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Recovery Strategy for the Red Mulberry (Morus rubra) in Canada

5. Population and Distribution

5.1 Population and Distribution Context

In 2000, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) designated Red Mulberry as Endangered because of its small and declining number of mature individuals (under 250) and its fragmented population (COSEWIC 2010).

Figure 1: North American Red Mulberry Distribution (Argus and White 1987)

Figure 1: North American Red Mulberry distribution (Argus and White 1987).

Red Mulberry reaches the northern edge of its range in southern Ontario where it is confined to the Carolinian Life Zone (Figure 1). While its range is somewhat diminished, there are no records to indicate that it was ever common or widespread here (Ambrose 1987). COSEWIC last assessed the species in 2000 based on 10 known locations, including six core populations of five or more trees, in two broad regions: 1) Essex County and the Municipality of Chatham-Kent, including Point Pelee National Park and Pelee, Middle, and East Sister Islands, adjacent to western Lake Erie and 2) Niagara, including the cities of Hamilton and Burlington. Occupation of these two areas may correspond to different historical migration paths from the central part of the range in the United States. The forest habitats in these two regions are quite different; Red Mulberry in Niagara occurs along the moist, calcareous9 Niagara Escarpment, while habitat along the Lake Erie shore is more open and sandy. These disparate ecological conditions appear to have given rise to genetic differentiation and local adaptation within Red Mulberry (K. S. Burgess pers. comm.).

Twenty-one extant populations (separated by at least 1 km) have now been confirmed (Table 2 and Figure 2). Of those, 10 are core populations of five or more trees (Thompson 2002b, Burgess et al. 2008a, Natural Heritage Information Centre [NHIC] unpub. data 2010). Of those 10, all are completely or almost entirely located on public or conservation lands. However, the impacts of nesting Double-crested Cormorants on the vegetation of Middle and East Sister Islands, in the western basin of Lake Erie, raise questions as to the long-term viability of these two populations. The remaining 11 populations, having only one or two trees each, may not be viable unless recovery work is undertaken to elevate their population size and minimize threats.

Overall, recruitment is low at all sites (K. S. Burgess pers. comm.). Red Mulberry seedlings are rarely encountered at Fish Point Provincial Nature Reserve (K. S. Burgess pers. comm.) and have only been infrequently observed in more stable forest communities, such as at Ball's Falls over the past 15 years. Although first year seedlings have been observed germinating at the edge of a gravel path under large fruiting trees at Point Pelee National Park, they did not survive for more than two years (Ambrose 1987). Numerous pollination experiments suggest that inbreeding depression in Red Mulberry is minimal (Burgess 2004a), although direct comparisons with Red Mulberry crosses within larger extant populations across the species range have not been made.

The largest population in the Niagara/Hamilton/Burlington region consists of approximately 155 trees of all age classes. The largest population in the Essex/Chatham-Kent region consists of approximately 55 trees of all ages. The total number of Red Mulberry trees, of all age classes, across the Canadian landscape is approximately 322 (Janas et al. 2001; Burgess et al. 2008a; Essex Region Conservation Authority unpub. data 2008; Parks Canada Agency unpub. data 2008; Ontario Parks unpub. data 2008, 2009; Niagara Peninsula Conservation Authority unpub. data 2009; Royal Botanical Gardens unpub. data 2009; NHIC unpub. data 2010; and G. Waldron, pers. comm. 2010). Apparent increases in overall population size in recent years are due to the location of older, previously undiscovered trees, rather than recovery of the population.

The Canadian distribution of Red Mulberry shows decline when compared to historical records (Figure 2). A total of 36 occurrences have been recorded for Red Mulberry. However, five of these are now considered extirpated and another 10 historic (not observed in the past 20 years). The northern limits of the Red Mulberry range once extended to Whitby, but have since contracted south to the Burlington area (Ambrose 1987). At Point Pelee National Park, the loss of three genetically pure trees has been documented since the late 1990s (Burgess et al. 2008a). At some other sites, only hybrid forms can now be located; indicating that Red Mulberry likely occurred there in the past, but has been swamped by hybridization with the exotic White Mulberry (Ambrose 1999). The recorded loss of occurrences, as well as individual trees within occurrences, combined with minimal observed recruitment, indicates a decline is continuing.

