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Principal foods versus preferred foods and their relations to stocking rate and range condition
PRINCIPAL FOODS VERSUS P R E F E R R E D FOODS A N D THEIR RELATIONS TO STOCKING RATE A N D RANGE CONDITION
GEORGE A. PETRIDES
Department of Fisheries and Wildlife, Michigan State University, East Lansing, Michigan, USA
A BS TRA C T
The principal foods of an animal population are those which it eats in greatest quantities. These foods, however, may or may not be those which are preferred Preferred foods are those which are proportionately more frequent in the diet than in the available environment. Several indexes to food preferences are discussed but there are special values in the rating which results when the percentage in the diet is divided by the percentage availability. The importance offood preference ratings in range management, habitat evaluation and otherwise is reviewed
INTRODUCTION
The concept of food preference has widespread ecological significance. It is basic to scientific range management, to the understanding of predator-prey relationships, and to other aspects of animal biology. Despite its fundamental importance, however, a lack of widespread understanding concerning it has been demonstrated. Cook & Stoddart (1953) have stated that 'the measurement and interpretation of (food) utilisation and preference.., are not well understood' and the National Academy of Sciences (1962) observed that 'researchers have not spent much time perfecting methods of measuring (food preference values).' Uncertainties concerning food preference analysis have been further emphasised by recent papers which referred in their titles to the food preferences of certain grazing animals yet devoted their discussions to the principal foods of those herbivores. Because of these evidences, it is felt that the important differences between evaluations of 'important' foods and 'preferred' foods deserve further clarification. Since it is believed easiest to visualise the feeding effects of large ungulates on herbaceous and shrubby vegetation, this discussion perhaps may best be followed 161 Biol. Conserv. (7) (1975)--© Applied Science Publishers Ltd, England, 1975 Printed in Great Britain
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G E O R G E A. P E T R I D E S
with that situation in mind. There is, however, no basic difference in food preference relationships whether the concern is with livestock and their pasturage, deer and their browse plants, predators and their animal foods, fish and their bottom organism or other prey, or other consumer species and their food resources. Prey preference ratings have been discussed elsewhere (Petrides & Pienaar, 1966; Pienaar, 1969).
C A L C U L A T I O N S OF F O O D C O N S U M P T I O N
VALUES
The principal foods of an animal population are here regarded as those which it eats in greatest quantities. These form the largest percentages of food items in the animals' diet. Calorific or other nutritive values are not involved here and whether an animal prefers those foods which comprise the greatest bulk of its diet is another matter. In studies designed to evaluate the comparative amounts of food species eaten, the total amounts ingested are not so important as the relative amounts of each species consumed. The quantities of various food species removed from the environment by herbivores (R, Table 1) thus comprise a measure of their dietary TABLE 1 CALCULATION OF FOOD PREFERENCE RATINGS AND THE DIETARY IMPORTANCE OF FORAGE SPECIES USING HYPOTHETICAL DATA
Quantities 2
Percentages
(p)
Forage
(A)
(R)
(a)
(d)
(r)
'Species "l
Available
Removed
Available
Die#
Removed 4
Preference ratings 5
B C E F G H I J
1000 1500 2000 2400 2500 400 100 100 S A = 10,000
900 750 600 300 210 150 90 0 S R : 3,000
10 15 20 24 25 4 1 I 100
30 25 20 I0 7 5 3 0 100
90.0 50.0 30.0 12.5 8.4 37.5 90.0 0 --
3.00 1.67 1.00 0.42 0.28 1.25 3.00 0.00 --
Totals
z Species are designated by letters which are without meaning; A and D are not used for species to avoid confusion with symbols for availability and diet. 2 Numbers or volumes of food items available and removed (eaten). 3 Percentage of each species removed as related to all food removed and consumed: d = IOOR/SR. 4 Percentage of available forage (A) which is consumed: r-- 100 R / A . 5 Note that the relative values of percentages of foods removed (r) directly parallel the preference rating values, p = d/a.
intake. By comparison with the total amount eaten (SR), these can be directly converted into percentages (d) which together comprise the total diet. In the example of Table l, forage species B through I are those materials which are eaten. Species B through F are identifiable as major foods in the diet.
