It’s All Connected

Habitat Enhancing Land Management (HELM)

In nature everything is connected, interwoven, subject to natural law. We cannot separate ourselves from that, no matter how hard we try. — Jeffrey R. Anderson, The Nature of Things— Navigating Everyday Life with Grace

Christine Middleton

The idea of linkages among ecosystem components is not new. Over 100 years ago, John Muir observed, "When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” Fifty years later in his seminal book, A Sand County Almanac, naturalist Aldo Leopold compared land to the human body, observing that the symptoms can lie in one organ and the cause in another. Leopold suggested that “The art of land doctoring is being practiced with vigor, but the science of land health has yet to be born.” Fortunately, in the seventy or so years since then, we have learned a lot about the connectedness of soil, plants, and animals. And there is still a lot we don’t yet completely understand.

Energy flows through the ecosystem

An ecosystem is a biological community of interacting organisms and their physical environment. This implies interconnectedness among both abiotic (non-living) and biotic (living) elements. The biotic and abiotic components of an ecosystem interact in a wide variety of ways, both above and below ground. The biotic components of an ecosystem include animals, plants, fungi, and bacteria. Abiotic components include climate, soil, rock, altitude, temperature, water, nutrients, and minerals. One of the major ways these components interact is through the transfer of energy in the form of food.

Back in grade school we learned about food chains. A simple above-ground food chain begins with the sun and its effect on plants. Plants initiate the food chain as the producers; they produce energy through photosynthesis. Vegetation is then consumed by herbivores, often referred to as primary consumers. Next come secondary consumers: carnivores, who eat only other animals and can be eaten by other carnivores and by omnivores, who also eat plants. At the top of the food chain are the carnivores referred to as tertiary consumers. They are sometimes also referred to as apex predators because nothing eats them—at least not while they are alive.

Food chains show a single path by which energy flows from producers to apex predators. But every habitat is a community of organisms interacting in a wide variety of ways. Food webs show how feeding relationships organize these communities. A stable ecosystem, where there is a balance in terms of productivity and abundance among the trophic levels—the feeding patterns of producers, primary consumers, secondary consumers, and tertiary consumers—can persist in a stable state for long periods of time. However, this balance can be easily disturbed. Shifts that impact interactions in one part of the web can quickly lead to undesirable changes across the entire ecosystem.

Think of a spider web. Spider webs are engineering marvels capable of surviving multiple tears or high winds. And the spider is quick to repair areas that have been compromised. But this resiliency has its breaking point. Enough tears in the right places and the whole structure will collapse. Similarly, the loss or overabundance of a single species at one trophic level may have only minor impacts on the overall community. Or the disruption can cascade across the entire spectrum of plants and animals who inhabit an ecosystem.

Maintaining ecosystem balance: trophic cascades

Certain species referred to as keystone species play an outsized role in maintaining the overall structure and function of an ecosystem. Whether the keystone species is an animal, a plant, a fungus, or a bacteria, its absence or overabundance can have devastating impacts on other species with whom it shares space. The keystone species concept originated in the 1960s, when ecologist Robert Paine removed a single starfish species from tide pools in Washington State. That particular starfish, Pisaster ochraceus, was the apex predator in a tightly confined ecosystem. Paine was simply trying to understand the food chain. But what he found surprised him. When the Pisaster starfish were eliminated, the entire ecosystem changed rapidly. Diversity decreased.

Keystone species are often, but not always, predators whose importance stems from their role in keeping the population and range of their prey in check. Removal of predators can result in a trophic cascade that reverberates throughout the food web and beyond. The classic example of a negative trophic cascade occurred after gray wolves were removed from Yellowstone National Park between the 1880s and 1920s. Without this apex predator, elk populations exploded and overbrowsed the park’s vegetation. Plants began dying, and soil erosion became a significant concern. Without competition from wolves, coyote populations increased, adversely impacting the pronghorn deer population. The impact of grey wolf removal continued to ripple through the park and was felt in aquatic as well as terrestrial ecosystems.

But there are positive trophic cascades. After the keystone grey wolves were reintroduced in 1995, a rebalancing took place that surprised even the experts. The elk populations were reduced. But that was just part of the story. Their fear of predation brought changes in the elks’ behavior: their herd size got smaller, and fear caused the elk to move around a lot more. Thus, vegetation had time to recover before the elk returned to browse the same area. Willows, an elk winter food staple, grew taller and more abundant. Beaver, who use willow to build dams, returned. The new ponds and marshes the beavers created provided rich habitat for moose, otters, mink, wading birds, fish, and other wildlife.

