Climate Change Impacts on Ecology in the Nashua River Watershed

In the Nashua River watershed, geology, hydrology, and climate have combined in a unique way to create one of the most complex landscapes and some of the most diverse habitat anywhere. The diversity of our landscape begins with geology. This area contains remarkable concentrations of glacial landforms: extensive areas of eskers, kames and kettles; an impressive “swarm” of drumlins; broad ground moraines; and areas scraped bare by the glacier. In the middle of our watershed once stood Glacial Lake Nashua, more than thirty miles long, stretching from Boylston, Massachusetts to Nashua, New Hampshire, and bounded by walls of ice. Remnants of the lake are all around, ranging from lakebed deposits to hanging deltas sitting high above the current valley floor. This set of landforms left intricate and varied topography and soils.

This area also lies precisely at the transition between two forest biomes. One is the northern hardwood-hemlock-pine forests, which extend north to Canada. The other is the central deciduous-oak-hickory forests, which extend south to Georgia. Biomes are communities of plants that tend to exist together. At the transition of biomes, both sets of vegetation exist together. Complex combinations of the vegetation characteristic of each biome are found throughout the watershed.

An ecosystem thrives on diversity and interconnections between habitats. The more types of habitat, the greater the density and diversity of plants and animals. The river system, in particular, creates such interconnections.

The confluence of diverse habitat and interconnection has yielded unusual biodiversity, including major concentrations of rare and endangered species. Perhaps the best measure of our biodiversity is the presence of several rare or threatened species of turtles. Many of these turtles are wanderers, moving overland between different wetland areas. The complex distribution of wetlands and streams, combined with the varied uplands, provides critical habitat for these species.

Throughout the watershed there is also an unusual concentration of vernal pools, where many unique but vulnerable creatures, such as our salamanders, breed. Vernal pools and our woodland marshes give rise to springtime peepers.

In addition to many rare species, this landscape is home to coyote, fisher, and, increasingly, bobcat, moose, and bear. It also supports a myriad of birds, butterflies, and dragonflies. This remarkable diversity has led to three Areas of Critical Environmental Concern (ACEC), representing more than a quarter of all the ACEC lands in Massachusetts, being established within the watershed and the designation by the federal government of significant portions of the Nashua, Squannacook, and Nissitissit Rivers as Partnership Wild and Scenic Rivers.

Extensive impacts to the ecology of the watershed are being caused by numerous climate-related changes. These impacts will significantly diminish the biodiversity and richness of the watershed unless actions are taken to increase resiliency and to better allow species to adapt.

Major Impacts to Ecology

The major ecological impacts of climate in the Nashua River watershed include the following:

Changing Landscapes

One of the major consequences of regional warming is shifting of our forest biomes. The two major biomes that intersect in the watershed are "migrating" northward in response to warming temperatures. Within the next 30 to 50 years, conditions within our local landscape may become better suited to the more southerly forest biome, and this will cause a significant reduction in the biodiversity and richness of our landscape.

As the forest biome zones move northward, not all plants will shift together, and therefore plant communities will also change. Many local species of wildlife have adapted over long periods of time to these plant communities and may be reduced in numbers or disappear as the habitat changes.

Ecosystems are interconnected, synergistic, and complex. The web of life is vulnerable in ways that are unpredictable. The imprint of humanity, developed lands, and fragmented landscapes makes ecological adaptation difficult. Also, the rate of change is currently very rapid, further challenging adaptation.

Projected Changes in Forest Biomes due to Climate Change. From 2009 National Climate Assessment “Global Climate Change Impacts in the United States.”

Invasives

Plants: Increased temperatures, especially less severe winters, will allow additional invasive (non-native) plant species to move into the watershed. As existing plant species become less competitive, invasives may be increasingly able to move in to replace them and take over areas. For example, some forests may be particularly vulnerable to increased invasions of Norway Maple and Buckthorn, which can outcompete native species. Over time, this can create monocultures with greatly reduced ecologic value. Another example is the increasing prevalence of Japanese Knotweed, a type of bamboo from Asia. It has become well established, especially in many disturbed locations along our rivers and streams and along roadsides, where it tends to form dense thickets and displace native plants.

Pests: Increased temperatures have led to the expansion of areas infested with new species of insects not previously in the watershed, some of which, such as the Elongated Scale and the Hemlock Wooly Adelgid, have caused extensive damage. As temperatures continue to rise, additional pests will invade, potentially causing the loss of key plant species.

Changing Seasons

The timing of seasons is shifting, affecting insects and the migrating and nesting birds that are dependent on them. The ecosystem of the watershed is complex, and the changing climate will have a detrimental impact on the flora and the fauna that is dependent upon it.

Declining Species

Insects: Significant declines in insect populations have been noted for many years. Although the focus has been on pollinator insects that are essential to many plant species, insects are an essential component of the entire ecosystem with many other species dependent on them.

Birds: A recent study, published in the journal Science, indicates that bird populations in the United States and Canada have declined by 29% over the past 50 years. This translates to a population loss of 2.9 billion birds. While there are many potential causes of the decline, such as land use changes and pesticide usage, climate change is involved. Changes to local habitat are certainly among the potential local causes.

General Wildlife: As temperatures warm and vegetation communities shift, habitat will decline and will not be adequate to support existing wildlife populations. Different wildlife species will seek ways to survive either by migrating to areas with adequate habitat or by adapting to the changes. The net result will be an overall decrease in the biological density.

Aquatic Habitats

Changing flow patterns will alter the suitability of streams as a habitat for aquatic life. This may include a decrease in the riffle-pool habitats preferred by many bottom-dwelling organisms that supply food for fish. Cobble-gravel stream bottoms may get clogged by increased siltation after scouring and erosion events. This may hinder spawning activities of fish. Changing sediment loads may alter the shape of channels. Together, these changes may decrease the quality and quantity of available habitat, inhibit reproduction and hinder the natural movement of aquatic organisms.

Adaptation-Migration Impediments

The interconnections among species and with landscape features are sometimes subtle but always powerful. The changing climate will disrupt the hydrologic and landscape systems in ways that will, in turn, disrupt the local and regional ecosystems.

Nature responds to change either by adapting to new conditions or through species migration. Some species, including some of our most vulnerable local species (such as those that require vernal pools or special habitat), do not have the ability to migrate and so must find ways to adapt if they are to survive.

Although the Earth has experienced major temperature swings and disruptions in the past, one of the major differences now is the rate of temperature change is too rapid to allow adaptation by some species. Another difference is the enormous number of barriers to natural migration due to human development. Major highways and heavily developed areas block movement, while fragmentation of the landscape prevents many species from moving to new areas where they can continue to survive.

Scouring by floods. Old undersized and poorly sited culverts can become scoured by flooding from excessive rain events leading to the disruption of stream continuity as the culvert now sits above the stream level which prevents movement of fish and other aquatic species.

Banner images at top (left to right): Indigo bunting, a neotropical migrant bird dependent on our watershed habitats during migration, photo by Nancy Lebedzinski; Nissitissit River, a valuable coldwater fishery with native trout populations, photo by Ken Hartlage; Bolton Flats Wildlife Management Area runs along the Nashua River in Lancaster, Bolton, and Harvard, and features specialized habitats like High-Terrace Floodplain Forest and Pitch Pine-Scrub Oak woods, photo by Chris Buelow.