By James Cash, Texas A&M WFSC '17
Many land owners spend a lot of time and money on improving
wildlife habitat in order to increase low population numbers or maintain stable
populations. It is important that land owners understand what the local game
population’s limiting factor is in order to effectively and efficiently use
their resources to improve the habitat. All wildlife species require four main
habitat components: food, water, cover, and space. The limiting factor is the
habitat component that is insufficient to the point that it prevents the
population from growing, or that causes a population to decline. More than one
of the four main habitat components may be deficient; however the limiting
factor is the most severe. Improving the other habitat components may lead to
slight population increases, but only improving the limiting factor will
significantly improve the population. In areas with good habitats the limiting
factor may be something such as disease or predation, however these two
examples could also be related to insufficient space (high population densities
are more prone to disease outbreaks) or cover (not enough cover can lead to
more predation).
Managing for quail is a good example of how important it is
to determine the limiting factor. Many land owners provide supplemental feed
for quail in an attempt to increase populations. However, nesting and escape
cover tend to be limiting factors in quail populations, not food. If that is
the case on your property, then supplemental feeding will have little or no
effect on the population, but improving cover through grazing management and
brush sculpting will have a better chance of increasing the population. There
may be instances on certain ranches with great cover where food is in fact the
limiting factor.
This area has poor quail nesting
cover, likely due to overgrazing the bunch grasses till they are too short. It
is important to address this deficiency before moving on to a supplemental feeding
program. Photo courtesy of Dale Rollins.
Predator control is another common practice attempted by land
owners to help increase the populations of many different game animals. The
problem with predator control is that new predators will always be attracted to
land with an abundant population of easily accessible prey. Shooting and
trapping predators may help reduce predation in the short term, but a more long
term solution to predation is to increase cover so that game animals can elude
predators. For quail this would mean increasing both nesting and loafing cover.
Recent research in the Rolling Plains of Texas shows that quail use a variety
of cover types to escape different predator threats. It was found that quail
prefer large, dense woody coverts when escaping from avian predators (Perkins
et al.2014). Keep in mind that reducing
predation will increase your game population significantly only if predation
mortality is the limiting factor. If food, water, or space is significantly
insufficient then reducing predation will not make a huge population difference.
One limiting factor that many people don’t think about is
space. If food, water, and cover are all sufficient then space becomes the
limiting factor. Extremely high population densities are highly conducive to
disease transmission. Populations of animals can also get so dense that even
with supplemental food and water there is simply not enough space for all the
resources necessary to increase the population. Behavioral traits such as
territoriality can also put a ceiling on how dense the population of a certain
species can get in an area. Some species simply will not live in dense
populations, even if a certain area has plenty of resources to go around. If
space has become the limiting factor on your land then the only thing that can
be managed may be your own expectations. At some point no amount of
supplemental food, water, and cover will increase the population.
There are several ways to go about determining the limiting
factor for a certain species on your property. Simple trial and error combined with good
population records can rule out what isn’t the limiting factor, or may confirm
that you’ve been managing for the limiting factor all along. For instance, if
you’ve been providing supplemental feed for years but haven’t seen any appreciable
population improvement then it’s probably safe to say that food isn’t the
limiting factor. If you are already
implementing management practices then you should step back and evaluate
whether you are actually improving the limiting factor.
Browse line in the Texas Hill
Country. A distinct line of nearly no palatable vegetation left from the ground
to about 5ft, the extent to which deer can easily feed. A good indication that
food may be a limiting factor on your land. Photo courtesy of Texas Parks and
Wildlife Department
Another way to determine the limiting factor is to conduct a
habitat evaluation, either by yourself or with the help of a wildlife
professional. AgriLife Extension publications such as B-6172: “Habitat
Monitoring for Quail on Texas Rangelands,” SP-317: “Habitat Appraisal Guide for
Rio Grande Wild Turkey”, or “Deer Range Appraisal for East Texas Forests” can
be used as guides if conducting the evaluation yourself.
The best approach would be to
combine habitat evaluations with trial and error. First, conduct a habitat evaluation
to get an overall estimation of the insufficient habitat components on your
property. If multiple habitat components are deemed insufficient then trial and
error with different management techniques can narrow down which is the most
limiting factor. If you have the time and money to correct all the insufficient
components then by all means do so. However, if funds are limited then focusing
on the most limiting factor will yield the best returns.
Keep in mind that the limiting
factor is not concrete, it will change over time. As you work on managing the
habitat to correct the limiting factor you will cause the limiting factor to
change. For instance, if the limiting factor had been water, but you added a
stock tank to correct this then the new limiting factor may become food.
Ideally a property will get to the point where the limiting factor is space,
meaning the habitat is sufficient to sustain and grow the populations but for a
simple lack of space for any additional animals. As mentioned before, this is
now the time to manage your own expectations.
Works
Cited
Perkins, R., C.
Boal, D. Rollins, and R. M. Perez. 2014 Northern bobwhite predator avoidance
behavior in response to varying types of threat. The Journal of Wildlife
Management 78: 1272-1281