**Introduction: **Functional responses describe the relationship
between an individualís rate of consumption and food density.** **They
have generally been divided into three types (see TYPE
II and TYPE III FUNCTIONAL RESPONSE). The type I functional response
is a linear increase in consumption rate as food densities rise, until
reaching a maximum consumption rate. The slope of the line is equal to
the consumerís attack rate (also called the capture or searching efficiency).
Examples of this type of functional response are somewhat rare; they are
most commonly found in herbivoreóplant interactions, and some invertebrate
predator-prey interactions.

**Importance: **A consumerís functional response
to changes in food density is an important component of population regulation.
The type I functional response is generally assumed to have a stabilizing
effect on population dynamics.

**Question:** Under what conditions is the type
I functional response observed? Why does its effect tend to be stabilizing?

**Methods:** The type I functional response can
be described by a linear equation of the form

y = ax + b ,

where *a* is the slope of the line and *b*
is the intercept. In the 1970ís, Batzli *et al*. measured the functional
response of brown lemmings foraging in arctic tundra. The data from early
summer in 1976 are graphed below (data redrawn from Batzli et al. 1981).

The relative foraging rate was calculated by dividing
the absolute foraging rate(mg/min) by the square root of body weight (gBW^{.5}).

**Interpretation: **A linear relationship between
food density and foraging rate is clearly exhibited. The equation for the
line fitted to these data is *y* = 0.101*x* + 2.62; the slope
of the line is 0.101, and the intercept is 2.62. The authors suggest that
the reason the line does not pass through the origin (coordinates [0,0]
on the graph) is that the lemmingsí diet included a constant rate of consumption
of mosses, whose biomass measurements were not included in the study.

The phase of linear increase in a type I functional response can have a different effect on predator-prey or herbivore-plant dynamics depending on the slope of the line. For a slope equal to one, the risk of being eaten is the same at all population densities. For a slope greater than one, the risk of being eaten increases as food density increases; this tends to stabilize population dynamics. For a slope less than one, the risk of being eaten decreases as food density increases, which also occurs when the consumption rates reach their maximum in a type I response. In this situation there is an inverse density-dependence in which food items in lower-density populations are at greater risk, and the effect of the type I response is destabilizing to population dynamics.

**Conclusions:** The foraging rates of the brown
lemmings did not reach a maximum in this study. Clearly, however, the consumption
rates cannot continue increasing indefinitely, regardless of food density!
How do we know that this curve does not simply represent the beginning
of a type II response? Batzli *et al*. argue that since they provided
the lemmings with the highest food densities found in their natural habitat,
it is reasonable to conclude that *within the range of naturally occurring
food densities*, lemmings do indeed exhibit a linear functional response.

A "true" type I functional response is possible when handling time is equal to zero, and predators do not become satiatedó not a realistic situation. However, if an organism exhibits a linear response to the food densities it encounters under natural conditions, as the lemmings did, then for all intents and purposes it is exhibiting a type I functional response and will have a corresponding effect on the prey/food population dynamics.

**Additional Question:**

1. Are there any other biologically realistic conditions under which a type I response would be seen? What are they?

**Sources:** Batzli, G. O., H.-J. G. Jung, and
G. Guntenspergen. 1981. Nutritional ecology of microtine rodents: linear
foraging-rate curves for brown lemmings. Oikos **37**:112-116.

Begon, M., J. L. Harper, and C. R. Townsend. 1996.
__Ecology:
Individuals, Populations, and Communities__, 3rd edition. Blackwell Science
Ltd., Cambridge, MA.

*copyright 1999 M. Beals, L. Gross, S. Harrell*