Wildlife Insider Spring Summer 2010 - Louisiana Department of ...

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CarryingCapacityBy Scott Durham,Deer Program LeaderThe term “carrying capacity” (CC) refersto the number of animals that a particularhabitat can support over time. It isnot constant or simple, but is an intricatemix of a number of factors that are changingall of the time. CC can change daily atfine levels or monthly with the changing ofthe seasons as environmental factors affecthabitat. On a longer term, landscape levelchanges such as human development, forestryand agricultural practices impact CC.Deer biologists spend a good deal of timetrying to determine the CC for individualtracts of lands within the different habitattypes across the state. From their assessments,they recommend harvest rates andthen compare them to historical data fora club or region. This process allows deerbiologists to make informed habitat managementand harvest recommendations tohunters, landowners and wildlife managers.There is no substitute for experience, includingworking knowledge of soils, plantsand forestry practices. There is usually notjust one way to accomplish your goals,truly illustrating why wildlife managementhas been coined as “an art and a science.”Determining CC is a perfect example ofthis adage.A very simple conceptual example ofCC is to think of a pasture with cows. Thenumber of cows that pasture can supportdepends on the kind of grasses, rainfall, soiltype and condition, temperature and timeof year, to name the basic variables. Eventhis simple example illustrates how CC isalways changing because the seasonal CCis less during a drought period than it wouldbe if rainfall was providing normal moisturelevels to the soil and grass. During thedormant season, the grass is not growing,but the cattle must be able to make it onwhatever food is still available. That is whycattle producers generally have a backupplan - hay, feed or a secondary place to takethose cattle if pasture conditions deterioratesufficiently to no longer support the herd.A mixed pine/hardwood forest is farmore diverse above the soil horizon thana simple pasture. Instead of only grass andcows, there are hundreds of herbaceousand woody plants, all competing for sunlightand nutrients. In addition to the plants,there are insects, amphibians, reptiles, birdsand mammals. Each group of plants and animalsis part of a complex web of life, andFigure 1. Young hardwood stand will produce hard mast in the future. Mast trees are especiallyimportant on poor soils or low productivity habitat. A period of lower browse availability must beendured in some portions of a tract to achieve the desired timber stand.Figure 2. Dense hardwood mid-stories must be inventoried. If undesirable regeneration is present,appropriate management action may need to be taken to promote more desirable species.each specimen in some way or another iscontributing or taking away something thatanother requires to survive in the long term.When things get too crowded in nature, thebackup plan initially is lower productivityor reproduction. Next may be dispersal oremigration and ultimately natural mortalitythrough increased predation, high parasitelevels and disease.The practical length of time to considerrelative to deer CC is probably at least ayear. Variations in terms of body characteristicssuch as antler development, weightsand lactation rates can easily be observedin the harvest data. Although it is extremelydifficult to quantify the impact of any one ofthese items individually, biologists considerannual fluctuations in precipitation, mastcrops, temperature, flooding, insect popula-tions, predator and competitor populations,and disease prevalence as factors affectingdeer production, abundance and, ultimately,harvest.Now we introduce another term, “lag,”which occurs when environmental andhabitat impacts are not fully expressed untilsome point in the future. For example,an extremely dry June could increase theincidence of hemorrhagic disease later thatsummer and fall, resulting in a 25 percentmortality event in an area. The deer populationis now lower there, possibly favoringthe deer that remain due to less competitionfor food sources.In another example, back to back hardspring freezes removes the white oak mastfor two years and the red oak mast for oneyear, reducing the average seasonal CC ofLDWF file photos8 Louisiana Wildlife Insider
Figure 3. Mixed pine hardwood stands often offer the most diverse array of deer browse species inthe understory.or even second, estrus cycle the next seasonif their body conditions do not improve.The resulting late born male fawns likelywill exhibit poorer antler development asyoung bucks due to the late and suppressedstart.To understand the long-term CC of atract of land or habitat type, an entire forestcycle must be considered. Even withinthe same soil region, a short-rotation pineplantation that is intensively managed forfast growing pulpwood or chip products,will have a different average annual deerCC than a more plant-diverse, long-rotationmixed pine/hardwood stand managedfor sawtimber. The forestry managementobjectives are different, and thus the forestmanagement regime is different.Recent research in pine plantations illustratesthe complexities of habitat quality.Mechanical, chemical and prescribed firetreatments and the timing of these managementtechniques have varying effects onplant composition and browse quantity andquality. Broadcast herbaceous weed controlreduces forage biomass significantly morethan banded herbaceous weed control (herbicidesapplied on a narrow strip along theplanted trees) (Jones et al. 2009). When anexotic woody plant, such as Chinese tallow(Triadica sebifera) or even a native likesweetgum (Liquidambar styraciflua), becomedominant in the under or mid-story,management action may be necessary toimprove wildlife habitat and forest regeneration.Selective herbicide and prescribedfire application can be used to increaseimportant deer forages in these situations(Mixon et al. 2009).The accompanying photos from thesame tract of land illustrate a variety of habitatconditions that are common to Louisiana.The department’s deer program is developinga joint research project with LouisianaState University that will provide furtherinsight into deer CC across several of thestate’s physiographic regions. Plants thatare important to deer will be measured andanalyzed for nutritional composition acrossthe state (Moreland 2005). This informationwill help biologists and deer managers betterunderstand habitat potential and assistthem in educating hunters about realisticexpectations concerning deer numbers.Literature citedFigure 4. This pine plantation has good browse availability, but unless complimented with mastbearing species along stream side management zones or adjoining tracts, it will not have the productivityor long -term carrying capacity desired by many hunters.deer, especially the second year, when thereis a complete mast failure. A pregnant femalewill go into the spring in poorer condition.If deer numbers are too high and thereis a summer drought, then browse and itsnutrient content will be even lower. Theremay be less milk for her fawns and morestress on the doe, perhaps making her a lesssuccessful mother. Her fawns will be smallerand have lower chances of survival. Ifshe were a normal 3.5-year-old doe on averageto good habitat, she should have twofawns. But if she loses one of them, there isa 50 percent reduction in recruitment fromthis individual. If this happens across alarger sample of females, the results will bean overall reduction in recruitment and deeravailable for harvest in succeeding years.The same does may not breed on their first,Spring/Summer 2010Jones, P.D., S.L. Edwards, and S. Demarais.2009. White-tailed deer foraging habitat inintensively established loblolly pine plantations.Journal of Wildlife Management73(4):488-496.Mixon, M.R., S. Demarais, P.D. Jones, andB.J. Rude. 2009. Deer forage response toherbicide and fire in mid-rotation pine plantations.Journal of Wildlife Management73(5):663-668.Moreland, D. 2005. Checklist of woody andherbaceous deer food plants of Louisiana.LDWF First ed. publication.9
Page 2 and 3: Rememberto tag yourdeer this season
Page 5: Photo courtesy of Walt Cottontree o
Page 8 and 9: Photos courtesy of Weeks MarineFigu
Page 12 and 13: State WildlifeGrant Program& the Lo
Page 14 and 15: A previously banded White-eyed Vire
Page 17 and 18: Wildlife Management Area Biologists
Page 19 and 20: Coastal Operations(State Wildlife,

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