PRINCIPAL FOODS VERSUS PREFERRED FOODS
163
CALCULATION OF FOOD PREFERENCE RATINGS
The term 'food preference' has been used in the literature for years although, as stated, not always with precision. Most often the term has been utilised, as it is here, to refer to the 'likeability' of a forage species (Sampson, 1952, p. 385) or the 'choice an animal makes' (Stoddart & Smith, p. 130) of available toods. While the word palatability has been employed in the range management literature as more or less synonymous with food preference, it has also been assigned other meanings (Sampson, 1952; Stoddart & Smith, 1955; National Academy of Sciences, 1962) so that the possibility of confusion makes its continued use undesirable. The reasons why a food species is preferred or neglected, it must be noted, is another matter. These reasons are not revealed in any way by the degree of preference or neglect which may be calculated to exist. It cannot be sa4d with certainty, for example, that any species is distasteful just because it is seldom eaten. It may be that taste sensations are involved but there is no assurance from availability and consumption data alone that this is true. Nor is there any evidence that harshness, chemical composition, succulence, or any other possible factor or combination of factors is necessarily involved. If we learn that there is preference for, or a tendency towards neglect of, a food, this is a starting point from which to undertake more searching studies. We are concerned here only with the detection and measurement of the degree to which a food is preferred or is treated with disdain. In terms of volume, numbers, or weight, several species may be equally significant in the total diet of an animal population at a given season or in a stated area. If so, then those several food species must comprise equal percentages of the diet. If one of those equally-consumed species is less available than others, then the animal feeding on it must generally have sought it out and that animal may be said to display a preference for it. A preferred food species, then, is one which is proportionately more frequent in the diet of an animal than it is in the available environment. Food preference is the extent to which a food is consumed in relation to its availability. Some foods may be proportionately less abundant in the total food consumed than in the available forage. Such species may be said to be neglected or even avoided completely by the feeding species. A food which is eaten exactly to the extent to which it is available in nature is neither preferred nor neglected. It is neutral with regard to the feeding preferences of the foraging species. It should be noted that a food item may be much sought after, but if it is difficult to find then it cannot be much utilised. Conversely, if foods which are not much liked are the only ones available, they may of necessity comprise a large proportion of the diet. As developed in the hypothetical example of Table 1, to determine food preference relationships in the field it is necessary to measure the amounts of
164
GEORGE A. PETRIDES
various foods available (A) for feeding and the extent to which those foods are actually removed (R) by herbivores. The amounts of foods available can be converted directly into percentages available (a). The quantities of forage plants eaten, however, can be calculated as percentages of two types. The first, as explained above under food consumption values, comprises the percentages (d) of each food species as components of the diet. The second is the relationship between the amount (R) of each species eaten and the amount available (A) of the food species. These latter values yield the percentages (r) of each species which is removed from the habitat by feeding. It is essential to distinguish between the two sets of feeding or 'utilisation' relationships (d and r). The percentage removal values (r) alone indicate the relative intensities of feeding and therefore the degree to which some species are preferred over others (as in Wehb, 1959). They do not, however, provide a reference point to indicate which species are the preferred forages and which tend to be neglected as foods. In contrast, though, if the percentage eaten (r) is divided by the percentage available (a), a preference value is determined which centres on 1.00 as a reference point. Species with preference values above 1.00 (species B, C, H, I of Table I) are those which are sought out as preferred foods. Ratings below 1.00 represent forage species (species F, G, and J of Table 1) which for some reason are neglected or avoided as foods. Species E of Table 1 has a preference rating of exactly 1.00, being neither preferred nor neglected but being eaten precisely in proportion to its abundance in the field. Some species of course may be totally avoided (e.g. J in Table 1), having a zero preference rating. In the calculation of preference ratings, the availability of each forage species is taken into account. It must be present to be eaten, of course, but no food can be said to be preferred 'became it is abundant', as has sometimes been reported in the literature. The measurement of vegetation available for feeding must include all accessible forage present prior to the feeding being evaluated. If both the foods 'available' and those 'eaten' are measured at the same time, the amounts missing and presumed eaten may have to he estimated by comparisons with uneaten samples. The amounts estimated to have been eaten must then be added to that which remains so that the original amount available is approximated. It is not only possible but of frequent occurrence that a species is highly preferred and yet relatively unimportant in the herbivore diet. In Table 1, both species H and I are of this type. In the species of this example, B and I have identical preference values, yet forage B is ten times as important by bulk, as species I in the total diet. In the hypothetical example of Table 1, all species from B through C were eaten more abundantly than H or I despite the high preference values of those latter two species. Hess & Swartz (1940) evidently were first to develop the idea of food preference ratings, which they termed 'forage ratios.' Working with trout and bottom insects,
PRINCIPAL
165
F O O D S VERSUS P R E F E R R E D F O O D S
however, they stated that several food species were unavailable due to their size or protective habits and advised that some derived 'forage ratios are due to availability rather than to preference.' The direct calculation of availability percentages in the environment, though, does yield food preference ratings which include availability as an essential component. ivlev (1961, as seeen in Siefert 1972) developed an electivity index as a modification of the forage ratio. His index (see Table 2) yielded values between + 1 TABLE 2 COMPARISONS OF PREFERENCE RATINGS AND ELECTIVITY INDEXES USING tIYPOTHETICAL DATA
Paired Forage 'species' K L M N
Amounts Available Removed
Availability
Percentages Diet
Removal
Indexes Preference Electivity
(A)
(R)
(a)
(d)
r = 100 R / A
p -- d / a
400 600 200 800
360 240 160 40
40 60 20 80
60 40 80 20
90 40 80 5
I. 50 0.67 4.00 0.25
E = d-a + 0.2 - 0.2 + 0.6 -0.6
and --1, 'the l~ormer value indicating complete positive selection and the latter, complete rejection of a food item' (Siefert, 1972). The electivity index has the advantage of computing selection values which lie between definite positive and negative limits, with an index of zero representing neither preference nor neglect. It offers the decided disadvantage, however, that its values are not proportional to the actual differences in preference indicated by the data (see columns r and p versus E, Table 2). From the p ratings of Table 2, for example, one can say that species M is eaten four times as much as is expected from its abundance or that species N is consumed only a quarter as much. It is difficult to infer these relationships from the electivity values. For this reason, Ivlev's index is not adopted here. V A L U E S OF F O O D P R E F E R E N C E D A T A
Of the forage species present in any area, those with highest and lowest preference ratings are especially useful as indicators of herbivore stocking density and range condition.
