Resource Partitioning and Why It Matters

Careful and detailed study has revealed some of the many ways in which potential competitors show differences in patterns of resource use.

Perhaps the most obvious way that species can partition resources is in terms of what they consume. This is often underpinned by differences in their morphological adaptations that allow differential resource use. For example, a detailed study of bumblebees in the mountains of Colorado (Figure 1) neatly shows how different species can be best adapted to specific forms of a resource (Pyke 1982). Bumblebee species all compete for nectar from flowers, but crucially these flowers vary in the length of their corolla. Matching this variation, different bumblebees in this area appear to be adapted to specific species of plant that have different corolla lengths in their flowers. Careful observations of bumblebee visits to different flowers revealed clear resource partitioning — different species preferred different length corollas in accordance with their proboscis length (i.e., long proboscis, long corolla; short proboscis, short corolla).

Ecologists have found it relatively easy to document the various differences in the ways that ecologically similar animal species use their environment and resources. In many cases nothing more than a pair of binoculars and careful observation is required. Studying resource partitioning in plants can be much more challenging, and the relative lack of such examples has led many ecologists to wonder whether plants really do show resource partitioning; after all, they all require a limited suite of resources (light, water, and nutrients). However, ecologists do not give up easily, and recent work has shown that coexisting plant species often differ in the forms of nitrogen (e.g., ammonium versus nitrate or organic v. inorganic) they prefer (Kahmen et al. 2006). Differences in rooting depth and light-use optima have also been documented. Nevertheless, how common or important resource partitioning is in plants remains uncertain and is an active area of current research.

Figure 1: Resource partitioning among bumble bees (Bombus spp.)

Species have proboscises of different lengths, enabling them to specialize in the exploitation of plants with different length corollas. Species with similar length proboscises occur at different altitudes (Pyke 1982).

© 2011 Nature Education Adapted from Begon et al. (1990). All rights reserved.

When species use a resource similarly in one respect (i.e., they show "overlap" in their use of a resource along one axis), they commonly show differences in some other respect (along another axis). For example, the bumblebee study mentioned above was conducted over sites varying in altitude. Pyke (1982), the author of this work, found that although several bumblebee species had similarly long proboscises and so could forage on similar species of plant, they were differentially specialized to altitude, so that sites at different altitudes were dominated by a different pair of long- and short-length proboscis species. Another striking example comes from tree-dwelling Anolis lizards on the Caribbean island of Bimini (Schoener 1974; Figure 2). In this case, species either foraged in the same places (as determined by the thickness of branches they perched on) or ate similar sized prey, but in no cases did two species do both of these. In contrast, individuals of the same species commonly showed a high degree of overlap along both of these resource axes (Figure 2).

Figure 2: Similarity in structural habitat and prey size in pairs of individual Anolis lizards from the Caribbean island of Bimini

Pairs of classes that do not belong to the same species (interspecific) do not show high overlap along both axes (i.e., there are no interspecific pairs in the dashed box).

© 2011 Nature Education Adapted from Schoener (1974). All rights reserved.

Ecological theory shows that interspecific competition will be less likely to result in competitive exclusion if it is weaker than intraspecific competition (Chesson 2000). Resource partitioning can result in exactly this! By consuming slightly different forms of a limiting resource or using the same limiting resource at a different place or time, individuals of different species compete less with one another (interspecific competition) than individuals of the same species (intraspecific competition). Species, therefore, limit their own population growth more than they limit that of potential competitors, and resource partitioning acts to promote the long-term coexistence of competing species. Other theories have been put forward that attempt to explain the coexistence of large numbers of species in local communities, and assessing their importance relative to resource partitioning is likely to be an active area of research for years to come. There is no doubt, however, that mechanisms reducing interspecific relative to intraspecific competition act to promote coexistence, and resource partitioning can achieve this.

So far we have discussed the phenomenon of resource partitioning and its role in reducing interspecific competition and therefore promoting coexistence. Where does resource partitioning come from in the first place (i.e., what causes species to be able to partition resources)?

Competition can limit the growth, and ultimately the reproductive success, of individuals. It can consequently serve as a selection pressure driving differential reproductive success and the evolution of traits that enable organisms to use resources differently compared to their competitors. This process has been clearly demonstrated in the evolutionary events that have followed the colonization of volcanic islands. For example, a single species of seed-eating finch originally colonized the Galapagos Islands and was faced with a diverse range of seed types and sizes. However, the beak of the founding species only allowed it to eat a small subset of the available seed types and sizes. The advantages gained by individuals that were able to exploit slightly different seed types drove evolution of many new species, each with different shaped beaks enabling them to specialize in a particular size of seed (Grant 1986).

There is convincing evidence that competition (and not another selection pressure such as predation) drove — and maintains — differences in beak sizes between these species. When species occur on their own on an island (i.e., there is no interspecific competition), they have similarly sized beaks and presumably exploit similarly sized seeds. When several species occur on the same island however, they show clear differences in beak shapes, showing that it is interspecific competition that maintains differences between species and resultant resource partitioning (Figure 3).

An interesting new twist has been added to this story of the evolution of resource partitioning. Around 25 years ago the island of Daphne Major, originally host to just a single species of Darwin's finch (Geospiza fortis) was invaded by another, larger beaked species (G. magnirostris). Amazingly, researchers have documented a rapid evolutionary shift in the sizes of beaks in G. fortis. In response to severe competition for larger seeds it has evolved to take full advantage of small seeds. This study is particularly important because the researchers were able to document the process of character displacement, and by monitoring the levels of resources, show that competition was the most likely possible cause (Grant & Grant 2006).

Figure 3: A classic example of character displacement

When multiple species of Darwin's finches co-occur on an island, they show differences in bill depth (and eat different sized seeds) compared to when they are alone on an island.

© 2011 Nature Education Adapted from Morin (1999). All rights reserved.