But that’s not all. The reintroduction of wolves helped control the coyote population and drove cougars out of the valleys and back into the hills. The carcasses of coyotes and other animals that wolves killed (to reduce competition for food rather than to consume) were scavenged by a wide variety of animals, including bald eagles, golden eagles, and magpies. As the willows recovered, the relative abundance of several species of songbirds increased. Pollinators now had more flowers from which to draw nectar. Even the buffalo benefitted, likely due to decreased competition with elk for the same resources.

Reintroducing wolves created a top-down trophic cascade.Trophic cascades can also happen from the bottom up. As vegetation disappears, herbivore populations are reduced and lack of prey reverberates up the food chain. This often happens after forest fires or massive land clearing. Revegetation can reverse this trend. A third kind, a subsidy trophic cascade, happens when some species find a secondary source of food outside their ecosystem. This often happens in farming communities when the natural balance is altered by the availability of crops or livestock, or when people provide extra food either directly or through unprotected food waste. Shown below is a setup that enables the feeding of quail and other small wildlife while preventing non-target species such as deer and hogs from consuming the food and water.

The mesopredator release effect

When we consider an ecosystem’s balance, another interesting concept is the mesopredator release effect. Meso- (= in the middle) predators are those secondary consumers in the food chain—the carnivores and omnivores who prey on other species but who can also themselves become prey. Apex predators influence mesopredators not just by consuming them. If two species share prey, the apex predator may kill the mesopredator simply to reduce competition. Or the mere presence of a larger predator can cause a mesopredator to limit its territory. Places the mesopredator avoids for fear of being eaten become safe havens for prey not shared with the apex predator. But once there is no longer a reason for the mesopredator to fear its nemesis, the dynamics change and the mesopredator’s prey no longer have safe places in which to live and breed.

Without apex predators, mesopredator populations tend to increase. An increasing population of mesopredators combined with behavior changes can result in dramatic declines in prey species. Human actions sometimes make the problem worse. An intriguing example is what happens when humans try to get rid of coyotes. Coyotes live in small packs where only the alpha male and alpha female breed. This provides a natural form of self regulation in terms of population. But when multiple members of the pack are killed, the pack structure becomes destabilized and the result is more rather than fewer individuals. https://www.humanesociety.org/resources/what-do-about-coyotes#trap

Humans also influence the size of mesopredator populations by providing them a more consistent source of food. This often happens with raccoons, whose cleverness enables them to tap unprotected food left out for other animals (e.g., bird feeders, pet food). Raccoons are also very good at raiding trash cans. An overpopulation of raccoons has been connected with declines in certain bird species, whose eggs they devour. Birds most impacted are those who nest on or close to the ground. Quail are considered particularly vulnerable.

Image: *2014 State of the Birds Report*, republished by CornellLab All About Birds.

But perhaps the mesopredator most responsible for bird declines is one we humans introduced—domestic cats. Estimates of how many birds are killed by cats across the U.S. range from between 1.3 and 4 billion birds annually. That makes the domestic cat second in bird mortality, with habitat loss the leading cause. Cats allowed outside kill in proportion to the amount of time they spend there. Feral cats, which represent 40% of the domestic cat population, are responsible for 70% of bird kills. And even well fed cats, whether owned or feral, will continue to kill. Cats are also responsible for the killing of lots of reptiles, amphibians, and small mammals. https://www.allaboutbirds.org/news/faq-outdoor-cats-and-their-effects-on-birds/# and the North American Bird Conservation Initiative U.S. Committee State of the Birds Report.

And what about the herbivores?

Mesopredator release can result in increased predation on smaller herbivores, potentially reducing their populations. In some cases, especially with island ecosystems, this leads to extinctions. But larger prey species benefit from the absence of predators capable of bringing them down. Absent other forms of control, the population of large herbivores generally increases. And, having lost their fear, herd animals tend to linger in one place for longer periods. As a result, the richness and abundance of diverse plant communities diminish as favored foods are devoured beyond their capacity to recover. Invasive plants—foreign food to the herbivore’s palate—often gain competitive advantage. And the whole ecosystem destabilizes.

The loss of large predators, reduced hunting, and new measures to control chronic wasting disease (CWD) have led to ever increasing deer populations across the United States. Surveys suggest that the deer population in the Edwards Plateau is the highest in the state of Texas. Between 2012 and the 2018/19 deer census, numbers in the Texas Hill Country grew from an estimated 1.5 million animals to almost 2.4 million. As a result, deer are having an impact not just on your landscaping. They are slowly redefining Hill Country ecosystems.