Stocking rate The number of animals occupying a measured area is the current stocking rate or stocking density of that species. That density may be higher or lower than the carrying capacity of the range. If the former condition prevails, the current stocking rate may not be one which can be sustained on a permanent basis. Overutilisation of the habitat may lead to deterioration of range condition and even to range destruction.
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G E O R G E A. P E T R I D E S
The reduction in animal population which may result from range depletion and consequent malnutrition or starvation typically will not alleviate grazing pressure on the range. Overstocking is not a matter of the absolute number of animals present. Rather, it is animal density with respect to carrying capacity which is significant. On an overpopulated range, the animals which survive starvation still may be too many for the range to support. Even with low numbers per unit of area, a stocking rate which exceeds range carrying capacity usually continues to prevent range recovery and to cause increasing range depletion. To appraise such circumstances, current range condition and trend must be established.
Range condition Where grazing animals are sufficiently abundant to remove vegetation faster than it can be replaced, changes are induced in the relative abundance of the plant species involved. The most-preferred forage species tend to be depleted first, with the remaining species affected more or less severely according to their preference ranking and the intensity and duration of cropping. Conversely, the avoided species tend to increase, as do neglected forages where grazing pressures are not overwhelmingly severe. The range management literature (for example see Dyksterhuis, 1958) often refers to various plant species as 'decreasers', 'increasers', or 'invaders'. In terms of this report, decreasers are preferred forage species, increasers are neglected foods, and invaders are avoided plants. The range condition, or current status of the vegetative habitat, is a reflection of the past and current stocking rates of the principal herbivore populations inhabiting the area.
Range trend A habitat trend towards improvement or degradation can be evaluated from the degree to which range plants are renewing themselves and providing soil protection. The severity and persistence of previous overuse of the range and the determination of whether over-utilisation is continuing, however, can be further evaluated from evidences that preferred and neglected foods are either being used heavily or not and that these and avoided forage species are either increasing or decreasing. Where preferred foods are scarce, neglected foods are heavily eaten and avoided foods form the bulk of the available food supply. Here, it can be expected that the consumer population is overpopulated and probably is in an impoverished physical condition. Under such circumstances food is probably the factor which prevents population increase.
167
PRINCIPAL FOODS VERSUS PREFERRED FOODS
ANALYSIS OF PREFERENCE DATA
Food preference ratings always represent conditions at the time that feeding occurred and relate principally to the area involved. Yet comparisons between areas and seasons may be instructive. The data of Tables 3 and 4 indicate how field data TABLE 3 VEGETATION DATA FROM APPROXIMATELY 100 MILACRE (4.840sq. yds.) PLOTS ON SOUTH FOX ISLAND, MICHIGAN, FOR WHITE-TAILEDDEER DURING THE WINTER OF 1969 (BASED ON DATA OF PALMER • KING, 1970)
Species Mountain maple (Acer spicatum Lam.) American yew (Tax'us canadensis Marsh.) Sugar maple (Acer saccharum Marsh.) American hazelnut ( Corylus americana Walt.) Chokeeherry wPrunus virginiana L.) hite cedar ( Thuja occidentalis L.) Totals
Kg / ha* Available Removed
Percentages Preference Available Diet Removed ratings
1.43
0.44
0.13
0.17
31.0
1075.38
250.81
95.13
98.02
23.3
1.03
6.00
1.16
0.54
0.45
19.3
0.83
3.61
0.64
0.33
0.25
17.8
0.76
1.58
0.12
0.14
0.05
7.8
0.36
42.01
2.72
3.73
1.06
6.5
0.28
255.89
100.00
100.00
22.6
1.00
1130.01
1.31
*Dry weights.
