Which niche is which: how to best use niches to model and predict species distribution

The ecological niche is a foundational idea in ecology, and the vigor of debate around how to best understand and visualize the niche is second only to that of disagreements about how the word is pronounced. The concept of a niche is closely related to the dynamics of distribution, and for many reasons the actual distribution of a species will often not exactly match the distribution made possible by the species' physiological requirements.  Accurate descriptions, predictions, and models of a species’ niche can be hard to come by because the specifics of how the ecological niche functions are so varied. However, when niches are modeled well, they can provide information not only on the role and requirements of a species in its current ecosystem, but also how the species will respond to larger changes in environmental conditions over time. The aquatic ecosystem in particular is home to several species which are well suited to a variety of environmental conditions, which makes them excellent local adapters but also well-suited to become invasive.

Kriticos and Brunwell’s 2016 paper “Assessing and Managing the Current and

 Future Pest Risk from Water Hyacinth, (Eichhornia crassipes), an Invasive Aquatic Plant Threatening the Environment and Water Security” uses niche modeling to predict the future habitat of a common invasive aquatic plant. By using CLIMAX, a niche modeling system, they sought to map the current global distribution of water hyacinth and how that distribution would change under the Global Climate Model’s projection of future climate conditions. CLIMAX uses a series of functions fitted to data on water hyacinth population growth rate under heat and cold stress to find the probability of water hyacinth introduction or persistence at a given location.  Climate is one of the most significant limiting factors of growth for water hyacinth, since it is winter hardy but sensitive to frost. Using CLIMAX to model niche possible niches, Kriticos and Brunwell generated the following maps of climate suitability for water hyacinth, both for the current climate (Figure 3) and predicted future climate (Figure 5). Given predicted climate change, heat stress will reduce the potential for water hyacinth to survive in Saharan Africa, the Middle East, and India, but it will likely able to expand its in low altitude areas such as Tasmania and the South Island of New Zealand. Knowing the many instances of suitable habitat it our current climate, the authors suggested a solution to the spreading invasive species in the form of legislation preventing sale and distribution of water hyacinth, as well as a widespread public education campaign about how to protect against it as an invasive species.

Figure 3. Possible distribution of water hyacinth given current climate conditions

Figure 5. Possible distribution of water hyacinth given future climate predictions

As is shown by the wide variety of suitable habitat for this floating aquatic plant, species distribution can be closely tied to physiological tolerances. Modeling niches based on the known growth rates at certain temperatures is a logical way to predict where water hyacinth will be able to spread. However, simulated niche models do not always take into account other environmental factors such as nutrient availability, and ignore the possibility of discrepancies between the fundamental niche of water hyacinth and its realized niche. Research designs which directly compare the physiological limits of an organism and the conditions of the environments they actually occupy provide details of species distribution that are left out by studies which assume growth rate as a result of physical limits as the only determinant in a species niche.  In my proposed research I intend to apply such a design in order to study another aquatic floating plant, Spirodela polyrhizia. S. polyrhizia, like water hyacinth, can also survive a variety of environmental conditions and spread so quickly as to be a problem in ponds and lakes. Learning more about how to best model and predict its niche will help the people who are trying to manage it, as well as add to the growing body of knowledge on the relationship between niche and species distribution.

Kriticos, D., S. Brunnel. 2016. Assessing and Managing the Current and

 Future Pest Risk from Water Hyacinth, (Eichhornia crassipes), an Invasive Aquatic Plant Threatening the Environment and Water Security. PLOS One. 11(8): 1-18.