr. 27, 1268-1276 (2018).
5. B. G. Freeman, M. N. Scholer, V. Ruiz-Gutierrez, J. W. Fitzpatrick, Climate change causes upslope shifts and mountaintop extirpations in a tropical bird community. PNAS 115, 11982-11987 (2018).
6. J. M. Alexander, J. M. Diez, J. M. Levine, Novel competitors shape species’ responses to climate change. Nature 525, 515-520 (2015).
7. C. M. Wong, L. D. Daniels, Novel forest decline triggered by multiple interactions among climate, an introduced pathogen and bark beetles. Glob. Change Biol. 23, 1926-1941 (2017).
8. J. K. Janes et al., How the mountain pine beetle (Dendroctonus ponderosae) breached the Canadian Rocky Mountains. Molecular Biology and Evolution 31, 1803-1815 (2014).
9. T. Roslin et al., Higher predation risk for insect prey at low latitudes and elevations. Science 356, 742-744 (2017).
10. A. L. Hargreaves et al., Seed predation increases from the Arctic to the Equator and from high to low elevations. Science Advances 5, eaau4403 (2019).
11. J. D. Thomson, Progressive deterioration of pollination service detected in a 17-year study vanishes in a 26-year study. New Phytol., (2019).