Lecture notes for ZOO 4400/5400 Population Ecology

Lecture 26 and 26a (1 and 3-Apr-13) Experimental insights on competition

Required reading.  Marquis and Whelan. 1994. Insectivorous birds increase growth of white oak through consumption of leaf-chewing insects. Ecology 75: 2007-2014.

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In the last several lectures we have examined competition and predation largely by considering mathematical models stemming from the work of Lotka and Volterra. Unfortunately, real-world systems are messier than mathematical models. How well do the results from these models hold up when tested experimentally? We will look at a few aspects of competition and predation from the vantage point of important empirical and experimental studies that illustrate some general concepts.

Experimental insights on competition:

1) Competition most evident when space is the limited (or limiting) resource. We actually see competition most readily when the limited resource is exactly the same for everyone. Even if two species eat almost the same diet, they will have different physiological capacities and so the "same" food item won't be the same resource (one species needs the nitrogen, the other needs the calcium). When IS the resource exactly the same? -- when it is space. That barnacle and that anemone don't care whether the rock is granite or limestone, one doesn't need aluminum from the rock, while the other needs iron. They both need SPACE. Space is space is space. With food, however, one person's meat is another person's poison. Where do we most often see strong importance of space as a limiting resource? In the intertidal marine environment. Best probable analog I can think of around here is space for lichens on rocks. See Paine (1984) for more details.

2) Perturbation (system never reaches equilibrium). In the lab., it is very hard to establish stable competitive coexistence. In nature, though, we often see a whole suite of ecologically very similar species (e.g., some parts of North America have 15 or more sympatric [co-occurring] species of warblers). How can that be? One very likely reason is that the environment is subject to wild fluctuations. For a few years we are headed toward competitive exclusion by the wet-year specialist. Then we have a few dry years and the dry-year specialist start to increase. Before they can take over we go to a few cold years and the cold-year specialists start to increase. One famous proponent of this view was G. Evelyn Hutchinson (1961) . He coined the term "the paradox of the plankton" to describe the puzzling coexistence of a huge number of ecologically very similar plankton species in marine systems. His basic argument was that the environment is so stochastic that the system never reaches equilibrium, and competitive exclusion can never run its course.

3) Indirect effects can be complicated but important. In experiments with seed-eating rodents and ants, (Brown et al. 1986) found that Dipodomys kangaroo rats and Pogonomyrmex harvester ants competed for seeds. In the long term, however, the rodents increase the proportion of small-seeded plants (benefiting the ants), while the ants increase the proportion of large-seeded plants (benefiting the rodents). Coexistence, here, may be mediated by the indirect effects of the competitors on the resources. Even more complexity enters in when other competitors of one species (but not the other) are considered. The harvester ants compete with the smaller ant, Pheidole. The rodents and the Pheidole, however, do not come even close to overlapping in seed size selection, so their direct interaction is negligible. The indirect interaction, however, is important -- again, the Pheidole remove small-seeded plants (and they don't even compete for the large-seeded ones). The whole area of indirect effects is a hot one in community ecology. See Brown et al. (1986) and Wootton (1994) for more details.

Experimental insights on predation:

1) Importance of spatial heterogeneity. In a classic experiment, Huffaker (1958) set up a system with a predatory mite, an herbivorous mite and oranges (food for the herbivore). In a structurally simple environment the predators always drove the prey extinct. In a more complex environment, however, persistence was greatly extended. In the complex environment, with barriers to dispersal, the prey could move to vacant, predator-free patches (oranges separated from others by barriers of vaseline). By moving they could stay "one jump ahead" of the predator that decimated the patches where it found them.

2) Predator-prey (and especially parasite-host) interactions can lead to coevolution. Where predator and prey coexist for long periods of time a factor may enter in that was not part of our simple models. The predator and prey may coevolve. Pimentel et al. (1963) showed such a coevolutionary interaction between houseflies (Musca domestica) and a parasitoid wasp (Nasonia). Over time, experimental populations housed together coevolved so that the flies became more resistant and the parasitoids became less virulent.

3) Competition and predation can interact. Very interesting work by Dybdahl and Lively (1996) shows that the New Zealand mud snail has a patchwork of different kinds of populations. Some are sexual (resistant to parasite) and others are asexual/clonal (very good intraspecific competitors). When the parasite "finds" a largely clonal population, the selection for parasite-resistance greatly increases the frequency of sexual reproduction (the sexual progeny have greater genetic variability as an arsenal to defend against the parasite). The parasite-free populations have strong selection for asexual reproduction (asexual populations can grow much more quickly). The result is a mosaic of interactions over time and space involving both predation and competition.

4) Birds (predators) affect herbivores and indirectly affect tree growth. Marquis and Whelan (1994; required reading). Kalka et al. (2008) found that a similar herbivore exclusion effect on tree growth may be driven as much by the impact of bats on herbivores as by the impact of birds on the herbivores. 

Kalka, M.B., A.R. Smith, and E.K.V. Kalko. 2008. Bats limit arthropods and herbivory in a tropical forest. Science 320: 71. (suggested reading folder on WyoWeb). 

Five factors that promote the stability of predator-prey systems.

We have looked at several ways that predators and prey can interact so as to move toward stable equilibria in some cases and unstable states in other cases.  Ricklefs (1997) pointed to five factors that will tend to stabilize predator-prey interactions:

1) Predator inefficiency (or prey refuges, prey escape strategies or prey defenses).
        Compare and contrast the outcomes of Fig. 24.1 and Fig. 24.2.

2) Density-dependent regulation of predators and prey by factors external to their interactions.
        See the outcome of two curved isoclines (density-dependence) in Fig. 22.5.

3)  Alternative food sources for the predator.
        Prey decline does not necessarily lead to predator crash.  (May also require prey refuges or predator inefficiency).

4)  Refuges from predation for the prey when they are at low density
        (as noted in the experiments by Huffaker, above)

5)  Reduced time lags -- the more quickly either predator or prey reacts to changes in the abundance or behavior of the other, the more likely that the interaction will be stable. Time lags (caused by, for example, discrete breeding seasons and developmental lags) are, however, very common and may often be working to destabilize systems that have several other stabilizing factors working on them


Brown, J.H., D.W. Davidson, J.C. Munger, and R.S. Inouye. 1986. Experimental community ecology: The desert granivore system. Chapter 3 In Community Ecology (J. Diamond, and T.J. Case, eds.) Harper and Row, NY.

Dybdahl, M.F. and C. M. Lively. 1996. The geography of coevolution: comparative population structures for a snail and its trematode parasite. Evolution 50:2264-2275.

Huffaker, C.B. 1958. Experimental studies on predation: Dispersion factors and predator-prey oscillations. Hilgardia 27: 343-383.

Hutchinson, G.E. 1961. The paradox of the plankton. American Naturalist 95: 137-145.

Paine, R.T. 1984. Ecological determinism in the competition for space. Ecology 65: 1339-1348.

Pimentel, D., W.P. Nagel, and J.L. Madden. 1963. Space-time structure of the environment and the survival of parasite-host systems. American Naturalist 97: 141-167.

Ricklefs, R.E. 1997. The Economy of Nature, 4th Edn. W.H. Freeman and Co., NY.

Wootton, J. T., 1994. The nature and consequences of indirect effects. Annual Review of Ecology and Systematics 25:443-466.

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