Table 2: Location and Landownership of Red Mulberry Trees in Canada

Critical Habitat Parcel #LocationLandowner(s)
Core Populations (5 or more trees under 1 km away from at least one other individual)
228_1Clappison Escarpment Woods, HamiltonConservation Halton
228_2Waterdown Escarpment Woods, BurlingtonConservation Halton and private
228_3Borer's Creek Conservation Area/Rock Chapel Escarpment/Berry Tract, HamiltonHamilton Conservation Authority, Royal Botanical Gardens and private
228_4Niagara Glen/Niagara Parkway, Niagara FallsNiagara Parks Commission and Hydro One
228_8Ball's Falls Conservation Area, VinelandNiagara Peninsula Conservation Authority
228_9Rondeau Provincial Park - North, MorpethOntario Parks
Point Pelee National Park - South, Leamington (although historically one population, loss of a single, centrally located tree now places extant trees over 1 km apart. Critical habitat for this population is therefore mapped in two pieces)Parks Canada Agency
228_15Fish Point Provincial Nature Reserve, Pelee Island, western Lake Erie basinOntario Parks
228_16Middle Island, Point Pelee National Park, western Lake Erie basinParks Canada Agency
228_17East Sister Island Provincial Nature Reserve, western Lake Erie basinOntario Parks
Non-Core Populations (4 or fewer trees under 1 km away from at least one other tree)
228_5St. David'sPrivate
228_6Leawood Court, St. CatharinesPrivate
228_7Pendale Plaza, St. CatharinesBrock University
228_10Rondeau Provincial Park - South, MorpethOntario Parks
228_11Point Pelee National Park - North, LeamingtonParks Canada Agency
228_14Stone Road Alvar, Pelee IslandEssex Region Conservation Authority
228_18Lot 6, Concession 3 East, KingsvillePrivate
228_19For the Birds, ColchesterPrivate
228_20Big Creek Study Site #40, AmherstburgPrivate
228_21Canard River Kentucky Coffee-tree Woods Environmentally Sensitive Area, McGregorPrivate
228_22LaSalle Candidate Natural Heritage Site CA5, LaSalleTown of LaSalle and private

Figure 2: Canadian Distribution of Red Mulberry (Updated from Thompson 2002b)

Figure 2: Canadian distribution of Red Mulberry (updated from Thompson 2002b).

5.2 Population and Distribution Objectives

The goal of this recovery strategy is to ensure persistence of Red Mulberry in Canada by conserving and restoring functioning metapopulations10 to long-term stability in the two broad regions of its occurrence.

The population and distribution objectives for Red Mulberry are:

  1. to maintain all currently existing populations of the species across its Canadian (Ontario) range and
  2. to prevent further decline in the number of individuals across the species' range.

These objectives will be revisited and potentially revised once new information becomes available. In particular, genetic work to confirm the total number and location of pure Red Mulberry trees in some of the largest populations in Canada may necessitate changes to the objectives. Recent research in the United States has identified a distinct species, Morus murrayana, in western Kentucky and the surrounding states, that has been previously identified as Red Mulberry (Galla et al. 2009). Further genetic research is required across the native Canadian population to determine if one or both native species of mulberry are present and to ascertain associated population size(s).

The impacts of nesting Double-crested Cormorants on the Middle Island and East Sister Island populations are severe enough that retention of these two populations cannot be guaranteed. In addition, while attempts need to be made to retain the 11 populations consisting of one or two trees each, given the extremely small size of these populations, the fact each tree bears either male or female flowers, and the potential for extirpation through natural events, long term maintenance of these populations remains uncertain, even if threats are reduced. For instance, the June 6, 2010 tornado that touched down from Harrow to Leamington passed very near one of the Essex County woodlands containing a Red Mulberry population of two trees. Other woodlands along the direct path of the tornado were devastated.

It should be noted that the second objective is not specifically to maintain the number of mature individuals. This is because: (1) the current population estimates are uncertain, given the possible occurrences of hybrids and of a new species; (2) a number of mature individuals are dying and their loss cannot likely be prevented; and (3) the only way to maintain populations is to facilitate regeneration or plant young (non-mature) trees.

The expectation under the objectives, and the best possible scenario, is that in future evaluations Red Mulberry will remain in the "Very Small Total Population" category of COSEWIC, but not fall in the "Small and Declining Number of Mature Individuals" category.

9 Calcareous refers to a calcium or calcium carbonate-based substrate.
10 A metapopulation is made up of a group of populations of the same species that are separated from one another, but that still experience exchange in individuals.