on forage availability and utilisation may be used in determining forage preference ratings. American yew is generally accepted as a highly preferred winter white-tailed deer forage throughout its range in the northeastern United States. Where it still occurs abundantly, as on some of the islands in the Great Lakes (Table 3), it may indeed comprise the bulk of the deer's diet. Yet it is not a highly preferred food there. White cedar is acknowledged as an important winter forage plant for white-tailed deer throughout most of its range. Yet on South Fox Island, where other plants are available (Table 3) or perhaps where soil conditions are different, it becomes a neglected species and is unimportant in the diet. Mountain maple is the most highly" preferred forage in winter on South Fox Island (Table 3) yet it is an avoided species in summer in the Adirondacks (Table 4). Similarly, sugar maple had a higher preference value on the island than in New York state at a different season. On South Fox Island (Table 3), decreases in mountain maple and American yew or increases in the other woody plants present should indicate that the deer herd there is increasing in size. The first five food plants listed in Table 4, and especially the first two (alternative-leaved dogwood and common elderberry), should show
168
GEORGE A. PETRIDES
TABLE 4 CALCULATION OF SUMMER BROWSE PREFERENCES OF ADIRONDACK MOUNTAINS, NEW YORK, WHITE-TAILED DEER (BASED ON DATA OF WEBB, 1959)
Species Alternate-leaf dogwood (Comus alternifolia L.f.) Common elderberry (Sambucus canadensis L.) Black cherry (Prunus serotina Ehrh.) Yellow birch (Betula lutea Michx. f.) White ash (Fraxinus americana L.) Red raspberry (Rubus idaeus L.) Sugar maple (Acer saccharum Marsh.) Others* Totals
Numbers of stems Percentages Available Browsed Available D i e t Removed
Preference ratings
16
Il
2.0
32.4
68.8
16.20
12
8
1.5
23.6
66.7
15.73
11
1
1.4
2.9
9.1
2.07
54
4
6.7
! 1.8
7.4
1.76
19
1
2.4
2.9
5.3
1.21
124
3
15.4
8.8
2.4
0.57
467
6
58.2
17.6
!.3
0.30
101
0
12.4
0
0
0
804
34
100.0
100.0
--
--
* Hobblebush (Virurnum alnifolium Marsh.), beech (Fagus grandifolia Ehrh.), fire .cherry (Prunus pensylvanica L.), quaking aspen (Populus tremuloides Michx.), striped maple (Acer spicatum Lam.), Canada honeysuckle (Lonicera canadensis Bartr.), mountain maple (Acer spicatum Lam.), red spruce (Picea rubens Sarg.) were present but avoided as summer foods. reduced availability u n d e r heavy use a n d should serve as good indicators of s u m m e r deer o v e r a b u n d a n c e o n the A d i r o n d a c k deer range.
CONCLUSIONS F o o d preference ratings provide useful evidence towards the u n d e r s t a n d i n g of trophic ecology a n d practical resource m a n a g e m e n t . T h e y yield i n f o r m a t i o n o n the suitability of ranges as a n i m a l habitats a n d enable appraisals of the a b u n d a n c e of c o n s u m e r organisms with respect to range carrying capacity. The relative a b u n d a n c e a n d c o n d i t i o n of food species present in a n area m a y f u r t h e r indicate the vigour of c o n s u m e r p o p u l a t i o n s a n d whether food factors limit c o n s u m e r a b u n d a n c e there. If calculated similarly on a widespread basis, preference values should e n a b l e m e a n i n g f u l c o m p a r i s o n s between research appraisals a n d m a n a g e m e n t results across the geographic ranges of i m p o r t a n t species. REFERENCES
C. W. • STODDART, L. A. (1953). The quandary of utilization and preference. J. Range Mgt, 6, 329-35. DYKSTERHUIS,E. J. (1958). Range conservation as based on sites and condition class. J. Soil Wat. Conserv. 13, 151-5.
COOK,
PRINCIPAL FOODS VERSUS PREFERRED FOODS
169
HESS, A. D. & SWARTZ,A. (1940). The forage ratio and its use in determining the food grade of streams. Trans. IV. Am. Wildl. Conf., 5, 162-4. IVLEV, V. S. (1961). Experimental ecology o/'./eeding of.fishes. New Haven, Yale University Press. NATIONALACADEMYOFSCIENCES.(1962). Basic problems and techniques in range research. ?Cam. Acad. ScL, Nat. Res. Coun. Publ. 890. Washington, D.C., 342 pp. PAI,M~R, W. L. & KING, D. R. (1970). South Fox Island Deer -1969. Mich. Dept. Nat. Res. R & D Rept, 204, I1 pp. PETRIDES. G. A. & PIENAAR, U. de V. (1966). Calculation and ecological interpretation of prey preferences for large predators in Kruger National Park, South Africa. Michigan State University, East Lansing (Mimeographed). PIENAAa, U. de V. (1969). Predator-prey relationships amongst the larger mammals of Kruger National Park. Koedoe, 12, 108-76. SAMPSON, A. W. (1952). Range management principles and practices. New York, Wiley. SIEVERT, R. E. (1972). First food of larval yellow perch, white sucker, bluegill, emerald shiner, and rainbow smelt. Trans. Am. Fish. Sot., 101,219-25. STODDAR1", L. A. & SMrrH, A. D. 0955). Range management. New York, McGraw-Hill. WEr~B, W. L. (1959). Summer browse preferences of Adirondack white-tailed deer. J. Wildl. Mgmt, 23, 455 -6.
