Native, Invasive, and the Problem of Choosing a Baseline

Debates over wildlife management often rely on the language of native and invasive as if these were fixed biological categories. They are not. While the terms sound scientific, they implicitly depend on a prior—and usually unspoken—decision that has already been made: the choice of a historical baseline.

Before any species can be labeled native or invasive, someone must decide which moment in time will serve as the reference community. Species present at that moment become “native.” Species that arrive later become “invasive.” Change the date, and the categories change with it.

This is not a minor semantic issue. It is a foundational problem that shapes policy, moral judgment, and public perception—often without being acknowledged.

History Gives Perspective, Not a Frozen Standard

In real time, decisions are made under pressure, using the information available and responding to immediate constraints. That is unavoidable. The value of history is that it allows us to revisit those decisions, evaluate their consequences, and reconsider the assumptions that guided them.

What history does not provide is a single, objectively correct ecological moment that can be frozen and treated as timeless. Ecosystems are dynamic. Climate changes, rivers shift, species migrate, populations fluctuate, and human activity alters landscapes continuously. Any baseline we choose is a human decision, not a biological fact.

How Baselines Change the Meaning of “Native”

The Salt River provides a clear illustration of how dramatically the native/invasive distinction shifts depending on the temporal baseline selected. The following examples are partial, not exhaustive.

~100 years ago (c. 1920s)

At this time, the animals present and free-roaming in the Salt River system included:

· Horses

· Cattle

· Coyotes

· Mule deer

· Javelina

· Beavers

· Native fish species

Under a 1920s baseline, these animals would be classified as native because they were already established on the landscape.

Animals that arrived later—and would therefore be labeled invasiveunder this definition—include:

· River otters (reintroduced to Arizona beginning in the 1980s)

· Desert bighorn sheep (reintroduced to central Arizona in the mid-20th century)

The key point is not whether these later arrivals are beneficial or harmful. It is that their classification depends entirely on the chosen baseline, not on biology alone.

~1,000 years ago (c. 1000 CE)

If this period were selected as the reference point, the list of native species would change substantially. Species such as:

· Coyotes

· Wolves

· Jaguars

· Beavers

· Riparian fish species

would be considered native.

Horses, cattle, and all modern livestock would be classified as invasive, because they were not present at that time. The moral certainty often attached to these labels would reverse completely.

~10,000–12,000 years ago (late Pleistocene / early Holocene)

Under this baseline, the native community in the desert Southwest would include:

· Horses

· Camels

· Giant ground sloths

· Dire wolves

· American lions

 By comparison, some species that dominate the modern landscape—including  animals we often treat as historically ‘normal,’ such as javelina and  bobcats—would be later arrivals relative to this Pleistocene fauna, and thus ‘non‑native’ if we froze the clock then. 

Each of these baselines is internally coherent. None is biologically privileged. The difference lies entirely in which moment in time we decide to freeze.

“Native” Is Not a Biological Category

This illustrates a broader point that is often overlooked: nativeis not a biological category in the way species, genus, or trophic role are. It is a management construct rooted in historical choice. It is a biogeographic status label that only makes sense once a reference period and spatial scale are chosen—both of which are management decisions. 

That does not mean ecological impacts are irrelevant. It means that the label itself carries assumptions. When those assumptions are left unstated, the language of native and invasive can project scientific certainty while quietly embedding normative judgments.

Selective Acceptance of Non-Native Species

The baseline problem becomes even clearer when we look at non-native species in the Salt River that are widely accepted and actively managed.

Rainbow trout, for example, are not native to the Salt River watershed. Their continued presence depends on ongoing stocking and altered flow regimes. Crayfish (crawdads) present in the system are also non-native and can have measurable ecological impacts, including predation on native species and modification of streambeds.

The same is true for largemouth bass, carp, and channel catfish.

All of these species fit standard definitions of non‑native, and several qualify as invasive given their ecological impacts. Yet they are often socially normalized, managed without controversy, and rarely framed as moral problems. Their presence is tolerated—or even valued—because they align with recreational, cultural, or economic preferences.

This selective concern suggests that the issue is not simply ecological impact, but which species fit comfortably within prevailing narratives.

Reintroduction and the Shifting Definition of “Native”

A related conceptual inconsistency appears in arguments about reintroduction. It is frequently said that bighorn sheep or river otters are native because they were once present in the region, and that reintroduction merely put them back where they “belong.”

This reasoning quietly changes the definition of nativeness yet again. Instead of baseline presence, historical occurrence at any point in the past becomes the criterion. A species is considered native if it existed in the region at some earlier time, even if it was absent for decades or longer and returned only through deliberate human action.

Once that definition is adopted, however, it must be applied consistently.

If historical presence alone is sufficient to establish nativeness, then the categorical rejection of other long-present, free-roaming animals becomes difficult to justify. This is where horses expose the inconsistency most clearly.

Many people reject horses as invasive while defending bighorn sheep and otters as native because they were “once here.” That framework is not coherent. It treats reintroduced species as legitimate because they fit a preferred restoration narrative, while denying legitimacy to another species because it does not.

The Only Coherent Frameworks

Logically, only a few consistent positions are available.

1. Baseline definition (time-specific)
Native means present during a selected historical reference period.
Under this approach, reintroduced species absent during that period are not native by baseline presence; they are managed restorations.

2. Historical-occurrence definition (time-expansive)
Native means the species existed here at some point in the past and can appropriately occupy habitat here.
If this approach is used, it cannot be selectively applied only to favored species. The focus must shift to impacts, resilience, and carrying capacity.

3. Impact-based management
The native/invasive distinction is treated as secondary. What matters is measurable ecological effect and long-term system viability under current conditions.

What does not work is a hybrid approach in which baseline presence is used to exclude some species, while historical occurrence is used to defend others.

Toward a More Honest Conversation

None of this argues for ignoring ecological limits or abandoning management. It argues for intellectual consistency.

History does not provide a pristine baseline. What it provides is perspective: the ability to see how categories are constructed, how assumptions harden into policy, and how reasonable decisions—made under pressure—can produce unintended outcomes over time.

Once that perspective is acknowledged, the conversation becomes more difficult but also more honest. Instead of relying on labels that depend on an arbitrary moment in the past, we are forced to confront the real work of stewardship: managing living systems in a world that has never been static, using concepts that are clear about their own limits.

That is the value of history—and the responsibility it imposes.

 A Final Note on Conservation Practice and Intent 

It should be noted that conservation biology and management already wrestle with many of these issues more explicitly than public debates often suggest. Practitioners use formal concepts like “indigenous range,” shifting baselines, and impact‑based assessments to guide decisions, and they are well aware that ecosystems change over time and that nativeness is partly a biogeographic judgment, not a taxonomic fact. My focus here is not to claim that these terms are meaningless or that current practice is naïve, but to highlight how, once the discussion moves into law, policy, and public rhetoric, the underlying choices of baseline, time scale, and acceptable impact are often left implicit. Making those assumptions visible—even where reasonable people will still disagree—does not solve the hard problems of management, but it does make our arguments more honest about the values and trade‑offs that shape them.