A variety of different evolutionary strategies promotes species coexistence in the Arctic – there is not one way to handle the short growing seasons and cold. This year we ran a new protocol, part of the ITEX network, which involves surveying the species pool around our community composition plots. We thought the protocol would take us a couple of hours, but little did we know that six hours in we would still be walking in concentric circles around our plots, identifying species after species. We were surprised by how many vascular plant species we found (55 near the plots in the Herschel vegetation type and 66 near the plots in the Komakuk vegetation type) in our two focal ecological communities, and we have been pondering what processes are shaping and maintaining diversity here on Qikiqtaruk ever since.
In “The Theory of Ecological Communities”, Mark Vellend singles out four high-level processes, which shape ecological communities – selection, dispersal, drift, and speciation. The relative importance of each of the four processes varies among biomes – for example, drift might strongly influence communities in tropical areas, whereas environmental filtering, a type of selection, might have a bigger role in cold or arid places where conditions are harsher and resources are limited. Dispersal does have a role on Qikiqtaruk, as for example the grass species Alopecurus alpinus is a recent arrival to our Komakuk monitoring plots between 2004 and 2009, and has been increasing in abundance ever since. Many tundra plants have small seeds and are adapted for long-distance wind or animal dispersal such as the feathery seeds Dryas integrifolia (mountain avens), the fluffy seeds of Eriophorum vaginatum (cottongrass), or the tiny seeds and fluff of willows. Nevertheless, out of the four high-level processes shaping ecological communities, selection seems to be the dominant force here in the Canadian Arctic.
One of the factors discussed by Vellend (2016) influencing community composition is negative frequency-dependent selection. That is a relative advantage for a species when rare relative to other species, because of inter-specific competition, disease, predation or other biological interactions. On Qikiqtaruk, trait-based negative frequency-dependent selection might be stronger than species-based negative frequency-dependent selection, because of the harsh environmental conditions might favour certain trait combinations. For example, evergreen species with small hardy leaves and large seeds or berries dispersed by animals versus deciduous species with large fleshy leaves and small wind dispersed seeds. We have observed high variance in traits both within and between species. Different evolutionary mechanisms and modes of selection could promote overdispersed tundra plant communities – with trait combinations spread across the phylogenetic tree.
Environmental variability within the growing season further maintains different reproductive strategies. For example, Salix richardsonii (Richardson’s willow) and Salix pulchra (diamond-leaf willow) flower early in the season before they leaf out in spring, whereas Salix arctica (Arctic willow) and Salix glauca (grayleaf willow) flower later, after green up and during the peak of the growing season. Here in the Arctic, selection fluctuates on both short timescales within the growing season and long timescales among seasons and over years. Most of the species are hardy perennials, so if conditions are really unfavourable in one year, they can hold off reproducing until the next, an example of “buffered population dynamics”, as outlined in Mark’s book. Even within the tundra landscape different environmental conditions exist with exposed ridges and snow patches where plant phenology, community composition and plant traits differ. Environmental variability over space and time likely dominate selection processes in the tundra biome.
In our first book club blog post , we told you about the two distinct vegetation communities we study – the Herschel type (dominated by Eriophorum vaginatum tussocks) and the Komakuk type (dominated by grassy species and bare ground patches). With the exception of perhaps little patches of tundra around and about, we haven’t observed an intermediate community state, suggesting that there are positive feedbacks in place, which push communities to either the Herschel or Komakuk vegetation type. But, what are the mechanisms which underpin the maintenance of these two different plant communities, and what selective forces are at play?
A highlight of Mark’s book for us have been the summary tables in each chapter on the four high-level processes. We love hypotheses, and the tables summarising the hypotheses linked to selection spurred much discussion in our island book club. We couldn’t help but pick out the ones which we consider to apply most to the Arctic tundra, as well as the ones we have been testing in our work – and make a table of our own.
Table 1. Hypotheses and predictions for selection in plant communities and the links to our work on Qikiqtaruk-Herschel Island.