GEORGE A. PETRIDES
Department of Fisheries and Wildlife, Michigan State University, East Lansing, Michigan, USA
A BS TRA C T
The principal foods of an animal population are those which it eats in greatest quantities. These foods, however, may or may not be those which are preferred Preferred foods are those which are proportionately more frequent in the diet than in the available environment. Several indexes to food preferences are discussed but there are special values in the rating which results when the percentage in the diet is divided by the percentage availability. The importance offood preference ratings in range management, habitat evaluation and otherwise is reviewed
INTRODUCTION
The concept of food preference has widespread ecological significance. It is basic to scientific range management, to the understanding of predator-prey relationships, and to other aspects of animal biology. Despite its fundamental importance, however, a lack of widespread understanding concerning it has been demonstrated. Cook & Stoddart (1953) have stated that 'the measurement and interpretation of (food) utilisation and preference.., are not well understood' and the National Academy of Sciences (1962) observed that 'researchers have not spent much time perfecting methods of measuring (food preference values).' Uncertainties concerning food preference analysis have been further emphasised by recent papers which referred in their titles to the food preferences of certain grazing animals yet devoted their discussions to the principal foods of those herbivores. Because of these evidences, it is felt that the important differences between evaluations of 'important' foods and 'preferred' foods deserve further clarification. Since it is believed easiest to visualise the feeding effects of large ungulates on herbaceous and shrubby vegetation, this discussion perhaps may best be followed 161 Biol. Conserv. (7) (1975)--© Applied Science Publishers Ltd, England, 1975 Printed in Great Britain
162
G E O R G E A. P E T R I D E S
with that situation in mind. There is, however, no basic difference in food preference relationships whether the concern is with livestock and their pasturage, deer and their browse plants, predators and their animal foods, fish and their bottom organism or other prey, or other consumer species and their food resources. Prey preference ratings have been discussed elsewhere (Petrides & Pienaar, 1966; Pienaar, 1969).
C A L C U L A T I O N S OF F O O D C O N S U M P T I O N
VALUES
The principal foods of an animal population are here regarded as those which it eats in greatest quantities. These form the largest percentages of food items in the animals' diet. Calorific or other nutritive values are not involved here and whether an animal prefers those foods which comprise the greatest bulk of its diet is another matter. In studies designed to evaluate the comparative amounts of food species eaten, the total amounts ingested are not so important as the relative amounts of each species consumed. The quantities of various food species removed from the environment by herbivores (R, Table 1) thus comprise a measure of their dietary TABLE 1 CALCULATION OF FOOD PREFERENCE RATINGS AND THE DIETARY IMPORTANCE OF FORAGE SPECIES USING HYPOTHETICAL DATA
Quantities 2
Percentages
(p)
Forage
(A)
(R)
(a)
(d)
(r)
'Species "l
Available
Removed
Available
Die#
Removed 4
Preference ratings 5
B C E F G H I J
1000 1500 2000 2400 2500 400 100 100 S A = 10,000
900 750 600 300 210 150 90 0 S R : 3,000
10 15 20 24 25 4 1 I 100
30 25 20 I0 7 5 3 0 100
90.0 50.0 30.0 12.5 8.4 37.5 90.0 0 --
3.00 1.67 1.00 0.42 0.28 1.25 3.00 0.00 --
Totals
z Species are designated by letters which are without meaning; A and D are not used for species to avoid confusion with symbols for availability and diet. 2 Numbers or volumes of food items available and removed (eaten). 3 Percentage of each species removed as related to all food removed and consumed: d = IOOR/SR. 4 Percentage of available forage (A) which is consumed: r-- 100 R / A . 5 Note that the relative values of percentages of foods removed (r) directly parallel the preference rating values, p = d/a.
intake. By comparison with the total amount eaten (SR), these can be directly converted into percentages (d) which together comprise the total diet. In the example of Table l, forage species B through I are those materials which are eaten. Species B through F are identifiable as major foods in the diet.
PRINCIPAL FOODS VERSUS PREFERRED FOODS
163
CALCULATION OF FOOD PREFERENCE RATINGS
The term 'food preference' has been used in the literature for years although, as stated, not always with precision. Most often the term has been utilised, as it is here, to refer to the 'likeability' of a forage species (Sampson, 1952, p. 385) or the 'choice an animal makes' (Stoddart & Smith, p. 130) of available toods. While the word palatability has been employed in the range management literature as more or less synonymous with food preference, it has also been assigned other meanings (Sampson, 1952; Stoddart & Smith, 1955; National Academy of Sciences, 1962) so that the possibility of confusion makes its continued use undesirable. The reasons why a food species is preferred or neglected, it must be noted, is another matter. These reasons are not revealed in any way by the degree of preference or neglect which may be calculated to exist. It cannot be sa4d with certainty, for example, that any species is distasteful just because it is seldom eaten. It may be that taste sensations are involved but there is no assurance from availability and consumption data alone that this is true. Nor is there any evidence that harshness, chemical composition, succulence, or any other possible factor or combination of factors is necessarily involved. If we learn that there is preference for, or a tendency towards neglect of, a food, this is a starting point from which to undertake more searching studies. We are concerned here only with the detection and measurement of the degree to which a food is preferred or is treated with disdain. In terms of volume, numbers, or weight, several species may be equally significant in the total diet of an animal population at a given season or in a stated area. If so, then those several food species must comprise equal percentages of the diet. If one of those equally-consumed species is less available than others, then the animal feeding on it must generally have sought it out and that animal may be said to display a preference for it. A preferred food species, then, is one which is proportionately more frequent in the diet of an animal than it is in the available environment. Food preference is the extent to which a food is consumed in relation to its availability. Some foods may be proportionately less abundant in the total food consumed than in the available forage. Such species may be said to be neglected or even avoided completely by the feeding species. A food which is eaten exactly to the extent to which it is available in nature is neither preferred nor neglected. It is neutral with regard to the feeding preferences of the foraging species. It should be noted that a food item may be much sought after, but if it is difficult to find then it cannot be much utilised. Conversely, if foods which are not much liked are the only ones available, they may of necessity comprise a large proportion of the diet. As developed in the hypothetical example of Table 1, to determine food preference relationships in the field it is necessary to measure the amounts of
164
GEORGE A. PETRIDES
various foods available (A) for feeding and the extent to which those foods are actually removed (R) by herbivores. The amounts of foods available can be converted directly into percentages available (a). The quantities of forage plants eaten, however, can be calculated as percentages of two types. The first, as explained above under food consumption values, comprises the percentages (d) of each food species as components of the diet. The second is the relationship between the amount (R) of each species eaten and the amount available (A) of the food species. These latter values yield the percentages (r) of each species which is removed from the habitat by feeding. It is essential to distinguish between the two sets of feeding or 'utilisation' relationships (d and r). The percentage removal values (r) alone indicate the relative intensities of feeding and therefore the degree to which some species are preferred over others (as in Wehb, 1959). They do not, however, provide a reference point to indicate which species are the preferred forages and which tend to be neglected as foods. In contrast, though, if the percentage eaten (r) is divided by the percentage available (a), a preference value is determined which centres on 1.00 as a reference point. Species with preference values above 1.00 (species B, C, H, I of Table I) are those which are sought out as preferred foods. Ratings below 1.00 represent forage species (species F, G, and J of Table 1) which for some reason are neglected or avoided as foods. Species E of Table 1 has a preference rating of exactly 1.00, being neither preferred nor neglected but being eaten precisely in proportion to its abundance in the field. Some species of course may be totally avoided (e.g. J in Table 1), having a zero preference rating. In the calculation of preference ratings, the availability of each forage species is taken into account. It must be present to be eaten, of course, but no food can be said to be preferred 'became it is abundant', as has sometimes been reported in the literature. The measurement of vegetation available for feeding must include all accessible forage present prior to the feeding being evaluated. If both the foods 'available' and those 'eaten' are measured at the same time, the amounts missing and presumed eaten may have to he estimated by comparisons with uneaten samples. The amounts estimated to have been eaten must then be added to that which remains so that the original amount available is approximated. It is not only possible but of frequent occurrence that a species is highly preferred and yet relatively unimportant in the herbivore diet. In Table 1, both species H and I are of this type. In the species of this example, B and I have identical preference values, yet forage B is ten times as important by bulk, as species I in the total diet. In the hypothetical example of Table 1, all species from B through C were eaten more abundantly than H or I despite the high preference values of those latter two species. Hess & Swartz (1940) evidently were first to develop the idea of food preference ratings, which they termed 'forage ratios.' Working with trout and bottom insects,
PRINCIPAL
165
F O O D S VERSUS P R E F E R R E D F O O D S
however, they stated that several food species were unavailable due to their size or protective habits and advised that some derived 'forage ratios are due to availability rather than to preference.' The direct calculation of availability percentages in the environment, though, does yield food preference ratings which include availability as an essential component. ivlev (1961, as seeen in Siefert 1972) developed an electivity index as a modification of the forage ratio. His index (see Table 2) yielded values between + 1 TABLE 2 COMPARISONS OF PREFERENCE RATINGS AND ELECTIVITY INDEXES USING tIYPOTHETICAL DATA
Paired Forage 'species' K L M N
Amounts Available Removed
Availability
Percentages Diet
Removal
Indexes Preference Electivity
(A)
(R)
(a)
(d)
r = 100 R / A
p -- d / a
400 600 200 800
360 240 160 40
40 60 20 80
60 40 80 20
90 40 80 5
I. 50 0.67 4.00 0.25
E = d-a + 0.2 - 0.2 + 0.6 -0.6
and --1, 'the l~ormer value indicating complete positive selection and the latter, complete rejection of a food item' (Siefert, 1972). The electivity index has the advantage of computing selection values which lie between definite positive and negative limits, with an index of zero representing neither preference nor neglect. It offers the decided disadvantage, however, that its values are not proportional to the actual differences in preference indicated by the data (see columns r and p versus E, Table 2). From the p ratings of Table 2, for example, one can say that species M is eaten four times as much as is expected from its abundance or that species N is consumed only a quarter as much. It is difficult to infer these relationships from the electivity values. For this reason, Ivlev's index is not adopted here. V A L U E S OF F O O D P R E F E R E N C E D A T A
Of the forage species present in any area, those with highest and lowest preference ratings are especially useful as indicators of herbivore stocking density and range condition.
Stocking rate The number of animals occupying a measured area is the current stocking rate or stocking density of that species. That density may be higher or lower than the carrying capacity of the range. If the former condition prevails, the current stocking rate may not be one which can be sustained on a permanent basis. Overutilisation of the habitat may lead to deterioration of range condition and even to range destruction.
166
G E O R G E A. P E T R I D E S
The reduction in animal population which may result from range depletion and consequent malnutrition or starvation typically will not alleviate grazing pressure on the range. Overstocking is not a matter of the absolute number of animals present. Rather, it is animal density with respect to carrying capacity which is significant. On an overpopulated range, the animals which survive starvation still may be too many for the range to support. Even with low numbers per unit of area, a stocking rate which exceeds range carrying capacity usually continues to prevent range recovery and to cause increasing range depletion. To appraise such circumstances, current range condition and trend must be established.
Range condition Where grazing animals are sufficiently abundant to remove vegetation faster than it can be replaced, changes are induced in the relative abundance of the plant species involved. The most-preferred forage species tend to be depleted first, with the remaining species affected more or less severely according to their preference ranking and the intensity and duration of cropping. Conversely, the avoided species tend to increase, as do neglected forages where grazing pressures are not overwhelmingly severe. The range management literature (for example see Dyksterhuis, 1958) often refers to various plant species as 'decreasers', 'increasers', or 'invaders'. In terms of this report, decreasers are preferred forage species, increasers are neglected foods, and invaders are avoided plants. The range condition, or current status of the vegetative habitat, is a reflection of the past and current stocking rates of the principal herbivore populations inhabiting the area.
Range trend A habitat trend towards improvement or degradation can be evaluated from the degree to which range plants are renewing themselves and providing soil protection. The severity and persistence of previous overuse of the range and the determination of whether over-utilisation is continuing, however, can be further evaluated from evidences that preferred and neglected foods are either being used heavily or not and that these and avoided forage species are either increasing or decreasing. Where preferred foods are scarce, neglected foods are heavily eaten and avoided foods form the bulk of the available food supply. Here, it can be expected that the consumer population is overpopulated and probably is in an impoverished physical condition. Under such circumstances food is probably the factor which prevents population increase.
167
PRINCIPAL FOODS VERSUS PREFERRED FOODS
ANALYSIS OF PREFERENCE DATA
Food preference ratings always represent conditions at the time that feeding occurred and relate principally to the area involved. Yet comparisons between areas and seasons may be instructive. The data of Tables 3 and 4 indicate how field data TABLE 3 VEGETATION DATA FROM APPROXIMATELY 100 MILACRE (4.840sq. yds.) PLOTS ON SOUTH FOX ISLAND, MICHIGAN, FOR WHITE-TAILEDDEER DURING THE WINTER OF 1969 (BASED ON DATA OF PALMER • KING, 1970)
Species Mountain maple (Acer spicatum Lam.) American yew (Tax'us canadensis Marsh.) Sugar maple (Acer saccharum Marsh.) American hazelnut ( Corylus americana Walt.) Chokeeherry wPrunus virginiana L.) hite cedar ( Thuja occidentalis L.) Totals
Kg / ha* Available Removed
Percentages Preference Available Diet Removed ratings
1.43
0.44
0.13
0.17
31.0
1075.38
250.81
95.13
98.02
23.3
1.03
6.00
1.16
0.54
0.45
19.3
0.83
3.61
0.64
0.33
0.25
17.8
0.76
1.58
0.12
0.14
0.05
7.8
0.36
42.01
2.72
3.73
1.06
6.5
0.28
255.89
100.00
100.00
22.6
1.00
1130.01
1.31
*Dry weights.
on forage availability and utilisation may be used in determining forage preference ratings. American yew is generally accepted as a highly preferred winter white-tailed deer forage throughout its range in the northeastern United States. Where it still occurs abundantly, as on some of the islands in the Great Lakes (Table 3), it may indeed comprise the bulk of the deer's diet. Yet it is not a highly preferred food there. White cedar is acknowledged as an important winter forage plant for white-tailed deer throughout most of its range. Yet on South Fox Island, where other plants are available (Table 3) or perhaps where soil conditions are different, it becomes a neglected species and is unimportant in the diet. Mountain maple is the most highly" preferred forage in winter on South Fox Island (Table 3) yet it is an avoided species in summer in the Adirondacks (Table 4). Similarly, sugar maple had a higher preference value on the island than in New York state at a different season. On South Fox Island (Table 3), decreases in mountain maple and American yew or increases in the other woody plants present should indicate that the deer herd there is increasing in size. The first five food plants listed in Table 4, and especially the first two (alternative-leaved dogwood and common elderberry), should show
168
GEORGE A. PETRIDES
TABLE 4 CALCULATION OF SUMMER BROWSE PREFERENCES OF ADIRONDACK MOUNTAINS, NEW YORK, WHITE-TAILED DEER (BASED ON DATA OF WEBB, 1959)
Species Alternate-leaf dogwood (Comus alternifolia L.f.) Common elderberry (Sambucus canadensis L.) Black cherry (Prunus serotina Ehrh.) Yellow birch (Betula lutea Michx. f.) White ash (Fraxinus americana L.) Red raspberry (Rubus idaeus L.) Sugar maple (Acer saccharum Marsh.) Others* Totals
Numbers of stems Percentages Available Browsed Available D i e t Removed
Preference ratings
16
Il
2.0
32.4
68.8
16.20
12
8
1.5
23.6
66.7
15.73
11
1
1.4
2.9
9.1
2.07
54
4
6.7
! 1.8
7.4
1.76
19
1
2.4
2.9
5.3
1.21
124
3
15.4
8.8
2.4
0.57
467
6
58.2
17.6
!.3
0.30
101
0
12.4
0
0
0
804
34
100.0
100.0
--
--
* Hobblebush (Virurnum alnifolium Marsh.), beech (Fagus grandifolia Ehrh.), fire .cherry (Prunus pensylvanica L.), quaking aspen (Populus tremuloides Michx.), striped maple (Acer spicatum Lam.), Canada honeysuckle (Lonicera canadensis Bartr.), mountain maple (Acer spicatum Lam.), red spruce (Picea rubens Sarg.) were present but avoided as summer foods. reduced availability u n d e r heavy use a n d should serve as good indicators of s u m m e r deer o v e r a b u n d a n c e o n the A d i r o n d a c k deer range.
CONCLUSIONS F o o d preference ratings provide useful evidence towards the u n d e r s t a n d i n g of trophic ecology a n d practical resource m a n a g e m e n t . T h e y yield i n f o r m a t i o n o n the suitability of ranges as a n i m a l habitats a n d enable appraisals of the a b u n d a n c e of c o n s u m e r organisms with respect to range carrying capacity. The relative a b u n d a n c e a n d c o n d i t i o n of food species present in a n area m a y f u r t h e r indicate the vigour of c o n s u m e r p o p u l a t i o n s a n d whether food factors limit c o n s u m e r a b u n d a n c e there. If calculated similarly on a widespread basis, preference values should e n a b l e m e a n i n g f u l c o m p a r i s o n s between research appraisals a n d m a n a g e m e n t results across the geographic ranges of i m p o r t a n t species. REFERENCES
C. W. • STODDART, L. A. (1953). The quandary of utilization and preference. J. Range Mgt, 6, 329-35. DYKSTERHUIS,E. J. (1958). Range conservation as based on sites and condition class. J. Soil Wat. Conserv. 13, 151-5.
COOK,
PRINCIPAL FOODS VERSUS PREFERRED FOODS
169
HESS, A. D. & SWARTZ,A. (1940). The forage ratio and its use in determining the food grade of streams. Trans. IV. Am. Wildl. Conf., 5, 162-4. IVLEV, V. S. (1961). Experimental ecology o/'./eeding of.fishes. New Haven, Yale University Press. NATIONALACADEMYOFSCIENCES.(1962). Basic problems and techniques in range research. ?Cam. Acad. ScL, Nat. Res. Coun. Publ. 890. Washington, D.C., 342 pp. PAI,M~R, W. L. & KING, D. R. (1970). South Fox Island Deer -1969. Mich. Dept. Nat. Res. R & D Rept, 204, I1 pp. PETRIDES. G. A. & PIENAAR, U. de V. (1966). Calculation and ecological interpretation of prey preferences for large predators in Kruger National Park, South Africa. Michigan State University, East Lansing (Mimeographed). PIENAAa, U. de V. (1969). Predator-prey relationships amongst the larger mammals of Kruger National Park. Koedoe, 12, 108-76. SAMPSON, A. W. (1952). Range management principles and practices. New York, Wiley. SIEVERT, R. E. (1972). First food of larval yellow perch, white sucker, bluegill, emerald shiner, and rainbow smelt. Trans. Am. Fish. Sot., 101,219-25. STODDAR1", L. A. & SMrrH, A. D. 0955). Range management. New York, McGraw-Hill. WEr~B, W. L. (1959). Summer browse preferences of Adirondack white-tailed deer. J. Wildl. Mgmt, 23, 455 -6.