So what do deer eat? Cattle are grazers; deer are primarily browsers. Deer lack compound stomachs so their ability to digest grass is limited. Under normal conditions, less than 15% of their diet is grass. Their diet also varies across the seasons, switching to more woody vegetation (browse) as broadleaf herbaceous plants (forbs) disappear. Like us, deer prefer some foods over others and consume different parts of plants, depending on what tastes good. When they eat the whole plant, constant nibbling to ground level often kills the plant. Or they may eat just the reproductive parts (flowers, nuts, seeds) of a specific plant. While the living plant survives, the size of its future generations is reduced. Or the deer may simply nibble new leaves and stems, leaving stunted trees like the cedar elm in the picture.

Image: White-tailed Deer Management in the Texas Hill Country https://kinney.agrilife.org/files/2011/08/whitetaildeermgtintxhillcountry_22.pdf

https://tpwd.texas.gov/huntwild/hunt/wma/wildlife_management/kerr_wma/research/field-studies.phtml#:~:text=Food%20Habits&text=A%20deer%20was%20placed%20in,browse%20when%20forbs%20were%20absent.

Some native species have evolved mechanisms for escaping or at least minimizing deer predation. Characteristics that make a species less palatable to deer include poisonous compounds, fuzzy or aromatic leaves, and tough, spiny, or bristly textures. “Aromatic” plants like salvias, lantanas, and verbenas tend to be high in deer resistance. And spiny leaves like those on Agarita and Texas Barberry can be effective deterrents. Location can also influence deer vulnerability. Plants that grow on cliffs, in steep ravines, under Ashe junipers, etc., are less likely to be ravaged. But if a deer is hungry enough, it will eat almost anything. Also keep in mind that different deer populations sometimes favor different plants.

When deer populations and behaviors are in balance with the ecosystem, their eating habits help control woody encroachment into grasslands and prevent understory from getting too dense. But what happens when the deer population grows beyond the carrying capacity of the area they inhabit? Biodiversity drops off. A property with majestic old Spanish (Red) oaks but no young trees of the same species likely has a deer problem. Another sign of overbrowsing is a stand of Turk’s Cap with lots of leaves and very few flowers. A property sown with a wildflower mix that after the first year or so only produces prairie coneflowers (Mexican hats) is likely overrun with deer.

But it’s not just biodiversity that suffers. Deer also influence the structure of habitats. They trim trees to a height of about 5 feet, often referred to as the browse line. Dense undergrowth is lost. Why does this matter? The most obvious impact is less food to support not only a growing deer population but also other wildlife dependent on the same vegetation. For example, with fewer acorns squirrels, wild turkeys, and other wildlife who depend on this nutritious winter food suffer.

There are also less obvious impacts of deer overbrowse. Fewer flowers lead to fewer pollinators, resulting in less food for birds who feed insects to their nestlings. Fewer insects also means reductions in lizards and other insectivores. And the impact is not confined to herbivores and insectivores. It can ripple up the food chain, affecting carnivores like foxes and hawks, who are also left with less food. Changes in habitat structure can also be devastating. A browse line can negatively impact birds like Painted Buntings, who nest in lower tree branches. And without thick cover, birds and other wildlife are left more vulnerable to predation.

So when thinking about managing your property, consider the impact your actions will have on wildlife. If you choose to clear an area, do it slowly. That will give the creatures inhabiting it time to adjust or simply move on. And think about creative ways to deal with our deer problem. In particular, consider creating “deer exclusion” areas on your property—places deer can’t get to or might visit only infrequently. This can be as simple as protecting individual seedlings you have planted or seen sprouting on your land. Or consider using brush piled to a height and depth that deer can’t jump. You’ll protect biodiversity across a larger portion of your property. Remember, it’s all connected!

Want to know more about deer exclosures or other ways you might refine your land management practices? As part of our HELM (Habitat Enhancing Land Management) program, we will send a team to walk your property and talk about a variety of issues related to land stewardship. We have completed our Spring 2024 HELM visits. But we’ll be back at it come fall. And we are continuing to take requests. So if you or one of your neighbors would like the HELM team to visit your/their property, simply go to https://www.beautifulhayscounty.org/helm/ and fill out our request form. And please help us spread the word!

Beasts in the Beauty

Wimberley Outdoor Educators