|Hypothesis||Prediction||Links to the tundra biome and our work||References and ongoing work|
|Constant and spatially variable selection||Composition-environment relationship across space
|We have been investigating how composition-environment and trait environment relationships shape the tundra biome, the extreme edge of life on planet earth.||Elmendorf et al. 2015, Bjorkman et al. in prep., Thomas et al. in prep.|
|Large variance in traits locally||Tundra traits do vary locally both within and between species and as intraspecific trait variation (ITV) is very important for studies at local scales (sites less than 100 km apart).||Bjorkman et al. in prep., Thomas et al. in prep.|
|Change in environment leads to change in species composition||A big disturbance event such as active layer detachments might have facilitated the establishment of Komakuk communities, which are then maintained by smaller-scale cryoturbation. Overtime climate change and reduced rates of cryoturbation might be leading to an increase in plant biomass and a change in community composition.||Qikiqtaruk Ecological Monitoring Team et al. in prep., Bjorkman et al. in prep., Elmendorf et al. 2015|
|Positive relationship between species diversity and spatial environmental heterogeneity||We are using drones to map microclimate, a metric of environmental heterogeneity, to test this prediction! And this is something that could be explored across tundra sites using future data from the HiLDEN network.||See future manuscripts…|
|Negative frequency-dependent selection||Species increase when rare or decrease when dominant at equilibrium||We haven’t found support for this per se, as from our observations, in the tundra rare species tend to stay rare, and dominant species tend to maintain their dominance – though some of the dominant species do seem to be becoming a bit more dominant, though this could be under non-equilibrium conditions with increasing growing seasons, changing active layer depth, etc.||Qikiqtaruk Ecological Monitoring Team et al. in prep.|
|Trait overdispersion locally||We have observed trait overdispersion in the tundra, as there are many different strategies to survive the cold conditions and very short growing seasons. Thus, trait combinations are represented across tundra plant phylogenies, covering almost the entire extent of global trait variation except along the height/seed size axis.||Thomas et al. in prep. and see future outputs of the ArcFunc Working Group|
|Temporally variable selection||Composition-environment relationship across time||The long-term environmental monitoring on Qikiqtaruk and elsewhere in the tundra biome are allowing us to study how composition-environment relationships in the tundra change through time.||Qikiqtaruk Ecological Monitoring Team et al. in prep., Bjorkman et al. in prep., Elmendorf et al. 2015|
|Community-level trait environmental relationship across time||Out of the species traits we study, height is the only one demonstrating a strong trait-environment relationship over time.||Bjorkman et al. in prep.|
|Positive relationship between species diversity and temporal environmental heterogeneity||We suspect this prediction is not valid in our island context, as the floodplain here is the most temporally heterogeneous habitat, yet it is not very diverse. Instead, topographic diversity might be the driver of higher species diversity.||No manuscripts planned at the moment, but maybe we should write something about this!|
|Positive frequency-dependent selection||Community dynamics sensitive to initial composition||Once the Herschel and Komakuk communities have established, they appear to be maintained across the landscape, thus the initial composition is influencing which, if any, new species colonise.||Again, no manuscripts planned, but it could be fun one day to experiment with artificial communities and initial conditions – though tundra plants are quite slow growing and long lived, making these experiments a challenge.|
|Positive intraspecific feedback via environmental modification||In the Herschel communities, the Eriophorum vaginatum tussocks create a specific microclimate, which is favourable for other species. For example, Stellaria longipes is present in both Herschel and Komakuk communities, but appears to do much better in the Herschel vegetation type. Similarly, nitrogen fixers, like Lupinus arcticus, alter the habitat and facilitate the persistence of other nitrogen-loving species.||Maybe it is time for a Qikiqtaruk plant biodiversity patterns manuscript?|
|Multimodal community composition||That is definitely the case on Qikiqtaruk, where there are two very different communities – Herschel and Komakuk. See Table 2 for environmental differences between the two communities as proxies for differences in selective pressures.||Qikiqtaruk Ecological Monitoring Team et al. in prep.|
Table 2. Different environmental factors potentially influencing selection in the two dominant vegetation communities on Qikiqtaruk-Herschel Island.
|Komakuk Vegetation Type||Herschel Vegetation Type|
|Deeper active layer||Shallower active layers|
|More cryoturbation||Wetter soils|
|More N fixing lupines||More acidic soils|
|More disturbance-loving grasses (Alopecurus alpinus, Arstagrostis latifolia)||More microtopography|
So, how do selective forces shape plant composition in the tundra biome and on Qikiqtaruk? Evolutionary adaptation and plastic responses to changing environmental conditions are as strong or stronger here as anywhere else on planet Earth. It is here in the tundra biome – one of the coldest places on the planet – that the environment and species interactions dominate to determine the species composition that we observe across the landscape. Probably Hubbell would not have come up with his ‘Neutral Theory’ if he were studying tundra plant communities instead of Barrow Colorado Island in the tropical forest of Panama, but that doesn’t mean that selection is the only force at play shaping tundra plant communities. To find out more, stay tuned for our next blog post on speciation, dispersal and drift in the tundra.
By Gergana and Isla
This blog post was written on Qikiqtaruk-Herschel Island in the Western Canadian Arctic as part of Team Shrub’s island book club, aiming to read and discuss Mark Vellend’s 2016 book “The Theory of Ecological Communities” while we are out in the field, right next to the communities we study. Team Shrub are a group of plant ecologists who often work in high-latitude tundra ecosystems on topics in community ecology.
The team’s book club discussions are summarised in four blog posts: