Figure 1. Food chain. An example of a food chain with four trophic levels (from left to right: primary producer, primary consumer, secondary consumer, and tertiary consumer). ©Karen Minot/AMNH

Food chains aren’t isolated. It was English biologist Charles Elton who recognized in the 1920s that food webs connect them. Food webs can be complex. Often it is easier to simplify these webs by either assigning species with similar trophic relationships to broad functional groups (trophic levels), or isolating portions that don’t interact much with the rest of the community (Figure 2).

Figure 2. Food web. An example of a food web focusing on a subset of species in a community. Note how some species can be categorized into more than one trophic level. For example, the mouse is both a primary consumer (eating a primary producer: wheat) and a secondary consumer (eating a primary consumer: grasshopper). ©Karen Minot/AMNH

Bottom-up and top-down controls

Once we’ve assigned species to trophic levels, an approach to understanding community structure is to model the relationships between the trophic levels. For example, let’s consider three possible relationships between vegetation and herbivores (Figure 3).

Figure 3. Relationships between plants and herbivores. Three possible relationships between vegetation (PP, for primary producers) and herbivores (PC, for primary consumers). The horizontal arrows depict how the change of biomass in one trophic level will lead to a change in the other trophic level. PP → PC shows the unidirectional relationship of an increase in primary producer biomass leading to an increase in primary consumer biomass. Conversely, PP ← PC represents a unidirectional relationship of an increase in primary consumer biomass leading to a decrease in primary producer biomass. When each trophic level responds to changes of biomass of the other, this can be represented with a double-headed arrow. ©Nadav Gazit/AMNH

The first two relationships in Figure 3 underlie what we call the bottom-up and top-down controls of communities, but we should keep in mind that most communities combine both. The unidirectional influence seen in the PP → PC relationship shows a bottom-up model where lower trophic levels influence higher trophic levels. For example, if fertilizer from a farm makes its way into a body of water, it could stimulate the growth of phytoplankton, leading to an increase of biomass in higher trophic levels (Figure 4A). In a bottom-up community, if you add or remove predators, lower trophic levels shouldn’t be affected because the effects are unidirectional.

The top-down model operates in the opposite direction: tertiary consumers limit secondary consumers, secondary consumers limit primary consumers, and primary consumers limit primary producers (Figure 4B). These are examples of the direct effects of one species on another. Because food webs connect so many species, the direct effect of one species on another may set off a chain of events that affects other species in the community (indirect effects). In Figure 4B, this is depicted with the direct effect represented by the solid arrow and the indirect effect represented by the dashed arrow: the number of tertiary consumers indirectly affects the number of primary consumers. When a predator initiates indirect effects on species lower in the food chain, it’s called a trophic cascade.

Figure 4. Bottom-up (A) and top-down (B) controls of communities. In (A), the addition of nutrients into the system (from agricultural runoff, for example) may increase the number of producers, which in turn increases the abundances of each successive trophic level. In (B), the number of predators in the highest trophic level has direct effects on its prey, which has both cascading direct effects (solid arrows) and indirect effects (dashed arrows) on the rest of the food chain. Adapted from Ricklefs & Relyea (2014) by Nadav Gazit/AMNH

Species with large effects

As with predatory fish in lakes, certain species can affect entire communities because their ecological roles, or niches, are pivotal. These type of species are known as keystone species. The wolves of Yellowstone National Park illustrate this concept. In 1995, fewer than 20 wolves were reintroduced to Yellowstone National Park and not only did elk numbers decline, but the effects cascaded through the ecosystem because the elk changed their behaviors, including where they foraged.

Sometimes a species’ impact on a community not due to its trophic interactions, but to its sheer abundance. Dominant species powerfully influence the occurrence and distribution of other species because of their prevalence or collectively large biomass. For example, mangrove trees and shrubs dominate and characterize the coastlines of many tidal marshes in the tropics. These mangrove trees may have become dominant in these areas because they are more able to sustain harsh conditions and better exploit limited resources such as fresh water. These dominant trees have abiotic effects on the ecosystem, including stabilizing soils and shading the water, which in turn affects which other species live in those areas. Dominant species might also become dominant because they succeed at avoiding predation or disease. We see this when invasive species are introduced to an area free of predators and disease agents, exploit this competitive advantage over native species, and attain a high biomass.

Some organisms exert their influence not through their abundance but by affecting the physical environment. They may create, the way corals build reefs by secreting calcium carbonate; significantly alter; maintain; or destroy their environments. Species that dramatically alter their physical environment are called “ecosystem engineers” or “foundation species.” For example, even a few beavers can transform a landscape by felling trees and building dams. The effects can be positive or negative, facilitating or inhibiting colonization or growth depending on the needs of other species (Figure 5).

Community stability and alternative stable states

As we’ve learned, a disturbance can change the community composition, species abundances, and the relationships between species. If the disturbance is an event like a landslide, the community may regain its original structure over time. This ability is called community stability and is measured by observing how much a community changes after a disturbance (resistance) and if, or how long, it takes to return to its original state (resilience).

When a community does not bounce back, a new and stable community may emerge that is resistant to further change. For example, if a beaver dams a stream that runs through a wetland, most of the native vegetation may die. When resources are reduced, the beaver will abandon its den, and when the dam gives way, the water will drain, exposing sediment-rich soils. If the invasive common reed (Phragmites spp.) is present and dominates this open niche, the community will shift from a one characterized by diverse native vegetation to a stable state dominated by dense common reed monocultures (Figure 5). Alternative stable states are not necessarily bad for ecosystems, and later in the course we will discuss an ongoing debate about the pros and cons of these “new” or “novel” ecosystems.

Figure 5A

Figure 5B

Figure 5A and B. Vegetation changes due to beaver activity. Beaver activity can change the community of species present in an area. The open and inundated areas created by beaver activity can be colonized by species such as the invasive common reed (Phragmites spp.) (see tall grass-like species in center and left-center of top image. In some cases, this invasion can lead to a shift from a diverse native species community to a new stable state dominated by a common reed monoculture, as seen in the bottom image. Top, Nicholas A. Tonelli/CC BY 2.0; bottom, ©Ontario Phragmites Working Group

Conclusion

This week we focused on community composition and how the interactions between species, including trophic relationships, can shape a community. The effect of an invasive species on the forests of Guam offers a dramatic example of these interconnections. In the mid-1940s, the brown tree snake (Boiga irregularis), was unintentionally introduced to the island, where it encountered no predators and ample food. To date, the snake has caused the local extinction of 12 native bird species, resulting in a trophic cascade in the food chain and indirect effects in how species interact. Trees in the forest rely on birds to disperse their seeds (known as a fruit-frugivore mutualism), and Guam has seen a 61% to 92% decline in tree seedling recruitment. This reduction in biodiversity of both dispersers and their plant mutualists will slow forest regeneration and, consequently, may reduce ecosystem resilience and carbon storage—two topics that will be discussed later in the course. Given our biodiversity crisis, the need to investigate the relationships between species is ongoing and urgent.

REFERENCES

Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2008). Biology. Boston: Pearson.

Morin, P. J. (1999). Community Ecology. Malden, MA: Blackwell Science.

Ricklefs, R. E., & Relyea R. (2014). Ecology: The Economy of Nature. New York: W. H. Freeman and Company.

Rogers, H. S., Buhle, E. R., Hillerislambers, J., Fricke, E. C., Miller, R. H., & Tewksbury, J. J. (2017). Effects of an invasive predator cascade to plants via mutualism disruption. Nature Communications, 8, 14557.